Divinylcyclopropanes sigmatropic

In accordance with this, the reaction of the electron-donor-substituted butadienes 170 (R=Ph, OMe) with the arylcarbene complexes 163 yields divinylcyclopropane intermediates 168 with high chemoselectivity for the electron-rich double bond in 170, which readily undergo a [3,3]-sigmatropic rearrangement to give the as-6,7-disubstituted 1,4-cycloheptadiene derivatives... 

The highly substituted cir-divinylcyclopropanes (20) and (21) do not undergo sigmatropic rearrangement at all. Apparently, the highly sterically congested nature of the transition states (F) precludes this possibility. Thermolysis of (20) and (21) at 170-180 C produces only equilibrium mixtures of these substances and the corresponding trans isomers (22) and (23), respectively (Scheme 3). ... 

Sperling, D., Reissig, H.-U., Fabian, J. [1,3]-sigmatropic rearrangements of divinylcyclopropane derivatives and hetero analogs in competition with Cope-type rearrangements. A DFT study. Eur. J. Org. Chem. 1999,1107-1114. 

Under the same conditions, reaction of 1 with buta-1,3-diene afforded the isomeric 1-chloro-l, 2-divinylcyclopropanes 3. However, trans-3 with its two vinyl groups cis to each other immediately rearranges via a 1,3-sigmatropic shift to produce 2-chloro-l,4-cycloheptadiene (4) (see also Sections 1.2.1.2.2 and l.B.2.4.5.1 for similar examples of the formation of seven-membered rings). 

Photolysis of diazomethane in benzene gives cycloheptatriene (1, X = Y = The nor-caradiene intermediate 2, although detected by UV spectroscopy, cannot be isolated in this case it readily undergoes a [3,3]-sigmatropic shift with ring opening of the cyclopropane moiety, as do most cw-l,2-divinylcyclopropane systems. ... 

The mechanism of the thermal and photochemical rearrangement is believed to proceed as shown (1 - 2), with the dilemma of concerted vs. diradical nature not definitely resolved for all cases. A comparison of energy parameters for the thermolysis of the parent 1,2-divinyl-cyclopropane and of l-(hex-l-enyl)-2-vinylcyclopropane ( - and Z-isomers, tram- and cis-cyclopropanes) has been reported. When both alkenes bear a Z-positioned substituent, the rearrangement of the divinylcyclopropane system becomes very slow and other processes, such as the [1,5] sigmatropic shift of alkyl(vinyl)cyclopropanes or the vinylcyclopropane to cyclo-pentene rearrangement, may compete (see also Section 2.4.3.). Both the mechanism and the applications of this rearrangement have been reviewed. 

The cyclopropanation of dienes with 17 is not very selective and results in the formation of a 4 1 ratio of cis- and irans-l,2-divinylcyclopropanes 18. The c/.v-isomers 18 immediately undergo a Cope rearrangement to yield 19 under the reaction conditions, whereas the more stable trans-isomers can be isolated. However, the Pww-compounds 18 (R1 = CH3) arc rapidly transformed to ring-opened trienes 20 via a [1,5] sigmatropic hydrogen shift on distillation or attempted chromatography. 

An interesting expansion of this methodology uses reactions of Fischer carbenes with dienynes to produce bicyclic and tricyclic cycloheptadienes, through cw-divinylcyclopropane intermediates. In the intermolecular version, a molybdenum alkylalkoxycarbene reacts with a substituted dienyne to produce the cfs-divinylcyclopropane intermediate which, under the reaction conditions, undergoes a [3,3]-sigmatropic rearrangement to produce a mixture of bicyclo[5.3.0]decanes [Eq. (31)] [71]. The electron-withdrawing nature of the carbethoxy... 

The process has been extended to the dienyne 3. Reaction of 3 with butylmethoxy-molybdenum carbene 4 at 60° results in a divinylcyclopropane (5), which undergoes [3.3]sigmatropic rearrangement to hexahydroazulenes 6 in 87% yield.- Use of the analogous chromium complex with 3 also results in a hexahydroazulene, but in much lower yield. An electron-withdrawing group in the diene is essential for formation of 5. 

The selection rules for sigmatropic reactions are powerful predictors of thermal rearrangements. They explain how some reactions can occur ster-eospecifically and with surprisingly low activation energies, while others do not. For example, cfs-l,2-divinylcyclopropane (44) undergoes a [3,3] sigmatropic rearrangement to cis,cis-l,4-cycloheptatriene (45) with a half-life of... 

Intermediate for the synthesis of natural products, terpenes, and substrates for Cope rearrangement, divinylcyclopropane and [3,3]-sigmatropic shift studies)... 

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