A DFT Study on the Mechanism of Nitrogen-directed Intramolecular Aromatic Borylation: C–H Bond Activation or Friedel–Crafts Reaction?

Metal‐Free C−H Borylation of N‐Heteroarenes by Boron Trifluoride

“…Importantly,n oe vidence of terminal alkyne CH bond activation was found even thoughthis substrate is known to form trialkynylborane SMe 2 adduct upon treatment with BF 3 •SMe 2 /4a system. [19] Allyltrimethylsilane 1p gave traces of borylation products according to 11 Ba nd 19 Fs pectra.A tt he same time, the characteristic peak for trimethylsilylfluoride multiplet found in the 19 FNMR spectrum at À157 ppm indicated TMS group cleavage. Aryldifluoroboranes 2a-j were generated andp artially characterized by the NMR spectroscopy in solution, however,a ttempted isolation of these compounds appeared to be unsuccessfuld ue to high reactivity and reversibility of the borylation reaction( Figure 1).…”

“…[c] Characteristic signalo f2a in 11 the weakly bound 2a•4 aH + /BF 4 À speciesy ield am ore stable product state (2a+ 4aH + /BF 4 À ). We carried out kinetic studies of the 1a borylation in the presenceo fv arying amountso fB F 3 •SMe 2 and 4a.A nalysis of initial rates revealed that the reaction is first order in 1a and a positiveo rder in BF 3 •SMe 2 .A tt he same time, addition of overstoichiometric 0.5 equivalentso f4a resulted in ap ronounced inhibiting effect (Figure 3).…”

“…Beside ansa ‐aminoboranes, it is also possible to activate C(sp 2 )−H bonds by strong Lewis acids such as B(C 6 F 5 ) 3 [9] or BX 3 (X=Cl, Br) [4c, 10] . Depending on the conditions, reactions proceed either by direct electrophilic attack of the boron halide [10e] or by in situ generated borenium cations derived from BCl 3 , [4c, 10b, 11] BBr 3 , [10a,d,e, 11] or catecholborane [12] . Regardless of the mechanisms, such borylation reactions with BF 3 , the weakest among boron halides Lewis acid, has not been reported to the best of our knowledge.…”

Intramolecular (directed) electrophilic C–H borylation

“…91 The mechanism of N-BX 3 mediated intramolecular C-H borylation was explored computationally by Uchiyama and Wang using 2-phenylbenzimidazole. 92 Starting from BBr 3 and BCl 3 borenium cations, 100, were found to be key intermediates on the C-H borylation pathway (boreniums may be more readily accessible in this case due to a BQN containing resonance form stabilising the borocation to a greater extent than possible with [pyridylBX 2 ] + , 93 Scheme 46, inset top right). From the borenium cations a C-H insertion mechanism proceeding via a four membered s-bond metathesis transition state (related to that calculated for B-H electrophiles) was found to be a high barrier process.…”

Acyl‐Directed ortho‐Borylation of Anilines and C7 Borylation of Indoles using just BBr3

“…Thed isparity between BCl 3 /B Br 3 also has been observed in N-heterocycle directed borylation and the origin of this has been examined previously. [18] The regioselectivity of borylation using BBr 3 was assessed by NMR spectroscopy in situ and post pinacol protection. This revealed that borylation of 1a led to C7 and C2 borylation products (with 3a and 4a formed in a4:1 ratio).…”