4-Methylene-1,2,5-hexatriene

Display the HOMO for cis-l,3,5-hexatriene. Which motion (conrotatory or disrotatory) insures bonding overlap Examine the geometry of the transition state for ring closure (hexatriene to cyclohexadiene). Is it consistent with the anticipated (conrotatory or disrotatory) motion of the terminal methylenes ... 

The simplest cross-conjugated polyene is 122, 3-methylene-1,4-pentadiene or 1,1-divinylethylene itself. Accepting the analysis in Reference 2 that was made using Roth s data, we find this species to be some 23 kJ mol 1 less stable than the simplest conjugated polyene, 80, (li)-, 3,5-hexatriene or 1,2-divinylethylene. The next simplest cross-conjugated polyenes are 3-methylene-l,4,6-heptatriene, 123, and 3,4-dimethylene-1,5-hexadiene, 124, that would naturally be compared with ( , )- ,3,5,7-octatetraene,... 

It should be emphasized that this analysis is based on the assumption of some appreciable 2—5 overlap in all-c/s and cis-gauche 1,3,5-hexatriene. Furthermore, it should be pointed out that our analysis of the conformational preference of 1,3,5-hexatriene is aiming at revealing electronic patterns. In reality, the all-c/s conformation of 1,3,5-hexatriene is unfavorable due to repulsive interactions between the two methylene groups, i.e. conformational preference varies in the order all-frans > cis-gauche. 

The structural variety increases if the second (and further) substituent(s) is (are) not bound to the allene nucleus. For vinylallene (2), the additional vinyl group can be introduced at C-5, leading to 1,2,4,6-heptatetraene (22 only. E-isomer shown) or at C-4, providing 4-methylene-l,2,5-hexatriene (23), the former being an important substrate for cyclization reactions, as will be discussed in Section 5.5 (Scheme 5.2). 

To prepare the other cross-conjugated allene, 4-methylene-l,2,5-hexatriene ( 2-allenyl-1,3-butadiene ) (23), the allene alcohol 215 was first converted into the phosphate 216, that readily underwent an SN2 -type substitution with allenylmagnesium bromide to yield the target hydrocarbon as a highly reactive allene derivative (Scheme 5.32) [76],... 

Figure B7.2. Electronic structure of cycloheptatrienylidene from the interaction of the orbitals of hexatriene and methylene. Symmetry labels refer to the p and n orbitals and a vertical mirror.

The 1,1-isomer of divinylethylene is to be recognized as 3-methylene-1,4-pentadiene (or 2-vinylbutadiene) while the 1,2-isomer is the 1,3,5-hexatriene discussed above. The enthalpy of formation of (E)-1,3,5-hexatriene is from the study of Rogers and his coworkers (cited in Reference 38). The enthalpy of formation of 3-methylene-1,4-pentadiene (or 2-vinylbutadiene) is derived by summing the enthalpies of demethylation (some 31 kj mol 1 from notes 38 and 39) and of formation of 3-ethylidene-l, 4-pentadiene (2-vinyl-1,3-pentadiene) from Reference 35. 

Product II can be identified empirically as a species related closely to l,6-diphenyl-l,3,5-hexatriene, the spectra of which are shown also in Figure 6. Fluorescence and excitation spectra from an alternating copolymer film doped with this triene are identical to those for the triene in methylene chloride and very close to the spectra from photolyzed copolymer film. A six-carbon chain fragment from the alternating copolymer would have phenyl substitution in the 1,5-positions, but neither this nor methyl or carbomethoxy substitution on the chain would be expected to produce spectra greatly different from those of 1,6-diphenyl-1,3,5-hexatriene. 

FIGLfRE 3. Absorption spectrum of 3-cm layer of 1 1 mixture of E-1,3,5-hexatriene and methylene-i odi de. 

To be photochromic, fulgides should have at least one aromatic ring or heteroaromatic ring (Ar) on the exo-methylene carbon atom, so that they form a 1,3,5-hexatriene structure that may undergo 6jt-electrocyclization. 

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