[37823-96-4] · C13H8O6W · Pentacarbonyl(methoxyphenylcarbene)tungsten(0) · (MW 444.06)
Alternate Name: pentacarbonyl[methoxy(phenyl)methylene]tungsten.
Physical Data: mp 59 °C.
Solubility: sol THF, ether, benzene, hexane.
Form Supplied in: orange-red crystals; not commercially available.
Preparative Method: prepared from Phenyllithium, Hexacarbonyltungsten, and Trimethyloxonium Tetrafluoroborate according to the procedure of Fischer.2
Purification: recrystallized from petroleum ether at -78 °C. Can also be purified by column chromatography on silica gel using pure hexane as the eluent.
Handling, Storage, and Precautions: slightly air sensitive; should be stored in a refrigerator.
Treatment of pentacarbonyl(methoxyphenylcarbene)tungsten (1) with Wittig reagents leads to the corresponding enol ether (eq 1),3a and pentacarbonyl(triphenylphosphine)tungsten. Alternatively, the same transformation can be effected using diazo compounds (eq 2).3b In each case the reaction occurs via nucleophilic attack at the carbene carbon followed by an elimination reaction. Similar transformations using the electronically similar compound methyl benzoate are considerably more difficult.4
Treatment of alkynes with (1) leads to intermediate vinylcarbene complexes (e.g. 2),5 which undergo subsequent transformations (eq 3).1 The reaction of Diphenylacetylene with complex (1) produces the indene in high yield (eq 4).6 Treatment of enyne (3) with complex (1) leads to vinylphenanthrene derivative (4) (eq 5).7 Formation of (4) involves alkyne insertion, followed by alkene insertion and alkene metathesis. The chromium analog of complex (1) (Pentacarbonyl(methoxyphenylcarbene)chromium(0)) has been better studied in its reaction with alkynes.1
Pentacarbonyl(methoxyphenylcarbene)tungsten effects cyclopropanation of some alkenes (eqs 6 and 7).8 The reaction is restricted to highly polarized alkenes such as a,b-unsaturated esters and enol ethers. Cyclopropanation of electron-rich alkenes proceeds through zwitterionic intermediates.9 Although stereoselectivity is poor, the method does generate highly functionalized cyclopropanes which would be difficult to prepare by other methods.
Treatment of complex (1) with isocyanides leads to the ketimine complex (5), whose reactivity is best characterized by the zwitterionic structure (6) (eq 8).10 The reactivity of the ketimine complex is very different from that of free ketimines, which are nucleophilic at the a-carbon. This reactivity profile is demonstrated by the cycloaddition reaction in eq 9, which leads to hydantoin (7) after oxidative removal of the metal (this result was obtained using the ethoxy analog of complex 1).11
Treatment of complex (1) with some azo compounds leads to the corresponding diaziridine derivatives (eq 10).12 The reaction is restricted to electron-deficient azo compounds since electron-rich azo compounds undergo metathesis reactions when treated with complex (1).
James W. Herndon
University of Maryland, College Park, MD, USA