Hydrogen–carbon, carbon–carbon double rearrangement induced by proximity effects. Part 3: Essential structural requirements in methoxydiphenylmethanes

Abstract

The hydrogen-carbon, carbon-carbon double rearrangement, responsible for the formation of benzyl or tropylium ions previously observed in the electron ionization mass spectra of several substituted 1,lbis(dimethoxydipheny1)methanes bearing an ortho methoxy group, also occurs for 2-methoxydiphenylmethane (1) itself, the simplest compound of the series. It was shown using deuterium and ''C labelled 2methoxydiphenylmethanes (2, 3) that in this instance benzyl or tropilium ions are also formed through two alternative fragmentation routes.

Studies in organic mass spectrometry. Part 25. Benzyl ion formation in chemical ionisation (methane or isobutane) of someortho-alkylhetero-substituted diphenylcarbinols

“…In order to ascertain whether the peak at m/z 91 of 1 and 2 corresponds to the rearrangement ion b (Scheme ), we examined the CI (methane) mass spectra of isotopomers 1d 3 and 2d 3 (Figs 1(b) and 2(b), respectively). The complete shift to m/z 93 confirms the formation of the ion b by the double rearrangement reaction of diphenylmethyl cations, previously observed in electron ionisation mass spectra 1–4. The peaks due to anisole loss, at m/z 107 and 121 for 1 and 2 , respectively, are not shifted in the spectra of the isotopomers 1d 3 and 2d 3 .…”

“…It has been reported that diphenylmethyl cations formed by the benzylic cleavage of the molecular ions of 1,1‐diphenylalkanes bearing, in the ortho position, an alkoxy, alkyl‐ or dialkylamino or alkylsulfanyl group undergo a double rearrangement process involving consecutively a H‐to‐C migration followed by a C‐to‐C displacement reaction1–4. The overall process affords benzyl (or tropylium) cations through migration of an alkylidene residue of the ortho ‐alkylhetero group to the other aromatic ring (Scheme ), and this constitutes a pathway for benzyl (or tropylium) cation formation not previously reported 5,, 6…”

“…This rearrangement has been studied in a series of compounds and it has been determined that the presence of an ortho ‐alkylhetero group alone constitutes the fundamental structural requirement for the occurrence of the reaction 3. The process even occurs for a series of derivatives bearing bulky groups linked to the benzyl carbon.…”

Substituent effects on the formation of benzyl ions from ortho-methoxy substituted 1,1-diarylalkanes under electron ionization: correlations between the abundance of the process and the 13C-NMR chemical shifts of the neutral precursors

“…2 The isotopic effect indicates that the hydrogen migration affording the rearrangement ion a' is the rate-determining process, and the close values in 70 eV-(k H /k D = 1.4), 1,3 and in MIKE-(k H /k D = 1.6), 2 spectra agree with a loose transition state for the hydrogen migration. 8 In order to achieve detailed mechanistic insight and information on the arrangement of the transition state related to the process involving the hydrogen migration, the determination of substituent effects on the abundance of the benzyl ion formation process could be used as a valuable tool.…”

“…2,3 It has been unequivocally demonstrated by experiments with 2 H-and 13 C-labeled compounds that the whole process of benzyl ion formation involves migration of the methylene residue of the methoxy group linked to the 2-position of an aromatic ring (ring A) to the other aromatic ring (ring B). [1][2][3][4] Further investigation revealed that such a reaction occurs also for ortho-alkoxy derivatives (OEt, OiPr) by migration of an alkylidene residue other than methylene, as well as for ortho-alkyl-hetero (NHMe, NMe 2 , SMe) 1,1-diarylalkanes. 4 Finally, it has been determined that the formation of the benzyl-(or tropylium) ions b constitutes the main unimolecular decomposition reaction of ortho-alkylhetero-substituted diaryl-or alkyldiaryl-or triaryl-methyl cations, a, generated either under electron ionization 5 Step 1…”