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Dediazoniations of Arenediazonium Ions. Part XXI. Dediazoniation of Arenediazonium Ions Complexed with Crown Ethers
Author(s) -
Nakazumi Hiroyuki,
Szele Ivanka,
Yoshida Katsuhira,
Zollinger Heinrich
Publication year - 1983
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19830660611
Subject(s) - chemistry , heterolysis , reaction rate constant , substituent , crown ether , ion , medicinal chemistry , hammett equation , kinetics , computational chemistry , organic chemistry , physics , quantum mechanics , catalysis
The evaluation of the dediazoniation kinetics of various m ‐ and p ‐substituted benzenediazonium tetrafluoroborates in 1,2‐dichloroethane at 50° in the presence of 18‐crown‐6, 21‐crown‐7 and dicyclohexano‐24‐crown‐8 demonstrates that the rate constant for the dediazoniation within the complex ( k 2 ) is smallest, and the equilibrium constant for complex formation (K) is largest for the complexes with 21‐crown‐7 (cf. Scheme 1) . The logarithms of the equilibrium constants (K) for complex formation with each of the crown ethers studied correlate well with Hammett 's substituent constants, σ, to give reaction constants ρ = 1.18–1.38. A linear correlation between the logarithms of the rate constants for the dediazoniation within the complex with those of the dediazoniation rate constants of uncomplexed diazonium ions (log k 2 vs. log. k 1 ), found for most substituted diazonium salts, indicates that the dediazoniation mechanism of the complexed diazonium ions is not significantly different from that of the free ions. For very electrophilic diazonium ions ( p ‐Cl, m ‐CN), k 2 was much larger than expected on the basis of the linear log k 2 vs. log k 1 relationship. Analysis of the dediazoniation products showed that this was due to a change in mechanism from heterolytic to homolytic dediazoniation. The complexation rate of diazonium salts by crown ethers ( k c ) is practically diffusion controlled and does not change much with the size of the crown ether. The decomplexation rate ( k d ), however, is significantly lower for complexes with 21‐crown‐7, than for those with 18‐crown‐6 and dicyclohexano‐24‐crown‐8, and is therefore the reason for the variations in the equilibrium constant (K) and thus for the fact that complexes of arenediazonium salts with 21‐crown‐7 are the most stable. The amounts of the N α ‐N β rearrangement, as well as those of the exchange of the 15 N‐labelled diazonio group with external nitrogen during dediazoniation of p ‐toluenediazonium salt were independent of the addition of crown ethers. A dediazoniation mechanism involving a charge transfer, as well as an insertion‐type diazonium ion‐crown ether complex is proposed. In this mechanism, dediazoniation of the insertion complex does not take place directly, but through the charge‐transfer complex.