Chiral Discrimination in Rhodium(I) Catalysis by 2,5-Disubstituted 1,3,4,6-Tetrahydropenatalene Ligands-More Than Just a Twist of the Olefins?

Chiral dienes are useful ligands in a number of asymmetric transition-metal-catalyzed reactions. Here, we evaluate the efficiency of 2,5-disubstituted 1,3,4,6-tetrahydropentalenes as ligands to rhodium(I). 2,5-Dibenzyl and diphenyl tetrahydropentalenes were synthesized in two steps and resolved, either chromatographically, or through fractional crystallization of diastereomeric rhodium(I) salts. When evaluated in a 1,4-arylation reaction, the 2,5-dibenzyl ligand gave up to 99% ee. The use of a well-defined rhodium complex as catalyst, CsCO as the base, and toluene/water as solvent was found to have a pronounced beneficial effect on the selectivity of the reaction. The homologous 2,5-diphenyl ligand on the other hand proved to be highly prone to racemization/loss of chirality during catalysis. Control experiments reveal that this rearrangement proceeds via a rhodium-mediated 1,3-hydride shift. Implications for ligand design and catalysis are discussed.