Dirhodium(II) Tetra(caprolactam)

[138984-26-1]  · C24H44N4O4Rh2  · Dirhodium(II) Tetra(caprolactam)  · (MW 658.54)

(catalyst for selective carbenoid reactions of diazo compounds1)

Solubility: sol MeOH, MeCN; slightly sol toluene, 1,2-dichloroethane, CH2Cl2.

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

Preparative Method: prepared by ligand substitution from Rh2(OAc)4.2

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

Introduction.

Dirhodium(II) tetra(caprolactam), Rh2(cap)4, which is much more soluble in organic solvents than is the corresponding acetamide,3 is prepared by ligand substitution of acetate by caprolactam on Dirhodium(II) Tetraacetate (eq 1) in refluxing chlorobenzene. When performed in a Soxhlet extraction apparatus where the extractor thimble contains sodium carbonate, acetic acid is trapped and ligand substitution is forced to completion. Four caprolactam molecules ligate one dirhodium(II) nucleus, and each rhodium is bound to two nitrogen and two oxygen donor atoms arranged in a cis configuration.4

Metal Carbene Transformations.

The principal advantages of Rh2(cap)4 are its solubility and its selectivity for product formation in reactions with diazocarbonyl compounds, relative to dirhodium(II) tetra(carboxylates). Having lower Lewis acidity than dirhodium(II) tetra(carboxylates), Rh2(cap)4 is less reactive towards diazo compounds; reactions with diazoacetates, diazoacetamides, and diazo ketones usually occur in refluxing dichloromethane, and reactions with diazoacetoacetates and diazoacetoacetamides ordinarily occur at temperatures above 80 °C.

Regioselectivity in Carbon-Hydrogen Insertion Reactions.

The use of Rh2(cap)4 provides high levels of regiocontrol in carbon-hydrogen insertion reactions of diazoacetate and diazoacetoacetate esters.2 Whereas 2,3,4-trimethyl-3-pentyl diazoacetate forms g-lactone products from insertion into primary and tertiary C-H bonds in a statistical distribution with Dirhodium(II) Tetrakis(perfluorobutyrate) (Rh2(pfb)4), only tertiary C-H insertion is observed with Rh2(cap)4 (eq 2). Similarly, when competition for C-H insertion is between primary and secondary C-H bonds, Rh2(cap)4 directs insertion to the secondary C-H bond with high selectivity (eq 3). Nearly identical selectivities are observed with Dirhodium(II) Tetraacetamide so, by extrapolation, Rh2(cap)4 will also be the catalyst of choice for regioselective cyclopropanation reactions with dienes.3

Chemoselectivity in Metal Carbene Transformations.

When there are two reaction centers for intramolecular carbenoid reactions, the use of Rh2(cap)4 often leads to the production of only one product.5,6 Cyclopropanation is favored over aromatic substitution (eq 4), over tertiary carbon-hydrogen insertion (eq 5), and over aromatic cycloaddition (eq 6). Product yields are high in each case. The order of reactivity for metal carbenes generated from Rh2(cap)4 is cyclopropanation > tertiary C-H insertion > secondary C-H insertion > aromatic cycloaddition,6 and the rate differences between them are as much as 100-fold.

Carbonyl ylide generation is favored over aromatic cycloaddition with Rh2(cap)4 (eq 7),7 but in competition with cyclopropanation both ylide generation and alkene cycloaddition occur with equivalent facility.5,8 Except for ylide generation, these catalytic metal carbene transformations exhibit an overwhelming preference for five-membered ring formation,9 but within this controlling limitation, both chemoselectivity and regioselectivity are greatly influenced by the ligand of dirhodium(II).

The selectivities achieved with changes in dirhodium(II) ligands are due, in part, to the degree of charge localization on the carbene carbon.6 However, conformational restrictions from the carbene system that is bound to dirhodium(II) can also influence selectivity,2 so broad generalizations regarding selectivity with Rh2(cap)4 are inappropriate.


1. Doyle, M. P. In Homogeneous Transition Metal Catalyzed Reactions; Moser, W. R.; Slocum, D. W., Eds.; American Chemical Society: Washington, 1992.
2. 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.
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. Bear, J. L.; Yao, C.-L.; Liu, L.-M.; Capdevielle, F. J.; Korp, J. D.; Albright, T. A.; Kang, S.-K.; Kadish, K. M. IC 1989, 28, 1254.
5. Padwa, A.; Austin, D. J.; Hornbuckle, S. F.; Semones, M. A.; Doyle, M. P.; Protopopova, M. N. JACS 1992, 114, 1874.
6. 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.
7. Cox, G. G.; Moody, C. J.; Austin, D. J.; Padwa, A. T 1993, 49, 5109.
8. Padwa, A.; Austin, D. J.; Hornbuckle, S. F.; Price, A. T. TL 1992, 33, 6427.
9. Padwa, A.; Austin, D. J. AG(E) 1994, 33, 1797.

Michael P. Doyle

Trinity University, San Antonio, TX, USA



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