Carbostannolysis Mediated by Bis(pentamethylcyclopentadienyl)lanthanide Catalysts. Utility in Accessing Organotin Synthons

Abstract

Facile carbon–tin bond activation in the reaction of 2-(trimethylstannyl)pyridine ( 1 ) with the organolanthanide complexes Cp* 2 LaCH(TMS) 2 ( 2a ) and [Cp* 2 LaH] 2 ( 2b ) yields Cp* 2 La(2-pyridyl) ( 3 ), as well as Me 3 SnCH(TMS) 2 and Me 3 SnH, respectively. At room temperature, ethylene then undergoes insertion into the resulting La–C(pyridyl) bond followed by carbostannolysis to catalytically generate 2-(2-(Me 3 Sn)ethyl)pyridine ( 4 ) or, with extended reaction times, 6-ethyl-2-(2-(trimethylstannyl)ethyl)pyridine ( 5 ). In contrast to 1, 6-methyl-2-(trimethylstannyl)pyridine ( 6 ) is unreactive, likely reflecting steric constraints. With terminal alkynes, this catalytic heterocycle–SnMe 3 activation/carbostannylation process affords tin-functionalized conjugated enynes. Thus, at 60 °C 2b catalyzes the conversion 1 + 1-hexyne to yield ( E )-2-butyl-1-(Me 3 Sn)-oct-1-en-3-yne in a 60:1 ratio E: Z isomer ratio. This reaction is available to α-monosubstituted and α-disubstituted terminal alkynes, while α-trisubstituted alkynes are too hindered for reaction. The catalytic cycle is proposed to proceed via a spectroscopically detectable Me 3 Sn–alkynyl intermediate which undergoes insertion into a Cp* 2 La–alkynyl bond to produce the conjugated alkynyl product, which is subsequently protonolyzed from the Cp* 2 La center by a new terminal alkyne substrate molecule. NMR spectroscopic and kinetic data support the proposed pathway and indicate turnover-limiting alkyne insertion.