On the possibility of conversion of strained ...

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Title On the possibility of conversion of strained bridgehead alkenes into carbenes via 1,2 hydrogen and 1,2 carbon migrations. A theoretical study of the rearrangements in the adamantene and protoadamantene systems
Author(s) G. V. Shustov, Michael T. H. Liu
Journal Canadian Journal of Chemistry / Revue Canadienne De Chimie
Date 1998
Volume 76
Issue 6
Start page 851
End page 861
Abstract Rearrangements of 2-adamantene 3, adamantylidene 4, and 4-protoadamantylidene 5 have been studied theoretically using high level ab initio computations. Adamantene 3 and adamantylidene 4 have singlet ground states. The conversion of 3 into 4 via 1,2 hydrogen migration (Delta G(not equal) = 69.1 kcal mol(-1)) cannot compete with 1,2 carbon migration of 3 to 5 (30.3 kcal mol-L). The singlet carbene 5 rearranges to 2,4-dehydroadamantane 6 and 4,5-protoadamantene 9 via 1,3H and 1,2H shifts, respectively, with identical activation barriers of 8.8 kcal mol(-1). The 1,2 migration of the bridge hydrogen in 5 leading to 3,4 protoadamantene 8 has a higher activation barrier (37.2 kcal mol(-1)). A retro Diels-Alder cycloaddition of 3 to 5-allyl-3-methylenecyclohex-1-ene 7 is the main channel of the unimolecular conversion of 3 (Delta G(not equal) = 20.5 kcal mol(-1)). A predicted ratio of 7 > > 6 = 9 of the products of thermolysis of 3 is in agreement with experimental data. The rearrangement of 4 into 6 via the 1,3H shift (Delta G(not equal) = 11.8 kcal mol(-1)) dominates over the 1,2C shift of 4 to 8 (13.5 kcal mol(-1)), also in accord with experiment.

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