Vinylcyclobutane-cyclohexene

Cycloaddition of OTr/n-bomyltriazolinedione 706 to 9-thia-l-azabicyclo[4.2.1]nona-2,4-diene 9,9-dioxide 705 gave compound 707 as a diastereoisomeric mixture (Scheme 112). The formation is attributed to the sequential operation of a regioselective [2+2] cycloaddition and a heteroatomic variant of the vinylcyclobutane-cyclohexene rearrangement <2006JOC6573>. 

No results have yet been reported for the kinetics of the pyrolysis of vinylcyclobutane though there is some indirect evidence that one of the reaction paths would yield cyclohexene. Kinetic results are available for isopropenylcyclobutane and by analogy with cyclopropane systems the behaviour of this compoimd should be very similar to vinylcyclobutane. It has been reported (Ellis and Frey, 1963) that the pyrolysis of isopropenylcyclobutane gives rise to ethylene, isoprene and 1-methyl-cyclohexene. These products arise by two simultaneous first-order processes which are both homogeneous ... 

Irradiation of cyclohexenes or cyclohexenones leads to valence isomerization with the formation of vinylcyclobutane derivatives85 (see Houben-Weyl, Vol. 4/4, p 108). 

Useful examples of the cyclohexene to vinylcyclobutane rearrangement for preparative purposes are given in Section 4.2.3. 

Photochemical [2 + 2] cycloadditions followed by vinylcyclobutane to cyclohexene rearrangements often yield the same products as the thermal Diels-Alder reactions. Strongly directing substituents on unsymmetrical dienes, however, may lead to constitutionally different isomers depending on the reaction path.19 21... 

Acceleration of the vinylcyclobutane to cyclohexene rearrangement of vinylcyclobutanols by conversion to the potassium alkoxide was first reported by Wilson and Mao.50 Typically, the [1,3] shift occurs at a satisfactory rate in tetrahydrofuran between room and reflux temperature. Sodium51 or lithium alkoxides52 also rearrange in this temperature range. The reaction rate can be further increased by addition of crown ethers or hexamethylphosphoric triamide.50,53... 

It has been suggested previously that the thermal cycloreversion of cyclohexene to ethylene plus buta-1,3-diene proceeds via a vinylcyclobutane intermediate and that, as a consequence, the stereochemistry of deuterium labels on the cyclohexene is not reflected in the deuterated ethenes obtained. This conclusion is supported by results of a study of the stereochemistry of thermal conversion of 1 -viny 1-2.3-r/.v-didcuteriocyclo-butane to butadiene and 1,2-dideuterioethylenes equal amounts of ( )-CHD=CHD and (Z)-CHD=CHD were formed.32... 

When the tetramethylene intermediate involved in [2 + 2] cycloadditions lacks stabilizing functionality at both termini, and the diene is free to assume, or is locked into, the (S)-cis conformation, the Diels-Alder process dominates ethylene and butadiene give vinylcyclobutane and cyclohexene in 1 5000 pro-... 

Styrene type [59a], As will be noted further on, the tendency of these initially formed vinylcyclobutane adducts to quickly rearrange via a cation radical mechanism to the cyclohexene (i.e., Diels-Alder) adducts is also more pronounced. In the case of a very highly electron rich substrate such as A-methyl-A-vinylacetamide, even the reaction with the rigidly s-cw-l,3-cyclohexadiene is CB periselective (Scheme 28) [61]. 

Vinylcyclobutanes can be similarly converted to cyclohexenes, but larger ring... 

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