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) Å] 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‘ = iPr (10), tBu (11)). The derivative with R‘ = Me (9) has also been synthesized. In 10 a Pd−Sn single bond [2.670(1) Å] is present. Complexes 10 and 11 (as well as 7 and 8 but not 9) react slowly with additional ethyne at 20 °C to reform the Pd(0)−ethyne complexes 5 and 6 with concomitant generation of the stannole (C4H4)SnR2 (2a). Likely intermediates of this reaction are the Pd(0)−η2-stannole complexes (R‘2PC2H4PR‘2)Pd(η2-C4H4SnR2) (R‘ = iPr (12), tBu (13)), which have been synthesized independently. The stannole ligand in 12, 13 is easily displaced by ethyne to yield 5 or 6 or by SnR2 to yield 7 or 8. Thus, the isolated complexes 5−8 and 10−13 are conceivable intermediates of the catalytic stannole formation, and from their stoichiometric reactions the catalysis cycle can be assembled. For the [(R‘3P)2Pd]-catalyzed system, the corresponding intermediates (Me3P)2Pd(C2H2) (15), (iPr3P)2Pd(C2H2) (17), (Me3P)2PdSnR2 (18), (iPr3P)2PdSnR2 (20), and (Me3P)2Pd(CHCH)SnR2 (Pd−Sn) (19) have been isolated or detected by NMR, and (iPr3P)2Pd(CHCH)SnR2 (Pd−Sn) (21) is postulated as an intermediate. The [(Me3P)2Pd] system (stannole formation above 0 °C) is catalytically more active than any of the [(R‘2PC2H4PR‘2)Pd] systems (slow stannole formation for R‘ = tBu at 20 °C). Most active is the [(iPr3P)2Pd] system, allowing a catalytic synthesis of the stannole 2a from SnR2 and ethyne at −30 °C [1% of 17; yield 2a: 87%; TON (turnover number): 87]. By carrying out the catalysis in pentane at 20 °C (0.04% of 17), the TON is increased to 1074 but the yield of 2a is diminished to 43% due to uncatalyzed thermal side reactions