BpCu-Catalyzed Cyclopropanation of Olefins: A Simple System That Operates under Homogeneous and Heterogeneous Conditions (Bp ) Dihydridobis(pyrazolyl)borate) † M. Mar Dı ´az-Requejo, M. Carmen Nicasio, and Pedro J. Pe ´rez* Departamento de Quı ´mica y Ciencia de Materiales, Universidad de Huelva, Carretera de Palos de la Fra., 21819-Huelva, Spain Received January 29, 1998 The complexes BpCu (1), BpCuL (L ) bipy, 2;L ) Ph 2 PCH 2 CH 2 PPh 2 , 3;L ) PCy 3 , 4) and BpCuL 2 (L ) py, 5;L ) PPh 3 , 6) catalyze the cyclopropanation of olefins in moderate to high yields in the homogeneous phase. These compounds can also be used under heterogeneous conditions, when supported on silica gel. Complex 1 converts an equimolar mixture of an olefin (styrene, cis-cyclooctene, 1-hexene) and ethyl diazoacetate in the corresponding cyclopropanes in high yield with no excess of olefin employed. For the heterogeneous system, the catalysts can be recovered and reused several times (6-12 cycles) up to ca. 1000 turnovers. The data available suggest the existence of a common active species: the 14-electron BpCu fragment 1. These data consist of competition experiments with para-substituted styrenes as well as kinetic studies carried out in the presence and in the absence of the alkene. Introduction Metal-catalyzed cyclopropanation of olefins, using diazo compounds as the carbene source, seems to be restricted to a few transition-metal-based systems: 1-3 among them, rhodium and copper appear as the ele- ments of choice for this type of transformation. Several systems with both high diastereo- and enantioselectivi- ties have been reported in the past decade using those metals. 2 Although rhodium-based systems have been extensively studied and usually afford cyclopropanes in better yields and selectivities, those of copper present the incentive of a low cost for potential use in industry. Copper-bronze is employed in the production of per- methric acid, whereas copper(II) complexes containing Schiff bases are used in the commercial synthesis of cilastatin, an antibiotic precursor. 4 A general problem in metal-catalyzed olefin cyclopropanation is the need for a large excess of the olefin to avoid the formation of undesired products due to decomposition of the diazo compound. In addition to this, catalyst recycling from the reaction mixture is difficult to achieve. We herein report an inexpensive, copper-based system for olefin cyclopropanation that avoids the outlined drawbacks. It provides cyclopropanes in moderate to high yields with small olefin excess, and it is usable under both homogeneous and heterogeneous conditions. In the heterogeneous case, the catalyst can be reused several times with no loss of activity or stereoselectivity being observed. Results and Discussion A. Synthesis of the Catalyst Precursors BpCu (1), BpCuL (2-4), and BpCuL 2 (5, 6). Very recently, Tolman et al. have reported 5 the synthesis and struc- tural characterization of 14-electron complexes of com- position Bp′Cu (Bp′ ) dihydridobis(3,5-dimethyl-1- pyrazolyl)borate, Bp Me 2 ; Bp′ ) dihydridobis(3-tert-butyl- 1-pyrazolyl)borate, Bp t-Bu ). Following this preparation, we have synthesized the related complex BpCu (1; Bp ) dihydridobis(pyrazolyl)borate), by direct reaction of copper(I) iodide and the potassium salt of the dihydri- dobis(pyrazolyl)borate ligand (Bp). The NMR spectra of complex 1 show the typical resonances for the coordinated bis(pyrazolyl)borate group. Tolman has reported the X-ray structure of the aforementioned Bp′Cu complexes, 5 showing a dimeric nature for the tert- butyl derivative and an oligomeric structure for the dimethyl derivative. Attempts to crystallize complex 1 as single crystals have failed: it decomposes with time † Dedicated to Professor Ernesto Carmona on the occasion of his 50th birthday. (1) Doyle, M. P. In Comprehensive Organometallic Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon Press: Oxford, U.K., 1995; Vol. 12, p 387. (2) (a) Doyle, M. P.; McKervey, M. A. Chem. Commun. 1997, 983. (b) Davies, H. M. L.; Bruzinski, P. R.; Lake, D. H.; Kong, N.; Fall, M. J. J. Am. Chem. Soc. 1996, 118, 6897. (c) Doyle, M. P.; Peterson, Ch. S.; Parker, D. L. Angew. Chem., Int. Ed. Engl. 1996, 35, 1334. (d) Evans, D. A.; Woerpel, K. A.; Himman, M. M.; Faul, M. M. J. Am. Chem. Soc. 1991, 113, 726. (e) Lowenthal, R. E.; Abiko, A.; Masamune, S. Tetrahedron Lett. 1990, 31, 6005. (f) Singh, V. K.; DattaGupta, A.; Sekar, G. Synthesis 1997, 137. (3) Other metals such as iron, palladium, or ruthenium have also been found to promote the addition of a carbene moiety to the carbon- carbon double bond. See for example: (a) Wolf, J. R.; Hamaker, C. G.; Djukic, J.-P.; Kodadek, T.; Woo, L. K. J. Am. Chem. Soc. 1995, 117, 9124. (b) Galardon, E.; Le Maux, P.; Simonneaux, G. Chem. Commun. 1997, 927. (c) Lo, W.-Ch.; Che, Ch.-M.; Cheng, K.-F.; Mak, T. C. W. Chem. Commun. 1997, 1205. (d) Heck, R. F. Palladium Reagents in Organic Synthesis; Academic Press: London, 1987. (4) Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis, 2nd ed.; Wiley-Interscience: New York, 1992. (5) Houser, R. P.; Tolman, W. Inorg. Chem. 1995, 34, 1632. 3051 Organometallics 1998, 17, 3051-3057 S0276-7333(98)00061-2 CCC: $15.00 © 1998 American Chemical Society Publication on Web 06/13/1998