Alkylation, Lewis acid catalysis

The mechanism of chemical modification reactions of PS were determined using toluene as a model compound with EC in the presence of BF3-0(C2H5)2 catalyst and the kinetics and mechanism of the alkylation reaction were also determined under similar conditions [53-55]. The alkylation reaction of toluene, with epichlorohydrin, underwent polymerization of EC in the presence of Lewis acid catalysis at a low temperature (273 K) as depicted in Scheme (9). 

Beside the Friedel-Crafts-type alkylation of arenes, the direct functionalization of 2,4-pentanediones is of great interest in Lewis acid catalysis. Although Pd-catalyzed Tsuji-Trost type allylations of 1,3-diketones are known, direct benzylation procedures catalyzed by Lewis acids are less explored [40-43]. Based on the previously described Friedel-Crafts alkylation of arenes and heteroarenes, the Rueping group developed a Bi(OTf)3-catalyzed benzylation of 2,4-pentanediones. Alcohols such as benzyl, allyl or cinnamyl alcohols were used as the electrophilic component to yield important 2-alkylated 1,3-dicarbonyl compounds. Initially, different Bi(III) salts were screened. In contrast... 

Alcohols can also be prepared from support-bound carbon nucleophiles and carbonyl compounds (Table 7.4). Few examples have been reported of the a-alkylation of resin-bound esters with aldehydes or ketones. This reaction is complicated by the thermal instability of some ester enolates, which can undergo elimination of alkoxide to yield ketenes. Traces of water or alcohols can, furthermore, lead to saponification or transesterification and release of the substrate into solution. Less prone to base-induced cleavage are support-bound imides (Entry 2, Table 7.4 see also Entry 3, Table 13.8 [42]). Alternatively, support-bound thiol esters can be converted into stable silyl ketene acetals, which react with aldehydes under Lewis-acid catalysis (Entries 3 and 4, Table 7.4). 

Polystyrene-bound allylsilanes react with /V-(alkoxycarbonyl)imincs under Lewis acid catalysis to yield /V-homoallylcarbamates (Entry 4, Table 14.9). Similarly, Wang resin bound carbamates have been successfully N-alkylated with allylsilanes and aldehydes in a Mannich-type reaction (Entry 5, Table 14.9). Resin-bound /V-(alkoxycarbo-nyl)imines can be generated either from unsubstituted carbamates ROCONH2 by... 

Another advantage of this approach is that we can now use electrophilic substitution on the pyrrole to add the rest of the molecule. So the secondary benzylic alcohol 106 might well cyclise to 105 with Lewis acid catalysis as the cation will be reasonably stable and the reaction is intramolecular. But the Friedel-Crafts alkylation to give 107 will not succeed as the cation would be primary. 

Various N-protected aziridines have been reacted with N-lithiated indoles to afford N-alkylated and 3-alkylated products, the exact ratios depending on the reaction solvent and the nature of the N-protecting group <1989CB2397>. Indoles and V-alkyl indoles afford tryptamine derivatives on reaction of aziridines under Lewis acid catalysis <1998SL754>. An improved technical process for the efficient N-alkylation of indoles 119 using the N-protected homochiral aziridine 123 has been developed (Equation 20) <20030PD22>. 

Electrophiles for aromatic substitution include the halonium ion, the nitronium ion and the carbonium ion. The latter may be generated from alkyl and acyl halides using Lewis acid catalysis in the Friedel-Crafts reactions. 

Activation of C=N double bonds by copper Lewis acids for nucleophilic addition has also been reported (Sch. 37) [73]. The a-imino ester 157 undergoes alkylation at the imine carbon with a variety of nucleophiles when catalyzed by copper Lewis acids. The presence of the electron-withdrawing ester group increases the reactivity of the imine and also assists in the formation of a stable five-membered chelate with the Lewis acid. Evidence for Cu(I) Lewis-acid catalysis and a tetrahedral chelate was obtained by FTIR spectroscopy, from the crystal structure of the catalyst, and from several control experiments. The authors rule out the intermediacy of a copper enol-ate in these transformations. The asymmetric alkylation of A,0-acetals with enol silanes mediated by a copper Lewis acid proceeding with high selectivity has been reported [74],... 

If you want to do a conjugate addition of a carbonyl compound without having a second anion-stabilizing group, you need some stable and relatively unreactive enol equivalent. In Chapters 27 and 28 you saw how enamines are useful in alkylation reactions. These neutral species are also perfect for conjugate addition as they are soft nucleophiles but are more reactive than ends and can be prepared quantitatively in advance. The reactivity of enamines is such that heating the reactants together, sometimes neat, is all that is required. Protic or Lewis acid catalysis can also be used to catalyse the reaction at lower temperature. 

SnI mechanism, 129-130 alkyl halides, 129 carbocation intermediates in, 106 leaving group, 130 Lewis acid catalysis, 130... 

Aldol reactions with specific enol equivalents Contrast with equilibrium methods Aldols with Lewis acid catalysis silyl enol ethers Application to the synthesis of gingerol Reaction at O or C Silylation, Acylation and Alkylation Naked enolates... 

Silyl enol ethers 64 need Lewis acid catalysis which generates at least a partial positive charge on the alkyl group so they react best with tertiary, allylic, and benzylic halides, and reasonably... 

Alkylation with tertiary halides is the special preserve of silyl enol ethers. Both the familiar isomers 22 and 25 give regiospecific alkylation in good yield with Lewis acid catalysis.23 The formation of 78 is remarkable as it puts two quaternary centres next to one another. 

Lithium derivatives of allyl silanes react in the y-position with alkyl halides, epoxides, and carbonyl compounds. The lithium derivative 110 of allyl silane 109 gives only the y-adduct 111 with ketones.31 Vinyl silanes such as 111 are usually converted into carbonyl compounds via epoxides which rearrange with Lewis acid catalysis and loss of silicon to give protected versions of ketones or aldehydes 112. 

Dithians 20 illustrate this approach and have been remarkably popular considering the problems in their use. They can be made from aldehydes with propane-1,3-dithiol and Lewis acid catalysis. They are deprotonated with BuLi and react with alkyl halides, epoxides, and carbonyl compounds (E+) to give 22 and hence 19 after hydrolysis. The hydrolysis is by no means easy there are many methods and this alone should warn us that none is very good.6... 

Due to the lability of the alkyl-oxygen bond in alkyl cyanates, the direct trimerization reaction to 2,4.6-trialkoxy-l,3,5-triazines runs into difficulties. Often side reactions, such as isomerization to isocyanates (Lewis acid catalysis)242 or alkylation reactions with nucleophilic catalysts or anions,243 are observed. Steric hindrance or strongly electron-accepting substituents enhance the stability of alkyl cyanates in these cases selective trimerization to the corresponding... 

Several routes are currently applied to synthesize cationic organolanthanide species, including the halide abstraction from heteroleptic Ln(III) compounds [Eq. (25)] [152], the oxidation of divalent metallocenes [Eqs. (26) and (27)] [153], the protolysis of lanthanide alkyl and amide moieties [Eqs. (28) and (29)] [154,155], and anion exchange [Eqs. (30) and (31)] [84,156]. In the absence of a coordinating solvent such extremely electrophilic species attain stabilization via arene interactions with the BPh4 anion (Sect.5.1) [153b]. Cationic rare earth species have been considered as promising candidates for Lewis acid catalysis [157]. 

Simple a,/3-unsaturated aldehydes, ketones, and esters participate preferentially in inverse electron demand (LUMOdlcne controlled) Diels-Alder reactions with electron-rich, strained, or simple olefinic and acetylenic dienophiles.3 5 The thermal reaction conditions for promoting the [4 + 2] cycloadditions of simple 1-oxabutadienes (R = H > alkyl, aryl > OR), cf. Eq. (1), are relatively harsh (150-250°C), and the reactions are characterized by competitive a,/3-unsaturated carbonyl compound dimerization or polymerization. Usual experimental techniques employed to compensate for poor conversions include the addition of radical inhibitors to the reaction mixture and the use of excess 1-oxabutadiene for promoting the [4 + 2] cycloaddition. Recent efforts have demonstrated that Lewis acid catalysis and pressure-promoted reaction conditions28-30 may be used successfully to conduct the [4 + 2] cycloaddition under mild thermal conditions (25-100°C). 

Slater determinant, 251, see also Determinantal wave function SnI mechanism, 129-130 alkyl halides, 129 carbocation intermediates in, 106 leaving group, 130 Lewis acid catalysis, 130 Sn2 mechanism, 130-136 alkyl halides, 130 carbocation intermediates in, 106 and E2, 143 gas phase, 144 geometry of approach, 131 leaving group, 130, 132 nucleophilicity, 131-132 substituent effects, 132-134 transition state, 132, 133 VBCM description, 134-135 Snoutene, 247 rearrangement, 289 Sodimn borohydride, 83, 278 Sodimn hydride, 83 Soft Electrophiles, 110 reaction with etiolate, 110 Spin function, 234... 

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