"Front Matter". In: Organosilanes in Radical Chemistry - Index of

150 Consecutive Radical Reactions
dominated the field using Bu 3SnH as the mediator. The rate (and efficiency) of
cyclization is strongly influenced by the presence of a heteroatom in the forming
cycle, as well as by geminal disubstituents (Thorpe–Ingold effect). Guidelines
for understanding the stereochemical outcome of radical cyclizations are well
developed and have been summarized in various papers and reviews [3,22,23].
The so-called Beckwith–Houk rules for radical cyclization have been found to
have an excellent predictive power allowing for the planning of radical steps in
complex natural product synthetic strategies. The study of the 5-exo cyclization
of methyl substituted hex-5-en-1-yl radicals has been the model for these rules
(Scheme 7.3). The major isomers result from chair-like transition states, where
the methyl substituent occupies an equatorial position, and the minor ones
form boat-like transition states.
65 �C
67% ds
1-Me 1,5-cis
80 �C
64% ds
2-Me 2,5-trans
80 �C
71% ds
3-Me 3,5-cis
80 �C
83% ds
4-Me 4,5-trans
Scheme 7.3 Cyclization of methyl subsituted hex-5-en-1-yl radicals (chair-like arrangements)
Over the last few years, silicon hydrides have been introduced as the radical
mediators and the intramolecular C w C bond formation (or cyclization) by
these reagents has been the subject of numerous publications.
7.3.1 CONSTRUCTION OF CARBOCYCLES
Reaction (7.13) shows the simplest and most popular type of hex-5-en-1-yl
radical cyclization mediated by (TMS) 3SiH. Thus, a 50 mmol solution of
5-hexenyl bromide and silane in the reported conditions leads to a 24:1 ratio
of cyclized vs uncyclized products [6]. Similar results were obtained by replacing
the bromide with isocyanide [7].
X
(TMS) 3SiH AIBN, 70 �C
X = Br
X = NC
+ +
93% 2% 4%
86% 2% 3%
(7.13)