Carbostannolysis mediated by bis(pentamethylcyclopentadienyl)lanthanide catalysts. Utility in accessing organotin synthons

Facile carbon-tin bond activation in the reaction of 2-(trimethylstannyl) pyridine (1) with the organolanthanide complexes Cp*₂LaCH(TMS) ₂ (2a) and [Cp*₂LaH]₂ (2b) yields Cp*₂La(2-pyridyl) (3), as well as Me₃SnCH(TMS) ₂ and Me₃SnH, respectively. At room temperature, ethylene then undergoes insertion into the resulting La-C(pyridyl) bond followed by carbostannolysis to catalytically generate 2-(2-(Me₃Sn)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-SnMe₃ 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-(Me₃Sn)-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 ₃Sn-alkynyl intermediate which undergoes insertion into a Cp*₂La-alkynyl bond to produce the conjugated alkynyl product, which is subsequently protonolyzed from the Cp*₂La center by a new terminal alkyne substrate molecule. NMR spectroscopic and kinetic data support the proposed pathway and indicate turnover-limiting alkyne insertion.