Phosphine catalysts enantioselective compounds

For a leading reference, see Willner, L, Maidan, R. and Shapira, M., Thermal and photochemical regeneration of nicotinamide cofactors and a nicotinamide model compound using a water-soluble rhodium phosphine catalyst, J. Chem. Soc., Perkin Trans. 2, 1990, 559. (b) For a recent reference, see Hollmann, F., Kleeb, A., Otto, K. and Schmid, A., Coupled chemoenzymatic transfer hydrogenation catalysis for enantioselective reduction and oxidation reactions. Tetrahedron Asymmetry, 2005, 16, 3512. 

The introduction of the activated allylic bromides and Morita-Baylis-HiUman acetates and carbonates pioneered the development of a number of phosphine-catalyzed reactions in subsequent years [45]. Interestingly, the asymmetric variant of this type of transformation only appeared in the literature seven years later. In 2010, Tang, Zhou, and coworkers disclosed a highly enantioselective intramolecular ylide [3-1-2] annulation using spirobiindane-based phosphine catalyst 31 (Scheme 20.27). BINAP was found inactive in this reaction even at an elevated temperature (70°C). Notably, both optically active benzobicyclo[4.3.0] compounds 32 and 32 with three continuous stereogenic centers could be obtained as major products in high yields and stereoselectivities just by a choice of an additive [Ti(OPr )4], which can block the isomerization of the double bond [46]. 

The enantioselective 1,4-addition addition of organometaUic reagents to a,p-unsaturated carbonyl compounds, the so-called Michael reaction, provides a powerful method for the synthesis of optically active compounds by carbon-carbon bond formation [129]. Therefore, symmetrical and unsymmetrical MiniPHOS phosphines were used for in situ preparation of copper-catalysts, and employed in an optimization study on Cu(I)-catalyzed Michael reactions of di-ethylzinc to a, -unsaturated ketones (Scheme 31) [29,30]. In most cases, complete conversion and good enantioselectivity were obtained and no 1,2-addition product was detected, showing complete regioselectivity. Of interest, the enantioselectivity observed using Cu(I) directly in place of Cu(II) allowed enhanced enantioselectivity, implying that the chiral environment of the Cu(I) complex produced by in situ reduction of Cu(II) may be less selective than the one with preformed Cu(I). 

The catalyst efficiency of these hydroalumination varies from a turnover number (TON) of 20-91. It is possible that the catalyst is deactivated by the presence of oxygen and water. Examination of the 31P NMR spectrum of the catalyst indicates that the phosphine monoxide and dioxide are formed in the presence of nickel prior to the addition of the substrate. Rigorous exclusion of oxygen and water is necessary in all these reactions. The enantioselective nickel-catalyzed hydroalumination route to dihydronaphthalenols may prove to be particularly important. Only one other method has been reported for the enantioselective syntheses of these compounds microbial oxidation of dihydronaphthalene by Pseudomonas putida UV4 generates the dihydronaphthalenol in 60% yield and >95% ee.1... 

Since the discovery and development of highly efficient Rh catalysts with chiral diphosphites and phosphine-phosphites in the 1990s, the enantioselectivity of asymmetric hydroformylation has reached the equivalent level to that of asymmetric hydrogenation for several substrates. Nevertheless, there still exist substrates that require even further development of more efficient chiral ligands, catalyst systems, and reaction conditions. Diastereoselective hydroformylation is expected to find many applications in the total synthesis of complex natural products as well as the syntheses of biologically active compounds of medicinal and agrochemical interests in the near future. Advances in asymmetric hydrocarboxylation has been much slower than that of asymmetric hydroformylation in spite of its high potential in the syntheses of fine chemicals. 

The high level of stereocontrol in the formation of complexes 25 and 27 suggests that compounds of this type may be useful as chiral catalysts. Indeed, several examples of enantioselective catalytic reactions carried out with half-sandwich complexes have been published recently [23, 25]. However, it seemed desirable to have access to complexes of the [21 Ru(solv)2] type, which have two easily removable solvent molecules coordinated to the central metal, in order to provide coordination sites for a substrate to be transformed. Although the chloride ligand could be easily removed from 23 and 25 all attempts to strip off the PPhs were unsuccessful. Therefore a new reaction scheme was developed which precluded the use of phosphine ligands, and the bis (acetonitrile) complex 28 could be obtained in a multi-step protocol via the T1 salt T1 21 (Scheme 1.5.12) [26]. 

High enantiomeric excess in organocatalytic desymmetrization of meso-diols using chiral phosphines as nucleophilic catalysts was achieved for the first time by Vedejs et al. (Scheme 13.21) [36a], In this approach selectivity factors up to 5.5 were achieved when the C2-symmetric phospholane 42a was employed (application of chiral phosphines in the kinetic resolution of racemic secondary alcohols is discussed in Section 12.1). A later study compared the performance of the phos-pholanes 42b-d with that of the phosphabicyclooctanes 43a-c in the desymmetrization of meso-hydrobenzoin (Scheme 13.21) [36b], Improved enantioselectivity was observed for phospholanes 42b-d (86% for 42c) but reactions were usually slow. Currently the bicyclic compound 43a seems to be the best compromise between catalyst accessibility, reactivity, and enantioselectivity - the monobenzoate of hydrobenzoin has been obtained with a yield of 97% and up to 94% ee [36b]. 

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