Lithium salts Lewis acids

Resoles are usually those phenolics made under alkaline conditions with an excess of aldehyde. The name denotes a phenol alcohol, which is the dominant species in most resoles. The most common catalyst is sodium hydroxide, though lithium, potassium, magnesium, calcium, strontium, and barium hydroxides or oxides are also frequently used. Amine catalysis is also common. Occasionally, a Lewis acid salt, such as zinc acetate or tin chloride will be used to achieve some special property. Due to inclusion of excess aldehyde, resoles are capable of curing without addition of methylene donors. Although cure accelerators are available, it is common to cure resoles by application of heat alone. 

Initially the LP-DE effect was ascribed to the high internal pressure generated by the solubilization of the salt in diethyl ether [34]. Today the acceleration is explained in terms of Lewis-acid catalysis by the lithium cation [35]. The contribution of both factors (internal pressure and lithium cation catalysis) has also been invoked [36]. 

The process is assumed to take place by a chemoselective attack of the dianion 2-223 at the bromomethyl group of 2-221 and subsequent nucleophilic attack of the resultant monoanion 2-224 onto the epoxide moiety to give 2-225. Use of the sodium-lithium-salt 2-223 of the dicarbonyl compound 2-220, the reaction temperature as well as the Lewis acid LiC104, are crucial. The reaction seems to be quite general, since various 1,3-dicarbonyl compounds can be converted into the corresponding furans. 

This complex is not the actual catalyst for the hydrovinylation, but needs to be activated in the presence of a suitable co-catalyst. The role of this additive is to abstract the chloride ion from the nickel centre to generate a cationic allyl complex that further converts to the catalytically active nickel hydride species. In conventional solvents this is typically achieved using strong Lewis acids such as Et2AlCl. Alternatively, sodium or lithium salts of non-coordinating anions such as tetrakis-[3,5-bis(trifluoromethyl)phenyl]borate (BARF) can be used to activate hydrovinylation... 

Tertiary A-allylthioamides have been converted into thioamidium salts by the formation of complexes with Lewis acid. Further treatment with lithium hexamethyldisilazide (LiHMDS) affords the corresponding 1,2-disubstituted pyrroles (Scheme 25).52... 

Finally, Cristau and coworkers have reported on a quite efficient preparation of triphenylphosphine oxide (Figure 2.13) by a similar addition-elimination reaction of red phosphorus with iodobenzene in the presence of a Lewis acid catalyst followed by oxidation of an intermediate tetraarylphosphonium salt.42 This approach holds the potential for the preparation of a variety of triarylphosphine oxides without proceeding through the normally used Grignard reagent. Of course, a variety of approaches is available for the efficient reduction of phosphine oxides and quaternary phosphonium salts to the parent phosphine, including the use of lithium aluminum hydride,43 meth-ylpolysiloxane,44 trichlorosilane,45 and hexachlorodisilane.46... 

The requirement for chemical inertness further excluded a family of lithium salts that have been widely used in primary lithium batteries LiAlX4 (X = halides).Since the Lewis acidities of the AIX3 are so strong, their complexation with the moderate... 

Lithium Salts Based on Heterocyclic Anions. Lithium salts based on organic anions where the formal charge is delocalized throughout substituted heterocyclic moieties were also reported sporadically, which included, for example, lithium 4,5-dicyano-l,2,3-triazolate ° and lithium bis(trifluoro-borane)imidazolide (Lild). ° The former was developed as a salt to be used for polymer electrolytes such as PEO, and no detailed data with respect to electrochemistry were provided, while the latter, which could be viewed as a Lewis acid—base adduct between LiBp4 and a weak organic base, was intended for lithium ion applications (Table 13). 

A procedure for enantioselective synthesis of carboxylic acids is based on sequential alkylation of the oxazoline 8 via its lithium salt. Chelation by the methoxy group leads preferentially to the transition state in which the lithium is located as shown. The lithium acts as a Lewis acid in directing the approach of the alkyl halide. This is reinforced by a steric effect from the phenyl substituent. As a result, alkylation occurs predominantly from the lower face of the anion. The sequence in which the groups R and R are introduced... 

Addition of LiBr or LiCl to a solution of Sml2 in THF causes a color change from blue to purple. Oxidation potential of Sml2 in THF changes from —1.33 V to —1.98 0.01 V upon addition of I2 or LiBr (more than 1 equiv.), or to —2.11 0.01 V by addition of 12 or more equiv. of LiCl. In the presence of 4-12 equiv. of the bromide or chloride salt, the pinacol coupling reaction of cyclohexanone is accelerated. These salts should be dried before use otherwise, simple reduction to cyclohexanol occurs. The co-existing lithium cation can also act as a Lewis acid to activate the carbonyl group by coordination. ... 

It has recently been reported that (3-(I-phenylthio)cyclopropyl enones 245 were more conveniently prepared from the lithium salt of P-hydroxymethylene ketone and the phenylthiocyclopropyllithium 116a (vida supra, Sect. 4.6.1) upon treatment with Lewis acids (e.g., A1C13, SnCl4, TiCl4, etc.) in CH2C12 or better with refluxing 50 % aqueous trifluoroacetic acid, they were converted to y-ketocyclobutanones such as 246, Eq. (75) 64>. 

If lithium aluminum deuteride is used, then controlled introduction of deuterium is achieved84. Other solvents such as THF or DME have also been employed76. Under normal circumstances, fluorides are inert but vinyl fluorides may be selectively replaced (equation 8)85-87. Other fluorides may also been reduced if cerium salts or other Lewis acids are added88. 

Much more impressive rate accelerations for several Diels-Alder (and other) reactions have been observed by employing solutions of lithium perchlorate (up to 5 m) in diethyl ether (LPDE solutions) [802-806]. The dramatic rate accelerations found for Diels-Alder reactions in LPDE solutions appear to stem from Lewis acid catalysis by the coordinative unsaturated Li+ ion (see the end of Section 3.1). The Lewis acid catalysis by LPDE is applicable to those Diels-Alder reactions in which the lithium cation can coordinate with suitable functional groups in the reactants e.g. Li+---0=C). Addition of lithium-specific crown ethers e.g. [12]crown-4) leads to a loss of the catalytic activity of the Li+. For a recent extensive review of salt effects on Diels-Alder reactions, see reference [802]. 

---