1999 R.U. Lemieux Award Lecture Adventures with azo-, azoxy-, and hydrazoarenes: from the Wallach to the benzidine rearrangement. Molecular electronics
Authors: Buncel Erwin 
Published: April 2011

The author's studies with aromatic azo-, azoxy-, and hydrazo-dye molecules, comprising kinetic and equilibrium investigations, as well as synthesis of novel molecules having photogenic properties, are described under the following highlights: A. Wallach rearrangement and cognate studies with azoxyarenes  (1) Elucidation of the mechanism of the Wallach rearrangement of azoxybenzene through the kinetic observation of a two-proton process which, together with a pKa study, was interpreted on the basis of formation of a deoxygenated, dicationic, symmetrical species as a key, short-lived reaction intermediate. (2) The proposal of a general acid-catalyzed pathway in concentrated sulfuric acid (catalysis by H2SO4 and H3SO+4. (3) Elucidation of the consecutive sulfonations of reaction products of azoxybenzene in the 100% H2SO4 region, and the diprotonation equilibria for p-hydroxyazobenzene, thus shedding light on past reaction pathway and product studies. (4) The observation of a novel reaction pathway for 2,4,6,2',4',6'-hexamethy lazoxybenzene. (5) The observation of a dichotomy of reaction pathways for α- and β-2-phenylazoxynaphthalenes: reaction via the dicationic intermediate and via quinoid intermediate species; comprising two isomeric compounds reacting by different pathways to give the same product. (6) Identification and structure proof of α- and β-isomers observed for the first time in the peracid oxidation of phenylazopyridine. (7) Observation of a rate constant ratio of 22 000 in the rearrangement of these α- and β-isomers, and the proposal of differential barriers for transition states leading to a tricationic intermediate. B. Benzidine rearrangement and cognate studies  (8) Observation of the acid-catalyzed hydroxylation of phenylazopyridine to p-hydroxyphenylazopyridine and the proposal of an SNAr mechanism with formation of an intermediate hydrazo species in the reaction. (9) First study of benzidine type rearrangement-disproportionation of phenylhydrazopyridine in acid media. (10) Proposal of a A [Formula: see text] B [Formula: see text] C [Formula: see text] D type reaction profile for the consecutive hydroxylation[Formula: see text]disproportionation processes of phenylazopyridine in aq H2SO4. (11) Proposal of 10-π and 14π-electron electrocyclic processes in the benzidine type rearrangement-disproportionation of phenylhydrazopyridine. (12) Identification and structural elucidation of a dimer formed from phenylazopyridine as a minor product and proposal of a reaction mechanism. C. Facile acid-catalyzed demethylation via SNAr/A-SE2 mechanisms and studies of tautomerism  (13) Observation of an abnormally facile acid-catalyzed cleavage (demethylation) of 4-methoxyphenylazopyridine via an SNAr mechanism. (14) Observation of two reaction pathways, SNAr and A-SE2, for the consecutive demethylations of 3,4-dimethoxyphenylazopyridine, with rate constant ratio of 7 000:1 favoring the SNAr process. (15) Quantitation of the tautomeric and protonation equilibria of 4-hydroxyphenylazopyridine, produced in (13). D. A new solvent polarity scale, molecular switches, and molecular electronics  (16) Establishment of a π*azo solvent polarity scale based on solvatochromism of a series of azomerocyanine molecules ("Buncel's dye"). (17) Some glimpses are presented of current forays into molecular electronics, as emanating from the above studies: (a) spiropyran (SP) <> merocyanine (MC) thermo- and photochromic "molecular switch" systems; (b) synthesis and characterization of azo-functionalized star-burst dendrimers with photoswitchable properties and potential applications in optical data storage systems, holographic gratings, and drug delivery systems as host molecules.Key words: Wallach rearrangement, benzidine disproportionation, azoarenes, azoxyarenes, dendrimers, hydrazoarenes, dendrimers, solvatochromism, photochromism, thermochromism, spiropyran-merocyanine molecular switch.