Cyanoethylenes, cycloaddition

The formation of various 1 1 adducts from dehydrodithizone 413, R = R = Ph, and various 1,3-dipolarophiles has been reported. Thus, dimethyl acetylenedicarboxylate yields 419, X = COOMe, tetra-cyanoethylene yields 420, and e oxycarbonylmethylenetriphenylphos-phorane yields the betaine 421. These transformations have been considered as 1,3-dipolar cycloaddition reactions of a novel type. It seems to us rather unlikely that these transformations are concerted 1,3-dipolar cycloadditions the alternative that they are reactions involving dipolar intermediates (e.g., 422, 423, and 424) should also be considered. The... 

A good example for which experimental data are available, involves activation energies for Diels-Alder cycloadditions of different cyanoethylenes as dienophiles with cyclopentadiene, relative to the addition of acrylonitrile with cyclopentadiene as a standard. 

Since the energy of D+A- is governed by the magnitude of /D — Aa, D + A" will be stabilized when the diene is substituted by electron-releasing substituents (/ is lowered) and the ene by electron-withdrawing ones (Aa is increased). Indeed, cycloadditions proceed more rapidly in such cases. For example, 1,2-dicyanoethylene undergoes cycloaddition with cyclopentadiene c. 80 times faster than cyanoethylene (Sauer, 1967). In the former case / — Aa is 7.8 eV while in the latter case it is 8.6 eV. 

For [2 + 2] cycloadditions of electron-poor and electron-rich pairs of alkenes, configuration interaction is not just marginally helpful it is of dominant importance. Without it, activation energies would ordinarily be not just a bit higher, but drastically higher. Interestingly, the nucleophilicities of the cyanoethylenes as two-bond nucleophiles in Diels-Alder reactions and as one-bond nucleophiles in [2 + 2] cycloadditions going by way of 1,4-dipolar intermediates are projected to be i proximately the same. ... 

Nitrogen-containing heterocyclic enone systems also reacted with allene to give (2 + 2)-cycloadducts, as was shown in alkaloid synthesis.178 As the keystep in the synthesis of an annotinine derivative, allene was added to 157 and the adduct 158 was obtained in quantitative yield. Various uracils have been modified by photochemical (2 + 2)-cycloaddition with olefins, e.g., vinylene carbonate,17,18° vinyl ethers, vinyl acetates, and (cetene acetals yielded 159.181 Very recently the photochemical addition of cyanoethylenes to 2-pyridones has been observed to yield mixtures of tetrahydroazocin-2-ones (160) and (2 + 2)-cycloadducts (161).182... 

Polar [2+ 2] cycloadditions have been studied using single determinant restricted Hartree-Fock calculations on model systems (cyanoethylene and hydroxethylene). All the calculations predict a transoid approach of the reaction partners, in clear contrast with the results of experimental studies. The authors suggest that, either the overall reaction with the transoid approach has a higher barrier than a cisoid reaction, or the RHF calculations are not appropriate for the study of these reactions. 

Vinylmethylketenes 423 react as dienes in cycloaddition reactions with cyanoethylene (1979JCS(CC)10), eneamines (1982HCA2230) and 4-phenyl-4H-l,2,4-triazole-3,5-dione 424. From the reaction of the latter with 423 the [4 + 2] cycloadduct 425 is provided (Scheme 130) (1987JOC3289). 

A metal atom can lead to significant changes in the reaction pattern toward electrophilic reagents. For example, tricarbonyliron derivatives of cyclo-heptatriene and cyclooctatetraene undergo facile, unprecedented, 1,3-cycloaddition reactions with electrophiles such as hexafluoroacetone and tetra-cyanoethylene. Removal of the tricarbonyliron group can lead to useful products [Eq. (58) (Green et al., 1973)]. 

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