Lewis bidentate

In summary, the effects of a number of important parameters on the catalysed reaction between 2.4 and 2.5 have been examined, representing the first detailed study of Lewis-acid catalysis of a Diels-Alder reaction in water. Crucial for the success of Lewis-acid catalysis of this reaction is the bidentate character of 2.4. In Chapter 4 attempts to extend the scope of Lewis-acid catalysis of Diels-Alder reactions in water beyond the restriction to bidentate substrates will be presented. 

Careful examination of literature reporting Lewis-acid catalysis of Diels-Alder reactions in combination with kinetic investigations indicate that bidentate (or multidentate) reactants are required in order to ensure efficient catalysis in water. Moreover, studies of a number of model dienophiles revealed that a potentially chelating character is not a guarantee for coordination and subsequent catalysis. Consequently extension of the scope in this direction does not seem feasible. 

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]. 

One of the problems related to the Lewis acid activation of a,/ -unsaturated carbonyl compounds for the reaction with a nitrone is the competitive coordination of the nitrone and the a,/ -unsaturated carbonyl compound to the Lewis acid [30]. Calculations have shown that coordination of the nitrone to the Lewis acid can be more feasible than a monodentate coordination of a carbonyl compound. However, this problem could be circumvented by the application of alkenes which allow a bidentate coordination to the Lewis acid which is favored over the monodentate coordination. 

Allylsilanes or allylstannanes in the presence of a bidentate Lewis acid such as tin(IV) chloride, titanium(IV) chloride, zinc chloride, and magnesium bromide as well as diallylzinc, are promising choices (Table 1). 

Allyltrialkoxy- or -tris(dialkylamino)titanium reagents are not capable of chelation-controlled addition reactions with oxy- or amino-substituted carbonyl compounds due to their low Lewis acidity87. To attain chelation control, the application of allylsilanes (Section 1.3.3.3.5.2.2.) and allylstannanes (Section I.3.3.3.6.I.3.2.) in the presence of bidentate Lewis acids like titanium(IV) chloride, tin(lV) chloride or magnesium bromide are the better options. 

Diiithium 2,2 -methyienebis(4,6-di-fert-butylphenoxide) as a bidentate Lewis acid in organic synthesis [111]... 

Bidentate binding of two Lewis acidic boron centers to one methoxide anion was first reported in 1967 [241]. Further examples did not appear until 1985 [242]. Today, other bis(boronates) like 152 and 154-158 (Fig. 41) are known that can be applied to the selective complexation of amines and diamines [243-247]. 

Keywords Group 13 metals (aluminum, gallium, indium, thalhum), Ambidentate ligands. Phosphorus-nitrogen bidentate ligands, Pyridyl phosphanes, Aminoiminophosphoranes, Lewis acid catalysis... 

Gabbai, F.P., Schier, A., Riede, J. and Tschrnkl, M.T. (1999) Micropore decoration with bidentate lewis adds spontaneous assembly of l,2-Bis(chloromercurio) tetrafluorobenzene. Angewandte Chemie International Edition, 38, 3547. 

The stereochemical outcome of the Mukaiyama reaction can be controlled by the type of Lewis acid used. With bidentate Lewis acids the aldol reaction led to the anti products through a Cram chelate control [366]. Alternatively, the use of a monoden-tate Lewis acid in this reaction led to the syn product through an open Felkin-Anh... 

Magnesium(II) is a milder Lewis acid than traditionally used ones such as boron(III), alumi-num(III), or titanium(IV). A characteristic feature of Mg11 is the presence of coordination sites which are occupied by Lewis bases other than counter anions. By using bidentate Lewis-basic ligands, it is therefore possible to form rigid stereochemical environments. 

Ally 1-tin compounds are employed as more reactive allylating agents. Because of their high reactivity, less active catalysts (TX species having mild Lewis acidity) or less reactive substrates are often required (Scheme 23).88,89 In addition to carbonyl compounds as substrates, allylation reactions of imines have been also reported.90 Also, a binuclear TiIV Lewis acid has been developed (compound (C) in Scheme 23), which shows higher catalytic activity than the mononuclear analogue (D) because of bidentate coordination to the carbonyl moiety of the substrate.91... 

In stereoselective reactions, Zn11 Lewis acids work well to achieve high selectivities (Scheme 54). Chiral complexes of Zn11 with chiral bis(oxazoline) ligands act as effective catalysts in Diels-Alder reactions of reactive dienes with dienophiles having bidentate chelating moieties such as... 

Since the regioselectivity of mercuration reactions is often hard to control, recent efforts have focused on the use of Lewis-basic substituents whose role is to direct the approach of the mercuric ion to a specific position. Reaction of the diazo derivative 74 with Hg(OAc)2 leads to mercuration of the naphthalene moiety in the 8-position rather than mercuration of the electron-rich imidazole carbon atoms. 4 Presumably, the diazo-imidazole bidentate moiety coordinates to the mercuric cation, thereby directing substitution at the naphthalene 8-position (Equation (27)). 

One doubly bidentate Lewis base 2,2 -bipyrimidine is coordinated with one germanium atom, while in the case of the analogous lead compound coordination takes place with two lead atoms. 

Complex with a chloride salt. The organotin compound is a powerful bidentate Lewis acid, binding the chloride ion. 

There have been several efforts geared toward reducing the oligomeric nature of the group 2 metallocene compounds. Reduction of the nuclearity has been accomplished via the use of sterically demanding cyclopentadienides and intermolecular Lewis bases (38-40). Hanusa and Williams have used bidentate amines to coordinatively saturate... 

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