Kinetic resolution of racemic propylene

Carbon dioxide is one of the most abundant carbon resources on earth. It reacts with an epoxide to give either a cyclic carbonate or a polycarbonate depending on the substrates and reaction conditions. Kinetic resolution of racemic propylene oxide is reported in the formation of both cyclic carbonate and polycarbonate. The fe ei value defined as ln[l-(conversion)(l+%ee)]/ln[l-(conversion)(l% ee)] reached 6.4 or 5.6 by using a Co(OTs)-salen complex with tetrabutylammonium chloride under neat propylene oxide or using a combination of a Co-salen complex and a chiral DMAP derivative in dichloromethane, respectively. 

FIGURE 4.19. Kinetic resolution of racemic propylene oxide in solvent-free conditions... 

SCHEME 24.7 Attempted kinetic resolution of racemic propylene oxide using 4. 

A so far still unsolved problem is the direct enantioselective epoxidation of simple terminal olefins. For example the epoxidation of propylene that was achieved with a 41% ee almost twenty years ago by Strukul and his coworkers using Pt/diphosphine complexes is still unsurpassed. Unfortunately such low ee s are of no practical interest. The problem was circumvented by Jacobsen using hydrolytic kinetic resolution of racemic epoxides (Equation 26) and is practised on a multi 100 kg scale at Chirex. The strategy used is to stereose-lectively open the oxirane ring of a racemic chiral epoxide leaving the other enantiomer intact. Reactions are carried out to a 50% maximum conversion. The catalyst belongs to the metal-salen class described above and can be recycled. The products are separated by fractional distillation. 

Rhodium and ruthenium complexes of CHIRAPHOS are also useful for the asymmetric hydrogenation of p-keto esters. Dynamic kinetic resolution of racemic 2-acylamino-3-oxobutyrates was performed by hydrogenation using ((5,5)-CHIRAPHOS)RuBr2 (eq 3). The product yields and enantiomeric excesses were dependent upon solvent, ligand, and the ratio of substrate to catalyst. Under optimum conditions a 97 3 mixture of syn and anti p-hydroxy esters was formed, which was converted to o-threonine (85% ee) and D-allothreonine (99% ee) by hydrolysis and reaction with propylene oxide. 

The first example of kinetic resolution catalyzed by an organometallic compound was the partially enantiomer-selective polymerization of racemic propylene oxide induced with a diethylzinc optically active alcohol system (50). 

In the presence of chiral polymerization catalysts, enantiomeric monomers are consumed at different rates (Scheme 75). Enantiomer-selective polymerization of racemic propylene oxide catalyzed by a diethylzinc-(-f)-bomeol system is a classical example of such kinetic resolution H 176). The polymeric product has an [a]D of +7.4°. The mechanism... 

Configurationally chiral, optically active polymers having stereogerric centers in the side chain or main chain can be obtained by enantiomer-selective pol5nnerization (lUPAC nomenclature asymmetric enantiomer-differentiating polymerization). In enantiomer-selective polymerization, one antipode of a racemic chiral monomer is preferentially polymerized to afibrd an optically active polymer. In this process, kinetic resolution of the racemic monomer takes place. The first clear polymerization of this typ>e was reported for propylene oxide. 

Chiral (salen)Co catalysts were found to be efficient catalysts for hydrolytic kinetic resolution of terminal epoxides with water [78]. The solvent-free reaction condition with racemic propylene oxide with 0.55 equiv of water and 0.2 mol%... 

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