Hydrophobic vitamin B12. Part 6. Carbon-skeleton rearrangement via formation of host–guest complexes derived from an ‘octopus’ azaparacyclophane and hydrophobic vitamin B12derivatives: a novel holoenzyme model system

Abstract

The alkylation reaction of a hydrophobic vitamin B₁₂ derivative with alkyl bromides in an ‘octopus’ azaparacyclophane having eight hydrocarbon chains have been investigated. Molecular discrimination has been shown to originate from electrostatic interaction between the octopus cyclophane and the alkyl bromides. Alkylation was enhanced by desolvation and proximity effects operating on the reacting species via formation of a ternary complex composed of the octopus cyclophane, the hydrophobic vitamin B₁₂ derivative, and an alkyl halide. Carbon-skeleton rearrangements reactions of alkyl ligands bound to the hydrophobic vitamin B₁₂ were found to be markedly favoured in the hydrophobic cavity provided by the octopus cyclophane, relative to the reactions in methanol and benzene, under anaerobic photolysis conditions at ordinary temperatures. The same reactions took place readily in solid benzene below 4°C under similar conditions. The central cobalt atom of the hydrophobic vitamin B₁₂ participates in the rearrangements reaction via formation of a tight pair with an alkyl radical species. Non-enzymic rearrangement reactions have been shown here to proceed quite efficiently by employing a relevant apoenzyme model.