Rhodium-phosphine catalysts achiral

As for other catalysts, chiral Rh(III) porphyrin catalysts 23a were also foiuid to catalyze asymmetric cyclopropanation of alkenes with EDA (84). Although moderate enantiocontrol (<60% ee) was observed, the reaction was cis-selective tic = 1 2.5) with >1800 turnover number. Other newly developed Rh(III) porphyrin catalysts 23b gave slightly improved enantioselectivity (68% ee) with moderate diastereoselectivity (85). Similar to their achiral covmterparts, a perpendicular approach of alkene to rhodium-carbene was proposed as a mechanism. Apart from porphyrins, phosphine catalysts also gave interesting results. Catalyst 24, which contains backbone chirality, catalyzed the cyclopropanation of styrene with EDA to give 91% ee for the cis-isomer and 90 10 of the cis trans ratio (86). 

The related chiral rhodium catalyst 4 has been used to effect kinetic resolution of these substrates.2 In this catalyst the achiral phosphine ligand of 1 is replaced by (R,R)-l,2-bis(o-anisylphenylphosphino)ethane (DIPAMP). Hydrogenation cat-... 

Interestingly, the enantioselectivity for the hydrosilation of acetophenone was found to be independent of the amount of chiral ligand present in the reaction mixture, although the reactions slow down as excess ligand is added. Thus, catalyst solutions prepared by adding from 1-10 equivalents of MDPP per rhodium to the (5-Binap)Rh dimer all gave ca. 55% e.e. of the expected 5-sec alcohol. The addition of excess achiral phosphines such as triphenylphosphine or tricyclohexylphosphine did not effect the optical yield of the alcohol as compared to reactions involving stoichiometric amounts of the monophosphine. 

In the aforementioned work, ligands are attached covalently to a polymer. An interesting non-covalent attachment was reported early on by Wilson and Whitesides [15]. An achiral phosphine ligand linked to a biotin unit (Structure 6) interacted specifically with the protein avidin in aqueous solution to impose stereoselectivity in catalytic hydrogenation. In the rhodium-catalyzed hydrogenation of a-acetamidoacrylic acid in buffered aqueous solution, ee values of up to 40% were observed at TONs of 500 in a 48 h mn. Catalyst recovery was not a motivation for this work, but during workup the catalyst was separated from the low molecular weight products by ultrafiltration. 

Complexes of rhodium compounds with enantiomerically pure phosphines facilitate enantiose-lective hydroboration of prostereogenic olefins with 1,3,2-benzodioxaborole (catecholboranc), an achiral hydroborating agent. The reactions are carried out using a 1 1.1 -1.2 molar ratio of the olefin and catecholboranc in the presence of 1 mol % of the catalyst which is generated in situ111"113. The following catalysts are used ... 

The NP2 unit and the resultant achiral [Rh(NP2)(NBD)] moiety can also be attached easily at a specific site in a protein. The protein structure then provides the chirality required for enantioselective hydrogenation. Thus, hydrogenation of a-acetamidoacrylic acid to A/ -acetylalanine catalyzed by [Rh(NP2)(NBD)] bound to avidin at RT and 1.5 atm of H2 showed —40% S enantiomeric excess. Although these hydrogenation results with avidin are modest, it does demonstrate that asymmetric synthesis is accomplished by the -phosphine rhodium catalyst attached covalently to a protein. 

---