Hydroboration of carbonyls and imines by an iminophosphonamido tin() precatalyst

A novel three-coordinated tin( ii ) chloride [Ph 2 P(N t Bu) 2 ]SnCl ( 1 ) supported by an N , N ′-di- tert -butyliminophosphonamide having two phenyl groups on the phosphorus atom was synthesized by the reaction of the starting lithium iminophosphonamide [Ph 2 P(N t Bu) 2 ]Li with SnCl 2 ·(dioxane) in toluene. The molecular structure of 1 was established by X-ray diffraction analysis. Tin( ii ) chloride 1 can act as an efficient precatalyst for the hydroboration of a wide variety of aldehydes, ketones, and imines at −10 °C. DFT calculations propose that hydroboration involves hydride transfer from the corresponding tin( ii ) hydride intermediate [Ph 2 P(N t Bu) 2 ]SnH ( 10 ) to the carbonyl substrates via four-membered transition states ( TS-12 ), affording three-coordinated tin( ii ) alkoxide intermediates [Ph 2 P(N t Bu) 2 ]SnOR ( 13 ), followed by the stepwise reaction of 13 with HBpin (pin = pinacolate) to release the boronate esters and regenerate the tin( ii ) hydride 10 . The stoichiometric reaction of the in site-generated 10 with benzophenone 2a at −10 °C led to the formation of 13 . Moreover, 13 also stoichiometrically reacted with HBpin at −10 °C, forming the corresponding boronate ester 3a and 10 based on the 1 H NMR spectrum of the reaction mixture. Iminophosphonamido tin( ii ) chloride 1 can act as a precatalyst for the hydroboration of aldehydes, ketones, and imines at −10 °C.