Lewis acids interactions

The anhydride or acyl chloride and the catalyst (proton acid or Lewis acid) interact leading to the acylating agent [formulated here for brevity as an acyl cation (83)]. ... 

Activation of the Dienophile by Lewis Acids, Interactions, Reaction Course, and Transition-state Structures... 

Figure 3.5-3 The hydrogen bond (Lewis acid) interaction of an...

A similar study performed by Welton and co-workers studied the rate and selec-tivities of the Diels-Alder reaction between cyclopentadiene and methyl acrylate in a number of neutral ionic liquids [44]. It was found that endo. exo ratios decreased slightly as the reaction proceeded, and were dependent on reagent concentration and ionic liquid type. Subsequently, they went on to demonstrate that the ionic liquids controlled the endo. exo ratios through a hydrogen bond (Lewis acid) interaction with the electron-withdrawing group of the dienophile. 

In the absence of MejSiCl, the catalytic reaction does not proceed. Sily-lation is considered to liberate the catalyst through the formation of the silyl ether (Scheme 6). The Lewis-acidic interaction of chlorosilanes with the carbonyl oxygen is also suggested to facilitate the electron transfer from... 

Boron being more electropositive than carbon, the fluorines of BF3 (-126 ppm) are more highly shielded than those of CF4 (-65 ppm). When this strong Lewis acid interacts with a Lewis base, as in BF3 OEt2... 

Konovalova, T. A. and L. D. Kispert (1998). EPR and ENDOR studies of carotenoid-solid Lewis acid interactions. J. Chem. Soc. Faraday Trans. 94 1465-1468. 

Earlier it was described how PH3 is a much weaker base than NH3. That is certainly true when the interaction of these molecules with H+ is considered. However, if the electron pair acceptor is Pt2+, the situation is quite different. In this case, the Pt2+ ion is large and has a low charge, so it is considered to be a soft (polarizable) Lewis acid. Interaction between Pt2+ and PH3 provides a more stable bond that when NH3 bonds to Pt2+. In other words, the soft electron acceptor, Pt2+, bonds better to the softer electron donor, PH3, than it does to NH3. The hard-soft interaction principle does not say that soft Lewis acids will not interact with hard Lewis bases. In fact, they will interact, but this is not the most favored type of interaction. 

AlBr3 and related compounds indicate that the Lewis acid interacts with a bridging CO group... 

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... 

Figure 3.5-3 The hydrogen bond (Lewis acid) interaction of an imidazolium cation with the carbonyl oxygen of methyl acrylate in the activated complex of the DIels-Alder reaction. 

Lewis acids are important catalysts for promoting organic reactions because they coordinate heteroatoms of functional groups. Lewis acids interact with carbonyl oxygen in its plane in either syn or anti fashion 248). Such perturbation of acceptor molecules lowers the LUMO level... 

SCHEME 104. Frontier MOs of free and Lewis acid-interacted acrolein. 

Another related feature is the observation that monomeric organo-lanthanides (Lewis acids) interact with nitrosyl groups. This occurs for example with CrCl(n-Cp)(NO)2 and (MeCp)3Sm. Adduct formation through the oxygen atom is proposed in order to explain the decreases that are found in the NO stretching frequencies.105... 

The fourth conceivable coordination mode H involves direct metal-Lewis acid interactions supported by phosphine coordination. The ability of Lewis acids to act as a-acceptor ligands was recognized early on, and referred to as Z-type ligands.79 80 Here, the transition metal is forced to behave as a Lewis base and to engage into dative M-Lewis acid interaction (Figure 35). 

Over the past few years, the coordination of preformed ambiphilic ligands, especially PBs, has been shown to afford straightforward access to complexes featuring metal-Lewis acid interactions. As discussed hereafter, this approach has allowed the significant extension of the scope of M-Z interactions, and to gain better understanding of their bonding situation. 

Charged metals (cations) in water behave as Lewis acids (willing to accept electrons). Water on the other hand, because it is willing to share its two unshared oxygen-associated pair of electrons, behaves as a Lewis base. Strong H2Q-metal (Lewis base-Lewis acid) interactions allow H+ on the water molecule to dissociate, hence, low pH water is produced. The degree of dissociation of water interacting with a cation (Mn+) is described by the metal hydrolysis constant (Table 2A)... 

Another important treatment of multiple interacting solvent effects, in principle analogous to Eq. (7-50) but more precisely elaborated and more generally applicable, has been proposed by Kamlet, Abboud, and Taft (KAT) [84a, 224, 226], Theirs and Koppel and Palm s approaches have much in common, i.e. that it is necessary to consider non-specific and specific solute/solvent interactions separately, and that the latter should be subdivided into solvent Lewis-acidity interactions (HBA solute/HBD solvent) and solvent Lewis-basicity interactions (HBD solute/HBA solvent). Using the solvato-chromic solvent parameters a, and n, which have already been introduced in Section 7.4 cf. Table 7-4), the multiparameter equation (7-53) has been proposed for use in so-called linear solvation energy relationships (LSER). 

The same approach was also used foi OsafCOn in a BBrj solvent 77]. Besides linear and branched hydrocarbons, alkylbromides were also formed. Using BCU ouly hydrocarbons were produced. The mechanism is undemonstrated. Chain elongation could occur by insertion into a M R bond and Lewis acid interaction of the type (30). 

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