1, 3-cycloheptadiene

A derivative of cyclopentyne has been trapped in a matrix. Although cycloheptyne and cyclohexyne have not been isolated at room temperatures, Pt(0) complexes of these compounds have been prepared and are stable." The smallest cyclic allene" so far isolated is l-/err-butyl-l,2-cyclooctadiene 107." The parent 1,2-cyclooctadiene has not been isolated. It has been shown to exist transiently, but rapidly dimerizes." " The presence of the rert-butyl group apparently prevents this. The transient existence of 1,2-cycloheptadiene has also been shown," and both 1,2-cyclooctadiene and 1,2-cycloheptadiene have been isolated in platinum complexes." 1,2-Cyclohexadiene has been trapped at low temperatures, and its structure has been proved by spectral smdies." Cyclic allenes in general are less strained than their acetylenic isomers." The cyclic cumulene 1,2,3-cyclononatriene has also been synthesized and is reasonably stable in solution at room temperature in the absence of air." ... 

Scheme 6.97 Products of dimerization, cycloadditions and protonation of 1,2-cycloheptadiene (465).

Scheme 6.99 Known 1-substituted 1,2-cycloheptadienes and generation and dimerization of 1-methoxy- (490) and l-phenyl-l,2-cycloheptadiene (497).

The l-oxa-2,4,5-cycloheptatrienes 602 and 603 were postulated to be intermediates in the rearrangement of certain (ethynylfuryl)oxiranes to furo[3,4-b]furans [251]. The replacement of the methylene groups of 1,2-cycloheptadiene (465) by SiMe2 groups and the introduction of substituents at the allene moiety allowed the preparation of isolable seven-membered ring allenes. Thus, Barton et ah [177] obtained 604 and Ando et al. [178] 605. A few reactions of these systems have also been studied [177, 252]. Both groups [178, 253] synthesized the [4.4]betweenallene 606 and determined its structure by X-ray diffraction. 

Calculations on the ring opening of fran -cyclopropylidene (113) to 1,3-dimethylallene predicted a barrier of 4.2 kcal mor via initial disrotatory motion of the substituents followed by a change to conrotatory motion. The di-cyclopropylidene rearrangement is barrierless and, in agreement with the elusive nature of 1,2-cycloheptadiene, the barrier to ring opening of bicyclic cyclopropylidene (114 n = 2) cannot be overcome at low temperatures. 

The iron complex of 1,2-cycloheptadiene was synthesized to study the relationship between 312 and a valence isomeric form (316) that has been referred to as the allyl cation form (Scheme 39).119 Because of ring strain, the latter had been predicted by EHMO calculations to lie rather close in energy to an allene ground state.126 The NMR of 312 confirmed the allene form as the ground state and fluxionality experiments on the terminal allene... 

Reactions of complexes of 1,2-cycloheptadienes have received only cursory attention. 1,2-cycloheptadiene is readily displaced from bisftriphenyl-phosphine)platinum(O)118 [Eq. (54)], but no reagent has been found that will displace the allene from iron.119 Reaction of the iron complex with alcohol in the presence of base (e.g., 312 — 322) is typical of Fp+ complexes of acyclic allenes.131132 The thermal chemistry of 312 is unusual in its decomposition to 324 (Scheme 41). This is probably attributable to the presence of the triflate counterion, since the corresponding fluoroborate salt is stable when warmed to 40°C for 16 h.119 A mechanism to 324 via carbene complex 321 appears likely. 

The internal allene bond angles of the platinum130 (343) and tungsten139 (346) complexes of cycloheptatetraene deviate from linearity by 45.1(9)° and 44.9(19)°, respectively. This is similar to the zirconium complex of 1,2-cycloheptadiene.116 As expected, the Pt-C2 bond [2.000(7) A] is shorter than Pt-Cl [2.111(8) A] because of the different hybridization of the two allene carbons. 

Thus we can say that elimination step leads essentially to 1,2-cycloheptadiene which easily dimerises. 

Cycloheptadiene may be an intermediate in reactions of 1-chlorocycloheptene with the enolate ions of alicyclic ketones. The thermal isomerization of cis-1,2-diethynylcyclobutane to bicyclo[4,2,0]octa-l,5,7-triene and bicyclo[3,3,0]octa-l,3,5-triene was suggested to involve cyclo-octa-l,2,4,5-tetraene as an intermediate. ... 

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