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

The complexes (R‘ 2 PC 2 H 4 PR‘ 2 )Pd(C 2 H 4 ) (R‘ = i Pr, t Bu) react with R 3 SnH (R = Me, n Bu) by displacement of the ethene ligand and oxidative addition of the Sn−H bond to generate the chelating phosphane stabilized cis Pd II hydrido stannyl complexes (R‘ 2 PC 2 H 4 PR‘ 2 )PdH(SnR 3 ) (R‘ = i Pr ( 1 ), t Bu ( 2 )). Complex 1a (R‘ = i Pr, R = Me), containing the smallest substituents, is only transiently formed but has been detected at −80 °C by NMR spectroscopy. It reacts further with Me 3 SnH, even at −120 °C, by eliminating hydrogen to give (d i ppe)Pd(SnMe 3 ) 2 ( 3 ). In contrast, the isolated (d i ppe)PdH(Sn n Bu 3 ) ( 1b ) is briefly stable at ambient temperature, whereas the sterically encumbered species (d t bpe)PdH(SnR 3 ) (R = Me ( 2a ), n Bu ( 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 C 2 R‘ ‘ 2 (R‘ ‘ = CO 2 Me) to give (d t bpe)Pd(C 2 R‘ ‘ 2 ) ( 4 ) and predominantly the corresponding ( E )-vinylstannane ( E )-(R‘ ‘)(H)C C(SnMe 3 )(R‘ ‘) ( E -5 ). Since 2a also catalyzes the hydrostannation of the alkyne, the cis Pd II 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 R 3 SnH into Sn 2 R 6 and H 2 .