J. Am. Chem. Soc. 1999, 121, 3939-3943
3939
Reactions of the “Stable” Nitroxyl Radical TEMPO with Ketenes:
Formation of a Unique Peroxidic Source of Aminyl Radicals
Wen-wei Huang, Huda Henry-Riyad, and Thomas T. Tidwell*
Contribution from the Department of Chemistry, UniVersity of Toronto,
Toronto, Ontario, Canada M5S 3H6
ReceiVed August 17, 1998. ReVised Manuscript ReceiVed February 18, 1999
Abstract: Calculations at the B3LYP level predict addition of the radical H2NO to the carbonyl carbon of
CH2dCdO to be exothermic by 18.7 kcal/mol. Consistent with this prediction, the ketene Ph2CdCdO reacts
at 25 °C with tetramethylpiperidinyloxy radical (TEMPO, TO) to yield an unstable species that reacts with
oxygen to form the peroxide (OCPh2CO2T)2 (6, T ) 2,2,6,6-tetramethylpiperidinyl), whose structure was
confirmed by X-ray crystallography. Heating of 6 at 100 °C in toluene with TEMPO leads to Ph2CdO,
tetramethylpiperidine, and PhCH2OT, indicating that 6 decomposes to form two 2,2,6,6-tetramethylpiperidinyl
radicals 15. Kinetic studies of the thermal decomposition of 6 show a 100-fold rate acceleration relative to
(PhMe2CO)2. Thermal reactions of TEMPO with the bisketene (Me3SiCdCdO)2 (23) at 90 °C and with the
allenylketene 26 also lead to deoxygenation of TEMPO, forming radicals 15, together with 2,3-bis(trimethylsilyl)-
maleic anhydride (24) and the alkylidenelactone 27, respectively.
Nitroxyl radicals such as 2,2,6,6-tetramethylpiperidine-1-oxyl
(TEMPO, TO) are remarkable for their stability toward dimer-
ization and their inertness toward typical organic molecules,
but react rapidly with most other free radicals, and have wide
application as radical traps.1 Recently nitroxyl radicals have also
attracted great interest in living free radical polymerizations,
where the nitroxyl radical reversibly combines with growing
radical chains in styrene polymerization.2a-e At 120 °C TEMPO
adds to styrene, giving the adduct PhCH(OT)CH2OT,2f and at
similar temperatures, TEMPO abstracts hydrogen from alkyl-
benzenes.2f,g Recently TEMPO was reported to react with tert-
butylperoxy radicals to form the unstable trioxide t-BuOOOT,
which cleaves to O2, t-BuO•, and T•.2j The thermolysis of cumyl-
TEMPO, forming TEMPO and cumyl radicals, has also been
observed.2k
are only a few scattered references to reactions of ketenes with
radicals.3a Recently we have carried out molecular orbital
calculations on the reactions of CH2dCdO with H, CH3, OH,
F, SiH3, and Cl radicals and predict these processes to be
strongly exothermic, with rather low barriers for addition to
either Câ or CR to form acyl or enolic radicals 1 and 2,
respectively, while attack at oxgyen to give vinyl radicals 3
(eq 1) is less favorable in all cases.3c
Reported experimental examples of interactions of free radical
with ketenes include reactions of R• (R ) H,4a t-Bu,4b OH,4c
F,4d and Cl4e,f) with CH2dCdO, resulting in either hydrogen
abstraction from the ketene (eq 2) or addition to Câ, forming
an acyl radical with subsequent decarbonylation (eq 3). These
Ketenes are an interesting class of organic molecules that
are usually susceptible to attack by most reactive intermediates.3a
In the first publication on ketenes,3b Staudinger noted in 1905
that diphenylketene reacted with O2, which strongly implies a
sensitivity to free radicals. Despite this early indication, there
(1) (a) Volodarsky, L. B.; Reznikov, V. A.; Ovcharenko, V. I. Synthetic
Chemistry of Stable Nitroxides; CRC Press: Boca Raton, FL, 1994. (b)
Keana, J. F. W. Chem. ReV. 1978, 78, 37-64. (c) Aurich, H. G. Nitroxides.
In Nitrones, Nitronates, Nitroxides; Patai, S., Rappoport, Z., Eds.; Wiley:
New York, 1989; Chapter 4.
reactions are highly exothermic, and because of the relatively
low barriers for decarbonylation of the acyl radicals, stable
products resulting from trapping of the species were not
observed. These experimental results agree with our calculations3c
(2) (a) Hawker, C. J. Acc. Chem. Res. 1997, 30, 373-382. (b) Georges,
M. K.; Veregin, R. P. N.; Kazmaier, P. M.; Hamer, G. K. Macromolecules
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H.; Suyama, S. J. Am. Chem. Soc. 1997, 119, 10987-10991. (e) Nakamura,
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J. Org. Chem. 1997, 62, 5578-5582. (f) Connolly, T. J.; Scaiano, J. C.
Tetrahedron Lett. 1997, 38, 1133-1136. (g) Opieda, I. A.; Matvienko, A.
G.; Ostrovskaya, O. Z. Kinet. Catal. 1995, 36, 441. (h) Braslau, R.; Burrill,
L. C., II; Siano, M.; Naik, N.; Howden, R. K.; Mahal, L. K. Macromoelcules
1997, 30, 6445-6450. (i) Hawker, C. J.; Barclay, G. G.; Overlang, A.;
Dao, J.; Devonport, W. Macromolecules 1996, 29, 5245-5254. (j) Barton,
D. H. R.; Le Gloahec, V. N.; Smith, J. Tetrahedron Lett. 1998, 39, 7483-
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Chem. Soc., Perkin Trans. 2 1998, 1553-1559.
(3) (a) Tidwell, T. T. Ketenes; Wiley: New York, 1995. (b) Staudinger,
H. Chem. Ber. 1905, 38, 1735-1739 (c) Sung, K.; Tidwell, T. T. J. Org.
Chem. 1998, 63, 9690-9697. (d) Barone, V.; Bencini, A.; Cossi, M.; Di
Matteo, A.; Mattesini, M.; Totti, F. J. Am. Chem. Soc. 1998, 120, 7069-
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Wallington, T. J.; Ball, J. C.; Straccia, A. M.; Hurley, M. D.; Kaiser, E.
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10.1021/ja982944i CCC: $18.00 © 1999 American Chemical Society
Published on Web 04/08/1999