[3 + 2] Cycloadditions and protonation by alcohols of photochemically generated nitrile ylides from 2H-Azirines. Formation and reactivities of azaallenium cations

Article abstract of DOI:10.1021/ja971648n

On photolysis of 2H-phenylazirines in acetonitrile or alcohol solution with 248 nm laser light, phenylnitrile ylides are formed by heterolytic cleavage of the C-C bond of the azirines. The absorption spectra of the ylides are characterized by two strong b

Full text of DOI:10.1021/ja971648n

J. Am. Chem. Soc. 1997, 119, 11605-11610
11605
[3 + 2] Cycloadditions and Protonation by Alcohols of
Photochemically Generated Nitrile Ylides from 2H-Azirines.
Formation and Reactivities of Azaallenium Cations
Evelyn Albrecht,1 Jochen Mattay,*,2 and Steen Steenken*,3
Contribution from the Institut fu¨r Organische Chemie der UniVersita¨t Mu¨nster,
Corrensstrasse 40, D-48149 Mu¨nster, Germany, Institut fu¨r Organische Chemie der UniVersita¨t
Kiel, Olshausenstrasse 40, D-24098 Kiel, Germany, and Max-Planck-Institut fu¨r Strahlenchemie,
D-45413 Mu¨lheim, Germany
ReceiVed May 20, 1997X
Abstract: On photolysis of 2H-phenylazirines in acetonitrile or alcohol solution with 248 nm laser light, phenylnitrile
ylides are formed by heterolytic cleavage of the C-C bond of the azirines. The absorption spectra of the ylides are
characterized by two strong bands, at ca. 240 and 280 nm, and a weak band, at 380 nm. Electron-deficient olefins
react with the nitrile ylides by 1,3-dipolar cycloaddition to yield 5-membered N-heterocycles (rate constants between
4 × 105 and 7 × 109 M-1 s-1, the Hammett-F value for the para-substituent on the phenyl ring is -0.9). In alcohols
as solvents, the nitrile ylides are protonated to yield azaallenium cations, whose spectroscopic properties and reactivities
with nucleophiles are described. The protonation rates of the ylides in alcohols increase with the acidity of the
alcohol. On the basis of the large kinetic isotope effect for protonation of ylide by alcohol (kH/kD ) 5.5), the transition
state for the ylide protonation is concluded to be linear.
Introduction
Scheme 1
N
H
–
C
+
C
H
hν
2H-Azirines are photochemically highly active substances.
Upon irradiation into their n-π* absorption bands the strained
3-membered azirine ring opens selectively at the C-C bond in
a heterolytic fashion resulting in the formation of a nitrile ylide
(NY)4,5 (Scheme 1). The nitrile ylide can be trapped by
dipolarophiles (AdB) or alcohols. Due to the pronounced
reactivity of nitrile ylides with dipolarophiles, azirines are
frequently used in [3 + 2] cycloaddition reactions.6 A recent
example is the facile synthesis of exohedrally functionalized
fullerenes by five-membered heterocycles.7 In the presence of
alcohols nitrile ylides form alkoxyimines,6,8 and a question is8,9
whether this reaction proceeds via the intermediate production
of azaallenium cations (Scheme 2). To shed light on this and
related questions, we have performed a systematic study of a
Ph
N
Ph
Ph
Ph
Scheme 2
N
+H+
–
C
+
C
+
N
hν
laser
H
R
H
Ar
H
R
Ar
N
C
C
H
Ar
R
2H-azirine
R = H, Ph
nitrile ylide
azaallenium ion
R′OH –H+
A
B
OR′
R
H
H
Ar
N
Ar
C
N C
R
H
A
B
alkoxyimine
adduct
group of 4-aryl-substituted 2H-azirines as identified below
X Abstract published in AdVance ACS Abstracts, November 15, 1997.
(1) Universita¨t Mu¨nster. Present address: SeaquistPerfect Dispensing
GmbH, Hildebrandstrasse 20, D-44319 Dortmund.
(2) Universita¨t Kiel (present address) and Universita¨t Mu¨nster.
(3) Max-Planck-Institut fu¨r Strahlenchemie.
N
N
Ar = C6H4-OMe,
C6H4-Me,
C6H4-H,
Ph
Ph
Ar
C6H4-Cl,
C6H4-CN
(4) Orahovats, A.; Heimgartner, H.; Schmid, H.; Heinzelmann, W. HelV.
Chim. Acta 1975, 58, 2662-2677.
(5) Padwa, A.; Dharan, M.; Smolanoff, J.; Wetmore, S. I. J. Am. Chem.
Soc. 1973, 95, 1954-1961.
and we report the results in the following.
(6) (a) Heimgartner, H. Angew. Chem. 1991, 103, 271-297; Angew.
Chem., Int. Ed. Engl. 1991, 30, 238. (b) Gilgen, P.; Heimgartner, H.; Schmid,
H.; Hansen, H.-J. Heterocycles 1977, 6, 143-212 and references therein.
(c) Padwa, A. Synthetic Organic Photochemistry; Horspool, W. M., Ed.:
Plenum Press, New York, 1984; pp 313-373. (d) Albrecht, E.; Averdung,
J.; Bischof, E. W.; Heidbreder, A.; Kirschberg, T.; Mu¨ller, F.; Mattay, J. J.
Photochem. Photobiol. A: Chem. 1994, 82, 219-232. (e) Mu¨ller, F.; Mattay,
J. Chem. ReV. 1993, 93, 99-117. (f) Mu¨ller, F.; Mattay, J. Chem. Ber.
1993, 126, 543-549. (g) Nair, V. The Chemistry of Heterocyclic Com-
pounds, Small Ring Heterocycles; Hassner, A., Ed.; Wiley Interscience:
New York, 1983; Vol. 1, pp 215-332.
(7) (a) Averdung, J.; Albrecht, E.; Lauterwein, J.; Luftmann, H.; Mattay,
J.; Mohn, H.; Mu¨ller, W. H.; ter Meer, H.-U. Chem. Ber. 1994, 127, 787-
789. (b) Averdung, J.; Abraham, W.; Albrecht, E.; Claus, K.-U.; Jakobi,
D.; Luftmann, H.; Mattay J. Recent AdVances in the Chemistry and Physics
of Fullerenes and Related Materials; Kadish, K. M., Ruoff, R. S., Eds.;
The Electrochemical Society: Pennington, NJ, 1995; Vol. 2, pp 1164-
1178. (c) Averdung, J.; Mattay, J. Tetrahedron 1996, 52, 5407-5420.
Results and Discussion
1. Formation of Nitrile Ylides. It has long been known
that nitrile ylides can be generated by photolysis of 2H-azirines
(AZ).8,10 We have now studied this reaction in more detail.
For example, on flash photolysis of 0.02 to 0.1 mM solutions
(8) (a) Padwa, A.; Rasmussen, J. K.; Tremper, A. J. Am. Chem. Soc.
1976, 98, 2605-2614. (b) Padwa, A.; Smolanoff, J.; Tremper, A. J. Am.
Chem. Soc. 1975, 97, 4682-4691. (c) Hansen, H.; Heimgartner, H. Nitrile
Ylides. In 1,3-Dipolar Cycloaddition Chemistry; Padwa, A., Ed.; John Wiley
& Sons: New York, 1984; Bd. 1, Kap. 2, p 208. (d) Naito, I.; Ishida, A.;
Takamuko, S.; Isomura, K.; Taniguchi, H. J. Chem. Soc., Perkin Trans. 2,
1992, 1985-1988.
(9) Hansen, H.; Heimgartner, H. In 1,3-Dipolar Cycloaddition Chemistry;
Padwa, A., Ed.; Wiley: New York, 1984; Vol. 1, pp 177-290.
S0002-7863(97)01648-X CCC: $14.00 © 1997 American Chemical Society

11606 J. Am. Chem. Soc., Vol. 119, No. 48, 1997
Albrecht et al.
Table 1. Absorption Maxima of the Nitrile Ylides Generated by Laser Flash Photolysis of 2H-Azirines in Acetonitrile at Room Temperaturea
+
C
–
C
H
H
X
N
+
C
–
C
H
Ph
N
X ) CN
X ) Cl
X ) H
X ) Me
X ) OMe
Ph
λ
max, nm
235, 345
238, 282
(385) [306]
240, 283
(385)
230, 280
(380)
236, 284
(380)
260, 288
(380) [300]
a Numbers in parentheses are for weak absorption. Numbers in square brackets are for a shoulder.
Figure 2. Absorption spectra of transients from the photolysis of 0.07
mM 3-(4-methylphenyl)-2H-azirine in deoxygenated acetonitrile con-
taining 30.4 mM acrylonitrile, recorded at several times after the laser
flash as indicated. The kinetic traces show that all bands belong to
only one species (the nitrile ylide).
Figure 1. Spectra of transients recorded on photolysis of deoxygenated
70 µM 3-(4-methylphenyl)-2H-azirine in acetonitrile, measured at
several times after the laser flash as indicated.
in acetonitrile of the 2H-azirines mentioned above with the 248
nm light of an excimer laser, optical absorptions were observed
with λmax values at ca. 205, 240, 280, and 380 nm. These signals
are assigned to nitrile ylides on the basis of their similarity to
those observed in matrices at 77 K.11 Since the formation of
the signals was complete after the 20 ns laser pulse, the rate
2H-azirine (eq 1) is monophotonic. In the absence of suitable
scavengers, nitrile ylides have lifetimes of more than 1 ms in
aprotic solvents. Under these conditions, they eventually form
dimeric products.4,5
constant k (AZ* f NY) must be >5 × 107 s-1
.
NY + AZ f dimer
AZ
9
hν8 AZ*
9
k8 NY
k > 5 × 107 s-1
(1)
However, in the presence of dipolarophiles, the lifetime of the
nitrile ylides is drastically shortened due to their trapping via a
[3 + 2] cycloaddition reaction, as known from product analysis
studies.13
An example is shown in Figure 1 for the case of 3-(4-
methylphenyl)-2H-azirine, dissolved in acetonitrile, and pho-
tolyzed with a 20 ns pulse of 248 nm light. There are positiVe
peaks at ca. 205, 236, 284, and 380 nm and negatiVe absorptions
at 210 and 260 nm. The latter result from depletion of the
parent, which absorbs at these wavelengths. The species
produced is quite long lived: there is essentially no decay within
74 µs. O2 does not decrease the lifetime of the transient, an
observation that is in agreement with the nitrile ylide nature of
the species.
kbi
NY + AdB
98 adduct
This is shown for the case of 3-(4-methylphenyl)nitrile ylide in
the presence of acrylonitrile (Figure 2).
In the presence of excess dipolarophile, the decay of the nitrile
ylides is by first-order kinetics. From the slopes of the linear
dependencies of kobs for the decay of nitrile ylide on [dipolaro-
phile] the bimolecular rate constants for these reactions were
determined. The results obtained for the reaction of different
nitrile ylides with different dipolarophiles are presented in Table
2.
Similar experiments were performed with the other 2H-
azirines, for which similar observations were made. The
spectroscopic data of the transients observed are collected in
Table 1.
It is evident that the reaction rate constants increase with
decreasing electron density of the olefinic compound and with
increasing electron density of the nitrile ylide.5,9b In Figure 3a
To obtain further information on the details of the photo-
chemical reaction leading to the formation of the ylides (eq 1),
the yields of the ylides were measured as a function of the laser
power by attenuating the laser light by placing neutral density
filters between the laser and the cell. In all cases the yield turned
out to be proportional to the laser power (in the range 0-10
mJ),12 which indicates that the photochemical conversion of the
is shown a Hammett plot of the data from Table 2 for the
(+)
reaction of 4-X-C6H4-C(-)dN-CH2
with acrylonitrile,
where the Hammett F value turns out to be -0.9. From the
decrease of reactivity with decreasing electron density of the
aromatic it may be concluded that the reactivity loss in this
direction is due to stabilization of the nitrile ylides. This
conclusion is supported by AM1 calculations.14 On this basis,
a negative inductive effect of the substituent at the phenyl group
(10) (a) Huisgen, R. In 1,3-Dipolar Cycloaddition Chemistry; Padwa,
A., Ed.; Wiley: New York, 1984; Vol. 1, pp 1-176. (b) Padwa, A.;
Rosenthal, R. J.; Dent, W.; Filho, P.; Turro, N. J.; Hrovat, D. A.; Gould, I.
R. J. Org. Chem. 1984, 49, 3174-3180. (c) Naito, I.; Ishida, A.; Yamamoto,
K.; Takamura, S.; Isomura, K.; Taniguchi, H. Chem. Lett. 1989, 1615-
1618. (d) Naito, I.; Morihara, H.; Ishida, A.; Takamura, S.; Isomura, K.;
Taniguchi, H. Bull. Chem. Soc. Jpn. 1991, 64, 2757-2761. (e) Albrecht,
E. Dissertation; Westfa¨lische Wilhelms-Universita¨t Mu¨nster, 1995.
(11) Sieber, W.; Gilgen, P.; Chaloupka, S.; Hansen, H.-J.; Schmid, H.
HelV. Chim. Acta 1973, 56, 687-696.
(13) (a) Padwa, A.; Smolanoff, J. J. Am. Chem. Soc. 1971, 93, 548-
550. (b) Giezendanner, H.; Ma¨rky, M.; Jackson, B.; Hansen, H.-J.; Schmid,
H. HelV. Chim. Acta 1972, 55, 745-748.
(14) The AM1 calculations were performed with the MOPAC program
(version 6.00) (Quantum Chemistry Program Exchange (QCPE), Department
of Chemistry, Indiana University, Bloomington, Indiana, 47405).
(12) At higher laser power saturation phenomena are observed.

Formation and ReactiVities of Azaallenium Cations
J. Am. Chem. Soc., Vol. 119, No. 48, 1997 11607
Table 2. Bimolecular Rate Constants kbi for the Addition of Acrylonitrile, Dimethyl Acetylenedicarboxylate, and Tetracyanoethylene to Nitrile
Ylides in Acetonitrile at 21 ( 2 °C
kbi [M-1 s-1
]
+
C
–
C
H
H
X
N
+
C
–
C
H
Ph
N
X ) CN
3.9 × 105
X ) Cl
X ) H
X ) Me
1.9 × 106
7 × 109
-0.170
X ) OMe
2.5 × 106
Ph
H2CdCHCN
MeO2CCtCCO2Me
(CN)2CdC(CN)2
8.9 × 105
1.2 × 107
4 × 109
9.4 × 105
1 × 107
5 × 109
1.4 × 106
1.7 × 107
6 × 109
σp(X)
0.628
0.226
0.00
-0.268
Figure 5. Digitizer traces (conductance (a) and optical density at 290
nm (new species) (b and c)) on photolysis of 0.06 mM 2,3-diphenyl-
2H-azirine in a mixture of 2-propanol/water, 4:6 (v/v), argon, at pH
10.9 and T ) 21 ( 2 °C.
Figure 3. Dependence of the logarithm of the reaction rate constants
of the [3 + 2] cycloaddition of 4-substituted nitrile ylides [4-X-C6H4-
C
(-)dN-CH2(+)] with acrylonitrile on the Hammett constants (X) (a),
generated by nucleophilic attack of the solvent to the nitrile
ylide, or (c) a species which is formed in a concerted addition
reaction. Also, the formation of a charge transfer complex (d)
or an anionic species (e) is conceivable.
and on the calculated energy differences of the frontier orbitals (b).
N
Ph
H
–
N
Ph
H
+
C
–OR
N
Ph
H
Ph
(c)
C
C
Ph
C
C
Ph
C
H
(a)
(b)
H
OR
H OR
+
–
N
Ph
H
–
N
Ph
Ph
(d)
CT-complex
C
C
Ph
(e)
C
C
H
+H
OR
H
OR
+
To distinguish between these possibilities, time-resolved
experiments with conductance detection were performed. It was
found that on pulsing a solution of an azirine in, e.g., 2-propanol/
water 4:6 (v/v), in the presence of base (OH-), there was a
conductance increase (Figure 5a), whereas in acidic solution
there wasswith the same ratesa conductance decrease (not
shown). The rate of the conductance increase (Figure 5a) was
the same as the rate of the optical density increase at 290 nm,
where the new species absorbs (Figure 5b).
The light-induced conductance changes are interpreted as
follows: The first step is the generation (within 20 ns) of the
zwitterionic nitrile ylide, with no change of the conductance
(see Figure 5a).
Figure 4. Absorption spectra of transients observed on photolysis of
0.06 mM 2,3-diphenyl-2H-azirine in a deoxygenated mixture of
2-propanol/water 4:6 (v/v), recorded at several times after the laser
flash as indicated. Insets: Decay kinetics at 345 nm (nitrile ylide) and
formation and decay at 290 nm (new species).
increases the energy difference between the frontier orbitals,
EHOMO(nitrile ylide) and ELUMO(acrylonitrile). It is interesting
that the relation between log kbi and the energy difference is a
linear one (Figure 3b), as with the Hammett relation.15
2. Azaallenium Cations. In aprotic solvents and in the
absence of scavengers, the nitrile ylides generated by a laser
flash slowly disappear by formation of dimers, as mentioned
above. However, in protic solvents, the nitrile ylides decay with
an increased rate and with the simultaneous development of
new absorptions. An example for this is given in Figure 4,
which contains the time-resolved absorption spectra of 1,3-
diphenylnitrile ylide in a mixture of water and 2-propanol.
On the basis of the existence of an isosbestic point and on
the equivalence of the nitrile ylide’s decay rate with the
formation rate of the new species (Figure 4, insets), the new
transient is the direct successor of the nitrile ylide from its
reaction with the protic solvent. In principle,9 the new
intermediate can be explained in terms of (a) a cationic species
(a protonated nitrile ylide), (b) a zwitterionic addition product,
N
H
–
C
+
C
Ph
H
hν
Ph
N
laser pulse:
Ph
Ph
The next step is the reaction of the ylide with ROH (R) H,
alkyl) by proton transfer to the ylide, which results in the
production of an azaallenium ion and RO- (Scheme 3, step A).
Due to the high mobility of OH- in solutions containing H2O,
there is a pronounced increase in the conductance as a result of
the formation of this ion.16 In acidic solution, the OH- produced
in Scheme 3, step A, is neutralized by the excess H+ present,
i.e. after reaction 3A is complete and the OH- neutralized, the
(16) The main part of the increase of the conductance is due to the
formation of additional hydroxide rather than to the less mobile large organic
cation. This excludes a reaction leading to an anionic intermediate (e), since
in this case the formation of H+ would neutralize the basic solution resulting
in a decrease of conductivity.
(15) For a review on orbital energy control of cycloaddition reactivity
see: Sustmann, R. Pure Appl. Chem. 1974, 40, 569-593.

11608 J. Am. Chem. Soc., Vol. 119, No. 48, 1997
Albrecht et al.
Table 3. Observed Reaction Rates kobs/s-1 for the Decay of p-X-Aryl Substituted Nitrile Ylides (d-NY), for the Formation of
p-X-Arylazaallenium Cations (f-CA), and for the Decay of the Cations (d-CA) in Different Protic Solvents at 21 ( 2 °C
substituent X
solvent
kobs(d-NY)/s-1
kobs(f-CA)/s-1
kobs(d-CA)/s-1
OMe
OMe
H2Oa
MeOH
MeOD
EtOH
>5 × 107
3.7 × 107
6.5 × 106
6.5 × 105
1.8 × 104
>5 × 107
8.8 × 106
2.2 × 105
7.9 × 103
=5 × 107
4.9 × 106
1.0 × 105
4.4 × 103
=2 × 107
2.2 × 106
3.5 × 105
3.0 × 104
9.2 × 102
>5 × 107
5.6 × 107
9.8 × 106
1.1 × 106
2.9 × 104
>5 × 107
1.7 × 107
3.1 × 105
1.1 × 104
=5 × 107
9.6 × 106
1.5 × 105
9.4 × 103
=2 × 107
2.3 × 106
5.3 × 105
4.9 × 104
2.1 × 103
5.2 × 105
7.5 × 106
9.4 × 105
1.7 × 105
1.5 × 103
1.1 × 106
1.2 × 106
2.7 × 104
1.0 × 103
1.6 × 106
4.3 × 105
8.8 × 103
7 × 102
OMe
OMe
Me
Me
Me
Me
H
H
H
H
Cl
2-PrOH
H2Oa
MeOH
EtOH
2-PrOH
H2Oa
MeOH
EtOH
2-PrOH
H2Oa
3.0 × 106
1.2 × 105
2.4 × 104
4.0 × 103
2.4 × 102
Cl
MeOH
MeOD
EtOH
Cl
Cl
2-PrOH
aThis solvent is water/acetonitrile 9:1 (v/v).
Figure 6. Decay of conductance and of optical density at 290 nm
(cationic species) on photolysis of 0.065 mM 2,3-diphenyl-2H-azirine
in a mixture of water/acetonitrile 9:1 (v/v), argon, at pH 9.8 and T )
21 ( 2 °C.
Figure 7. (a) Dependence of the rate of protonation on the pKa of the
alcohol ROH. (b) Hammett plot of the rate of the reaction 4-X-NY +
MeOH [average of the decay rate of the 4-X-aryl-substituted NY (d-
NY) and the formation rate of the 4-X-aryl-substituted CA)] (b), and
of kobs, the reaction of the cation (CA) with MeOH as a function of the
Hammett constants F(X) (0).
Scheme 3
–
C
+
C
+ROH
H
+
N
+ROH
–H+
H
H
Ph
N
alkoxy(hydroxy)imine
C
C
–RO– Ph
Ph
Ph
Table 4. Extrapolated Decay Rates k/s-1 of the Nitrile Ylides in
Water (pKa ) 15.75) and in Trifluoroethanol (pKa ) 12.5)
R = H, alk
A
B
k(d-NY)/s-1
H+ concentration is less than it was before the pulse (before
generation of the ylide), i.e., there is a conductance decrease.
To summarize, the conductance results unambiguously show
the formation of a positively charged organic species (a cation).
As is visible from Figure 5c, at longer times the azaallenium
cation produced (λmax ) 290 nm) decays (with exponential
kinetics). With the same rate, the conductance signal decays
to zero, i.e., exactly to the same level as that before the pulse.
This is shown for the case of 2,3-diphenyl-2H-azirine in water/
acetonitrile (9:1) at pH 9.8, Figure 6. The conductance behavior
is explained as follows: As a result of the reaction of the cation
with water, protons are released, which are neutralized rapidly
in the basic solution, leading to a decrease of the conductance
(k ) 5.1 × 104 s-1). This corresponds with the decay of the
cation (absorbing at 290 nm, k ) 5.0 × 104 s-1) by which a
product is formed which does not absorb at 290 nm.
For a more thorough examination of the protonation step A,
photolyses were performed with different azirines in different
alcohols, including O-deuterated ones. In all cases the decay
rates of the nitrile ylides were found to correspond to the
formation rates of the cationic primary reaction products (the
azaallenium cations).17 The reaction rates of the ylides with
and in the alcohols are presented in Table 3, which also contains
solvent
X ) MeO
X ) Me
X ) H
X ) Cl
H2O
TFE
3.1 × 108
7.8 × 107
3.2 × 107
1.4 × 107
1.2 × 1011
1.5 × 1010
7.7 × 109
5.3 × 109
data on the decay rates of the resulting azaallenium cations in
the alcohols.
From Table 3 it is evident that the protonation rates of the
nitrile ylides depend on the acidity of the alcohol and on the
substituent at the nitrile ylide. Concerning the effect of the
alcohol, the decay rates of the nitrile ylides correlate with the
pKa of the protic solvents: There is a linear dependence of log
k(decay of NY ) buildup of azaallenium cation) on the acidity
of the alcohol (see Figure 7a).18 This diagram allows the
estimation of decay rates of the nitrile ylides in water (pKa )
15.75)19 and in trifluoroethanol (pKa ) 12.5)20 (Table 4), where
the corresponding lifetimes could not be measured because they
were within the pulse length of the laser flash (20 ns), e.g., for
X ) MeO, the calculated lifetimes of the NY are 3 ns in H2O
and 8 ps in CF3CH2OH.
(18) A dependence of the decay rate of the nitrile ylide from 3-(biphenyl-
4-yl)-2H-azirine on the acidity of the reaction medium has previously been
found by Ishida et al. (ref 8d).
(19) Kosower, E. M. An Introduction to Physical Organic Chemistry;
John Wiley & Sons: New York, 1986.
(17) The fact that the agreement in the rates is not perfect (see Table 5)
is due to the formation rates of the cations being distorted by the (large)
rates of decay of the cations, kobs(d-CA).
(20) Mukherjee, L. M.; Grunwald, E. J. Phys. Chem. 1958, 62, 1311-
1314.

Formation and ReactiVities of Azaallenium Cations
J. Am. Chem. Soc., Vol. 119, No. 48, 1997 11609
Table 5. Rate Constants, ks for the Addition of
On the other hand, for the protonation rates of the nitrile
ylides, the electron density of the NY (as varied by the para
substituent X) also is important, as can be seen from the (linear)
Hammett plot (F ) -2.5) in Figure 7b. Thus, electron donating
substituents in the para position of the aryl ring accelerate the
protonation of the nitrile ylide, apparently by allowing a better
delocalization of the positive charge in the transition state of
the protonation reaction.
The rate of reaction of ylide was also studied in O-deuterated
methanol, MeO-D. It was found that the rate of reaction in
MeOD was lower, on average, by the factor 5.5 than in MeOH
(see Table 3), i.e., the kinetic isotope effect, kie ) k(ROH)/
k(ROD) ) 5.5. Such a high value, which may be compared
with that (kie ) 4.5) for protonation of CN-,21 which also
involves protonation on carbon, clearly indicates22 that the
transition state for protonation of the ylide carbon carrying the
negative charge is a linear one, i.e., the proton donor (OH) is
linearly aligned with the proton acceptor (C), in other words,
the three atoms O‚‚‚H‚‚‚C- are arranged linearly. This situation
is completely different from the case of the protonation of
carbenes, where carbocations are also the products.23
1,3-Diphenylazaallenium Cation to Solvents, and kNu to Different
Nucleophiles, in Water/Acetonitrile, 9:1 (v/v), at 21 ( 2 °Ca
nucleophile
kNu/M-1 s-1
ks/s-1 b
H2O/AN, 9:1
MeOH (neat)
F-
4.7 × 104
1.0 × 106
3.0 × 105
2.6 × 106
2.5 × 107
2.7 × 107
1.0 × 108
6.0 × 108
7.3 × 108
1.1 × 109
CH3COO-
OH-
-
NO2
CN-
SCN-
2-
SO3
-
N3
a In the case of the halides Cl-, Br-, and I-, no influence on the
rate of decay was observed. b Rate constant for reaction with/in solvent.
In Figure 7 is also shown the dependence on X of the
reactivities in MeOH of the azaallenium cations, F ) -3.4 (in
EtOH, F ) -3.1, in 2-PrOH, F ) -1.6). The fact that the
Hammett F value for this reaction is negative (i.e., that the
reactivitiy of the azaallenium cation decreases with decreasing
electron density of the cation) is opposite to the expectation
and to the behavior of “normal” carbocations.24 This “exotic”
behavior of the azaallenium cations would be understandable25
if it was assumed that the reaction of ROH with the cation
involves H-bonding/proton transfer to the nitrogen in the
azaallenium cation followed by attack of RO- (or a second
ROH) to the cationic center on the distal carbon, as shown
below:26
Figure 8. Two different decay components observed on photolysis of
deoxygenated 6 × 10-5 M 2,3-diphenyl-2H-azirine in water/acetonitrile,
9:1 (v/v), in the presence of 7.7 mM thiocyanate at T ) 21 ( 2 °C:
(A) fast decay to a preliminary plateau; (B) slow decay to the baseline.
first-order rates of decay were measured for the 1,3-dipheny-
lazaallenium cation, monitored by its absorption at 290 nm, in
the presence of a number of different Nu-. Bimolecular rate
constants (collected in Table 5) were obtained from the slopes
of the plots of the observed rates, kobs, versus the nucleophile
concentrations.
+
The behavior of thiocyanate merits special comment. Most
of the examined reactions between cation and nucleophile are
irreversible, i.e. the products formed are thermally stable within
the detection time. However, in the case of SCN-, an
equilibrium reaction between the cation and thiocyanate was
observed to take place. The decay curve of the cation consisted
of two components. The two different decay reactions pro-
ceeded in different time domains. This is shown in Figure 8.
For the fast reaction, a plot of the observed rate of decay versus
the concentration of thiocyanate reveals a straight line with
positive slope. For the slow reaction, however, the dependence
is not linear: kobs(slow) decreases smoothly with increasing
[SCN-] (see Figure 9).
ArHC
ArHC
N
CH2
+
N
CH2
(+ROH, –H+)
fast
+
+
+ROH
slow
ArHC
ArHC
CH2
CH2
ArHC
CH2OR
N
N
H
O
N
R
The large kinetic isotope effect of 6.5 ( 2 for decay of the
p-Cl- and MeO-substituted phenylazaallenium cations (see Table
3) is in agreement with this mechanism.27
There is another, final proof for the identification of the
intermediates observed in the alcohols as cations: This is their
reactivity toward anionic nucleophiles, Nu-. As an example,
These features can be explained as follows (Scheme 4): The
fast reaction corresponds to the reversible bimolecular combina-
tion reaction of the cation with the nucleophile. The observed
reaction rate kobs for the fast reaction consists of the rate of the
(25) It has been conclusively established that the product of this reaction
has the alkoxide function on this carbon and not on the ArCH carbon, see:
Padwa, A.; Rasmussen, J. K.; Tremper, A. J. Am. Chem. Soc. 1976, 98,
2605.
(26) It has recently been shown by quantum chemical calculations
(Bo¨ttger, G.; Geisler, A.; Fro¨hlich, R.; Wu¨rthwein, E.-U. J. Org. Chem.
1997, 62, 6407). that the equilibrium between the linear-orthogonal
2-azaallenium type ion and the bent-planar azaallylium ion (see structures
below) is shifted toward the latter by electron donor groups. If it is assumed
that the bent structure is the reactive one, the greater reactivity for electron-
donating substituents would be the consequence of the shifted equilibrium
(we thank referee IV for pointing this out).
(27) It should be noted that in the solvent effects on the decay actually
different species are compared, one cation being ArCH, the other ArCD.
However, it is unlikely that this is the source of the large kinetic isotope
effect (we thank referee IV for pointing this out).
(28) The acitivity was calculated from the concentration with use of
Debye-Hu¨ckel theory.
(21) Bednar, R. A.; Jencks, W. P. J. Am. Chem. Soc. 1985, 107, 7117.
(22) See, e.g.: Westheimer, F. H. Chem. ReV. 1961, 61, 265-273.
Saunders, W. H. Kinetic Isotope Effects. In Techniques of Chemistry; Lewis,
E. S., Ed.; Wiley: New York, 1974; Vol. VI, pp 211-255. More O’Ferral,
R. A. J. Chem. Soc. B 1970, 785.
(23) Kirmse, W.; Guth, M.; Steenken, S. J. Am. Chem. Soc. 1996, 118,
10838.
(24) See, e.g.: McClelland. R. A.; Kanagasabapathy, V. M.; Banait. N.;
Steenken, S. J. Am. Chem. Soc. 1989, 111, 3966-3972. Steenken, S.;
McClelland, R. A. J. Am. Chem. Soc. 1990, 112, 9648-9649. Kanagasa-
bapathy, V. M.; Banait, N. S.; Steenken, S. J. Am. Chem., Soc. 1991, 113,
1009-1014. Cozens, F. L.; Mathivanan, N.; McClelland, R. A.; Steenken,
S. J. Chem. Soc., Perkin Trans. 2 1992, 2083-2090. Patz, M.; Mayr, H.;
Bartl, J.; Steenken, S. Angew. Chem. 1995, 107, 519-521. Verbeek, J.-M.;
Stapper, M.; Krijnen, E. S.; van Loon, J.-D.; Lodder, G.; Steenken, S. J.
Phys. Chem. 1994, 98, 9526-9536. See also refs 23 and 29.

11610 J. Am. Chem. Soc., Vol. 119, No. 48, 1997
Albrecht et al.
Figure 9. Observed reaction rates for the fast decay component (left)
and for the slow decay component (right) as a function of the activity28
of thiocyanate, a(SCN-).
Figure 10. Determination of the equilibrium constant of the combina-
tion reaction of 1,3-diphenylazaallenium ion with thiocyanate based
on ref 29.
Scheme 4
Ph
C+
H
Ph
kf
+ SCN–
PhHC
N
PhHC
N
C(SCN)
H
deficient olefins with rate constants between 4 × 105 and 7 ×
109 M-1 s-1, depending on the olefin and on the substituent on
the phenyl group, whereby the Hammett F value is -0.9. The
nitrile ylides are also scavenged by and in alcohols (rate
constants from 103 to 107 s-1, increasing with increasing acidity
of the alcohol and with the electron density of the nitrile ylide
as varied by a substituent (for the case of the reaction of NY
with MeOH, Hammett F ) -2.5)). In this reaction, which is
characterized by a linear transition state O‚‚‚H‚‚‚C- for the ylide
protonation as concluded from kinetic isotope effect studies,
azaallenium cations are formed, as shown by time-resolved
conductance measurements. The cations react with nucleophiles
with rate constants between 3 × 105 M-1 s-1 for F- and 1 ×
109 M-1 s-1 for the much more nucleophilic N3-. In the case
of SCN-, the reaction with the cations is reversible. The same
is probably true for Cl-, Br-, and I-.
kd
H2O
kSolvent
alkoxy(hydroxy)imine
formation of the addition product, kf, and of the rate of the
decomposition of this product, kd.29
kobs(fast) ) kf[SCN-] + kd
The slow reaction represents the irreversible reaction of the
cation in the equilibrating mixture with the solvent water. In
the equilibrium an increase of the concentration of the anion
results in a decrease in the concentration of the cation. Thus,
by favoring the formation reaction, with increasing thiocyanate
concentration the slow reaction of the cation with the solvent
is delayed. Conditions as in Scheme 4 have previously been
analyzed. On the basis of that29 interpretation, the equilibrium
constant of the fast reaction can be evaluated. A plot of kobs
for the fast reaction versus the concentration of thiocyanate is
linear (Figure 9), providing the rate constants for formation and
decomposition and also the equilibrium constant as their ratio
Experimental Section
The syntheses of the azirines were performed in a manner analogous
to the method of Hassner.30 Solutions were prepared by using organic
solvents of spectroscopic grade purity and water from a Millipore
Milli-Q system.
The laser flash photolysis measurements were conducted with use
of a computer-controlled apparatus. A Lambda Physik excimer laser
provided pulses of 248 nm light (KrF*) with a duration of 20 ns and
a power ranging from 0 to 100 mJ/pulse. The relaxation of the
nonequilibrium situation produced by the light pulse was followed with
time-resolved optical or conductance detection (time resolution 1-2
ns).
Solutions were 50-100 µM in azirine and had OD/cm-1 ca. 1.0 at
248 nm. The optical and conductance signals from the photolysis were
digitized by Tektronix 7612 and 7912 transient recorders interfaced
with a DEC 11/73 computer, which was also used for on-line (pre)-
analysis of the experimental data and for their storage and documenta-
tion.
K ) kf/kd ) 324 M-1
.
A second method to evaluate the equilibrium constant
involves the use of the equation29
K ) (ODo - ODint)/(ODint × [Nu])
whereby ODo and ODint correspond to the magnitude of the
absorption directly after the pulse and that of the intermediary
plateau, respectively. The value obtained for K is 243 M-1 (as
taken from Figure 10). This value agrees quite well with the
equilibrium constant based upon the kinetic method (K ) 324
M-1 (taken from Figure 9)).
It is reasonable to assume that the combination reaction is
reversible also in the case of the halides Cl-, Br-, and I-, this
being the reason for the absence of a visible effect of halide on
the lifetime of the cation (see above). This means that in this
case the equilibrium is such that the ionic reactants are favored
(kd > 106 s-1).
Acknowledgment. E.A. is indebted to M. Toubartz (Mu¨l-
heim) for her helpful assistance. Financial support for this work
was provided by a doctoral fellowship of the “Graduierten-
fo¨rderung des Landes NRW” and especially by the “Deutsche
Forschungsgemeinschaft” and the “Fonds der Chemischen
Industrie”.
Summary and Conclusions
It has been demonstrated that the nitrile ylides generated in
JA971648N
the 248 nm photolysis of 2H-phenylazirines react with electron-
(30) (a) Fowler, F. W.; Hassner, A.; Levy, L. A. J. Am. Chem. Soc. 1967,
89, 2077-2082. (b) Hassner, A.; Fowler, F. W. J. Am. Chem. Soc. 1968,
90, 2869-2875.
(29) McClelland, R. A.; Banait, N.; Steenken, S. J. Am. Chem. Soc. 1986,
108, 7023-7027.