Additions of Other Nucleophiles to Arenediazonium Ions

As discussed in Sections 5.1-5.3, arenediazonium ions are Lewis acids in which the (3-nitrogen forms the center of electrophilic character. This was demonstrated by the addition of hydroxide ions and water molecules. Other nucleophiles can also be added and, in principle, these reactions display the same mechanistic characteristics as those with OH and H20. According to the nature of the atom of the nucleophile that provides the lone pair of electrons, O-, S-, Se-, N-, P-, or C-coupling can occur. With N- and C-coupling, important and large groups of compounds are formed, namely azo compounds (mainly important as azo dyes) and triazenes, respectively. These compounds will be discussed in Chapters 12 and 13, respectively. 

Diazo Chemistry I Aromatic and Heteroaromatic Compounds. By Heinrich Zollinger Copyright 1994 VCH Verlagsgesellschaft mbH ISBN 3-527-29213-6 

With alkoxide and phenoxide ions diazo ethers are formed. The latter played a considerable part in the classic controversy about diazo isomerism between Hantzsch and Bamberger at the turn of the 19th century (see Sec. 1.1). Von Pechmann and Frobenius (1894 a) showed that the most convenient synthesis for preparative pur- 

An analogous regioselective effect of silver ions on an addition reaction of arene-diazonium ions was found by Ignasiak et al. (1975) with cyanide salts. Potassium cyanide yields diazocyanides (Ar— N2 — C = N see Sec. 6.6), i.e., C-coupling products, but with silver cyanide - albeit in low yield (7-9%) - diazoisocyanides (Ar—N2 — N = C) are formed (a better synthesis of diazoisocyanides is described in Sec. 6.4). 

It is very interesting, however, that in alkane potassium diazoate alkylations with Meerwein s reagent (triethyloxonium tetrafluoroborate, Et30+BF4) in CH2C12 suspensions or with alkyl halides in hexamethylphosphoric triamide solutions, azoxy compounds (6.4) are formed, i.e., alkylation takes place at the (3-nitrogen (Moss et al., 1972). 

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The volumes of activation for some additions of anionic nucleophiles to arenediazonium ions were determined by Isaacs et al. (1987) and are listed in Table 6-1. All but one are negative, although one expects — and knows from various other reactions between cations and anions — that ion combination reactions should have positive volumes of activation by reason of solvent relaxation as charges become neutralized. The authors present various interpretations, one of which seems to be plausible, namely that a C — N—N bond-bending deformation of the diazonium ion occurs before the transition state of the addition is reached (Scheme 6-2). This bondbending is expected to bring about a decrease in resonance interaction in the arenediazonium ion and hence a charge concentration on Np and an increase in solvation. 

The fundamental understanding of the diazonio group in arenediazonium salts, and of its reactivity, electronic structure, and influence on the reactivity of other substituents attached to the arenediazonium system depends mainly on the application of quantitative structure-reactivity relationships to kinetic and equilibrium measurements. These were made with a series of 3- and 4-substituted benzenediazonium salts on the basis of the Hammett equation (Scheme 7-1). We need to discuss the mechanism of addition of a nucleophile to the P-nitrogen atom of an arenediazonium ion, and to answer the question, raised several times in Chapters 5 and 6, why the ratio of (Z)- to ( -additions is so different — from almost 100 1 to 1 100 — depending on the type of nucleophile involved and on the reaction conditions. However, before we do that in Section 7.4, it is necessary to give a short general review of the Hammett equation and to discuss the substituent constants of the diazonio group. 

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