Heteroleptic Nickel Complexes for the Markovnikov-Selective

Communication pubs.acs.org/Organometallics

Heteroleptic Nickel Complexes for the Markovnikov-Selective Hydroboration of Styrenes Eric E. Touney, Ryan Van Hoveln, Carl T. Buttke, Michael D. Freidberg, Ilia A. Guzei, and Jennifer M. Schomaker* Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706, United States S Supporting Information *

ABSTRACT: First-row transition metal catalysis offers a cheaper, more environmentally sustainable alternative to second- and third-row transition metal catalysts. Nickel has shown great promise as a tool for the borylation of unsaturated compounds to yield boronic esters, but Markovnikov-selective hydroborations of simple styrenes have not been well-explored. Herein, we report the synthesis of benzyl boronic esters via nickel-catalyzed hydroboration of styrenes using a heteroleptic N-heterocyclic carbene (NHC)−phosphine nickel complex, IMes(Cy3P)NiCl2. The IMes(Cy3P)NiCl2 complex displays a broad substrate scope and maintains the integrity of yield and regioselectivity when challenged with substrates bearing increased steric hindrance. The heteroleptic complexes also tolerate both electron-withdrawing and -donating groups, in contrast to traditional bis-phosphine and Ni(0) complexes.

F

Table 1. Initial Screening of Ni-Catalyzed Hydroboration

irst-row transition metal catalysts exhibit many advantages over second- and third-row transition metals, including decreased cost, lower environmental impact, and increased sustainability. Transformations of alkenes using first-row metal catalysts are particularly attractive, as the substrates are readily available and the products serve as useful synthetic building blocks. For example, metal-catalyzed alkene hydroborations have received much attention over the last several years, with the resulting organoboranes functioning as versatile intermediates for the preparation of diverse synthetic building blocks and crosscoupled products.1,2 Traditionally, precious metals, including rhodium, iridium, and platinum, have been employed in alkene hydroboration protocols, but recent work has seen the emergence of first-row transition metals, most notably copper, cobalt, and iron, as alternative catalysts.1,3,4 While nickel has shown promise for the borylations of aryl and allyl halides as well as borylative carbonyl-diene couplings and 1,4-hydroborations of 1,3-dienes,5−7 surprisingly, there are only two reports of nickelcatalyzed hydroboration of styrenes.8,9 In addition, neither of these reported protocols yields good Markovnikov-selective addition to styrenyl or terminal alkyl olefins.8,9 Herein, we report heteroleptic Ni catalysts selective for the Markovnikov hydroboration of styrenes to benzyl boronic esters; both electrondonating and -withdrawing groups on the aromatic ring are welltolerated. Efforts to tune the regioselectivity of the hydroboration through the identity of the ligand are also described. We began our investigations of nickel-catalyzed hydroboration by exploring a variety of readily available mixed-donor Ni complexes (Table 1). Encouragingly, the reaction of styrene 1a in the presence of pinacolborane (HBpin) and a t-BuOK additive produced the desired product 2a in good to excellent yields with several Ni complexes, including Ni(cod)2 3a (entry 1). However, Ni(cod)2 exhibited limited reactivity with other styrenes (entries 2−4), particularly with electron-rich substrates, such as p© XXXX American Chemical Society

entry

R

Ni complex

1 2 3 4 5 6 7 8 9 10 11 12 13

H 1a o-Me 1b p-OMe 1c o-CF3 1g H 1a o-Me 1b p-OMe 1c o-CF3 1g H 1a H 1a H 1a H 1a H 1a

Ni(cod)2 Ni(cod)2 Ni(cod)2 Ni(cod)2 (Cy3P)2NiCl2 (Cy3P)2NiCl2 (Cy3P)2NiCl2 (Cy3P)2NiCl2 (tBu3P)2NiCl2 (tBu2MeP)2NiCl2 (nBu3P)2NiCl2 (Et3P)2NiCl2 (IMes)2NiCl2b

yielda 3a 3a 3a 3a 3b 3b 3b 3b 3c 3d 3e 3f 3g

99% 27% 0% 58% 73% 43% 10% 53% 10% 23% 0% 0% 0%

2a 2b 2c 2g 2a 2b 2c 2g 2a 2a 2a 2a 2a

a

NMR yields determined using 1,1,1,2-tetrachloroethane as an internal standard. bIMes: 1,3-bis(2,4,6-trimethylphenyl)-imidazolium.

methoxystyrene 1b (entry 3). Bidentate ligands, including Ni(dppp)Cl2 (dppp = 1,3-bis(diphenylphosphino)propane) and Ni(dCype)Cl2 (dCype = 1,2-bis(dicyclohexylphosphino)ethane) resulted in mixtures that included reduction, linear boronic ester, and vinyl boronic ester products. Switching to precatalyst (Cy3P)2NiCl2 3b still produced variable results for different substrates (entries 5−8), but this catalyst exhibited more consistent reactivity as compared to Ni(cod)2. With this in Received: August 15, 2016

A

DOI: 10.1021/acs.organomet.6b00652 Organometallics XXXX, XXX, XXX−XXX

Communication

Organometallics mind, a series of Ni complexes supported by bis-phosphine and bis-NHC ligands were synthesized in hopes of identifying a Ni catalyst suitable for a broader scope of styrenes (Table 1, entries 9−13). Increasing the sterics of the ligand in 3c,d reduced the yield (entries 9−10), while using less bulky phosphines in 3e,f gave no desired product at all (entries 11−12). The mixed-donor (IMes)2NiCl2 complex 3g also shut down reactivity (entry 13). One possibility for the poor reactivity using mixed-donor Ni complexes might be the slow ligand dissociation from the metal center. As heteroleptic metal complexes are known to exhibit different reactivity profiles than their mixed-donor counterparts, we were curious if combining one phosphine ligand with one NHC ligand might offer Ni complexes with good reactivity across a wider range of styrenes.10 The influence of the trans effect of the NHC was of particular interest,10 as it would be expected to increase the rate of phosphine dissociation. Accordingly, the heteroleptic carbene/phosphine nickel complex, IPr(Cy3P)NiCl2 3h (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2ylidene), was synthesized using a method similar to that reported by Matsubara et al.11 Interestingly, 3h restored reactivity in the hydroboration (Table 2, column 2) and furnished the most

Table 3. Substrate Scope of Hydroboration Catalyzed by a Heteroleptic Ni Complex

Table 2. Comparison of Nickel Complex Reactivity

R H o-Me p-OMe

1a 1b 1c

(Cy3P)2NiCl2 3ba

IPr(Cy3P)NiCl2 3ha

IMes(Cy3P)NiCl2 3ia

73% 43% 10%

77% 72% 50% (6%)

99% 73% 86% (3%)

a

NMR yields determined using 1,1,1,2-tetrachloroethane as an internal standard. (Yield of linear product shown in parentheses.)

reproducible results with both electron-rich and -poor styrenes 1b and 1c. Further optimization of the carbene ligand showed that IMes(Cy3P)NiCl2 3i improved the yields compared to those of 3h (Table 2, column 3). The substrate scope was explored using IMes(Cy3P)NiCl2 3i as the precatalyst (Table 3). In contrast to Ni(cod)2 and mixeddonor phosphine and NHC-supported Ni complexes, the yield was not dramatically affected by the electronics of the substrate. Both electron-rich styrenes 1b−e (entries 2−5) and electrondeficient styrenes 1f−h (entries 6−8) gave comparable isolated yields. Additionally, the reaction conditions were tolerant of substitution on the olefin (entries 9−10), as well as polyaromatics (entries 11−12). Hydroboration was not overly affected by ortho substitution near the reactive site, in spite of the steric bulk of the catalyst complex (entries 2, 5, and 7). In all cases, selectivity for the branched boronic ester was strongly preferred over the linear boronic ester; in most cases, no linear product was observed. Notably, halogenated aromatics containing bromine did not perform well in the reaction, presumably due to competing borylation of the aryl halide (Figure 1).5 However, a moderate yield of 2m was noted in the hydroboration of 1m, containing an aryl chloride (entry 13). Unfortunately, a number of more functionalized substrates did not perform well in the hydroboration (Figure 1), but these substrates have also proved to be challenging for other transition metal hydroboration catalysts.

a

Isolated yield on a 1.0 mmol scale. bNMR yields shown in parentheses determined using 1,1,1,2-tetrachloroethylene as an internal standard on a 0.25 mmol scale. cCatalyst loading was doubled.

Figure 1. Substrates that perform poorly in the Ni-catalyzed hydroboration.

It was noted that the selectivity for the branched benzyl boronic ester resulting from borylation of p-methoxystyrene 1c (Table 2) was improved using IMes-supported Ni complex 3i (25:1 branched:linear) over IPr complex 3h (8.3:1 branched:B

DOI: 10.1021/acs.organomet.6b00652 Organometallics XXXX, XXX, XXX−XXX

Communication

Organometallics linear). As IPr is sterically bulkier than IMes, we wondered if the selectivity of the hydroboration could potentially be reversed to favor the linear product by changing the sterics of the NHC ligand to a greater extent. To test this possibility, the sterically demanding nickel complex IPr*(Cy3P)NiCl2 3j was synthesized and employed for the hydroboration of selected styrenes (Table 4).12 Despite its increased bulk, 3j exhibited reasonable reactivity,

Scheme 1. Potential Catalytic Cycle

Table 4. Effect of Steric Bulk of the Catalyst on Regioselectivity in the Ni-Catalyzed Hydroborationa

a

entry

substrate (R)

1b

2

4

b:l ratio

1 2 3 4

1a (H) 1c (p-MeO) 1f (p-Ph) 1h (p-CF3)

0% 20% 17% 5%

48% 36% 41% 39%

33% 27% 31% 35%

1.5:1 1.3:1 1.3:1 1.1:1

Table 5. Effect of Added Cy3P on the Yield of the Hydroboration Reaction Catalyzed by IPr(Cy3P)NiCl2 (3h)

Catalyst:

b

NMR yields determined using 1,1,1,2-tetrachloroethane as an internal standard.

entry

added Cy3P

yield 2aa

1 2 3 4

0 equiv 0.1 equiv 0.5 equiv 1.0 equiv

77% (Table 2) 29% 8% 1%

a

NMR yields determined using 1,1,1,2-tetrachloro-ethane as an internal standard.

providing good conversion of the starting materials and significant amounts of the linear products. These unoptimized results indicate that product distribution could be controlled by judicious choice of the NHC ligand but likely at the cost of decreased conversion. There are several potential mechanisms for hydroboration catalyzed by heteroleptic nickel complexes; one possibility is illustrated in Scheme 1. Ni(II) precatalyst A is proposed to be reduced by HBpin to generate 14 e− Ni(0) species C. It is likely that the phosphine ligand dissociates, as the strong trans effect of the NHC ligand would be expected to facilitate such an event, presumably prior to oxidative addition of C into the B−H bond of HBpin to form D. Further evidence in support of phosphine dissociation is provided by the observation that doping the reaction using IPr(Cy3P)NiCl2 as the catalyst with excess Cy3P leads to a significant loss in yield and eventual shutdown of the reaction (Table 5). In any case, ligand dissociation frees a site for the alkene to coordinate to the Ni to give E. At this point, the alkene can potentially insert into the Ni−H bond in two ways, eventually yielding either the branched or linear boronic ester products. Smaller NHC ligands prefer 1,2-addition to produce the benzyl nickel to yield proposed intermediates F/G, which are stabilized by conjugation with the π-system of the aromatic ring. Indeed, the Markovnikov regioselectivity observed in our Nicatalyzed hydroborations supported by small NHC ligands mirror the stabilization of the catalyst via π-benzylic interactions, as described by Hayashi et al. in Rh-catalyzed hydroboration.13 Species represented by F/G have been hypothesized as

intermediates in other metal-catalyzed reactions, as well.14,15 In contrast, if the NHC is sufficiently bulky, as in the case where IPr* is the carbene ligand, then the π-benzyl intermediate may be disfavored by steric repulsion between the aromatic ring and the ligand. Thus, the Ni can also add to the β carbon of the olefin to yield significant amounts of H. Nickel intermediates F−H would then be expected to undergo reductive elimination to produce the respective branched or linear boronic esters and regenerate the Ni(0) catalyst. Presumably, an even bulkier NHC ligand than 3j could disfavor the formation of the benzyl nickel species to a greater extent; however, reactivity may suffer given the bulk of the nickel complex and attempts were not undertaken to further optimize a linear selective catalyst at this time. Studies of Rh-catalyzed hydroboration carried out by Halpern et al.16 showed that when the alkene substrate was too electronrich the equilibrium between the free and coordinated alkene heavily favored species analogous to D (Scheme 1). Ligand dissociation in mixed-donor complexes may not be facile enough to favor effective coordination of electron-rich alkenes to the Ni center; however, the trans effect exhibited by the NHC ligand on the dissociation of the phosphine may be helpful in this regard. Once the alkene has been coordinated to Ni, migratory insertion into the M−H bond should be facilitated by the presence of resonance electron-donating groups. We have shown that nickel can effectively catalyze the hydroboration of both electron-rich and -poor styrenes. Use of new, heteroleptic carbene/phosphine nickel complexes enables C

DOI: 10.1021/acs.organomet.6b00652 Organometallics XXXX, XXX, XXX−XXX

Communication

Organometallics

K.; Ito, H. Chem. Commun. 2016, 52, 5916−5919. (r) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457−2483. (s) Burgess, K.; van der Donk, W. A.; Westcott, S. A.; Marder, T. B.; Baker, R. T.; Calabrese, J. C. J. Am. Chem. Soc. 1992, 114, 9350−9359. (t) Waltz, K. M.; He, X.; Muhoro, C.; Hartwig, J. F. J. Am. Chem. Soc. 1995, 117, 11357−11358. (3) For selected references, see (a) Lata, C. J.; Crudden, C. M. J. Am. Chem. Soc. 2010, 132, 131−137. (b) Cipot, J.; Vogels, C. M.; McDonald, R.; Westcott, S. A.; Stradiotto, M. Organometallics 2006, 25, 5965−5968. (c) Crudden, C. M.; Hleba, Y. B.; Chen, A. C. J. Am. Chem. Soc. 2004, 126, 9200−9201. (d) Männig, D.; Nöth, H. Angew. Chem., Int. Ed. Engl. 1985, 24, 878−879. (4) For selected references, see (a) Noh, D.; Yoon, S. K.; Won, J.; Lee, J. Y.; Yun, J. Chem. - Asian J. 2011, 6, 1967−1969. (b) Noh, D.; Chea, H.; Ju, J.; Yun, J. Angew. Chem., Int. Ed. 2009, 48, 6062−6064. (c) Schmid, S. C.; Van Hoveln, R.; Rigoli, J. W.; Schomaker, J. M. Organometallics 2015, 34, 4164−4173. (d) Palmer, W. N.; Diao, T.; Pappas, I.; Chirik, P. J. ACS Catal. 2015, 5, 622−626. (e) Zheng, J.; Sortais, J.-B.; Darcel, C. ChemCatChem 2014, 6, 763−766. (f) Zhang, L.; Zuo, Z.; Leng, X.; Huang, Z. Angew. Chem., Int. Ed. 2014, 53, 2696−2700. (5) For examples of coupling to make boronic esters, see (a) Sogabe, Y.; Namikoshi, T.; Watanabe, S.; Murata, M. Synthesis 2012, 44, 1233− 1236. (b) Zhang, P.; Roundtree, I. A.; Morken, J. P. Org. Lett. 2012, 14, 1416−1419. (c) Yi, J.; Liu, J.-H.; Liang, J.; Dai, J.-J.; Yang, C.-T.; Fu, Y.; Liu, L. Adv. Synth. Catal. 2012, 354, 1685−1691. (d) Tobisu, M.; Nakamura, K.; Chatani, N. J. Am. Chem. Soc. 2014, 136, 5587−5590. (e) Furukawa, T.; Tobisu, M.; Chatani, N. Chem. Commun. 2015, 51, 6508−6511. (f) Huang, K.; Yu, D.-G.; Zheng, S.-F.; Wu, Z.-H.; Shi, Z.-J. Chem. - Eur. J. 2011, 17, 786−791. (g) Molander, G. A.; Cavalcanti, L. N.; Garcia-Garcia, C. J. Org. Chem. 2013, 78, 6427−6439. (h) Zhang, H.; Hagihara, S.; Itami, K. Chem. Lett. 2015, 44, 779−781. (6) For examples of diboration, see (a) Cho, H. Y.; Yu, Z.; Morken, J. P. Org. Lett. 2011, 13, 5267−5269. (b) Ely, R. J.; Morken, J. P. Org. Lett. 2010, 12, 4348−4351. (c) Cho, H. Y.; Morken, J. P. J. Am. Chem. Soc. 2008, 130, 16140−16141. (d) Cho, H. Y.; Morken, J. P. J. Am. Chem. Soc. 2010, 132, 7576−7577. (7) For examples of 1,4-hydroboration, see. (a) Ely, R. J.; Yu, Z.; Morken, J. P. Tetrahedron Lett. 2015, 56, 3402−3405. (b) Crotti, S.; Bertolini, F.; Macchia, F.; Pineschi, M. Org. Lett. 2009, 11, 3762−3765. (8) Pereira, S.; Srebnik, M. Tetrahedron Lett. 1996, 37, 3283−3286. (9) Kabalka, G. W.; Narayana, C.; Reddy, N. K. Synth. Commun. 1994, 24, 1019−1023. (10) For selected references on heteroleptic Ni and first-row metal complexes, see (a) Kaeser, A.; Mohankumar, M.; Mohanraj, J.; Monti, F.; Holler, M.; Cid, J.-J.; Moudam, O.; Nierengarten, I.; KarmazinBrelot, L.; Duhayon, C.; Delavaux-Nicot, B.; Armaroli, N.; Nierengarten, J.-F. Inorg. Chem. 2013, 52, 12140−12151. (b) Toyoda, R.; Tsuchiya, M.; Sakamoto, R.; Matsuoka, R.; Wu, K.-H.; Hattori, Y.; Nishihara, H. Dalton Trans. 2015, 44, 15103−15106. (11) Matsubara, K.; Ueno, K.; Shibata, Y. Organometallics 2006, 25, 3422−3247. (12) Berthon-Gelloz, G.; Siegler, M. A.; Spek, A. L.; Tinant, B.; Reek, J. N. H.; Marko, I. E. Dalton Trans. 2010, 39, 1444−1446. (13) Hayashi, T.; Matsumoto, Y.; Ito, Y. Tetrahedron: Asymmetry 1991, 2, 601−612. (14) For selected examples of metals engaging in π-benzyl interactions, see (a) Cotton, F. A.; Marks, T. J. J. Am. Chem. Soc. 1969, 91, 1339− 1346. (b) Cotton, F. A.; LaPrade, M. D. J. Am. Chem. Soc. 1968, 90, 5418−5422. (c) Becker, Y.; Stille, J. K. J. Am. Chem. Soc. 1978, 100, 845− 850. (d) Crascall, L. E.; Litster, S. A.; Redhouse, A. D.; Spencer, J. L. J. Organomet. Chem. 1990, 394, C35−C38. (e) Fryzuk, M. D.; McConville, D. H.; Rettig, S. J. J. Organomet. Chem. 1993, 445, 245− 256. (f) Bleuel, E.; Schwab, P.; Laubender, M.; Werner, H. J. Chem. Soc., Dalton Trans. 2001, 266−273. (15) Tasker, S. Z.; Standley, E. A.; Jamison, T. F. Nature 2014, 509, 299−309. (16) For selected references, see (a) Halpern, J.; Okamoto, T.; Zakhariev, A. J. Mol. Catal. 1977, 2, 65−68. (b) Halpern, J.; Okamoto, T. Inorg. Chim. Acta 1984, 89, L53−54. (c) Burgess, K.; Van der Donk, W. A.; Kook, A. M. J. Org. Chem. 1991, 56, 2949−2951.

excellent selectivity for the branched hydroboration product over the linear hydroboration product as well as overall good yields. Future studies will focus on the use of these new nickel complexes to carry out a range of other useful transformations, particularly in an asymmetric manner.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00652. Detailed experimental procedures; characterization and crystallographic data (PDF) NMR spectra (PDF) Crystallographic information file for 3h (CIF) Crystallographic information file for 3i (CIF) Crystallographic information file for 3j (CIF) Cartesian coordinates for 3h (XYZ) Cartesian coordinates for 3h (collection of three models) (MOL)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research was supported by start-up funds provided by the University of Wisconsin−Madison and the ACS-PRF #53146ND1. We thank Brian Dolinar and Kelsey Miles of the University of Wisconsin-Madison for their assistance with X-ray crystallography. Steve Schmid of UW-Madison is acknowledged for helpful suggestions and editorial assistance.

■

REFERENCES

(1) For selected references on metal-catalyzed hydroboration, see (a) Evans, D. A.; Fu, G. C.; Hoveyda, A. H. J. Am. Chem. Soc. 1988, 110, 6917−6918. (b) Beller, M.; Bolm, C. MetalCatalyzed Hydroboration Reactions, in Transition Metals for Organic Synthesis: Building Blocks and Fine Chemicals; Wiley-VCH Verlag GmbH: Weinheim, Germany, 1998. (c) Vogels, C. M.; Westcott, S. A. Curr. Org. Chem. 2005, 9, 687−699. (d) Huang, S.; Xie, Y.; Wu, S.; Jia, M.; Wang, J.; Xu, W.; Fang, H. Curr. Org. Synth. 2013, 10, 683−696. (e) Burgess, K.; Ohlmeyer, M. J. Chem. Rev. 1991, 91, 1179−1191. (2) (a) Ritleng, V.; Sirlin, C.; Pfeffer, M. Chem. Rev. 2002, 102, 1731− 1770. (b) Lyons, T. W.; Sanford, M. S. Chem. Rev. 2010, 110, 1147− 1169. (c) Diedrich, F., Strang, P. J., Eds. Metal-Catalyzed Cross-coupling Reactions; Wiley-VCH: Weinheim, 1998. (d) Phapale, V. B.; Cardenas, D. Chem. Soc. Rev. 2009, 38, 1598−1607. (e) McDonald, R. I.; Liu, G.; Stahl, S. S. Chem. Rev. 2011, 111, 2981−3019. (f) Farina, V. Adv. Synth. Catal. 2004, 346, 1553−1582. (g) Jana, R.; Pathak, T. P.; Sigman, M. S. Chem. Rev. 2011, 111, 1417−1492. (h) Xu, L.-M.; Li, B.-J.; Yang, Z.; Shi, Z.-J. Chem. Soc. Rev. 2010, 39, 712−733. (i) Tamaru, Y., Ed. Modern Organonickel Chemistry; Wiley: Weinheim, 2005. (j) Krause, N., Ed. Modern Organocopper Chemistry; Wiley: Weinheim, 2002. (k) Rosen, B. M.; Quasdorf, K. W.; Wilson, D. A.; Zhang, N.; Resmerita, A.; Garg, N. K.; Percec, V. Chem. Rev. 2011, 111, 1346−1416. (l) Alonso, D. A.; Najera, C. In Science of Synthesis; de Meijere, A., Ed.; Thieme: New York, 2009; Vol. 47a, pp 439−479. (m) Jana, R.; Pathak, T. P.; Sigman, M. S. Chem. Rev. 2011, 111, 1417−1492. (n) Li, B.; Xu, L.; Wu, Z.; Guan, B.; Sun, C.; Wang, B.; Shi, Z. J. Am. Chem. Soc. 2009, 131, 14656−14657. (o) Han, F.-S. Chem. Soc. Rev. 2013, 42, 5270−5298. (p) Kono, H.; Ito, K.; Nagai, Y. Chem. Lett. 1975, 4, 1095−1096. (q) Iwamoto, H.; Kubota, D

DOI: 10.1021/acs.organomet.6b00652 Organometallics XXXX, XXX, XXX−XXX