Zinc catalysed electrophilic C–H borylation of heteroarenes

Abstract

Cationic zinc Lewis acids catalyse the C-H borylation of heteroarenes using pinacol borane (HBPin) or catechol borane (HBCat). An electrophile derived from [IDippZnEt][B(C6F5)4] (IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) combined with N,N-dimethyl-p-toluidine (DMT)...

Two‐Coordinate NHC‐Supported ZnII Organocations: Steric and Electronic Tunability and Use in Alkyne Hydroboration Catalysis

“…Borenium B–H cations are known to readily undergo B−H insertion reactivity with various alkynes to afford the corresponding cis ‐hydroboration vinylborane products [22] . In addition, Lewis acid activation (by similar Zn II organocations) of pinBH for borenium‐type reactivity was recently proposed to be an operative mechanism for Zn‐catalyzed C−H borylation of heteroarenes [12a] …”

“…Zn‐catalyzed functionalization of organic molecules with hydroboranes remain little studied though NHC‐supported Zn cations were recently reported to be effective catalysts for alkyne diboration and C−H borylation of heteroarenes, for which the Lewis acidity of the Zn II plays an important role for catalytic activity [12a,c] . We investigated Lewis acidic cations [ 3 a ] + , [ 3 b ] + , [ 5 ] + and [(IPr)ZnC 6 F 5 ] + in alkyne hydroboration catalysis.…”

“…successfully used the cation [(IPr)Zn−C 6 F 5 ] + as a π ‐Lewis acid for alkynylcycloheptatrienes rearrangement, hydroarylation and transfer hydrogenation catalysis, [14] while Ingleson and coll. demonstrated that cation [(IPr)Zn−Et] + may be used as pre‐catalyst for the C−H borylation of heteroarenes [12a] . Thus far, known stable cations [(NHC)Zn−R] + only incorporate IPr as a carbene source: as a comparison, two‐coordinate Zn(II) cations stabilized with the less sterically hindered IMes carbene are not thermally stable [12d] .…”

Recent advances in borenium catalysis

“…Very recently Ingleson reported a new strategy to generate electrophilic borenium species for C–H borylation, which relies on coordination of an oxygen atom of HBpin to a highly oxyophilic zinc centre. 80 With 5 mol% of [IDippZnEt][B(C 6 F 5 ) 4 ] as precatalyst and HBpin or HBcat as borylation reagents, a number of heteroarenes, such as indoles, pyrroles, thiophenes and furans, can be converted to the corresponding arylboronate esters at 80 °C (Scheme 43). The real catalyst is proposed to be [[IDippZnH][B(C 6 F 5 ) 4 ]], which is generated via ligand redistribution between [IDippZnEt][B(C 6 F 5 ) 4 ] and HBpin or HBcat.…”

Transition‐Metal‐Free Heterocyclic Carbon‐Boron Bond Formation

“…But, recently Ingleson group reported a zinc‐catalyzed C−H borylation of weakly activated heteroarenes (Scheme 15). [76] The Cat.–7 in combination with N , N ‐dimethyl‐ p ‐toluidine (DMT) catalyzes the C−H borylation of weakly activated heteroarenes, and the borylated products were obtained in good to excellent yields. DMT and catecholborane act as optimal Brønsted base and borylating agents respectively.…”