Nickel-Catalyzed Regioselective 1,4-Hydroboration of N

Letter Cite This: ACS Catal. 2018, 8, 6186−6191

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Nickel-Catalyzed Regioselective 1,4-Hydroboration of N‑Heteroarenes Sem Raj Tamang, Arpita Singh, Daniel K. Unruh, and Michael Findlater* Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409, United States S Supporting Information *

ABSTRACT: Combinations of Ni(acac)2 with phosphine ligands were found to catalyze the regioselective hydroboration of Nheteroarenes with pinacolborane, affording N-borylated 1,4-reduction products. Preliminary mechanistic studies have focused on the isolation and study of potential intermediates in the catalytic cycle. KEYWORDS: 1,4-regioselective, dearomatization, N-heteroarene, pyridine, nickel

T

upon earth-abundant (Fe)18 and even metal-free (borane and phosphane)13a,b,14 catalysts are particularly noteworthy. Catalysis based upon earth-abundant metals has recently garnered much attention,19 and nickel has emerged as a leading nonprecious metal alternative.20 Thus, regioselective reduction of N-heteroarenes utilizing nickel is significant and desirable. Considerable progress has already been made in nickelcatalyzed reduction of unsaturated hydrocarbons such as ketones,21 esters,21b,22 amides,22,23 and nitriles.24 However, to the best of our knowledge, nickel-catalyzed dearomative reduction of pyridines has not been reported. Our group has a long-standing interest in developing base-metal-catalyzed processes,25 and we have focused on employing commercially available metal salts in operationally convenient transformations.26 Herein, we report the first nickel-based catalyst system for the 1,4-regioselective hydroboration of N-heteroarenes in good to excellent yields. Preliminary experiments focused on the ability of commercially available nickel salts to affect the hydroboration of pyridine with HBpin in benzene-d6. Little to no activity was observed using Ni(acac)2 (1, 10 mol %), even after prolonged reaction times at elevated temperatures (24 h at 50 °C). Initial attempts at reaction optimization focused on the use of additives (e.g., employing NaOtBu) resulted in improved

he dearomatization of N-heteroarenes to afford dihydropyridines (DHPs) is an important transformation as it provides a pathway to a range of substrates which are biologically useful agents (e.g., nicotinamide adenine dinucleotide (NADH)1 and in the synthesis of pharmaceutically important drug molecules).2 Moreover, 1,4-DHPs are also known to serve as reducing agents in organocatalysis by functioning as organohydride donors,3 and they are also products of the multicomponent Hantzch ester synthesis4 which are of potential use as NADH mimics.5 Thus, a range of methods have been reported which allow synthetic access to DHPs, though achieving regioselectivity remains a challenge. Typically, regioselectivity may be achieved employing multistep procedures which utilize metal hydrides or alkali metals2d,6 or mixtures of products (1,2- and 1,4-DHPs) requiring separation are observed.2a,4d,7 For example, the hydrogenation of pyridines require harsh conditions and are susceptible to over-reduction to piperidine products.8 Therefore, the development of a mild and regioselective method is desirable.9 Hill and co-workers published the seminal work on the utilization of hydroboration in the reduction of pyridines to yield a mixture of 1,2- and 1,4-reduction products.10 Further reports of regioselective pyridine hydroboration have appeared (Scheme 1); 1,4-products may be obtained (catalytically) using Ru,11 Zr- and Hf-based metal−organic frameworks,12 organoboranes,13 and N-heterocyclic phosphanes,14 while 1,2-products are accessible using Rh,15 La or Th,16 Zn,17 and Fe.18 In particular, the development of regioselective methods based © XXXX American Chemical Society

Received: March 26, 2018 Revised: May 29, 2018 Published: June 4, 2018 6186

DOI: 10.1021/acscatal.8b01166 ACS Catal. 2018, 8, 6186−6191

Letter

ACS Catalysis Scheme 1. Recent Advances in 1,2- and 1,4-Regioselective Hydroboration of Pyridines

Scheme 2. Preliminary Screening of Pyridine (2a) and Picoline Substrates (2b−d) in Hydroboration Catalysis Employing Ni(acac)2/PR3†

†

All conversions and product ratios were determined by 1H NMR spectroscopy using tetraethylsilane as an internal standard. All reactions were monitored for 1 h unless otherwise noted: a= 5.5 h; b= 18 h.

activity and afforded a mixture of 1,2- and 1,4-N-borylated dihydropyridines (2:1) at room temperature (Figure S2).27 Further experiments revealed that upon addition of tricyclohexylphosphine, nickel loading could be lowered to 5 mol % with no concomitant erosion in substrate conversion.28 Additionally, the use of phosphine ligand afforded enhanced 1,4regioselectivity with no need for salt additives (Table S1). With these findings in hand, the hydroboration reactions of pyridine (2a) and picolines (2b−d) were used to screen metal/ phosphine combinations: 1/L1, 1/L2, and 1/L3 (Scheme 2; L1 = tricyclohexylphosphine, L2 = dicyclohexylphenylphosphine, L3 = tricyclopentylphosphine). In all combinations, pyridine (2a) was converted quantitatively to the 1,4-DHP product (3). In contrast, 2-picoline (2b) exhibited low conversions which likely arise from the presence of unfavorable steric interactions between the o-methyl group and Ni center (vide infra). Importantly, all 1/L x combinations afforded 1,4-DHPs regioselectively, moderate conversions (51−59%) were observed in the case of the hydroboration of 4-picoline (2d).

Significantly higher conversions (73−95%) could be obtained using 3-picoline (2c) as substrate, although it should be noted that small amounts of 1,2-product (3c) are formed in this case. Across these pyridine substrates, L3 affords consistently higher conversion of substrates at lower reaction times. The combination of 1/L3 was used in all subsequent reactivity studies. The scope of pyridines amenable to hydroboration using 1/ L3 (Table 1) was explored. Substrates bearing electrondonating functional groups at the meta position showed high regioselectivity for 1,4-DHP products in good to excellent 1H NMR yields (3i 97%, 3j/3j′ 65%, 3c/3c′ 86%). In the case of 3-methoxypyridine (3j), only moderate conversion (65%) was obtained. However, this substrate failed to react with the previously reported B(Me)ArF2 catalyst,13a and in the case of a recently disclosed 1,3,2-diazaphosphonium triflate catalyst afforded the opposite (1,2-DHP) regioselectivity.14a Thus, our system affords a high degree of complementarity to known hydroboration catalysts. Little to no reactivity was seen upon 6187

DOI: 10.1021/acscatal.8b01166 ACS Catal. 2018, 8, 6186−6191

Letter

ACS Catalysis Table 1. Ni(II)-Catalyzed 1,4-Hydroboration of Heteroarenes

a

Reaction conditions: pyridine substrates (0.5 mmol), HBpin (0.6 mmol), Ni(acac)2 (5 mol %), PCyp3 (5 mol %), C6D6 (0.7 mL). NMR yields and ratios of regioisomers (1,4 DHP: 1,2-DHP) were determined by 1H NMR spectroscopy using tetraethylsilane as an internal standard. b1 g of substrate, isolated yield in parentheses.

introduction of electron-withdrawing groups at the meta position (3k/3k′/k″ 51%, 3n n.r.); moreover, when the meta-substituent is a halide, dehalogenation chemistry was observed. Interestingly, we obtained products of complete 1,4selectivity for para substituted pyridines in good to excellent yields. (3d 88%, 3e/3e′ 95%, 3f 71%, 3g 85%, 3h 94%). This is rather surprising, as para substituted pyridines are not known to be reactive for regioselective 1,4 hydroboration reactions; typically, these substrates are only capable of undergoing 1,2hydroboration. The substrate scope is not limited to just pyridines. The expansion to include other heteroarenes revealed quinoline,

benzoquinoline, isoquinoline, acridine and phenanthrolines to be viable substrates for hydroboration. Until now, successful (regioselective) examples of 1,4-hydroboration of benzofused N-heteroarenes has been limited.12,13b,18 However, our results show good to excellent catalytic activity in 1,4-hydroboration of benzofused N-heterocycles (3t 55%) including quinolines (3r/ 3r′ 89%, 3s/3s′ 95%), benzo[f]quinoline (3u 96%,), and phenanthroline (3v 57%). In the case of the hydroboration of pyridines catalyzed by Ru,11 formation of the 1,4-addition product was proposed to occur via intramolecular 1,5-hydride migration. However, 1 is stable toward HBPin, and under no experimental conditions 6188

DOI: 10.1021/acscatal.8b01166 ACS Catal. 2018, 8, 6186−6191

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ACS Catalysis

Scheme 3. Left: Preparation of 4. Right: Solid-State Structure of 4 (40% Probability of Thermal Ellipsoids), Hydrogen Atoms Omitted for Clarity

Scheme 4. Left: Preparation of 5. Right: Solid-State Structure of 5 (40% Probability of Thermal Ellipsoids), Hydrogen Atoms Omitted for Clarity

Figure 1. Reaction profile of reduction of 3,5-lutidine by HBPin in the presence of 1/L3 (■), 4 (red ●), and 5 (blue

▲).

complexes 6 and 7 arising from treatment of 1 with PCy3 and PCy2Ph, respectively.29 Exposure of 5 to 3,5-lutidine results in rapid phosphine displacement and the generation of complex 4 (Figure S24). Finally, no reaction was observed between HBpin and either PCyp3 or 3,5-lutidine.30 Preliminary kinetic analysis was performed for the catalytic hydroboration of 3,5-lutidine (2i). A plot of the initial rate for the formation of 2i vs [1/L3] indicated the reaction was firstorder in [1/L3], while the rate of formation was found to be independent of the concentration of 2i (Figures S26−S28). The zero order for 2i suggests that the binding of the substrate to the nickel center is not rate-limiting. Interestingly, varying the concentration of HBPin appeared to reveal saturation kinetics, whereby the reaction is first-order in [HBPin] up to ∼1.5 equiv, after which it becomes zero-order. Finally, analysis

have we been successful in detecting any signal attributable to “Ni−H” by 1H NMR spectroscopic analysis. To gain further insight into the mechanism of the nickel-catalyzed hydroboration, several stoichiometric reactions were examined using 3,5-lutidine (2i) as the model heteroarene. We found that 1 reacted preferentially with 2i rather than activating HBPin. Based upon NMR and X-ray crystallography, 1 is stable toward HBPin. The treatment of a green solution of 1, in benzene, with two equivalents of 2i leads to a rapid color change and precipitation of a purple solid. The resulting octahedral complex Ni(acac)2(2i)2 (4) was characterized by single-crystal X-ray diffraction experiments (Scheme 3). Similarly, treatment of a benzene solution of 1 with an equimolar amount of PCyp3 affords crystals of the 1:1 phosphine ligated complex, 5, as a five-coordinate species (Scheme 4). This approach was also used to obtain the related 6189

DOI: 10.1021/acscatal.8b01166 ACS Catal. 2018, 8, 6186−6191

ACS Catalysis

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of the reaction profile (Figure 1) revealed the presence of an induction period. Interestingly, when isolated crystals of complex 4 were used to catalyze the hydroboration of 3,5-lutidine, a kinetic profile very similar to the in situ catalysis (1/L3) is observed (Figure 1). This supports the conclusion that the first step of the hydroboration mechanism is formation of the bis(heteroarene) complex prior to generation of an active catalyst. Subsequently, we probed the ability of isolated phosphine complex, 5, to catalyze the hydroboration reaction. Complex 5 affords a completely different kinetic profile with a much longer induction period. Gunanathan and co-workers have proposed a hydroboration mechanism in which the active catalyst (Rubased) contained both a bound heteroarene and a bound phosphine.11 Unfortunately, all efforts to independently synthesize an analogous Ni(acac)2(PR3)(heteroarene) complex failed. There are several potential mechanisms for the hydroboration catalyzed by 1. Analogous nitrile hydroboration employing 1 proposes complete reduction of 1 from Ni(II) to Ni(0) by HBCat (Cat = catechol).24b We attempted to perform catalytic hydroboration of 3,5-lutidine employing a redox innocent catalyst, Zn(acac)2 under otherwise identical reaction conditions. In this case, no reduced product was observed (Figure S22), implying that the role of nickel in the catalysis is not confined to that of a pure Lewis acid. In summary, we have developed an efficient and regioselective 1,4-hydroboration of N-heteroarenes using a commercially available and air-stable nickel precatalyst. This catalysis exhibits excellent regioselectivity and broad functional group compatibility. Preliminary kinetic and mechanistic studies have also been reported. Future work will focus on a detailed mechanistic analysis and an expansion of catalytic reactions based upon Ni(acac)2/PR3.

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REFERENCES

(1) Pollak, N.; Dölle, C.; Ziegler, M. The power to reduce: pyridine nucleotides − small molecules with a multitude of functions. Biochem. J. 2007, 402, 205−218. (2) (a) Wender, P. A.; Schaus, J. M.; White, A. W. General methodology for cis-hydroisoquinoline synthesis: synthesis of reserpine. J. Am. Chem. Soc. 1980, 102, 6157−6159. (b) Cozzi, P.; Carganico, G.; Fusar, D.; Grossoni, M.; Menichincheri, M.; Pinciroli, V.; Tonani, R.; Vaghi, F.; Salvati, P. Imidazol-1-yl and pyridin-3-yl derivatives of 4-phenyl-1,4-dihydropyridines combining Ca2+ antagonism and thromboxane A2 synthase inhibition. J. Med. Chem. 1993, 36, 2964−2972. (c) Kumar, P. P.; Stotz, S. C.; Paramashivappa, R.; Beedle, A. M.; Zamponi, G. W.; Rao, A. S. Synthesis and Evaluation of a New Class of Nifedipine Analogs with T-Type Calcium Channel Blocking Activity. Mol. Pharmacol. 2002, 61, 649−658. (d) Lavilla, R. Recent developments in the chemistry of dihydropyridines. J. Chem. Soc., Perkins Trans. 1 2002, 1141−1156. (e) Duttwyler, S.; Chen, S.; Lu, C.; Mercado, B. Q.; Bergman, R. G.; Ellman, J. A. Regio- and Stereoselective 1,2-Dihydropyridine Alkylation/Addition Sequence for the Synthesis of Piperidines with Quaternary Centers. Angew. Chem., Int. Ed. 2014, 53, 3877−3880. (3) (a) Ouellet, S. G.; Walji, A. M.; Macmillan, D. W. C. Enantioselective Organocatalytic Transfer Hydrogenation Reactions using Hantzsch Esters. Acc. Chem. Res. 2007, 40, 1327−1339. (b) Richter, D.; Mayr, H. Hydride-Donor Abilities of 1,4Dihydropyridines: A Comparison with π Nucleophiles and Borohydride Anions. Angew. Chem., Int. Ed. 2009, 48, 1958−1961. (c) Zheng, C.; You, S.-L. Transfer hydrogenation with Hantzsch esters and related organic hydride donors. Chem. Soc. Rev. 2012, 41, 2498−2518. (d) Li, G.; Chen, R.; Wu, L.; Fu, Q.; Zhang, X.; Tang, Z. Alkyl Transfer from C-C Cleavage. Angew. Chem., Int. Ed. 2013, 52, 8432−8436. (e) McSkimming, A.; Colbran, S. B. The coordination chemistry of organo-hydride donors: new prospects for efficient multi-electron reduction. Chem. Soc. Rev. 2013, 42, 5439−5488. (f) Prier, C. K.; Rankic, D. A.; MacMillan, D. W. C. Visible Light Photoredox Catalysis with Transition Metal Complexes: Applications in Organic Synthesis. Chem. Rev. 2013, 113, 5322−5363. (g) Lo, H. C.; Ryan, J. D.; Kerr, J. B.; Clark, D. S.; Fish, R. H. Bioorganometallic chemistry: Co-factor regeneration, enzyme recognition of biomimetic 1,4-NADH analogs, and organic synthesis; tandem catalyzed regioselective formation of Nsubstituted-1,4-dihydronicotinamide derivatives with [Cp*Rh(bpy)H]+, coupled to chiral S-alcohol formation with HLADH, and engineered cytochrome P450s, for selective C-H oxidation reactions. J. Organomet. Chem. 2017, 839, 38−52. (4) (a) Hantzsch, A. Condensationsprodukte aus Aldehydammoniak und ketonartigen Verbindungen. Ber. Dtsch. Chem. Ges. 1881, 14, 1637−1638. (b) Hantzsch, A. Ueber die Synthese pyridinartiger Verbindungen aus Acetessigäther und Aldehydammoniak. Justus Liebigs Ann. Chem. 1882, 215, 1−82. (c) Stout, D. M.; Meyers, A. I. Recent advances in the chemistry of dihydropyridines. Chem. Rev. 1982, 82, 223−243. (d) Wan, J.-P.; Liu, Y. Recent advances in new multicomponent synthesis of structurally diversified 1,4-dihydropyridines. RSC Adv. 2012, 2, 9763−9777. (5) (a) Karrer, P.; Ringier, B. H.; Büchi, J.; Fritzsche, H.; Solmssen, U. Modellversuche betreffend die wasserstoffübertragenden Wirkungsgruppen der Cofermente. Helv. Chim. Acta 1937, 20, 55−71. (b) Mauzerall, D.; Westheimer, F. H. 1-BenzyldihydronicotinamideA Model for Reduced DPN. J. Am. Chem. Soc. 1955, 77, 2261−2264. (6) (a) Olah, G. A.; Hunadi, R. J. Carbanions. 5. Preparation and proton and carbon-13 NMR spectroscopic structural study of the 4hydridopyridyl anion and 4,4′-bis(hydridopyridyl) dianion. Absence of homoazacyclopentadienyl ion character. J. Org. Chem. 1981, 46, 715− 718. (b) Sundberg, R. J.; Hamilton, G.; Trindle, C. Synthesis and Diels-Alder reactions of N-carbalkoxydihydropyridines. Substituent effects on the regiochemistry of reduction of N-carbalkoxypyridinium ions. J. Org. Chem. 1986, 51, 3672−3679. (c) Bull, J. A.; Mousseau, J. J.; Pelletier, G.; Charette, A. B. Synthesis of Pyridine and

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.8b01166. X-ray data (CIF) X-ray data (CIF) X-ray data (CIF) X-ray data (CIF) X-ray data (CIF) X-ray data (CIF) X-ray data (CIF) Experimental details, NMR spectra, details of kinetics experiments, and crystallographic information (PDF)

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Letter

AUTHOR INFORMATION

Corresponding Author

*E-mail: michael.fi[email protected]. ORCID

Daniel K. Unruh: 0000-0002-2594-5786 Michael Findlater: 0000-0003-3738-4039 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support of the Robert A. Welch Foundation (Grant No. D1807) is gratefully acknowledged. 6190

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ACS Catalysis Dihydropyridine Derivatives by Regio- and Stereoselective Addition to N-Activated Pyridines. Chem. Rev. 2012, 112, 2642−2713. (7) (a) Fowler, F. W. Synthesis of 1,2- and 1,4-dihydropyridines. J. Org. Chem. 1972, 37, 1321−1323. (b) Donohoe, T. J.; McRiner, A. J.; Sheldrake, P. Partial Reduction of Electron-Deficient Pyridines. Org. Lett. 2000, 2, 3861−3863. (8) (a) Adkins, H.; Kuick, L. F.; Farlow, M.; Wojcik, B. Hydrogenation of Derivatives of Pyridine. J. Am. Chem. Soc. 1934, 56, 2425−2428. (b) Hinman, R. L.; Shull, E. R. The Structure of Diskatole1. J. Org. Chem. 1961, 26, 2339. (c) Lunn, G.; Sansone, E. B. Facile reduction of pyridines with nickel-aluminum alloy. J. Org. Chem. 1986, 51, 513−517. (d) Glorius, F. Asymmetric hydrogenation of aromatic compounds. Org. Biomol. Chem. 2005, 3, 4171−4175. (e) Takasaki, M.; Motoyama, Y.; Higashi, K.; Yoon, S.-H.; Mochida, I.; Nagashima, H. Ruthenium Nanoparticles on Nano-Level-Controlled Carbon Supports as Highly Effective Catalysts for Arene Hydrogenation. Chem. - Asian J. 2007, 2, 1524−1533. (f) Buil, M. L.; Esteruelas, M. A.; Niembro, S.; Oliván, M.; Orzechowski, L.; Pelayo, C.; Vallribera, A. Dehalogenation and Hydrogenation of Aromatic Compounds Catalyzed by Nanoparticles Generated from Rhodium Bis(imino)pyridine Complexes. Organometallics 2010, 29, 4375−4383. (g) Wang, D.-S.; Chen, Q.-A.; Lu, S.-M.; Zhou, Y.-G. Asymmetric Hydrogenation of Heteroarenes and Arenes. Chem. Rev. 2012, 112, 2557−2590. (9) Park, S.; Chang, S. Catalytic Dearomatization of N-Heteroarenes with Silicon and Boron Compounds. Angew. Chem., Int. Ed. 2017, 56, 7720−7738. (10) Arrowsmith, M.; Hill, M. S.; Hadlington, T.; Kociok-Köhn, G.; Weetman, C. Magnesium-Catalyzed Hydroboration of Pyridines. Organometallics 2011, 30, 5556−5559. (11) Kaithal, A.; Chatterjee, B.; Gunanathan, C. RutheniumCatalyzed Regioselective 1,4-Hydroboration of Pyridines. Org. Lett. 2016, 18, 3402−3405. (12) Ji, P.; Feng, X.; Veroneau, S. S.; Song, Y.; Lin, W. Trivalent Zirconium and Hafnium Metal−Organic Frameworks for Catalytic 1,4-Dearomative Additions of Pyridines and Quinolines. J. Am. Chem. Soc. 2017, 139, 15600−15603. (13) (a) Fan, X.; Zheng, J.; Li, Z. H.; Wang, H. Organoborane Catalyzed Regioselective 1,4-Hydroboration of Pyridines. J. Am. Chem. Soc. 2015, 137, 4916−4919. (b) Keyzer, E. N.; Kang, S. S.; Hanf, S.; Wright, D. S. Regioselective 1,4-hydroboration of pyridines catalyzed by an acid-initiated boronium cation. Chem. Commun. 2017, 53, 9434−9437. (c) Gandhamsetty, N.; Joung, S.; Park, S.-W.; Park, S.; Chang, S. Boron-Catalyzed Silylative Reduction of Quinolines: Selective sp3 C−Si Bond Formation. J. Am. Chem. Soc. 2014, 136, 16780−16783. (d) Gandhamsetty, N.; Park, S.; Chang, S. Selective Silylative Reduction of Pyridines Leading to Structurally Diverse Azacyclic Compounds with the Formation of sp3 C−Si Bonds. J. Am. Chem. Soc. 2015, 137, 15176−15184. (14) (a) Rao, B.; Chong, C. C.; Kinjo, R. Metal-Free Regio- and Chemoselective Hydroboration of Pyridines Catalyzed by 1,3,2Diazaphosphenium Triflate. J. Am. Chem. Soc. 2018, 140, 652−656. (b) Hynes, T.; Welsh, E. N.; McDonald, R.; Ferguson, M. J.; Speed, A. W. H. Pyridine Hydroboration with a Diazaphospholene Precatalyst. Organometallics 2018, 37, 841−844. (15) Oshima, K.; Ohmura, T.; Suginome, M. Regioselective Synthesis of 1,2-Dihydropyridines by Rhodium-Catalyzed Hydroboration of Pyridines. J. Am. Chem. Soc. 2012, 134, 3699−3702. (16) (a) Dudnik, A. S.; Weidner, V. L.; Motta, A.; Delferro, M.; Marks, T. J. Atom-efficient regioselective 1,2-dearomatization of functionalized pyridines by an earth-abundant organolanthanide catalyst. Nat. Chem. 2014, 6, 1100−1107. (b) Liu, H.; Khononov, M.; Eisen, M. S. Catalytic 1,2-Regioselective Dearomatization of NHeteroaromatics via a Hydroboration. ACS Catal. 2018, 8, 3673− 3677. (17) Lortie, J. L.; Dudding, T.; Gabidullin, B. M.; Nikonov, G. I. Zinc-Catalyzed Hydrosilylation and Hydroboration of N-Heterocycles. ACS Catal. 2017, 7, 8454−8459.

(18) Zhang, F.; Song, H.; Zhuang, X.; Tung, C.-H.; Wang, W. IronCatalyzed 1,2-Selective Hydroboration of N-Heteroarenes. J. Am. Chem. Soc. 2017, 139, 17775−17778. (19) For example, see the following: (a) Chirik, P.; Morris, R. Getting Down to Earth: The Renaissance of Catalysis with Abundant Metals. Acc. Chem. Res. 2015, 48, 2495−2495. (b) Chirik, P. J.; Gunnoe, T. B. A Meet-ing of Metals-A Joint Virtual Issue between Organometallics and ACS Catalysis on First-Row Transition Metal Complexes. ACS Catal. 2015, 5, 5584−5585. (20) Tasker, S. Z.; Standley, E. A.; Jamison, T. F. Recent advances in homogenous nickel catalysis. Nature 2014, 509, 299−309 and references therein.. (21) (a) Li, Y.-Y.; Yu, S.-L.; Shen, W.-Y.; Gao, J.-X. Iron-, Cobalt-, and Nickel-Catalyzed Asymmetric Transfer Hydrogenation and Asymmetric Hydrogenation of Ketones. Acc. Chem. Res. 2015, 48, 2587−2598. (b) Chakraborty, S.; Bhattacharya, P.; Dai, H.; Guan, H. Nickel and Iron Pincer Complexes as Catalysts for the Reduction of Carbonyl Compounds. Acc. Chem. Res. 2015, 48, 1995−2003. (22) Yue, H.; Guo, L.; Lee, S.-C.; Liu, X.; Rueping, M. Selective Reductive Removal of Ester and Amide Groups from Arenes and Heteroarenes through Nickel-Catalyzed C−O and C−N Bond Activation. Angew. Chem., Int. Ed. 2017, 56, 3972−3976. (23) Simmons, B. J.; Hoffmann, M.; Hwang, J.; Jackl, M. K.; Garg, N. K. Nickel-Catalyzed Reduction of Secondary and Tertiary Amides. Org. Lett. 2017, 19, 1910−1913. (24) (a) Caddick, S.; Judd, D. B.; Lewis, A. K. d. K.; Reich, M. T.; Williams, M. R. V. A generic approach for the catalytic reduction of nitriles. Tetrahedron 2003, 59, 5417−5423. (b) Nakamura, G.; Nakajima, Y.; Matsumoto, K.; Srinivas, V.; Shimada, S. Nitrile hydroboration reactions catalysed by simple nickel salts, bis(acetylacetonato)Nickel(II) and its derivatives. Catal. Sci. Technol. 2017, 7, 3196−3199. (25) (a) Brown, L. A.; Wekesa, F. S.; Unruh, D. K.; Findlater, M.; Long, B. K. BIAN-Fe(η6-C6H6): Synthesis, characterization, and llactide polymerization. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 2824−2830. (b) Wekesa, F. S.; Arias-Ugarte, R.; Kong, L.; Sumner, Z.; McGovern, G. P.; Findlater, M. Iron-Catalyzed Hydrosilylation of Aldehydes and Ketones under Solvent-Free Conditions. Organometallics 2015, 34, 5051−5056. (26) (a) Tamang, S. R.; Findlater, M. Iron Catalyzed Hydroboration of Aldehydes and Ketones. J. Org. Chem. 2017, 82, 12857−12862. (b) Arias-Ugarte, R.; Wekesa, F. S.; Schunemann, S.; Findlater, M. Iron(III)-Catalyzed Dimerization of Cycloolefins: Synthesis of HighDensity Fuel Candidates. Energy Fuels 2015, 29, 8162−8167. (c) AriasUgarte, R.; Wekesa, F. S.; Findlater, M. Selective aldol condensation or cyclotrimerization reactions catalyzed by FeCl3. Tetrahedron Lett. 2015, 56, 2406−2411. (27) The 1,4-product (87:13) is favored upon carrying out the reaction at 50 °C (Figure S3). (28) Note: we observe minimal reaction with PPh3 and slow reaction with PCyPh2. Order of reactivity based on ligand: PCy3> PCy2Ph > PCyPh2≫PPh3. (29) Complex 6 has previously been reported: Zheng, J.; Roisnel, T.; Darcel, C.; Sortais, J.-B. Nickel Catalysed reductive Amination with Hydrosilanes. ChemCatChem 2013, 5, 2861−2864. (30) (a) Schomberg, F.; Zi, Y.; Vilotijevic, I. Lewis-base-catalysed selective reductions of ynones with a mild hydride donor. Chem. Commun. 2018, 54, 3266−3269. (b) Dureen, M. A.; Lough, A.; Gilbert, T. M.; Stephan, D. W. B-H Activation by frustrated Lewis pairs: borenium or boryl phosphonium cation? Chem. Commun. 2008, 4303−4305.

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DOI: 10.1021/acscatal.8b01166 ACS Catal. 2018, 8, 6186−6191