C–H Borylation of Diphenylamines through Adamantane-1-carbonyl Auxiliary by BBr₃

Abstract

A method for ortho-C−H borylation of diphenylamines using BBr 3 as the boron source has been reported. The noncatalytic adamantane-1-carbonyl directed reaction exhibited site exclusivity and good functional group tolerance. Generally, the borylation occurred at the more electron-rich aromatic ring and the borylated products could be converted to various useful intermediates. Besides, the derived arylation and removal of auxiliary of the product could be achieved in a one-pot fashion.A s a highly important class of chemicals, diphenylamines are frequently found among drugs, dyes, agrochemicals, explosive stabilizers, and radical trapping antioxidants, such as diclofenac sodium, mefenamic acid, and clausine E (Figure 1). 1

Transient Imine as a Directing Group for the Metal-Free o-C–H Borylation of Benzaldehydes

“…Some reports have recently appeared on metal-free C–H borylation reactions in which a removable or modifiable directing auxiliary is used . For example, the metal-free directed o -C–H borylation of phenol derivatives using either a pyridine or a phophinite directing group, an acyl-group-directed o -C–H borylation of anilines, the C7–H borylation of indoles, − and the o -C–H borylation of diphenylamine were also achieved . In these reports, the directing groups were installed prior to the borylation step.…”

“…Thus, a unified directed strategy continues to be needed for metal-free C–H borylation, in which the installation and removal of a directing auxiliary can proceed with ease. It should be noted that previously reported metal-free directed C–H borylation systems have been applicable to only electron-rich arene systems, such as phenol, aniline, and indole derivatives, due to the favorable electrophilic aromatic substitution (S E Ar) step (Figure E), − which originated from the previous concept of the nondirected Lewis acid mediated p -C–H borylation of an electronically rich system via an S E Ar reaction (Figure F). − We chose benzaldehyde derivatives for metal-free directed C–H borylation, since they are highly electron deficient molecules and generally unreactive in S E Ar reactions (Figure G). Using such derivatives would emphasize the efficacy of a directing group over an electronic effect in this rection.…”

“…25 For example, the metal-free directed o-C−H borylation of phenol derivatives using either a pyridine 26 or a phophinite directing group, 27 an acyl-groupdirected o-C−H borylation of anilines, the C7−H borylation of indoles, 28−30 and the o-C−H borylation of diphenylamine were also achieved. 31 In these reports, the directing groups were installed prior to the borylation step. In some cases, further functionalization of the borylated product was carried out before the removal of the directing group, due to the low stability of the boron moiety (Figure 1D).…”

Regioselective Transition‐Metal‐Free C(sp2)−H Borylation: A Subject of Practical and Ongoing Interest in Synthetic Organic Chemistry

“…Following these reports, Ma, Ji, and co-workers reported the ortho-CÀ H borylation of diphenylamines by using an adamantane-1-carbonyl group as a directing auxiliary (to give 98). [110] The efficiency of the acyl directing group was further confirmed by the fact that a N-pivaloylpyrrole was smoothly borylated at the C2-position (as shown in 99). Notably, the site selectivity was controlled by both electronic effects and the use of a directing group.…”

Boron‐Mediated Regioselective Aromatic C−H Functionalization via an Aryl BF2Complex

“…The reactions generally proceed via a stable 5‐ or 6‐membered boracycle [7] that governs the regioselective functionalization of the aromatic or heteroaromatic system. These synthetic strategies have been used in borylative hydroxylation of N‐heterocycles [5d,m] and for installing boronic acid pinacol esters (Bpin) on aldehydes, [5i] anilines, [5b,e] alkenes, [5g] indoles, [5b,c] pyrroles, [5h] phenoxypyiridines, [5a] pyrimidylanilines, [5j] phosphines, [5k,l] thiopyridines, [5f] and are synthetically useful due to their widespread use in the Suzuki reaction (Figure 1B).…”