1.3.5- Hexatrienes vinylation

Such cross-conjugation (where two of the vinyls are conjugated with each other through boron but not simultaneously with the third vinyl35) always lowers the total delocalization energy (DE) of the conjugated atomic set In simple Hiickel MO theory, for example, ( )-l,3,5-hexatriene (37) has a DE of 0.988/3, while 2-vinyl-1,3-butadiene (38) yields a DE of 0.900/3. 

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). 

It should be noted that products like 443 and 447 are the normal products of photochemical reactions of acyclic 1,3,5-hexatrienes, as well as of divinyl aromatics, but are quite unusual for thermal transformations of such substrates. Presumably, the electrostatic repulsion between CF2 groups prevents the formation of conformation 450 which is necessary for the electrocyclic ring closure (i.e. 438 — 439 and 445 -> 446). Instead, it leads to conformation 451 which is favorable to generate the diradical and then the fused vinyl-cyclopropane intermediates 452 (equation 170). Note that the rearrangement 452 —> 453... 

There is no unity of opinion in the literature concerning a classification, i.e, whether to call these transformations aza-Claisen or aza-Cope rearrangements. It is accepted that the term aza-Claisen should be reserved only for those processes in which a carbon atom in the allyl vinyl ether system has been replaced by nitrogen357. Three different types of aliphatic 3-aza-Cope reactions which were studied theoretically are the rearrangements of 3-aza-l,5-hexadienes (610, equation 262), 3-azonia-l,5-hexadienes (611, equation 263) and 3-aza-l,2,5-hexatrienes (612, equation 264) (the latter is a ketenimine rearrangement )357. 

The first nonintroductory section of the text starts with the observation made early in this century that cyclopropanes have significant olefinic character. That is, corresponding ethylene and cyclopropane derivatives have significant similarities. There are literature comparisons of the thermochemistry of direct counterparts such as the parent species (1 and 2, X = H), propene and methylcyclopropane with X = Me, and of methyl acrylate and methyl cyclopropanecarboxylate 8 with X = COOMe. But the chemistry of substituted eth-ylenes is far richer than just that of vinyl compounds. One can retrieve enthalpy of formation data for cumulenes ( cumulenated olefins) such as allene (3) and both cis- and trans-2,3,4-hexatriene (4a and 4b), and for conjugated olefins such as 1,3-butadiene (5) and both (Z)- and (E)-1,3,5-hexatriene (6a and 6b). For the cyclopropane chemist it is natural to... 

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. 

Many conjugated dienes react twice with methyl 3-bromo-2-propenoate and triethylamine, making this procedure convenient for preparing a variety of dimethyl 2,4,6,8-decatetraenedioates.98 1,3,5-Hexatriene and its substituted derivatives undergo vinylation also, to form tetraenes and pentaenes in low yields.83... 

Furthermore, given the phenyl/vinyl comparison, we also note interesting features of the energetics of cross-conjugated vs conjugated 2-vinylbutadiene and 1,3,5-hexatriene (see Reference 8). 

It is noteworthy that the product of a possible 1,4-cycloaddition of dichlorogermylene to hexatriene, i.e. l,l-dichloro-2-vinyl-l-germacyclo-3-pentene, was not observed at all in the reaction mixture. 

There are abundant examples in the literature which reveal that the thermal electrocyclization of 8e systems to form eight-membered rings proceeds with lower activation energies than for the lower vinyl-og, (Z)-hexatrienes. The helical geometry of the transition state leads to less steric congestion about Ae reacting termini of the octatetraene and this likely accounts for the facility of the thermal 8e conrotatory process. Minor structural differences can induce cycloreversion of the process. ... 

The introduction of a vinyl group in position 1 in a molecule of butadiene markedly increases its reactivity, i. e. that of 1,3,5-hexatriene, toward isobutene. In... 

A similar reaction with 1,3,5-hexatriene and dimethyldichlorosilane yields the 1,1-dimethyl-2-vinyl-l-silacyclopent-3-ene when THE and CU2CI2 are employed [110],... 

MethyI-hexatrien-(l,3,5) l-Methyl-3-vinyl-cydobuten + 3-Methyl-bicydo[3.1.0 hexen-(2) — 4... 

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