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.
The addition of singly and doubly activated dienophiles to TpW(NO)(PMe3)(5,6-η2-anisole) and TpW(NO)(PMe3)(5,6-η2-1,3-dimethoxybenzene) to generate dihapto-coordinated [2.2.2]bicyclooctadienes and -trienes is explored. The highly functionalized bicyclic scaffolds are isolated from the metal fragment intact. Although moderate yields hinder their synthetic application, a unique stereoelectronic effect is uncovered. Coordination diastereomers of anisoles with the methoxy substituent distal to the PMe3 ligand have faster reaction rates relative to the proximal diastereomer. DFT calculations reveal the metal fragment possesses a dipole that directs positive charge away from the PMe3 ligand, and a polarized cycloaddition transition state is invoked to explain this observation.
TpRe(CO)(MeIm)(η 2 -2-methylfuran) (1; Tp ) hydridotris(pyrazolyl)borate and MeIm ) N-methylimidazole) and TpRe(CO)(MeIm)(η 2 -2,5-dimethylfuran) (9) undergo Lewis acid promoted cyclopentannulation reactions with enones and enals to generate 3-acetylcyclopentene complexes. During the reaction, a rearrangement occurs such that the R-and -carbons of the enone or enal are incorporated into the new carbocycle. The reactions are completely regioselective. Yields vary from 26 to 78%, depending on reaction conditions. Diastereomer ratios vary from 50:50 to >95:5. In a similar fashion, these furan complexes undergo reactions with aldehydes to form acetyldihydrofuran complexes, where a new dihydrofuran ring is generated from the aldehyde and C3-C5 of the original furan. These dihydrofuran complexes are readily converted into the corresponding trisubstituted furans via oxidative demetalation. Attempts to synthesize TpRe(CO)(MeIm)(η 2 -2-methoxyfuran) for the synthesis of cyclopentene esters resulted in the formation of a novel rhenium carbene.
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