The cyclization of parent and cyclic hexa-1,3-dien-5-ynes - A combined theoretical and experimental study

The thermal cycloisomerization of both parent and benzannelated hexa-1.3-di en-5-yne, as well as of carbocyclic 1,3-dien-5-ynes (ring size 7-14), was i nvestigated by using pure density functional theory (DFT) of Becke, Lee, Ya ng, and Parr (BLYP) in connection with the 6-31G* basis set and the Brueckn er doubles coupled-cluster approach [BCCD(T)] with the cc-pVDZ basis set fo r the parent system. The initial cyclization product is the allenic cyclohe xa-1,2,4-triene (isobenzene), while the respective biradical is the transit ion structure for the enantiomerization of the two allenes. Two consecutive [1,2]-H shifts further transform isobenzene to benzene. For the benzannela ted system, the energetics are quite similar and the reaction path is the s ame with one exception: the intermediate biradical is not a transition stat e but a minimum which is energetically below isonaphthalene. The cyclizatio n of the carbocyclic 1,3-dien-5-ynes, which follows the same reaction path as the parent system, clearly depends on the ring size. Like the cyclic ene diynes, the dienynes were found to cyclize to products with reduced ring st rain. This is not possible for the 7- and 8-membered dienynes, as their cyc lization products are also highly strained. For 9- to 11-membered carbocycl es, all intermediates, transition states, and products lie energetically be low the parent system; this indicates a reduced cyclization temperature. Al l other rings (12- to 14-membered) have higher barriers. Exploratory kineti c experiments on the recently prepared 10- to 14-membered 1,3-dien-5-ynes r ings show this tendency, and 10- and 11-membered rings indeed cyclize at lo wer temperatures.