Lewis adds complexation with

FIGURE 4. The crystal structure of the Lewis add complex of a chiral dienophile with titanium... 

Hoshino et al. [33] reported the first example of an enantioselective radical reaction employing a chiral Lewis add complex. The enantioselective reduction of a-methoxy-methyl-a-iodolactone 41 with tributyltin hydride (BusSnH) in the presence of stoichiometric amounts of the chiral complex of a chiral diamine 42 and Mgl2, gave the reduced product 43 in 88 % yield with 62 % ee (Sch. 17). Reaction using Mg(C104)2, TiCU, Znl2,... 

Chiral Lewis acids combined with less reactive organometallics like organotins (organostannanes) can add to carbonyls to give one enantiomer selectively. The chiral Lewis acid complex with the carbonyl makes one face of the carbonyl more accessible to the organometallie nucleophile. The product chirality is set by the chiral catalyst. 

Lewis add complexes formed by the reactions of various aminoalcohols with Et2AlG [778, 824] or by the reaction of Et2Zn with a chiral sulfamide [806] have displayed a low efficiency in the asymmetric condensations of ketene and thioketene silyiacetals derived from acetic acid with aldehydes. Disappointing se-lectivities have also been observed with some binaphtol-titanium complexes [778]. However, Mikami and Matsukawa [1296] recently performed the enantioselective condensation of various aldehydes with acetic acid derivatives in the presence of a chiral binaphtol-titanium complex. Good selectivities were observed when the reaction was performed at 0°C in toluene (Figure 6.95). Quaternary ammonium fluorides derived from cinchona alkaloids have been proposed as catalysts to perform additions of enoxysilanes derived from ketones to PhCHO, but the observed selectivities are modest [1303],... 

Whatever the cause for deviation of sequence distribution in SAN, there has been considerable interest in the development of polymerization methods to minimize the AN sequence length [6,7,58,68-75]. Many researchers [6,7,71-73,75,76] have shown that the addition erf Lewis acids to SAN copolymerization has a strong influence upon sequence distribution. Electron poor Lewis adds ormplex with the electron rich nitrile group of acrylonitrile. Complexation of the nitrile group increases its inductive effect resulting in a decrease of electron density of the acrylonitrile double bond (its Alfrey-Price e-value becomes more positive). Thus oopolymerization of the acrylonitrile ... 

Several alternative attempts have been made to quantify Lewis-acid Lewis-base interaction. In view of the HSAB theory, the applicability of a scale which describes Lewis acidity with only one parameter will be unavoidably restricted to a narrow range of struchirally related Lewis bases. The use of more than one parameter results in relationships with a more general validity ". However, a quantitative prediction of the gas-phase stabilities of Lewis-acid Lewis-base complexes is still difficult. Hence the interpretation, not to mention the prediction, of solvent effects on Lewis-add Lewis-base interactions remains largely speculative. 

Fortunately, azachalcone derivatives (2.4a-g, Scheme 2.4) turned out to be extremely suitable dienophiles for Lewis-add catalysed Diels-Alder reactions with cyclopentadiene (2.5). This reaction is outlined in Scheme 2.4 and a large part of this thesis will be devoted to the mechanistic details of this process. The presence of a chromophore in 2.4 allows kinetic studies as well as complexation studies by means of UV-vis spectroscopy. Furthermore, the reactivity of 2.4 is such that also the... 

Table 2.7. Hammett p-values for complexation of 2.4a-e to different Lewis-adds and for rate constants (kcat) of the Diels-Alder reaction of 2.4a-e with 2.5 catalysed by different Lewis-acids in water at 2.00 M ionic strength at 25°C.

Physical Properties. Both (1) and (2) are weak bases, showing 4.94 and 5.40, respectively. Their facile formation of crystalline salts with either inorganic or organic acids and complexes with Lewis acids is in each case of considerable interest. Selected physical data for quinoline and isoquinoline are given in Table 1. Reference 4 greatly expands the range of data treated and adds to them substantially. 

As mentioned above, the ability of TiCl4 (OR) to function as a Lewis add decreases as n increases. Few adduct complexes of Ti(OR)4 are known, although Ti2(OEt)8 en, Ti2(OPri)8-en and Ti(OPri)4-en have been reported,14 but these complexes almost completely dissodate in solution. However, Ti(OPh)4 readily forms 1 1 adducts with PhOH, NH3, NH2Me, NHMe2, NMe3, py, dioxane, PhNH2, Me2CO- These complexes have been the subject of a review.62 The difference in the behaviour of the Ti(OPh)4 to the Ti(OR)4 (R = Me, Et) towards Lewis bases is undoubtedly related to the monomeric, and thus the coordinatively unsaturated, nature of the phenoxides in solution. 

The chemical reactions of M(BH )4 complexes have received very little attention. Both compounds behave as Lewis adds in reactions with tetraalkylammonium and lithium tetrahydroborates, yielding [(C8Hi7)3NPr][M(BH4)5], [NBu4][M(BH4)5] and LiM(BH4)5.620 Zr(BH4)4 reacts with LiAlH4 in ether to give Zr(AlH4)4, an unstable white solid, which decomposes within several hours at room temperature to a pyrophoric black solid.617... 

Metalloenzymes contain a bound metal ion as part of their structure. This ion can either partidpate directly in the catalysis, or stabilize the active conformation of the enzyme. In Lewis acid catalysis (typically with zinc, vanadium, and magnesium), the M"+ ion is used instead of H+. Many oxidoreductases use metal centers such as V, Mo, Co, and Fe in much the same way as homogeneous catalysis uses ligand-metal complexes. Figure 5.7 shows a simplified mechanism for the halide oxidation readion catalyzed by vanadium chloroperoxidase. The vanadium atom ads as a Lewis add, activating the bound peroxide [30]. 

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