Syntheses of strained cyclic dienes were accomplished via palladium(II)-catalyzed oxidative cyclizations of terminal bis(vinylboronate esters). The reactions generated strained (E,E)-1,3-dienes which, due to their instability at room temperature, underwent 4π electrocyclizations to bicyclic cyclobutenes. The strain driving the second cyclizations was set in place by thermodynamically favored reductive eliminations from palladium(II). Unique selectivity for cyclobutenes, as opposed to typically more stable 1,3-dienes, arises from strain in the intermediate medium-sized (E,E)-1,3-diene rings. Thermal ring opening of the cyclobutenes gives (Z,Z)-1,3-diene products, as opposed to the (E,E) products. DFT calculations verified the thermodynamic versus kinetic control of the reactions and kinetic studies were in excellent agreement with the calculated energy changes. Further exploration of the tandem coupling/electrocyclization pathway was demonstrated by a palladium(II) oxidative homocoupling/8π electrocyclization cascade.

Using ring strain to control 4π electrocyclization reactions: Torquoselectivity in ring closing of medium ring dienes and ring opening of bicyclic cyclobutenes

251st National Meeting (2016)

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