THE RATE-LIMITING STEP IN THE ONE-ELECTRON OXIDATION OF AN ALKENE BY OXO[MESO-TETRAKIS(2,6-DIBROMOPHENYL)PORPHINATO]-CHROMIUM(V) IS THE FORMATION OF A CHARGE-TRANSFER COMPLEX

We have proposed, in previous studies, that the rate-limiting step in the oxidation of alkenes by hypervalent metal-oxo porphyrin species is the formation of a charge-transfer (CT) complex. The CT complex then partitions, dependent upon the alkene and the metal, to various oxidation products as epoxide and carbocation radical. In this paper we provide evidence that a carbocation radical intermediate is formed on oxidation of (Z)-1,3-bis(trans-2,trans-3-diphenylcyclopropyl)ethene (1-Z) by (Br8TPP)Cr(V)(O)(X). We also show that formation of carbocation radical is not rate-limiting. Oxidation of 1-Z by (Br8TPP)Cr(V)(O)(X) in the presence of the nonoxidative electrolyte (n-Bu)4NBF4 provides as the major product a mixture of the two isomeric (3E,5E)-1,8-dichloro-1,2,7,8-tetraphenylocta-3,5-diene (referred to as trans,trans-diene) and (3E,5Z) 1,8-dichloro-1,2,7,8-tetraphenylocta-3,5-diene (referred to as trans,cis-diene). Detailed H-1 NMR decoupling and 2-D COSY provided the assigned structures for trans,trans-diene and trans,cis-diene. The same diene mixture is obtained as the major product (and same ratio of the configurational isomers) on 1e- oxidation of 1-Z by controlled-potential bulk electrolysis (CPBE) in CH2Cl2 using the nonoxidative (n-Bu)4NBF4 as the supporting electrolyte. This shows that the oxidation of 1-Z by (Br8TPP)Cr(V)(O)(X) provides both trans,trans-diene and trans,cis-diene by way of an intermediate carbocation radical. The diene products arise from carbocation radical by opening one cyclopropyl ring via a cyclopropylcarbinyl to homoallylcarbinyl radical rearrangement, while the other cyclopropyl ring opens in a cyclopropylcarbinyl to homoallylcarbinyl carbocation rearrangement. In contrast to the oxidation of 1-Z by (Br8TPP)Cr(V)(O)(X) in the presence of the nonoxidative electrolyte (n-Bu)4NBF4, which provides the mixture of dienes plus (Br8TPP)Cr(IV)(O), oxidation of 1-Z by (Br8TPP)Cr(V)(O)(X) in the presence of the cooxidant (n-Bu)4NClO4 provides trans-2,trans-3-diphenylcyclopropanecarboxaldehyde as a major product plus the immediate product (Br8TPP)Cr(III)(X). Since the second-order rate constants are much the same for the reaction of 1-Z with (Br8TPP)Cr(V)(O)(X) in the presence of (n-Bu)4NBF4 or (n-Bu)4NClO4, it is concluded that the rate-limiting step for reaction of 1-Z with (Br8TPP)Cr(V)(O)(X) precedes and is separate from the product-forming reactions. The intermediate in the formation of a carbocation radical on 1e- oxidation of an alkene is most reasonably a CT complex.