Electrocyclic reactions orbitals symmetry properties

If the reverse back reaction is prevented or is forbidden by other considerations, the energy remains stored in the photoproducts. Some simple photorearrangement reactions which are governed by Woodward-Hoffman rules have been found useful. These rules provide the stereochemical course of photochemical rearrangement based on symmetry properties of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the molecule (Section 8.6). A reaction which is photochemically allowed may be thermally forbidden. Front the principle of microscopic reversibility, the same will be true for the reverse reaction also. Thermally forbidden back reaction will produce. ble - photoproducts. Such electrocyclic rearrangements are given in . ..ure... 

Analysis of a reaction by frontier orbital theory has additional benefits, particularly for predicting reactivity and stereochemistry. Woodward and Hoffman pointed out "that electrocyclic reactions followed the stereochemistry dictated by the symmetry, or nodal properties of the HOMO of the polyene".This concept of orbital symmetry will be important for discussions of all pericyclic reactions. Of particular importance is the difference in energy between the HOMO one Ji system and the LUMO of a second Jt-system, because this will be used to predict reactivity in pericyclic reactions (see below). 

How do the symmetry properties of 1 2 influence electrocyclic reactions For convenience, let us examine the microscopic reverse of the ring-opening of a cyclobutene to a butadiene, realizing that any factors that appear on this reaction path also appear on the forward reaction path. For bonding to occur between the carbon atoms at the end of the ir-system, the positive lobe on C(l) must overlap with the positive lobe on C(4) (or negative with negative). This overlap can be accomplished only by conrotatory motion. Disrotatory motion causes overlap of orbitals of opposite sign, and precludes bond formation. Since similar symmetry properties of the HOMO exist for other 4n -systems, the conrotatory mode will also be preferred for all thermal electrocyclic reactions in these systems. 

Analysis of the symmetry properties of hexatriene MO s (Fig. 10.2) follows the same reasoning and leads to a strikingly different conclusion, which is in complete agreement with the experimental observations. Since there are six ir-electrons, 3 is the HOMO, and a bonding interaction will occur only for disrotatory closure. Consideration of orbital symmetries for other it-systems leads to the conclusion that concerted electrocyclic reactions in systems containing 4n + 2 7r-electrons should be disrotatory. 

This stereocontrol is observed in many other electrocyclic transformations and is governed by the symmetry properties of the relevant tt molecular orbitals. The Woodward-Hoffmann rules describe these interactions and predict the stereochemical outcome of all electrocyclic reactions as a function of the number of electrons taking part in the process and whether the reaction is carried out photochemically or thermally. A complete treatment of this subject is best left to a more advanced course in organic chemistry. However, the predicted stereochemical course of electrocyclic reactions can be summarized in the simple manner shown in Table 14-2. 

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