Organic & Biomolecular Chemistry
Communication
of 8.1 kcal mol−1 for ΔSE, defined as the difference between
the strain energy of the spiro compound and the sum of the
strain energies of the separate rings. In contrast, spiro[2.3]-
7 B. H. Northrop and K. N. Houk, J. Org. Chem., 2006, 71,
3–13.
8 H. M. Frey and A. T. Cocks, J. Chem. Soc. A, 1971, 2564–
2566; J. D. Bender, P. A. Leber, R. R. Lirio and R. S. Smith,
J. Org. Chem., 2000, 65, 5396–5402.
hexane, with an estimated value of ΔSE = 1.3 kcal mol−1 29
,
would appear to experience relatively little ring strain. While
these values do not preclude a potential ring strain contri-
9 D. Hasselmann, Tetrahedron Lett., 1972, 13, 3465–3468.
bution to the observed rate effect, we surmise that cyclopropyl 10 B. K. Carpenter, J. Am. Chem. Soc., 1995, 117, 6336–6344.
hyperconjugation exerts a more pronounced rate effect. This 11 C. P. Suhrada, C. Selçuki, M. Nendel, C. Cannizzaro,
analysis is supported by prior research on azoalkane decompo-
sition reactions30 by Martin and Timberlake, who concluded a
K. N. Houk, P.-J. Rissing, D. Bauman and D. Hasselmann,
Angew. Chem., Int. Ed., 2005, 44, 3548–3552.
rate enhancement by a cyclopropyl substituent attached to an 12 D. J. Fenick and D. E. Falvey, J. Org. Chem., 1994, 59, 4791–
incipient radical center can be explained by “a postulated 4799.
stabilization of product radicals by cyclopropyl conjugation.” A 13 D. Griller and K. U. Ingold, Acc. Chem. Res., 1980, 13, 317–
reported value of 3 kcal mol−1 of corresponding stabilization
323.
in the cyclopropylcarbinyl radical31 can account for most if not 14 P. J. Wagner, K.-C. Liu and Y. Noguchi, J. Am. Chem. Soc.,
all of the rate effect. 1981, 103, 3837–3941.
In summary, selective kinetic cyclopropanation converts 3 15 A. Rudolph and A. C. Weedon, Can. J. Chem., 1990, 68,
to 7. A thermal study of 7 has provided definitive experimental 1590–1597.
evidence for 1,4-diradical intermediate D by an observed rate 16 W. Adam, M. Dörr, K. Hill, E.-M. Peters, K. Peters and
enhancement and for CPC diradical intermediate C due to H. G. von Schnering, J. Org. Chem., 1985, 50, 587–595.
product 9 formation. We intend to examine vinylcyclobutanes 17 P. S. Engel, D. E. Keys and A. Kitamura, J. Am. Chem. Soc.,
related to 7 for further evidence of CPC rearrangements. In 1985, 107, 4964–4975.
particular, we believe that the bicyclo[4.2.0] analog of com- 18 I. V. Kazimirchik, K. A. Lukin, G. F. Bebikh and
pound 7 is a suitable substrate to test our earlier prediction N. S. Zefirov, Zh. Org. Khim., 1983, 19, 105–110.
that 1,4-diradicals derived from bicyclo[4.2.0]oct-2-enes are 19 J. Furukawa, N. Kawabata and J. Nishimura, Tetrahedron,
less tightly associated5 than those derived from bicyclo[3.2.0]-
1968, 24, 53–58.
hept-2-enes. If so, they might well be more prone to CPC 20 M. Nee and J. D. Roberts, J. Org. Chem., 1981, 46, 67–70.
rearrangements.
21 G. A. Antoniadis, M. T. M. Clements, S. Peiris and
J. B. Stothers, Can. J. Chem., 1987, 65, 1557–1562.
22 K. J. Stone and R. D. Little, J. Org. Chem., 1984, 49, 1849–
1853.
Acknowledgements
23 H. M. R. Hoffmann and O. Koch, J. Org. Chem., 1986, 51,
2939–2944.
24 R. D. Bach and O. Dmitrenko, J. Am. Chem. Soc., 2006, 128,
4598–4611.
25 N. H. Werstiuk and W. B. Sokol, Can. J. Chem., 2011, 89,
409–414.
We acknowledge the Donors of the American Chemical Society
Petroleum Research Fund and the Franklin & Marshall College
Hackman Program for support of this research at Franklin &
Marshall College. RMB was the recipient of both a Snavely
Summer Research Award and the 2010 Snavely Research
Award.
26 As compound
9 undergoes an irreversible thermal
rearrangement to an isomer that elutes at 13.0 min, the
total CPC contribution is the sum of the concentrations of
the primary product 9 and its secondary product.
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