Pericyclic Mechanisms

Like the Diels-Alder reaction discussed in Sections 14.4 and 14.5, the Claisen rearrangement reaction takes place through a pericyclic mechanism in which a concerted reorganization of bonding electrons occurs through a six-membered, cyclic transition state. The 6-allyl-2,4-cyclohexadienone intermediate then isomerizes to o-allylpbenol (Figure 18.1). 

Propose a pericyclic mechanism to account for the following transformation ... 

Jones and coworkers200 found that a variety of sulphenic acids may be generated by thermolysis of the readily available /J-cyanosulphoxides (equation 81) and observed their highly regiospecific addition also to non-conjugated alkynes (Table 12). As expected for a pericyclic mechanism, the reaction afforded the product of a stereospecific cis-addition. However, the regioselectivity of the addition suggests that the partial carbon-sulphur bond in the transition state 148 is polarized in such a way that the carbon atom has some cationic character (equation 82). 

These reactions can take place by either heterolytic or pericyclic mechanisms. Examples of the latter are shown on page 1322. Free-radical P eliminations are extremely rare. In heterolytic ehminations, W and X may or may not leave simultaneously and may or may not combine. 

The HX compounds are electrophilic reagents, and many polyhalo and polycyano alkenes, (e.g., Cl2C=CHCl) do not react with them at all in the absence of free-radical conditions. When such reactions do occur, however, they take place by a nucleophilic addition mechanism, (i.e., initial attack is by X ). This type of mechanism also occurs with Michael-type substrates C=C—Z, where the orientation is always such that the halogen goes to the carbon that does not bear the Z, so the product is of the form X—C—CH—Z, even in the presence of free-radical initiators. Hydrogen iodide adds 1,4 to conjugated dienes in the gas phase by a pericyclic mechanism ... 

These reactions, called electrocyclic rearrangements, take place by pericyclic mechanisms. The evidence comes from stereochemical studies, which show a remarkable stereospecificity whose direction depends on whether the reaction is induced by heat or light. For example, it was found for the thermal reaction that cis-3,4-dimethylcyclobutene gave only cw,tran5-2,4-hexadiene, while the trans isomer gave only the trans-trans diene... 

Because of the nature of the transition state in the pericyclic mechanism, optically active substrates with a chiral carbon at C-3 or C-4 transfer the chirality to the product, making this an enantioselective synthesis (see p. 1451 for an example in the mechanistically similar Claisen rearrangement). ... 

An illustration of how the overall pericyclic mechanism might apply to the decomposition of 1,2 diphenylethane, a model substrate, in the presence of A1-dihydronaphthalene, a model hydrogen-donor, has recently been given (17). In the present work, attention is focussed on the hydrogen-transfer step. 

Allenyl ketones can undergo a Conia-ene cyclization, as shown in Eq. 13.56 [65]. Heating ketone 166 at 275 °C for 15 min leads to acetylcyclopentene 168 in 70% yield. Cyclization presumably takes place through a pericyclic mechanism involving enol 221. 

The formation of cyclic nitrones (150) from computational studies have provided evidence for the involvement of a novel pseudo-pericyclic electrocyclization in the conversion of o-vinylphenyl isocyanates into quinolin-2-ones. " Such reactions have also provided evidence of torquoselectivity in a 6jt system. Hash vacuum thermolysis of triazoles (151) has been found to afford dihydroquinolines (155), presumably by generation of a-oxoketenimines (152) which can undergo a [1,5]-hydrogen shift to the o-quinoid imines (153)7(154) and subsequent electrocyclization (see Scheme 57). 

Several mechanisms have been proposed for the reaction with singlet oxygen.221 One of these is a pericyclic mechanism, similar to that of the ene synthesis (5-16) and to the first... 

The Woodward-Hoffmann pericyclic reaction theory has generated substantial interest in the pathways of forbidden reactions and of excited state processes, beginning with a paper by Longuet-Higgins and Abrahamson,54 which appeared simultaneously with Woodward and Hoffmann s first use of orbital correlation diagrams.55 We have noted in Section 11.3, p. 586, that the orbital correlation diagram predicts that if a forbidden process does take place by a concerted pericyclic mechanism,56 and if electrons were to remain in their original orbitals, an... 

Organic reaction mechanisms may be classified into three main groups depending on the manner in which the covalent bonds are cleaved, namely (a) ionic mechanisms (b) radical mechanisms and (c) pericyclic mechanisms. 

In the disconnection approach, bonds are usually disconnected by either homolytic or heterolytic fission (Figure 10.11(a) and (b)). However, some bonds may be disconnected by a reverse pericyclic mechanism (Figure 10.11(c)). 

This chapter ends with a survey of the role of stereochemistry in the determination of mechanism. Though we have left stereochemistry to the last, it is one of the most important tools in unravelling complex mechanisms. You have already seen how inversion of configuration is a vital piece of evidence for an Sn2 mechanism (Chapter 17) while retention of configuration is the best evidence for participation (Chapter 37). You have seen the array of stereochemical evidence for pericyclic mechanisms (Chapters 35 and 36). The chapters devoted to diastereoselectivity (33 and 34) give many examples where the mechanism follows from the stereochemistry. We shall not go over that material again, but summarize the types of evidence with new examples. The first example looks too trivial to mention. 

The mechanism of 1,3-cycloaddition of nitrile oxides has been a subject of sometimes bitter controversy. While these reactions displayed the regioselec-tivity and stereoselectivity appropriate to concerted pericyclic mechanisms,62 it was suggested that the results were equally in accord with a diradical mechanism.86 However, no direct evidence for a diradical intermediate has been produced, and the balance of the theoretical argument now favors the... 

There have also been a few examples of ring expansions involving azetines. One such instance, which follows a pericyclic mechanism, is the thermolysis of 4-cyano-l-t-octyl-3-t-octylamino-2-t-octyliminoazetine (160), producing the 4-amino-5-cyanoimidazole (Scheme 89). Such azetine species have been implicated as intermediates in the photolysis of enaminonitriles to imidazoles (Section 4.08.1.1.1 Scheme 17). In strongly basic medium the azetidinone (161), which possesses a lactam group, is ring expanded to the 4H-imidazolinone (163), probably via the anionic acylic species (162 Scheme 89) <80AHC(27)241>. 

Two studies on the mechanism of this type of [4 + 2] cycloaddition which have led to very di erent interpretations have appeared. Mock and Nugent suggested that the Diels-Alder reactions of N-sulfi-nyl-p-toluenesulfonamide are stepwise, ionic processes. On the other hand, Hanson and Stockbum prefer a concerted, pericyclic mechanism in accord with frontier molecular orbital theory. Both proposals satisfactorily rationalize the observed regioselectivity of these reactions. 

A stmctural variation of azodicarboxylate is arenediazocyanide (151) prepared from the corresponding diazonium salt (equation 61)." Dienophiles of this class add regioselectively to 1,3-dienes in a Diels-Alder sense. This selectivity is consistent with both stepwise polar and concerted pericyclic mechanisms. 

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