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The Woodward–Hoffmann rules, devised by Robert Burns Woodward and Roald Hoffmann, are a set of rules in organic chemistry predicting the barrier heights of pericyclic reactions based upon conservation of orbital symmetry. The Woodward–Hoffmann rules can be applied to understand electrocyclic reactions, cycloadditions (including cheletropic reactions), sigmatropic reactions, and group transfer reactions. Reactions are classified as ''allowed'' if the electronic barrier is low, and ''forbidden'' if the barrier is high. Forbidden reactions can still take place but require significantly more energy. The Woodward–Hoffmann rules were first formulated to explain the striking stereospecificity of electrocyclic reactions under thermal and photochemical control. Thermolysis of the substituted cyclobutene ''trans''-1,2,3,4-tetramethylcyclobutene (1) gave only one diastereomer, the (E,E)-3,4-dimethyl-2,4-hexadiene (2) as shown below; the (Z,Z) and the (E,Z) diastereomers were not detected in the reaction. Similarly, thermolysis of ''cis''-1,2,3,4-tetramethylcyclobutene (3) gave only the (E,Z) diastereomer (4). Due to their elegance and simplicity, the Woodward–Hoffmann rules are credited with first exemplifying the power of molecular orbital theory to experimental chemists. Hoffmann was awarded the 1981 Nobel Prize in Chemistry for this work, shared with Kenichi Fukui who developed a similar model using frontier molecular orbital (FMO) theory; because Woodward had died two years before, he was not eligible to win what would have been his second Nobel Prize for Chemistry.〔(The Nobel Prize in Chemistry 1981 ). Nobelprize.org.〕 ==Original formulation== The Woodward–Hoffmann rules were first invoked to explain the observed stereospecificity of electrocyclic ring-opening and ring-closing reactions at the termini of open chain conjugated polyenes either by application of heat (thermal reactions) or application of light (photochemical reactions). In the original publication in 1965, the three rules distilled from experimental evidence and molecular orbital analysis appeared as follows: *In an open-chain system containing 4n-electrons, the orbital symmetry of the highest occupied molecule orbital is such that a bonding interaction between the termini must involve overlap between orbital envelopes on opposite faces of the system and this can only be achieved in a conrotatory process. *In open systems containing 4n + 2 electrons, terminal bonding interaction within ground-state molecules requires overlap of orbital envelopes on the same face of the system, attainable only by disrotatory displacements. * In a photochemical reaction an electron in the HOMO of the reactant is promoted to an excited state leading to a reversal of terminal symmetry relationships and reversal of stereospecificity. Using this formulation it is possible to understand the stereospecifity of the electrocyclic ring-closure of the substituted 1,3-butadiene pictured below. 1,3-butadiene has 4 -electrons in the ground state and thus proceeds through a conrotatory ring-closing mechanism. Conversely in the electrocyclic ring-closure of the substituted 1,3,5-hexatriene pictured below, the reaction proceeds through a disrotatory mechanism. In the case of a photochemically driven electrocyclic ring-closure of 1,3-butadiene, electronic promotion causes becomes the HOMO and the reaction mechanism must be disrotatory. Organic reactions that obey these rules are said to be symmetry allowed. Reactions that take the opposite course are symmetry forbidden and require substantially more energy to take place if they take place at all. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Woodward–Hoffmann rules」の詳細全文を読む スポンサード リンク
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