Woodward-Hofmann rule

The photochemical cycloaddition of a carbonyl compound 1 to an alkene 2 to yield an oxetane 3, is called the Patemo-Buchi reaction - This reaction belongs to the more general class of photochemical [2 + 2]-cycloadditions, and is just as these, according to the Woodward-Hofmann rules, photochemically a symmetry-allowed process, and thermally a symmetry-forbidden process. 

The problem of competition of the molecular reaction (direct route) and chain reaction (complicated, multistage route) was firstly considered in the monograph by Semenov [1], The new aspect of this problem appeared recently because the quantum chemistry formulated the rule of conservation of orbital symmetry in chemical and photochemical reactions (Woodward-Hofmann rule [4]). Very often the structure of initial reactants suggests their direct interaction to form the same final products, which are also obtained in the chain reaction, and the thermodynamics does not forbid the reaction with AG < 0. However, the experiment often shows that many reactions of this type occur in a complicated manner through several intermediate stages. For example, the reaction... 

In the absence of catalysts the Diels-Alder reaction occurs as a bimolecular reaction with the concerted rearrangement of 7t-orbitals. The 7t-orbitals of diene with the 7t-orbital of olefin interact in the transition state, which is favored by the geometry of the reactants (a six-membered cycle appears) and the character of (properties of symmetry) of the interacting orbitals. According to the rules of orbital symmetry (Woodward—Hofmann rule), the efficient overlap of the interacting orbitals occurs only when the bonding orbital of one reactant and the antibonding orbital of another reactant have appropriate symmetry. As seen from the scheme presented in Fig. 9.1, the reaction of diene synthesis obeys this condition and, hence, occurs comparative-... 

The Woodward—Hofmann selection rules for the generalized case are the same as for cycloaddition reactions (1) For supra—supra or an antara—antara double group transfer, the reaction is symmetry allowed under thermal conditions when p + q = 4n -I- 2, and it is photochemically allowed when p + q = 4n. (2) For supra—antara double group transfer, the reaction is symmetry allowed under thermal conditions when p -I- q = 4n, and it is photochemically allowed when p - - q = 4n - - 2. 

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