Ring Opening Reactions - Heterogeneous

The ring opening of cyclopentanes occurs more readily on metal catalysts (Pt, Pd, Ir, Rh, etc.) than hydrogenolysis of open-chain alkanes because of the hindered free rotation. Different mechanisms proposed are reviewed. The position of ring opening is usually hindered close to the alkyl substituent(s) on metals but favored on acidic catalysts where C5C6 ring enlargement takes place. Product distribution on different metals and bimetallic systems is discussed. Smaller (C3, C4) rings give open-chain products more readily. Catalyst composition affects the stereochemistry of opening of substituted cyclopropanes. Larger cycles also undergo ring contraction to aromatics, together with the formation of bicyclic products. The practical importance of opening of substituted C6 cycles to alkanes in upgrading fuel components (rather than their aromatization) is presently increasing. Spirocyclic hydrocarbons also undergo isomerization and subsequent aromatization. Oxacycloalkanes (C3C5 heterocycles containing oxygen) produce alcohols in a hydrogenolytic ring-opening process or carbonyls with skeletal rearrangement. Keywords: ring opening; CC bond hydrogenolysis; cyclopentane; methylcyclopentane; alkylcyclopentanes; cyclopropane; cyclobutane; alkylcyclopropanes; alkylcyclobutanes; oxacycloalkanes; oxiranes; oxetanes; spirocyclic compounds; metal catalysts; platinum; rhodium; nickel; bimetallic catalysts