Dirhodium(II) Tetraacetamide

Rh2(NHCOMe)4

[87985-40-8]  · C8H16N4O4Rh2  · Dirhodium(II) Tetraacetamide  · (MW 438.10)

(catalyst for selective carbenoid reactions of diazo compounds1)

Physical Data: UV/vis (MeCN) 500 (2.2), 345 (shoulder) nm.2 NMR (CD3CN): d 2.20 (s).3

Solubility: sol MeOH, MeCN, pyridine, DMSO; insol CH2Cl2, toluene.

Form Supplied in: blue solid for anhydrous form; purple solid as hydrate.

Preparative Method: by ligand substitution from Rh2(OAc)4.3

Handling, Storage, and Precautions: air stable, weakly hygroscopic; stored in desiccator.

Introduction.

Dirhodium(II) tetraacetamide, Rh2(acam)4, was first prepared from Dirhodium(II) Tetraacetate in a melt of acetamide.4 However, this method gave a mixture of Rh2(OAc)4 - n(acam)n, of which Rh2(acam)4 was the dominant product but could not be conveniently separated. The preferred procedure is to treat Rh2(OAc)4 with acetamide in refluxing chlorobenzene under conditions where acetic acid is trapped by sodium carbonate in a Soxhlet extraction apparatus (eq 1).3 Four acetamidates are ligated to one dirhodium(II) nucleus, and each rhodium is bound to two nitrogen and two oxygen donor atoms arranged in a cis geometry.5 Incomplete substitution, when only three acetamides have replaced acetate, yielding Rh2(acam)3(OAc), produced a catalyst whose selectivity is not optimum.

Metal Carbene Transformations.

Although insoluble in the solvents in which catalytic metal carbene transformations are performed, Rh2(acam)4 enters solution after addition of the diazo compound. The principal advantage of this catalyst is its selectivity for product formation from reactions with diazocarbonyl compounds, but its reactivity towards dinitrogen extrusion is less than that of dirhodium(II) tetra(carboxylates).

Stereoselectivity in Cyclopropanation Reactions.

Use of Rh2(acam)4 for intermolecular cyclopropanation of alkenes results in higher trans (anti) selectivity which, when the diazo compound is 2,6-di-t-butyl-4-methylphenyl diazoacetate (BDA), is exceptional (e.g. eq 2: 98% trans).3 Product yields are high (75-96%), and byproducts are often minimal. Relative reactivities are also enhanced by Rh2(acam)4, which has made possible highly regioselective cyclopropanation of selected dienes (e.g. eq 3).3 However, Rh2(acam)4 is unsuitable, relative to Dirhodium(II) Tetraacetate, for intermolecular cyclopropanation of styrene by the pantolactone ester of trans-2-diazo-4-phenyl-3-butenoate.6 Substitution of Rh2(acam)4 by the more soluble Dirhodium(II) Tetra(caprolactam), RH2(cap)4, does not provide any obvious advantage in reactivity or selectivity for cyclopropanation.

Carbon-Hydrogen Insertion Reactions.

Use of Rh2(acam)4 provides an increase in regioselectivity for competitive insertion into carbon-hydrogen bonds (tertiary > secondary > primary) that result in the formation of five-membered ring carbonyl compounds (e.g. eq 4; pfb = perfluorobutyrate).7,8

Both diazoacetoacetates and diazoacetates show exceptional selectivity enhancement with Rh2(acam)4. However, the same degree of control is not evident in the competition from diazoacetoacetamides for b-lactam versus g-lactam formation.9 -11 The use of Rh2(cap)4 in place of Rh2(acam)4 does not provide any obvious advantage in regioselectivity for carbon-hydrogen insertion.

Chemoselectivity.

Few comparisons have been made with Rh2(acam)4 and Rh2(cap)4, but those that have suggest that Rh2(cap)4 holds an advantage.11 N-(2-Arylethyl)-N-t-butyldiazoacetamides, for example, exhibit competition between aromatic cycloaddition and carbon-hydrogen insertion (e.g. eq 5), and chemoselectivity for C-H insertion with Rh2(cap)4 is greater than with Rh2(acam)4, but both are more selective than is Rh2(OAc)4.


1. Doyle, M. P. In Homogenous Transition Metal Catalyzed Reactions; Moser, W. R.; Slocum, D. W., Eds.; American Chemical Society: Washington, 1992.
2. Chavan, M. Y.; Zhu, T. P.; Lin, X. Q.; Ahsan, M. Q.; Bear, J. L.; Kadish, K. M. IC 1984, 23, 4538.
3. Doyle, M. P.; Bagheri, V.; Wandless, T. J.; Harn, N. K.; Brinker, D. A.; Eagle, C. T.; Loh, K.-L. JACS 1990, 112, 1906.
4. (a) Zhu, T. P.; Ahsan, M. Q.; Malinski, T.; Kadish, K. M.; Bear, J. L. IC 1984, 23, 2. (b) Best, S. P.; Chandley, P.; Clark, R. J. H.; McCarthy, S.; Hursthouse, M. B.; Bates, P. A. JCS(D) 1989, 581.
5. Ahsan, M. Q.; Bernal, I.; Bear, J. L. IC 1986, 25, 260.
6. Davies, H. M. L.; Cantrell, W. R., Jr. TL 1991, 32, 6509.
7. Doyle, M. P.; Bagheri, V.; Pearson, M. M.; Edwards, J. D. TL 1989, 30, 7001.
8. Doyle, M. P.; Westrum, L. J.; Wolthuis, W. N. E.; See, M. M.; Boone, W. P.; Bagheri, V.; Pearson, M. M. JACS 1993, 115, 958.
9. Doyle, M. P.; Taunton, J.; Pho, H. Q. TL 1989, 30, 5397.
10. Doyle, M. P.; Pieters, R. J.; Taunton, J.; Pho, H. Q.; Padwa, A.; Hertzog, D. L.; Precedo, L. JOC 1991, 56, 820.
11. Padwa, A.; Austin, D. J.; Price, A. T.; Semones, M. A.; Doyle, M. P.; Protopopova, M. N.; Winchester, W. R.; Tran, A. JACS 1993, 115, 8669.

Michael P. Doyle

Trinity University, San Antonio, TX, USA



Copyright 1995-2000 by John Wiley & Sons, Ltd. All rights reserved.