Page 1029 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

Reactions of 4-substituted 4-pentenyl bromoacetates with tri-n-butylstannane 1013
resulted in modest yields of eight-membered endo-cyclization products, along with
reduction products. 117 SECTION 11.2
Characteristics of
Reactions Involving
O
Radical Intermediates
O X O O X
Bu 3 SnH
CO(CH ) C CH CO(CH ) C CH
BrCH 2 2 3 CH 2 + 3 2 3 2
AIBH
X

Product yield
X Cyclization Reduction
H 38% 31%
38% 18%
CH 3
(CH 3
3 Si 54% 32%

This type of cyclization can be further improved by use of a BF catalyst. 118
3

O O O
10% (Bu 3 Sn) 2
CH(CH ) OCCH I
CH 2 2 3 2
3 eq BF , hv
3
I
Systems with the potential for forming nine-, ten-, or sixteen-membered rings gave only
reduction. The relatively favorable formation of eight-membered rings is attributed to
the s-trans conformation of the ester group. The relative energies of the 8-endo and
7-exo TS were calculated for both the s-trans and s-cis conformations by ROHF/MP2/3-
21G computations. 117 The most favorable TS is the 8-endo-cis structure. The relative
energies are shown in Figure 11.10. The general pattern that emerges from these
experimental and computational results is that trajectory of approach, steric effects,
and reactant conformation are the controlling factors in the ring size selectivity for
radical cyclizations.

11.2.3.4. Other Radical Reactions Section D of Table 11.3 includes several examples
of radical fragmentation reactions. Entries 44, 48, and 49 are examples of -scission
reactions. The facile decarboxylation of acyloxy radicals is an example.

O O
RCOOC(CH ) RCO . R . + CO 2
3 3

117 E. Lee, C. H. Yoon, T. H. Lee, S. Y. Kim, T. J. Ha, Y. Sung, S.-H. Park, and S. Lee, J. Am. Chem.
Soc., 120, 7469 (1998).
118
J. Wang and C. Li, J. Org. Chem., 67, 1271 (2002).