Origin of High Regio-, Diastereo-, and Enantioselectivities in 1,6-Addition of Azlactones to Dienyl N-Acylpyrroles: A Computational Study.

Chiral P-spiro triaminoiminophosphorane (1) was developed to promote the highly regio-, diastereo-, and enantioselective 1,6- and 1,8-additions of azlactones (2·H) to dienyl and trienyl N-acylpyrroles (3 and 4). DFT calculations enabled us to gain deep insight into the whole reaction mechanism as well as the origin of the high regio- and stereoselectivities. The present reaction consists of three steps: (1) formation of the phosphonium-enolate ion-pair complex by deprotonation of 2·H with 1, (2) C-C bond formation of 2 with 3 and 4, and (3) protonation of the resulting enolate anion. The C-C bond formation is irreversible, and the rate- and stereodetermining step. The Cα-protonation preferentially proceeds rather than the thermodynamically and kinetically disfavored O- and Cγ-protonation, respectively. The high regio- and enantioselectivities are mainly attributed to the steric and electronic features of 1·H and 3/4. The hydrogen bonds (NH-O and CH-O) and the attractive CH-π interaction between 1·H and 2 and 3 play a key role in achieving high stereocontrol. The high regioselectivity is mainly controlled by the structural distortion of 1·H and the disruption of the π-conjugated system of 3 (1,4-system) and 4 (1,4- and 1,6-systems).