Stepwise [4 + 2]- and [4 + 4]-cyclodimerizations of 1,1,2,2,3,3-hexamethyl-4,5-bis(methylene)cyclopentane

Abstract

Solvent Dependence of the Product Ratio from la and 5. The reaction was carried out with equimolar amounts of la and 5 a t ambient temperature in different solvents as described for ether (see above).lg The crude product mixture was analyzed by 'H NMR using the singlets at 6 5.04 and 5.07 for 8, the singlets at 6 5.58 and 6.43 for 9, and the singlet at 6 6.09 for 10. The small (CDCl,) 6 22.12 (t), 31.10 (t), 40.01 (t), 45.14 (t), 92.78 (s), 108.50The deviation of the 8:9:10 product ratio (ether) from that reported above is due to the use of la and 5 in a 1:l ratio (2:l in the previous experiment), which reduces the probability of the sequential reaction of la with 9 to give 10.amounts of 11 have been neglected. Relative Reactivities of Dipolarophiles toward Benzonitrile Oxide (la). A mixture of two dipolarophiles and benzohydroxamoyl chloride were dissolved in 4 mL of CClb Triethylamine was added, and the mixture was stirred a t 20.5 OC for 8 h. The mixture was filtered to remove triethylammonium hydrochloride, and the filtrate was concentrated under reduced pressure to give a mixture of products, which was analyzed by 'H NMR or GC.The The thermal dimerization of the title compound 1 yields the [4 + 21-cyclodimer 3 and the [4 + 41-cyclodimer 4 in a ratio of approximately 3.5:l. Comparison of activation enthalpies ( " ( 3 ) = 80.1 kJ/mol, "(4) = 76.6 kJ/mol) and activation entropies (LS'(3) = -143.1 J/mol K, AS'(4) = -163.3 J/mol K) indicates both cycloadditions

Activation and Reaction Volumes in Solution. 3

“…The several ene reactions studied at high pressure (entries 518-524) are clearly of a concerted nature 536,537 except in the case of the azo compounds; in that case, the much reduced contraction in the transition state suggests that a diradical mechanism is operating. Sigmatropic shifts (entries [525][526][527][528][529][530][531][532][533][534][535] are subject to moderate pressure acceleration resulting from the assumption of a cyclic conformation; the bond-making and -breaking parts should largely cancel. 520 A very nice confirmation comes from a study of the 1,2-divinylcyclobutanes.…”

Correlation between pressure and steric interactions in organic reactions

“…77 Another example is provided by the [4C2] and [4C4] cyclodimerization of a highly substituted 4,5-bis(methylene)-cyclopentane which proceeds via stepwise mechanism at variance with the route involving the unhindered monomer. 78 In the Diels-Alder reaction of hexachlorocyclopentadiene, q decreases with increasing ring strain. The variation is ascribed to the possible intrusion of a two-step mechanism (see Ref.…”