[14523-22-9]  · C4Cl2O4Rh2  · Tetracarbonyl(di-m-chloro)dirhodium  · (MW 388.76)

(catalyst or catalyst precursor for rearrangement of strained rings;1 carbonylative ring expansion;2 carbonylative coupling of organometallics;3 hydroformylation;4 silylformylation5)

Physical Data: mp 124 °C; sublimes 80 °C/1 mmHg.

Solubility: sol most organic solvents; slightly sol saturated hydrocarbons.

Form Supplied in: red or orange solid; widely available.

Purification: recryst from hexanes.

Handling, Storage, and Precautions: stable in dry air but should be protected from moisture. Solutions decompose in air; solvents should be deoxygenated before dissolving reagent.

Rearrangement of Strained Rings.

Rearrangements of many strained polycyclic hydrocarbons have been carried out with [Rh(CO)2(m-Cl)]2, but most of these hold more mechanistic interest than synthetic utility.6 On the other hand, rearrangements of three-membered rings have proven more generally useful. Thermolysis of alkoxy- or siloxycyclopropanes with catalytic [Rh(CO)2(m-Cl)]2 gives enol ethers,1a,b although stereoselectivity is slight (eq 1). Catalyzed rearrangement of 3-acylcyclopropenes yields substituted furans in a regioselective manner (eq 2).1c,2a When the reaction is carried out under CO, these same substrates are rearranged with carbonylation to give a-pyrones (eq 3).2a In a similar way, vinyl cyclopropenes are carbonylated to give phenols.2a Carbonylative ring expansion of aryl aziridines occurs stereo- and regiospecifically to give b-lactams (eq 4); however, the reaction fails in the absence of the aryl substituent.2b Cyclopropene (1) reacts with 1-hexyne to give oxepin (2) in the presence of [Rh(CO)2Cl]2, which then was isomerized with acid to phenol (3) (eq 5) in near quantitative yield.11

Organometallic Coupling.

The [Rh(CO)2(m-Cl)]2-catalyzed carbonylative coupling of vinylmercury(II) chlorides provides an efficient synthesis of symmetric divinyl ketones (eq 6).3a Arylmercury(II) chlorides or triarylbismuthines3b are coupled to give diaryl ketones, while in the absence of CO the mercury reagents are directly coupled to 1,3-dienes and biaryls.7

Hydroformylation of Alkenes.

[Rh(CO)2(m-Cl)]2 is a common catalyst precursor for hydroformylation in the presence of phosphine or phosphite ligands.4a,b These reactions are not often used in synthetic schemes because regioselectivity can be poor and alkene isomerization and hydrogenation can interfere. Nevertheless, with some functionalized alkenes, useful selectivity can be achieved,4a,c including modest enantioselectivity (eq 7).4b,d


In reactions related to hydroformylation, [Rh(CO)2(m-Cl)]2 catalyzes the addition of hydrosilanes and CO to enamines, giving silyl enol ethers (eq 8).5a Hydrolysis provides a-siloxy ketones. b-Siloxy aldehydes arise stereospecifically from the ring-opening silylformylation of epoxides (eq 9).5b As in many of the reactions catalyzed by [Rh(CO)2(m-Cl)]2, other RhI catalysts such as [Rh(cod)(m-Cl)]2 and Rh(CO)2(acac) can also be used.

Catalytic Hydrogenation of Alkenes.

Alkenes are catalytically hydrogenated to alkanes with [Rh(CO)2Cl]2. The reagent is usually bound to either alumina8 or phosphine-functionalized polystyrene,9 although [[Rh(CO)2Cl]2 has also been used as a heterogeneous catalyst. trans-1,3-Pentadiene, for example, has been reduced regiospecifically at the terminal double bond in good yield.10

Related Reagents.


1. (a) Ikura, K.; Ryu, I.; Ogawa, A.; Kambe, N.; Sonoda, N. TL 1989, 30, 6887. (b) Doyle, M. P.; Van Leusen, D. JACS 1981, 103, 5917. (c) Padwa, A.; Kassir, J. M.; Xu, S. L. JOC 1991, 56, 6971.
2. (a) Cho, S. H.; Liebeskind, L. S. JOC 1987, 52, 2631. (b) Calet, S.; Urso, F.; Alper, H. JACS 1989, 111, 931.
3. (a) Larock, R. C.; Hershberger, S. S. JOC 1980, 45, 3840. (b) Cho, C. S.; Ohe, T.; Itoh, O.; Uemura, S. CC 1992, 453.
4. (a) Botteghi, C.; Ganzerla, R.; Lenarda, M.; Moretti, G. J. Mol. Catal. 1987, 40, 129. (b) Ojima, I.; Hirai, K. In Asymmetric Synthesis; Morrison, J. D., Ed.; Academic: New York, 1985; Vol. 5, pp 102-146. (c) Neibecker, D.; Réau, R. NJC 1991, 15, 279. (d) Sakai, N.; Nozaki, K.; Mashima, K.; Takaya, H. TA 1992, 3, 583.
5. (a) Ikeda, S.-i; Chatani, N.; Kajikawa, Y.; Ohe, K.; Murai, S. JOC 1992, 57, 2. (b) Fukumoto, Y.; Chatani, N.; Murai, S. JOC 1993, 58, 4187.
6. (a) Bishop, K. C. III CRV 1976, 76, 461. (b) Alper, H. In Transition Metal Organometallics in Organic Synthesis; Alper, H., Ed.; Academic: New York, 1978; Vol. 2, pp 150-163.
7. Larock, R. C.; Bernhardt, J. C. JOC 1977, 42, 1680.
8. Lausarot, P. M.; Vaglio, G. A.; Valle, M. JOM 1981, 204, 249.
9. Drago, R. S.; Nyberg, E. D.; El Armma, A. G. IC 1981, 20, 2461.
10. Lausarot, P. M.; Vaglio, G. A.; Valle, M. JOM 1981, 215, 111.
11. Padwa, A.; Xu, S. JACS 1992, 114, 5881.

Mark A. Huffman

Merck Research Laboratories, Rahway, NJ, USA

Michael S. Shanklin & Michael P. Doyle

Trinity University, San Antonio, TX, USA

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