Phosphine-dihydrooxazole

The hydrogenation of alkenes without privileged functional groups has not been investigated systematically, probably because much more effort is required to achieve good enantioselectivity. Successful examples are Structures 37-39. Of special interest are the Ir phosphine dihydrooxazole (P OXAZ) catalysts [27], even though their functional group tolerance is relatively low. 

Systems modified by hybrid P-N ligands (68-75) were also found to be active for the copolymerization of styrene. In contrast to the N-N hgand [8, 9], their productivity is increased by increasing the pressure of carbon monoxide. In the case of the phosphine-dihydrooxazole ligands (68-72) the geometry of the ligand is very important for the steric control. The presence of only one substituent on position 4 of the dihydrooxazole ring (e.g., 69) is essential to achieve isotactic copolymerization [98]. Chirality associated with the presence of two different substituents in that position (Scheme 8.14, 72) is not sufficient to cause efficient i-enantio-face discrimination [99]. On the basis of these results, the model for styrene coordination (76) reported in Scheme 8.15 was assumed. 

Furthermore, the phosphine-dihydrooxazole hgands show an unusual behavior with respect to ethene and styrene. The productivity of those systems is larger for styrene than for ethene under equal reactions conditions nevertheless, in the terpolymerization experiments ethene, and not styrene, is prevailingly inserted. Considering that ethene was inserted more rapidly than styrene into model acetyl complexes [103], the poisoning" effect of ethene can be explained by assuming that ethene is coordinated more easily, without rapid olefin dissociation, and that rate-determining carbon monoxide insertion into the two different alkyl intermediates occurs. 

The diphosphine (R,S)-BPPFA [(R,pS)-9] reacts analogously with acetic anhydride to give the corresponding acetate which can be derivatized. Replacement of the acetate by hydroxide leads to a useful ligand BPPFOH 1534, which has been used for the rhodium-catalyzed enantioselective reduction of a-oxo acids to a-hydroxy acids (Section D.2.3.1.). Recently, the chemistry of gold(I) complexes of such chiral phosphines has been developed they catalyze aldol-type cycloadditions of isocyanides to carbonyl compounds to give chiral dihydrooxazoles. which can be hydrolyzed to synthetically important chiral amino alcohols and amino acids 30,39,40. 

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