Photorearrangement of α-azoxy ketones and triplet sensitization of azoxy compounds

Article abstract of DOI:10.1021/jo040274v

(Chemical Equation Presented) Although some aspects of azoxy group radical chemistry have been investigated,¹ unhindered α-azoxy radicals remain poorly understood. Here we report the generation of α-azoxy radicals under mild conditions by irradiation of α-azoxy ketones 4a,b. These compounds undergo α-cleavage to yield radicals 5a,b, whose oxygen atom then recombines with benzoyl radicals to produce presumed intermediate 15. Formal Claisen rearrangement gives α-benzoyloxyazo compounds 8a,b, which are themselves photolabile, leading to both radical and ionic decomposition. The ESR spectrum of 5a was simulated to extract the isotropic hyperfine splitting constants, which showed its resonance stabilization energy to be exceptionally large. Azoxy compounds have been found for the first time to be good quenchers of triplet excited acetophenone, the main sensitized photoreaction of 7Z in benzene being deoxygenation. While this reaction has been reported previously, it was always in hydrogen atom donating solvents, where chemical sensitization occurred. The principal direct irradiation product of 4bZ and model azoxyalkane 7Z is the E isomer, whose thermal reversion to Z is much faster than that of previously studied analogues.

Full text of DOI:10.1021/jo040274v

Photorearrangement of r-Azoxy Ketones and Triplet Sensitization
of Azoxy Compounds
Paul S. Engel,*,‡ Konstantin P. Tsvaygboym,‡ Sergei Bachilo,‡ William B. Smith,†
JinJie Jiang,§ Colin F. Chignell,§ and Ann G. Motten§
Department of Chemistry, Rice University, P.O. Box 1892, Houston, Texas 77005,
Department of Chemistry, Texas Christian University, Fort Worth, Texas 76129, and
Laboratory of Pharmacology and Chemistry, National Institute of Environmental Health Sciences,
Research Triangle Park, North Carolina 27709
engel@rice.edu
Received October 15, 2004
Although some aspects of azoxy group radical chemistry have been investigated,1 unhindered R-azoxy
radicals remain poorly understood. Here we report the generation of R-azoxy radicals under mild
conditions by irradiation of R-azoxy ketones 4a,b. These compounds undergo R-cleavage to yield
radicals 5a,b, whose oxygen atom then recombines with benzoyl radicals to produce presumed
intermediate 15. Formal Claisen rearrangement gives R-benzoyloxyazo compounds 8a,b, which
are themselves photolabile, leading to both radical and ionic decomposition. The ESR spectrum of
5a was simulated to extract the isotropic hyperfine splitting constants, which showed its resonance
stabilization energy to be exceptionally large. Azoxy compounds have been found for the first time
to be good quenchers of triplet excited acetophenone, the main sensitized photoreaction of 7Z in
benzene being deoxygenation. While this reaction has been reported previously, it was always in
hydrogen atom donating solvents, where chemical sensitization occurred. The principal direct
irradiation product of 4bZ and model azoxyalkane 7Z is the E isomer, whose thermal reversion to
Z is much faster than that of previously studied analogues.
Introduction
Several years ago, we reported that γ-azoxy radical 1
generated from a perester precursor undergoes fragmen-
tation to ethylene with a rate constant below 2 × 105 s-1
at 120 °C.9 Mysteriously, we were unable to find any
Despite its occurrence in a number of biologically active
molecules,2-7 the azoxy functional group has received less
attention than its lower oxidation state analogue, the
aliphatic azo group.8 The latter is a widely used source
of free radicals, but as pointed out in a recent review,1
the radical chemistry of azoxy compounds is relatively
sparse.
‡ Rice University.
† Texas Christian University.
§ National Institute of Environmental Health Sciences.
(1) Engel, P. S.; Duan, S.; He, S.-L.; Tsvaygboym, K.; Wang, C.-R.;
Wu, A.; Ying, Y.-M.; Smith, W. B. Arkivoc 2003, 12, 89-108.
(2) Brough, J. N.; Lythgoe, B.; Waterhouse, P. J. Chem. Soc. 1954,
4069-4079.
(3) Matsumoto, H.; Nagahama, T.; Larson, H. O. Biochem. J. 1965,
95, 13C-14C.
product attributable to the expected R-azoxy radical 2,
also named hydrazonyl oxide 3. On the other hand,
(4) Fujiu, M.; Sawairi, S.; Shimada, H.; Takaya, H.; Aoki, Y.; Okuda,
T.; Yokose, K. J. Antibiot. 1994, 47, 833.
(5) Ohwada, J.; Umeda, I.; Ontsuka, H.; Aoki, Y.; Shimma, N. Chem.
Pharm. Bull. 1994, 42, 1703-1705.
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(7) LaRue, T. A. Lloydia 1977, 40, 307-321.
(8) Engel, P. S. Chem. Rev. 1980, 80, 99-150.
(9) Engel, P. S.; He, S.-L.; Wang, C.; Duan, S.; Smith, W. B. J. Am.
Chem. Soc. 1999, 121, 6367-6374.
10.1021/jo040274v CCC: $30.25 © 2005 American Chemical Society
Published on Web 02/25/2005
2598
J. Org. Chem. 2005, 70, 2598-2605

Photorearrangement of R-Azoxy Ketones
TABLE 1. UV Extinction Coefficients of Azoxy and Azo
Compounds
compd
ꢀ (313 nm)
ꢀ (366 nm)
energy than biphenyl, decreased the conversion of 4a to
8a by 18.1%, corresponding to a Stern-Volmer kqτ of
1.77. To determine whether R-cleavage of the ketone
moiety was on the pathway from 4a to 8a, we trapped20,21
the benzoyl radicals with tert-butyl thiol. Irradiation of
4a at 313 nm with 0.16 M tert-butyl thiol afforded
benzaldehyde in 25% yield, as determined by NMR. A
corresponding reduction in the amount of 8a was ob-
served but no product from trapping of 5a could be
identified.
4a
128
830
4.7
212
32
262
41
16
4bZ
22
8a
26
80
1.9
7.7
6.7
8b
6
7Z
PhCOMe
bromination of azoxy compounds is known to occur at the
distal carbon, presumably via R-azoxy radicals.3,10
The photolysis of 4a was also investigated by ESR
spectroscopy. Irradiation of 4a in toluene-d8 at +8 °C
with a full 500 W mercury arc lamp gave a weak but
structured ESR signal. Suspecting that the signal was
due to radical 5a, we generated this intermediate inde-
pendently by irradiation of di-tert-butyl peroxide22,23
containing tert-butyl(ONN)azoxy-2-propane 9 at -78 °C.
A stronger signal ∼60 G wide containing about 37 lines
was observed and was very similar in appearance to the
one from 4a (cf. Supporting Information). The GC trace
Results
The goal of the present work was to generate R-azoxy
radicals under mild conditions in hopes of learning more
about their chemistry. Having failed to synthesize azo
or perester precursors to 2, we turned to the Norrish Type
I photochemical cleavage of R-azoxy ketone 4, a previ-
of the irradiated solution showed several peaks, none of
which were in the right region for the C-C dimer of 5a.9
To support the assignment of the ESR signals to 5a, the
experiment with 9 was repeated at -50 °C using better
ESR equipment. The superior resolution (cf. Supporting
Information) allowed isotropic simulation (correlation
coefficient 0.926) with the program WinSim,24 which led
to the following splitting constants: N1 13.75 G, N2 1.91
G, 3H 5.64 G, 3H 5.12 G. A second radical was present
in the spectrum but since its intensity was much lower
than that of 5a, this minor species was not studied
further.
Photochemistry of 4bZ. As shown in Table 1, 4bZ
absorbs about three times more strongly at 313 nm than
the sum of its two chromophores, which are represented
by acetophenone and 7Z.25-27 Irradiation of 4bZ at 313
nm both at 25 and -78 °C gave the intensely yellow 8b,
but the major reaction was Z f E photoisomerization
ously unknown structural type. Photolysis of some phe-
nones and benzyl ketones yields alkyl radicals in
solution;11-17 hence, it is not unreasonable that 4a-c
might also exhibit R-cleavage.
Photochemistry of 4a. The extinction coefficients of
4a at two photochemically useful wavelengths are in-
cluded in Table 1, and the spectra can be found in the
Supporting Information. The bichromophoric molecule
absorbs twice as strongly as the sum of its two individual
chromophores, represented by acetophenone and azoxy-
tert-butane 6.18 Irradiation of 4a in benzene at 313 nm
and 25 °C caused clean rearrangement to an unexpected
product, azoester 8a, whose UV data are included in
Table 1. The quantum yield for appearance of 8a was
0.02 at 23, 60, and 100 °C, and the reaction even
proceeded at -78 °C.
The disappearance rate of 4a was not diminished by
the inclusion of 0.1 M biphenyl19 as quencher, but 0.1 M
1,3-cyclohexadiene, a quencher of much lower triplet
(20) Brown, C. E.; Neville, A. G.; Rayner, D. M.; Ingold, K. U.;
Lusztyk, J. Aust. J. Chem. 1995, 48, 363-379.
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P.; Parker, A. W.; Towrie, M.; Turro, N. J.; Gill, P. M. W.; George, M.
W. J. Am. Chem. Soc. 2002, 124, 14952-14958.
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7159.
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(11) Gould, I. R.; Zimmt, M. B.; Turro, N. J.; Baretz, B. H.; Lehr, G.
F. J. Am. Chem. Soc. 1985, 107, 4607-4615.
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(13) Heine, H.-G. Jusuts Liebigs Ann. Chem. 1970, 732, 165-180.
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C. J. Org. Chem. 1998, 1935, 5-1943.
(24) http://epr.niehs.nih.gov/.
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1751-1754.
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2133.
(27) Sullivan, F. R.; Luck, E.; Kovacic, P. J. Org. Chem. 1974, 39,
2967-2970.
J. Org. Chem, Vol. 70, No. 7, 2005 2599

Engel et al.
TABLE 2. Relative GC Peak Areas in the Direct and Sensitized Photolysis of 7Z at 366 nm in Toulene-d8
a Conditions: -5 °C, 4 h, 62% of 7Z left unreacted. b Conditions: PhCOMe, -5 °C, 4 h, 58% of 7Z left unreacted.
SCHEME 1. Photoreactions of 4bZ
is the first reported triplet sensitized reaction of an azoxy
compound, we examined the azoxy group as a triplet
quencher. Acetophenone was chosen as the donor because
of its high triplet energy (73.6 kcal/mol),36 its structural
similarity to the ketone moiety of 4, and because it is
known to phosphoresce in solution.37 Azoxy-tert-butane
6 (0.012 M) and phenylazoxy-tert-butane 7Z (0.0021 M)
were found to be strong quenchers of acetophenone
emission intensity. The quenching rate constant in
isooctane was obtained by quenching the triplet lifetime
of acetophenone, giving kq values of 9.8 × 108 M-1 s-1 for
6 and 4.4 × 109 M-1 s-1 for 7Z.
Since 6 and 7Z proved to be good triplet quenchers,
we decided to investigate the triplet sensitized photo-
chemistry of 7Z. Two degassed and sealed NMR tubes
were prepared, one containing 0.0191 M 7Z in toluene-
d8 and the other containing the same concentration of
7Z plus 0.437 M acetophenone. The high concentration
of acetophenone was needed to make it the main light
absorber because the extinction coefficient of 7Z exceeded
that of acetophenone at 366 nm (cf. Table 1). Irradiating
both tubes in parallel for 4 h at 366 nm led to the same
products but in different amounts (cf. Table 2). Whereas
Z f E isomerization was dominant under direct irradia-
tion, this process is actually negligible with acetophenone
present. We calculate that 4.6% of the incident light was
absorbed directly by 7Z in the sensitized experiment, as
compared to 16% in the direct irradiation, where the
overall absorbance was only 0.073. Since the contribution
of direct photolysis in the sensitized reaction was 4.6/16
) 29%, the bulk of the 7E seen in the latter case arose
by direct irradiation of 7Z.
about the azoxy double bond.28-30 The E isomer (4bE)
reverted to the Z isomer 4bZ with a half-life of 14 min
at 50 °C. This behavior supports the structural assign-
ment of 4bE but additionally, the chemical shift changes
in toluene-d8 upon Z-E isomerization (4bZ Me2 1.716
ppm vs 4bE Me2 at 1.377 ppm) are similar to those of
7Z f 7E (1.472 vs 0.943 ppm). The magnitude of the
upfield shift upon Z f E isomerization is larger than that
in the methyl and ethyl analogues.29 Irradiation of 4bZ
at ambient temperature and 313 nm in acetone-d6 as
solvent and triplet sensitizer31-34 exhibited the same
rearrangement quantum yield as in toluene-d8. Irradia-
tionof 4bZ in toluene at -53 °C in the ESR cavity led to
a weak signal about 50 G wide consisting of 13 peaks,
which were much broader than those from 4a. This
spectrum was definitely not due to phenyldiazenyl radi-
cals35 arising from secondary photolysis of 8b. Instead,
the similarity of the major nitrogen coupling to that of
5a suggests that this radical is 5b.
Irradiation of 4c. This compound was designed to
decarbonylate photochemically but it proved to be quite
photostable. Thus 4c upon irradiation with an unfiltered
500 W mercury lamp gave neither 8c nor the E azoxy
isomer but instead slowly decomposed to a mixture of
many products.
The photoisomer 7E was thermally labile, exhibiting
a half-life for reversion to 7Z of 2.3 h at 25 °C. Due to
steric repulsion, 7E is much more labile than its ethyl
analogue, which reverts completely to the Z isomer in 5
h at 110 °C.29 On the other hand, 7E is slightly more
stable than 10Z, whose half-life for conversion of 10E is
1.36 h at 25 °C.38 The set 7E, 10Z provides a rare
opportunity to compare the thermal isomerization rate
of a cis azoalkane with that of its related azoxy com-
pound.
Triplet Sensitization of Model Azoxy Compounds.
Because the photorearrangement of 4bZ in acetone-d6
(28) Taylor, K. G.; Issac, S. R.; Swigert, J. L. J. Org. Chem. 1976,
41, 1146-1152.
(29) Taylor, G. K.; Riehl, T. J. Am. Chem. Soc. 1972, 94, 250-255.
(30) Bunce, N. J. Can. J. Chem. 1975, 53, 3477-3480.
(31) Engel, P. S.; Schexnayder, M. A. J. Am. Chem. Soc. 1972, 94,
4357-4359.
(32) Engel, P. S.; Schexnayder, M. A.; Ziffer, H.; Seeman, J. I. J.
Am. Chem. Soc. 1974, 96, 924.
(33) Warzecha, K.-D.; Leitich, J.; Demuth, M. J. Photochem. Pho-
tobiol. A: Chem. 2002, 152, 103-108.
(34) Cristol, S. J.; Kaufman, R. L.; Opitz, S. M.; Szalecki, W.; Bindel,
T. H. J. Am. Chem. Soc. 1983, 105, 3226-3234.
(35) Suehiro, T.; Tashiro, T.; Nakausa, R. Chem. Lett. 1980, 1339-
1342.
Deoxygenation was the main process observed under
triplet sensitization but the fate of oxygen is unknown.
The structure of the deoxygenated product, phenylazo-
(36) Engel, P. S.; Monroe, B. M. In Advances in Photochemistry;
Pitts, J. N. H.; Hammond, G. S.; Noyes, W. A., Eds.; John Wiley and
Sons: New York, 1971; Vol. 8.
(37) Takemura, T.; Aikawa, M.; Baba, H. Bull. Chem. Soc. Jpn.
1974, 47, 2476-2481.
(38) Porter, N. A.; Funk, M. O. J. Chem. Soc., Chem. Commun. 1973,
263-264.
2600 J. Org. Chem., Vol. 70, No. 7, 2005

Photorearrangement of R-Azoxy Ketones
TABLE 3. 15N Chemical Shiftsa
ppm), just as in the azo precursors (10E 1.311; 10Z
1.090).29 After 10Z had disappeared, the oxidation of 10E
continued at a much slower rate and oxygen was intro-
duced proximal18 to the tert-butyl group to yield major
product 11 accompanied by ∼6% 7Z. The reversal of
oxidation regiochemistry between 10E and 10Z is note-
worthy and apparently unprecedented since the only
acyclic cis azoalkane oxidized previously was symmetri-
cal.28
Secondary Photolysis of Azoesters. As seen in
Table 1, the azoester products 8a,b absorb enough light
to undergo secondary photoreactions and in fact these
can complicate the mechanistic interpretation of azoxy
ketone photolysis. Irradiation of authentic 8a,b yielded
nitrogen plus a mixture of products, as shown below. In
8b only, decomposition was accompanied by isomeriza-
tion to the cis azo isomer 13. Because it was initially
7Z calc
7Z obs
7E calc
7E obs
12 calc
Nb
Nc
1.2
-39.7
-20.2
-52.1
30.8
-23.9
1.4
-42.0
-115.4
-164.3
a ppm from nitromethane standard. b Nitrogen proximal to the
tert-butyl group. c N-O.
tert-butane 10Z,E, was proven by comparison with an
authentic sample.39,40 The sensitized irradiation was
repeated in C6D6 to rule out hydrogen abstraction from
the solvent, but the outcome was the same.
Because direct irradiation of azoxy compounds can led
to oxadiaziridines28,41-43 as well as Z f E isomeriza-
tion,28-30 it was important to verify the structure of 7E.
1
Surprisingly,44 no H NOE was seen for the tert-butyl
protons and the ortho H’s of the aromatic ring. This
negative result prompted us to examine the NOE of a
model compound 10Z, whose trans-cis photoisomeriza-
tion is well-known.38 Since 10Z also failed to exhibit
NOE, we resorted to comparing experimental 15N chemi-
cal shifts with those calculated theoretically,45 as shown
in Table 3. The proximal and distal nitrogens were
1
assigned unambiguously by H-15N HMBC.
Although the calculated shifts are 10-20 ppm down-
field from the observed value for 7Z, the predicted
direction and magnitude of the changes upon Z f E
isomerization lie within 8 ppm of the observed value. The
chemical shifts of oxadiaziridine 12 are calculated to fall
drastically upfield from those of 7Z,E, ruling out 12 as
the photoisomer.
Additional support for the structure of 7E was provided
by its independent synthesis from 10Z. A solution of 10E
in toluene-d8 was irradiated in an NMR tube with a 150
W xenon lamp until the 10Z:10E ratio was 0.35. The
mixture was oxidized in situ with MCPBA, resulting in
the clean conversion of all 10Z to 7E within 10 min. The
tert-butyl group of 7E (δ ) 0.943 ppm) exhibited a sizable
upfield shift in toluene-d8 relative to that of 7Z (1.472
surprising to find benzoic acid as the major product of
8a,b, the irradiation of 8a was repeated in MeOH-d4. We
found that the benzoyloxy group was replaced in part by
CD3O to yield 14, consistent with ionic dissociation of the
substituent R to the azo group.46,47
(39) Porter, N. A.; Marnett, L. J. J. Am. Chem. Soc. 1973, 95, 4361-
4367.
(40) Curtin, D. Y.; Ursprung, J. A. J. Org. Chem. 1956, 21, 1221-
1225.
(41) Greene, F. D.; Hecht, S. S. J. Org. Chem. 1970, 35, 2482.
(42) Swigert, J.; Taylor, K. G. J. Am. Chem. Soc. 1971, 93, 7337-
7338.
(43) Olsen, H.; Pedersen, C. L. Acta Chem. Scand. B 1982, 36, 701-
706.
(44) Anderson, J. E.; Barkel, D. J. D. J. Chem. Soc. Perkin Trans.
II 1984, 1053-1057.
(45) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.,
Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.;
Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.;
Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo,
C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.;
Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen,
W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.;
Pople, J. A. Gaussian 98, revision A.4; Gaussian, Inc.: Pittsburgh, PA,
1998.
Discussion
Mechanism of 4 to 8. The simplest mechanism for
the photorearrangement begins with R-cleavage to ben-
zoyl radical plus R-azoxy radical 5a. These radicals
(46) Levi, N.; Malament, D. S. Isr. J. Chem. 1974, 12, 925-936.
(47) Levi, N.; Malament, D. S. J. Chem Soc. Perkin Trans. II 1976,
1249-1256.
J. Org. Chem, Vol. 70, No. 7, 2005 2601

Engel et al.
TABLE 4. Calculated Activation Energy for Claisen
Rearrangement of 15 to 8
whether it came from 4a or from secondary photolysis of
8a. However, a plot of benzaldehyde versus time showed
a sizable initial slope while a similar plot of acetone, a
decomposition product of 8, showed an initial slope of
zero. If benzaldehyde arose solely from secondary pho-
tolysis of 8a, both plots would have exhibited an initial
slope of zero. The ESR experiment with 4a also supports
initial R-cleavage since essentially the same spectrum
was observed from 4a and from irradiation of 9 with di-
tert-butyl peroxide. The nitrogen splitting constants
extracted from the latter spectrum (13.75, 1.91 G) are in
accord with those of known heavily substituted hydra-
zonyl oxide radicals (e.g. for t-Bu2CdN2-N1(O•)-Ph,
a(N1) ) 12.1 G, a(N2) ) 2.7 G54 and for t-BuPhCdN2-
N1(O•)-t-Bu, a(N1) ) 12.5 G, a(N2) ) 1.7 G55 and in
acceptable agreement with the splittings calculated
quantum mechanically45 (cf. Supporting Information).
The proton splittings of 5a are exceptionally small,
reflecting the high degree of radical stabilization that one
might expect for this nitroxyl structure.56 Thus 1,1-
dimethylallyl radical exhibits a(CH3) ) 12.22, 15.35 G
while 5a exhibits a(CH3) ) 5.12, 5.64 G. Application of
Ruchardt’s eq 4,23 which relates â hyperfine splittings to
radical stability, reveals that the resonance stabilization
energy of 5a is 28.1 kcal/mol, which is twice that of the
1,1-dimethylallyl radical.
HF,
hartrees
TS,
hartrees
∆Hq,
kcal/mol
R1
R2 R3
H
H
H
H
H
H
H
H
-338.465 327 5 -338.419 204 8
-415.868 578 8 -415.828 300 7
-569.530 707 4 -569.490 234 4
28.9
25.3
25.4
21.7
19.3
21.0
H2CdCH-
Ph
H
H
Me -417.103 647 5 -417.069 106 5
Me Me -456.418 324 7 -456.387 535 6
H2CdCH- Me Me -533.822 631 1 -533.789 195 6
recombine at azoxy oxygen to afford intermediate 15,
which then rearranges to the final product 8. However,
careful scrutiny of the reaction by low-temperature NMR
failed to reveal the presence of 15. We were misled
initially by the appearance of broad 1H NMR peaks from
4a (but not from 4bZ) that built up to a 1:5 ratio relative
to starting material (cf. Supporting Information). Since
these peaks reverted to those of 4a on warming to -25
°C, fell at unreasonable chemical shifts for 15, and were
too labile to be the E azoxy isomer, their nature remains
obscure.
A literature search revealed no published structures
quite like 1548-51 and an attempt to generate 15 from
acetone tert-butyl hydrazone and benzoyl peroxide52 at 0
°C led cleanly to 8a and unknown NMR peaks not
consistent with 15a. In hopes of understanding why we
failed to observe this postulated intermediate by NMR,
the activation energy for Claisen rearrangement was
calculated theoretically at the B3LYP/6-31G* level.45
Table 4 shows that the activation enthalpy lies in the
same range as that for the Cope rearrangement.53 Radical
delocalizing substituents R1 and methyl groups R2 and
R3 decrease ∆Hq but not enough to explain our inability
to detect 15. Homolytic dissociation of 15 was calculated
to require more energy than the figures in Table 4.
Perhaps 15 absorbs UV light as does a peroxide or
N-chloroamine and 15 f 8 occurs photochemically.
Lacking experimental evidence for 15, we sought to
verify that R-cleavage of 4a really is the first step of the
reaction. Irradiation of this azoxy ketone with tert-butyl
thiol led to benzaldehyde20,21 but it was not clear initially
There remains the possibility that R-cleavage is a side
reaction not on the path from 4a to 8a. However, in a
t-BuSH trapping experiment where 60% of 4a reacted,
the yield of PhCHO was 25% while that of 8a and its
photolysis products was 21% and 15%, respectively. It
follows that t-BuSH diminishes the amount of 8a by an
amount very nearly equal to the yield of PhCHO, thus
placing PhCO• on the reaction pathway. Since t-BuSH
failed to trap 5a, our only evidence for this radical comes
from the ESR experiment.
The low quantum yield for photorearrangement of 4a,b
may be attributed to efficient recombination of benzoyl
radicals at the original site of attachment. However, the
spin density of 5a is higher at oxygen than at carbon, as
shown by the calculated spin densities below. Moreover,
cage recombination of other “allylic” radicals occurs at
both ends.57 Another explanation for the low quantum
yield is radiationless decay of excited 4a,b, which could
be accompanied by Z f E isomerization of the azoxy
group. To explore the importance of these factors, we
undertook a brief investigation of the azoxy group as a
triplet energy acceptor.
(48) D’Anello, M.; Erba, E.; Gelmi, M. L.; Pocar, D. Chem. Ber. 1988,
121, 67-73.
(49) Steglilch, W.; Kubel, B.; Gruber, P. Chem. Ber. 1973, 106, 2870-
2876.
Triplet Energy Transfer to Model Azoxy Com-
pounds. We report for the first time that azoxyalkanes
(50) Wieland, H. Justus Liebigs Ann. Chem. 1907, 353, 65-105.
(51) Yanai, M.; Kinoshita, T. Chem. Pharm. Bull. 1968, 16, 1221-
1227.
(54) Ahrens, W.; Berndt, A. Tetrahderon Lett. 1974, 3741-3742.
(55) Aurich, H. G.; Lotz, I.; Weiss, W. Tetrahedron 1978, 34, 879-
885.
(52) Edward, J. T.; Samad, S. A. Can. J. Chem. 1963, 41, 1638-
1640.
(56) Creary, X.; Engel, P. S.; Kavaluskas, N.; Pan, L.; Wolf, A. J.
Org. Chem. 1999, 64, 5634-5643.
(53) Hrovat, D. A.; Beno, B. R.; Lange, H.; Yoo, H.-Y.; Houk, K. N.;
Borden, W. T. J. Am. Chem. Soc. 1999, 121, 10529-10537.
(57) Engel, P. S.; Gerth, D. B. J. Am. Chem. Soc. 1983, 105, 6849-
6851.
2602 J. Org. Chem., Vol. 70, No. 7, 2005

Photorearrangement of R-Azoxy Ketones
SCHEME 2. Photolysis of 8a
are surprisingly rapid triplet quenchers. The quenching
rate constant of acetophenone triplets was in the range
ever, Wille recently reported that photolysis of 17 gave
cyclohexanone in 33% yield by fragmentation of radical
18,69 exactly analogous to the behavior of 16.
of 109 M-1 -1, which is similar to that for azo-tert-
s
butane58 and for tetramethyldiazetine dioxide quenching
benzanthrone triplets.59 The “triplet sensitized” deoxy-
genation of azoxybenzene60 was shown to be a case of
chemical sensitization;61,62 that is, ketyl radicals derived
from the sensitizer reduced the azoxy group.63 For this
reason, we initially suspected that our acetophenone-
sensitized deoxygenation of 7Z in toluene (cf. Table 2)
followed the same pathway. However, the product dis-
tribution was unchanged in C6D6 solvent, suggesting that
triplet sensitized deoxygenation64 is a true photoreaction
of 7Z. In contrast, direct irradiation gave cis-trans
isomerization, already a well-established process.28,29,65
Investigating the triplet energy of azoxy compounds and
the fate of oxygen are potential topics for further re-
search.
The major photolysis product of 8a,b is benzoic acid
but Scheme 2 cannot explain its presence. In a rarely
cited set of papers dating from nearly 30 years ago, Levi
and Malament reported that acyclic azoalkanes contain-
ing R-chloro or R-acyloxy groups underwent heterolytic
cleavage even in nonpolar solvents.46,47 The same unusual
mechanism can be applied to formation of benzoate and
the known highly stabilized R-azo cation 19.70 Most likely,
Although the azoxy group quenches ketone triplets
intermolecularly, it is inappropriate to discuss 4a,b in
terms of individual chromophores because the enhanced
UV absorption (Table 1) indicates mixing of electronic
states.66 The experimental facts are that upon direct
irradiation, neither compound undergoes deoxygenation,
only 4bZ exhibits cis-trans isomerization, and both
compounds undergo inefficient R-cleavage of the benzoyl
group.
Photolysis of R-Benzoyloxyazoalkanes. The initial
products 8a,b are themselves photolabile, giving rise to
a product mixture dominated by benzoic acid and acetone.
Part of the likely mechanism, shown in Scheme 2,
involves the usual nitrogen loss from azoalkanes, pre-
sumably via thermolysis of the labile cis isomer8 followed
by plausible reactions of the formed tert-butyl and
acyloxyalkyl radicals 16. Published information on 1-acyl-
oxyalkyl radicals is sparse and mainly concerns their
inter- and intramolecular addition to alkenes.67,68 How-
benzoic acid arises by proton transfer from adventitious
water to benzoate, as the ortho protons of the acid were
clearly visible by NMR of the unopened, degassed, sealed
tubes. The ionic mechanism cannot operate exclusively,
even though acetone could arise when 19 traps water,
because it cannot rationalize the obviously free-radical-
derived products isobutane, benzaldehyde, benzophenone,
and isopropyl benzoate. Thus 8a undergoes competitive
C-N homolysis and C-O heterolysis. To support the
ionic mechanism, we irradiated 8a in CD3OD, monitoring
the course of the reaction by 1H NMR. The starting
material disappeared over 19 h and gave mainly 14, as
proven by comparison with an authentic sample of the
nondeuterated analogue. On further irradiation, 14 also
decomposed.
(58) Wamser, C. C.; Medary, R. T.; Kochevar, I. E.; Turro, N. J.;
Chang, P. L. J. Am. Chem. Soc. 1975, 97, 7.
(59) Singh, P.; Ullman, E. F. J. Am. Chem. Soc. 1976, 98, 3018-
3019.
(60) Tanikaga, R. Bull. Chem. Soc. Jpn. 1968, 41, 1664-1668.
(61) Monroe, B. M.; Wamser, C. C. Mol. Photochem. 1970, 2, 213-
223.
(62) Drewer, R. J. In The Chemistry of the Hydrazo, Azo, and Azoxy
Groups; Wiley: London, UK, 1975; Vol. Part 2, pp 1006-1010.
(63) Engel, P. S.; Wu, W.-X. J. Am. Chem. Soc. 1989, 111, 1830-
1835.
Irradiation of 8b in C6D6 caused azo trans-cis isomer-
ization, in accord with the known photochemistry of
phenylazoalkanes.39 Several of the products were similar
to those of 8a, again suggesting a competition between
(64) Hata, N.; Ono, I.; Kawasaki, M. Chem. Lett. 1975, 25-28.
(65) Basch, H.; Hoz, T. Mol. Phys. 1997, 91, 789-803.
(66) DeSchrijver, F. C.; Boens, N.; Put, M. A. Adv. Photochem. 1977,
10, 359.
(67) Beckwith, A. L. J.; Glover, S. A. Aust. J. Chem. 1987, 40, 157-
173.
(68) Giese, B.; Harnisch, H.; Leuning, U. Chem. Ber. 1985, 118,
1345-1351.
(69) Wille, U. J. Am. Chem. Soc. 2002, 124, 14-15.
(70) Malament, D. S.; McBride, J. M. J. Am. Chem. Soc. 1970, 92,
4586.
J. Org. Chem, Vol. 70, No. 7, 2005 2603

Engel et al.
132.2, 127.78, 127.72, 76.1, 68.6, 27.6, 22.5. NMR (toluene-d8)
δ 198.2, 136.0, 131.8, 128.2, 127.7, 75.8, 68.7, 27.4, 22.6.
2-Phenyl(ONN)azoxy-2-benzoylpropane (4bZ), PhCOC-
Me2-NdN(O)-Ph. 2-Benzoyl-2-(N,N-dichloroamino)propane
was reacted with 1.1 equiv of nitrosobenzene in MeCN as
described above, except that the nitroso dimer dissociation step
was omitted because nitrosobenzene exists largely as the
monomer. The product was purified on silica gel, eluting with
5:1 hexane:ethyl acetate, Rf 0.43. Vacuum drying afforded 325
mg (68%) of product that was further purified by recrystalli-
zation from hot hexane (2 mL), mp 56-58 °C. 1H NMR
(toluene-d8) δ 8.02 (m, 2H), 7.86 (m, 2H), 6.80-6.96 (m, 6H),
1.72 (s, 6H). 13C NMR (toluene-d8) δ 197.5, 147.3, 135.5, 132.2,
131.6, 128.7, 128.2, 127.9, 122.2, 69.6, 22.9.
2-Amino-2-phenylacetylpropane Hydrochloride, Ph-
CH2-COCMe2-NH3Cl. To a mixture of 3-methyl-1-phenyl-
2-butene (0.7 g, 4.79 mmol) and isopentyl nitrite (0.77 mL, 5.75
mmol) cooled to 5 °C was added dropwise concentrated HCl
(0.96 mL, 11.5 mmol, 12 N). After the solution was stirred for
15 min, the nitroso chloride dimer precipitated as a greenish
solid. This product was washed with a small portion of warm
acetone, dried in vacuo, and used in the next step without
further purification. Yield 0.50 g (49%) of colorless crystals,
mp 133-134 °C (lit. mp 136-137 °C).72 The nitroso dimer (0.25
g, 1.18 mmol) was mixed with MeOH:EtOH (3 mL:3 mL) and
the solution was stirred for 24 h under under 10 psi of NH3 at
45 °C,73 causing eventual dissolution of the dimer. The solvent
was evaporated and the solid was dissolved in 50 mL of 6 M
HCl by heating to 50 °C. This solution was extracted with
ether, and then the aqueous phase was made alkaline with
sodium carbonate (note: CO2 evolution!). Upon raising the pH
to 10, the color changed from yellow to blue/green. The
precipitate was dissolved in ether and dried over Na2SO4, and
the solvent was evaporated. The residue (190 mg) was found
by NMR to be a mixture of oxime and ketone. A solution of 6
N aq HCl (10 mL) was added and the mixture was stirred for
1.5 h at 50 °C. The solvent was evaporated and the residue
dissolved in a small amount of isopropyl alcohol by heating to
50 °C. The salt was precipitated by addition of ether, filtered,
and dried in vacuo. Yield 91 mg (36%) of PhCH2-COCMe2-
NH3Cl, mp 134-141 °C. 1H NMR (MeOH-d4) δ 7.23-7.34 (m,
5H), 1.95 (s, 2H), 1.64 (s, 6H). 13C NMR (MeOH-d4) δ 207.0,
134.8, 130.9, 129.7, 128.3, 63.4, 43.1, 23.1.
ionic and homolytic decomposition. However, the yield
of benzophenone and benzaldehyde from 8a was much
higher than that from 8b, indicating that that the C-N
homolysis pathway is more important in 8a (cf. Scheme
2). The R-azo cation from 8b is surely more stabilized
than 19, favoring ionic decomposition of 8b.
Conclusions
Despite our failure to identify any products from 5a,
5b, their involvement in the photorearrangement of
R-azoxy ketones 4a, 4bZ to 8a, 8b and their observation
by ESR shows that sterically unhindered R-azoxy radicals
are viable intermediates. The small hydrogen hyperfine
splittings in the ESR spectrum of 5a indicate a very high
degree of resonance stabilization. Direct or acetone
sensitized irradiation of 4bZ also induces azoxy Z f E
isomerization, but in model compound 7Z acetophenone
triplets cause deoxygenation without Z f E isomeriza-
tion. Azoxy compounds are surprisingly rapid quenchers
of acetophenone triplets. In both 4bZ and 7, steric
repulsion causes thermal reversion of the E azoxy isomer
to be much faster than in previously reported homo-
logues. Azoesters 8a,b undergo photochemical C-O
heterolysis in competition with C-N homolysis to 1-acyl-
oxy radicals, which fragment to ketones plus acyl radi-
cals.
Experimental Section
2-tert-Butyl(ONN)azoxy-2-benzoylpropane, PhCOC-
Me2-NdN(O)-Bu-t (4a). A solution of nitroso-tert-butane
dimer (250 mg, 1.43 mmol, 0.8 equiv) in CH3CN (10 mL) was
stirred for 3.5 h at 25 °C in the dark to allow dissociation to
monomer. Meanwhile, a solution of 2-benzoyl-2-aminopropane
hydrochloride71 (795 mg, 3.98 mmol, 1 equiv) in 1.5 N aq HCl
(12 mL, 4.5 equiv) was added dropwise to a suspension of Ca-
(OCl)2 (1.424 g, 5.97 mmol, 60 wt %, 1.5 equiv) in CH2Cl2 (24
mL) and water (24 mL) at 5 °C. After the mixture was stirred
for 1 h at 5 °C, the organic layer was separated and the water
layer was extracted with CH2Cl2 (2 × 8 mL). The combined
organic phase was dried over MgSO4, filtered, and concen-
trated to a yellow oil (0.806 g, 87% yield). The crude N,N-
dichloroamine was light sensitive and was used immediately
in the next step. 1H NMR (CDCl3) δ 8.23 (m, 2H), 7.56 (m,
1H), 7.45 (m, 2H), 1.74 (s, 6H). Following Nelson et al.,25 KI
(298 mg, 1.79 mmol) was added to the above nitroso-tert-
butane solution at 25 °C, the temperature was lowered to 0
°C, and the N,N-dichloroamine (416 mg, 1.79 mmol) in MeCN
(5 mL) was added. The mixture was stirred for 2-3 h at 5 °C,
then the temperature was gradually raised to 25 °C and the
mixture was stirred overnight. Water (50 mL) and ether (25
mL) were stirred in and the water layer was separated and
extracted with ether (2 × 8 mL). The combined organic layer
was extracted with sufficient aq Na2S2O3 (∼312 mg, 1.97
mmol) to change the color from dark brown to light green. After
drying over MgSO4 and removal of the solvent, the crystalline
product was purified by silica gel chromatography, eluting with
4:1 hexane:ethyl acetate, Rf 0.52. Solvent evaporation yielded
273 mg (61%) of R-azoxy ketone 4a, mp 70.5-71 °C. Further
purification was effected by recrystallization from a small
2-tert-Butyl(ONN)azoxy-2-phenylacetylpropane, Ph-
CH2-COCMe2-NdN(O)-Bu-t (4c), was prepared in the same
manner as PhCO-CMe2-NdN(O)-Bu-t. Purification on silica
gel eluting with 4:1 hexane:ethyl acetate (Rf 0.51) yielded 32
mg of clear, oily product (42% based on PhCH2-CO-CMe2-
NH3Cl). 1H NMR (toluene-d8) δ 7.00-7.21 (m, 5H), 3.45 (s, 2H),
1.34 (s, 6H), 1.29 (s, 9H). 13C NMR (toluene-d8) δ 203.8, 137.5,
130.3, 128.4, 126.7, 76.1, 69.3, 43.5, 27.8, 21.5.
Azoxy-tert-butane was made according to the method of
Freeman18 and fractionally distilled twice, bp 66 °C/40 mm.
1H NMR (C6D6) δ 1.40 (s, 9H), 1.35 (s, 9H). 13C NMR (C6D6) δ
76.39, 57.83, 28.25, 25.69.
tert-Butyl(O,N,N)azoxy-2-propane, i-Pr-NdN(O)-Bu-t,9
was synthesized by a modified literature procedure.74 Nitroso-
tert-butane dimer (217.5 mg, 1.25 mmol) in absolute EtOH (2.5
mL) was stirred for 3 h at 25 °C in the dark. In a separate
vessel, i-PrNHOH‚HCl (290 mg, 2.6 mmol) was added in one
portion to a solution of KOH (154 mg, 2.75 mmol) in absolute
EtOH (2.5 mL). After brief stirring, the ethanolic suspension
of i-PrNHOH was added to a dark blue solution of t-BuNO.
The mixture was stirred for 2 h at 25 °C and for 16 h at 38 °C,
1
amount of MeOH. H NMR (CDCl3) δ 7.84 (m, 2H), 7.46 (m,
1H), 7.35 (m, 2H), 1.59 (s, 6H), 1.31 (s, 9H). NMR (C6D6) δ
7.99 (m, 2H), 7.08 (m, 1H), 6.99 (m, 2H), 1.64 (s, 6H), 1.07 (s,
9H). NMR (toluene-d8) δ 7.89 (m, 2H), 7.10 (m, 1H), 7.01 (m,
2H), 1.59 (s, 6H), 1.09 (s, 9H). 13C NMR (CDCl3) δ 199.7, 135.0,
(72) Gnichtel, H. e. a.; Beier, M. Justus Liebigs Ann. Chem. 1981,
312-316.
(73) Al-Hassan, S. S.; Cameron, R. J.; Curran, A. W. C.; Lyall, W.
J. S.; Nicholson, S. H.; Robinson, D. R.; Stuart, A.; Suckling, C. J.;
Stirling, I.; Wood, H. C. S. J. Chem. Soc. Perkin Trans. 1 1985, 1645-
1659.
(71) Zawalski, R. C.; Lisiak, M.; Kovacic, P.; Leudtke, A.; Timber-
lake, J. W. Tetrahedron Lett. 1980, 21, 425-428.
(74) Taylor, K. G.; Issac, S. R.; Clark, M. S. J. Org. Chem. 1976, 41,
1135-1140.
2604 J. Org. Chem., Vol. 70, No. 7, 2005

Photorearrangement of R-Azoxy Ketones
after which it was diluted with 1 N aq HCl (5 mL), extracted
with pentane (10 × 3 mL), and dried over Na2SO4. The solvent
was removed by careful bulb-to-bulb distillation at 50 mmHg,
cooling the receiver in a dry ice-2-propanol bath. Since the
azoxyalkane product was highly volatile, some loss was
unavoidable. The residue was distilled by reducing the pres-
sure and the product was purified by preparative HPLC on a
silica gel column, eluting with 10:1 pentane:ethyl ether. Bulb-
to-bulb distillation at 50 mmHg removed the solvent, after
which the residue was distilled at 0.001 mm to a trap at -196
°C. Yield 55 µL. GC analysis of the collected solvent indicated
that it contained approximately 67 µL of i-Pr-NdN(O)Bu-t.
1H NMR (C6D6) δ 4.32 (septet, 1H, J ) 6.4 Hz), 1.36 (s, 9H),
1.15 (d, 6H, J ) 6.4 Hz). 13C NMR (C6D6) δ 75.5, 51.0, 28.1,
19.5.
2-tert-Butylazo-2-benzoyloxypropane, PhCOO-CMe2-
NdN-Bu-t (8a). Crude 2-tert-butylazo-2-chloropropane, t-Bu-
NdN-CMe2-Cl,75 was treated with silver benzoate as in the
synthesis of 8b above. The product was purified on silica gel,
eluting with 9:1 hexane:ethyl acetate, Rf 0.48. Yield 0.956 g
(53% from acetone-tert-butyl hydrazone). The azo compound
crystallized at 25 °C over time, yielding ice-like, transparent
crystals that melted at 29-30 °C to a light beige oil. 1H NMR
(C6D6) δ 8.20-8.22 (m, 2H), 7.02-7.12 (m, 3H), 1.65 (s, 6H),
1.20 (s, 9H). 13C NMR (C6D6) δ 164.75, 132.67, 132.02, 130.04,
128.42, 101.32, 67.09, 26.69, 24.59.
2-tert-Butylazo-2-methoxypropane,
t-Bu-NdN-C-
(Me)2-OMe (protiated 14).76 To t-Bu-NdN-CMe2-Cl (0.934
g, 7.29 mmol) prepared as above and cooled to -78 °C was
added 2 mL of MeOH.77 In a separate vessel, sodium methoxide
(0.45 g, 8.39 mmol) was dissolved in MeOH (4 mL) and the
solution was added to the reaction mixture dropwise at -78
°C. The mixture was stirred at -78 °C for 10 min and the
temperature was allowed to increase gradually to 25 °C over
3 h. Water and CH2Cl2 were added, the mixture was shaken,
and the aqueous phase was extracted with CH2Cl2 several
times. The combined organic phase was dried over Na2SO4 and
the solvent was evaporated. Removal of residual solvent in
vacuo at ∼40 mmHg yielded 0.509 g (44% from t-Bu-NH-
NdCMe2) of volatile liquid. 1H NMR (MeOH-d4) δ 3.39 (s, 3H),
1.21 (s, 6H), 1.19 (s, 9H).
t-Bu-NdN(O)-Ph (7Z) was made according to Sullivan
et al.27 and distilled, bp 44-62 °C/0.001 mm. Further purifica-
tion was effected by silica gel chromatography, eluting with
9:1 hexane:EtOAc, Rf 0.54. The pure compound retains a
yellow coloration. 1H NMR (toluene-d8) δ 8.13 (m, 2H), 6.90-
7.02 (m, 3H), 1.47 (s, 9H). 13C NMR (toluene-d8) δ 149.6, 131.4,
128.9, 122.8, 59.2, 26.3. 15N NMR (δ vs MeNO2) -52.1 (N-O),
-20.2. The 15N HMBC experiment was optimized for a long-
range N-H coupling of 3 Hz.
t-Bu-NdN(O)-Ph (7E): 1H NMR (toluene-d8) δ 6.92-7.03
(m, 3H), 6.78 (m, 2H), 0.94 (s, 9H). 15N NMR (δ vs MeNO2)
-42.0 (N-O), 1.4.
2-Phenylazo-2-benzoyloxypropane (8b), PhCOO-C-
(Me)2-NdN-Ph. To freshly distilled PhNHNH2 (3.85 g, 35.6
mmol) and water (29 mL) at 5 °C was added acetic acid (0.95
mL, 16.6 mmol), then dropwise 5.8 mL of acetone. After the
solution was stirred for 1.5 h, the air-sensitive hydrazone was
quickly filtered off, washed twice with ice water, and dried in
vacuo. Yield 90%. NMR (C6D6) δ 1.04 (s, 3H), 1.75 (s, 3H),
6.30-6.50 (br, 1H), 6.72-7.30 (m. 5H). To the freshly prepared
hydrazone (1.446 g, 9.77 mmol) in CH2Cl2 (8 mL) at -78 °C
was added dropwise t-BuOCl (1.219 g, 11.24 mmol). The
mixture was stirred for 2.5 h as the temperature rose to 25
°C. The solvent and tert-butyl alcohol byproduct were evapo-
rated and the residual Ph-NdN-CMe2-Cl was dissolved in
benzene (5 mL). Because neat Ph-NdN-CMe2-Cl decom-
poses within 30 min at 25 °C, this freshly prepared solution
was added dropwise to silver benzoate (1.79 g, 7.80 mmol) in
benzene (10 mL) at 5 °C. The heterogeneous mixture was
stirred for 1 h at 5 °C, then at 25 °C for 3 days. The azoester
suspension was filtered through a cotton plug and then
through filter paper and the filtrate was concentrated to a deep
yellow oil. The crude product was purified on silica gel, eluting
with 9:1 hexane/ethyl acetate, Rf 0.47. Yield 1.71 g (81%) of
deep yellow solid with mp 48-49 °C. The compound crystal-
lized from hexane after maintaining the solution at -20 °C
for several weeks. 1H NMR (C6D6) δ 8.26 (m, 2H), 7.79 (m,
2H), 7.03-7.16 (m, 6H), 1.78 (s, 6H). 13C NMR (C6D6) δ 165.3,
152.3, 133.2, 132.2, 131.4, 130.5, 129.5, 128.9, 123.4, 102.6,
25.2.
Acknowledgment. We thank the Robert A. Welch
Foundation for support of this work and the National
Science Foundation for the 400 MHz spectrometer,
grant no. CHEM 0075728. Professors Graham Palmer
and R. Bruce Weisman generously provided ESR and
laser flash photolysis equipment, respectively. Dr. Mar-
ian Fabian kindly assisted with the ESR work at Rice
University, Dr. Larry B. Alemany provided invaluable
help with the NMR experiments, and Mr. Artur F.
Izmaylov carried out some of the quantum chemical
calculations.
Supporting Information Available: Calculated isotropic
Fermi contact couplings for 5a, ESR spectra of 5a, UV spectra
of 4a, 6, 8a, 4bZ, 7Z, and 8b, computed structures of 15, 8,
5a, and NMR spectra. This material is available free of charge
via the Internet at http://pubs.acs.org.
JO040274V
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