Abstract
An atom-economical methodology to access substituted acyl-cyclohexenes from pentamethylacetophenone and 1,5-diols is described. This process is catalyzed by an iridium(I) catalyst in conjunction with a bulky electron rich phosphine ligand (CataCXium A) which favors acceptorless dehydrogenation over conjugate reduction to the corresponding cyclohexane. The reaction produces water and hydrogen gas as the sole byproducts and a wide range of functionalized acyl-cyclohexene products can be synthesized using this method in very high yields. A series of control experiments were carried out, which revealed that the process is initiated by acceptorless dehydrogenation of the diol followed by a redox-neutral cascade process, which is independent of the iridium catalyst. Deuterium labeling studies established that the key step of this cascade involves a novel base-mediated [1,5]-hydride shift. The cyclohexenyl ketone products could readily be cleaved under mildly acidic conditions to access a range of valuable substituted cyclohexene derivatives.
Highlights
The synthesis of cyclohexenes in a regio- and stereocontrolled manner is of fundamental importance in the preparation of natural products, functional materials, and medicinally relevant compounds.[1]
We recently reported that pentamethylphenyl (Ph*) ketones can be directly alkylated with alcohols via hydrogen borrowing catalysis.[6,7]
We subsequently showed that this approach could be extended to an iridium catalyzed synthesis of cyclohexanes by double alkylation of pentamethylacetophenone with 1,5diols (Scheme 1B).[8,9]
Summary
The synthesis of cyclohexenes in a regio- and stereocontrolled manner is of fundamental importance in the preparation of natural products, functional materials, and medicinally relevant compounds.[1]. Intermolecular reactions used to synthesize sterically demanding, multisubstituted cyclohexenes are much less well documented, and new methods for cyclohexene synthesis that complement the Diels−Alder approach are highly desirable
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