Flow vacuum thermolysis (500-650") of 1,2-cyclonona-and 1,2-cyclodecadienes yields, as primary products, terminal enynes and enedienes. Secondary intramolecular Diels-Alder reactions of the enedienes afford cis and trans bicyclics. --During the course of studies on allene photochemistry,1*2 we have found it desirable to carefully examine the high temperature thermal reactions of homologous allenes, 1,2cyclononadiene and 1,2-cyclodecadiene. We report here facile ring fragmentation, which results from two distinct modes of intramolecular hydrogen transfer. Previous studies of unimolecular allene thermal chemistry3 have concerned principally the rotational barrier, 4 Cope rearrangements,5 interconversion with propynes and cyclopropenes,3cy6 sigmatropic shifts in vinylallenes,l and more complex rearrangements.3a*b Among simple alkyl substituted allenes, Ward and Karafiath reported* that pyrolysis of 1,2hexadiene (1) gives [Scheme II ethylene, butadiene, and cyclization'products 2 and 3. The same products were observed in photochemical reactions. Crandall and Watkins have described Scheme I 350" ) gas phase = + 1 2 3 0 > 4 5 6 1 thermal rearrangement of 1,2-cyclononadiene (4) to enyne 5 and bicyclics 6 and 7.' The authors speculated that intramolecular hydrogen abstraction by biradical 8 provides a route to products. More recently, it has been recognized that the analogous bent (~a. 130") biradical 10 should correspond to the allene thermal rotational barrier." This is a potential surface