Stereoconvergent Synthesis of Z-1,3-Disubstituted-1,3-Dienes by Uphill Photocatalysis

A variety of 1-aryl-1,3-dienes were isomerized from E to Z isomers by photocatalysis using Ru(bpy)₃ [PF₆ ]₂ and blue LED light. Enrichment of the Z-isomer is thought to occur by selective triplet energy transfer from the photocatalyst to the stereoisomeric mixture. The 1,3-diene starting materials are easily made by catalytic ene-yne metathesis (EYM). To access 1,3-diene Z-stereoisomers directly, a one pot procedure was developed. Additional 1,3-dienes were investigated for both isomerization and Z-enrichment. The combination of cross EYM with photocatalysis allows for the stereoconvergent synthesis of Z-1,3-dienes.

Preparation of Dienyl Boronates by Tandem Ene-Yne Metathesis/Dienyl Isomerization: Ready Access to Diene Building Blocks for the Synthesis of Polyenes

The ene-yne metathesis of alkenyl boronates with terminal alkynes is reported. These challenging metatheses were accomplished using a Grubbs catalyst bearing the cyclic alkyl amino carbene (CAAC) ligand, whereas N-heterocyclic carbene (NHC) derived catalysts gave lower yields. Subsequent dienyl isomerization via a cobalt-catalyzed hydrogen atom transfer (HAT) furnished the more substituted dienyl boronate with high EE/EZ ratios. Finally, the resulting dienyl boronate products were successfully used in Suzuki-Miyaura cross-coupling reactions and in a Diels-Alder cycloaddition.

Ene-Yne Metathesis of Allylphosphonates and Allylphosphates: Synthesis of Phosphorus-Containing 1,3-Dienes

A variety of ene-yne cross metathesis reactions were performed using unsaturated phosphonate and phosphate reagents, affording the corresponding phosphorylated 1,3-diene products in good to excellent yields. These difficult ene-yne metatheses employed a Grubbs catalyst bearing a cyclic amino alkyl carbene ligand. A variety of terminal alkynes of varying substitution underwent the reaction, and different phosphorus-containing alkenes were found to give the conjugated diene products in high yields. The resulting dienes were further transformed by Horner-type Wittig reactions and a Diels-Alder cycloaddition.