An Oxy-anion Accelerated [1,5]-o-Quinone Methide Shift During the Nucleophilic Epoxidation of Salicylfulvene.

A new facet of nucleophilic fulvene epoxidations has been uncovered. 6-Arylfulvenes containing an ortho or para hydroxyl group react with basic hydrogen peroxide in an unusual manner; the epoxidation of the fulvene exocyclic double bond is followed by a phenoxide ion initiated epoxide ring opening to form an o-quinone methide (o-QM) intermediate. The resulting cyclopentadienolate undergoes an unusual oxy-anion accelerated [1,5]-sigmatropic o-QM shift. Computational studies reveal that the activation energy for the [1,5]-QM-shift in the cyclopentadienolate intermediate is quite low, signifying the acceleration caused by the oxy-anion group. Placement of a second hydroxyl group in the 6-aryl ring at C5 epoxidation via electron donation to the o-QM carbon; instead, an intramolecular oxa-6-π-electrocyclization of the o-QM intermediate onto the cyclopentadiene is observed.

Unusual hydroxyl effect on fulvene endoperoxide decompositions.

The thermal decomposition of fulvene endoperoxides ordinarily proceeds via an allene oxide intermediate affording oxepin-2(3H)-one derivatives. We have now uncovered new, unusual pathways in these decompositions where the presence of a hydroxyl group on the alkyl or aryl attached to the fulvene exocyclic double bond has a profound effect on the fate of the reactive intermediates derived from the unstable endoperoxides. Computational work supports the proposed mechanistic pathways.