Metal-Free Boron-Mediated ortho-C–H Hydroxylation of N-Benzyl-3,4,5-tribromopyrazoles

Abstract

A novel route has been reported for C–H hydroxylation of benzyl compounds directed by a 3,4,5-tribromopyrazole auxiliary via boronation/oxidation using BBr3 and NaBO3·4H2O. The strategy exhibits outstanding site selectivity and affords the corresponding phenols in moderate to excellent yields under metal-free conditions. Besides, this protocol can be achieved in one pot, which is highly promising as a practical method for use in a multistep organic synthetic process.

BX3-mediated borylation for the synthesis of organoboron compounds

“…The MacLean group realized the borylation of 2-phenylbenzimidazoles using BCl 3 , and studied their subsequent hydroxylation 71 (Scheme 25a). Then, the Ma group demonstrated C–H borylation of benzyl compounds with a 3,4,5-tribromopyrazole auxiliary directing group using BBr 3 , as well as their subsequent hydroxylation with NaBO 3 ·4H 2 O 72 (Scheme 25b).…”

Metal‐Free Directed Site‐Selective Csp3‐H Borylation of Saturated Cyclic Amines

“…Recently, the Shi [5] and Ingleson [4] groups have independently reported a general strategy for the oxygen atom directed C−H borylation of (hetero)arenes using BBr 3 under metal‐free conditions [3,58,59] . In addition, the Chatani and Ji groups have independently reported the nitrogen/oxygen atom directed metal‐free ortho −C−H borylation of aryl compounds [60–64] …”

“…[3,58,59] In addition, the Chatani and Ji groups have independently reported the nitrogen/ oxygen atom directed metal-free orthoÀ CÀ H borylation of aryl compounds. [60][61][62][63][64] In contrast, there are only a limited number of reports on metal-free Csp 3 À H borylation. Indeed, Vedejs et al [65] showed that the CÀ H borylation of neopentylamine at the γposition can be achieved through an intramolecular nitrogen-tethered borenium species.…”

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).…”