Bicyclo silicon effect

Adcock, Shiner and coworkers found a significant 48.6-fold rate enhancement for the 4-trimethylsilyl bicyclo[2.2.2]octyl mesylate 265 compared with the silicon-free analogue 266102. The effects of trimethylgermyl and trimethyltin groups in the 4-position (in 267... 

Alkynylsilanes have been used as terminators in allylic alcohol and ketene thioacetal initiated cycliza-tions. In the late 1970s Heathcock and Johnson independently established the utility of alkynylsilanes in cyclization reactions. The example reported by Heathcock illustrated the electronic effect of silicon in the formic acid promoted transformation of the alkynylsilane (76) to the bicyclic ketone (78) in 76% yield. In contrast cyclization of the analogous alkyne (77) produced the bicyclo[2.2.2]octene (79), as illustrated in Scheme 38. 

Flowever, Adcock, Shiner and coworkers found increased rates of solvolysis for 4-metalloidal-substituted bicyclo[2.2.2]-oct-l-yl p-nitrobenzenesulphonates and methanesulphonates, in the order tin>germanium>silicon , as also observed for the p-effect. The relative rate of solvolysis of the 4-trimethylsilyl mesylate 63 compared to the non-substituted parent compound was 49 1. 

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