The Chemistry of 1,4-Dehydrobenzenes

Upon heating, alkyl substituted cis 1,2-diethynyl olefins undergo cyclization to yield reactive 1,4-dehydrobenzenes; the products isolated may be derived from either unimolecular or bimolecular reactions of the intermediate. Z-4,5-Diethynyl-4- octene (19) undergoes rearrangement to yield 2,3-di-n-propyl-1,4- dehydrobenzene (33). Solution pyrolysis of 19 in inert aromatic solvents produces three unimolecular products, (Z-dodeca-4,8- diyn-6-ene (23), benzocyclooctene (25) and o-allyl-n-propylbenzene (26)) in high yield. When 1,4-cyclohexadiene is added to the pyrolysis solution as a trapping agent, high yields of the reduced product o-di-n-propylbenzene (28) are obtained. The kinetics of solution pyrolysis of 19 in the presence and absence of trapping agent establish that 2,3-di-n-propyl-1,4- dehydrobenzene is a discrete intermediate on the pathway leading to products. When the reaction was run in the heated probe of an NMR spectrometer, CIDNP was observed in 26. This observation, along with kinetic and chemical trapping evidence, indicates the presence of two additional intermediates, formed from 33 by sequential intramolecular [1,5] hydrogen transfer, on the pathway to products. The observation of CIDNP, coupled with the reactivity exhibited by 33 and the other two intermediates, implicates a biradical description of these molecules. Two approaches have been used to determine the spin state(s) of 1,4-dehydrobenzenes produced in the solution reaction of diethynyl olefins. The first method relies on the "spin correlation effect" which postulates a relationship between the spin state of a caged radical pair and the ratio of cage and escape reactions (C/E) which may occur in the pair. When the 2,3-di-n-propyl-1,4-dehydrobenzene biradical abstracts hydrogen from 1,4-cyclohexadiene, a radical pair is generated. If a mixture of l,4-cyclohexadiene-do and -d4 is employed it is possible, by performing a VPC-MS analysis, to determine the ratio C/E leading from the radical pair to the reduced product, 28. Applying this method to the reaction of 19, C/E was found to be 0.6, independent of the concentration of 1,4-cyclohexadiene (between 0.1 and 10 M) in the chlorobenzene reaction solution. This result suggests the presence of the singlet state of 33 in the reaction of 19. Independent support for this analysis came from the reaction of 3,4-dimethyl-1,5-diyn-3-ene (38) in hexachloroacetone sol vent in an NMR probe. The major product, l,4-dichloro-2,3-dimethylbenzene (39), obtained by chlorine abstraction from the sol vent, showed polarization (emission) in the aromatic protons. The interpretation of this result is straightforward and indicates solvent trapping of the singlet state of the intermediate 2,3-dimethyl-1,4-dehydrobenzene. Both of these experiments indicate that only the singlet state of 1,4- dehydrobenzenes is generated upon thermal reaction of diethynyl olefins. The failure to observe evidence for the triplet state of the 1,4-dehydrobenzenes under the reaction conditions requires that, if the triplet is the ground electronic state, the rate of intersystem crossing from the singlet must be <109 sec-1.