Bisallenes, formation

What about measurements of enthalpies of combustion of condensed phase species 49 and 50 and accompanying enthalpies of vaporization Enthalpies of formation of the gaseous hydrocarbons can be directly obtained from these studies as well. There are two recent studies that provide us with useful information. The first42 results in the values of 104.6 0.6 and 104.8 0.6 kJmol-1 respectively. The second accompanies the earlier cited cyclic bisallene (and polycyclic monoolefin) study, in which the authors20... 

We can narrow the difference from 10 kJmol-1 even further once it is remembered that in the comparison of meso-bisallene, 27, and (Z, Z)-diene, 29, there are two extra alkylallene and alkylolefin interactions for which a stabilization of ca 3 kJ mol-1 for the latter was already suggested. Admittedly, comparison with the corresponding 1,5-cyclooctadiyne suggests strain-derived anomalies. From the enthalpy of hydrogenation, and thus derived enthalpy of formation, of this diyne from W. R. Roth, H. Hopf and C. Horn, Chem. Ber., 127, 1781 (1994), we find 1/2S (bis-allene, bis-acetylene) equals ca — 80 kJ mol-1. We deduce that the discrepancy of this last 5 quantity from the others is due to strain in the cyclic diyne. 

Thermal rearrangements of 1,5-diynes result in the formation of bisallenes, but the products readily cyclize to give bismethylenecyclobutene derivatives 231122,1123. 

However, in an important unpublished work, Huntsman and Onderak showed that the major product from the threo isomer at 220°C in a static system was nearly a 1 1 mixture of both the anti,anti- and syn.syn- diethylidenecyclobutene isomer. At higher temperatures, a 92 8 mixture of the two isomers, respectively, are formed, presumably by equilibration via the threo dimethylbisallene. Scheme 7.16. The observations are consistent with the mechanism proposed above with a ratedetermining, stereospecific 3,3-shift followed by a rapid, reversible conrotatory formation of the dimethylenecyclobutenes. The 3,3-shift must proceed in a fashion that transfers asymmetry from C3 and C4 to the bisallene, presumably via an orbital symmetry controlled reaction. 

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