A palladium-catalyzed stannole synthesis

A palladium-catalyzed (2 + 2 + 1) cycloaddition reaction of two C2H2 and one SnR(2) to form C-unsubstituted stannoles (C4H4)SnR(2) [R = CH(SiMe(3))(2) 2a, R(2) = {C(SiMe(3))(2)CH2}(2) 2c] is described. Catalysts are (R'(2)PC(2)H(4)PR'(2))Pd complexes (slow reaction) and (R'P-3)(2)Pd complexes (fast reaction). The mechanism of the catalysis has been elucidated in detail from stoichiometric reactions based on R = CH(SiMe(3))(2). For the [(R'(2)PC(2)H(4)PR'(2))Pd]-catalyzed system, the starting Pd(0)-ethene complexes (R'(2)PC(2)H(4)PR'(2))Pd(C2H4) (R' = Pr-i (3),(t)Bu (4)) react both with ethyne to give the Pd(0)-ethyne derivatives (R'(2)PC(2)H(4)PR'(2))Pd(C2H2) (R' = Pr-i (5), (t)Bu (6)) and with SnR(2) to yield the Pd(0)-Sn(II) adducts (R'(2)PC(2)H(4)PR'(2))Pd=SnR(2) (R' = Pr-i (7), (t)Bu (8)). The Pd-Sn bond [2.481(2) Angstrom] of 7 is very short, indicative of partial multiple bonding. Subsequent reactions of the Pd(0)-ethyne complexes 5 and 6 with SnR(2) or of the Pd(0)-Sn(II) complexes 7 and 8 with ethyne afford the 1,2-palladastannete complexes (R'2PC2H4-PR'(2))Pd(CH=CH)SnR(2) (Pd-Sn) (R' = Pr-i (10), (t)Bu (11)). The derivative with R' = Me (9) has also been synthesized. In 10 a Pd-Sn single bond [2.670(1) Angstrom] is present. Complexes 10 and 11 (as well as 7 and 8 but not 9) react slowly with additional ethyne at 20 degrees C to reform the Pd(0)-ethyne complexes 5 and 6 with concomitant generation of the stannole (C4H4)SnR(2) (2a). Likely intermediates of this reaction are the Pd(0)-eta(2)-stannole complexes (R'(2)PC(2)H(4)PR'(2))Pd(eta(2)-C(4)H(4)SnR(2)) (R' = Pr-i (12), (t)Bu (13)), which have been synthesized independently. The stannole ligand in 12, 13 is easily displaced by ethyne to yield 5 or 6 or by SnR(2) 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'P-3)(2)Pd]-catayyze system, the corresponding intermediates (Me(3)P)(2)Pd(C2H2) (15), ((Pr3P)-Pr-i)(2)Pd(C2H2) (17), (Me(3)P)(2)Pd=SnR(2) (18), ((Pr3P)-Pr-i)(2)Pd=SnR(2) (20), and (Me(3)P)(2)Pd(CH=CH)SnR(2) (Pd-Sn) (19) have been isolated or detected by NMR, and ((Pr3P)-Pr-i)(2)Pd(CH=CH)SnR(2) (Pd-Sn) (21) is postulated as an intermediate. The [(Me(3)P)(2)Pd] system (stannole formation above 0 degrees C) is catalytically more active than any of the [(R'(2)PC(2)H(4)PR'(2))Pd] systems (slow stannole formation for R' = (t)Bu at 20 degrees C). Most active is the [((Pr3P)-Pr-i)(2)Pd] system, allowing a catalytic synthesis of the stannole 2a from SnR(2) and ethyne at -30 degrees C [1% of 17; yield 2a: 87%; TON (turnover number): 87]. By carrying out the catalysis in pentane at 20 degrees 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.