Lewis acid behaviour

In addition, a Lewis acid behaviour was proposed in the cyclopropyl carbonyl rearrangement catalyzed by [pmim][Br] as depicted in Scheme 78 [200]. The products were obtained in good yields up to 95% when stirred at rt in the ionic liquid. By the application of sonication, the reaction time was decreased to 0.75 h. 

Some articles review the coordination chemistry of /3-diketones1529,1530 and, specifically, isomerism1531 and Lewis acid behaviour of the complexes.1532 Amongst the carbon-bonded /3-diketone complexes, those of nickel(II) have not been reported.1533... 

The vacant orbital is able to accept a lone pair of electrons from a Lewis base to give a neutral species or can combine with a nucleophile to form a negatively charged tetrahedral anion. Thcj reducing agent boran e-dimethyl sulfide is an example of the Lewis acid behaviour while the borohy-j dride anion would be the result of the imaginary reaction of borane with a nucleophile hydride. The vacant orbital makes borane a target for nucleophiles. 

The complex compounds formed under non-aqueous conditions are largely those derived from Lewis acid behaviour of VFS and the oxo halides. Thus addition of KF to VF5 in ligand HF gives KVF6 which is hydrolyzed... 

These insights into the nature of the chemical bonding also help us understand patterns of framework substitution. In particular, strongly ionic metals readily replace aluminium in phosphates, but less readily in silicates. Once substitution has occurred, QM-based methods can be applied to understand the Lewis acid behaviour of such dopants as they coordinate to adsorbed molecules. The activation of hydrogen peroxide by tetrahedral Ti is of particular importance for selective oxidation and is discussed further in Chapters 7 and 9. 

Aluminium.—A. review on the organometallic chemistry of aluminium contains some mechanistic work, particularly n.m.r. studies of exchange reactions. Kinetic studies of the exchange between bridging and terminal sites for alkyl aluminium dimers indicate that, for the methyl compound, reaction proceeds by a dissociative mechanism but that for meta- and para-to y alkyls the mechanism involves only partial dissociation of the brid. For the addition of (Bu 2AlH)3 to oct-4-yne, the kinetic order of i for the aluminium compound indicates that the monomer, Bu AlH, is the active species. Further studies on the Lewis acid behaviour of a range of species, including aluminium compounds, have been published. ... 

Thus, in these formally 14-electron system the Ln atom is coordinatively unsaturated causing a pronounced Lewis acidic behaviour. The electron deficiency of the Ln atom causes the formation of an "a-agostic" Ln...CH interaction. Due to these interaction the systems can be considered as effectively 16-electron systems. The remaining electron deficiency of the Ln atom in these compounds is decreased by an additional intramolecular y-agostic Ln...HC interaction between the Ln atom and the methyl groups of the carbyl ligands [141]. 

On the basis of the studies described in the preceding chapters, we anticipated that chelation is a requirement for efficient Lewis-acid catalysis. This notion was confirmed by an investigation of the coordination behaviour of dienophiles 4.11 and 4.12 (Scheme 4.4). In contrast to 4.10, these compounds failed to reveal a significant shift in the UV absorption band maxima in the presence of concentrations up to one molar of copper(ir)nitrate in water. Also the rate of the reaction of these dienophiles with cyclopentadiene was not significantly increased upon addition of copper(II)nitrate or y tterbium(III)triflate. 

A different type of behaviour is observed with the chloro Lewis acid BCI3. With this reagent halogen exchange occurs to produce the acyclic cation [N(SC1)2] , as the [BCU] salt, rather than NSCf. " Thiazyl trichloride NSCI3 is predicted to be unstable with respect to NSCl -1-Cl2. ... 

Lewis acid (fluoride-ion acceptor) behaviour is exemplified by reactions with NOF and MF to give [N0]+[C1F2] and M+[C1F2] respectively (M = alkali metal or NH4). Lewis base (fluoride ion donor) activity includes reactions with BF3 and ASF5 ... 

Group 4 metal complexes of the dianion [ BuNP( -N Bu)2PN Bu] polymerize ethylene in the presence of a co-catalyst, but they are readily deactivated [10,14]. This behaviour is attributed to coordination of the lone-pair electrons on the phosphorus(III) centers to Lewis acid sites, which initiates ring opening of the ligand [15]. 

For the studied catechol methylation reaction the catalyst structure and surface properties can explain the catalytic behaviour As mentioned above, the reaction at 260-350°C has to be performed over the acid catalysts. Porchet et al. [2] have shown, by FTIR experiments, the strong adsorption of catechol on Lewis acid/basic sites of the Y-AI2O3 surface. These sites control the reaction mechanism. 

The extent of the ionization produced by a Lewis acid is dependent on the nature of the more inert solvent component as well as on the Lewis acid. A trityl bromide-stannic bromide complex of one to one stoichiometry exists in the form of orange-red crystals, obviously ionic. But as is. always the case with crystalline substances, lattice energy is a very important factor in determining the stability and no quantitative predictions can be made about the behaviour of the same substance in solution. Thus the trityl bromide-stannic bromide system dilute in benzene solution seems to consist largely of free trityl bromide, free stannic bromide, and only a small amount of ion pairs.187 There is not even any very considerable fraction of covalent tfityl bromide-stannic bromide complex in solution. The extent of ion pair and ion formation roughly parallels the dielectric constant of the solvents used (Table V). The more polar solvent either provides a... 

The experimental evidence shows that the carbon of terminal carbonyl groups is positively polarized (or polarizable) and, contrary to the behaviour of free carbon monoxide, is easily attacked by strong nucleophiles (OH-, OR-) a behaviour which is general in the chemistry of metal carbonyls. Moreover, the negative polarisation (or polarizability) of oxygen atoms of carbonyl groups, particularly bridging carbonyls, is illustrated by the facile formation of adducts with Lewis acids as shown inEq. (16) 7. ... 

These results led to a separation of the observed Diels-Alder reactivities into three categories (a) increase of the rate constants on increasing the Lewis acid character of the solvent as quantified by the AN parameter this behaviour reflects the interactions between the LUMO of the solvent and the HOMO of the reactants and is similar to Lewis acid catalysis (vide supra) (b) reaction retardation by electron donation, as quantified by the D-ji parameter the HOMOsoivent-LUMOreactant interactions are held responsible for this effect, representing an anti-Lewis acid interaction which increases the HOMO-LUMO gap and hence hampers the reaction (c) the Diels-Alder reactions show very small solvent effects and are relatively insensitive to specific reactant-solvent interactions, and... 

The most general conclusion is that in almost all of the Lewis acids tested the normal Evans jyn-aldol 101 is formed in only trace amount, if at all. The unique behaviour of the different Lewis acids may be summarised as follows TiCl4 shows syn selectivity regardless of stoichiometry, Et2AlCl is anti selective regardless of stoichiometry and SnCl4 shows syn or anti, depending on whether an excess of aldehyde (entry 5) or Lewis acid (entry 7) is used. 

By 1989 Mukaiyama had already explored the behaviour of phosphonium salts as Lewis acid catalysts. It was possible to show that the aldol-type reaction of aldehydes or acetals with several nucleophiles and the Michael reaction of a,j3-unsatu-rated ketones or acetals with silyl nucleophiles gave the products in good yields with a phosphonium salt catalyst [116]. In addition, the same group applied bisphosphonium salts as shown in Scheme 45 in the synthesis of ]3-aminoesters [117]. High yields up to 98% were obtained in the reaction of A-benzylideneaniline and the ketene silyl acetal of methyl isobutyrate. Various analogues of the reaction parteers gave similar results. The bisphosphonium salt was found to be superior to Lewis acids like TiCl and SnCl, which are deactivated by the resulting amines. 

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