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Cite This: Org. Lett. XXXX, XXX, XXX−XXX

Markovnikov-Selective Co(I)-Catalyzed Hydroboration of Vinylarenes and Carbonyl Compounds Piyush Kumar Verma, Sethulekshmi A. S., and K. Geetharani* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India

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S Supporting Information *

ABSTRACT: An NHC-supported Co(I) catalyst has been developed for selective Markovnikov hydroboration of vinylarenes under mild reaction conditions. The hydroboration allows highly selective synthesis of a wide range of secondary and tertiary alkyl boronates in excellent yields. Our protocol also enables hydroboration of aldehydes and ketones with diverse functional groups to access the corresponding borate esters.

T

Scheme 1. (a) Base-Free Markovnikov-Selective Hydroboration of Styrenes. (b) Markovnikov-Selective Hydroboration of 1,1-Disubstituted Terminal Alkenes Using Base as an Activator; (c) This work: Co(IMes)2ClCatalyzed Selective Markovnikov Hydroboration of Monoas Well as 1,1-Disubstituted Terminal Alkenes

he importance of boronic esters in current scenarios of chemical, medicinal, and materials science is manifested by its ubiquity in these fields mainly due to their stability and selective transformation into a wide range of functional groups via various available protocols.1 In particular, alkylboronic acid derivatives are interesting compounds in medicinal chemistry (e.g., bortezomib as an anticancer agent) and also used as versatile intermediates for transition-metal-catalyzed crosscoupling reactions.1d Conventionally, alkylboronic acid derivatives can be prepared by using alkyllithium or alkylmagnesium reagents or by using transition-metal-catalyzed borylations of hydrocarbons,2 alkyl halides, and pseudohalides.3 Nevertheless, hydroboration of alkenes by either direct4 or by metal-catalyzed reactions5 offers an atom-economical and selective route for the synthesis of alkyl boronates. Although precious metal complexes (Rh, Ru, and Ir)5 have been developed for the preparation of alkylboronic esters, it suffers from inherent toxicity, high cost, and a lack of sustainability.6 Earth-abundant metal catalysts free from such drawbacks have been reported to show equally high reactivity and selectivity and are increasingly viable alternatives to precious metals for alkene hydroboration.7 Most of the base-metal catalysts in alkene hydroboration promote anti-Markovnikov selectivity,5,8 while the reactions with Markovnikov selectivity are scarce. The first example of an unusual Markovnikov-selective hydroboration using a Rh catalyst was reported in 1989 by Hayashi and co-workers.9 In 1992, Marder and Baker et al. reported an active Rh-catalyst ([Rh(η3-2-Me-allyl){(Pri2PCH2)2}]) system for the highly selective Markovnikov hydroboration of both aryl- and aliphatic alkenes with HBcat.10a Further, notable achievements include the well-established efficacy of Rh9,10 and Pd11 systems and earth-abundant metal catalysts, such as Mn,12 Fe,13 Co,14 Ni,15 and Cu.16 A base-free protocol was developed by Zhang et al.12,14a and Thomas et al.13a for the synthesis of secondary boronic esters (Scheme 1a). A copper-based system in the presence of excess KOtBu and Pd(OAc)2-based catalyst in the presence of methanol was reported to provide branched © XXXX American Chemical Society

alkylboranes upon reaction with 1,1-disubstituted alkenes via Markovnikov regioselectivity (Scheme 1b).11,16a Recently, Findlater et al. reported that commercially available Co(acac)3 can be used as precatalyst for selective Markovnikov hydroboration of alkenes with HBpin in the presence of PPh3 as a ligand and NaOtBu as a base.17 Despite noteworthy progress in the development of satisfactory Markovnikov selectivity, most of the catalytic systems require base assistance and have limited substrate scope, particularly for the hydroboration of 1,1,2-trisubstituted alkenes to obtain tertiary alkyl boronates. Therefore, there has been a considerable Received: October 20, 2018

A

DOI: 10.1021/acs.orglett.8b03356 Org. Lett. XXXX, XXX, XXX−XXX

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can be performed at room temperature, giving the desired hydroborated product in 92% yield with 96:4 regioselectivity (entry 10). To investigate the substrate scope and limitation of this Co(I)-catalyzed Markovnikov hydroboration, a diverse array of alkenes was used, as summarized in Scheme 2. This method

challenge for the efficient transformations with broad substrate scope and improved selectivity. We recently reported the borylation of aryl halides, including aryl chlorides, using a Co(II) catalyst bearing an NHC ligand.18 Mechanistic studies suggested that Co(I) is the active catalyst to afford the targeted aryl boronic esters. This catalytic reactivity inspired us to explore the hydroboration of alkenes using Co(I) as a catalyst. Herein, we disclose a base-free approach for the synthesis of both secondary and tertiary alkyl boronates via hydroboration of terminal as well as internal alkenes using HBpin and Co(IMes)2Cl (A, IMes = 1,3bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) as a catalyst with excellent Markovnikov selectivity (Scheme 1c). In addition, this catalyst system shows the selective hydroboration of aldehydes and ketones. We embarked on our investigation with hydroboration of styrene (1) using 3 mol % of A, 1.1 equiv of HBpin at 50 °C in the absence of solvent which gave 76% of hydroborated product with 58:42 Markovnikov/anti-Markovnikov regioselectivity (Table 1, entry 1). Gratifyingly, performing the

Scheme 2. Substrate Scope of Co(I)-Catalyzed Markovnikov-Selective Hydroboration of Alkenesa,b

Table 1. Optimization of the Reaction Conditions for the Co(I)-Catalyzed Hydroboration of Styrene (1)a

entry

catalyst

1 2 3 4 5 6d 7e 8 9 10f

Co(IMes)2Cl Co(IMes)2Cl Co(IMes)2Cl Co(IMes)2Cl Co(IMes)2Cl Co(IMes)2Cl Co(IMes)2Cl Co(IMes)2Cl2 Co(IMes)2Cl

solvent

yieldb (%)

ratioc (1a/1b)

THF DME MTBE Toluene THF THF THF THF THF

76 97 95 78 45 70 97 0 0 92

58:42 95:5 95:5 83:17 68:32 96:4 95:5 a

Yields (a + b) were determined by 1H NMR analysis using nitromethane as an internal standard. Isolated yields after chromatographic workup are given in parentheses. bProduct ratios determined from 1H NMR of crude reaction mixtures. cReaction was performed with 2.2 equiv of HBpin. dReaction was performed at 70 °C. eThe structure of 12a was confirmed by single-crystal X-ray diffraction. f With allyl benzene (22), 15a was obtained as major product. For product distribution, see the Supporting Information.

0 96:4

a

Reaction conditions: 1 (0.1 mmol), HBpin (0.11 mmol), Co(IMes)2Cl (3 mol %) in 0.4 mL of THF at 50 °C for 2 h unless otherwise stated. bCombined yields (1a + 1b) were determined by 1H NMR analysis using nitromethane as an internal standard. cProduct ratios determined from 1H NMR of crude reaction mixtures. dThe reaction was performed using 1 mol % of A. eThe reaction was performed using 5 mol % of A. fThe reaction was performed at room temperature.

worked efficiently for vinylarenes that bear electron-donating groups (2−8), such as p-methyl (2), p-methoxy (3), pdimethylamine (4), p-amine (5), and m-methoxy (6) styrenes gave the secondary boronic ester in good yields and selectivity. A slightly lower yield of 68% was observed in the case of p(trimethlsilyl)styrene (7). Styrene derivatives bearing electronwithdrawing halogen substituents at the para (9) and meta (10) position gave high yields of the hydroborated products, albeit with slightly reduced Markovnikov selectivity. 4(Trifluoromethyl)styrene (11) gave lower yield and selectivity (61:39). We observed the Markovnikov selective hydroboration of 1,1-disubstituted (12−13) and 1,1,2-trisubstituted (17) vinylarenes to afford tertiary boronic esters in excellent yield and selectivity, which are consistent with that of Rh-,10a Cu-,16 and Pd-based catalyst systems.11 These products are highly useful precursors due to their extensive applications in organic synthesis, particularly C−C bond-forming reactions.1d

reaction in THF resulted in 97% yield with excellent Markovnikov selectivity (entry 2). Given this observation, the solvent effect was explored. The use of polar 1,2dimethoxyethane (DME) gave the hydroboration product in comparable yield and selectivity (entry 3), while the use of less polar MTBE (methyl tert-butyl ether) provided lower yield and selectivity (83:17; entry 4). Using toluene as a solvent gave a lower yield with poor regioselectivity (entry 5). When the catalyst loading was reduced to 1 mol %, hydroborated product was obtained in moderate yield (entry 6), whereas increasing the catalyst loading to 5 mol % failed to improve the reaction (entry 7). No evidence of hydroborated product was observed while using Co(II) as a catalyst (entry 8). A control experiment demonstrated that no boronic ester formed in the absence of a catalyst (entry 9). Importantly, the reaction B

DOI: 10.1021/acs.orglett.8b03356 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters These results are in contrast with the recent report on the Co/ PPh3 catalytic system,17 where the α-methylstyrene (13) gave anti-Markovnikov-selective product.12,19 Sterically congested 1,3,5-trimethyl-2-vinylbenzene (14) was hydroborated in good yield, but with poor selectivity. This Co(I)-catalyzed hydroboration reaction can also be successfully applied to internal olefins (15−17) with HBpin, affording the Markovnikov addition products in high isolated yields. Interestingly, hydroboration of 15 gave exclusively a branched isomer, 15a, in 99% yield. Aliphatic alkenes, internal (18) as well as terminal (19 and 20), gave hydroborated products with anti-Markovnikov selectivity. N-Vinylcarbazole (21) gave a 90% yield of anti-Markovnikov product. The hydroboration of allylbenzene (22) with HBpin undergoes alkene isomerization−hydroboration, yielding 1-phenylpropylpinacolborane (15a) as a major product. This selectivity was previously observed in the rhodium-catalyzed reaction of allylbenzene with BH3·THF20 and with the cobalt−phosphine complex reported by Chirik.21 The gram-scale reaction could be carried out smoothly for styrene (1) to afford 1a in 80% yield with regioselectivity. Encouraged by the olefin hydroboration, we then turned our attention to exploring the activity of A with aldehydes and ketones (Scheme 3).22 The hydroboration of benzaldehyde

Scheme 4. Selective Hydroboration of Carbonyl Compounds by A

(IMes)2Cl], A, and HBpin at 50 °C in THF afforded a colorless product, I (Scheme 5a; see the experiment S3 in Scheme 5. Mechanistic Investigations

Supporting Information).23 This four-coordinate boron compound results from the intramolecular C(sp 3)−H activation of a methyl C−H bond of one of the mesityl groups of NHC ligand, confirmed by single-crystal X-ray diffraction analysis (Figure S8). The high-field 11B NMR signal of I (δ = 3.9 ppm) is consistent with a tetracoordinate boranebased adduct. Presumably, one of the NHC ligands dissociates from A to give a free site for the alkene to coordinate to the Co center. To confirm this, the catalytic reaction was performed in the presence of excess IMes ligand, which gave a trace amount of hydroborated product. Notably, compound I was observed to be inactive throughout the reaction (see the Supporting Information). This observation obviously suggested an in situ generated active catalyst. We speculate that the elimination of I generates the (IMes)CoCl species “B”, which undergoes oxidative addition of HBpin to furnish the Co(III) intermediate (C, supported by HRMS; see the Supporting Information) followed by alkene coordination and its insertion into the Co−H bond. Subsequent reductive elimination gives vinylboronic esters and regenerates the active catalyst species “B” as described by Fout et al.5l and others.8f,15,24 The lack of formation of vinylboronate ester as a side product and the observed regioselectivity supports the insertion of alkene into the Co−H rather than Co−B bond.10b A similar mechanism has been proposed for the cobalt-catalyzed hydrosilylation of alkynes by Deng and co-workers.25 However, this is different from the other Co-catalyzed hydroboration or hydrosilylation reactions, where the active Co(I) species is capable of undergoing a σ-bond metathesis reaction pathway.26 We speculate that the Markovnikov regioselectivity obtained in styrene derivatives might be due to the η 3 -benzylic coordination of the styryl groups, as observed in the case of Ni15 and Rh10a,c,27 catalyst systems. Further evidence for the assumption of Markovnikov selectivity came from the observation that aliphatic alkenes (19 and 20) and vinyl groups having no benzene rings as substituents (21) gave selectively anti-Markovnikov product, which might be due to the lack of π-benzylic interactions.

Scheme 3. Co(I)-Catalyzed Hydroboration of Carbonyl Compounds.a

a Reaction conditions: carbonyl (0.5 mmol), A (1 mol %), HBpin (1.1 equiv), THF (0.8 mL), at room temperature for 2 h unless otherwise stated. Yields were determined by 1H NMR, using nitromethane as an internal standard. bReaction was performed at 50 °C for 2 h.

(23a) with HBpin proceeded in 98% yield, albeit at a low catalyst loading of 1 mol %, as assayed by 1H NMR spectroscopy. Carbonyls bearing electron-donating (23b and 23f) as well as electron-withdrawing (23c, 23d, 23g, and 23i) groups were found to be compatible with this catalyst system, giving the desired borate esters in good to excellent yields. Seeking to probe the chemoselectivity of this Co(I) catalyst system further, the hydroboration of trans-cinnamaldehyde (23j) was attempted, showing selective hydroboration at the carbonyl part, with no observable reduction of the olefin functional group. Moreover, the chemoselectivity was upheld in intermolecular hydroboration reactions involving equimolar mixtures of styrene (1), carbonyl compounds (23a or 23e), and HBpin, which gave selectively the reduction of CO bonds (Scheme 4). Having an established the scope and utility of the Markovnikov-selective hydroboration method, attention was devoted toward understanding the mechanistic features of this transformation. Heating an equimolar mixture of [CoC

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Catalyzed Cross-Coupling Reactions of Organoborane Compounds. Chem. Rev. 1995, 95, 2457−2483. (d) Boronic Acids: Preparation and Applications in Organic Synthesis, Medicine and Materials, 2nd ed.; Hall, D. G., Ed.; Wiley-VCH: Weinheim, 2011. (2) (a) Shimada, S.; Batsanov, A. S.; Howard, J. A. K.; Marder, T. B. Formation of Aryl and Benzylboronate Esters by Rhodium-Catalyzed C−H Bond Functionalization with Pinacolborane. Angew. Chem., Int. Ed. 2001, 40, 2168−2171. (b) Mkhalid, I. A. I.; Barnard, J. H.; Marder, T. B.; Murphy, J. M.; Hartwig, J. F. C-H Activation for the Construction of C-B Bonds. Chem. Rev. 2010, 110, 890−931. (c) Partyka, D. V. Transmetalation of Unsaturated Carbon Nucleophiles from Boron-Containing Species to the Mid to Late DBlock Metals of Relevance to Catalytic C−X Coupling Reactions (X = C, F, N, O, Pb, S, Se, Te). Chem. Rev. 2011, 111, 1529−1595. (d) Hartwig, J. F. Borylation and Silylation of C−H bonds: A Platform for Diverse C−H Bond Functionalizations. Acc. Chem. Res. 2012, 45, 864−873. (e) Manna, K.; Ji, P.; Lin, Z.; Greene, F. X.; Urban, A.; Thacker, N. C.; Lin, W. Chemoselective Single-Site EarthAbundant Metal Catalysts at Metal−Organic Framework Nodes. Nat. Commun. 2016, 7, 12610−12620. (f) Zuo, Z.; Wen, H.; Liu, G.; Huang, Z. Cobalt-Catalyzed Hydroboration and Borylation of Alkenes and Alkynes. Synlett 2018, 29, 1421−1429. (3) (a) Yang, C.-T.; Zhang, Z.-Q.; Tajuddin, H.; Wu, C.-C.; Liang, J.; Liu, J.-H.; Czyzewska, M.; Steel, P. G.; Marder, T. B.; Liu, L.; Fu, Y. Alkylboronic Esters from Copper-Catalyzed Borylation of Primary and Secondary Alkyl Halides and Pseudohalides. Angew. Chem., Int. Ed. 2012, 51, 528−532. (b) Ito, H.; Kubota, K. Copper(I)-Catalyzed Boryl Substitution of Unactivated Alkyl Halides. Org. Lett. 2012, 14, 890−893. (c) Dudnik, A. S.; Fu, G. C. Nickel-Catalyzed Coupling Reactions of Alkyl Electrophiles, Including Unactivated Tertiary Halides, to Generate Carbon−Boron Bonds. J. Am. Chem. Soc. 2012, 134, 10693−10697. (d) Yi, J.; Liu, J.-H.; Liang, J.; Dai, J.-J.; Yang, C.T.; Fu, Y.; Liu, L. Alkylboronic Esters from Palladium- and NickelCatalyzed Borylation of Primary and Secondary Alkyl Bromides. Adv. Synth. Catal. 2012, 354, 1685−1691. (e) Bedford, R. B.; Brenner, P. B.; Carter, E.; Gallagher, T.; Murphy, D. M.; Pye, D. R. IronCatalyzed Borylation of Alkyl, Allyl, and Aryl Halides: Isolation of an Iron(I) Boryl Complex. Organometallics 2014, 33, 5940−5943. (f) Bose, S. K.; Fucke, K.; Liu, L.; Steel, P. G.; Marder, T. B. ZincCatalyzed Borylation of Primary, Secondary and Tertiary Alkyl Halides with Alkoxy Diboron Reagents at Room Temperature. Angew. Chem., Int. Ed. 2014, 53, 1799−1803. (g) Zhang, Z.-Q.; Yang, C.-T.; Liang, L.-J.; Xiao, B.; Lu, X.; Liu, J.-H.; Sun, Y.-Y.; Marder, T. B.; Fu, Y. Copper-Catalyzed/Promoted Cross-Coupling of gem-Diborylalkanes with Nonactivated Primary Alkyl Halides: An Alternative Route to Alkylboronic Esters. Org. Lett. 2014, 16, 6342−6345. (h) Atack, T. C.; Cook, S. P. Manganese-Catalyzed Borylation of Unactivated Alkyl Chlorides. J. Am. Chem. Soc. 2016, 138, 6139−6142. (i) Bose, S. K.; Brand, S.; Omoregie, H. O.; Haehnel, M.; Maier, J.; Bringmann, G.; Marder, T. B. Highly Efficient Synthesis of Alkylboronate Esters via Cu(II)-Catalyzed Borylation of Unactivated Alkyl Bromides and Chlorides in Air. ACS Catal. 2016, 6, 8332−8335. (4) (a) Brown, H. C.; Gupta, S. K. Catecholborane (1,3,2Benzodioxaborole) as a New, General Monohydroboration Reagent for Alkynes. A Convenient Synthesis of Alkeneboronic Esters and Acids from Alkynes via Hydroboration. J. Am. Chem. Soc. 1972, 94, 4370−4371. (b) Brown, H. C.; Gupta, S. K. 1,3,2-Benzodioxaborole (Catecholborane) as a New Hydroboration Reagent for Alkenes and Alkynes. A General Synthesis of Alkane and Alkeneboronic Acids and Esters via Hydroboration. Directive Effects in the Hydroboration of Alkenes and Alkynes with Catecholborane. J. Am. Chem. Soc. 1975, 97, 5249−5255. (c) Tucker, C. E.; Davidson, J.; Knochel, P. Mild and Stereoselective Hydroborations of Functionalized Alkynes and Alkenes Using Pinacolborane. J. Org. Chem. 1992, 57, 3482−3485. (5) (a) Evans, D. A.; Fu, G. C.; Hoveyda, A. H. Rhodium(I)Catalyzed Hydroboration of Olefins. The Documentation of Regioand Stereochemical Control in Cyclic and Acyclic Systems. J. Am. Chem. Soc. 1988, 110, 6917−6918. (b) Baker, R. T.; Ovenall, D. W.; Calabrese, J. C.; Westcott, S. A.; Taylor, N. J.; Williams, I. D.; Marder,

Further, kinetic studies were performed for the catalytic hydroboration of trans-β-methylstyrene (15) by HBpin. A firstorder dependence of the reaction rate in both [catalyst] and [HBpin] as well as zero-order dependence in [15] was observed. These results imply that the reaction between catalyst and HBpin is the rate-limiting step of the operating catalytic cycle. In summary, we have developed a Markovnikov-selective, NHC-stabilized, cobalt-catalyzed hydroboration of alkenes with pinacolborane. The reaction proceeds under mild conditions, displays broad substrate scope and functional group tolerance, and furnishes secondary boronic esters in both high yield and regioselectivity. Importantly, this method gives access to valuable branched alkylboronic esters from challenging multisubstituted internal alkenes hydroboration. Additionally, this protocol is amenable to the hydroboration of various aldehydes and ketones with HBpin. Further studies on details of the reaction mechanism will be the subject of future work.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b03356. Experimental and spectroscopic data, 1H, 13C{1H}, and 11 1 B{ H} spectra, and GC−MS data (PDF) Accession Codes

CCDC 1864217 and 1864220 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

K. Geetharani: 0000-0003-2064-1297 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to AllyChem Co. Ltd., China, for a gift of B2pin2 and the SERB (EMR/2016/000367), DST inspire faculty award (DST/INSPIRE/04/2015/000785), and Indian Institute of Science (IISc) start-up research grant for funding. P.K.V. thanks IISc Bangalore for a research fellowship. We also thank Prof. S. Ramakrishnan, IISc Bangalore, for the GC−MS analysis.

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REFERENCES

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DOI: 10.1021/acs.orglett.8b03356 Org. Lett. XXXX, XXX, XXX−XXX