Coordination of Lewis acids

The complexation of a-enones [( )-MeCH=CHCORj with SnCU has been studied by NMR spectroscopy and by semi-empirical and ab initio SCF-MO methods, which indicate a preference for linear coordination of Lewis acids with a-enones because of the steric effect of R groups in the s-cis conformation (Table 3) [73]. 

When considering factors that may influence the reactivity and conformation of carbonyl-Lewis acid complexes, primary attention should be given to the modes of coordination of Lewis acids to carbonyl groups, analyzing the exact location of the Lewis acid with respect to its carbonyl ligand. There are several different possible modes of coordination. First, purely electrostatic interaction can be considered, in which the metal is situated at the negative end of the C=0 dipole [C-0-M= 180°... 

Numerous investigations of reactions of allyfic stannanes with aldehydes have demonstrated that changes in steric interactions and electronic effects are introduced by the coordination of Lewis acids. In fact, the oxocarbenium species serves as the reactive partner in the allylation process, and deficiencies of detailed knowledge regarding the structure, reactivity, and steric requirements of oxocarbenium complexes leads to some uncertainties in providing a simple model to forecast stereoselectivity. Based upon a body of results accumulated from a variety of investigators, the pre-complexation with a specific Lewis acid and aldehyde may sustain the inherent attributes of the synclinal transition state, or the coordination complex may present features that override this tendency in favor of antiperiplanar arrangements. 

But examples are also known for die coordination of Lewis acids to terminal CO ligands ... 

One-electron oxidation, coordination of Lewis acids, and attack by Lewis bases can accelerate the insertion processes. 

CO insertion into M-C bonds has been shown to be accelerated by coordination of Lewis acids to the carbonyl group and by oxidation of 18-electron complexes to generate 17-electron species. The origin of these effects and examples of such acceleration by additives are described in the following two sections. 

Several studies show that Lewis acids accelerate the insertion of CO into alkyl groups. This effect has been shown to result from two different reactivities of Lewis acids. In one case, this acceleration is likely related to stronger coordination of Lewis acids to the oxygen of the acyl product than to tlie oxygen of the CO ligand. In a second case, the acceleration is due to an affect on ion pairing. 

The reaction between a known phosphide bridging diplatinum(I) compound 174 [132] and [AgOTf] produced a Lewis acid adduct at the Pt(I)-Pt(I) bond (175) (Scheme 10.76) [126]. The coordination of Lewis acid has only minor influence on the Pt-Pt-P core. 

Modes for Coordination of Lewis Acids to Carbonyl Croups and Designed Lewis Acids... 

Yamamoto and coworkers extensively discussed several different modes for coordination of Lewis acids to carbonyl groups, which they call selective coordination (Figure 6.1) [21-23]. Model A shows a purely electrostatic interaction, in which the... 

The less polar nature of disulfides and sulfenamides makes the addition of these reagents to alkenes inefficient. Although thiolates can be generated under reducing conditions,the activation of disulfides and sulfenamides is routinely performed by Lewis acids to enable the introduction of new u bonds other than C—S bonds. As a consequence, the stereospecificity and reactivity observed in the Lewis-acid-promoted addition of disulfides and sulfenamides to alkenes is similar to that of sulfenylation by sulfenyl halides and thi-osulfonium salts. Likewise, the accepted reaction mechanism includes the coordination of Lewis acid, resulting in a thio-sulfonium-like species that can further react with alkene either directly or indirectly by way of dithiosuhonium ions. 

Coordination of Lewis acids occurs at carbonyl groups and also at sp oxygen and nitrogen atoms. Coordination to a carbonyl group enhances its polarity and alters susceptibility to nucleophilic attack in the same way as does protonation. 

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