Concerted transition structure

DFT calculations have been used to study the mechanistic pathway of the intramolecular Diels-Alder cycloaddition involved in the biosynthesis of natural products paraherquamide A and VM55599. The cycloaddition involves a dihydropyrolo[l,2- ]pyrazine as the azadiene and a standard alkene as the dienophile (Scheme 1). Analysis of the results reveals that these cycloadditions take place through concerted transition structures associated with [4+2]... 

Calculations on the experimental system and a model reaction were unable to locate a transition structure for formation of the diradical intermediate of a stepwise mechanism. The stepwise mechanism and the asynchronous concerted mechanism start out geometrically similar, and the two pathways appear to have merged at a highly asynchronous concerted transition structure. 

The reaction of butadiene and ethylene has been studied at many computational levels and serves as a prototype for pericyclic reactions [6,7], The concerted transition structure with Cs symmetry, shown in Fig. 5, is found to be lowest in energy. The calculated activation energy varies widely, from over 45 kcal/mol at the Hartree-Fock limit to 17.6 kcal/mol, with MP2/6-31G calculations. Inclusion of dynamic correlation energy, such as provided by QCISD(T) calculations [35], is necessary to give accurate activation energies near the experimental values of 24 kcal/mol [6, 36]. 

Fig. 5. Concerted transition structure of the Diels-Alder reaction of butadiene and ethylene...

Isobe, H. Takano, Y. Kitagawa, Y. Kawakami, T. Yamanaka, S. Yamaguchi, K. Houk, K. N. Extended Hartree-Fock (EHF) theory of chemical reactions VI hybrid DFT and post-Hartree-Fock approaches for concofed and non-concerted transition structures of the Diels-Alder reaction, Mol. Phys. 2002,100, IVl-lH. 

Ab initio for the concerted transition structure for the 1,3-dipo-lar addition of ozone to ethene. (Adapted from reference 121.)... 

The numerical value gradient and curvature of these surfaces, in the variable R and a, at the concerted transition structure are given in table 1. 

Table 1. Gradient and Curvature of the potential energy surfaces at the "concerted transition structure" for cycloaddition reaction of two ethylene molecules, the Diels Alder reaction of Butadiene and ethylene and the 1,3 Dipolar cycloaddition of Fulminic acid and Acetylene.

The two reactions studied are both predicted to proceed via the radical-pair mechanism in the gas phase. In the first case there is over 150 kJ mol difference between the two pathways, although the introduction of an electron-withdrawing carbonyl group in the second case is seen to stabilize the concerted transition structure with respect to the radical intermediates. The biradical mechanism is certainly favored, although the two pathways are considerably closer in energy than is the case for the smaller ylide. These ylides, however, are very simplistic compared with the experimental systems and the effects of substitution are discussed in the next stageon. 

Figure 11 PM3 optimized geometries of clusters of six solvating acetonitrile molecules surrounding the ylide (a) and concerted transition structure (b) of Stevens rearrangement 4...

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