Homolysis of carbenes. Free radicals from dialkoxycarbenes [1]

Full text of DOI:10.1021/ja982566h

11182
J. Am. Chem. Soc. 1998, 120, 11182-11183
Communications to the Editor
Scheme 1
Homolysis of Carbenes. Free Radicals from
Dialkoxycarbenes
Paul C. Venneri and John Warkentin*
Department of Chemistry, McMaster UniVersity
Hamilton, ON, Canada, L8S 4M1
ReceiVed July 20, 1998
We report the first clear cases of thermal fragmentation of
acyclic dialkoxycarbenes in solution to radical pairs consisting
of methoxycarbonyl and allylic radicals, Scheme 1, in which both
the carbenes and the radicals could be trapped.
Scheme 2
2-Methoxy-2-allyloxy-5,5-dimethyl-∆3-1,3,4-oxadiazolines (2a,
2b) were prepared by the acid-catalyzed exchange reaction1-4 of
the 2-acetoxy-2-methoxy analogue (1) with 1-phenyl-2-propen-
1-ol and with cinnamyl alcohol, respectively, Scheme 2. Ther-
molysis of 2 in benzene (sealed tube) at 110 °C afforded the esters
(6), Scheme 3. Both 2a and 2b afforded 6a and 6b in 2:1 and
1:2 ratio, respectively (total yields 60%, isolated).
These results might be accounted for with competitive [1,2]-
migrations and [2,3]-sigmatropic rearrangements of carbene
intermediates or with another mechanism, possibly bypassing a
carbene entirely. Carbene trapping with t-BuOH in benzene to
afford the expected5-7 orthoformate 7 in 70% yield (isolated),
confirmed that carbene 3b is indeed formed upon thermolysis of
2b (Scheme 4). The yields of 6 and 7 as a function of [t-BuOH]
were shown to be interdependent, Figure 1, indicating that the
carbene 3b is the precursor of radicals 4 and 5b, a conclusion
that was supported by interception of the radicals. Thus,
thermolysis of 2b in benzene containing TEMPO afforded adducts
8 and 9, respectively (Scheme 5).8 The yields of 6 dropped with
increased [TEMPO], but traces of the esters could always be
detected by GC. That is not surprising because TEMPO should
not trap caged radical pairs and because TEMPO adducts of
radicals can regenerate those radicals upon heating.9
Scheme 3
Scheme 4
“â-Scissions” of alkylidenes to an alkyl radical and an
unsaturated radical, analogous to the demonstrated fragmentations
of Scheme 1, are very rare10 for either carbenes with a triplet
ground state or for ground-state singlets with a readily accessible
triplet state. Ring opening of cyclic oxacarbenes to acyl alkyl
biradicals (dotted arrow, Scheme 6) has been proposed,11-12 and
biradical combination, either from the photoreaction of cyclobutan-
(1) Kassam, K.; Pole, D. L.; El-Saidi, M.; Warkentin, J. J. Am. Chem. Soc.
1994, 116, 1161.
(2) Kassam, K.; Warkentin, J. Can. J. Chem. 1997, 75, 120.
(3) Pole, D. L.; Sharma, P. K.; Warkentin, J. Can. J. Chem. 1996, 74, 1335.
(4) Kassam, K.; Venneri, P. C.; Warkentin, J. Can. J. Chem. 1997, 75,
1256.
Figure 1. Relative yields of 2 and 7 as a function of [t-BuOH].
(5) Kirmse, W.; Loosen, K.; Sluma, H.-D. J. Am. Chem. Soc. 1981, 103,
5935.
ones14-19 or from diazenes,12 to cyclic oxacarbenes (Scheme 6)
is fairly well established. Good yields of alkenes that are
apparently biradical-derived have been obtained from presumed
cyclic dioxacarbenes.20
(6) Moss, R. A.; Wostowski, M.; Terpinski, J.; Kmiecik-Lawrynowicz, G.;
Krogh-Jespersen, K. J. Am. Chem. Soc. 1987, 109, 3811.
(7) Kirmse, W.; Guth, M.; Steenken, S. J. Am. Chem. Soc. 1996, 118,
10838.
(8) A trace amount of the methyl adduct of TEMPO, presumably from
decarboxylation of the methoxycarbonyl radical, was detected also. Its identity
was confirmed by comparison with an authentic sample, prepared by
thermolysis of di-tert-butyl peroxide in the presence of TEMPO.
(9) Fukuda, T.; Terauchi, T.; Goto, A.; Ohno, K.; Tsujii, Y.; Miyamoto,
T.; Kobatake, S.; Yamada, B. Macromolecules 1996, 29, 6393.
(10) (a) Cummins, J. M.; Porter, T. A.; Jones, M., Jr. J. Am. Chem. Soc.
1998, 120, 6473. (b) Berdick, T. E.; Levin, R. H.; Wolf, A. D.; Jones, M., Jr.
J. Am. Chem. Soc. 1973, 95, 5087.
Analogous processes involving fragmentations of acyclic
alkoxycarbenes, in condensed phase reactions, have been sug-
gested to account for minor products. For example, methoxy-
carbene, generated in solution from a tosylhydrazone precursor,
afforded traces of methane,21 possibly from methyl radicals, and
(13) Ramnauth, J.; Lee-Ruff, E. Can. J. Chem. 1997, 75, 518.
(14) Yates, P.; Loutfy, R. O. Acc. Chem. Res. 1975, 8, 209.
(15) Altmann, J. A.; Csizmadia, I. G.; Robb, M. A.; Yates, K.; Yates, P.
J. Am. Chem. Soc. 1978, 100, 1653.
(11) Ayral-Kaloustian, S.; Agosta, W. C. J. Org. Chem. 1982, 47, 284.
(12) Miller, R. D.; Go¨litz, P.; Janssen, J.; Lemmens, J. J. Am. Chem. Soc.
1984, 106, 7277.
10.1021/ja982566h CCC: $15.00 © 1998 American Chemical Society
Published on Web 10/16/1998

Communications to the Editor
J. Am. Chem. Soc., Vol. 120, No. 43, 1998 11183
Scheme 5
Scheme 7
Scheme 8
planar singlet ground state and the nonplanar triplet state of ca.
76 kcal mol-1 (in dimethoxycarbene).30 Thus the triplet mech-
anism is most unlikely. Moreover, it is difficult to accommodate
the observed preference for the ester (6) that is most closely related
to the geometry of the starting carbene with a triplet mechanism.
That preference, although it is not understood at this time, implies
very fast coupling, competitive with separation by diffusion. The
[2,3]-Wittig rearrangement (Scheme 8), which occurs with partial
retention of configuration,31-33 is reminiscent of the “memory
effect” observed here.
There have been only a few theoretical investigations of the
mechanism by which dialkoxycarbenes dissociate into radical
pairs.15,34 Computation of the singlet and triplet potential energy
surfaces of carbene 3b is not possible at a currently reasonable
cost, but calculations with the model system, CH2dCHCH2O-
(HO)C:, are in progress.
Scheme 6
diethoxycarbene from a similar precursor afforded ethylene (1.5
parts) and ethane (1 part) out of 51.5 parts of identified gaseous
products. Ethylene and ethane were attributed to ethyl radicals
formed from diethoxycarbene, although neither the radical nor
the carbene were trapped.22 Carbenes 3b, on the other hand,
afford esters 6 as major products (60%, isol) and trapping of the
carbene with t-BuOH and of radicals with TEMPO established
that 6 are derived from the sequence oxadiazoline f dialkoxy-
carbene f radicals f esters. A further indicator of radical
intermediates from both 2a and 2b was the formation of bibenzyl
as a major coproduct of their thermolysis in toluene. Formation
of bibenzyl must be attributed to attack of radicals on toluene.
Given that sigmatropic rearrangement of analogous (bishet-
eroatom)carbenes is well-known,23-26 the fragmentation of car-
benes 3 to radicals is surprising. One mechanism for the
fragmentation would involve intersystem-crossing to the triplet
carbene and formation of the double bond of the methoxycarbonyl
radical by â-scission (Scheme 7). Although the singlet and triplet
states of nonplanar dihydroxycarbene are almost degenerate at
lower levels of theory27 and separated by ca. 20-37 kcal mol-1
at higher levels of calculation,28,29 there is a barrier between the
Acknowledgment. We are grateful to the Natural Sciences and
Engineering Research Council of Canada for financial support and to
Darren L. Reid for helpful discussions.
Supporting Information Available: Spectroscopic data for com-
pounds 2a, 2b, 6a, 6b, 7, 8, 9, and the methyl adduct of TEMPO (3
pages, print/PDF). See any current masthead page for ordering informa-
tion and Web access instructions.
JA982566H
(26) Baldwin, J. E.; Walker, J. A. J. Chem. Soc., Chem. Commun. 1972,
354.
(27) Feller, D.; Borden, W. T.; Davidson, E. R. J. Chem. Phys. 1979, 71,
4987.
(28) Ra¨sa¨nen, M.; Raaska, T.; Kunttu, H.; Murto, J. J. Mol. Struct.
(THEOCHEM) 1990, 208, 79.
(16) Stohrer, W.-D.; Wiech, G.; Quinkert, G. Angew. Chem., Int. Ed. Engl.
1974, 13, 199.
(29) Reddington, R. L.; Bock, C. W.; Aboab, B. J. Mol. Struct.
(THEOCHEM) 1990, 224, 89.
(17) Boate, D. R.; Johnston, L. J.; Kwong, P. C.; Lee-Ruff, E.; Scaiano, J.
C. J. Am. Chem. Soc. 1990, 112, 8858.
(30) Moss, R. A.; Wostowski, M.; Shen, S.; Krogh-Jespersen, K.; Matro,
A. J. Am. Chem. Soc. 1988, 110, 4443.
(18) Morton, D. R.; Lee-Ruff, E.; Southam, R. M.; Turro, N. J. J. Am.
Chem. Soc. 1970, 92, 4349.
(31) Scho¨llkopf, U. Angew. Chem., Int. Ed. Engl. 1970, 9, 763.
(32) Mikami, K.; Kimura, Y.; Kishi, N.; Nakai, T. J. Org. Chem. 1983,
48, 279.
(19) Quinkert, G.; Jacobs, P.; Stohrer, W.-D. Angew. Chem., Int. Ed. Engl.
1974, 13, 197.
(20) Borden, W. T.; Hoo, L. H. J. Am. Chem. Soc. 1978, 100, 6274.
(21) McDonald, R. M.; Krueger, R. A. J. Org. Chem. 1966, 31, 488.
(22) Crawford, R. J.; Raap, R. Proc. Chem. Soc., London 1963, 370.
(23) Nakai, T.; Mikami, K. Chem. Lett. 1978, 11, 1243.
(24) Nakai, T.; Mikami, K. Chem. Lett. 1979, 1081.
(25) Hoffmann, R. W. Angew. Chem., Int. Ed. Engl. 1979, 18, 563.
(33) Tomooka, K.; Yamamoto, H.; Nakai, T. Liebigs Ann./Recueil 1997,
1275.Feller, D.; Borden, W. T.; Davidson, E. R. J. Comput. Chem. 1980, 1,
158.
(34) Feller, D.; Borden, W. T.; Davidson, E. R. J. Comput. Chem. 1980
1, 158.