Phosphine catalysts Lewis bases

These homogeneous catalysts can in addition contain a Lewis base, such as a phosphine or phosphite, and can accordingly be divided into... 

This preparation is carried out in an aprotic solvent (e.g. benzene, chloroform) with no special provision other than working in a well-ventilated fume hood to avoid ill-smelling sulfur compounds. Various ligands have proved successful phosphines, pyridines, imidazoles, tetra-m ethyl thiourea, etc. When the same reaction is carried out in the absence of the Lewis base L, a dimer 6 is obtained, which is a useful catalyst in its own right and sometimes a much more active one see Section VILA. The chemical equation for that reaction is,... 

The rate of polymerization can be controlled by a gel modification additive. A gel modification additive is a substance that cooperates with the catalyst to change the rate of the catalyzed reaction. Most generally, a gel modification additive may be any electron donor or Lewis base. Particularity suitable compounds acting in this way are tricyclohexylphosphine, tricyclopentylphosphine, triisopropyl-phosphine, triphenyl phosphine, and pyridine. 

Our proposed transition state model for this catalytic enantioselective cyanosilylation of ketone is shown as 35.30a The titanium acts as a Lewis acid to activate the substrate ketone, while the phosphine oxide acts as a Lewis base to activate TMSCN. The intramolecular transfer of the activated cyanide to the activated ketone should give the ( )-cyanohydrin in high selectivity. The successful results described above clearly demonstrate the practicality of our asymmetric catalyst for cyanosilylation of ketones. 

Asymmetric organocatalytic Morita-Baylis-Hillman reactions offer synthetically viable alternatives to metal-complex-mediated reactions. The reaction is best mediated with a combination of nucleophilic tertiary amine/phosphine catalysts, and mild Bronsted acid co-catalysts usually, bifunctional chiral catalysts having both nucleophilic Lewis base and Bronsted acid site were seen to be the most efficient. Although many important factors governing the reactions were identified, our present understanding of the basic factors, and the control of reactivity and selectivity remains incomplete. Whilst substrate dependency is still considered to be an important issue, an increasing number of transformations are reaching the standards of current asymmetric reactions. 

Both and H NMR spectroscopic studies indicated the bifunctional role of the catalyst. The phosphine acts as a Lewis base to initiate the reaction sequence, whereas... 

The catalyst must act as a Lewis base. The structural features of Kocovsky s catalysts (e.g., 35) are detailed right below Table 4.1 (a) Valine i Pr (optimized) (b) N Me (c) N CH—O (formyl) and (d) another anilide group in the molecule. Other catalysts share similar features, in some cases the formamide group can be replaced by a picolinyl (41 43) of sulfinamide group (46). Other functional groups that can be considered are, for example, imidazolyl (high affinity to Si), oxazolidinyl, N oxide, phosphine oxides, and so on. 

Catalysts (25) are the Lewis acid-Lewis base bifunctional catalysts in which Lewis acid-Al(III) moiety activates acyl iminium ion and the Lewis base (oxygen of phosphine oxide) does TMSCN, simultaneously (Scheme 5.7). Halogen atoms at the 6-position enhanced both yields and enantioselectivity in Reissert-type cyanation of the imino part of 26. However, the order for the activation is not parallel to the electronegativity of the halogen atoms and, moreover, the strong electron-withdrawing trifluoromethyl group provided unexpectedly the worst result for the activation [13]. It is not simple to explain this phenomenon only in terms of the increased Lewis acidity of the metal center. Trifluoromethylated BINOL-zirconium catalysts (28) for asymmetric hetero Diels-Alder reaction (Scheme 5.8) [14], trifluoromethylated arylphosphine-palladium catalyst (32) for asymmetric hydrosilylation (Scheme 5.9) [15], and fluorinated BINOL-zinc catalyst (35) for asymmetric phenylation (Scheme 5.10) [16] are known. 

Nucleophilic catalytic reactions are usually addition and substitution reactions. A diverse array of Lewis bases (e.g., tertiary phosphines, tertiary amines, pyridines, and imidazoles) have been shown to serve as nucleophilic catalysts. Nucleophilic reactions typically occur at C=X and activated C=C multiple bonds. In a general form for a reaction... 

In the preceding examples, the asymmetric catalyst is a Lewis acid and hence the catalytic processes reported so far involve electrophilic activation by a metal-centred chiral Lewis acid. There is another strategy, although less explored, which consists of designing chiral Lewis bases for nucleophilic catalysis. It is well known that Lewis bases such as nitrogen heterocycles and tertiary phosphines and amines catalyse a variety of important chemical processes. For instance 4-(dimethylamino)pyridine (DMAP) catalyses the acylation of alcohols by anhydrides the mechanism by which DMAP accelerates this process provides an instmctive illustration of how nucleophiles can... 

Ishihara developed a highly efficient Mukaiyama aldol reaction between ketones and trimethylsilyl enolates catalysed by sodium phenoxide-phosphine oxides (46) as simple homogeneous Lewis-base catalysts (0.5-10 mol%) (Scheme 2.29). For a variety of aromatic ketones and aldimines, aldol and Mannich-type products with an a-quaternary carbon centre were obtained in good to excellent yields. Remarkably, a retro-aldol reaction was not observed. On a scale of up to 100 mmol, benzophenone and trimethylsilyl enolate gave the product in 97% yield (34.8 g) using 0.5 mol% of catalyst. 

In 2003, we first demonstrated that l,l -bi-2,2 -naphthol (BINOL)-derived chiral LBBA (Lewis base and Bronsted add) bifunctional phosphine CP17 (LB = PPhs, BA = Ph-OH) could be used as an effective catalyst in asymmetric aza-MBH reaction of A-tosylimines with MVK and phenyl acrylate, affording the corresponding adducts in good yields with high ees (Scheme 2.119). The addition of molecular sieves increased chemical yields because they removed the ambient moisture that caused the decomposition of A-sulfonated imines. The asymmetric induction of this catalyst is comparable to that of the quinidine... 

Figure 2.9 Bifunctional chiral phosphine Lewis bases for use as catalysts.

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