Syntheses of β,β′-Disubstituted Tungstacyclopentanes from Terminal Olefins and Their Conversions to Alkylidenes
Richard R. Schrock, René Riedel, Milan Maji, Matthew P. Conley, Veronica Carta
Abstract
β,β′-Disubstituted tungstacyclopentane complexes have been prepared through addition of propylene, 1-heptene, 1,6-heptadiene, 1,7-octadiene, or diallylaniline to unsubstituted tungstacyclopentane complexes, W(NR)(OR′) 2 (C 4 H 8 ); tungstabicycloalkanes are formed from the dienes. Several tungstacyclopentanes have been prepared from W(NR)(OR′) 2 Cl 2 complexes through alkylation and reduction with diethylzinc in the presence of the olefin. The synthesized complexes include those in which NR = NPh, NCPh 3, or N-2,6- i -Pr 2 C 6 H 3 and OR′ = OSiPh 3 or OCMe 2 (CF 3 ). The tungstacyclopentane complexes are stable in the dark, but upon exposure to violet (405 λ max ) or blue (445 nm λ max ) LED light, they form alkylidenes, largely through α hydrogen abstraction; ring-contraction to give a tungstacyclobutane is a minor reaction pathway only for the β,β′-dimethyltungstacyclopentane. Preliminary DFT calculations suggest that one electron is promoted from the HOMO (composed of the two W–C σ bonds) into an empty d xy orbital to give what could be viewed as a W(V)CH 2 CHRCHRCH 2 · complex. Abstraction of an H α by the radical chain end yields an alkylidene, while abstraction of an H γ leads to ring-contraction to give a tungstacyclobutane. The rates of the photochemical reactions for the β,β′-disubstituted tungstacyclopentanes do not vary dramatically with solvent (toluene, THF, CDCl 3 ). Twelve compounds were characterized through single crystal X-ray diffraction studies.