On the Mechanism of SnCl4-Promoted Additions of Allylstannanes to Aldehydes: A Response to Denmark, Wilson and Willson

Article abstract of DOI:10.1021/ja00203a012

The reaction of allyltri-n-butylstannane with the complexes formed from three aldehydes (1-3) with SnCl4 has been investigated via variable-temperature (119)Sn NMR spectroscopy under carefully controlled conditions.With aldehyde 1, which forms a tight bidentate chelate with SnCl4, addition products are produced without the intermediacy of the transmetalation product allyltrichlorostannane at 1:1 stoichiometry, as revealed by appropriate control experiments with this reagent.The same is true of 2:1 (aldehyde:SnCl4) complexes derived from 1 and 2 provided that such complexes are formed stoichiometrically at low temperature and free SnCl4 is not present.With 3, free SnCl4, chelate, and 2:1 complex are all in equilibrium at -80 to -90 deg C at 1:1 SnCl4:aldehyde stoichiometry, and transmetalation with free SnCl4 is an important component of the overall reaction pathway.Competition experiments between the chelate and the 2:1 complex derived from 1 reveal that the chelate is more reactive but that the rate difference is modest.In addition, a mechanistic/spectroscopic study of allylstannane additions to aldehydes reported recently by Denmark, Wilson, and Willson has been reinvestigated, and results contrary to those reported are presented.In particular, it is shown that transmetalation pathways involving conversion of allyltri-n-butylstannane to allyltrichlorostannane or diallyldichlorostannane prior to reaction with aldehyde 5 and 6 do not occur from stable 2:1 (RCHO)2*SnCl4 complexes at low temperature.The sensitivity of such experiments to experimental details is emphasized.