cis-(R‘₂PC₂H₄PR‘₂)PdH(SnR₃) Complexes:  Trapped Intermediates in the Palladium-Catalyzed Hydrostannation of Alkynes

The complexes (R'2PC2H4PR'2)Pd(C2H4) (R' = iPr, tBu) react with R3SnH (R = Me, nBu) by displacement of the ethene ligand and oxidative addition of the Sn−H bond to generate the chelating phosphane stabilized cis PdII hydrido stannyl complexes (R'2PC2H4PR'2)PdH(SnR2) (R' = iPr (1), tBu (2)). Complex 1a (R' = iPr, R = Me), containing the smallest substituents, is only transiently formed but has been detected at −80 °C by NMR spectroscopy. It reacts further with Me3SnH, even at −120 °C, by eliminating hydrogen to give (dippe)Pd(SnMe3)2 (3). In contrast, the isolated (dippe)PdH(SnnBu3) (1b) is briefly stable at ambient temperature, whereas the sterically encumbered species (dtbpe)PdH(SnR3) (R = Me (2a), nBu (2b)) are stable well above 100 °C. The molecular structure of 2a has been determined by X-ray crystallography. Complex 2a reacts with 2 equiv of C2R''2 (R'' = CO2Me) to give (dtbpe)Pd(C2R''2) (4) and predominantly the corresponding (E)-vinylstannane (E)-(R'')(H)CC(SnMe3)(R'') (E-5). Since 2a also catalyzes the hydrostannation of the alkyne, the cis PdII hydrido stannyl complexes 1a,b and 2a,b represent trapped intermediates in the Pd-catalyzed hydrostannation of alkynes. The existence of the complexes also sheds light on the mechanism of the Pd-catalyzed degradation of R3SnH into Sn2R6 and H2