Mechanism of cyclopropanol to cyclopropanol isomerization mediated by Ti(IV) and a Lewis acid

Abstract

Isomerization of trans-3-deutero-r-1-methyl-cis-2-phenylcyclopropan-1-ol (1-trans-d) to three isomeric cyclopropanols was facilitated by reaction with a mixture of Ti(O-i-Pr)4 and BF3·OEt2. The more Lewis acidic Cl2Ti(O-i-Pr)2 catalyzed this reaction in the absence of BF3·OEt2. This cyclopropanol to cyclopropanol rearrangement involves reversible ring opening to a β-titanaketone. When the major species in solution prior to quenching was a titanium cyclopropoxide, a 40:40:10:10 mixture of cyclopropanols 1-trans-d:1-cis-d:2-trans-d:2-cis-d was obtained; this is close to the equilibrium ratio of the titanium cyclopropoxides. When a catalytic quantity of Ti(O-i-Pr)4 and a large excess of cyclopropanol was used, quenching gave a 29:29:21:21 mixture; this is closer to the equilibrium ratio of the cyclopropanols than the cyclopropoxides. Extrapolation to 0% and to 100% cyclopropoxide gave equilibrium constants for both cyclopropanols (Keq = [2]/[1] = 1.3) and cyclopropoxides (Keq = [2-Ti]/[1-Ti] = 0.18). A mechanism for these isomerization processes that involves ring opening and (or) ring closing with both retention and inversion of configuration at the carbon bearing phenyl is proposed.Key words: cyclopropanol, titanium isopropoxide, Kulinkovich hydroxycyclopropanation.