Woodward-Hoffmann rales

All recent ab initio studies on the head-to-tail dimerization reaction for the parent silene H2Si=CH2 predict a very exothermic reaction with a low barrier, despite the fact that the reaction is formally forbidden12. It is believed that the strong polarization of the Si=C double bond leads to a relaxation of the Woodward-Hoffmann rales. A detailed analysis of the symmetry of the head-to-tail silene dimerization reveals, however, that a concerted [2jrs + 2jrs] reaction in the appropriate point group is not forbidden by symmetry (see Figure l)186. 

According to the Woodward-Hoffmann rales, five concerted transition states are possible for the Claisen as well as the closely related Cope rearrangements chair, boat, twist, cross and plane (Table 6). Only the chair and boat TS have to be considered, as twist, cross and plane are antarafacial-anta-rafacial processes and require highly elevated temperatures. - For the correct prediction of product stereochemistry it is nevertheless crucial to know the preference for chair- or boat-like transition state in the actual 3,3-sigmatropic shift. 

However, despite their proven explanatory and predictive capabilities, all well-known MO models for the mechanisms of pericyclic reactions, including the Woodward-Hoffmarm rules [1,2], Fukui s frontier orbital theory [3] and the Dewar-Zimmerman treatment [4-6] share an inherent limitation They are based on nothing more than the simplest MO wavefunction, in the form of a single Slater determinant, often under the additional oversimplifying assumptions characteristic of the Hiickel molecular orbital (HMO) approach. It is now well estabhshed that the accurate description of the potential surface for a pericyclic reaction requires a much more compUcated ab initio wavefunction, of a quality comparable to, or even better than, that of an appropriate complete-active-space self-consistent field (CASSCF) expansion. A wavefunction of this type typically involves a large munber of configurations built from orthogonal orbitals, the most important of which i.e. those in the active space) have fractional occupation numbers. Its complexity renders the re-introduction of qrralitative ideas similar to the Woodward-Hoffmann rales virtually impossible. 

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