Unsymmetrical Diels-Alder transition structures

The transition-state structure of the hetero-Diels-Alder reaction is generally found to be unsymmetrical. Houk et al. have for the reaction of formaldehyde with 1,3-butadiene calculated the C-C and C-0 bond lengths to be 2.133 A and 1.998 A, respectively, in the transition state using ab-initio calculations shown in Fig. 8.12 [25 bj. The reaction of formaldimine follows the same trend for the transition-state structure. 

In contrast to the great number of calculations concerning the all-carbon Diels-Alder reaction [39], there are only a few theoretical studies on the hetero Diels-Alder reaction [41,42,45 - 53 ]. The general mechanism of the Diels-Alder reaction is still in discussion however, in most cases a concerted reaction is assumed,but there is also evidence for a two-step path. The ab initio calculations carried out for the butadiene/ethene system by Houk, Ortega, Bernardi und Gajewski gave a symmetrical transition structure only using the semiempirical AM1/CI method (half electron approximation) an unsymmetrical diradicaloid intermediate was found [40]. 

The former reaction is unsymmetrical, like many Diels-Alder reactions, and the transition structure will also be unsymmetrical. This will make f3, which is distance-dependent, different for each of the CC bonds being formed, and will effectively increase the difference between the Zc2-valucs calculated above. Asynchronicity involving initial overlap of the two largest lobes will also be enhanced by the stabilisation of whatever radical character the transition structure has, and hence will augment the frontier-orbital explanation for the site selectivity in several other conjugated systems. 

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