1,3,6-heptatriene isomers

Prompted by W. M. Jones work on the formation of heptafulvene from phenyl-carbene in the gas phase,which implied a seven-membered ring intermediate, and by the implications of the possible reversibility of the process, Baron et al. rediscovered Vander Stouw s cryptic work and showed that all three possible tolyl-carbenes gave benzocyclobutene and styrene, albeit in different ratio from the ortho isomer than from the meta or para species.Baron et al. ° proposed a mechanism in which a methylcycloheptatrienylidene (CH3-7I) was the active seven-membered species, and which additionally feamred the intermediacy of methylbicyclo[4.1.0]-heptatrienes 73 and 73 (Scheme 7.35). 

A 1,6-cycloaddition was wrongly claimed to occur with benzyne and cyclo-heptatriene. The structure of the cycloadduct is reliably established as 25, and the isomer 26 derived from an ene reaction was also obtained.32 1,8-Cycloaddition of benzyne occurs with 8-cyanoheptafulvene (27) to give 28.3 3 It is clearly possible to envisage similar reactions occurring with heterocyclic compounds, although none has yet been reported. 

The Z,Z,Z-isomer is also formed, but with a slightly higher activation energy. The kinetic isotope for the transferred hydrogen is around 7, consistent with C-H bondbreaking being involved in the rate-determining step. A similar has been measured for the unsubstituted Z,Z-l,3,5-heptatriene. ... 

Pyrolysis of a methyl-labeled derivative of 1,2,6-heptatriene, namely, optically active i -5-methyl- -l,2,6-octatriene gave a 4 1 mixture of E and Z-4-methyl-3-methylene-l,5-octatriene, but the E isomer was racemized to the extent of 32% while the starting material was optically stable.This data is consistent with closure to two stereoisomeric, chair-like 1,4-biradicals in an 84 16 ratio (due to equatorial/ axial preferences) which undergo ring flip and cleavage (Scheme 8.50). 

A third product of the iron carbonyl-cyclooctatriene reaction is a red complex of formula CgHioFe2(CO)g, which was initially formulated as C9Hi2Fe2(CO)e (61). This is derived from the 1,3,5-isomer and probably has a bis(7r-allyl) structure like the corresponding complexes with cyclo-heptatriene (Section II, A) and cyclooctatetraene (Section III, B) (46, 90). 

Heptatrienyl(di- -propyl)borane 28 was prepared from 1,3,6-heptatriene with 70% yield (Scheme 2.8). Analysis of the NMR spectra of 28 shows that all four possible isomers with a terminal di- -propylboryl group 28a-d are equilibrating in solution (the equilibrium ratio 28a 28b 28c 28d is 82 12 5 1 at 298 K) [34]. 

Non-fluxional vinylic boranes do not react with carbonyl compounds. Nevertheless, the vinylic borane 29 reacts with acetone (2 h under reflux) yielding the Z-isomer of the homoallylic borinic ester 30b. The cw-configuration of the reaction product 30b corresponds to the following sequence of transformations (Scheme 2.11). The [1,7]-H shift in the vinylic borane 29 gives the allylic Z, Z-isomer 28d which immediately reacts with acetone before the equilibrium among the allylic isomers is established. On the other hand, cyclopentanone reacts directly with 29 under mild conditions yielding Z, Z-1,3,5-heptatriene 31 and borinic ester 32 (Scheme 2.12). Apparently 29 reacts with the enol form of cyclopentanone, and a direct splitting of the B-Q ,2 bond takes place. Similar reaction of 29 with acetic acid was used for the preparative synthesis of previously unknown hydrocarbon 31 (Scheme 2.12) [35]. 

Heptatriene 18 having electron-withdrawing substituents, undergoes rapid conversion into bicyclo[4.1.0]-hepta-2,4-diene 19 and both the isomers remain in equilibrium because of low activation energy Ea < 10 kcal/mol. This transformation is known as valence tautomerism [18]. 

Fluorenylidene attacks benzene to give a mixture of the valence isomers norcaradiene 185 and cyclo-heptatriene 186 (Scheme 35). ... 

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