Concerted reactions symmetry allowed

The 1,4- and 3,2-rearrangements are thermal, concerted, orbital symmetry allowed processes possessing suprafacial-suprafacial characteristics. They are facile reactions, occurring readily and under mild conditions. The 3,2-rearrangement will take place with allylic inversion, as demanded by orbital symmetry control. Thus, the sulfonium salt (30) gave the allylic sulfide (31), whereas (32) gave the isomeri-cally pure (33 Scheme 9). ... 

Woodward-Hoffmann rules These rules are based upon the principle of the conservation of orbital symmetry, and are used to predict which concerted reactions are allowed and which are forbidden. 

Symmetry allowed reaction (Section 10 14) Concerted reac tion in which the orbitals involved overlap in phase at all stages of the process The conrotatory ring opening of cy clobutene to 1 3 butadiene is a symmetry allowed reaction... 

The reaction of carbon atoms with A-unsubstituted aziridines leads to alkenes and hydrogen cyanide (72IA3455), probably via extrusion from the initially formed adduct (285). The fragmentation does not appear to be concerted, although this would be a symmetry-allowed process, since only about half the alkene formed retains the aziridine stereochemistry in the case of cM-2,3-dimethylaziridine. 

According to the Woodw ard-Hofmann rules the concerted thermal [2n + 2n] cycloaddition reaction of alkenes 1 in a suprafacial manner is symmetry-forbidden, and is observed in special cases only. In contrast the photochemical [2n + 2n cycloaddition is symmetry-allowed, and is a useful method for the synthesis of cyclobutane derivatives 2. 

An explanation for the finding that concerted [4 -I- 2] cycloadditions take place thermally, while concerted [2 + 2] cycloadditions occur under photochemical conditions, is given through the principle of conservation of orbital symmetry. According to the Woodw ard-Hofmann rules derived thereof, a concerted, pericyclic [4 -I- 2] cycloaddition reaction from the ground state is symmetry-allowed. 

Symmetry-allowed, symmetry-disallowed (Section 30.2) A symmetry-allowed reaction is a pericyclic process that lias a favorable orbital symmetry for reaction through a concerted pathway. A symmetry-disallowed reaction is one that does not have favorable orbital symmetry for reaction through a concerted pathway. 

Although the orbital-symmetry rules predict the stereochemical results in almost all cases, it is necessary to recall (p. 1070) that they only say what is allowed and what is forbidden, but the fact that a reaction is allowed does not necessarily mean that the reaction takes place, and if an allowed reaction does take place, it does not necessarily follow that a concerted pathway is involved, since other pathways of lower energy may be available.Furthermore, a forbidden reaction might still be made to go, if a method of achieving its high activation energy can be found. This was, in fact, done for the cyclobutene butadiene interconversion (cis-3,4-dichloro-cyclobutene gave the forbidden cis.cis- and rran.y, ra i -l,4-dichloro-1,3-butadienes,... 

The chemical reactions through cyclic transition states are controlled by the symmetry of the frontier orbitals [11]. At the symmetrical (Cs) six-membered ring transition state of Diels-Alder reaction between butadiene and ethylene, the HOMO of butadiene and the LUMO of ethylene (Scheme 18) are antisymmetric with respect to the reflection in the mirror plane (Scheme 24). The symmetry allows the frontier orbitals to have the same signs of the overlap integrals between the p-or-bital components at both reaction sites. The simultaneous interactions at the both sites promotes the frontier orbital interaction more than the interaction at one site of an acyclic transition state. This is also the case with interaction between the HOMO of ethylene and the LUMO of butadiene. The Diels-Alder reactions occur through the cyclic transition states in a concerted and stereospecific manner with retention of configuration of the reactants. 

Within the isolobal formalism, the conversion of 47 to 48 is a symmetry-allowed process, if it were to proceed as a concerted reaction (50). Structure 47 represents a transoid-2-meta.Wa-1,3-butadiene. In the bonding description, complex 48 represents formally a 1-metalla-bicyclo[1.1.0]butane. Therefore, the conversion of 47 to 48 represents a thermally allowed, concerted [ 2a + 2S] ring closure, in analogy to the pericyclic ring opening of bicyclo[1.1.0]butanes to give trans,trans-, 3-butadienes. 

In this way Hoffmann and Woodward have established the following simple rule a concerted m + n cycloaddition will be symmetry allowed in the ground state and symmetry forbidden in the excited state if m + n = 4q + 2 (q = 0, 1, 2...) if m + n = 4q the reaction will be symmetry allowed in the excited state and symmetry forbidden in the ground state. This rule applies to ms + ns cycloadditions and to ma + na processes. 

Reactions.—Nucleophilic Attack at Carbon. (/) Carbonyls. Methyl arylglyoxylates react with trisdimethylaminophosphine (TDAP) to form m-a/S-dimethoxycarbonyl-stilbene oxides.63 The initially formed zwitterion (61) reacts with a second molecule of the ester to form a fra/ -diphenyl-1,4,2-dioxaphospholan intermediate, which undergoes a concerted symmetry-allowed retrograde n2s + 4 cycloaddition to give a carbonyl ylide, conrotatory cyclization of which leads to the cw-oxirans (62) (Scheme 3). 

This system covers concerted reactions of the n electron systems on two reactants to form new a bonds yielding carbocyclic rings with a single unsaturation. If the reaction follows the rule of maximum orbital overlap, then it is a suprafacial, suprafacial process and is termed a [,r4 + r t] reaction. By the Woodward-Hoffmann rules this is a symmetry-allowed thermal reaction [13]. 

The other major reaction pathway for oxonium ylide is [l,2]-shift (Stevens rearrangement). Compared with [2,3]-sigmatropic rearrangement, which is an orbital symmetry-allowed concerted process, the [l,2]-shift has higher activation barrier, [1,2]-Shift is generally considered as stepwise process with radical pair as possible intermediates. 

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. 

More than one mechanism can account for the experimental observation of the Diels-Alder reaction.521,522,528 However, most thermal [4 + 2]-cycloadditions are symmetry-allowed, one-step concerted (but not necessarily synchronous) process with a highly ordered six-membered transition state.529 Two-step mechanisms with the involvement of biradical or zwitterion intermediates can also be operative.522,528... 

---