STEREOSPECIFICITY OF THE BETA-HYDROXYL ELIMINATION FROM THE (HYDROXYALKYL)CHROMIUM COMPLEX (H2O)5CRIII-CH(CH3)CH(CH3)OH2+

The reaction of the aliphatic free radical .CH(CH3)CH(OH)CH3 (formed by .OH addition to cis-/trans-2-butene or by hydrogen abstraction from 2-butanol by .OH free radicals) with chromous ions in aqueous solutions was investigated. The radical reacts with Cr2+(aq), k = 1.1 x 10(8) M-1 s-1, to yield a transient complex with a chromium-carbon sigma-bond. The transient complex decomposes via an acid-catalyzed beta-elimination reaction, k = (230 + 9.8 x 10(4)[H3O+]) s-1, to form a complex in which the tervalent chromium is bound to 2-butene by a d --> pi-bond. This complex decomposes to produce cis- or trans-2-butene. At pH > 2 cis- and trans-2-butene are produced in equal amounts whereas in acidic solutions, pH < 1, the thermodynamically less stable cis-2-butene is produced in considerable excess. These results are discussed and tentatively explained by the higher rate of dissociation of the cis d --> pi-complex coupled with an acid-catalyzed isomerization of the two d --> pi-complexes. The rate of the hydrolysis of the chromium d --> pi-complex obeys the rate law k = (0.8 + 6.7[H3O+]) s-1. The same mechanism also explains the effect of pH on the relative yields of 1-butene and 2-butene obtained in the oxidation of 2-butanol by Cr2+(aq) and H2O2. The rates of the beta-elimination reactions of (H2O)5Cr(III)-CH2CH2OH2+ and (H2O)5Cr(III)-CH(CH3)CH2OH2+ are also reported; the results indicate that methyl substituents considerably affect the rate of beta-elimination reactions.