Butadiene twisted

The rotational strength calculated for I is as large as that of a butadiene twisted by 20°. In II, with an out-of-plane methyl, R increases by a factor of about 2. This shows that the contributions to R of dissymmetric substituents of chiral cisoid dienes may be comparable to and even outweigh the contributions arising from the intrinsic dissymmetry of the chromophore. 

As the triplet energy of the sensitizer becomes less than that necessary to excite either form of the butadiene (Et < 50 kcal/mole), it is proposed that energy is transferred via nonvertical excitation to lower energy twist forms of the diene triplets.(11> The product distribution again reflects the ground state population of s-cis and s-trans forms ... 

Di-tert-butyl- 1,3-butadiene behaves like a nonconjugated dienes because the bulky tert-butyl groups twist the structure and prevent the double bonds from lying in the same plane => the p orbitals at C2 and C3 do not overlap and delocalization (and therefore resonance) is prevented. 

Triplet photoaddition of simple non-cyclic monoolefins is unknown. The sensitized dimerization of ethyl vinyl ether gives exclusively head-to-head adducts, Eq. 21, and probably should not be classed as an example of simple acyclic olefin. Usually the triplets have high energies and are severly twisted. 55> Some cyclic rigid molecules, Eq. 20, that do dimerize 63> do not incorporate substituents that allow regioselectivity to be determined. Butadiene gives principally head-to-head dimerization, Eq. 19, concordant with the PMO prediction, and so does indene, Eq. 22. The anti dimer that is formed would not be expected from a singlet excimer reaction. 

The main interest of this molecule resides in the fact that the principal source of rotational strength of the it - it lowest energy transition has been attributed40 to the twist of one of the two double bonds (a = —136°, as in fraws-cyclooctene) rather than to the twist of the 1,3-butadiene moiety (6 = +50.2°)... 

More recent high level ab initio calculations on 1,3-butadiene confirm that zwitterionic allylmethylene twisted species correspond to energy minima on the 11B potential... 

Intriguingly, the conical intersection model also suggests that E,Z-isomerization of acyclic dienes might be accompanied by conformational interconversion about the central bond, reminiscent of the so-called Hula-Twist mechanism for the efficient ,Z-photo-isomerization of the visual pigment rhodopsin in its rigid, natural protein environment101. A study of the photochemistry of deuterium-labelled 2,3-dimethyl-l,3-butadiene (23-d2) in low temperature matrices (vide infra) found no evidence for such a mechanism in aliphatic diene E,Z -photoisomerizations102. On the other hand, Fuss and coworkers have recently reported results consistent with the operation of this mechanism in the E,Z-photoisomerization of previtamin D3 (vide infra)103. 

The cation-radical of permethyldithia[6]radialene provides one, even more pertinent, example of the steric control over spin delocalization (Gleiter et al. 1996). As described, the unpaired electron is delocalized only in one half of this cation-radical, namely, within the limits of the 2,3-dithiatetramethyl-butadiene unit. Owing to the steric demand of the isopropilidene groups, two of the four methylene groups are twisted, whereas the other two are coplanar (see Scheme 3.16). It seems... 

The computed transition state of the [4+2]-cycloaddition between ethene and butadiene is shown in Figure 15.2 (top), along with the computed transition state of the [4+2]-cycloaddi-tion between acetylene and butadiene. It is characteristic of the stereochemistry of these transition states that ethene or acetylene, respectively, approaches the cw-conformer of butadiene from a face (and not in-plane). Figure 15.2 also shows that the respective cycloadducts— cyclohexene or 1,4-cyclohexadiene—initially result in the twist-boat conformation. 

Photo-[4+2] reactions of the dienone steroid 105 illustrates interesting regio-, stereo-, and site selectivities (Sch. 24) [75-78]. Reaction with 1-acetoxy-1,3-diene 106 gives trans adduct 107 in good yield, epimeric at the acetate. The trans cycloaddition was attributed to a triplet pathway rather than a twisted enone intermediate [75]. Reaction with 2,3-dimethyl-1,3-butadiene 108 leads to four [4+2] adducts, with reaction at both alkenes groups of the dienone. Note that the products of reaction at the y,5-alkene are both cis. 

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