Thermal [pi]-route rearrangement of 4-cycloheptane-1-methyl acetate Public Deposited

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  • Pyrolysis of 4-cycloheptene-l-methyl acetate under flow conditions at 285° gives only rearranged bicyclic acetates and hydrocarbons rather than the anticipated simple elimination product methylenecyclohept- 4-ene. These rearranged acetates have the same structures and are formed in virtually the same relative percentages as those observed in the "π- route" solvolysis of the corresponding brosylate of 4-cycloheptene-l-methanol in the liquid phase. The possibility of the involvement of an ion-pair mechanism leading to the formation of the same carbonium ion intermediate in the gas phase as that proposed for the solvolytic rearrangement is considered but not conclusively proved because the pyrolysis could not be clearly demonstrated to be a homogeneous unimolecular gas phase reaction. Pyrolysis of 4- cycloheptene- 1-methyl acetate in a flow system appears to be largely heterogeneous as indicated by the steady downward drift in the calculated first order rate constant and by the serious retardation of the rate when the flow reactor was conditioned with the products from the thermal decomposition of allyl bromide. A consideration of the energy required for the formation of fully developed ion pairs in the gas phase in the complete absence of any stabilization from solvation leads to the conclusion that any mechanism invoking the formation of these intermediates is highly unlikely. On the other hand, the energy required for the formation of a highly polar transition state containing semi- ion pairs (but not leading to any discrete ion pair intermediates) is not energetically prohibitive and a pathway of this type seems probable in the flow pyrolysis of 4-cycloheptene-l-methyl acetate. The heterogeneity which is observed under these conditions may be a direct consequence of polar species interacting with the pyrex surface of the flow reactor. This surface interaction may thus be responsible for catalyzing the formation of products characteristic of carbonium ion rearrangement rather than simple elimination in the flow system. In contrast, pyrolysis of 4-cycloheptene-1-methyl acetate under static conditions takes place only at a higher temperature (3500) and leads only to the anticipated simple elimination product and its Cope rearrangement isomer 3- vinylmethylenecyclopentane. Kinetic studies show that this reaction is a homogeneous unimolecular gas phase elimination. An estimate of the activation energy indicates that it is very close to the values typically observed for the gas phase eliminations of acetates of most unsubstituted primary aliphatic alcohols. The simple elimination product methylenecyclohept-4-ene was synthesized and shown to undergo Cope rearrangement readily to 3-vinylmethylenecyclopentane under the prevailing pyrolysis conditions. The higher temperature required for reaction under static conditions and the absence of any heterogeneity caused by surface catalysis indicate that the simple elimination of 4- cycloheptene-1- methyl acetate involves a higher energy pathway than its rearrangement to the bicyclic acetates and hydrocarbons observed in the flow system. The general mechanism for the homogeneous pyrolysis of esters in the gas phase is also reviewed in light of these and other recently reported results.
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