The Art of Writing Reasonable Organic Reaction Mechanisms by Robert B. Grossman

Author:Robert B. Grossman
Language: eng
Format: epub, pdf
ISBN: 9783030287337
Publisher: Springer International Publishing

Example

Problem 4.1

Draw a reasonable mechanism for the following preparation of pentamethylcyclopentadiene, a useful ligand for transition metals.

The electrocyclic equilibrium between cyclopropyl cations and allyl cations normally favors the allyl cation, because the cyclopropyl cation is both more strained and less conjugated than the starting allylic cation. For example, the cyclopropyl halide shown undergoes concerted loss of Br– and electrocyclic ring opening to the allyl cation upon distillation. (In principle, a ring opening that is not concerted with loss of the leaving group could be drawn, but the cyclopropyl cation is very high in energy.) Addition of Br− to the allylic cation then gives the product.

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