Lewis salt

A mass of experimental evidence on MPV reactions, much of which has been summarized by Morrison and Mosher, points to hydride transfer through a six-membered Lewis salt (50 equation 26). No account of aggregation is taken in this model. The observation (equation 26) that the major enantiomer obtained from reductions with optically pure aluminum alkoxides is correctly predicted by (50) provides additional support for the six-membered intermediate postulate. Examples of reductions by alkoxides wherein this geometry is unattainable will shortly be discussed, however. 

Calculational results also support a transition state formed from the six-membered Lewis salt (50). As calculated at the 3-21G level the transition state for the reaction of lithium methoxide with formaldehyde... 

These complexes are Lewis salts BH3 is a Lewis acid that accepts a lone pair of electrons from the basic ether or sulfide. 

Included in this group are any of the isoelectronic Lewis salt-forming reactions. Path Crosschecks... 

Double addition example (path Adj, then An, followed by the Lewis salt serving as a hydride source, then AdN again) ... 

Second-addition example (path Adfj then Ep, Lewis salt formation with the aluminum species to create the nucleophilic aluminate, followed by a second AdN) ... 

The electrophilic addition of a Lewis acid to a lone pair to form a Lewis salt is an isoelectronic reaction. An example is the first step of borate ester hydrolysis ... 

Related studies employing (V-protonated vinyl-165 or aryliminium salts,64 161 Lewis salts of aryl-63a and vinylimines,63b 1,3,4-oxadiazolium salts,166 and nitrilium salts167 constitute additional examples of reaction processes proceeding through cationic azadienes which formally participate in [4+ + 2] cycloaddition reactions.168... 

A final point concerning Lewis complex possibilities should be mentioned. The adducts formed between Group III organometallics and alkali metals are somewhat remindful of Lewis salt formation except only one electron is involved (compare oxidation of R—M, Section IV.C.l) ... 

A Lewis salt (192) of 3,4-dimethyl-1-phenylphosphole with BHj is formed by reaction with H3B SMe2. The reaction occurs rapidly at 0°C and is near quantitative <940M925>. The salt can be deprotonated at a methyl group at — 80°C with sec-butyllithium the allylic ion (193) so produced reacts at the 2-position on protonation or methylation, giving the novel phosphine complexes (194) and (196) that are isomeric with phosphole complexes and can generate the free phosphole isomers (195) and (197) on decomplexation (Scheme 44). 

Lewis acids such as BF3, SO3, AICI3, etc., readily react with pyridine to form Lewis salts (Scheme 6.5), and many coordination complexes have been prepared with metallic ions. 

Steric hindrance by substituents at the nitrogen atom of amines has been determined by preparing certain Lewis salts and measuring their dissociation constants and heats of formation. Quinuclidine (98) was found to yield the most stable adduct with trimethylboron , while the adduct in the case of tiiethylamine is unstable . These results show that the approach to the nitrogen atom is hindered to a certain extent by the ethyl groups, which are free to rotate and flip... 

Note that BF3 neither provides hydrogen ions in solution nor is a proton donor as required by the Arrhenius and Bronsted-Lowry definitions, respectively. Similarly, NH3 neither provides hydroxide ions in solution nor acts as a proton acceptor. (There are other instances in which ammonia is a proton acceptor, but it does not play that role in this reaction.) Therefore, this is not an acid-base reaction under these more restricted definitions. Boron trifluoride is, however, an electron-pair acceptor and ammonia an electron-pair donor, so the reaction can be classified as acid-base under the Lewis definitions. (In a sense, the product of a reaction between a Lewis acid and a Lewis base could be called a Lewis salt. However, the more technical term for such a product is a Lewis adduct.)... 

Note that the metal cation is an electron-pair acceptor (Lewis acid) and each ammonia molecule is an electron-pair donor (Lewis base). The resulting coordination compound can then be thought of as a Lewis salt or adduct. 

As discussed in Chapter 4, coordination compounds are Lewis salts or adducts composed of a Lewis acid (the metal atom or ion) and various numbers of Lewis bases (the ligands). Lewis acids accept electron pairs, while Lewis bases donate them. In the early 1960s, Ralph Pearson introduced the idea of hard and soft acids... 

Since BF3 is such a reactive, corrosive gas, it is difficult to handle directly. Often it is used in the form of the dimethyl ether adduct (shown in Figure 14.8), which can be thought of as a Lewis salt. The etherate is a liquid that boils at 126 C. AU the boron halides, but particularly the chloride and the bromide, react with water to produce boric acid and the corresponding hydrohalic acid as shown in Equation (14.8) ... 

The "etherate" of BF3 is a Lewis salt. Dimethyl ether (the Lewis base) contributes a lone pair of electrons to the unfilled, unhybridized 2p orbital of BF3 (the Lewis acid). The boron atom is rehybridized to A molecule characterized by a bond in which both the bonding electrons come from one atom is known as an adduct or sometimes an addition compound. The bond itself is called a dative bond, or sometimes a coordinate-covalent bond. 

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