Lewis acid/base intramolecular

Thirdly, the intramolecular assodation of a solvent affects the Lewis acid - base equilibrium Upon... 

Closely related to, but distinct from, the anionic boron and aluminum hydrides are the neutral boron (borane, BH3) and aluminum (alane, A1H3) hydrides. These molecules also contain hydrogen that can be transferred as hydride. Borane and alane differ from the anionic hydrides in being electrophilic species by virtue of the vacant p orbital and are Lewis acids. Reduction by these molecules occurs by an intramolecular hydride transfer in a Lewis acid-base complex of the reactant and reductant. 

The intramolecular Lewis acid-base interaction of type B is of course always in competition with an intermolecular interaction, as indicated by formula C. Again, a bulky group in a-position to X can favor the formation of monomer B. 

Before studying some examples more closely, let us consider some cases which are not listed in Table 13. There are numerous compounds SnX2 which are definitely monomeric but are nevertheless no carbene analogs since their valence electron number at the tin atom is at least eight. These compounds contain chelating ligands which can stabilize the carbenoid tin atom due to intramolecular Lewis acid-base interactions as shown by structure A and B (see also Chapter 3). 

Substitution of the dimethylsilyl group by bis(tert-butyl)-stannyl does not change the structure in solution, e.g. 33 is found to be monomeric. A very interesting dimer is 26. In contrast to the centrosymmetrical dimer of 1 (C-Symmetry), 26 has a twofold axis (C2, see Fig. 9). This special structure may be due to intramolecular Lewis acid-base interactions between the boron and nitrogen atoms 39). Nevertheless,... 

Cycloheptane annelation.1 A new route to cycloheptanes is based on a Lewis acid-catalyzed, intramolecular addition of an allylsilane group to a 3-vinylcycloal-kenone (equation I). This annelation has been applied to a synthesis of the ses-... 

Its unique reactivity comes from the fact that borane first forms a Lewis acid-base complex with the acid and then a boron-carboxylate intermediate which increases the reactivity of the boron hydride and delivers the hydride by an intramolecular reaction. As such it provides a selective way to reduce acids and produce alcohols in the presence of most other functional groups. 

The silatrane molecules with their intramolecular Si N Lewis acid/base bond have already been listed together with other nitrogen compounds (Table 4). For 1,4-bis(trimethylsiloxy)benzene, a crystal structure analysis proves a 60° dihedral angle for the conrotatory twist of the RsSiO substituents around the OC6H4O axis, reducing the ng/jr interaction to 9.24 — 7.96 = 1.28 eV (cf. Figure 15). 

A simple example of an intramolecular Lewis acid/base reaction (thus avoiding the association step) is the xanthene dye rhodamine B, which exists in solution either in the red-coloured zwitterionic form (26a) or as the colourless lactonic form (26b) [175, 221, 222]. Solutions of rhodamine B in non-HBD solvents such as dimethyl sulfoxide,... 

Analogous solvent-dependent intramolecular Lewis acid/base equilibria between laetonie and zwitterionie forms have also been found for the xanthene dyes fiuoreseein and eosin [176]. Strueturally related eolourless 3,3-diarylphthalides form highly eoloured triarylearbenium ions by interaetion with acidic compounds such as phenols. This aeid/ base reaetion has been used to design optochemical sensors suitable for the deteetion of vapours of polar solvents in the air or in waste water [271]. 

Table 4-6. Thermodynamic data for the intramolecular Lewis acid/base reaction (27a) (27b) at 25 °C [99aj.

The related addition reaction of allylzuic bromide to alkynil zinc reagents in refluxing THF leads to a mixture of vinyhc 1,1-organo-ggw-dimetallic species and gem-trimetalhc species, which respectively result from single and double additions (equation 51). The presence of a Lewis basic (see Lewis Acids Bases) group suitably placed for intramolecular (see Intramolecular) chelation, and bearing a secondary substituent, avoids the formation of double addition products and allows the reaction to take place in mild conditions (equation 52). 

Several inter- and intramolecular combinations of bulky Lewis acid-base pairs were effectively probed for the heterolytic cleavage of hydrogen. Highly active FLPs of Figure 15.2 (B, C) with a linked design were reported from the groups of Erker, Repo, Rieger, and Tamm [15]. 

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