Suprafacial, Woodward-Hoffmann rules

As we saw in 15-58, the Woodward-Hoffmann rules allow suprafacial concerted cycloadditions to take place thermally if the total number of electrons is 4n 4- 2 and... 

Cycloadditions of ketenes and alkenes have synthetic utility for the preparation of cyclobutanones.163 The stereoselectivity of ketene-alkene cycloaddition can be analyzed in terms of the Woodward-Hoffmann rules.164 To be an allowed process, the [2ir + 2-tt] cycloaddition must be suprafacial in one component and antarafacial in the other. An alternative description of the TS is a 2irs + (2tts + 2tts) addition.165 Figure 6.13 illustrates these combinations. Note that both representations predict formation of the d.v-substituted cyclobutanone. 

Thermal concerted 2 + 2 reactions are predicted to occur between an alkene and a ketene. According to Woodward-Hoffmann rules, addition must be suprafacial to one component and antarafacial to the other if the process is to be concerted. 

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 suprafacial thermal addition of an allylic cation to a diene (a 3 + 4 cycloaddition) is allowed by the Woodward-Hoffmann rules (this reaction would be expected to follow the same rules as the Diels-Alder reaction1095). Such cycloadditions can be carried out1096 by treatment of a diene with an allylic halide in the presence of a suitable silver salt, e.g,1097... 

The Woodward-Hoffmann rules predict high activation energies for the suprafacial-suprafacial addition of two carbon-carbon double bonds, these may be lowered by polar effects (74AF(3)75i). [2 + 2]... 

So far we have only defined what suprafacial and antarafacial mean on rc-systems (Fig. 2.7), but we need to see how o-bonds are treated by the Woodward-Hoffmann rules. Just as a suprafacial event on a rc-bond has overlap developing to the two upper lobes that contribute to the bond, so with o-bonds (Fig. 3.5a), overlap that develops to the two large lobes of the sp3... 

The Woodward-Hoffmann rules also allow the prediction of the stereochemistry of pericyclic reactions. The Diels-Alder reaction is an example of (re4s + re2s) cycloaddition. The subscript s, meaning suprafacial, indicates that both elements of the addition take place on the same side of the re-system. Addition to opposite sides is termed antarafacial. The Woodward-Hoffmann rules apply only to concerted reactions and are derived from the symmetry properties of the orbitals involved in the transition state. These rules may be summarised as shown in Table 7.1. 

The possibility of rearrangement in pentadienyl anions must be borne in mind when they are employed synthetically. When 1- or 5-alkyl groups are present, intramolecular 1,6-sigmatropic hydrogen shifts are possible and the stereochemistry follows Woodward-Hoffmann rules, being thermally antara-facial but photochemically suprafacial. Bates, for example, showed that the same equilibrium mixture of isomers results at 40°C from the deprotonation of either 5-methyl-1,4-hexadiene or 2-methyl-1,4-hexadiene (79). The tendency is to form isomers with fewer alkyl groups in the 1,3, and 5 positions of the delocalized system (50). 

Whether the reaction is supra- or antarafacial ought to be reflected in the relative stereochemistry of the cyclized products—and indeed it is. This reaction gives solely the diastereoisomer on the left, with the methyl groups syn—clear proof that the reaction is suprafacial. This is a difficult result to explain without the enlightenment provided by the Woodward-Hoffmann rules ... 

The Woodward-Hoffmann rules predict high activation energies for the suprafacial-suprafacial addition of two carbon-carbon double bonds, which can be lowered, however, by polar effects. [2 + 2] Photocycloadditions are common and usually involve diradical intermediates e.g., photoexcited ketones react with a variety of unsaturated systems (Scheme 1). Both the singlet and triplet (n, 7t ) excited states of the ketones will form oxetanes with electron-rich alkenes. With electron-deficient alkenes only the singlet states give oxetanes. Diradicals are the immediate precursors to the oxetanes in all cases, but the diradicals are formed by different mechanisms, depending on the availability of electrons in the two components. 

Group Transfer Reactions. There are so few of these reactions that a fully general rule for them can wait until the next section, where we see the final form of the Woodward-Hoffmann rules. For now, we can content ourselves with a simplified rule which covers almost all known group transfer reactions. When the total number of electrons is a (4 +2) number, group transfer reactions are allowed with all-suprafacial stereochemistry. 

Then there is the problem of assessing whether the reaction is symmetry-allowed or not using the Woodward-Hoffmann rule. All reactions using (An + 2) electrons (an odd number of curly arrows) are allowed in the all-suprafacial mode, and so it is helpful to draw the dashed or solid lines (or better still use a line with a distinctive colour) to show the developing overlap with only suprafacial components. The (4q + 2)s components will then add up to an odd number, and the task is done. 

Under thermal conditions, the [2+2]-cycloaddition of olefins is symmetrically forbidden, according to the Woodward-Hoffmann rules. However, under photochemical conditions, [2+2]-cycloadditions become a suprafacial process for both components The orbital geometry of the interacting orbitals is equal and therefore the entire reaction is symmetrically allowed. 

Sigmatropic shifts represent another important class of pericyclic reactions to which the Woodward-Hoffmann rules apply. The selection rules for these reactions are best discussed by means of the Dewar-Evans-Zimmerman rules. It is then easy to see that a suprafacial [1,3]-hydrogen shift is forbidden in the ground state but allowed in the excited state, since the transition state is isoelectronic with an antiaromatic 4N-HQckel system (with n = 1), in which the signs of the 4N AOs can be chosen such that all overlaps are positive. The antarafacial reaction, on the other hand, is thermally allowed, inasmuch as the transition state may be considered as a Mobius system with just one change in phase. 

The reaction of ketenes with alkenes is assumed to occur via a concerted nonsynchronous mechanism, where the approach of the reacting partners is orthogonal. " As a consequence, the bulkier substituent of the ketene will end up on the sterically more crowded face of the cyclobutanone product. There are two descriptions that explain the experimental results 1) according to the Woodward-Hoffmann rules, the LUMO of the ketene reacts antarafacially with the HOMO of the alkene that reacts suprafacially " 2) the HOMO of the alkene forms a bond with the pz orbital of... 

The Woodward-Hoffmann rules for electrocyclic reactions can also be formulated using the terms suprafacial and antarafacial (Table 4.3). A it system is said to react suprafacially in a pericyclic reaction when the bonds being made to the two termini of the it system are made to the same face of the 77 system. It reacts an-tarafacially when the bonds are made to opposite faces of the 7r system. In electrocyclic reactions, disrotatory reactions are suprafacial, and conrotatory reactions are antarafacial. 

The Woodward-Hoffmann rules for cycloadditions (Table 4.4) are as follows. Both components of a cycloaddition involving an odd number of electron pairs are suprafacial under thermal conditions under photochemical conditions, one component must be antarafacial. Both components of a cycloaddition involving an even number of electron pairs are suprafacial under photochemical conditions under thermal conditions, one component must be antarafacial. 

The application of the Woodward-Hoffmann rules to cheletropic reactions is not straightforward. In the [2+1] cycloaddition of singlet carbenes to alkenes, the stereochemistry of the alkene is preserved in the product, so the alkene must react suprafacially. The Woodward-Hoffmann rules suggest that the carbene component of this thermal, four-electron reaction must react antarafacially. However, what this means for a species lacking a 77 system is difficult to interpret. 

Thermal [1,3] H shifts such as the conceited rearrangement of enols to carbonyl compounds are disallowed. The allylic C-C-O unit itself can only react suprafa-cially, as it is geometrically impossible for the H(ls) orbital to bond simultaneously to a top lobe on one terminus and a bottom lobe at the other terminus, and the H atom itself must also react suprafacially, as the H(ls) orbital has only one lobe. The Woodward-Hoffmann rules, though, say that one of the two components of this four-electron rearrangement must react antarafacially for it to be allowed. Therefore this rearrangement reaction always proceeds through a nonconcerted mechanism and requires acidic or basic catalysis. 

Problem 4.17. A small amount of 4-allylphenol is often obtained from the Claisen rearrangement of allyl phenyl ether. Draw a concerted mechanism for this reaction, name the mechanism, and determine whether the suprafacial-suprafacial rearrangement is thermally allowed or disallowed by the Woodward-Hoffmann rules. If it is not allowed, draw a multistep mechanism for the reaction. 

All the reactions we have looked at so far involve only the geometrically easy, and hence commonly observed, suprafacial processes. If we restrict ourselves to all-suprafacial processes, we may note that the allowed reactions involve an aromatic (4n + 2) number of electrons, and the forbidden reactions an antiaromatic (4n) number of electrons. (The number of electrons involved is easily counted it is twice the number of curly arrows.) This is the simplest version of the Woodward-Hoffmann rules it was first pointed out by Evans126 in 1939 and recalled many years later by Dewar.127... 

There is a special kind of site-selectivity which has been called periselectivity. When a conjugated system enters into a reaction, a cycloaddition for example, the whole of the conjugated array of electrons may be mobilized, or a large part of them, or only a small part of them. The Woodward-Hoffmann rules limit the total number of electrons (to 6, 10, 14 etc. in all-suprafacial reactions, for example), but they do not tell us which of 6 or 10 electrons would be preferred if both were feasible. Thus in the reaction of cyclopentadiene (355) and tropone (356), mentioned at the beginning of this book, there is a possibility of a Diels-Alder reaction, leading to 354, but, in fact, an equally allowed, ten-electron reaction is actually observed,121 namely the one leading to the adduct (357). The product is probably not thermodynamically much preferred to the... 

According to the Woodward-Hoffmann rules, the suprafacial apical-equatorial fragmentation cannot occur by a concerted pathway. It will require either a zwitterionic or a biradical transition state in order to take place. 2... 

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