Diels-Alder forming bond lengths

FIGURE 2.3 Endo and exo TS geometries of the Diels-Alder reaction between o-QM and MVE, also in the presence of an ancillary CHC13 molecule (MEV-TSexoCHCl3), at B3LYP/ 6-31G(d,p) level, in the gas phase. Forming bond lengths are in A. 

An important contribution for the endo selectivity in the carho-Diels-Alder reaction is the second-order orbital interaction [1], However, no bonds are formed in the product for this interaction. For the BF3-catalyzed reaction of acrolein with butadiene the overlap population between Cl and C6 is only 0.018 in the NC-transi-tion state [6], which is substantially smaller than the interaction between C3 and O (0.031). It is also notable that the C3-0 bond distance, 2.588 A, is significant shorter than the C1-C6 bond length (2.96 A), of which the latter is the one formed experimentally. The NC-transition-state structure can also lead to formation of vinyldihydropyran, i.e. a hetero-Diels-Alder reaction has proceeded. The potential energy surface at the NC-transition-state structure is extremely flat and structure NCA (Fig. 8.6) lies on the surface-separating reactants from product [6]. 

In the Diels-Alder transition state, the two alkene carbons and carbons 1 and 4 of the diene rehybridize from sp2 to sp 5 to form two new single bonds, while carbons 2 and 3 of the diene remain sp2-hybridized to form the new double bond in the cyclohexene product. We ll study this mechanism at greater length in Chapter 30 but will concentrate for the present on learning more about the characteristics and uses of the Diels-Alder reaction. 

In each transition state, the bonds which are antiperiplanar to the forming Hnu-C bond are elongated, axial C-Hs for axial attack, and C-C bonds for equatorial (Fig. 6-1). This effect has also been noted in calculations of the Diels-Alder transition states [39, 40]. Using the calculated length of the adjacent axial C-H bond of cyclohexanone (1.087 A) as a reference, a further lengthening [58] to 1.091 A (0.37%) in the axial transition state occurs, with no extension of the C -C/j bond. 

Cycloadditions are characterised by two components coming together to form two new a bonds, one at each end of both components, joining them together to form a ring, with a reduction in the length of the conjugated system of orbitals in each component (Fig. 6.1a). Cycloadditions like the Diels-Alder reaction are by far the most abundant, varied, featureful and useful of all pericyclic reactions. They are inherently reversible, and the reverse reaction is called a retro-cycloaddition or a cycloreversion. 

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