C-H Activation and Cyclizations using Stannylenes.

Abstract

The direct formation of Sn-C bonds from allylic and propargylic CH-bonds has been demonstrated by employing the mixed reagent Sn[C2(SiMe3)4C2H4] / PhI. Alkane and ether C-H activation has been demonstrated with Sn[N(SiMe3)2]2 / PhX (X = Br, I). Palladium catalyzed cross-coupling was carried out with allylic tin compounds obtained via C-H activation of cyclohexene with Kira stannylene (Sn[C2(SiMe3)4C2H4] / PhI) and primary tin compounds obtained with C-H activation of tert-butyl methyl ether and pentane with Sn[N(SiMe3)2]2 / PhI. A moderate degree of steric control of C-H activation with substrates possessing different C-H bonds was accomplished using sterically bulky aryl halides. Statistically averaged tertiary/primary C-H bond abstraction ratio (3o/1o) of trans-4-methyl-2-pentene and 4-methyl-2-pentyne was measured with iodobenzene, 2,4,6-trimethyl-iodobenzene and 2,4,6-tri-isopropyl-iodobenzene. The decrease of 3o/1o from 4.7 to 1.1 was observed for the alkene and 15 to 4 for the alkyne, respectively, as the steric bulk of the aryl halide was increased. The effects of steric bulk and electronics have been analyzed in detail and it was found that sterically bulky aryl halides suppress the formation of oxidative addition byproduct. The electronic influence of substituents on the aryl halide was found to operate mainly through inductive effects. Sigma electron withdrawing substituents in the para position of the aryl halide suppressed oxidative addition formation. The unexpected addition of phenyl and the tin complex across double bonds was also discovered. This reaction proceeded diastereoselectively in case of cyclopentene providing product with tin and phenyl in 1,2 position and trans to each other. Stereoselective radical cyclizations have been demonstrated with 2-iodophenyl substrates having alkene or alkyne functionality. 2-iodophenyl alkenes were used as radical clocks and the rate of alkene and alkyne C-H activation was found to be comparable, on the order of 1.1x 108 M-1s-1. This mechanistic study showed that transient phenyl radicaloid species are involved in the C-H activation reaction with divalent group 14 reagent and aryl halide. Methyl and trimethyl silyl functionalized alkynes cyclized stereoselectively to obtain benzofuran products with exocyclic double bond with E stereochemistry and tin functionality.