Catalysts bidentate

Binuclear Al111 complexes for MPV reduction have been developed. In the presence of 5mol.% of the bidentate catalyst (Scheme 69, compound (R)), the reduction proceeds smoothly at room... 

Glaxo employed a bidentate catalyst, pyridinium dichloromethylphosphonate, in... 

Rhodium-catalyzed nucleophilic ring opening of 1,4-epoxy-dihydronaphthalenes to form amino-alcohols was reported by Tautens <2002JOC8043>. An enantioselective rhodium-catalyzed version of this approach was recorded <2003JA14884> in which 268 was converted to an amino-alcohol 275, as illustrated in Equation (162). Biaryl and binaphthyl monodentate and bidentate catalysts have also been shown by Pregosin to provide similar results <2004OM2295>. 

Scheme 8.12. (a) An example of a bidentate catalyst for copper salts used in asymmetric aziridination reactions, (b) An asymmetric aziridination reaction [58]. 

Energetics and Dynamics of a Hydrogen-Bonded Bidentate Catalyst... 

The early phosphoramides 21.9 and 21.10 (Figure 21.1), developed by Denmark, exhibited modest enantioselectivity in the allylation reaction but played an important role in the mechanistic elucidation and development of the second generation of catalysts. Thus, kinetic measurements and the observation of a nonlinear relationship between the enantiopurity of 21.9/21.10 and the product 21.7 indicated that two molecules of the catalyst are coordinated to the silicon centre.However, when the concentration of the catalyst is low, a second mechanism may compete, namely that with only one molecule of the catalyst coordinated, which apparently attenuates the enantioselectivity. As a logical progression from these mechanistic observations, bidentate catalysts, such as 21.12 and 21.13, were designed. The latter exhibited higher enantios-electivities and also was more reactive, so that the loading could be reduced from 10-20 to 5 Excellent diastereoselectivity was... 

According to the generally accepted mechanism of enantioselective hydrogenation by Rh complexes, co-ordination of the substrate to the solvated Rh complex to form the bidentate catalyst-substrate complex, outlined in Fig. 2.5, was invoked. 

The red tetrathiomolybdate ion appears to be a principal participant in the biological Cu—Mo antagonism and is reactive toward other transition-metal ions to produce a wide variety of heteronuclear transition-metal sulfide complexes and clusters (13,14). For example, tetrathiomolybdate serves as a bidentate ligand for Co, forming Co(MoSTetrathiomolybdates and their mixed metal complexes are of interest as catalyst precursors for the hydrotreating of petroleum (qv) (15) and the hydroHquefaction of coal (see Coal conversion processes) (16). The intermediate forms MoOS Mo02S 2> MoO S have also been prepared (17). 

The chain-growth catalyst is prepared by dissolving two moles of nickel chloride per mole of bidentate ligand (BDL) (diphenylphosphinobenzoic acid in 1,4-butanediol). The mixture is pressurized with ethylene to 8.8 MPa (87 atm) at 40°C. Boron hydride, probably in the form of sodium borohydride, is added at a molar ratio of two borohydrides per one atom of nickel. The nickel concentration is 0.001—0.005%. The 1,4-butanediol is used to solvent-extract the nickel catalyst after the reaction. 

Reaction of the cyclopropyl-substituted pivalate (25) with dimethyl benzylidenema-lonate in the presence of a palladium catalyst gave a mixture of alkylidenecyclo-propane (26) and vinylcyclopropane (27). The ratio of these two adducts is found to be quite sensitive to the choice of ligand and solvent. While triisopropyl phosphite favors the formation of the methylenecyclopropane (26), this selectivity is completely reversed with the use of the bidentate phosphite ligand dptp (12). Interestingly there was no evidence for any products that would have derived from the ring opening of the cyclopropyl-TMM intermediate (Scheme 2.8) [18]. 

The major developments of catalytic enantioselective cycloaddition reactions of carbonyl compounds with conjugated dienes have been presented. A variety of chiral catalysts is available for the different types of carbonyl compound. For unactivated aldehydes chiral catalysts such as BINOL-aluminum(III), BINOL-tita-nium(IV), acyloxylborane(III), and tridentate Schiff base chromium(III) complexes can catalyze highly diastereo- and enantioselective cycloaddition reactions. The mechanism of these reactions can be a stepwise pathway via a Mukaiyama aldol intermediate or a concerted mechanism. For a-dicarbonyl compounds, which can coordinate to the chiral catalyst in a bidentate fashion, the chiral BOX-copper(II)... 

In most TiCl2-TADDOLate-catalyzed Diels-Alder and 1,3-dipolar cycloaddition reactions oxazolidinone derivatives are applied as auxiliaries for the alkenoyl moiety in order to obtain the favorable bidentate coordination of the substrate to the catalyst... 

The enantioselective inverse electron-demand 1,3-dipolar cycloaddition reactions of nitrones with alkenes described so far were catalyzed by metal complexes that favor a monodentate coordination of the nitrone, such as boron and aluminum complexes. However, the glyoxylate-derived nitrone 36 favors a bidentate coordination to the catalyst. This nitrone is a very interesting substrate, since the products that are obtained from the reaction with alkenes are masked a-amino acids. One of the characteristics of nitrones such as 36, having an ester moiety in the a position, is the swift E/Z equilibrium at room temperature (Scheme 6.28). In the crystalline form nitrone 36 exists as the pure Z isomer, however, in solution nitrone 36 have been shown to exists as a mixture of the E and Z isomers. This equilibrium could however be shifted to the Z isomer in the presence of a Lewis acid [74]. 

The exo selecdvity of the TiCL-TADDOLate-catalyzed 1,3-thpolar cycloadthdon is improved by the tise of sticcinimide instead of oxazoiidinone as attxiliary for the a,fi-imsatitrated carbonyl moiety (Eq. 8.55). A strong bidentate coo rdinadon of the alkenyl moiety to the metal catalyst is impcrtant in these re... 

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