James W. Wilt

Reactivity and selectivity in the cyclization of sila-5-hexen-1-yl carbon-centered radicals

Abstract A trio of sila-5-hexen-1-yl radicals has been prepared from the corresponding halides by reaction with tri- n -butyltin hydride (deuteride). The radicals possessing a dimethylsilyl function α or β to the carbon radical center demonstrated marked reduction in total (but especially exo-trig ) cyclization compared to the all-carbon system. The γ-silyl radical behaved, contrariwise, quite comparably to the all-carbon system. The difference in cyclization found in the α-silyl radical was demonstrated to result from both a pronounced decrease in cyclization rate via the expected exo-trig mode and and from a significantly enhanced rate of hydrogen abstraction from TBTH . Both the α- and γ-silyl radicals cyclized via the endo-trig mode at rates close to that of the parent 5-hexen-1-yl radical itself. The cyclizations studied were demonstrated to be irreversible. The kinetic control thus shown by the preferred formation of endo cyclized product from the α- and β-silyl radicals is highly unusual and represents the first report of carbon-centered 5-hexen-1-yl type radicals violating the Baldwin-Beckwith rule exo-trig cyclization preferred by 5-hexen-1-yl radicals). Rationalization of the cyclization behavior of the α- and γ-silyl radicals involves both steric and electronic factors. The behavior of the most unusual case, the β-silyl radical, which has the lowest cyclization propensity and no exo mode product, remains largely unexplained because its hydrogen abstraction rate from TBTH is unavailable as yet. Some speculative considerations involving the preferred radical conformation in this system and its relation to cyclization are given.

Radical Reactions of Silanes

Silicon, after oxygen the most abundant element in the earth’s crust (28% by weight), has deservedly an equally abundant chemistry.1 Although obscured somewhat by the shadow of its congener carbon and its overwhelming array of transformations, the chemistry of silicon has been an ever-increasing area of importance. Two astounding reactions, the “direct synthesis” of halosilanes (1945)2 and the transition-metal-complex-catalyzed “hydrosilylation” process (1957),3 opened the door to organosilicon chemistry especially, even though the subject dates back considerably prior to them both.4 Direct synthesis made the silicones5 commercially available, and one need not elaborate on the significance of that achievement. Hydrosilylation made thousands of new organosilanes available,6 which in turn led exponentially to their further exploitation.

endo-3,3-Diphenyltricyclo[3.2.1.02,4]oct-6-ene

The main feature in the title compound, C 20 H 18 , is the system of fused three-, five- and six-membered rings. The two phenyl substituents are bonded to one C atom of the three-membered ring and that ring adopts an endo conformation relative to the larger rings. This results in a phenyl-group atom being close to the diene bond in the fused rings. The contact distances are 2.912 (4) and 2.952 (3) A. The plane of this phenyl group is nearly parallel to the diene bond. Planes of the two phenyl rings make an angle of 76.1 (2)°

π Participation in addition reactions of endo-3,3-diphenyltricyclo[3.2.1.02,4]oct-6-ene

Radical initiated LRAMERO (long range aryl migration coupled with electrophilic ring opening) is one example of π participation occurring from the syn phenyl ring of (1) in both electrophilic and radical addition reactions.