J. Org. Chem. 1999, 64, 7271-7273
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Anomalous Products from Intramolecular C-H Insertion by a Rhodium Carbenoid. Possible Involvement of a Zwitterionic Mechanism James D. White* and Peter Hrnciar Department of Chemistry, Oregon State University, Corvallis, Oregon 97331-4003 Received May 17, 1999
Metal-catalyzed decomposition of R-diazoketones to yield metal carbenoids is a technique for generating a reactive species that can undergo remarkably selective intramolecular C-H insertion.1 The reaction has been widely applied in ring construction, particularly to the synthesis of cyclopentanones.2 A mechanism has been put forward by Taber for carbenoid C-H insertion mediated by rhodium(II) in which the new C-C bond is formed directly via a four-centered transition state (Scheme 1).3 This mechanism is also favored by Doyle who has buttressed the argument with calculations using an MM2 force field.4 A central feature of the mechanism shown in Scheme 1 is that C-C bond formation is accompanied by simultaneous transfer of hydride from carbon to rhodium and is concluded by reductive elimination at the metal atom. Quite frequently, it is found that C-H insertion reactions of rhodium carbenoids give rise to anomalous products which cannot be easily explained by this “concerted” mechanism. Thus, Clark in a study of rhodium-catalyzed decomposition of R-diazo-R′-alkoxy ketones, observed products that are more readily accommodated by a zwitterionic intermediate.5 In this scenario, hydride transfer takes place from carbon to rhodium, leading to a discrete intermediate A that precedes C-C bond formation (Scheme 2). This pathway would be favored in those cases where a stable carbocation is generated after hydride transfer. A key reaction in our recently published synthesis of (+)-codeine involved rhodium(II)-catalyzed decomposition of diazoketone 1.6 In addition to the desired pentacyclic ketone 2, three products were obtained: methyl ketone 3 containing a benzofuran nucleus, olefin 4 resulting from fragmentation, and the fused cyclobutanone 5. The ratio of 2, 3, 4, and 5 exhibited a marked dependence upon the rhodium catalyst employed in the decomposition of 1 (Table 1). The highest yield of 2 was obtained with dirhodium(II) tetrakis(acetamide) (Rh2(acam)4, 6), a catalyst that Doyle has shown can lead to high selectivity for carbene insertion into electron-rich methine C-H bonds.7 When dirhodium(II) tetra(trifluoroacetate) (Rh2(TFA)4, 7)8 was employed as catalyst, a significant quan(1) (a) Taber, D. F. In Comprehensive Organic Synthesis; Pattenden, G., Ed.; Pergamon Press: Oxford, 1991; Vol. 3, Chapter 4.2, p 1045. (b) Doyle, M. P. In Comprehensive Organometallic Chemistry II; Hegedus, L. S., Ed.; Pergamon Press: New York, 1995; Vol. 12, Chapter 5.2, p 421. (2) Ye, T.; McKervey, M. A. Chem. Rev. 1994, 94, 1091. (3) Taber, D. F.; Ruckle, R. E., Jr. J. Am. Chem. Soc. 1986, 108, 7686 (footnote 31). (4) Doyle, M. P.; Westrum, L. J.; Wolthuis, W. N. E.; See, M. M.; Boone, W. P.; Bagheri, V.; Pearson, M. M. J. Am. Chem. Soc. 1993, 115, 958. (5) Clark, J. S.; Dossetter, A. G.; Russell, C. A.; Whittingham, W. G. J. Org. Chem. 1997, 62, 4910. (6) White, J. D.; Hrnciar, P.; Stappenbeck, F. J. Org. Chem. 1997, 62, 5250.
tity of 4 was obtained, and both 3 and 4 were produced in addition to 2 when Rh2(OAc)4 (8) was used. Benzofuran 3 was not formed with the sterically demanding dirhodium(II) tetra(triphenylacetate) (Rh2(TPA)4, 9)9 as catalyst. Scheme 1. “Concerted” Mechanism of Rhodium(II) Carbenoid C-H Insertion
Scheme 2. Zwitterionic Mechanism of Rhodium(II) Carbenoid C-H Insertion
(7) Doyle, M. P.; Bagheri, V.; Wandless, T. J.; Harn, N. K.; Brinker, D. A.; Eagle, C. T.; Loh, K.-L. J. Am. Chem. Soc. 1990, 112, 1906. (8) Johnson, S. A.; Hunt, H. R.; Neumann, H. M. Inorg. Chem. 1963, 2, 960. (9) Hashimoto, S.; Watanabe, N.; Ikegami, S. Tetrahedron Lett. 1992, 33, 2709.
10.1021/jo990797g CCC: $18.00 © 1999 American Chemical Society Published on Web 09/01/1999
7272 J. Org. Chem., Vol. 64, No. 19, 1999
Notes
Table 1. Rhodium(II)-Catalyzed Decomposition of Diazoketone 1 yield of product (%) catalyst
2
3
4
5
Rh2(acam)4 (6) Rh2(TFA)4 (7) Rh2(OAc)4 (8) Rh2(TPA)4 (9)
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