Reductive allylation of pyrrole with allylboranes.: Synthesis of trans- and cis-2,5-disubstituted pyrrolidines

Pyrrole undergoes reductive mono- and diallylation on successive treatment with beta,gamma-unsaturated organoboron derivatives (triallylborane, allyl(dipropyl)borane, and triprenylborane) and alcohols to give 2-allylated 3-pyrrolines and trans-2,5-diallylated pyrrolidines. The addition of both the first and second boron-allylic fragment to the heterocycle proceeds with rearrangement. A method for transformation of the trans-2,5-diallylpyrrolidine into the cis-isomer (heating with triallylborane at 190 degrees C) was developed and a series of N-substituted derivatives of these pyrrolidines was synthesized. A method for the preparative synthesis of nonsymmetrically substituted trans- and cis-2-alkyl(phenyl)-5-allylpyrrolidines, based on reductive allylboration of pyrrole followed by 1,2-addition of RLi to the 5-allyl-1-pyrroline that formed, was also developed. A direct confirmation of intermediate formation of 2H- and 3H-pyrrole tautomers under the action of allylboranes was obtained. The adduct of 2H-pyrrole with BF3 was detected by NMR spectroscopy.