Structural studies butadienes

The ground state structure of butadiene has been extensively studied using different kinds of theoretical methods19,21,23,31,34,36. For this molecule, several conformations associated with rotation around the single C—C bond are possible. Experimental evidence shows that the most stable one is the planar s-trans conformation. All theoretical calculations agree with this fact. 

Because of the similarity between the structures of the Ge(100)-2 x 1 and Si(100)-2x1 surfaces, cycloaddition products like those observed on Si(100)-2 x 1 are also expected to form at the Ge surface. Indeed, studying butadiene, Teplyakov et al. [240] showed that a similar Diels-Alder product formed on the surface of Ge(100)-2 x 1. Studies of alkenes have also revealed the formation of [2 + 2] cycloaddition products on germanium. For example, cyclopentene has been shown to form the [2 + 2] cycloaddition product on both surfaces [224,296,297]. In further studies of several other dienes and alkenes (including ethylene [298-303], acetylene [304-306], and cyclohexadiene [307]), cycloaddition products were found for Ge(100)-2 x 1 similar to those observed for Si(100)-2 x 1. 

Teplyakov, A. V., Kong, M. J. and Bent, S. F. Diels-Alder reactions of butadienes with the Si(100)-2 x 1 surface as a dienophile vibrational spectroscopy, thermal-desorption and nearedge X-ray-absorption fine-structure studies. Journal of Chemical Physics 108, 4599 1606 (1998). 

R1 = N3 R2 = OH) were useful compounds (see Section II,D). The starting materials 55 had been synthesized via Diels-Alder reaction of acetylene-dicarboxylic acid with 2,3-dialkyl-1,3-butadienes, followed by oxidation (85USP4532301 85USP4556512). Yoshida et al. condensed the doubly cross-conjugated cyclopentadiene 57 with hydrazine to obtain 1,2-diazocine 58 (84TL4223) (For structural studies of 58, see Section II,B.)... 

Dienes adopt conformations in which the double bonds are coplanar, so as to permit optimum TT-orbital overlap and electron delocalization. The two alternative planar conformations for 1,3-butadiene are referred to as s-trans and s-cis. In addition to the two planar conformations, there is a third conformation, referred to as the skew conformation, which is cisoid but not planar. Various types of structural studies have shown that the s-trans conformation is the most stable one for 1,3-butadiene. A small amount of the skew conformation is also present in equilibrium with the major conformer. The planar s-cis conformation incorporates a van der Waals repulsion between the hydrogens on C(l) and C(4), which is relieved in the skew conformation. 

Yashino and Shinomiya [31] have published Raman spectra of solutions of various polymers. The technique has also found a limited application in structural studies of polymers [32-44]. For example, the three C=C stretching bands in poly butadiene corresponding to the three possible configurations can be observed by Raman spectroscopy. Raman spectroscopy has many applications in the identification of polymers in which additives obscnre the polymer peaks obtained in the IR spectrum. 

The carbo-Diels-Alder reaction of acrolein with butadiene (Scheme 8.1) has been the standard reaction studied by theoretical calculations in order to investigate the influence of Lewis acids on the reaction course and several papers deal with this reaction. As an extension of an ab-initio study of the carbo-Diels-Alder reaction of butadiene with acrolein [5], Houk et al. investigated the transition-state structures and the origins of selectivity of Lewis acid-catalyzed carbo-Diels-Alder reactions [6]. Four different transition-state structures were considered (Fig. 8.4). Acrolein can add either endo (N) or exo (X), in either s-cis (C) or s-trans (T), and the Lewis acid coordinates to the carbonyl in the molecular plane, either syn or anti to the alkene. 

In an investigation by Yamabe et al. [9] of the fine tuning of the [4-1-2] and [2-1-4] cycloaddition reaction of acrolein with butadiene catalyzed by BF3 and AICI3 using a larger basis set and more sophisticated calculations, the different reaction paths were also studied. The activation energy for the uncatalyzed reaction were calculated to be 17.52 and 16.80 kcal mol for the exo and endo transition states, respectively, and is close to the experimental values for s-trans-acrolein. For the BF3-catalyzed reaction the transition-state energies were calculated to be 10.87 and 6.09 kcal mol , for the exo- and endo-reaction paths, respectively [9]. The calculated transition-state structures for this reaction are very asynchronous and similar to those obtained by Houk et al. The endo-reaction path for the BF3-catalyzed reaction indicates that an inverse electron-demand C3-0 bond formation (2.635 A... 

The transition state assembly 55 (Figure 3.8), that rationalizes the stereochemistry of the cycloadduct, is consistent with the structure of the chiral catalyst determined by an X-ray diffraction study. Interestingly it has been shown [58] that in the cycloadditions of maleimides 56 with 2-methoxy-l,3-butadiene, the enantioselection depends on the bulkiness of Ar and Ari groups of catalyst 54 and dienophile 56, respectively (Scheme 3.13). The importance of the bulky Ari... 

Another type of interaction is the association of radical ions with the parent compounds. Recently (118), a theoretical study was reported on the interaction of butadiene ions with butadiene. Assuming a sandwich structure for the complex, the potential curve based on an extended Hiickel calculation for two approaching butadienes (B + B) revealed only repulsion, as expected, while the curves for B + and B + B" interactions exhibit shallow minima (.068 and. 048 eV) at an interplanar distance of about 3.4 A. From CNDO/2 calculations, adopting the parameter set of Wiberg (161), the dimer cation radical, BJ, appears to be. 132 eV more stable than the separate B and B species, whereas the separate B and B species are favored by. 116eV over the dimer anion radical, BJ. This finding is consistent with experimental results formation of the dimer cation radical was proved in a convincing manner (162) while the attempts to detect the dimer anion radical have been unsuccessful. With other hydrocarbons, the reported formation of benzene dimer anion radical (163) represents an exceptional case, while the dimeric cation radical was observed... 

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