Oxo(methoxy)molybdenum(V) 5,10,15,20-Tetraphenylporphyrin

[74751-79-4]  · C45H31MoN4O2  · Oxo(methoxy)molybdenum(V) 5,10,15,20-Tetraphenylporphyrin  · (MW 755.74)

(able to catalyze selective and specific oxidation of alkenes by peroxy-bond heterolysis1,2)

Solubility: sol methylene chloride, benzene.

Preparative Methods: the alkoxo complex is obtained1,2 by chromatography of O=Mo(TPP)Cl1 over alumina using 20% MeOH in methylene chloride as eluant, followed by crystallization from MeOH-CH2Cl2.

Stereoselective Epoxidation.2

The reagent has been used as a catalyst in the selective epoxidation of alkenes with t-Butyl Hydroperoxide. In a typical experiment, OMo(TPP)OMe (5 × 10-6 mol) was dissolved in dry oxygen-free benzene. Cyclohexene (24.6 mmol) was added. The accurate catalyst concentration was measured by UV-Vis spectrometry, and the mixture was heated to 80 °C under argon. After equilibration, t-BuOOH (5 mmol) was added and aliquots were periodically monitored.

O=Mo(TPP)OMe, O=Mo(TPP)Cl, and cis-O2Mo(TPP) catalyze the epoxidation of cyclohexene with t-butyl hydroperoxide with formation of the insoluble [OMo(TPP)]2O. These Mo complexes are comparable to Hexacarbonylmolybdenum, but exhibit a longer induction period. A high selectivity to cyclohexene oxide was obtained at total conversion of the hydroperoxide. The rates of epoxide formation, expressed as the turnover number of the catalyst, are, respectively, Mo(CO)6 283 and OMo-(TPP)OMe 51 h-1. Monitoring the reaction by UV-Vis spectrometry revealed a complicated spectrum evolution during the induction period. However, after a time of reaction corresponding to three times the duration of the induction period, i.e. when the reaction had reached its stage of full catalytic activity, the concentration of MoV porphyrin complexes in solution was still 98-100% of that measured before addition of t-BuOOH.

OMo(TPP)Cl-catalyzed epoxidation of cis- and trans-2-hexene was found to be highly stereoselective, affording, respectively, 97% and 99% of the corresponding cis- and trans-2-epoxides. Relative rates of formation of cis- and trans-2-hexene oxides are significantly different. The cis-2-hexene oxide is formed 3.5 times faster than the trans isomer, showing a specific behavior of the porphyrin complex. The higher rate of formation observed for cis-2-hexene oxide indicates a more efficient steric control, as expected if the bulky rigid porphyrin ligand is present in the activated complex.

This effect is clearly evidenced in the case of 2-methylbutadiene (isoprene) where 3-methyl-3,4-epoxy-1-butene and 2-methyl-3,4-epoxy-1-butene, corresponding respectively to the epoxidation of the di- and monosubstituted double bond, are obtained in a 4:1 ratio by using Mo(CO)63 compared to 0.7:1 with OMo(TPP)OMe (eq 1).

OMo(TPP)X complexes represent the first example of metalloporphyrins able to catalyze selective and specific oxidation of alkenes by peroxy-bond heterolysis.2 With other transition metals like iron or cobalt, radical chain processes are observed for the oxidation of alkenes, giving only extremely low selectivity to epoxide4 or the hydroxylation of saturated hydrocarbons.5 This oxomolybdenum porphyrin-t-butyl hydroperoxide system may be considered as a simple chemical model for activation of hydroperoxides by cytochrome P-450 dependent monooxygenases.

1. (a) Ledon, H. J.; Mentzen, B. ICA 1978, 31, L393. (b) Ledon, H. J.; Bonnet, M. C.; Brigandat, Y.; Varescon, F. IC 1980, 19, 3488.
2. Ledon, H. J.; Durbut, P.; Varescon, F. JACS 1981, 103, 3601.
3. Sheng, M. N.; Zajecek, J. G. JOC 1970, 35, 1839. US Patent 3 538 124.
4. Paulson, D. R.; Ullman, R.; Sloane, R. B. CC 1974, 186.
5. Mansuy, D.; Bartoli, J. F.; Chottard, J. C.; Lange, M. AG(E) 1980, 19, 909.

Tapan Ray

Sandoz Research Institute, East Hanover, NJ, USA

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