Page 965 - Advanced Organic Chemistry Part B - Reactions & Synthesis

Bicyclo[3.2.2]non-1-ene, a strained bridgehead alkene, is generated by rearrangement 941
when bicyclo[2.2.2]octyldiazomethane is photolyzed. 227
SECTION 10.2
hν Reactions Involving
Carbenes and Related
Intermediates
N CH :CH
2
Carbene centers adjacent to double bonds (vinyl carbenes) usually cyclize to
cyclopropenes. 228
H
CH CH 2 CH 3 CH CH 3
3
2
CH 3 CH NNHSO Ar CH 3
2
CH 3 Ref. 229
Cyclopropylidenes undergo ring opening to give allenes. Reactions that would be
expected to generate a cyclopropylidene therefore lead to allene, often in preparatively
useful yields.
Ph Ph
N O
H Ph H
LiOC 2 5
: C C
NCNH 2 C
Ph O Ph H Ph 79% Ref. 230

CH 3
Cl BuLi
2
Cl CH 3 CH C CHCH 2 CH CH 3
CH CH CH 2 Ref. 231
2
3
10.2.7. Related Reactions
There are several reactions that are conceptually related to carbene reactions but
do not involve carbene, or even carbenoid, intermediates. Usually, these are reactions
in which the generation of a carbene is circumvented by a concerted rearrangement
process. Important examples of this type are the thermal and photochemical reactions of
-diazo ketones. When -diazo ketones are decomposed thermally or photochemically,
they usually rearrange to ketenes, in a reaction known as the Wolff rearrangement. 232
+
O N N
–
R C CH O C CHR concerted mechanism
O O
+ – :
R C CH N N R C CH O C CHR carbene mechanism
:
:
O
R oxirene
227
M. S. Gudipati, J. G. Radziszewski, P. Kaszynski, and J. Michl, J. Org. Chem., 58, 3668 (1993).
228 G. L. Closs, L. E. Closs, and W. A. Böll, J. Am. Chem. Soc., 85, 3796 (1963).
229
E. J. York, W. Dittmar, J. R. Stevenson, and R. G. Bergman, J. Am. Chem. Soc., 95, 5680 (1973).
230
W. M. Jones, J. W. Wilson, Jr., and F. B. Tutwiler, J. Am. Chem. Soc., 85, 3309 (1963).
231 W. R. Moore and H. R. Ward, J. Org. Chem., 25, 2073 (1960).
232
W. Kirmse, Eur. J. Org. Chem., 2193 (2002); T. Ye and M. A. McKervey, Chem. Rev., 94, 1091 (1994).