Butadiene s-cis

As mentioned in the Introduction, the ring closure of s-cis butadiene to cyclobutene has been at the very center of the evolution of theoretical understanding of polyene photochemistry to its current state25,87-89,151. Early ab initio calculations recognized the crucial role of the 21Ag state in the isomerization, and successfully accounted for the disrotatory stereospecificity of the reaction in terms of a two-dimensional model in which the planarity of the carbon framework is more or less maintained throughout12,13,15. 

TABLE 1. H NMR spectral data (chemical shift <5 in ppm coupling constants J in Hz) for (s-cis-butadiene)metal complexes... 

In the area of reaction energetics. Baker, Muir, and Andzehn have compared six levels of theory for the enthalpies of forward activation and reaction for 12 organic reactions the unimolecular rearrangements vinyl alcohol -> acetaldehyde, cyclobutene -> s-trans butadiene, s-cis butadiene s-trans butadiene, and cyclopropyl radical allyl radical the unimolecular decompositions tetrazine -> 2HCN -F N2 and trifluoromethanol -> carbonyl difluoride -F HF the bimolecular condensation reactions butadiene -F ethylene -> cyclohexene (the Diels-Alder reaction), methyl radical -F ethylene -> propyl radical, and methyl radical -F formaldehyde -> ethoxyl radical and the bimolecular exchange reactions FO -F H2 FOH -F H, HO -F H2 H2O -F H, and H -F acetylene H2 -F HC2. Their results are summarized in Table 8.3 (Reaction Set 1). One feature noted by these authors is... 

To obtain a state symmetry, say, of s-cis-butadiene, we note that... 

Butadiene, the parent conjugated diene, can in principle attain two planar conformations, namely s-frans-butadiene and. v-m-butadiene. In reality, the majority of the acyclic 1,3-butadiene derivatives exhibit global conformational minima that are at least close to the s-trans-diene situation.1,2 For butadiene itself the s-trans-C4H6 conformer is more stable than the i-cw-isomer by ca. 3 4 kcal mol-1, although the s-trans- - s-cis-butadiene interconversion is kinetically rapid (AG 7 kcal mol-1). Consequently, reactions via the less favorable conformations are not uncommon (e.g., the Diels-Alder reaction) (Scheme 1). 

Zirconocene complexes (with s-cis geometry) of isoprene, 2,3-dimethylbutadiene, and 3-methyI-l,3-pentadiene are reported to give exclusively 1 1 addition with carbonyl substrates even if these are used in excess and the reaction temperature is fairly high (ca. lOO C). On the contrary, zirconocene complexes of s-cis-butadiene, 1,3-pentadiene, and 2,4-hexadiene ca. 1 1 mixture of the s-cis and s-trans isomers) easily accept (equation 53) 2 equiv. of either butanal or 3-pentanone, at low temperature (ca. 30 C) in high yields (95%). - ° This can be exploited for the stepwise insertion of two different electrophiles... 

The Diels-Alder reaction is one of the most powerful carbon-carbon bond forming processes in organic synthesis [69]. Considerable experimental work has been carried out to improve the rate as well as the selectivity of Diels-Alder reactions [69]. Theoretical work in understanding this important reaction is relatively small compared to the huge amount of available experimental data (see references in Ref. 17). As a result, the Diels-Alder reaction is well studied, but not completely understood. From our research efforts accumulated over the last few years, we summarize the differences discovered between the computed transition structures of the Diels-Alder reaction in vacuum, microsolvated environments, and fully solvated systems for one of the simplest Diels-Alder reactions, acrolein, and s-cis butadiene, as schematically illustrated in Fig. 4. Molecular origins leading to the rate enhancement and selectivities are discussed, and then are related to the issues surrounding enzymatic catalysis. 

We have recently reported a detailed discussion on solvation effects in this particular reaction [17]. Briefly, the experimental activation energy value at 298 K in the gas phase has been reported to be 19.7 kcal/mol [70]. In toluene, the experimental activation enthalpy was reported at 15.8 1.4 kcal/mol, with an activation entropy of —38 4 cal/mol K [71]. Four possible reaction pathways are possible for the acrolein and s-cis butadiene reaction. Consistent with previous conventions [17,72,73], the transition structures are denoted as NC (endo, s-cis acrolein), XC (exo, s-cis acrolein), NT (endo, s-trans acrolein), and XT (exo, s-trans acrolein), as illustrated for the parent reaction in vacuum in Fig. 5. 

The net result of such electrocylclic reactions is the interconversion of a n- and a a-bond. The reactions usually proceed towards formation of the a-bond because a-orbitals are lower in energy than 7C-, but formation of a strained ring may reverse this energetic effect. Let us now consider a prototype electrocyclic reaction, the ring-closure of s-cis butadiene to cyclobutene (Fig. 4.9). 

The HOMO and LUMO are often easy to identify, as in Fig. 1.5, where the TT-orbitals of s-cis-butadiene are stacked in order of increasing energy, alongside those of ethylene - its reaction partner in the prototypical [,r4+7r2]-cycloaddition. Its HOMO, i/>2, is less stable than x of ethylene by virtue of the phase discontinuity between the two central atoms, so it is assumed to be the orbital bearing the frontier electrons when the reaction takes place on the ground-state surface. On photoexcitation, one of these two electrons is raised to the less antibonding of the two unoccupied orbitals, which becomes the SOMO. In both the ground-state and excited state reactions, the frontier orbital of butadiene, HOMO or SOMO respectively, is presumed to be stabilized by interaction with... 

Garavelli M et al (2003) A simple approach for improving the hybrid MMVB force field application to the photoisomerization of s-cis butadiene. J Comput Chem 24 1357-1363... 

Cramer and Barrows investigated computationally the intermolecular cycloaddition between s-cis butadiene and various oxyallyl dienophiles [4]. The four studied in order of increasing electrophilicity are oxyallyl (neutral) < sodium oxyallyl cation < lithium oxyallyl cation < 2-hydroxyallyl cation. Computations showed that the more electrophilic oxyallyl cations showed a higher preference for a stepwise pathway, while the weakly electrophilic oxyallyl dienophiles followed concerted cycloadditions. 

Early discussions of the conformational equilibrium in Tachy have settled on the tEc and the tEt conformers as the most energetically favored, with the former being more abundant. A recent molecular mechanics-based (MMX) conformational search confirms the placing of tEc as the most abundant Tachy conformer (63%) but predicts cEc (18%) to be slightly more abundant than tEt (13%). The weak structureless band at the onset of the UV spectrum of HOTachy, Figure 27.8, should then be assigned to either the cEc or the tEt conformer, or both, and 254 nm should favor tEc-HOTachy excitation (note that s-cis diene moieties normally absorb to the red of s-trans diene moieties). The s-cis-butadiene moiety in tEc-HOTachy may explain the much lower HOTachy to HOPre quantum yield at 254 nm than... 

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