Iodorhodium(III) Tetraphenylporphyrin

(R = Ph)

[69509-35-9]  · C44H28IN4Rh  · Iodorhodium(III) Tetraphenylporphyrin  · (MW 842.57) (R = mesityl)

[85990-32-5]  · C56H52IN4Rh  · Iodorhodium(III) Tetramesitylporphyrin  · (MW 1010.93)

(catalyst for cyclopropanation of alkenes by diazoesters1)

Alternate Name: Rh(TPP)I.

Physical Data: NMR (d in CDCl3) 7.7 and 8.2 (multiplets, 20H, phenyl), 8.8 (singlet, 8H, pyrrole); visible (CH2Cl2) lmax = 575 nm (ε = 4000), 535 nm (15 700), 422 nm (151 000).2

Solubility: sol dichloromethane, chloroform, acetone, ethyl acetate; insol pentane.

Preparative Method: readily prepared from the commerically available tetraphenylporphyrin and Tetracarbonyl(di-m-chloro)dirhodium using a modification of Callot's method.2 The initially formed RhI species is oxidized to RhIII using Iodine or N-Iodosuccinimide.

Handling, Storage, and Precautions: not air or moisture sensitive. For prolonged storage, the reagent should be protected from light. Toxicity has not been thoroughly investigated.

Numerous transition metal compounds are known to catalyze the cyclopropanation of alkenes by diazoesters.3 Among these, compounds based on copper, palladium, and rhodium have been the leading choices for catalysts. Callot and co-workers reported that iodorhodium(III) tetraphenylporphyrin [Rh(TPP)I] (1) is also an efficient catalyst for this transformation.1

Porphyrin catalysts offer two distinct advantages over other transition metal species. The most important of these is their propensity to provide more of the syn cyclopropyl ester than most commonly used catalysts, which produce predominantly the anti diastereomer (eq 1).1,4,5 The syn preference can be enhanced considerably by using bulkier macrocycles such as iodorhodium(III) tetramesitylporphyrin [Rh(TMP)I] (2) (eq 1).5 Syn/anti ratios of up to 7.8:1 have been reported when cis disubstituted alkenes are used as substrates.6

The second advantage is the higher number of turnovers porphyrin catalysts afford: several thousand compared to several hundred for other catalysts. Therefore, considerably less catalyst is needed to promote the reaction. In addition, the reactivity of the porphyrin catalysts approaches that of rhodium carboxylates, which have been reported to be the most efficient catalysts for this process.7 Yields typically vary from 50-80% with (1) depending on the nature of the alkene substrate. Dimerization products (maleates and fumarates) resulting from diazoester attack on the putative metallocarbene intermediate are minimized by using large excesses of alkene or by slow addition of the diazoester.

Competitive cyclopropanation reactions have revealed steric interactions to be the dominant factor in determining relative reactivity for unfunctionalized alkenes.6 In general, aromatic alkenes are slightly more reactive than aliphatic alkenes, and tetrasubstituted alkenes are poor substrates for the porphyrin catalysts. Differences between other substituted alkenes are modest but follow the trend reported by Doyle for rhodium acetate-catalyzed reactions.8 The difference in reactivities can be enhanced by use of (2).

At present, rhodium porphyrins are the most useful catalysts for obtaining syn cyclopropyl esters from alkenes and diazoesters. Enantioselectivities reported using chiral porphyrin ligands are not synthetically useful.6,9 When the chiral systems are fully developed, they should be particularly useful since optically pure syn cyclopropyl esters are not readily accessible with current methodology.10

1. Callot, H. J.; Piechocki, C. TL 1980, 21, 3489.
2. Callot, H. J.; Schaeffer, E. NJC 1980, 4, 311.
3. (a) Maas, G. Top. Curr. Chem. 1987, 137, 75. (b) Doyle, M. P. CRV 1986, 86, 919. (c) For a review of stoichiometric carbene-transfer agents, see: Brookhart, M.; Studabaker, W. B. CRV 1987, 87, 411.
4. Doyle, M. P.; Dorow, R. L.; Buhro, W. E.; Griffin, J. H.; Tamblyn, W. H.; Trudell, M. L. OM 1984, 3, 44.
5. Callot, H. J.; Metz, F.; Piechocki, C. T 1982, 38, 2365.
6. Maxwell, J. L.; O'Malley, S.; Brown, K. C.; Kodadek, T. OM 1992, 11, 645.
7. Anciaux, A. J.; Hubert, A. J.; Noels, A. F.; Petiniot, N.; Teyssié, P. JOC 1980, 45, 695.
8. Doyle, M. P.; Griffin, J. H.; Bagheri, V.; Dorow, R. L. OM 1984, 3, 53.
9. O'Malley, S.; Kodadek, T. OM 1992, 11, 2299.
10. For some substrates, very high enantiomeric excesses can be realized for the anti diastereomer. See: (a) Aratani, T. PAC 1985, 57, 1839. (b) Fritschi, H.; Leutenegger, U.; Pfaltz, A. HCA 1988, 71, 1553. (c) Lowenthal, R. E.; Abiko, A.; Masamune, S. TL 1990, 31, 6005. (d) Evans, D. A.; Woerpel, K. A.; Hinman, M. M.; Faul, M. M. JACS 1991, 113, 726. (e) Doyle, M. P.; Pieters, R. J.; Martin, S. F.; Austin, R. E.; Oalmann, C. J.; Müller, P. JACS 1991, 113, 1423.

Thomas Kodadek & David W. Bartley

University of Texas, Austin, TX, USA

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