Dimethylenecyclobutane Rearrangements

These new developments place allene-olefin and allene-allene thermal cycloadditions in a new context framed by the questions Are there reactive intermediates separating reactants and products in the cycloadditions, or in the degenerate rearrangements Might there be a common reactive intermediate for the allene-olefin addition and the methyl-enecyclobutane rearrangement, and another for the allene-allene addition and the 1,2-dimethylenecyclobutane rearrangement How may the experimentally observed stereochemical features of these four processes be reconciled with orbital symmetry theory ... 

For the 1,2-dimethylenecyclobutane rearrangement, Gajewski and Shih 53> have demonstrated preferred conrotatory ring opening and closing. For the allene-allene cycloaddition, then, the sequence would be disrotatory motion as two allenes approach to form the perpendicular biallylene intermediate, followed by conrotatory closure of that species. 

Tetramethylene-ethane (TME), or 2,2/-bis-allyl diradical 81, was suggested as an intermediate in the thermal dimerization of allene, as well as in the interconversions of 1,2-dimethylenecyclobutane 82, methylenespiropentane 83, bis-cyclopropylidene 84 and other bicyclic systems (equation 30)45. The isolation of two different isomeric dimethylene cyclobutanes 87 and 88 (in a ca 2 I ratio) after the thermal rearrangement of the deuteriated 1,2-dimethylene cyclobutane 85 suggests that the rearrangement proceeds via a perpendicular tetramethyleneethane diradical (2,2/-bisallyl) 86 (equation 31)45. 

Analogous transformations have also been reported for the tetramethylbi-cyclopropylidenes 24b,c [99, 1001, the bicyclopropylidene 22a of Moore and Ward [1011, tetramethyldichlorobicyclopropylidene 28a [46], and 1,1-dideuter-iobicyclopropylidene [51]. The mechanistic and kinetic aspects of these rearrangements as well as subsequent transformations of the resulting methylene-spiropentanes to dimethylenecyclobutane derivatives at higher temperatures have been reviewed [1021. 

Gajewsld and Shih S2> synthesized l,2-bis(dideuteriomethylene)cyclo-butane and found it rearranged at 275 °C to a mixture of tetradeuterio-1,2-dimethylenecyclobutanes containing 26—31% of the total protium on exocyclic methylene groups. 

Because both 1-methylenespiropentane (89) and 1-cyclo-propylidenecyclopropane (95) include methylenecyclopropane moiety in their structures, the methylenecyclopropane-type reversible interconversion between 89 and 95 is expected to occur upon pyrolysis, involving a trimethylenemethane biradical intermediate. However, such a rearrangement formally does not take place, though 95 rearranges to 89. Instead, on pyrolysis at 320 C, 89 rearranges to dimethylenecyclobutanes (91 and 93) through the tetramethylene-ethane biradical 90 and the vinylic-allylic biradical 92, respectively. Presumably, biradical 94 formed by C-2-C-5 bond fission... 

Dimethylenecyclobutane Degenerate Rearrangement, Tetramethyleneethane, and the Allene Dimerization... 

Details of the coding system developed to handle the interconversions of 1,2-dimethylenecyclobutanes and the related biradicals have now appeared. Gajewski has presented stereochemical evidence for the interconversion of planar and orthogonal bisallyl radicals in the thermal rearrangement of trans-3,4-dimethyl-l,2-dimethylenecyclobutane (442). Pyrolysis of (442) at 230°C for 80 minutes gave a 9 % conversion into an 18 1 1 mixture of the isomers (443), (444), and (445). The recovered (442) was 19 % racemized, and the isomer (443) also appeared to be racemic. The pyrolysis products show the expected preference for conrotatory motions about the 1,2- and 3,4-bonds. The conrotatory out motion leads to the anti-anti-bismethallyl radical (446) which can only close to anri,anri-diethylidene-cyclobutane (445) or anti-l-ethylidene-2-methylene-3-methylcyclobutane (443). The small percentage of the syn-isomer (444) found in the products could arise from the... 

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