EVALUATION OF THE SELECTIVITY OF THE EPOXIDATION OF CIS-ALKENES VERSUS TRANS-ALKENES BY OXO[MESO-TETRAKIS(2,6-DIBROMOPHENYL)PORPHINATO]CHROMIUM(V) - DYNAMICS, PRODUCTS, AND VANDERWAALS ENERGIES

Studies of the reaction of oxo[meso-tetrakis(2,6-dibromophenyl)porphinato]chromium(V) [(Br8TPP)Cr(V)(O)(X)] with a number of trans-alkenes have been carried out with the objective of clarifying the nature of the reported stereoselectivity of the epoxidation of cis-alkenes as compared to trans-alkenes. The alkenes employed were trans-p,p'-dimethoxystilbene, trans-stilbene, trans-beta-methylstyrene, trans-p,p'-dicyanostilbene, trans-2-pentene, trans-5-decene, trans-2-hexenyl acetate, (E)-1,2-bis(trans-2,trans-3-diphenylcyclopropyl)ethene, and also (Z)-1,2-bis(trans-2,trans-3-diphenylcyclopropyl)ethene. The influence of electronic and steric factors in determining the second-order rate constants (k1) for the reactions of trans-alkenes with (Br8TPP)Cr(V)(O)(X) were separated by use of the linear free-energy relationship of log k1 and the one-electron oxidation potentials (E 1/2) established by the reaction of a series of cis-alkenes with (Br8TPP)Cr(V)(O)(X). Quantification of the steric factors established that there is no appreciable steric retardation of the second-order rate constants for reaction of trans-p,p'-dimethoxystilbene, trans-beta-methylstyrene, trans-2-pentene, trans-5-decene, and trans-2-hexenyl acetate with (Br8TPP)Cr(V)(O)(X). Steric hindrance is quite marked with trans-stilbene, trans-p,p'-dicyanostilbene, and (E)-1,2-bis-(trans-2, trans-3-diphenylcyclopropyl)ethene. In comparison with trans-stilbene, the lack of a kinetic steric effect with trans-p,p'-dimethoxystilbene requires a change of mechanism. The standard free energy (DELTA-G-degrees) for one-electron oxidation of trans-p,p'-dimethoxystilbene [(Br8TPP)Cr(V)(O)(X) + > C = C < --> (Br8TPP)Cr(IV)(O)(X) + > C + C-degrees <] is favored over the DELTA-G-degrees values for trans-stilbene and cis-stilbene by 0.46 and 0.59 V, respectively. Thus, trans-p,p'-dimethoxystilbene but not trans-stilbene can undergo facile le- oxidation by (Br8TPP)Cr(V)(O)(X). This can be shown to be allowed by comparison of DELTA-G-degrees and DELTA-G(a) values. One-electron transfer can take place at significantly longer distances than can covalent bond formation. Computer docking experiments have defined the stereochemistry associated with the closest approach of minimal van der Waals repulsive forces in the reaction of trans- and cis-stilbene. For trans-stilbene the closest approach is at a distance of 3.88 angstrom from directly above the Cr(V)(O) while for cis-stilbene lowest energy closest approach is at 2.81 angstrom with the C = C bond: (i) lined parallel to opposite meso carbons; and (ii) above the Cr(V)(O) oxygen with an offset (alpha) from the perpendicular Cr(V)-O bond of alpha = 24-degrees. Product yields are in accord with assigned steric effects and the nature of the products and mode of formation are discussed.