Furan [3 + 2] Dipolar Cycloadditions Promoted by a π-Basic Tungsten Metal Fragment

Reactions are reported for the TpW(NO)(PMe3) complex of either 2-methylfuran or 2,5-dimethylfuran
with various dipolarophiles (N-methylmaleimide, N-phenylmaleimide, acrylonitrile). The resulting
7-oxanorbornene complexes are thought to be formed from a two-step reaction sequence. The first step
involves the metal shifting the coordination site from C4−C5 to C3−C4, an action that renders the furan
similar to a carbonyl ylide. This is followed by a 1,3-dipolar cycloaddition. In the case of the maleimides,
a high preference (>10:1) for exo stereochemistry is observed. For acrylonitrile, the preference for exo
is only modest (2:1). Attempts to liberate the intact 7-oxanorbornene were unsuccessful. Two crystal
structures are presented that support the stereochemical assignments of the complexed cycloadducts.