The synthesis and study of some (CH)14# hydrocarbons and their derivatives.

Abstract

The (CH) n hydrocarbons .have presented a challenge to the synthetic chemist to construct molecules of a wide variety of structural types, often with a high degree of symmetry and strain. The study of the thermal and photochemical rearrangements of polymethine hydrocarbons serve as a test of the Woodward-Hoffmann rules for conservation of orbital symmetry. The lower (CH) n homologues have been studied extensively, but the (CH) 14 set has been largely unexplored. In this work the synthesis of two (CH) 14 hydrocarbons, bicyclo-[4.4.4]tetradeca-2,4,7,9,11,13-hexaene (J.J.) and cis,transoid,cis-tetracyclo[8.2.2.02 ' 9 .03 ' 8 ]tetradeca-4,6,11,13-tetraene (78) was attempted. The synthesis of J1.. required that the unknown ring system bicyclo[4.4.4]tetradecane be prepared. The approach employed involved coupling reactions of symmetrical derivatives of 1,6-diethylidine cyclodeca-3,8-diones. The Grignard coupling reaction gave intramolecular cyclization products cis- and trans-9,10-divinyl hexalins (124) and (125) but did not give the desired ring system. The isomeric hydrocarbons 124 and 125 provided a unique insight into the mechanism of the Retro Diels-Alder reaction in the mass spectrometer. Substituted derivatives of 78 were prepared and the products of their thermal and photochemical reactions were identified. A general synthesis of derivatives of barrelenes and cyclooctatetraenes was proposed. The synthesis of 78 was accomplished in two steps from Nenitzescu's hydrocarbon (20). A Diels-Alder reaction with o-benzoquinone afforded -ivtwoisomeric a-diones 181 and 182. Photochemical bisdecarbonylation of the endo adduct 182 gave the (CH) 14 polymethine hydrocarbon. _ Two minor products form this reaction were also observed in the photochemical reaction of the dione 181. Mass spectral evidence suggested that they, too, may have been (CH) 14 hydrocarbons; 78 thus appeared a viable entry into the (CH) 14 energy surface.