Pentacarbonyl(methoxyphenylcarbene)chromium(0)

[27436-93-7]  · C13H8CrO6  · Pentacarbonyl(methoxyphenylcarbene)chromium(0)  · (MW 312.21)

(metal-carbene complex1)

Alternate Name: pentacarbonyl[methoxy(phenyl)methylene]chromium.

Physical Data: mp 46 °C.

Solubility: sol THF, ether, benzene, hexane.

Form Supplied in: orange-red crystals; not commercially available.

Preparative Method: prepared from Phenyllithium, Hexacarbonylchromium, 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. Chromium compounds display variable toxicity. Care is recommended. Use in a fume hood.

Benzannulation.

Reaction of pentacarbonyl(phenylmethoxycarbene)chromium (1) with alkynes leads to p-methoxynaphthol derivatives (eq 1).1 The reaction is tolerant of a variety of substitution patterns on the alkyne. The regiochemistry is such that the larger alkyne substituent preferentially ends up ortho to the hydroxy group, and the smaller substituent ends up ortho to the methoxy group. The observed regioselectivity is proportional to the size difference of the alkyne substituents. Terminal alkynes react with complete control of regiochemistry (eq 2), but unsymmetrical internal alkynes provide mixtures of products (eq 3).3 The initially obtained arene complexes (2) are not very stable, and are converted to the free arene (3) upon exposure to air and light. Treatment of the arene complex with oxidants leads to naphthoquinone (4) or naphthoquinone monoacetal derivatives (5). In most cases, naphthalene derivatives are the major products from reaction of complex (1) and alkynes; however, in some cases other products such as cyclobutenones4 or furans5 are significant products in the reaction.

Photochemical Conversion to the Ketene and Subsequent Trapping.

Upon irradiation by UV light, complex (1) undergoes conversion to a ketene-metal complex (7), which can subsequently be intercepted (eq 4) if the irradiation is performed in the presence of suitable traps. Irradiation of complex (1) in the presence of imines leads to b-lactam derivatives (8),6 while treatment with alkenes leads to cyclobutenones (9).7 Compounds (8) and (9) are produced with excellent control of relative stereochemistry.8 Irradiation in the presence of alcohols leads to a-hydroxy esters (10).9

Cyclopropanation.

Complex (1) effects cyclopropanation of selected alkenes (eqs 5-7). The reaction is restricted to highly polarized alkenes such as a,b-unsaturated esters (eqs 5 and 6)10,11 and enol ethers (eq 7).12 The cyclopropanation of electron-rich alkenes must be conducted under Carbon Monoxide pressure; in the absence of CO, alkene metathesis is the major reaction pathway. In the case of dienoic acid esters, cyclopropanation occurs primarily at the 4,5-double bond (eq 6).

Alkenation.

Treatment of complex (1) with sulfur ylides leads to vinylogous carbonate derivatives (eq 8).13 Similar ylide-type alkenation reactions have been reported for the tungsten analog of complex (1),14 and although not reported, should proceed similarly for the molybdenum complex. Similar transformations using the electronically similar compound methyl benzoate are considerably more difficult.15

Reaction with Isocyanides.

Treatment of complex (1) with isocyanides leads to the ketimine complex (11), whose reactivity is best characterized by the zwitterionic structure (12) (eq 9).16 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 addition of methanol (eq 10), which leads to carbene complex (13) (this result was obtained using the ethoxy analog of complex 1).17

Insertion into s-Bonds.

Certain compounds undergo net bond insertion reactions upon treatment with carbene complexes (e.g. complex 1). Notable classes of compounds undergoing this transformation with complex (1) include Group 14 metal hydrides (eq 11)18 and 1,2-cyclobutenediones.19 In the latter case, the initial insertion product (15) might undergo isomerization to the observed product, alkylidenefuranone (16) (eq 12).


1. For reviews, see: (a) Wulff, W. D. COS 1991, 5, 1065. (b) Dötz, K. H. AG(E) 1984, 22, 587.
2. Fischer, E. O.; Maasböl, A. CB 1967, 100, 2445.
3. (a) Wulff, W. D.; Tang, P.-C.; McCallum, J. S. JACS 1981, 103, 7677. (b) Dötz, K. H.; Mühlemeier, J.; Schubert, U.; Orama, O. JOM 1983, 247, 187.
4. (a) Yamashita, A.; Toy, A. TL 1986, 27, 3471. (b) Chan, K. S.; Peterson, G. A.; Brandvold, T. A.; Faron, K. L.; Challener, C. A.; Hyldahl, C.; Wulff, W. D. JOM 1987, 334, 9.
5. (a) Semmelhack, M. F.; Jeong, N.; Lee, G. R. TL 1990, 31, 609. (b) McCallum, J. S.; Kunng, F.-A.; Gilbertson, S. R.; Wulff, W. D. OM 1988, 7, 2346.
6. Hegedus, L. S.; McGuire, M. A.; Schultze, L. M.; Yijun, C.; Anderson, O. P. JACS 1984, 106, 2680.
7. Söderberg, B. C.; Hegedus, L. S.; Sierra, M. A. JACS 1990, 112, 4364.
8. Valentí, E.; Pericàs, M. A.; Moyano, A. JOC 1990, 55, 3582.
9. Hegedus, L. S.; de Weck, G.; D'Andrea, S. JACS 1988, 110, 2122.
10. Dötz, K. H.; Fischer, E. O. CB 1972, 105, 1356.
11. Buchert, M.; Reissig, H.-U. TL 1988, 29, 2319.
12. (a) Dötz, K. H.; Fischer, E. O. CB 1972, 105, 3966. (b) Wulff, W. D.; Yang, D. C.; Murray, C. K. JACS 1988, 110, 2653.
13. Alcaide, B.; Domínguez, G.; Rodríguez-López, J.; Sierra, M. A. OM 1992, 11, 1979.
14. (a) Casey, C. P.; Burkhardt, T. J. JACS 1972, 94, 6543. (b) Casey, C. P.; Bertz, S. H.; Burkhardt, T. J. TL 1973, 1421.
15. Maryanoff, B. E.; Reitz, A. B. CRV 1989, 89, 863.
16. Aumann, R. AG(E) 1988, 27, 1456.
17. Aumann, R.; Fischer, E. O. CB 1968, 101, 954.
18. Connor, J. A.; Rose, P. D.; Turner, R. M. JOM 1973, 55, 111.
19. Zora, M.; Herndon, J. W. OM 1993, 12, 248.

James W. Herndon

University of Maryland, College Park, MD, USA



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