A Palladium-Catalyzed Stannole Synthesis

A palladium-catalyzed (2 + 2 + 1) cycloaddition reaction of two C2H2 and one SnR2 to form C-unsubstituted stannoles (C4H4)SnR2 [R = CH(SiMe3)2 2a, R2 = {C(SiMe3)2CH2}2 2c] is described. Catalysts are (R‘2PC2H4PR‘2)Pd complexes (slow reaction) and (R‘3P)2Pd complexes (fast reaction). The mechanism of the catalysis has been elucidated in detail from stoichiometric reactions based on R = CH(SiMe3)2. For the [(R‘2PC2H4PR‘2)Pd]-catalyzed system, the starting Pd(0)−ethene complexes (R‘2PC2H4PR‘2)Pd(C2H4) (R‘ = iPr (3), tBu (4)) react both with ethyne to give the Pd(0)−ethyne derivatives (R‘2PC2H4PR‘2)Pd(C2H2) (R‘ = iPr (5), tBu (6)) and with SnR2 to yield the Pd(0)−Sn(II) adducts (R‘2PC2H4PR‘2)PdSnR2 (R‘ = iPr (7), tBu (8)). The Pd−Sn bond [2.481(2) A] of 7 is very short, indicative of partial multiple bonding. Subsequent reactions of the Pd(0)−ethyne complexes 5 and 6 with SnR2 or of the Pd(0)−Sn(II) complexes 7 and 8 with ethyne afford the 1,2-palladastannete complexes (R‘2PC2H4PR‘2)Pd(CHCH)SnR2 (Pd−Sn) (R‘ =...