MECHANISM OF THE DIELS-ALDER REACTION¹

Catalysis of the Cope and Claisen rearrangements

“…0^0 The title compound, 370, R = C16H33, as well as a related substance, R = Me, undergo a degenerate rearrangement that is detected by NMR, with coalescence of signals at about 70 °C (chloroform, benzene).235 The Me rearrangement is intramolecular on the basis of cross reactions, and was formulated as a [3,3]-sigmatropic shift;235® the structure imposes a boatlike transition state analogous to that for related l-0,4-0 systems such as methacrolein dimer. 236 The rearrangement was acid catalyzed, with coalescence of signals occurring at "constant temperature" in 0.2 M trichloroacetic acid in benzene.237 Acid catalysis was accounted for by a heterolytic process involving a univalent oxygen cation (phenoxylium ion),235*5 but a charge-accelerated [3,3] rearrangement analogous to that observed in the re- tro-Claisen rearrangement of cyclohexadienones (cf. 188) may well be involved here.…”

Claisen rearrangement of 3,4-dihydro-2H-pyranylethylenes. Synthesis of cyclohexenes

“…Conden-sation14 of methyl dimethylphosphonoacetate with acrolein dimer (1) gave a mixture of ,/3-unsaturated esters containing 82% of trans-18 and 18% of cAisomer 19. Combination of the same ylid with the dimer of methyl vinyl ketone (4) resulted in a 2:1 mixture of the trans-zster 23 and its stereoisomer 24.16 Thermolysis of the trans-ester 18 gave a mixture of products containing the cA-aldehyde 20 and the ,ßunsaturated ester 22. Similarly isomerization of the trans-zster 23 was rapid at 230°and afforded a mixture of rearranged products consisting of cA-25 and trans-26 epimers.…”

“…Journal of the American Chemical Society / 92:10 / May 20,1970 1.4790; nmr (CC14) 6.35 (1 H, d ofd, 7 = 11.5, 7.0), 6.31 (1 H, doft,7 = 6.5, 1.5), 5.75(1 H, d of d, 7 = 11.5, 1.5), 5.32(1 H, m), 4.65 (1 H, m), 3.68 (3 H, s), and 2.2-1.5 (4 H, m).…”

Recrossing and Dynamic Matching Effects on Selectivity in a Diels–Alder Reaction

“…The 3:4 ratio was previously reported as 10:1, but the product ratio changes with time as these isomers interconvert by what was proposed to be a Cope-type rearrangement. [14,16] The formation of mixtures of products complicates the direct determination of the reaction kinetics, so we opted for an indirect process. Looking at the simple Diels-Alder dimerization of 1, the activation energy was 21.9 AE 1.2 kcal mol À1 , based on initial rates over a temperature range from 100-180 8C.…”