Cyclobutanes dimethylene

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. 

In general the thermal reaction of allene gives a complex mixture of dimers, trimers, and higher oligomers including small amounts of spiro compounds (140). A highly selective dimerization to 1,2-dimethylene-cyclobutane is achieved by thermal reaction of dilute solutions Theoretically the process [2s + 2a] of allenes and related cumulenes may be facilitated by participation of the orthogonal pir-orbital in the addition 142). However, the concertedness of the cyclodimerization is still in dispute. Selective cyclotrimerization and pentamerization of allene have not... 

See Cyclobutane-1 2-dicarboxylic Acid. EUlylene sulphide Dimethylene svlphide)... 

The dimerization of ketens has been used to prepare the spirocyclic cyclobutane-1,3-diones (273), (274), and (275). The dimethylene and trimethylene ketens were generated by thermolysis of the malonate esters (276) and (277). In the cyclohexane case, the necessary keten was prepared by elimination of hydrogen chloride from the acid chloride. Thermal dimerization of benzylphenylketen gives mainly the cis-dimer (279). An acyclic dimer is also formed. 

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