engleski

Chiral Discrimination in Rhodium(I) Catalysis by 2, 5-Disubstituted 1, 3a, 4, 6a-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, 3a, 4, 6a-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, Cs2CO3 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.

Addition reaction ; Carbonyl compounds (organic) ; Catalysts ; Group 13 compounds ; Molecular structure ; Reaction mechanism

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