Methyl acrylate conformation

A similar study was done with methyl acrylate as the dienophile.28 The uncatalyzed and catalyzed TSs are shown in Figure 6.7. As with propenal, the catalyzed reaction is quite asynchronous with C(2)-C(3) bonding running ahead of C(l)-C(6) bonding. In this system, there is a shift from favoring the exo-s-cis TS in the thermal reaction to the endo-s-trans TS in the catalyzed reaction. A large component in this difference is the relative stability of the free and complexed dienophile. The free dienophile favors the s-cis conformation, whereas the BF3 complex favors the s-trans conformation. 

Because of the repulsion of the cyanide groups the polymer backbone assumes a rod-like conformation. The fibers derive their basic properties from this stiff structure of PAN where the nitrile groups are randomly distributed about the backbone rod. Because of strong bonding between the chains, they tend to form bundles. Most acrylic fibers actually contain small amounts of other monomers, such as methyl acrylate and methyl methacrylate. As they are difficult to dye, small amounts of ionic monomers, such as sodium styrene sulfonate, are often added to improve their dyeability. Other monomers are also employed to improve dyeability. These include small amounts (about 4%) of more hydrophilic monomers, such as -vinyl-2-pyrrolidone (Equation 6.69), methacrylic add, or 2-vinylpyridine (Equation 6.70). 

Theoretical calculations (6-31G ) have been used to compare the energies of four possible transition states for Diels-Alder reaction of the BF3 complex of methyl acrylate with 1,3-butadiene. The results are summarized in Fig. 6.4. The endo transition state with the s-trans conformation of the dienophile is preferred to the others by about 2kcal/mol.14... 

The conformational entropies of copolymer chains are calculated through utilization of semiempirical potential energy functions and adoption of the RIS model of polymers. It is assumed that the glass transition temperature, Tg, is inversely related to the intramolecular, equilibrium flexibility of a copolymer chain as manifested by its conformational entropy. This approach is applied to the vinyl copolymers of vinyl chloride and vinylidene chloride with methyl acrylate, where the stereoregularity of each copolymer is explicitly considered, and correctly predicts the observed deviations from the Fox relation when they occur. It therefore appears that the sequence distribution - Tg effects observed in many copolymers may have an intramolecular origin in the form of specific molecular interactions between adjacent monomer units, which can be characterized by estimating the resultant conformational entropy. 

The ease of racemization of chiral a-amino aldehydes under MBH conditions is undoubtedly a major difficulty in studying diastereoselective reactions [53]. Epi-merization can be essentially avoided by conducting the reaction at low temperature [54, 67], or it can be minimized at room temperature when a conformation-ally restricted amino aldehyde, such or N-trityl-azetidine 2-(S)-carboxyaldehyde is used [54]. The use of ultrasound also increases the rate of the MBH reaction, avoiding racemization almost completely, even at room temperature [55]. When adding various a-amino acid-derived aldehydes to methyl acrylate using DABCO... 

In order to form the activated complex required for the formation of product D, rotational changes of the less dipolar anti-form A to the more dipolar s jn-conformer B are necessary, to give an activated complex C with more parallel bond dipoles, which is thus more dipolar and better solvated than the reactant molecule. In agreement with this explanation is the observation that the reverse refro-Diels-Alder reaction exhibits no large solvent effect, since the activated complex C is quite similar to the reactant D [807], A very subtle solvent effect has been observed in the Diels-Alder addition of methyl acrylate to cyclopentadiene [124], The polarity of the solvent determines the ratio of endo to exo product in this kinetically controlled cycloaddition reaction, as shown in Eq. (5-43). The more polar solvents favour endo addition. 

Photoresponsive membranes of cellulose-2,4-diacetate incorporating 6-nitro-l, 3, 3 -trimethylspiro-(2H-l-benzopyran-2,2 -indoline) have been prepared and chiroptical and fluorescence properties of optically active co-polymers of acenaphthalene with methyl acrylate/methacrylate have been investigated. Marked optical activity is induced in the aromatic units only for the co-polymer with methyl methacrylate. This difference in behaviour is associated with an overall higher main-chain flexibility and conformational freedom in acrylates compared with methacrylates. Photoisomerization in polyurethanes containing azo-links has been found to be dependent upon the thermal history of the polymer, and photoisomerization of linoleic acid and... 

Atropisomers of conformationally restricted bis-phenols have been popular ligands in several applications (Sch. 5, 8, 9, 11-13 Tables 4 and 5). Aluminum compounds prepared from bis-l,T-binaphth-2,2 -ol (BINOL) 40 and the derivatives 97 were examined as chiral catalysts in the reaction of methyl acrylate and cyclopentadiene by Maruoka, Concepcion and Yamamoto [50] and by Ketter, Glahsl and Hermann [47]. Four catalysts prepared from four derivatives of the 3,3 -bis-triarylsilyl derivatives of 97 and trimethylaluminum were examined in both toluene and dichloromethane the results are summarized in Sch. 26 [50]. Slightly higher asymmetric induction was observed in toluene and for the f-butyldiphenysilyl derivative 97b. The catalyst prepared... 

The borane complexes of acrylic acid and methyl acrylate were studied next (Table 5). The syn, s-trans conformers are again preferred. Notably, the steric arguments discussed previously can nicely rationalize these results. Single point calculations on a linearly restricted BH3 complex (a = 3 = 180°) revealed only a 0.1 kcal mol preference for the s-trans structure, thereby supporting the notion that, in contrast to the Li case, the steric gearing effect is dominant in determining the structure here. 

The general preference for the s-trans conformer carries over to some extent for carboxylic esters. Ab initio calculations for the borane-methyl acrylate complex show a 1.4 kcal/mol preference for the s-trans conformer (Fig. 10), presumably due to reduced steric interactions (B-H for the s-trans vs. B-CH2 for the s-cis) [33]. Solid state structures, however, show that both conformers can be observed for esters (Figs. 11 and 12). 

The predominant conformation of a,P-unsaturated carbonyl compounds (aldehydes, ketones, esters) can vary when Lewis adds are coordinated to the carbonyl group, ft has been experimentally shown that the s-trans conformer of acrolein is more stable than the s-ds conformer, in the absence of any Lewis add. The two conformers are of almost equal stability in the case of acrylic acid or methyl acrylate 83] (Figure L21). The barriers between these conformers are low (4-9 kcal mol ). Interconversion is rapid, and the Curtin-Hammett principle ( 1.3) can be applied. 

In the presence of Lewis adds, 2-methacrolein, acrolein, E-2-heptenal, acrylic acid, methyl acrylate and ethyl cinnamate are more stable in the s-trans conformation [76, 83, 85, 88] by about 1.5-3.2 kcal mol. The calculated barrier to conformational interconversion is higher (12 kcal mol ). 

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