Woodward—Hoffmann orbital symmetry rules

This is one of the observations that led to the Woodward-Hoffmann orbital symmetry rules.86... 

Woodward-Hoffmann orbital symmetry rules (Section 30.9) a series of rules for predicting the stereochemistry of pericyclic reactions. Even-electron species react thermally through either antarafacial or conrotatory pathways, whereas odd-electron species react thermally through either suprafacial or disrotatory pathways. 

The introduction of Valence Bond theory has motivated the search for structural regularities that can be interpreted by models of local electronic features, such as the powerful model of Valence Shell Electron Pair Repulsion [93,94] theory. Alternative approaches, based on Molecular Orbital theory, have led to the discovery of important rules, such as the Woodward-Hoffmann orbital symmetry rules [95] and the frontier orbital approach of Fukui [96,97], As a result of these advances and the spectacular successes of ab initio computations on molecular... 

Photoinduced [2 + 2] cycloaddition (Section 4.9) of alkenes (alkynes) to form cyclobutane (cyclobutene) derivatives is one of the best studied reactions in photochemistry.680 682 According to the Woodward Hoffmann orbital symmetry rules,336 the cycloaddition of one singlet excited (Si) and one ground-state alkene is allowed by a suprafacial suprafacial concerted stereospecific pathway (Scheme 6.45) 695 699 700 Rare concerted [4 + 2] and [4 + 4] photocycloadditions of conjugated singlet excited dienes must occur in a suprafacial antarafacial and suprafacial suprafacial manner, respectively.690 Since the suprafacial antarafacial reactant approach is geometrically difficult to achieve, [4 + 2] reactions usually proceed stepwise (involving biradical intermediates). [2 + 2] or [4 + 4] photocycloadditions can occur in either a concerted or stepwise fashion. 

The Woodward-Hoffmann orbital symmetry rules are not limited in application to the neutral polyene systems that have been discussed up to this point. They also apply to charged systems, just as the Htickel aromaticity rule can be applied to charged ring systems. The conversion of a cyclopropyl cation to an allyl cation is the simplest... 

Woodward—Hoffmann orbital symmetry rules can be applied to the charged systems as well. The conversion of a cyclopropyl cation to an allylic cation is the simplest one, which involves only 27r-electrons (Figure 2.13). This is an electrocyclic reaction of (4n + 2) type (n = 0) and should, therefore, be a disrotatory process. 

In a sense, the shape and relative phase of molecular orbitals are a physical observable because these influence reaction mechanisms as described by the Woodward-Hoffmann orbital symmetry rules. One can see what individual MOs look like either by computing electron density or orbital maps, which is fairly easy with modern programs, or by manually examining the LCAO-MO coefficient matrix. To interpret this matrix, one needs to know how the molecule is oriented in the coordinate system (Table 4). Knowing this, the orbital lobes can be sketched in a way consistent with the signs of the coefficients (Figure 5). Such sketches show qualitatively what the MOs look like. In ac-... 

Woodward and Hoffmann presented an orbital symmetry rule for pericychc reactions [12, 13]. 

The interpretation of chemical reactivity in terms of molecular orbital symmetry. The central principle is that orbital symmetry is conserved in concerted reactions. An orbital must retain a certain symmetry element (for example, a reflection plane) during the course of a molecular reorganization in concerted reactions. It should be emphasized that orbital-symmetry rules (also referred to as Woodward-Hoffmann rules) apply only to concerted reactions. The rules are very useful in characterizing which types of reactions are likely to occur under thermal or photochemical conditions. Examples of reactions governed by orbital symmetry restrictions include cycloaddition reactions and pericyclic reactions. 

Orbital Symmetry Rules See Woodward-Hoffmann Rules. 

We have emphasized that the Diels-Alder reaction generally takes place rapidly and conveniently. In sharp contrast, the apparently similar dimerization of olefins to cyclobutanes (5-49) gives very poor results in most cases, except when photochemically induced. Fukui, Woodward, and Hoffmann have shown that these contrasting results can be explained by the principle of conservation of orbital symmetry,895 which predicts that certain reactions are allowed and others forbidden. The orbital-symmetry rules (also called the Woodward-Hoffmann rules) apply only to concerted reactions, e.g., mechanism a, and are based on the principle that reactions take place in such a way as to maintain maximum bonding throughout the course of the reaction. There are several ways of applying the orbital-symmetry principle to cycloaddition reactions, three of which are used more frequently than others.896 Of these three we will discuss two the frontier-orbital method and the Mobius-Huckel method. The third, called the correlation diagram method,897 is less convenient to apply than the other two. 

The series of articles written by Woodward and Hoffmann in the middle of the 1960s caused a considerable stir in the organic chemistry community. For decades afterwards organic chemists were checking and trying out reactions proposed by the orbital symmetry rules. In 2003, the first paper of their series [14] was the 88th most cited paper in the Journal of the American Chemical Society [15],... 

Professors Kenichi Fukui (Kyoto University) and Roald Hoffmann (Cornell University) received the 1981 Nobel Prize in Chemistry for their quantum mechanical studies of chemical reactivity. Their applied theoretical chemistry research is certainly at the core of computational chemistry by today s yardstick. Professor Fukui s name is associated with frontier electrons, which govern the transition states in reactions, while that of Hoffmann is often hyphenated to R. B. Woodward s name in regard to their orbital symmetry rules. In addition, Professor Hoffmann s name is strongly identified with the extended Hiickel molecular orbital method. Not only was he a pioneer in the development of the method, he has continued to use it in almost all of his over 300 papers. 

Woodward saw organic synthesis as a way to advance science and to solve practical problems. One need only look to his vitamin Bj2 work to illustrate this. A reaction that Woodward had planned to use as part of the early stages of the synthesis of vitamin B12 gave a prodnct with nnexpected stereochemistry, leading the perplexed Woodward to look for similar reactions in the organic literatnre. He found them, and with Roald Hoffmann, a theoretical chemist at Harvard, formulated what are now known as the Woodward-Hoffmann mles for the conservation of orbital symmetry. These rules explained the ontcomes of a series of seemingly unrelated chemical reactions and correctly predicted the outcomes of many others. For his con-tribntions to the orbital symmetry rules, Hoffmann shared the 1981 Nobel Prize in chemistry with Kenichi Fukui of Japan, who had reached similar conclnsions independently. Woodward died before the 1981 Nobel Prize was awarded, and had he lived longer, he certainly would have received his second Nobel Prize. 

MOs, while tlie two 7t c orbitals lead to the tt and tt MOs. In the initial stage of (he dimerization, the interaction between two ethylencs is weak so that 7t+ and tt. lie far below the n+ and tt levels, so that only 7t+ and rr are occupied. Of the a orbitals of cyclobutane described earlier, only those related to the tt., 7t1 and nl levels by symmetry are shown in Figure 11.1. Not all the occupied MOs of the reactant lead to occupied orbitals in the product. In particular, tt. correlates with one component of the empty set in cyclobutane. The tt+ combination ultimately becomes one component of the filled set in cyclobutane. So the reaction is symmetry forbidden. The reader should carefully compare the correlation diagram for ethylene dimerization here with the Ho + O2 reaction in ITgure 5.8. flie two correlation diagrams are very similar, as they should be, since in this instance the spatial dfstributions of tt and n " are similar to those of and respectively, in H2. These two reactions are probably the premier examples of symmetry-forbidden reactions. A related symmetry-allowed example is the concerted cycloaddition of ethylene and butadiene, the Diels-Alder reaction. We shall not cover the orbital symmetry rules for organic, pericyclic reactions. There are several excellent reviews that the reader should consult.But it should be pointed out that the orbital symmetry rules have stereochemical implications in terms of the reaction path and products formed. The development of these rules by Woodward and Hoffmann... 

The Woodward-Hoffmann (W-H) rules are qualitative statements regarding relative activation energies for two possible modes of reaction, which may have different stereochemical outcomes. For simple systems, the rules may be derived from a conservation of orbital symmetry, but they may also be generalized by an FMO treatment with conservation of bonding. Let us illustrate the Woodward-Hoffmann rules with a couple of examples, the preference of the 4 + 2 over the 2 + 2 product for the reaction of butadiene with ethylene, and the ring-closure of butadiene to cyclobutene. 

These orbital correlation diagrams subsequently played a very important role in the development of Walsh diagrams and most importantly in the elucidation of the orbital symmetry rules developed by Woodward and Hoffmann which accounted for the stereochemistries of pericyclic reactions of organic molecules [174—184]. 

The mechanism of the Diels-Alder reaction was first fully described by Woodward and Hoffmann using their orbital symmetry rules for cycloadditions. The Woodward-Hoffmann rules state that the Diels-Alder reaction is a thermal [4jts + 2 Tis] cycloaddition involving overlap of the diene s highest occupied molecular orbital (HOMO) t / with the dienophile s lowest unoccupied molecular orbital (LUMO) /. In the case of an inverse electron demand Diels-Alder reaction, the dienophile s HOMO / combines with the diene s LUMO... 

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