Wheland complexes substitution

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. 

In aromatic electrophilic substitutions, 60 corresponds to the Wheland complex. 

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. 

Electrophilic Aromatic Substitutions via Wheland Complexes ( 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). 

According to Section 5.1.1 electrophilic ipso substitutions via Wheland complexes occur, for example, when a proton attacks at the substructure Csp2—ferf-Bu or Csp2—S03H of appropriately substituted aromatic compounds. After expulsion of a ferf-butyl cation or an HSO3 ion, an aromatic compound is obtained, which has been defunctionalized in the respective position. 

Which Wheland complexes are the most stable This is determined to a small extent by steric effects and to a considerably greater extent by electronic effects As a car-bocation, a substituted Wheland complex is considerably more stable than an unsubstituted one only when it carries one or more donor substituents, and unsubstituted Wheland complexes E—C6Hj are still considerably more stable than Wheland complexes that contain one or more acceptor substituents. Therefore, donor-substituted benzenes are attacked by electrophiles more rapidly than benzene, and acceptor-substituted benzenes are attacked more slowly. 

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 ... 

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]. 

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]. 

The rate-determining step for the overall process of Eq. (5.13) is, as a rule, the formation of an adduct, i.e., the substituted cyclohexadienyl cation XLV or, much less frequently (e.g., in a sulfonation reaction), the decomposition of XLV. The XLV type cations were termed the <7-complexes or the Wheland complexes, although, as has recently been noted [127], the first to have pointed, in a well-founded manner, to the formation of ion intermediates in the electrophilic substitution reactions were, back in 1928, Pfeiffer and Wizinger [128]. 

M.o. theory and the transition state treatment In 1942 Wheland proposed a simple model for the transition state of electrophilic substitution in which a pair of electrons is localised at the site of substitution, and the carbon atom at that site has changed from the sp to the sp state of hybridisation. Such a structure, originally proposed as a model for the transition state is now known to describe the (T-complexes which are intermediates in electrophilic substitutions... 

As the o-complexes in these azo coupling reactions are steady-state intermediates (Wheland intermediates, named after Wheland s suggestion in 1942), their stereochemistry cannot be determined directly. Bent structures like that in Figure 12-6 can, however, be isolated in electrophilic substitutions of 1,3,5-triaminobenzene... 

Electrophilic substitution of pyrrole can, however, be carried out under specialised conditions (e.g. acylation with (MeC0)20/BF3, sulphonation with a pyridine/S03 complex, C5H5N-S03, cf. (67)) leading to preferential attack at the 2-, rather than the 3-, position. This reflects the slightly greater stabilisation of the Wheland intermediate for the former (70) compared with that for the latter (71) ... 

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