Reactions of Diarylnitrenium Ions with Electron Rich Alkenes:  An Experimental and Theoretical Study

Abstract

Photolysis of N-(diphenylamino)-2,4,6-trimethylpyridinium tetrafluoroborate (1a) and N-[bis(4-methylphenyl)amino]-2,4,6-trimethylpyridinium salt (1b) gives products attributable to diarylnitrenium ion (Ar(2)N(+), 2). The major products of these reactions include products from nucleophilic addition of various pi-nucleophiles (e.g. electron rich alkenes) to the ortho and para positions of one of the phenyl rings. Nanosecond and EPR spectroscopy show that radicals also form. These radicals are thought to give rise to the diarylamines isolated as minor products from the photolysis of 1aand 1b. In addition to the para addition products and Ph(2)NH, N-phenylindoles and N-phenylindolinones are isolated when silyl enol ethers and silyl ketene acetals are used as trapping agents, respectively. The indoles and indolinones are generated from initial addition of the nucleophile to the ortho position on 2 followed by cyclization of the resulting intermediate. A product resulting from N addition of the nucleophile to 2 is isolated only when silyl ketene acetals are used. A number of electronic sturcture calculations at different levels of molecular orbital and density functional theory were carried out on Ph(2)N(+). There do not seem to be effects associated with either the charge distribution or the LUMO that would strongly influence ortho/para/N selectivity in nucleophilic trapping. Laser flash photolysis on 1a provides absolute rate constants for the nucleophilic addition of various alkenes to Ph(2)N(+). These fall in the range of 10(9)-10(10) M(-)(1) s(-)(1) and correlate with the oxidation potential of the alkene. From these data it is clear that the more easily oxidized the alkene the faster it will react with Ph(2)N(+).

Chemical trapping experiments support a cation-radical mechanism for the oxidative polymerization of aniline

“…Again, all attempts failed to give reasonable molecular weight polymers. 39 This and our earlier observations show that aniline does not polymerize chemically through an electrophilic substitution reaction by nitrenium ion.…”

“…34 Nitrenium ion has long been known to react even with benzene or toluene to form a diarylamine, [35][36][37][38] or with water to form p-aminophenol. 39 Our attempted polymerization experiments with N-arylhydroxylamines (below) have also confirmed that nitrenium ion is a reactive species which is difficult, if not impossible, to be used as an intermediate to form clean polymers.…”

Singlet and triplet states in the reactions of nitrenium ions

“…More recent attention has turned to the diphenylnitrenium ion (Ph 2 N ). 48,49 Although an extensive analysis of its singlet-triplet state dynamics is not complete, it is clear that the major decay pathways occur from the singlet state. For example, Ph 2 N reacts rapidly with nucleophiles such as halides, alcohols, water and amines giving adducts analogous to 5.…”

Synthesis and crystal structure of 1-ethyl-3-(phenyl)-1,2,3-triazolium percholorate

“…From experimental data and theoretical calculations it emerged that these molecules are stabilized by electronic delocalization [9]. Intramolecular rearrangement reaction of nitrenium ions have been reported, and established as useful intermediates in wide variety of biological applications [10]. In our previous work, we described the synthesis of some stable nitrenium ions [8] and their comparison study with structurally related carbenes and found that stable nitrenium ions (as their carbene analogues) are electronically di®erent from non stable ones.…”