Reaction discovery and optimizationThe silyl group plays a crucial role in synthetic chemistry, particularly as a valuable partner in C–C bond cross-coupling reactions27,28. In addition, the carbon−silicon exchange is becoming one of the most important strategies in the area of drug design (Fig. 2a)29. Therefore, the direct borylation of organosilicons is highly attractive because the borylated products would provide a wide variety of downstream derivatives via C−Si and/or C−B-based cross-coupling reactions. While silyl-directed C–H borylation reactions have demonstrated effectiveness in iridium catalysis, as shown by Hartwig and colleagues30,31,32, several challenges need to be addressed for the realization of the nickel-catalyzed borylation process (Fig. 2b). Firstly, the Si−H bond borylation reaction of hydrosilanes with B2pin2 is known to be catalyzed by N-heterocyclic carbene (NHC)-nickel complexes. This suggests that the Si−B coupling pathway (Fig. 2b, A) may compete with the C–H borylation process (B)33. Secondly, unlike the aforementioned processes involving a Ni(0)/Ni(II) catalytic cycle, it remains uncertain whether the corresponding nickel species, after the oxidative addition of the Si−H bond at the nickel center (C)34,35, can further activate the more inert C–H bonds to form a high-valent nickelacycle intermediate (D).Fig. 2: Structural importance of Si-containing compounds and potential challenges.a Applications of organosilicons in medicinal chemistry. b Synthetic challenges for nickel-catalyzed hydrosilyl-directed C(sp2)−H borylation reactions.To overcome the challenges mentioned above, a thorough screening of ligands, bases as well as nickel precursors is necessary. Initially, we attempted a Ni(cod)2-catalyzed C(sp2)−H borylation reaction of easily accessible benzyldimethylsilane 1a using B2pin2 2 (Table 1, and see Supplementary Tables 1–6 for details). After some trials, we found that the utilization of Ni(cod)2 (10 mol%) and PMe3 (10 mol%), and potassium bis(trimethylsilyl)amide (KHMDS) (20 mol%) provided the best results, affording the borylation products 3a and 3a’ in 83% combined yield with a mono-/bis- ratio of 71:29 (Table 1, entry 1). The utilization of PMe3 as the ligand avoids the formation of the Si−H borylation product. This is distinct from recent studies on the nickel-catalyzed Si−H borylation reaction with B2pin2 by Ito and co-workers33, in which 1,3-dicyclohexylimidazol-2-ylidene (ICy) was used as the ligand. Replacing PMe3 with ICy only affords 3a and 3a’ in 42% yield (Supplementary Table 1). When using the air-stable 16e Ni(0) complex (such as Ni(4‑CF3stb)3, Ni(4‑tBustb)3) developed by Cornella et al.36 as the precatalyst, we found that Ni(4‑CF3stb)3 is an efficient Ni(0) source (Table 1, entry 2), while Ni(4-tBustb)3 failed to catalyze the reaction (Table 1, entry 3). The Ni (II) precatalysts, such as Ni(acac)2 and NiCl2, are ineffective (Table 1, entry 4). However, Ni(OTf)2 could be a suitable candidate, producing the related borylation product in moderate yield (Table 1, entry 5). The addition of KHMDS was also essential, and there was hardly any formation of borylation product in the absence of KHMDS (Table 1, entry 6). In addition to KHMDS, both tBuOK, and KH are suitable bases for this transformation, yielding 3a and 3a’ in moderate yield (Table 1, entries 7–8). The use of a weaker base KOAc did not promote the reaction at all (Table 1, entry 9). It was found that increasing the amount of KHMDS to 1.0 equivalent results in the formation of bis-borylated product 3a’ in 69% yield along with a trace amount of 3a, and 52% isolated yield of 3a’ could be obtained in 2.5 mmol scale (Table 1, entry 10). Investigation into the ligand effect shows that in the absence of PMe3, the yield is greatly reduced (Table 1, entry 11), and the reaction cannot occur normally when the ratio of Ni(cod)2 and PMe3 is 1:4 (Table 1, entry 12). Other trialkyl phosphines (e.g., PEt3 and PCy3) are also effective, although slightly lower yields were observed (Table 1, entries 13–14). No desired product was observed when replacing PMe3 with PPh3 or a bidentate ligand 1,2-bis(dicyclohexylphosphino)ethane (Dcype) (Table 1, entry 15). These results reveal that both the steric and electronic effects of the phosphine ligand are important factors for this silyl-directed borylation process. A diminished yield was observed at lower or elevated temperatures (Table 1, entries 16–17).Table 1 Optimization of reaction conditionsaSubstrate ScopeWith the optimal reaction conditions in hand, we explored the substrate scope. As shown in Fig. 3a, electronic and steric effects of the substituent on the aryl ring were investigated with primary benzylic silanes 1b‒1g (para-), 1h‒1j (meta-), 1k‒1m (ortho-). For the prototype reaction, this ortho-selective borylation process afforded 3a and 3a’ in 73% combined yield after purification. Both the electron-donating and electron-withdrawing substituents decorated at the para-position of the phenyl ring are well tolerated to deliver the borylated products in moderate to excellent yields (3b/3b’‒3g/3g’) with mono-/bis- ratios ranging from 58:42 to 66:34, including those with trifluoromethyl (3f), and boronate groups (3g). The groups of Marder and Martin et al. have demonstrated that C(sp2)−F or C(sp2)−O bonds might be converted into the C−B bonds in the presence of a nickel catalyst37,38,39,40,41. Here these chemical bonds (1c, 1i, 1l, etc.) were well compatible with the current conditions. For the substrates with a substituent at the meta-position of the phenyl ring, the regioselectivity is governed by steric hindrance. The borylation reactions occurred at the less hindered positions, producing the corresponding mono-borylated products in moderate to good yields (3h‒3j). Similarly, substrates with ortho-substituents did participate in the borylation reaction, affording the corresponding 1,2,3-trisubstituted arenes in moderate yields (3k‒3m). Tri-substituted substrates were also applicable, providing the corresponding tetrasubstituted arenes in 42–94% yields (3n‒3q). For substrates derived from naphthalenes (1r and 1s), the borylation reactions selectively occurred at the C3 and C2 sites of the naphthalene ring, respectively (1r → 3r, 51% yield; 1s → 3s, 62% yield). For the polyaromatic hydrocarbon-type substrates triphenylene and pyrene, the reaction can also undergo smoothly to afford the related monoborylation product (1t → 3t, 58% yield; 1u → 3u, 50% yield). It is worth mentioning that the selectivity borylation at the 2-position of pyrenes was also achieved by Marder et al. when employing iridium-based catalysts42,43,44. In comparison with substrates with a dimethyl-silyl directing group (1a, 1c), the use of a larger directing group, diisopropyl-silyl (−SiHiPr2, 1v and 1w), could improve the mono-/bis- ratios, but lower yields were observed.Fig. 3: Substrate scope.Reaction conditions: benzylic silanes (0.2 mmol), B2(pin)2 (0.6 mmol), Ni(cod)2 (10 mol%), PMe3 (10 mol%), KHMDS (20 mmol%), THF (0.5 mL), 12 h, 80 °C. Yields refer to the combined yield of mono- and bis-borylated products, and the ratios were determined after purification. a Primary benzylic hydrosilanes. b Secondary & tertiary benzylic hydrosilanes. c Borylation of substrates relevant to medicinal chemistry. aBenzylsilylboronate was obtained in 15% yield. bwith Ni(OTf)2.Substrates with a methyl group at the benzylic position proceed well with improved mono-/bis- ratios (Fig. 3b, 4a‒d). With a strong electron donating group at the para position (dimethylamino, 2d), the reaction only afforded the mono-borylated product 4d in 71% yield under the standard conditions. Further increasing the steric hindrance of the substrate through the introduction of an ‒OMe group at the ortho position or a phenyl group at the benzylic position leads to lower borylation efficiency (4e and 4f). Using the ‒SiHEt2 as the directing group (2g → 4g/4g’ and 2 h → 4h), the borylation reaction outcomes were similar to that with primary derivatives (1a → 3a/3a’ and 1r → 3r). Furthermore, tertiary hydrosilane could also be converted into the mono-borylated product 4i in 35% yield accompanied by the formation of Si−H bond borylation product in 15% yield.Furthermore, we examined the synthetic utility of this silyl-directed C–H functionalization reaction in the regioselective functionalization of drug-relevant substrates (Fig. 3c). L-Menthol-derived hydrosilane 2j successfully underwent the borylation reaction under standard conditions to afford a combined yield of 4j and 4j’ (81%). D-Galactopyranose derivative 2k is also tolerated to afford the related borylation products 4k and 4k’ in 89% yield with Ni(OTf)2 as the catalyst. The reaction of tyrosine derivative 2l also proceeded well under the standard condition to give mono- and bis-borylation products 4l and 4l’ in 81% yield. The substrate 2m derived from hydrocholesterol proceeded with the borylation reaction well to generate the ortho-borylated products 4m/4m’ in 89% yields.Synthetic applicationsFigure 4 provides an overview of the practicality of this method and the synthetic applications of the borylation products. In the presence of 2.5 mol% Ni(cod)2, mono-borylated product 3a and bis-borylated product 3a’ could be prepared in 49% and 16% yields on a 10 mmol scale (Fig. 4a). As both hydrosilanes and boronic esters are easily modifiable groups, the ortho-borylated products could be converted into Si−O−C or Si−O−B−C heterocycles. For example, the Si−H bond and −Bpin in 3a can be readily converted to an eight-membered heterocycle 7 by applying known methods (Fig. 4b). Upon simply treating the ortho-borylated product with 2 M NaOH aqueous solution, a library of benzo[d][1,2,6] oxasilaborinic heterocycles could be readily constructed (3 → 8, Fig. 4c). For the bis-borylation product 3a’, a similar heterocycle 8a’ could also be obtained leaving the second boryl group untouched. Recently, the carbon−silicon switch strategy has been successfully applied in the agrochemical industry (e.g., flusilazole and silafluofen), and increasing interest has also been drawn in the pharmaceutical industry45,46. In addition, boron-containing heterocycles are important in a variety of applications from drug discovery to materials science47. This nickel-catalyzed C–H borylation reaction provides economic access to precursors for the construction of Si−O, or Si−O−B containing heterocycles, potentially relevant to medicinal applications.Fig. 4: Synthetic applications.a Gram-scale synthesis. b, c Synthetic applications in the construction of Si-containing heterocycles. aTHF as the reaction solvent.Mechanistic studiesWe then investigated the mechanistic insights into this nickel-catalyzed C(sp2)−H borylation reaction. Adding an excess amount of mercury did not significantly inhibit the borylation reaction, suggesting that a homogeneous nickel species might be responsible for this silyl-directed borylation process (Fig. 5a)48. The experimental outcomes collected in Fig. 5b show that the replacement of the −SiHR2 moiety by −SiMe3 (9) or −SiMe2Bpin (10) did not produce the corresponding borylation products (11/11’ or 12/12’) under standard conditions. Besides, the reaction of deuterated benzylic hydrosilane 1a-[D] did proceed well. As determined by 1H NMR, a certain level of deuterium scrambling was observed in the borylation products (3a-[D] and 3a’-[D]). We noticed that the deuteration rate of 3a’-[D] was lower than that of 3a-[D], suggesting that a C–H/Si−D exchange occurred at the nickel center during the borylation reaction (Fig. 5b). Recently, Marder et al. have thoroughly characterized a series of anionic sp2-sp3 diboron adducts49,50,51, which might be formed in the current borylation system. Therefore, we analyze the equimolar mixture of KHMDS and B2pin2 with 11B{H} NMR (Supplementary Fig. 16), which exhibited two distinct peaks: one at 31.0 ppm (associated with the sp2 boron atom) and the other at approximately 3.8 ppm (attributed to the sp3 tetrahedral boron atom). These results suggest that this base-boron equimolar solution likely contains both B2pin2 and the anionic sp2-sp3 adduct ([(TMS)2NB2pin2]-). When replacing the base with sp2-sp3 diboron complex [tBuOB2pin2]-K+ or the premixed solution of B2pin2 with KHMDS (see Supplementary Figs. 15 and 16), these reactions could also proceed well to afford the related borylation products (Fig. 5c). These results suggest that the Lewis adduct might be a potential intermediate in this borylation reaction. To examine the by-product of the reaction (Fig. 5d), GC-MS analysis of the reaction mixture of borylation of 1a under standard conditions was conducted, and (TMS)2NBpin and HBpin were observed (Supplementary Fig. 6), which might be associated with the formation of [Ni]−Bpin intermediate. The reaction of phenylethyl silane 13 did not yield the normally borylated product 16 (Fig. 5e). Instead, it resulted in the formation of the intramolecular C–H silylated product benzo[b]silole 15 in 72% yield (n = 1). For (3-phenylpropyl)silane 14, the reaction produced the intramolecular C–H silylated product 17 (n = 2). These findings suggest that the oxidative addition of the Si−H bond at the nickel center may take place during the reaction to afford the Ni(II)−silyl species C (Fig. 5e)34,35. The resulting C could then undergo C–H bond activation via oxidative addition or concerted metalation deprotonation (CMD) mechanism, leading to the formation of cyclometallic intermediates. The outcome of the reaction (C–H borylation versus intramolecular C–H silylation) might be influenced by the ring size of the related cyclometallation intermediate. To probe the possibility of a free radical-involved pathway, we examined the reaction in the presence of radical scavenger 9,10-dihydroanthracene (Fig. 5f). No significant influence in the reaction outcome was observed. Besides, electron paramagnetic resonance (EPR) analysis on the standard reaction mixture did not detect any radical signal. These results suggest that the Ni-catalyzed borylation reaction does not involve free radical intermediates or Ni(III) complexes.Fig. 5: Control experiments.a Hg poisoning test. b Importance of hydrosiyl group. c With Lewis base adduct as the base precursor. d Detection of by-products. e Involvement of cyclometallic intermediate. f Effect of radical scavenger for the standard reaction and EPR experiment on the reaction mixture.Monitoring the C(sp2)−H borylation reaction of 1a in THF-d8 in the presence of B2pin2 (1.2 equiv.), Ni(cod)2 (10 mol%) and PMe3 (10 mol%), and potassium bis(trimethylsilyl)amide (KHMDS) (20 mol%) shows four apparent stages (Fig. 6a)52. The first one is the dormant period (Stage I); the second one is the smooth induction period, with only ~10% yield borylation product formed after 30 min (Stage II); the third one is the fastest, the yield of borylation product increased to 58% within 20 min (Stage III); and the fourth one corresponds to the consumption of B2pin2 or the possible deactivation of the system (Stage IV).Fig. 6: Mechanistic experiment.a Plot of the concentration of borylation product (total concentration of 3a and 3a’) versus time (min) for the nickel-catalyzed borylation of 1a. b Stacked 1H NMR (600 MHz, THF-d8) spectra of the reaction. c Stacked 31P{H} NMR (162 MHz, THF-d8) spectra of the reaction (with PPh3 as an external standard). d Parallel kinetic isotopic effects (KIE).Based on the kinetic profile, we then collected the 1H NMR spectra from these four stages (Fig. 6b). At the upfield (0.14 ppm), the formation of (TMS)2NBpin could be detected since Stage I, the signal strength of which rapidly increased during Stage II accompanied by the gradual decrease of KHMDS (at 0.00 ppm). When the reaction entered Stage III, the amount of (TMS)2NBpin species did not change anymore (about 9% yield, see Supplementary Fig. 10). It has been demonstrated that metal alkoxides (e.g., tBuOK, KOMe) are suitable bases to promote this nickel-catalyzed borylation reaction (see Supplementary Table 2). Further 1H NMR analysis reveals the explicit formation of the tBuOBpin (or MeOBpin) species during the reaction when using metal alkoxide as the additive (see Supplementary Fig. 11). These results suggest that the additive KHMDS (or other effective bases) plays a role in inducing the formation of the [Ni]−Bpin intermediate, a possible catalytically reactive species.The in situ 31P{H} NMR spectra (Fig. 6c, Supplementary Fig. 17) disclose the presence of two peaks, one at −10.6 ppm which is attributed to Ni(cod)(PMe3)253 and another at −23.3 ppm, during both Stage I and II. However, by the time the reaction enters stage III (~45 min), the Ni(cod)(PMe3)2 is nearly depleted. This observation implies that Ni(cod)(PMe3)2 is not a resting state or a reactive intermediate in the course of the reaction. The attribution of the principal phosphorus signal at −23.3 ppm in stage III is uncertain, but our further control experiments show that Ni(PMe3)4 (at about −22.0 ppm) is also inactive for the borylation reaction (see Supplementary Figs. 18 and 19). Thus, we speculate the observed peak at −23.3 ppm might be assigned to a new phosphine-ligated nickel species that potentially catalyzes the borylation reaction. We also conducted parallel kinetic isotope effect (KIE) experiments by measuring the initial rate of the reaction at the steady state (stage III). A relatively obvious KIE (kH/kD) of ~1.8 was observed between secondary benzylic silane 2g and its deuterated form 2g-[D] (Fig. 6d, see Supplementary Fig. 5 for the reaction outcome of H/D exchange experiment with 2g-[D] as the substrate). These KIE experiments demonstrate that the C–H activation step is likely the rate-determining step in the catalytic reaction.DFT calculationsTo get more mechanistic insight into this nickel-catalyzed borylation reaction, we also performed DFT calculations with B3LYP-D3 functional using the PCM model to treat the solvent effect54,55,56,57,58 (basis set details are collected in Supplementary Discussion section). The pathways for the key C–H bond activation were located with the combined molecular dynamics and coordinate driving method (MD/CD)59. The borylation reaction between 1a and B2pin2 was chosen as the model reaction. Under the experimental conditions (10 mol% Ni(cod)2 catalyst with 10 mol% PMe3 and 20 mol% KHMDS, and 3.0 equivalents of B2pin2), some different types of Ni(0) complexes might be generated. The energetic details of these species were computed (see Supplementary Fig. 31 for details). Combined control experimentals and DFT computations reveal that Ni(cod)(PMe3)2 is the most stable nickel Ni(0) complex with exergonic by 15.3 kcal/mol. According to our experimental studies and DFT calculations, the most possible C–H borylation pathway started with the formation of Ni(cod)(PMe3)2 species IM1 through the ligand exchange reaction between Ni(cod)2 and PMe3 (see Supplementary Fig. 42 for the second borylation pathway).As shown in Fig. 7a, the 18 electron Ni(cod)(PMe3)2 species IM1 occurs ligand exchange with B2pin2 to form Ni(B2pin2)(PMe3)2 complex IM2, which can further undergo an oxidative addition reaction to form a Ni(II)-boryl species IM360,61,62,63. Then, ligand exchange between IM3 and KHMDS affords a new Ni(II)-ate complex IM4, accompanied by the release of one PMe3 molecule. Subsequently, IM4 experiences a reductive elimination of the B–N bond via TS4/5 to generate the intermediate IM5. This step carries an activation barrier of 21.8 kcal/mol relative to IM1. In IM5, the B–N bond coordinates with the nickel center in an η2-manner. Its further complexation with 1a generates a Ni complex IM6. A comparison with the bond lengths of B–H in free HBpin (1.19 Å) and Si–H in 1a (1.49 Å) reveals that the B–H and Si–H bonds are cleaved in IM6 (dB–H = 1.45 Å, and dSi–H = 3.12 Å, Fig. 7b). Therefore, IM6 might be assigned as a Ni(II) species. The formation of IM6 is exergonic by 19.9 kcal/mol. It is noteworthy that while IM1 can undergo successive ligand exchange by reacting with B2pin2 and KHMDS, subsequent nickel species (IM2-IM5) are all less energetically favorable than IM1. The computed 31P{H} NMR chemical shift for IM6, which is −24.7 ppm, closely matches the experimentally observed resonance at −23.3 ppm (Fig. 6c). Moreover, the high-resolution mass spectrometry (HRMS) analysis has confirmed the likelihood of the formation of Ni(II)-ate complex IM6 and its analogous complex IM12, related to the second borylation step (Fig. 7c and Supplementary Figs. 23 and 24).Fig. 7: DFT calculations.a Calculated free energy profile for the formation of nickelate intermediate IM6 through the reaction of Ni(cod)2, B2pin2, KHMDS, PMe3 and 1a. Computed at PCM (THF)/B3LYP-D3/[6-311 + G(d,p) (C, H, O, N, B, Si, P), SDD(Ni)]//B3LYP-D3/[6-31 G(d) (C, H, O, N, B, Si, P), SDD (Ni)] level. b The 3D structure and 31P{H} NMR characterization of the nickelate intermediate IM6 (Distances are in Å). Color code: H, white; C, gray; B, pink; O, red; P, orange; Si, brown; Ni, green. c HRMS characterization of IM6 and IM12.In addition, we also considered other potential pathways related to the formation of IM6. These pathways involve the initial Si−H bond oxidative addition toward Ni(PMe3)2, followed by the reaction of the diboron compound with the H-Ni(II)-SiR3 complex or the boryl transfer from the sp2-sp3 diboron adduct to the Ni(0) center (see Supplementary Fig. 34 for details). Although the Si−H bond oxidative addition of 1a with Ni(PMe3)2 is thermodynamically favorable (∆G = −8.2 kcal/mol), further boryl group transfer from the (TMS)2N-B2pin2 adduct to IMA1 is kinetically unfavorable (∆Gǂ = 39.0 kcal/mol). Besides, our results indicate that the transfer of the nucleophilic boryl group from the anionic sp2-sp3 diboron adduct to the Ni(0) center requires a higher activation barrier compared to the reaction of Ni(PMe3)2 with the neutral B2pin2. This result aligns with our in situ 11B{H} NMR studies, which revealed no detection of the anionic sp2-sp3 diboron adduct during the borylation reaction in the presence of catalytic amounts of KHMDS (see Supplementary Fig. 21). However, when tBuOK was used as the model base, it was found that both neutral B2pin2 and the anionic sp2-sp3 diboron adduct can react with the related Ni species with an accessible activation barrier. These results suggest that for the formation of key intermediate IM6, different bases might lead to distinct mechanistic pathway. Because the tBuOK is not the optimal base in this method, the related pathway and discussion are collected in the Supplementary Information (see Supplementary Fig. 36).Then, IM6 undergoes a ligand-to-ligand hydrogen transfer64,65,66 to form a five-membered cyclometallic nickelate intermediate IM7 with an activation barrier of 28.5 kcal/mol (Fig. 8a, via TS6/7). In TS6/7, the formation of the Ni−C bond occurs synergistically with a hydrogen transfer from the phenyl ring to the B atom, accompanied by the hydride migration from Ni to the B atom. This C–H activation transition state could be identified through IRC analysis (see Supplementary Fig. 44). IM7 resembles a nickel-borohydride complex62, in which two hydrogen atoms are closely bounded to the nickel center. This outcome is consistent with the findings from the deuterium-labeling experiment, which employed a Si−D substrate and demonstrated substantial H/D exchange activity at the −SiDMe2 moiety (cf., Fig. 5b). The two Ni–H and B–H bonds in IM7 are not equivalent (e.g., B−H1 = 1.37 Å, Ni−H1 = 1.58 Å, and B−H2 = 1.27 Å, Ni−H2 = 1.80 Å). Thus IM7 has 3 formally anionic ligands and a negative charge and could be assigned to a Ni(II) complex. IM7 readily dissociates HBpin to generate a Ni(II) intermediate IM8 (via TS7/8, ΔGǂ = 4.5 kcal/mol). Subsequently, B2pin2 react with IM8 to generate a Ni(IV) intermediate IM10 (IM8 → IM9 → IM10, via TS9/10). In IM10, the bond lengths of two Ni–B bonds are 1.92, and 1.96 Å, which are shorter than the sum of the covalent radii of nickel and boron atoms (2.09 Å). Besides, the B−B distance (2.13 Å) is much longer than that in the free B2pin2, indicating a nearly broken B−B bond. The Ni−H distance (1.59 Å) is slightly shorter than the Si−H distance (1.63 Å). Therefore, IM10 could be assigned to a Ni(IV) species67,68. Finally, the reductive elimination of the C−B bond via transition state TS10/11 gives the mono-borylated product 3a and regenerates the Ni(II)-1a-ate complex IM6. Along the whole free energy profile, the C–H activation step is the rate-limiting step with a barrier of 28.5 kcal/mol, and this borylation process is exergonic by 26.4 kcal/mol. The barrier for reductive elimination is only 5.7 kcal/mol (with respect to IM10), indicating the IM10 could be considered as a fleeting intermediate. We also studied possible single electron transfer (SET) processes between the Ni(IV) intermediates (IM10) and the reducing agents (HBpin, B2pin2, and [(TMS)2N-B2pin2]-) in the reaction system. Our results show that the single electron reduction of Ni(IV) by the boron species are unlikely to happen in the reaction system (Supplementary Table 7). For the rate-determining step, calculations with larger basis sets and other functionals lead to somewhat lower energy barriers (with PWPB95-D4 functional, ∆Gǂ = 20.1 kcal/mol; with ωB97M-V functional ∆Gǂ = 25.6 kcal/mol, see Supplementary Table 8). Thus, the calculated free energy profile is qualitatively consistent with the reaction temperature of 80 °C.Fig. 8: DFT Calculations.a Calculated free energy profile and important nickel species for Ni-catalyzed ortho-C–H borylation reaction of benzylic hydrosilane 1a with nickelate complex IM6 as the key intermediate. Computed at PCM (THF)/B3LYP-D3/[6-311 + G(d,p) (C, H, O, B, Si, P), SDD(Ni)]//B3LYP-D3/[6-31G(d) (C, H, O, B, Si, P), SDD (Ni)] level. b Competing Si–H borylation pathway. Color code: H, white; C, gray; B, pink; O, red; P, orange; Si, brown; Ni, green.It has been shown that the C(sp2)−H activation via a CMD mechanism is feasible at a Ni(II) center69. However, when employing KHMDS as the base, it was found that the C(sp2)−H activation of Ni(II)-ate complex IM6 through a classical CMD transition state is thermodynamically unfavorable due to the higher activation barrier required (∆Gǂ = 53.0 kcal/mol, see Supplementary Fig. 38 for details). In addition to the C–H borylation pathway, IM6 could also coordinate with B2pin2 to form a Ni(II) intermediate species, IMS1. Then, the oxidative addition of the B−B bond toward the nickel center affords a Ni(IV) intermediate IMS2, which further undergoes a Si−B reductive elimination to generate the Si−H borylation product. Computational results in Fig. 8b revealed that this pathway is unlikely to occur due to the high barrier required for Si−B elimination (see Supplementary Fig. 45 and the related discussions for details). These results can account for the observed selectivity for C–H borylation under experimental conditions.Proposed catalytic cycleBased on the control experiments and computational studies, a possible pathway involving a Ni(II)/Ni(IV) cycle might be responsible for this silyl-directed C–H borylation reaction as summarized in Fig. 9. The ortho C–H bond activation step starts with the formation of a [Ni(II)]-(H)(Bpin)(SiR3) complex I (IM6) that might be generated from Ni(cod)(PMe3)2, KHMDS, B2pin2, and 1a via sequential B–B bond oxidative addition and B–N bond elimination event. Intermediate I undergoes a ligand-to-ligand hydrogen transfer event at the Ni(II) center to form a cyclometallation species II (IM7). After the dissociation of HBpin, a cyclometallic Ni(II) intermediate III (IM8) is formed. Further, the oxidative addition of B2pin2 with concomitant elimination of the C–B bond results in a similar Ni(II)-ate complex V (IM11). Intermediate V further undergoes a ligand exchange with 1a, delivering the final product 3a and regenerating the reactive catalytic species I (IM6). The detection of Ni(II)-ate complex I and two other by-products ((TMS)2NBpin and HBpin) provides strong evidence for the proposed catalytic cycle.Fig. 9: Proposed catalytic cycle for nickel-catalyzed silyl-directed C(sp2)–H borylation reaction.The borylation reaction proceeds through a Ni(II)/Ni(IV) catalytic cycle with Ni(II)-Bpin-ate complex as the key in key intermediate.
