Phosphate-pyridine ligands

The molybdenum and tungsten complexes catalyze reactions of soft nucleophiles, such as malonates, related 1,3-dicarbonyl compoimds, and nitroalkanes. Azlactones are also soft carbanions, and Trost has shown that complexes formed from molybdenum and the bis(pyridine) ligands catalyze enantioselective and diastereoselective allylation of azlactones with allylic phosphates to form quaternary amino acids (Equation 20.40). In these reactions, the nucleophile adds to the more substituted position of the allylic electrophile, and a stereocenter is formed at both the allyl carbon and the azlactone carbon. One route to the protease inhibitor tipranavir by the molybdenum-catalyzed allylation with 1,3-dicarbonyl compounds was demonstrated by Trost (Equation 20.41), and the Merck process group used related allylation chemistry with Trost s bis(pyridine) ligand to prepare the cyclopentanone precursor to various analogs of tipranavir (Equation 20.42). 

The solubility of NdX3 catalysts is improved by the addition of electron donors (D). Catalyst activity is remarkably increased without substantial deterioration of the cis- 1,4-content. Typical donor ligands applied in NdX3 Dn/AlR3 type systems are alcohols such as EtOH [92,112,113], 2-ethylhexanol [114] or various pentanol isomers [115]. Furthermore, tetrahydrofuran (THF) [35], tributyl phosphate (TBP) [116-119], alkyl sulfoxides [116,117,120,121], propion amide [122,123], B(0-CH2-CH2-0-CH2-CH2-0H)3/B(0-CH2-CH2-0-C2H5)3 [124,125], pyridine [126] and dioxides [127,128] are applied as donors. The increase in catalyst activity by donor ligands is attributed to the improved solubility of the active species in hydrocarbon solvents [129,130]. 

The impact of electron donor ligands in NdX3-systems has already been discussed in Sect. 2.1.1.1. ligands such as alcohols, trialkyl phosphates, alkyl sulfoxides, alkyl amides, THF, N-oxides, pyridine etc. are added in order to facilitate water removal from NdCl3 6H20 by azeotropic distillation and in order to increase solubility and activity of NdCl3-based catalyst systems in organic solvents. 

Ligands having oxygen as donor atom are also able to form many complexes with Zn and Cd. These include phosphine oxide complexes such as tetrahedral [Zn(0PPh3)4](C104)2,67 pyridine oxide complexes, the simplest of which is [Cd(pyO)6][CdCl4],68 alkoxides, M(OR)2, carbamates,69 and phosphates.70... 

Inanaga and co-workers developed another type of lanthanide catalyst for asymmetric hetero Diels-Alder reaction (Sch. 3) [36]. Benzaldehyde reacted with 1-meth-oxy-3-(trimethylsiloxy)-l,3-butadiene in the presence of chiral Yb(III) phosphate to afford the corresponding adduct in 77% yield and in 70% ee. Because the reaction mixture was heterogeneous, they tried to make a clear solution by addition of ligands and examined their effects on the reactions. Pyridine and pyridine derivatives dissolved the catalyst and chemical yields and ee were usually improved. The best result (93% ee) was obtained in the reaction of p-anisaldehyde with 2,6-lutidine as additive. 

Proton NMR spectroscopy was employed to follow complex formation on titration of V,7V-bis[ 6-(hydroxymethyl)-pyridine-2-yl methyl]-p-tosylamide (= L) with Cu(I) and Zn(II) salts (giving (L)Cu1, (L)Zn11).1163 31P NMR spectra were used to follow oxidative-addition reactions of (L)AuX + X2 (X = Cl, Br L = phosphines, phosphates), showing a reluctance by very bulky L ligands to undergo such reactions.1164 Similar data were obtained for redox reactions of (R3P)2AuBr with diselenides (R = Me, Et).1165... 

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