Benzylic C−H Functionalisation by [Et₃SiH+KO^tBu] leads to Radical Rearrangements in o‐tolyl Aryl Ethers, Amines and Sulfides

Abstract

Reaction of Et 3 SiH + KO t Bu with diaryl ethers, sulfides and amines that feature an ortho alkyl group leads to rearrangement products. The rearrangements arise from formation of benzyl radicals, likely formed through hydrogen atom abstraction by triethylsilyl radicals. The rearrangements involve cyclisation of the benzyl radical onto the partner arene, which, from computation, is the rate determining step. In the case of diaryl ethers, Truce-Smiles rearrangements arise from radical cyclisations to form 5-membered rings, but for diarylamines, cyclisations to form dihydroacridines are observed.

Et3SiH + KOtBu provide multiple reactive intermediates that compete in the reactions and rearrangements of benzylnitriles and indolenines

“…Notably, it can add to arene rings in the substrate to generate intermediates 49 that feature a labile H atom. [2][3][4][5][6]13 This can react with a silyl radical 45 to form trimethylsilane or with silyl anion 46 to form trimethylsilane as shown in Scheme 5. This would mean that the missing trialkylsilane reagent (Me 3 SiH in this experiment) would be generated in situ, starting from the disilane.…”

“…We have recently shown that benzylic C-H bonds can undergo abstraction of an H-atom under the conditions of these reactions, by triethylsilyl radicals 1. 13 In this case, this would lead to radical 105 (Scheme 12). Cyclopropylcarbinyl radical rearrangement would lead to ringexpansion to radical 107, which, following deprotonation, would expel a benzyl radical to yield quinoline anion 109.…”

“…A novel reducing system, consisting of the reagent-pair, triethylsilane and potassium tert-butoxide was reported by Stoltz, Grubbs et al in 2013. 1 The combination of the two reagents has since been investigated by a number of research groups [2][3][4][5][6][7][8][9][10][11][12][13][14][15] and provides a range of distinctive reaction types, arising through an unprecedented menu of reactive intermediates formed in the reaction, including triethylsilyl radicals 1, silanates 2 as hydrogen atom donors to both closed shell molecules and to radicals, and as potential hydride ion donors, and tert-butoxytriethylsilyl radical anions 3 as a very powerful electron donor. Exposing substrates simultaneously to multiple reactive intermediates is not routinely encountered in organic chemistry, other than in modelling of prebiotic conditions, 16 and so the variety of reactive intermediates produced by this reagent pair provides opportunities to witness unusual outcomes.…”