Direct electrophilic borylation using Y(2)BCl (Y(2) = Cl(2) or o-catecholato) with equimolar AlCl(3) and a tertiary amine has been applied to a wide range of arenes and heteroarenes. In situ functionalization of the ArBCl(2) products is possible with TMS(2)MIDA, to afford bench-stable and easily isolable MIDA-boronates in moderate to good yields. According to a combined experimental and computational study, the borylation of activated arenes at 20 °C proceeds through an S(E)Ar mechanism with borenium cations, [Y(2)B(amine)](+), the key electrophiles. For catecholato-borocations, two amine dependent reaction pathways were identified: (i) With [CatB(NEt(3))](+), an additional base is necessary to accomplish rapid borylation by deprotonation of the borylated arenium cation (σ complex), which otherwise would rather decompose to the starting materials than liberate the free amine to effect deprotonation. Apart from amines, the additional base may also be the arene itself when it is sufficiently basic (e.g., N-Me-indole). (ii) When the amine component of the borocation is less nucleophilic (e.g., 2,6-lutidine), no additional base is required due to more facile amine dissociation from the boron center in the borylated arenium cation intermediate. Borenium cations do not borylate poorly activated arenes (e.g., toluene) even at high temperatures; instead, the key electrophile in this case involves the product from interaction of AlCl(3) with Y(2)BCl. When an extremely bulky amine is used, borylation again does not proceed via a borenium cation; instead, a number of mechanisms are feasible including via a boron electrophile generated by coordination of AlCl(3) to Y(2)BCl, or by initial (heteroarene)AlCl(3) adduct formation followed by deprotonation and transmetalation.

Hail boration! 2‐Dimethylaminopyridine‐ligated dihaloborocations [X2B(2‐DMAP)]+ with a strained four‐membered boracycle were used for the haloboration of terminal and dialkyl internal alkynes (see scheme). Esterification then provided vinyl boronate esters as useful precursors to tetrasubstituted alkenes. Following mechanistic studies, the scope of the haloboration was expanded simply by variation of the amine. Pin=2,3‐dimethyl‐2,3‐butanedioxy.

Electrophilic direct borylation is facilitated, and arene substrate scope enhanced, by using electrophiles derived from inexpensive reagents; specifically an amine, BCl(3) and AlCl(3).

Hydride abstraction from N,N′-bis(adamantyl)-1-hydrido-1,3,2-benzodiazaborole with catalytic [Ph 3 C][closo-CB 11 H 6 Br 6 ] resulted in a low yield of arene borylation and a major product derived from migration of both adamantyl groups to the arene backbone. In contrast, the related arylsubstituted diazaborole N,N′-(2,6-diisopropylphenyl)-1-bromo-1,3,2-diazaborole did not borylate benzene or toluene, being resistant to halide abstraction even with strong halide acceptors: e.g., [Et 3 Si][closo-CB 11 H 6 Br 6 ]. The reactivity disparity arises from greater steric shielding of the boron p z orbital in the 2,6-diisopropylphenyl-substituted diazaboroles. Boron electrophiles derived from 1-chloro-1,3,2-benzodithiaborole ((CatS 2 )BCl) are active for arene borylation, displaying reactivity between that of catecholato-and dichloro-boron electrophiles.[(CatS 2 )B(NEt 3 )][AlCl 4 ] is significantly less prone to nucleophile-induced transfer of halide from [AlCl 4 ]¯to boron compared to catecholato and dichloro borocations, enabling it to borylate arenes containing nucleophilic −NMe 2 moieties in high conversion (e.g., N,N,4-trimethylaniline and 1,8-bis(dimethylamino)naphthalene). Calculations indicate that the magnitude of positive charge at boron is a key factor in determining the propensity of chloride transfer from [AlCl 4 ]¯to boron on addition of a nucleophile.

Crystallographic and NMR spectroscopic studies on allyl complexes of lithium, potassium and magnesium containing an O-donor functionality are described.