[47829-28-7]  · C37H33I2P2Rh  · Diiodo(methyl)bis(triphenylphosphine)rhodium(III)  · (MW 896.35)

(used in cross-coupling of organomercurials with alkyl halides2)

Physical Data: mp 170 °C (dec); 1H NMR (CD2Cl2) d 2.8 (dt, J = 2.9, 5.3 Hz, CH3), 7.39 (m, Hm,p of Ph), 7.63 (m, Ho of Ph); lmax (log ε in CH2Cl2) 250 (sh), 340 (3.71), 500 (sh), 610 (2.31); d 1.67 g cm-3.

Solubility: sparingly sol benzene and chloroform; sol CH2Cl2 and nitromethane; insol acetone or petroleum.

Form Supplied in: dark green crystalline solid.

Preparative Methods: Chlorotris(triphenylphosphine)rhodium(I) (1.0 g, 1.08 mmol) is allowed to stir in Iodomethane (20 mL) for 1 h; excess solvent is removed in vacuo and the resulting mass extracted with acetone; recrystallization from benzene affords green crystals of MeRhI2(PPh3)2.C6H6;1 an alternate method of preparing uncomplexed MeRhI2(PPh3)2 involves addition of iodomethane in dichloromethane to (Ph3P)3Rh+HC(SO2CF3)2-.2

Handling, Storage, and Precautions: the dry solid is moderately air-stable but rapidly air-oxidizes in solution.

Methylation of Organomercurials.3

The stoichiometric cross-coupling of organomercurials with alkyl halides proceeds in moderate to excellent yields using MeRhI2(PPh3)2. Optimal conditions for cross-coupling alkenylmercurials include addition of excess Lithium Chloride, using HMPA as solvent, and running the reaction at 70 °C for 24 h. Indeed, addition of MeRhI2(PPh3)2 to styrylmercury(II) chloride under these conditions affords propenylbenzene in excellent yield (eq 1). Only small amounts of product formed, however, in reactions where the rhodium complex was prepared in situ with subsequent attempts at cross-coupling.

Dialkynylmercury compounds transmetalate with MeRhI2(PPh3)2 to give products with complete methylation of both organic moieties (eq 2). Attempted methylation of n-decylmercury(II) chloride gives only n-decane, 1-decene, and other isomeric decenes. Methylation of benzylmercury(II) chloride and a-chloromercurioacetophenone has also failed.

Arylmercurials react with MeRhI2(PPh3)2 in the presence of excess iodomethane to give the corresponding methyl-substituted arenes (eqs 3 and 4). This reaction is general for arylmercurials containing electron donating and withdrawing groups, as well as heterocyclic organomercurial derivatives.

Unfortunately, attempts to catalyze the methylation of alkenylmercurials using MeRhI2(PPh3)2 afford significant amounts of dimerized product.4 Better yields of cross-coupling products are obtained in catalyzed reactions of dialkynylmercurials (ca. 60%). This catalyst system is also effective for the cross-coupling of arylmercurials with alkenyl halides.

1. (a) Lawson, D. N.; Osborn, J. A.; Wilkinson, G. JCS(A) 1966, 1733. (b) Troughton, P. G. H.; Skapski, A. C. CC 1968, 575. (c) Douek, I. C.; Wilkinson, G. JCS(A) 1969, 2604.
2. (a) Siedle, A. R.; Newmark, R. A.; Pignolet, L. H.; Howells, R. D. JACS 1984, 106, 1510. (b) Siedle, A. R.; Newmark, R. A.; Pignolet, L. H. OM 1984, 3, 855.
3. (a) Larock, R. C.; Hershberger, S. S. TL 1981, 22, 2443. (b) Larock, R. C.; Hershberger, S. S. JOM 1982, 225, 31.
4. Larock, R. C.; Bernhardt, J. C. JOC 1977, 42, 1680.

Stephen A. Westcott

University of North Carolina at Chapel Hill, NC, USA

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