Pyran-2-ylidene carbene complexes

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Abstract
Pentacarbonyl(diphenylcyclopropenylidene)chromium(0)(1a) and the corresponding molybdenum complex (1b) have been synthesised and converted, by reaction with pyridinium ylides, into the pyran-2-ylidene complexes (2), a new type of carbene complex stabilised by 6 π-electron delocalisation. The 6-ethoxy-groups in the pyran-2-ylidene complexes (2b and e) are readily displaced by nucleophiles such as methanol, ammonia, ethylamine, morpholine, and phenyl-lithium. Pentacarbonyl-(6-ethoxy-3,4-diphenylpyran-2-ylidene)molybdenum(0) is converted into 4,5-diphenyl-2-pyrone by sodium hydroxide or by trifluoroacetic acid, and oxidised to 6-ethoxy-4,5-diphenyl-2-pyrone by lead tetra-acetate. Pentacarbonyl-(3,4,6-triphenylpyran-2-ylidene)molybdenum(0) reacts with N-phenacylpyridinium bromide and triethylamine to give 2-phenacylidene-3,4,6-triphenyl-2H-pyran (6). The complex also reacts with benzyne in a Diels–Alder reaction to give 1,2,4-triphenylnaphthalene and hexacarbonylmolybdenum. The reactivity of the cyclopropenylidene and pyranylidene complexes is compared with that of the corresponding carbonyl compounds.

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