Hexatrienes, formation

Scheme 11 Formation of the cyclopentenyl zwitterion derivative 55 from a l-tungsta-2-diethylamino-l,3,5-hexatriene 54 [58,59]...

Thiemann and coworkers [68] sought novel types of steroids with different biological activity, and in doing so prepared areno-annulated compounds such as 6/1-133 (Scheme 6/1.35). This is achieved with a Heck reaction of 6/1-132 with an acrylate, followed by an electrocydic ring closure of the formed hexatriene. The reaction is then terminated by removal of the nitro group, with formation of the aromatic ring system. 

The photoequilibrium between 1,3-cyclohexadienes and 1,3,5-hexatrienes 3t5,3i6) s key step jn synthesis of vitamin D, as shown in the formation of vitamin D3 (R = C8H17) via a [l,7]sigmatropic H-shift from previtamin D which is obtained by irradiating provitamin D (3.9) 317). 

The thermochemistry of totally cumulated trienes, i.e. species with the C=C=C=C substructure, is very limited. Indeed, the sole examples we know are those reported by Roth, namely (Z)- and ( )-2,3,4-hexatrienes MeCH=C=C=CHMe, species 17 and 18. Their enthalpies of formation are identical to within experimental error, 265 kJ mol-1. This equality is altogether reasonable given the small Me—Me interaction across the 4-carbon, linear, cumulene chain in contradistinction to the 4.3 kJ mol-1 difference that is found for the isomeric (Z)-and (E)-2-butenes with their significantly smaller Me...Me distance. Are cumulated trienes unstable relative to cumulated dienes much as cumulated dienes are unstable relative to simple olefins Briefly regressing to cumulated dienes, this assertion is corroborated by the finding that species 3, i.e. 1,3-dimethylallene, has an enthalpy of decarbonization 18 of 144.5 kJmol-1 (reaction 12)... 

Agreement is somewhat poorer for substituted hexatrienes. Consider now the ( )-isomer of 2,5-dimethyl-l,3,5-hexatriene (81) for which Roth gives an enthalpy of formation of 95.8 kJ mol-1. Simple olefin additivity, as done above for the parent hexatriene, results in a value of 103 kJ mol-1. Modifying the above 5 to 3.5 kJ mol-1 as found for the relatedly branched conjugated diene (isoprene, 82) gives a new value of ca 100 kJmol-1 for the enthalpy of formation of 81. The discrepancy has shrunk to ca 4 kJmol-1. 

Consider now the 1,1,6,6-tetramethylated derivative of (Z)-l,3,5-hexatriene (83), a species more properly named (Z)-2,6-dimethyl-2,4,6-octatriene and occasionally and trivially called ds-allo-ocimene . To estimate its enthalpy of formation, let us use simple olefin additivity along with ... 

This class of compounds is defined to have some of the three conjugated double bonds found in the ring and others not. This class includes the isomeric 3,3 -bis(cyclohexenylidenes), 100 and 101. Roth shows us that the two isomers have the same enthalpy of formation within ca 1 kJmol-1, a difference somewhat smaller than the 4 kJmol-1 found for the totally acyclic 1,3,5-hexatrienes, 79 and 80 respectively. Naively these two sets of trienes should have the same (E)/(Z) enthalpy difference. Given experimental uncertainties, we will not attempt to explain the difference69. We may compare 100 and 101 with phenylcyclohexane, 102, an isomeric species which also has the same carbon skeleton. There is nearly a 110 kJ mol-1 enthalpy of formation difference between the semicyclic and cyclic trienes. We are not surprised, for the word cyclic is customarily replaced by aromatic when in the context of the previous sentence. 

While we know of no experimental thermochemical data for 123, Roth informs us that the enthalpy of formation of 124 is 259 kJmol-1. There are no experimental thermochemical data for 125 either, but it is easy to estimate the desired enthalpy of formation. We may either use the standard olefin approach with ethylene, 1,3-butadiene and (E)-l,3,5-hexatriene (i.e. with CH2=CH2, 33 and 79) or linearly extrapolate these three unsaturated hydrocarbons. From either of these approaches, we find a value of ca 225 kJ mol-1. Cross-conjugation costs some 35 kJ mol-1 in the current case. Interestingly, the directly measured cross-conjugated 1,1-diphenylethylene (126) is only ca 10 kJmol-1 less stable than its directly measured conjugated (E)- 1,2-isomer (40) despite the expected strain effects that would additionally destabilize the former species. 

We naturally exclude here the cyclopropenyl, cyclopentadienyl and cycloheptatrienyl radicals, all of which can also be recognized as cyclic C H species much as we did not include in our discussion the enthalpies of formation of allyl and pentadienyl radical as part of our analysis of polyenes such as butadiene and hexatriene. 

Benzene remains safely aromatic by this definition as well. After all, its enthalpy of formation is 82.6 kJmol-1 while that of the real 1,3,5-hexatriene, the reference acyclic species, is 165.1 kJmol-1, considerably higher. 

Sulfenyl chlorides and halogens react with 1,2-alkadienylphosphonic acids to afford phosphorus-containing heterocydes [72], However, the electrophilic addition of dialkyl 4-methyl-2,3,5-hexatrien-2-yl phosphonates with sulfenyl or selenyl chloride afforded 2-thienyl methylphosphonates or the seleno analogues [73, 74]. The conjugate addition of sulfenyl or selenyl chloride with the 2,4-diene moiety in the starting allene leads to the formation of the five-membered skeleton (Scheme 10.69). 

Cyclohexadiene itself undergoes smooth photochemical ring opening to Z-l,3,5-hexatriene in both the gas phase (d> = 0.13)176 and in solution (d> = 0.41)71,177. As is almost always the case, extended irradiation in solution leads to the formation of a variety of isomeric products due to secondary irradiation of the Z-triene and its E-isomer (vide infra)11. 

Direct irradiation of orf/io-divinylbenzene (164) leads to the formation of the benzobi-cyclo[3.1.0]hex-2-ene isomer 167 as the major product. Deuterium labelling and methanol trapping experiments suggested the intermediacy of both 165 and 166, the expected products of irradiation of a benzannelated 1,3,5-hexatriene (equation 62)144. 

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... 

Similar experiments showed that neither cyclohexane nor cyclohexene is formed from labeled 1-hexene. However, the formation of 1,3,5-hexatriene... 

Tracer Studies on the Possibility of Formation of Hexatriene and Various Six-Membered Rings During Dehydrocyclization of 1-Hexene (20, 53)... 

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