Phosphine Lewis base activity

Shi, M., Chen, L-H., and U, C.-Q. (2005) Chiral phosphine Lewis bases catalyzed asymmetric aza-Baylis-Hillman reaction of N-sulfonated imines with activated olefins. J. Am. Chem. Soc., 127, 3790-3800. 

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,... 

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. 

The origin of the highly enantioselective catalysis by 55 is the simultaneous activation of aldehydes and TMSCN by the Lewis acid (Al) and the Lewis base (the oxygen atom of the phosphine oxide), respectively. The enantioselectivity of the reaction may be explained by the working model depicted in Fig. 1, with the... 

The phosphine-alane is expected to displace PPh at Ni, but no reaction takes place in the absence of substrate due to strong intramolecular P Al interactions. Lewis bases such a triethylamine are able to split the head-to-tail phosphine-alane dimer and consistently, further increase catalytic activity. The precise role of the Lewis acid moiety in the catalytic cycle remains unknown. It is supposed to interact with the methyl group at Ni and to facilitate methyl/silyl exchange. The key active species 36a could not be characterized, but its Lewis base adduct 36b was identified by NMR (Figure 19). 

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... 

The activity in MMA polymerization can be dramatically affected by the apical ligands. Apical aquo or alcohol ligands are labile and rapidly exchange with the polymerization medium. Lewis base ligands (e.g. pyridine, triphenyl phosphine) are comparatively stable. In MMA polymeri/ation, it is found that activity increases with the basicity of the ligand. With alkyl Co " complexes, a different order is found possibly because the type of apical ligand also controls the rate of initial generation of the active Co" complex. 

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... 

Does the presence of excessive steric bulk when a Lewis acid and a Lewis base attempt to form an adduct automatically render these species inert towards each other The unique behavior of sterically frustrated Lewis pairs (FLPs), pioneered by Stephan, is a vigorous research area with applications for small molecule activation and catalysis." The highly Lewis acidic and sterically bulky tris(pentafluorophenyl)borane" plays a role in many FLP reactions. The great promise of FLP chemistry was revealed by reactions between tris(pentafluorophenyl)borane and tertiary and secondary phosphines, where sterics preclude formation of classic adducts. A seminal example of a frustrated Lewis pair is that of the secondary phosphine di(2,4,6-trimethylphenyl)phosphine that is precluded from forming a classic adduct with tris(pentafluorophenyl)borane. The phosphine Lewis pair is frustrated since it cannot interact with boron to form the adduct. 

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