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Abstract

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.