Borylation of Arenes with Bis(hexylene glycolato)diboron

Abstract

A series of diolate-substituted diboron compounds were investigated as reagents for iridium-catalyzed C-H borylations of arenes. These studies showed that commercially available bis(hexylene glycolato)diboron reacts with arenes in the presence of the catalyst generated from [Ir(COD)OMe] 2 and di-tert-butylbipyridine. This reagent is prepared from a less expensive diol than the more commonly used bis(pinacolato)diboron reagent.Arylboronate esters are versatile synthetic intermediates that undergo Suzuki-Miyaura and Chan-Lam-Evans cross-coupling reactions to form C-C and C-X bonds. 1,2 A traditional method to form these aromatic organoboron reagents is the addition of a reactive organometallic species, such as an aryllithium or aryl Grignard reagent, to an electrophilic boron compound. More recently, metal-catalyzed reactions of aryl halides have been employed to form arylboronate esters. 3 The Ir-catalyzed C-H borylation of arenes (Scheme 1) has become a powerful method to access arylboronate esters directly from aryl C-H bonds instead of a prefunctionalized aryl halide. 4-6 Besides being a more facile approach to the synthesis of arylboronate esters than the borylation of aryl halides, the selectivity of the Ir-catalyzed C-H borylation of arenes is distinct. The regioselectivity of the C-H borylation of arenes is controlled by the steric properties of the substrate. 7 Therefore, it complements C-H functionalization methods that rely on directing groups 8-10 and FriedelCrafts reactions of electron-rich substrates that are controlled by the electronic properties of the arene. Scheme 1 Iridium-catalyzed borylation of arenesAlthough the scope of the arenes that undergo Ir-catalyzed C-H borylation is broad, and many functional groups are tolerated, the scope of the diboron reagents that undergo C-H borylation is limited. Only a few boron reagents undergo Ir-catalyzed C-H functionalization reactions. Most commonly, the C-H borylation of arenes is conducted with bis(pinacolato)diboron (B 2 pin 2 ) or pinacolborane (HBpin) as the boron source. In addition to the wide range of reports on the reactions of this reagent, Suginome and co-workers reported the C-H borylation with 1,8-naphthalenediaminatoborane (HBdan) as the boron reagent, 11 and our group reported the C-H borylation of arenes with bis(catecholato)diboron (B 2 cat 2 ). 12 Both reagents were less reactive for the C-H borylation of arenes than B 2 pin 2 and required neat substrate to obtain significant yields.We have proposed that the origin of this difference in reactivity stems from electronic effects. The addition of B 2 pin 2 to the iridium center forms a more electron-rich complex than that from addition of B 2 cat 2 , and the C-H bond cleavage of arenes by a complex containing a more electron-rich metal center occurs faster than that by a complex containing a more electron-poor metal center. 12,13 Although the C-H borylation occurs with the highest turnover numbers with B 2 pin 2 as the boron reagent, the pinacolate ester products generated in these reactio...

Steric Shielding Effects Induced by Intramolecular C–H···O Hydrogen Bonding: Remote Borylation Directed by Bpin Groups

“…To begin addressing this question, we ran CHB reactions with larger diboron partners such as B 2 hg 2 and B 2 pp 2 (Figure a). B 2 hg 2 proved less reactive than B 2 pin 2 in accordance with a previous report; however, the selectivity for the para position improved. We tested a novel diboron partner for CHB, B 2 pp 2 , and interestingly the conversion to the borylated product was greater than with B 2 hg 2 .…”

Balancing Reactivity, Regioselectivity, and Product Stability in Ir-Catalyzed Ortho-C–H Borylations of Anilines by Modulating the Diboron Partner

“…14 Traditionally, the focus in achieving high selectivities in CHB reactions has centered on extensive ligand screening, while the influence of the diboron reagent employed has been a less-explored facet. 12,13,15,16 We proposed that using boronic partners with larger substituents than B 2 eg 2 could lead to more stable orthoborylated anilines, possibly enabling direct isolation of the product. 17 However, there is a risk of reducing the regioselectivity of ortho CHB in the process.…”

“…Recently, the Yan group discovered that employing mesoionic carbene-Ir catalysts enables high regioselectivities in the ortho CHB of anilines with B 2 pin 2 . Traditionally, the focus in achieving high selectivities in CHB reactions has centered on extensive ligand screening, while the influence of the diboron reagent employed has been a less-explored facet. ,,, …”

“… 14 Traditionally, the focus in achieving high selectivities in CHB reactions has centered on extensive ligand screening, while the influence of the diboron reagent employed has been a less-explored facet. 12 , 13 , 15 , 16 …”