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

Abstract

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 a ,4,6 a -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, Cs 2 CO 3 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.

Adventures and Detours in the Synthesis of Hydropentalenes

“…Such a Rh-catalyzed C=C isomerization is much more likely, according to DFT calculations, and agrees well with experimental findings obtained by the Strand group. 101 More recently, bicyclo[3.3.0]octadienes and the resulting Rh complexes were probed in microemulsions (MEs) as nonconventional solvents. 102 MEs are stable single-phase emulsions of a hydrophobic solvent (oil), a polar solvent (e.g., H 2 O), and a surfactant, such as a sugar amphiphile C 8 G 1 .…”

“…100 Strand was able to obtain diastereomerically pure monomeric Rh diene binaphthylamine complexes by treatment of the rac-Rh diene complex with (R)-binaphthyldiamine. 101 The enantiomerically pure diene was finally isolated after decomplexation with COD.…”

Planar Chiral Rhodium Complexes of 1,4‐Benzoquinones

“…Much less common but no less interesting are the complexes that contain achiral dienes, such as 1,5‐diphenyl‐cyclooctadiene or 5,11‐diphenyl‐dibenzo‐cyclooctatetraene [19] . Coordination of these ligands with rhodium gives racemic complexes, which have to be separated into enantiomers by crystallization of their diastereomeric adducts with the chiral auxiliary, namely R ‐binaphthyl‐diamine [20–23] …”

Synthesis of Rhodium Complexes with Chiral Diene Ligands via Diastereoselective Coordination and Their Application in the Asymmetric Insertion of Diazo Compounds into E−H Bonds

“…Following an interesting alternative approach, racemic diene ligands can be initially coordinated with rhodium and then the resulting complexes can be separated by crystallization of diastereomeric adducts with R ‐binaphthyl‐diamine [13–16] . However, it is difficult to predict if this method can be applied for any particular diene because of the unpredictable nature of the crystallization process itself.…”

“…[10][11][12] Following an interesting alternative approach, racemic diene ligands can be initially coordinated with rhodium and then the resulting complexes can be separated by crystallization of diastereomeric adducts with R-binaphthyl-diamine. [13][14][15][16] However,i ti sd ifficult to predict if this method can be applied for any particular diene because of the unpredictable nature of the crystallization process itself.T his motivated us to propose and develop an ew approach to the chiral diene complexes based on their diastereoselective coordination with the auxiliary salicyloxazoline ligands.T he approach has two important differences from the crystallization method:firstly,only one of two enantiomers coordinates with an auxiliary ligand, and secondly,t he selectivity of such coordination can be efficiently predicted before the synthesis by DFT calculations.…”