Methyl acrylate Lewis acid complexes

The reaction rates for the cycloaddition of several of the mentioned dienophiles to electron-rich dienes are significantly increased upon addition of a catalytic amount of a Lewis acid. The A1C13 complex of methyl acrylate reacts 100,000 times faster with butadiene than pure methyl acrylate (Figure 15.21). Apparently, the C=C double bond in the Lewis acid complex of an acceptor-substituted dienophile is connected to a stronger acceptor substituent than in the Lewis-acid-free analog. A better acceptor increases the dienophilicity of a dienophile in a manner similar to the effect several acceptors have in the series of Table 15.1. 

In 1979, Koga and coworkers disclosed the first practical example of a catalytic enantioselective Diels-Alder reaction [44] promoted by a Lewis acidic complex, presumed to be menthoxyaluminum dichloride (1), derived from menthol and ethylaluminum di chloride, whose structure remains undefined [45]. This complex catalyzed the cycloaddition of cyclopentadiene with acrolein, methyl acrylate, and methacrolein with enantioselectivities as high as 72% ee. Oxidation of 2 (predominantly exo) followed by recrystallization actually lowered the ee ... 

Figure 3.5-3 The hydrogen bond (Lewis acid) interaction of an imidazolium cation with the carbonyl oxygen of methyl acrylate in the activated complex of the DIels-Alder reaction. 

Chiral 1,2-dihydropyridine complex 78 has been shown to undergo Lewis acid-initiated [5+2] cycloadditions with electron-deficient alkenes, such as methyl acrylate, to give, after demetalation using ceric ammonium nitrate, functionalized tropanes 80 (Scheme 22) <20000L3909, 1999JA5811>. Short reaction times and substoichiometric amounts of Lewis acid are sufficient to ensure regio- and enantioselectivity. It is thought that excess Lewis acid mediates a slow reversal... 

To the extent that the enolate resulting from conjugate addition at the (3-carbon can be stabilized, the rate of this reaction pathway is enhanced. For example, (3-Michael additions are observed for MVK, acrolein, and acetylenic electrophiles even without the presence of a Lewis acid. Furthermore, MVK reacts with the 2,5-dimethylpyrrole complex (22) to form a considerable amount of (3-alkylation product, whereas only cycloaddition is observed for methyl acrylate. The use of a Lewis acid or protic solvent further enhances the reactivity at the (3-position relative to cycloaddition. While methyl acrylate forms a cycloadduct with the 2,5-dimethylpyrrole complex (22) in the absence of external Lewis acids, the addition of TBSOTf to the reaction mixture results in exclusive conjugate addition (Tables 3 and 4). 

Benzotrichloride Method. The central carbon atom of the dye is supplied by the trichloromethyl group fromy>-chlorobenzotrichloride. Both symmetrical and unsymmetrical triphenylmethane dyes suitable for acrylic fibers are prepared by this method. 4-Chlorobenzotrichloride is condensed with excess chlorobenzene in the presence of a Lewis acid such as aluminium chloride to produce the intermediate aluminium chloride complex of 4,4, 4"-trichlorotriphenylmethyl chloride (18). Stepwise nucleophilic substitution of the chlorine atoms of this intermediate is achieved by successive reactions with different a.rylamines to give both symmetrical (51) and unsymmetrical dyes (52), eg, N-(2-chlorophenyl)-4-[(4-chlorophenyl) [4-[(3-methylphenyl)imino]-2,5-cyclohexadien-l-yhdene]methyl]benzenaminemonohydrochloride [85356-86-1] (19) from / -toluidine and a-chloroaniline. 

It was noted that a fourth low energy complex for methyl acrylate is structure (7), which is conceptually related to (Z)-acrylic acid by coordination of the Lewis acid Me syn to the double bond. 

The copolymerizations of 2-methyl-1-propene (isobutylene) and acrylamides 13a f were studied in Lewis acid-promoted copolymerizations (Fig. 8). Although 13a-f can be homopolymerized in the presence of Lewis acids, poor conversions are obtained except with 13a. Presumably, complexation renders the radical and monomer too electron-deficient to react efficiently. This effect, however, should enhance the reactivity of the complexed radical toward more electron-rich alkenes and has been observed to increase the alternating character of copolymers of isobutylene and methyl acrylate [9], Isobutylene also is an ideal choice for a comonomer as it does not homopolymerize by radical pathways, and the analysis of the copolymer s tacticity is not complicated by additional stereocenters as would be the case with monosubstituted vinyl comonomers. 

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