Validity of the Woodward-Hoffmann Rules

In this language, the implications of the W-H rules are that allowed reactions will proceed in a concerted fashion and that forbidden reactions will not, the argument in the latter case being that some other, non-concerted mechanism is likely to provide a more stable pathway. It is now fairly clear that there are exceptions to both of these generalizations. In this connection the concept of orbital isomerism is helpful. This was introduced by Dewar, Kirschner, and Kollmar,39 who postulated that a group of isomeric molecules can be divided into sets in such a way that members of any one set can be interconverted by paths which involve no crossing between the highest 

Schmidt81 has commented that symmetry-forbidden, concerted modes of reaction may be quite accessible if the reaction path is short, the force constants in reactant and product are both small, and the anharmonicities large. He cites a case where a forbidden reaction occurs readily with an activation energy of only 25 kJ mol-1. 

Student, A. Brown, R. C. Bingham, D. H. Lo, C. A. Ramsden, H. Kollmar, P. Weiner, and P. K. Bischof, Quantum Chemistry Program Exchange, Indiana University Program 279 (CDC version) Programs 308, 309 (IBM versions). 

Archie84 who pyrolysed c/s-3,4-dimethylcyclobutene at 280 °C and found about 0.005 % of the W-H-forbidden product. They inferred that the difference in activation energy was at least 60 kJ mol-1. Earlier Brauman and Golden65 obtained the same result from a consideration of the isomerization of bicyclo[2,l,0]pent-2-ene (14) 

Stevens, and Herschbach88 have carried out accurate calculations on a number of possible transition states for the H2 + D2 2HD reaction. The most likely candidate for a concerted process is a trimolecular, hexagonal structure, which has an energy of 288 kJ mol-1 above three separated molecules. This is to be compared with 517 kJ mol-1 for the square bimolecular species67 68 and 432 kJ mol-1 for dissociation of H2 into atoms. Other H4n+2 species would be allowed intermediates according to the W-H rules, but only H6 has an energy lower than is required for the atomic process. 

George, Chem. Phys., 1975, 11, 259. 

Lathan, R. Ditchfield, M. D. Newton, and J. A. Pople, Quantum Chemistry Program Exchange, Indiana University, Program 236 W. J. Hehre, R. F. Stewart, and J. A. 

---