Protium–deuterium exchange of alkylated benzenes in dilute acid at elevated temperatures

Abstract

tetralin (16) have been labelled with deuterium by the high temperature -dilute acid (HTDA) method. In dilute DCl/D20 (0.14-0.27 M) at temperatures of 250-285"C, the ring hydrogens of these compounds equilibrate with the deuterium pool. At temperatures above 250°C the methyl groups of 3, 4, 5, 6 and 11 undergo slow HID exchange; the methyl groups of 8, 9, 10, and 13 undergo exchange much more readily and hexamethylbenzene (13) can be readily perlabelled. Hexamethylbenzene exhibits a lower reactivity than 13 at the benzylic sites.

Hexaethylbenzene: A Sterically Crowded Arene and Conformationally Versatile Ligand

“…Side-chain deuteration proceeded to am uch greater extent in HMB (96 %a t2 85 8Cf or 48 h) than for HEB (14 %a t2 70 8Cf or 44 h). [107] In the latter case, exchange occurred only at the methylene position, in accordance with the mechanism depictedi nS cheme 19.…”

“…A cognate mechanism has been advanced for the high‐temperature dilute acid hydrogen–deuterium exchange process in HMB and HEB. Side‐chain deuteration proceeded to a much greater extent in HMB (96 % at 285 °C for 48 h) than for HEB (14 % at 270 °C for 44 h) . In the latter case, exchange occurred only at the methylene position, in accordance with the mechanism depicted in Scheme .…”

“…[126] Later work on HMB by Birchall and Gillespie, in the more acidic medium fluorosulfuric acid, suggested that intermolecular exchange processes are also operative. [127] Although HEB has been protonated to form the corresponding [C 6 Et 6 H] + ion, as in the deuteration experiment described above, [107] we are unaware of ac omparable NMR exchange study to establish whether the hydrogen can migrate around the ring in as imilarf ashion to the behaviour of [C 6 Me 6 H] + .W e note, however,t hat such ap rocess could have ar elativelyh igh barrier, because to maintain its position on the same face of the HEB ring would require successive rotationso ft he ethyl substituents.…”

Overhauser Dynamic Nuclear Polarization with Selectively Deuterated BDPA Radicals

“…Investigation of the OE in 1,3-bisdiphenylene-2-phenylallyl (BDPA) radicals required the development of synthetic routes that would enable selective 2 H labeling of orthogonal positions on the fluorene moieties, i.e., 1, 3, 6, 8 and 2, 4, 5, 7. Conventionally, the deuteration of aromatic compounds requires harsh reaction conditions, costly catalysts, and/or extended reaction times as aromatic hydrogens are not labile and often lead to poor deuterium incorporation. − Efficient direct H/D exchange at alternating positions on arenes is typically achieved by using activating groups, specifically ortho/para directors such as phenols and anilines. − …”

Noncatalytic kinetic study on site-selective H/D exchange reaction of phenol in sub- and supercritical water

“…The reaction under hydrothermal conditions is a possible answer to the clean H/D conversion with desired site selectivity. The H/D exchange reaction under hydrothermal conditions have been investigated recently for various chemicals with or without catalysts, [25][26][27][28][29][30][31][32][33][34][35] as reviewed in brief by Savage. 16 Hot water is a convenient and effective reaction medium since it serves both as a reactive species and as a solvent by surrounding the reactant and lowering the activation energy of the transition state.…”