Cyclizative radical carbonylations of azaenynes by TTMSS and hexanethiol leading to alpha-silyl- and thiomethylene lactams. Insights into the E/Z stereoselectivities.
Free-radical mediated cyclizative carbonylations of azaenynes were carried out using TTMSS as a radical mediator to compare the efficiency and the stereochemistry with those using tributyltin hydride. Using a substrate concentration of 0.1 M, the reactions gave good yields of alpha-silylmethylene lactams having four to seven-membered rings. The observed E-diastereoselectivity of the resulting vinylsilane moiety is in sharp contrast to the Z-selectivity observed during the analogous carbonylation using tributyltin hydride. When hexanethiol was used as the radical mediator, alpha-thiomethylene lactams were formed with E-favoring stereoselectivity again. Ab initio and DFT molecular orbital calculations on the stability of E and Z products were carried out for a set of five-membered methylene lactams bearing SnH3, SiH3, and SMe groups. The distinct thermodynamic preference for the Z-isomer was only predicted for the Sn-bearing lactam. A steric effect due to the bulky (TMS)3Si group is proposed for the E-selectivity observed in the TTMSS-mediated reaction.
Tojino
Mami
M
Department of Chemistry, Faculty of Arts and Sciences, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan.
Otsuka
Noboru
N
Fukuyama
Takahide
T
Matsubara
Hiroshi
H
Schiesser
Carl H
CH
Kuriyama
Hiroki
H
Miyazato
Hironari
H
Minakata
Satoshi
S
Komatsu
Mitsuo
M
Ryu
Ilhyong
I