Wheland complexes reactions

It should be pointed out that the existence of stable structures of the intermediate-complex type (also known as a-complexes or Wheland complexes) is not of itself evidence for their being obligate intermediates in aromatic nucleophilic substitution. The lack of an element effect is suggested, but not established as in benzene derivatives (see Sections I,D,2 and II, D). The activated order of halogen reactivity F > Cl Br I has been observed in quantita-tivei36a,i37 Tables II, VII-XIII) and in many qualitative studies (see Section II, D). The reverse sequence applies to some less-activated compounds such as 3-halopyridines, but not in general.Bimolecular kinetics has been established by Chapman and others (Sections III, A and IV, A) for various reactions. 

The accepted reaction mechanism for the electrophilic aromatic nitration was postulated by Ingold in 1969[3] and involves several steps (Scheme 5.1). Firstly, the nitric acid is protonated by a stronger acid (sulfuric). The protonated nitric acid gives water and the nitronium ion (N02+) which is the electrophilic active species for nitration of aromatics. Nitric acid heterolysis is considered to be accelerated by the polarity of the solvent, and solvation of nitronium ion in different media affects its reactivity and the selectivity of the reaction. Combination of nitronium ion and an aromatic molecule form an intermediate named the Wheland complex or er-complex. The loss of a proton from the er-complex gives the aromatic nitrocompound (Scheme 5.1). 

The electrophilic aromatic substitution via sigma (Wheland) complexes, or the Ar-SE reaction, is the classical method for functionalizing aromatic compounds. In this section, we will focus on the mechanistic foundations as well as the preparative possibilities of this process. 

In the first step of the actual Ar-SE reaction, a substituted cyclohexadienyl cation is formed from the electrophile and the aromatic compound. This cation and its derivatives are generally referred to as a sigma or Wheland complex. Sigma complexes are described by at least three carbenium ion resonance forms (Figure 5.1). There is an additional resonance form for each substituent, which can stabilize the positive charge of the Wheland complex by a pi electron-donating (+M) effect (see Section 5.1.3). This resonance form is an all-octet formula. 

There is also some debate over whether nitrations like the one in the last example may alternatively proceed via a nitrosyl cation (NO ) instead of a HN03 molecule as the electrophile. Small amounts of the nitrosyl cation occur in diluted nitric acid and would—via a Wheland complex intermediate—initially lead to a nitrosoaromatic compound (Ar-N=0) as the Ar-SE product. (Remember Figure 5.23 presented a different reaction mode between nitrosyl cations and—less electron-rich—aromatic compounds.) This nitrosoaromatic compound would subsequently undergo rapid oxidation by the diluted nitric acid to finally yield the nitroaromatic compound. 

Electrophilic Aromatic Substitutions via Wheland Complexes ( Ar-SE Reactions )... 

Ar-SE reactions (cf. Figure 5.1). This, in turn, means that Wheland complexes are also a good—even the best—model for the transition state of Ar-SE reactions. 

In a few cases, cations other than the proton are eliminated from the Wheland complex to reconstitute the aromatic system. The ferf-butyl cation (Figure 5.1, X=ferf-Bu) and protonated S03 (Figure 5.1, X=S03H) are suitable for such an elimination. When the latter groups are replaced in an Ar-SE reaction, we have the special case of an ipso substitution. Among other things, ipso substitutions play a role in the few Ar-SE reactions that are reversible (Section 5.1.2). 

Because of completely analogous considerations, every acceptor-substituted Wheland complex E—C6H5—EWG+ is less stable than the reference compound E—C6Hg (Figure 5.9). From this analysis, one derives the following expectations for Ar-SE reactions of acceptor-substituted benzenes ... 

The regioselectivity and reactivity of Ar-SE reactions of naphthalene are explained correctly by comparing the free activation enthalpies for the formation of the Whe-land complexes 1-E—C10Hi"0 and 2-E—C10H 0 from the electrophile and naphthalene and for the formation of the Wheland complex E—C6Hg from the electrophile and benzene, respectively. 

The regioselectivity of Ar-SE reactions with naphthalene follows from the different stabilities of the Wheland complex intermediate of the 1-attack (Figure 5.10, top) compared with that of the 2-attack (Figure 5.10, bottom). For the Wheland complex with the electrophile at Cl these are five sextet resonance forms. In two of them the aromaticity of one ring is retained. The latter forms are thus considerably more stable than the other three. The Wheland complex with the electrophile at C2 can also be described with five sextet resonance forms. However, only one of them represents an aromatic species. The first Wheland complex is thus more stable than the second. The 1-attack is consequently preferred over the 2-attack. 

Aryldiazonium salts are weak electrophiles. Consequently, they undergo Ar-SE reactions via Wheland complexes (azo couplings) only with the most strongly activated aromatic compounds. Phenolates and secondary as well as tertiary aromatic amines are therefore solely attacked. Primary aromatic amines react with diazonium salts, too, but via their N atom. Thus, triazenes, that is, compounds with the structure Ar—N=N—NH—Ar are produced. Phenol ethers or nondeprotonated phenols can be attacked by aryldiazonium salts only when the latter are especially good electrophiles, for example, when they are activated by nitro groups in the ortho or para position. 

Reaction of pentafluorophenol with rerf-butyl hypobromite starts as an electrophilic substitution in the benzene ring. The electrophile is formed by dissociation of tert-butyl hypobromite to tert-butoxy anion and bromine cation. The bromine cation attacks the para position and forms a positively charged Wheland complex, a nonaromatic species that is converted by ejection of proton from the phenolic hydroxyl to a quinon-oid compound, 4-bromopentafluorocyclohexa-2,5-dienone [39]. 

This important stabilization has other consequences on the reaction pathway of isomerization (see Figure 11). The investigation of the activation of toluene by proton attack reveals a completely different picture than the one obtained with the cluster approach. The protonation step of toluene becomes an inflection point in the reaction pathway, which gives as product a metastable Wheland complex. 

Two charge-transfer complexes are formed during the course of this reaction.261 The first will be 340 and the second (341) is generated during loss of the proton in the aromatization process. In the chlorination reaction, the rate-determining step (the slowest) is conversion of the charge transfer complex to the Wheland ctimplex (340 - complex).269 Fonuation of 340. conversion of the Wheland complex to 341, and elimination of the proton are all fast processes. 

The proposed [43, 44e] mechanism of the reaction between PtCla and the arene to afford/ ara and meta isomers of the a-arylplatinum(IV) complex (VII-14 and VII-14m in Scheme VII.8) involves the formation of a weak 7i-arene complex of platinum(IV) VII-11 which is transformed into an intermediate Wheland-type complex. The isomerization of the Wheland complexes VII-12p and VH-12/n possibly proceeds though the transition state VII-13, and/or is due to the reversibility, to some extent, of the stage of VII-12 formation. The transarylation mentioned above (equation VII.6) is perhaps also due to the reversibility of the Wheland complex formation. 

The electrophilic substitution of hydrogen in arenes is known both for nontransition - Hg(ll), Tl(lll), Pb(IV) - and transition metals, such as Au(lll), Pd(II), Pt(IV), Rh(III). All these reactions apparently proceed with the intermediate formation of Wheland complexes. Some parameters for these reactions are summarized in Table Vlll. 1 [2]. 

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