Anatomy of ene and Diels–Alder reactions between cyclohexadienes and azodicarboxylates

Abstract

In contrast with other (C H N) hydrogen transfers, the high-pressure kinetics of the ene reaction between cyclohexa-l,4-diene and diethyl azodicarboxylate show a concerted transition state. The discrepancy is assigned to the enhanced rigidity of the cyclohexadiene molecule with orthogonal hydrogen transfer to the nitrogen atom. Cyclohexa-l,3-diene reacts with diethyl azodicarboxylate according to a concerted Diels-Alder reaction.

Correlation between pressure and steric interactions in organic reactions

“…Simultaneously, we suggested that the higher q values might also be ascribed to the strain increase in agreement with the Hammond postulate. Retrospectively, we now strongly support the latter interpretation for two reasons: (i) the diradical character of ene processes involving diethyl azodicarboxylate has been generalized, 63 with only one exception when the azo compound was reacted with 1,4-cyclohexadiene (in that case, the reaction would follow a concerted pathway due to the quasi planar structure of the diene favoring a rigid transition state, 82 ) and (ii) the analogy of the pressure effect on cyclic strain with the pressure effect on mere steric hindrance, as demonstrated in the radical copolymerisation of maleic anhydride and cycloalkenes. 83 …”

High‐pressure mechanistic delineation based on activation volumes

“…A striking dichotomy in six-membered cycloalkene ene reactions could be evidenced through the activation volume. 24 Whereas is 0.58 in the addition of DEAD to cyclohexene (Table 13) it is 0.88 (with DV 25 * = À30 cm 3 mol À1 ) in the corresponding ene reaction of 1,4-cyclohexadiene, suggesting a concerted hydrogen transfer process. The reason may be due to the enhanced rigidity of 1,4-cyclohexadiene (additional double bond) which adopts a quasi-coplanar structure like the product (Scheme 14).…”

Solvothermal Reactions between Barium, Manganese, and Tellurium in Methylamine – the Formation of Polytellurides and Unexpected Cyanido Complexes

“…Major confusion arose over the competition between the DADA reaction and the addition‐abstraction reaction (aza‐ene reaction), and instead of obtaining bicyclo‐1,4‐ endo ‐azocyclohexane 7 via N,N′ ‐diethoxycarbonyl‐1,4‐ endo ‐hydrazocyclohexene 8 a ,10 by analogy with the published synthesis of N,N′ ‐diethoxycarbonyl‐1,3‐ endo ‐hydrazocyclo pentene ­ 9 (Scheme ),11 other products were obtained. During the 1960s, it became clear that the product of the ene reaction ( 11 a ) often dominated over the DADA reaction to produce 8 a 12–16. In short, the chemoselectivity of the addition of ethyl azodicarboxylate to 1,3‐cyclohexadiene appeared to be highly dependent on the experimental conditions.…”