Catalytic Hydrosilylation of the Carbonyl Functionality via a Transient

Mar 10, 2009 - Sabuj Kundu , William W. Brennessel , and William D. Jones. Inorganic Chemistry 2011 50 (19), 9443-9453. Abstract | Full Text HTML | PD...
0 downloads 0 Views 374KB Size
Download PDF
2234

Organometallics 2009, 28, 2234–2243

Catalytic Hydrosilylation of the Carbonyl Functionality via a Transient Nickel Hydride Complex Ba L. Tran, Maren Pink, and Daniel J. Mindiola* Department of Chemistry, Indiana UniVersity, Bloomington, Indiana 47405 ReceiVed December 6, 2008

We report the syntheses and characterization of two new sterically demanding chelate ligands, denoted PNMe3 (N-(2-(diisopropylphosphino)-4-methylphenyl)-2,4,6-trimethylanilide) and PNiPr3 (N-(2-(diisopropylphosphino)-4-methylphenyl)-2,4,6-triisopropylanilide), as well as the preparation of their nickel(II) halide complexes [(PNiPr3)Ni(µ2-Br)]2 (1) and [(PNMe3)Ni(µ2-Cl)]2 (2). The combination of complex 1 with KOtBu and Et3SiH as the hydride source can promote the hydrosilylation of a variety of carbonyl compounds in excellent yield and in a catalytic manner. The reactivity for a scope of substrates and a plausible mechanism along the hydrosilylation reaction are discussed in this work. 1. Introduction The reduction of the carbonyl functionality, specifically the aldehyde and ketone groups, to alcohol via hydride transfer is an important transformation in organic synthesis.1-3 The application of stoichiometric quantities of main-group-metal hydrides composed of boron and aluminum for the reduction of aldehydes and ketones to alcohol is ubiquitous but is of concern, due to waste production being generated when these reactions are performed on larger scales. More importantly, reduction of the carbonyl group followed by protection of the alcohol moiety would appeal from a synthetic standpoint if such a substrate is being used for subsequent multistep synthesis. Consequently, metal-catalyzed hydrosilylation reactions constitute a practical protocol in synthetic organic chemistry, because both the reduction and the protection steps are performed in a single, atom-efficient fashion. Traditionally, catalytic hydrosilylation of the carbonyl functionality has been employed with precious, heavy metals ranging from Re, Rh, and Ru to Ir.4-14 However, the cost affiliated with these metals * To whom correspondence should be addressed. E-mail: mindiola@ indiana.edu. (1) Smith, M. B.; March, J. March’s AdVanced Organic Chemistry; Wiley-Interscience: New York, 2001. (2) Ohkuma, T.; Noyori, R. In ComprehensiVe Asymmetric Catalysis; Jacobsen, E. N., Pfaltz, A., Yamamoto, H., Eds.; Springer: Berlin, 1999. (3) Nishiyama, H.; Itoh, K. In Catalytic Asymmetric Synthesis; Ojima, I., Ed.; Wiley-VCH: New York, 2000. (4) Nolin, K. A.; Krumper, J. R.; Pluth, M. D.; Bergman, R. G.; Toste, F. D. J. Am. Chem. Soc. 2007, 129, 14684–14696. (5) Du, G.; Abu-Omar, M. M. Organometallics 2006, 25, 4920–4923. (6) Ison, E. A.; Trivedi, E. R.; Corbin, R. A.; Abu-Omar, M. M. J. Am. Chem. Soc. 2005, 127, 15374–15375. (7) Gade, L. H.; Cesar, V.; Bellemin-Laponnaz, S. Angew. Chem., Int. Ed. 2004, 43, 1014–1017. (8) Nishiyama, H.; Sakaguchi, H.; Nakamura, T.; Horihata, M.; Kondo, M.; Itoh, K. Organometallics 1989, 8, 846–848. (9) Sawamura, M.; Ryoichi, K.; Ito, Y. Angew. Chem., Int. Ed. 1994, 33, 111–113. (10) Tao, B.; Fu, G. C. Angew. Chem., Int. Ed. 2002, 41, 3892–3894. (11) Zhu, G.; Terry, M.; Zhang, X. J. Organomet. Chem. 1997, 547, 97–101. (12) Nishibayashi, Y.; Takei, I.; Uemura, S. Organometallics 1998, 17, 3420–3422. (13) Chianese, A. R.; Crabtree, R. H. Organometallics 2005, 24, 3066– 3073. (14) Ojima, I.; Nihonyanagi, M.; Nagai, Y. J. Chem. Soc., Chem. Commun. 1972, 938a.

inasmuch as disposal concerns of the heavier congeners have prompted the search for alternative hydrosilylation catalysts such as Ti(IV), Cu(I), Fe(II), and Zn(II).15,16,19-35 To our knowledge, analogous hydrosilylation catalysts involving well-defined Ni(II) precursors are exceedingly rare; however, there have been earlier reports of nickel-mediated hydrosilylation of olefins,36 alkynes,37 and R,β-unsaturated aldehydes.38 Specifically, the few reduction studies of the carbonyl moiety using nickel complexes often invoke the generation of a nickel hydride species as the active species for converting the carbonyl motif to an alcohol.39,40 Recently, Guan and co-workers have spectroscopically observed (15) Carter, M. B.; Schiott, B.; Guiterrez, A.; Buchwald, S. L. J. Am. Chem. Soc. 1994, 116, 11667–11670. (16) Yun, J.; Buchwald, S. L. J. Am. Chem. Soc. 1999, 121, 5640– 5644. (17) Mahoney, W. S.; Brestensky, D. M.; Stryker, J. M. J. Am. Chem. Soc. 1988, 110, 291–293. (18) Mahoney, W. S.; Stryker, J. M. J. Am. Chem. Soc. 1989, 111, 8818– 8823. (19) Deutsch, C.; Krause, N.; Lipshutz, B. H. Chem. ReV. 2008, 108, 2916–2927. (20) Lipshutz, B. H.; Lower, A.; Noson, K. Org. Lett. 2002, 4, 4045– 4048. (21) Lipshutz, B. H.; Frieman, B. A. Angew. Chem., Int. Ed. 2005, 44, 6345–6348. (22) Lipshutz, B. H.; Noson, K.; Chrisman, W.; Lower, A. J. Am. Chem. Soc. 2003, 125, 8779–8789. (23) Diez-Gonzalez, S.; Scott, N. M.; Nolan, S. P. Organometallics 2006, 25, 2355–2358. (24) Diez-Gonzalez, S.; Nolan, S. P. Acc. Chem. Res. 2008, 41, 349– 358. (25) Kaur, H.; Zinn, F. K.; Stevens, E. D.; Nolan, S. P. Organometallics 2004, 23, 1157–1160. (26) Shaikh, N. S.; Enthaler, S.; Junge, K.; Beller, M. Angew. Chem., Int. Ed. 2008, 47, 2497–2501. (27) Shaikh, N. S.; Junge, K.; Beller, M. Org. Lett. 2007, 9, 5429– 5432. (28) Langlotz, B. K.; Wadepohl, H.; Gade, L. H. Angew. Chem., Int. Ed. 2008, 47, 4670–4674. (29) Tondreau, A. M.; Lobkovsky, E.; Chirik, P. J. Org. Lett. 2008, 10, 2789–2792. (30) Furuta, A.; Nishiyama, H. Tetrahedron Lett. 2008, 49, 110–113. (31) Nishiyama, H.; Furuta, A. Chem. Commun. 2007, 760–762. (32) Mimoun, H.; Laumer, J. Y. d. S.; Giannini, L.; Scopelliti, R.; Floriani, C. J. Am. Chem. Soc. 1999, 121, 6158–6166. (33) Bette, V.; Mortreux, A.; Ferioli, F.; Martelli, G.; Savoia, D.; Carpentier, J.-F. Eur. J. Org. Chem. 2004, 3040–3045. (34) Ge´rard, S.; Pressel, Y.; Riant, O. Tetrahedron: Asymmetry 2005, 16, 1889–1891. (35) Bette, V.; Mortreux, A.; Savoia, D.; Carpentier, J.-F. AdV. Synth. Catal. 2005, 347, 289–302.

10.1021/om801160j CCC: $40.75  2009 American Chemical Society Publication on Web 03/10/2009

Hydrosilylation Via a Transient Ni Hydride Complex

Organometallics, Vol. 28, No. 7, 2009 2235

the insertion of the carbonyl functionality across the nickel-hydride bond by 1H NMR spectroscopy, using the catalyst [2,6(iPr2PO)2C6H3]NiH.41 In 1995, the group of Iyer and co-workers reported catalytic homogeneous hydrogenations of ketones and aldehydes to the corresponding alcohol using NiCl2(PPh3)2 and NaOH in isopropyl alcohol.40 However, the conditions for the system are somewhat inefficient, considering the high catalyst loading needed (∼15 mol % NiCl2(PPh3)2) as well as prolonged reaction times required in refluxing isopropyl alcohol (12-36 h). These reactions also require basic conditions, plausibly due to the deprotonation step of the alcohol for subsequent transmetalation to the nickel center. Whether the isopropoxide nickel(II) system presented by Iyer could be amenable to β-hydride elimination to generate a nickel hydride is speculative at present, but such a step seems intuitive to propose, on the basis of the fact that alcohols containing β-hydrogens appear to be indispensable for catalytic turnover. In their studies, it was also observed that exchanging PPh3 in NiCl2(PPh3)2 for a bidentate phosphine such as dppe (dppe ) Ph2PCH2CH2PPh2) resulted in loss of catalytic activity. On the basis of this observation, it was inferred that dppe might be coordinatively saturating the metal center by confining it to a square-planar d8 configuration, whereas a triphenylphosphine ligand could dissociate from the metal, thus exposing a vacant site for subsequent binding of the substrate. These sets of observations suggest that a low-coordination Ni(II) environment, possibly three-coordinate,42-53 is a prerequisite for the catalytic hydrogenation of the organic substrate. Aside from high catalyst loadings and extended reaction times required by the NiCl2(PPh3)2 hydrogenation system,40 such work laid the

premise for nickel(II), specifically a nickel(II) hydride, being a viable candidate for carbonyl reduction catalysis. In recent years, Cu(I), in the form of (IPr)CuOtBu (IPr ) N,N′-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), has been also shown to catalyze the hydrosilylation of the carbonyl functionality in the presence of various silanes.23-25 Moreover, Sadighi and co-workers have recently isolated the product of the alkoxide-silyl σ-bond metathesis reaction, in the form of a [(IPr)Cu(µ2-H)]2 dimer, by reacting the precursor complex (IPr)Cu(OtBu) with stoichiometric triethoxysilane, (EtO)3SiH. In addition, the byproduct, (EtO)3SiOtBu, was observed by 1H NMR spectroscopy when the reaction was conducted in a stoichiometric fashion, therefore confirming the role of the silane in the σ-bond metathesis step.54 The surprising dearth of Ni(II) systems in carbonyl reduction chemistry coupled with the precedent of the NiCl2(PPh3)2 hydrogenation catalysis prompted us to investigate hydrosilylation chemistry using a nickel(II) system supported by a bidentate and sterically encumbering amido-phosphine ligand. Expanding on original work by Stryker17,18 with [(Ph3P)CuH]6 and H2 for the catalytic reduction of R,β-unsaturated ketones, as well as a similar strategy reported by Lipshutz19-22 and Nolan,23-25,55 we report a Ni(II) precursor capable of conducting the hydrosilylation of ketones and aldehydes, utilizing Et3SiH as the hydride source. In these studies we have also detected a transient nickel hydride as the active species mediating the catalytic hydrosilylation reaction, via a series of independent reactions using various hydride sources.

(36) (a) Bennett, E. W.; Orenski, P. J. J. Organomet. Chem. 1971, 28, 137–144. (b) Kiso, Y.; Kumada, M.; Maeda, K.; Sumitani, K.; Tamao, K. J. Organomet. Chem. 1973, 50, 311–318. (c) Kiso, Y.; Tamao, K.; Kumada, M. J. Organomet. Chem. 1974, 76, 95–103. (d) Kumada, M.; Kiso, Y.; Umeno, M. J. Chem. Soc. D 1970, 611. (e) Lappert, M. F.; Nile, T. A.; Takahashi, S. J. Organomet. Chem. 1974, 72, 425–439. (f) Yamamoto, K.; Hayashi, T.; Uramoto, Y.; Ito, R.; Kumada, M. J. Organomet. Chem. 1976, 118, 331–348. (g) Yamamoto, K.; Uramoto, Y.; Kumada, M. J. Organomet. Chem. 1971, 31, C9–C10. (h) Hyder, I.; Jimenez-Tenorio, M.; Puerta, M. C.; Valerga, P. Dalton Trans. 2007, 3000–3009. (37) (a) Chaulagain, M. R.; Mahandru, G. M.; Montgomery, J. Tetrahedron 2006, 62, 7560–7566. (b) Tamao, K.; Miyake, N.; Kiso, Y.; Kumada, M. J. Am. Chem. Soc. 1975, 97, 5603–5605. (c) Tamao, K.; Kobayashi, K.; Ito, Y. J. Am. Chem. Soc. 1989, 111, 6478–6480. (38) (a) Lappert, M. F.; Nile, T. A. J. Organomet. Chem. 1975, 102, 543–550. (b) Boudjouk, P.; Choi, S.-B.; Hauck, B. J.; Rajkumar, A. B. Tetrahedron Lett. 1998, 39, 3951–3952. (39) Fontaine, F.-G.; Nguyen, R.-V.; Zargarian, D. Can. J. Chem. 2003, 81, 1299–1306. (40) Iyer, S.; Varghese, J. P. Chem. Commun. 1995, 465–466. (41) Chakraborty, S.; Krause, J. A.; Guan, H. Organometallics 2009, 28, 582–886. (42) Bradley, D. C.; Hursthouse, M. B.; Smallwood, R. J.; Welch, A. J. J. Chem. Soc., Chem. Commun. 1972, 872. (43) Nilges, M. J.; Barefield, E. K.; Belford, R. L.; Davis, P. H. J. Am. Chem. Soc. 1977, 99, 755. (44) Ellis, D. D.; Spek, A. L. Acta Crystallogr. 2000, C56, 1067. (45) Eaborn, C.; Hill, M. S.; Hitchcock, P. B.; Smith, J. D. Chem. Commun. 2000, 691. (46) Mindiola, D. J.; Hillhouse, G. L. J. Am. Chem. Soc. 2001, 123, 4623. (47) Melenkivitz, R.; Mindiola, D. J.; Hillhouse, G. L. J. Am. Chem. Soc. 2002, 124, 3846. (48) Kitiachivili, K. D.; Mindiola, D. J.; Hillhouse, G. L. J. Am. Chem. Soc. 2004, 126, 10554. (49) Kogut, E.; Wiencko, H. L.; Zhang, L.; Cordeau, D. E.; Warren, T. H. J. Am. Chem. Soc. 2005, 127, 11248. (50) Eckert, N. A.; Dinescu, A.; Cundari, T. R.; Holland, P. L. Inorg. Chem. 2005, 44, 7702. (51) Bai, G.; Wei, P.; Stephan, D. W. Organometallics 2005, 24, 5901. (52) Nilges, M. J.; Barefield, E. K.; Belford, R. L.; Davis, P. H. J. Am. Chem. Soc. 1979, 99, 755. (53) Arriortua, M. I.; Cortes, A. R.; Lezam, L.; Rojo, T.; Solans, X.; Font-Bardia, M. Inorg. Chim. Acta 1990, 174, 263.

2.1. Synthesis of the PNMe3 (N-(2-(Diisopropylphosphino)4-methylphenyl)-2,4,6-trimethylanilide) and PNiPr3 (N-(2(Diisopropylphosphino)-4-methylphenyl)-2,4,6-triisopropylanilide) Scaffold. Recently, Liang and Peters reported amido-phosphine ligands for the stabilization of late-transition-metal complexes composed of Ni(II), Zn(II), Cu(I), and Ag(I).56-58 In their synthesis, the incorporation of the asymmetric diarylamine skeleton involved a Buchwald-Hartwig C-N cross-coupling step. Nucleophilic aromatic substitution of the fluoride group on the diarylamine backbone with potassium and/or lithium salts of diphenylphosphide (-PPh2) and diisopropylphosphide (-PiPr2), respectively, elicited formation of the amidophosphine ligand. Unfortunately, it was observed by us that incorporation of the phosphide group onto the aryl moiety becomes problematic when the phosphide salt in question was both electron rich and sterically demanding. Hence, the caveat to their scaffold synthesis was restricted to the installation of large peripheral groups on either the P or N site, but not both. As a result, the latter limitation along with the long reaction times needed to complete the assembly of these interesting ligands led us to adopt a different synthetic approach to prepare similar monoanionic amidophosphine ligands, but having encumbering groups at both the N and P sites. Moreover, a bulky scaffold is desirable for the purpose of generating lowcoordinate nickel complexes, on the basis of the observation

2. Results and Discussion

(54) Mankad, N. P.; Laitar, D. S.; Sadighi, J. P. Organometallics 2004, 23, 3369–3371. (55) Diez-Gonzalez, S.; Kaur, H.; Zinn, F. K.; Stevens, E. D.; Nolan, S. P. J. Org. Chem. 2005, 70, 4784–4796. (56) Liang, L. C.; Lee, W. Y.; Hung, C. H. Inorg. Chem. 2003, 42, 5471–5473. (57) Liang, L.-C.; Lee, W.-Y.; Yin, C.-C. Organometallics 2004, 23, 3538–3547. (58) Miller, A. J. M.; Dempsey, J. L.; Peters, J. C. Inorg. Chem. 2007, 46, 7244–7246.

2236 Organometallics, Vol. 28, No. 7, 2009

Tran et al.

Scheme 1. Synthetic Outline of the PN Ancillary Liganda

a Legend: (a) Pd(OAc) , 1,1′-bis(diphenylphosphino)ferrocene (DPPF), sodium t-pentoxide, toluene, reflux, 4 days (R ) Me, 72%; iPr, 72%); (b) 2 N-bromosuccinimide (NBS), acetonitrile, 12 h, 0-30 °C (R ) Me, 94%; iPr, 92%); (c) (1) nBuLi (2 equiv), Et2O, -78 °C, 3 h; (2) ClPiPr2, 48 h; (3) degassed water with Ar.

that the choice of ligand is critical for catalytic activity by a Ni(II) precursor (vide supra).40 The synthesis of the PNAr scaffold, where Ar ) 2,4,6-R3C6H2 (R ) CH3, iPr), involved a synthetic modification from the pincer ligand PNP (PNP ) N[2-P(CHMe2)2-4-MeC6H3]2) procedure devised by Ozerov and co-workers (Scheme 1) and utilized extensively in our laboratory.59-68 Our choice of Ar ) 2,4,6-R3C6H2 is ideal for steric protection of the anilide moiety, while the incorporation of the tolyl group allows for construction of a rigid backbone. Preparing the free bases (PNMe3)H and (PNiPr3)H involved first a Buchwald-Hartwig C-N coupling reaction of commercially available aniline with the appropriate aryl bromide (see the Experimental Section). Having the para positions blocked at both aryl moieties results in smooth ortho bromination utilizing N-bromosuccinimide (NBS), in acetonitrile, to afford the desired bromo-containing precursor N-(2bromo-4-methylphenyl)-2,4,6-triisopropylanilide and N-(2bromo-4-methylphenyl)-2,4,6-trimethylanilide in 92% and 94% isolated yields, respectively (Scheme 1). Lithiation of the latter substrates with 2 equiv of nBuLi in a frozen ethereal solution under N2, followed by quenching of the mixture with ClPiPr2, (59) Fan, L.; Foxman, B. M.; Ozerov, O. V. Organometallics 2004, 23, 326–328. (60) Ozerov, O. V.; Guo, C.; Fan, L.; Foxman, B. M. Organometallics 2004, 23, 5573–5580. (61) Zhu, Y.; Fan, L.; Chun-Hsing; Finnell, S. R.; Foxman, B. M.; Ozerov, O. V. Organometallics 2007, 26, 6701–6703. (62) Fan, L.; Parkin, S.; Ozerov, O. V. J. Am. Chem. Soc. 2005, 127, 16772–16773. (63) Fafard, C. M.; Adhikari, D.; Foxman, B. M.; Mindiola, D. J.; Ozerov, O. V. J. Am. Chem. Soc. 2007, 129, 10318–10319. (64) Adhikari, D.; Mossin, S.; Basuli, F.; Huffman, J. C.; Szilagyi, R. K.; Meyer, K.; Mindiola, D. J. J. Am. Chem. Soc. 2008, 130, 3676–3682. (65) Adhikari, D.; Huffman, J. C.; Mindiola, D. J. Chem. Commun. 2007, 4489–4491. (66) Kilgore, U. J.; Sengelaub, C. A.; Pink, M.; Fout, A. R.; Mindiola, D. J. Angew. Chem., Int. Ed. 2008, 47, 3769–3772. (67) Bailey, B. C.; Fout, A. R.; Fan, H.; Tomaszewski, J.; Huffman, J. C.; Mindiola, D. J. Angew. Chem., Int. Ed. 2007, 46, 8246–8249. (68) Fout, A. R.; Bailey, B. C.; Tomaszewski, J.; Mindiola, D. J. J. Am. Chem. Soc. 2007, 129, 12640–12641.

produced crude Li(PNAr) salts. To prepare pure lithium salts, however, the crude product must be quenched slowly with meticulously deoxygenated water under an inert atmosphere. Subsequent workup of the mixture and drying of the viscous orange oil allows for isolation of the crude, free base aminophosphines (PNMe3)H and (PNiPr3)H. Mass spectrometry of (PNMe3)H and (PNiPr3)H yielded the parent ion peaks at 341.00 and 426.33 [M+], respectively. It is worth emphasizing that during quenching of the lithio salts and workup, to afford the aminophosphines (PNMe3)H and (PNiPr3H, the water must be completely deoxygenated with an argon purge for at least 30-50 min to avoid formation of aminophosphine oxide ligand. Under an inert atmosphere, deprotonation of the aminophosphines (PNMe3)H and (PNiPr3)H, in thawing pentane, with 1 equiv of n BuLi resulted in formation of the bright yellow lithio salts Li(PNMe3) and Li(PNiPr3). Isolation of pure lithium salt can be accomplished either by precipitation from pentane, in the case of Li(PNMe3), or by formation of the THF adduct followed by trituration with cold hexane, in the case of Li(PNiPr3), to form (THF)2Li(PNiPr3) (see the Experimental Section, Scheme 2). Compounds Li(PNMe3) and (THF)2Li(PNiPr3) have been fully characterized by multinuclear (1H, 13C, 31P) NMR spectroscopy and high-resolution mass spectrometry. The isopropylmethyl groups for both lithium salts of Li(PNMe3) and (THF)2Li(PNiPr3) are diastereotopic, as evinced by their 1H NMR spectra. Moreover, the two coordinated THF molecules in (THF)2Li(PNiPr3) are easily identified and integrated at 3.19 and 1.21 ppm. The 31P{1H} spectra of Li(PNMe3) and (THF)2Li(PNiPr3) display chemical shifts at -13.2 and -4.5 ppm, respectively. 2.2. Synthesis of Dinuclear Nickel(II) Halide Precursors Supported by the Sterically Encumbering PNAr Scaffold. The treatment of 1.1 equiv of anhydrous NiBr2 with (THF)2Li(PNiPr3) in THF initially produced a dark green solution, which upon stirring overnight at 70 °C resulted in the isolation of a dark red solid product subsequent to filtration and removal of all volatiles. Crystallization of the

Hydrosilylation Via a Transient Ni Hydride Complex

Organometallics, Vol. 28, No. 7, 2009 2237

Scheme 2. Lithiation of the Free Ligand and Synthesis of the Nickel Complexes 1 and 2

crude material from slow evaporation of hexane at room temperature under an inert atmosphere gave the complex [(PNiPr3)Ni(µ2-Br)]2 (1) in 70% yield (Scheme 2). The 1H NMR spectrum of 1 in C6D6 was consistent with a squareplanar Ni(II) diamagnetic species likely arising from bridging of the bromide ligand. The 31P NMR spectrum of 1 displayed a broad resonance at 63.9 ppm (∆ν1/2 ) 95.8 Hz), which is shifted significantly downfield in comparison to the sharp resonance at -4.5 ppm for (THF)2Li(PNiPr3) and -16.6 ppm for the free base (PNiPr3)H. Moreover, X-ray diffraction analysis of single crystals of 1 grown from the evaporation of toluene at ambient temperature confirmed the degree and mode of aggregation as well as the geometry about each nickel(II) center. Accordingly, the solid-state structure of 1 features two square-planar nickel centers bridged by the bromide ligands (Figure 1). The phosphine and triisopropylaryl groups are oriented anti to one another in order to minimize steric congestion. The Ni-Namido and Ni-P bond distances are 1.8853(19) and 2.1391(6) Å, respectively, and are within range of the expected distances for these donor atoms in comparison to the complex [(PNiPr2)NiCl(PMe3)] (PNipr2 ) N-(2-(diphenylphosphino)phenyl)-2,6-diisopropy-

Figure 1. Molecular structure of 1 with 50% thermal ellipsoids. The structure of 1 is depicted in its entirety, with hydrogen omitted for clarity (top). The bottom drawing displays a simplified version of 1 showing only first- and second-coordination-sphere atoms. Selected bond lengths (Å) and angles (deg) for 1: Ni(1)-N(1), 1.8853(19); Ni(1)-P(1), 2.1391(6); Ni(1)-Br(1), 2.3337(4); Ni(1)-Br(1A),2.3949(4);N(1)-Ni(1)-P(1),85.98(6);N(1)-Ni(1)-Br(1), 176.85(6); N(1)-Ni(1)-Br(1A), 96.44(6); P(1)-Ni(1)-Br(1), 91.35(2); P(1)-Ni(1)-Br(1A), 176.57(2).

lanilide)reportedbyLiangandco-workers.57 TheP(1)-Ni(1)-Br(1) angle is 91.35(2)°, while the P(1)-Ni(1)-Br(1A) angle is 176.57(2)°, consistent with a Ni(II) system confined in a square-planar environment. A complete description of the crystallographic metric parameters for the structure of compound 1 is presented in the Experimental Section. The preparation of the closely related complex [(PNMe3)Ni(µ2Cl)]2 (2) was accomplished analogously to that of 1, in 75% isolated yield, by reacting Li(PNMe3) with NiCl2(THF)1.5 in THF over 12 h at room temperature. The purification of 2 involved the removal of the THF solvent from the mixture via reduced pressure, and the red solids were extracted into toluene and filtered through a Celite-filled medium-porosity frit to remove unreactive NiCl2(THF)1.5 and salt side product. Subsequently, the toluene solvent was evaporated from the filtrate by reduced pressure, affording red solids, which were extracted into hexane and stored at -37 °C to precipitate pure material. The 1H and 31P NMR spectra of 2 are also consistent with a square-planar complex essentially analogous to 1. Due to the fact that complexes 1 and 2 are dimers in the solid state, and since the focus of this study is to generate unsaturated Ni(II) fragments, only the catalytic proficiency of the more sterically congested complex, 1, will be presented herein. 2.3. Catalytic Hydrosilylation Studies with Complex 1. When complex 1 is combined with 2 equiv of KOtBu, the mixture can mediate the catalytic hydrosilylation of carbonyl functionalities such as aldehydes and ketones to the silyl ether products at 100 °C when Et3SiH is used as a coreagent. No catalysis is observed in the absence of silane. Hydrosilylation reactions were monitored by 1H NMR spectroscopy in C6D6 by following the decay of the aldehyde resonance with concomitant growth of the methylene protons of the silyl ether product. In the case of ketones, the reaction was monitored by the growth of the proton located on the R-carbon with respect to the silyl ether moiety. We examined hydrosilylation catalysis of benzaldehyde with 2 mol % of 1 in benzene or toluene at 100 °C and in the presence of KOtBu (4 mol %) and triethylsilane (1.2 equiv with respect to the carbonyl compound) to afford the desired silyl ether product PhCH2OSiEt3. Alternatively, complex 1 can be treated with 2 equiv of KOtBu and utilized for subsequent catalysis by addition of the silane and aldehyde. The hydrosilylation reaction was also performed in d5-bromobenzene, with benzaldehyde under the same catalytic conditions, producing results identical with those observed in benzene as a solvent, thus indicating that the solvent polarity essentially did not play a significant role in this transformation. A compiled list of substrates along with the reaction time, yield of products, turnover number, and turnover frequencies is presented in Table 1. The library of substrates was chosen to probe for degree of tolerance, for halogen and amino groups on the aryl, for heteroatom-containing heterocycles, as well as differences in electronic and steric

2238 Organometallics, Vol. 28, No. 7, 2009 Table 1. Catalytic Hydrosilylation Conditions using Complex 1 as the Precatalyst

a All products were analyzed and confirmed by 1H NMR spectroscopy and GC-MS. Unless otherwise noted, yields are from isolated product from column chromatography employing pentane as the eluent. All yields are an average of two runs. b The reaction was performed at room temperature. c The yield was determined by 1H NMR spectroscopy. TON ) turnover number ((mol of product) (mol of cat.)-1), TOF ) turnover frequency ((mol of product) (mol of cat.)-1 h-1), and NR ) no reaction.

factors. On the basis of our compiled data, it was observed that substrates which are liquids can be hydrosilylated in the absence of solvent media to afford product in yields comparable to those

Tran et al.

for reaction mixtures diluted in solvents such as benzene and toluene. For most substrates, the reaction is rapid and complete within 15 min to 4 h. The successful hydrosilylations of 2-fluorobenzaldehyde (entry 2) and 4-chlorobenzaldehyde (entry 3) indicate that the catalyst, under our reaction conditions, can tolerate these functional groups on the aromatic ring. However, placing an electron-donating group in the para position of the aryl ring slowed the reduction significantly. For instance, the electron-rich 4-(dimethylamino)benzaldehyde substrate (entry 4) required 31 h for completion, in comparison to the values observed in entries 2 and 3. This difference in reactivity may be due to competitive binding to the metal center from the dimethylamino and aldehyde groups. Despite this, the donating nature of the NMe2 moiety in the para position can allow for a resonance structure whereby the aldehyde oxygen becomes more nucleophilic than it generally is. This feature might explain why the dimethylamino group does not appear to poison the catalyst. A similar observation was previously reported for copper(I)catalyzed hydrosilylations.55 The substrate 2,6-dimethylbenzaldehyde (entry 5) and benzophenone (entry 6) were both examined to probe the effect of steric about the carbonyl functionality on the rate of the reaction. Not surprisingly, both substrates required longer reaction times, ∼2 h, thereby producing a moderate yield of the silyl ether product. When an aliphatic-based aldehyde, trimethylacetaldehyde (entry 7), was subjected to the established catalytic conditions, the expected silyl ether along with the ether (tBuCH2)2O were formed in the reaction mixture (inferred by 1H NMR spectroscopy and GCMS). Similar products have been previously obtained in the hydrosilylation of isopropylaldehyde with cationic monooxo-rhenium(V) salen complexes utilizing Et3SiH as the hydride source, in which the iPrCH2OCH2Pri ether product was obtained as the major product.5 In their studies, it was found that iPrCH2OSiMePh2 could be produced free of the ether product, iPrCH2OCH2Pri, by varying the silane source from Et3SiH to the bulkier Ph2MeSiH compound. In our system, it was determined that one of the byproducts, (tBuCH2)2O, could be suppressed by performing the reaction at room temperature for 4 h to afford the silyl ether product, tBuCH2OSiEt3, in quantitative yield. The mechanism of formation of the ether is presently unclear; however, Olah and co-workers have observed the reductive condensation of carbonyl compounds and alkoxysilanes to produce ether products.69 Unfortunately, substrates containing good donor motifs such as 2-pyridinecarboxaldehyde (entry 8) and 3-thiophenecarboxaldehyde (entry 9) inhibit the catalytic reaction, whereby no traces of product are detected by NMR spectroscopy. It is very likely that these substrates coordinatively saturate the catalyst by forming a square-planar nickel complex upon migratory insertion of the hydride into the carbonyl group. Consistent with our observations, Kuhl and co-workers have also noted unsuccessful hydrogenation reactions of the pyridyl-containing substrate N-benzylidene-1(pyridin-2-yl)methanamine, with a Ni(0)/IMes (IMes ) N,N′bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) system utilizing Et2CHONa as a hydrogen donor.70 As our final substrate, the reduction of a aliphatic-substituted ketone such as cyclohexanone (entry 10) required a longer reaction time of 2.5 h and produced product in moderate yield (70%). Unfortunately, attempts to perform the selective hydrosilylation of a vinyl ketone such as 2-cyclohexen-1-one yielded a mixture of (69) Sassaman, M. B.; Kotian, K. D.; Prakash, G. K. S.; Olah, G. A. J. Org. Chem. 1987, 52, 4314–4319. (70) Kuhl, S.; Schneider, R.; Fort, Y. Organometallics 2003, 22, 4184– 4186.

Hydrosilylation Via a Transient Ni Hydride Complex

intractable materials. We have also determined the turnover number (TON) and the turnover frequency (TOF) of the hydrosilylation reaction mediated by complex 1, which is presented in Table 1. The TON and TOF values obtained are moderate in comparison to those for the reported Cu and Rh systems.24 Furthermore, the high temperature (100 °C) utilized in our system renders it a marginal catalyst when considering the Cu and Rh systems. 2.4. Proposed Mechanism for Catalytic Hydrosilylation of Aldehydes and Ketones by Precursor 1. It was assessed that no catalytic activity was observed under similar conditions in the absence of 1, therefore implying that the nickel complex must be necessary for hydrosilylation and catalytic turnover. The use of KOtBu as an alkoxide source is also critical in the reaction medium, since compound 1 alone failed to promote hydrosilylation of the carbonyl group (i.e., benzaldehyde) in the presence of triethylsilane. In fact, treatment of 1 with stoichiometric triethylsilane in the absence of carbonyl substrate does not result in a reaction. Therefore, we propose that the transient “(PNiPr3)Ni(OtBu) (A)” must be generated along the reaction coordinate, followed by exchange of the alkoxide for hydride with triethylsilane, a process that is now readily observed in copper-based hydrosilylation catalysis.19-26,54,55 This proposal is further corroborated by the observation that red 1 undergoes a rapid color change to green when treated with 2 equiv of KOtBu. Unfortunately, we have been unable to isolate the putative A from independent and stoichiometric reactions involving 1 and 2 equiv of KOtBu. Likewise, we have also been unable to cleanly isolate the hypothetical nickel hydride species “(PNiPr3)NiH” presumably being generated in the course of the reaction. In fact, attempts to produce the nickel(II) hydride by independent routes such as oxidative addition of the N-H bond in PNiPr3H by Ni(COD)2 have been hampered by formation of multiple products. We do, however, observe formation of a nickel hydride in solution (albeit not in pure form), on the basis of a combination of 1H and 31P NMR spectral data for reactions conducted in situ using the same types of reagents involved in the catalytic reactions. Accordingly, treating 1 with 2 equiv of KOtBu (15 min) followed by addition of a stoichiometric amount of triethylsilane in C6D6 over 1 h at room temperature revealed a diagnostic triplet resonance in the 1 H NMR spectrum at -20.66 ppm (JP-H ) 18 Hz). The 31P{1H} NMR spectrum of the reaction mixture also evinced a new singlet at 54.7 ppm, recorded in d6-benzene at 25 °C. The small JP-H value suggests a hydride bridge dimer where the hydride is likely oriented cis with respect to the two phosphine residues.71 We therefore tentatively assign this resonance to the hydride dimer [(PNiPr3)Ni(µ2-H)]2 (3). Slightly larger JP-H values of 22-24 Hz have been observed in the hydride-bridged Ni(I) dimers [(R2PCH2CH2PR2)Ni(µ2-H)]2 (R ) iPr, tBu) reported by Jones72 and Porschke.73 Likewise, our JP-H value and multiplicity negate the possibility of 3 being a monomer in solution, since the hydride monomer, (PNP)NiH,60,74 displays a much higher JP-H value (triplet, 63.3 Hz). In addition, Liang has reported a series of analogous (R-PNP)NiH complexes with similar JP-H values, where the PNP core is [N(o-C6H4PR2)2](71) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals; Wiley: Hoboken, NJ, 2005. (72) Vicic, D. A.; Jones, W. D. J. Am. Chem. Soc. 1997, 119, 10855– 10856. (73) Bach, I.; Goddard, R.; Kopiske, C.; Seevogel, K.; Prschke, K.-R. Organometallics 1999, 18, 10–20. (74) Adhikari, D.; Mossin, S.; Basuli, F.; Dible, B. R.; Chipara, M.; Fan, H.; Huffman, J. C.; Baik, M.-H.; Meyer, K.; Mindiola, D. J. Inorg. Chem. 2008, 47, 10479–10490.

Organometallics, Vol. 28, No. 7, 2009 2239

and R ) Ph, iPr, Cy.75 Attempts to isolate and fully characterize 3 have been hampered by its gradual decomposition as well as the presence of other impurities in the reaction mixture. Hence, our inability to isolate 3 might stem from its lack of stability. A recent report by Limberg describes the isolation of [(βdiketiminato)Ni(µ2-H)]2 ([ArNC(CH3)]2CH-, Ar ) 2,6i Pr2C6H3) and the crystal structure along with the innate reactivity of the compound.76 The complex is extremely sensitive to external stimulus, including heat, solvent, and Lewis base (4-dimethylaminopyridine, propionitrile), leading to the reduction of the Ni(II) center to Ni(I) via elimination of H2, which was detected by 1H NMR spectroscopy. Furthermore, the subjection of [(β-diketiminato)Ni(µ2-H)]2 to heat in hexane led to the formation of a nickel mirror with detectable protonated ligand and unidentified paramagnetic products.76 To probe for whether a hydride was the active species along the catalytic cycle, we explored an independent synthesis to such a complex while subjecting the mixture to the appropriate carbonyl substrate. Accordingly, it was found that treatment of complex 1, with 2 equiv of NaHB(OMe)3 or NaHBEt3 and in the presence of benzaldehyde and triethylsilane, effectively promoted the catalytic hydrosilylation of the aldehyde. Furthermore, the combination of NaHB(OMe)3 and triethylsilane with benzaldehyde, in the absence of 1, failed to yield hydrosilylated product, thus implying that a mild Lewis acid such as B(OMe)3 could not be responsible for the observed catalysis or reduction of the carbonyl group. As a result, our control reactions strongly endorse the possibility of a transient nickel hydride complex, 3, being the responsible species for the reduction the carbonyl group. At present, we cannot discard the possibility of 3 dissociating into the more reactive three-coordinate monomer “(PNiPr3)NiH” during the catalytic cycle. Whether the carbonyl substrate or solvent promotes dissociation in 3 is speculative at present and we have been unable to trap the monomer with Lewis bases such as phosphines and pyridines. The unsuccessful reduction of the substrates in entries 8 and 9 (Table 1) and the hypothesis that a low-coordinate nickel hydride might be responsible for catalytic activity prompted us to investigate if the vacant site was necessary for hydrosilylation to proceed. Accordingly, we turned our attention to the known complex (PNP)NiH, originally reported by Ozerov and co-workers,60,74 since this species is a d8 square-planar species and, therefore, a coordinatively saturated hydride. The treatment of 2 mol % of (PNP)NiH with benzaldehyde and Et3SiH did not result in formation of the silyl ether product, PhCH2OSiEt3, despite heating the mixture at 100 °C for 12 h. Monitoring the stoichiometric reaction of (PNP)NiH and benzaldehyde by 31P NMR spectroscopy revealed the starting material (PNP)NiH (31P NMR 56.7 ppm) along with traces of a minor byproduct at 33.7 ppm. Interestingly, the 1H NMR spectrum of this mixture did not display the expected methylene protons for the anticipated insertion product (PNP)Ni(OCH2Ph) (4). The distribution of products did not change, even with heating at 100 °C overnight, as the majority of (PNP)NiH remained unreacted. To assess if the minor product formed from the reaction of (PNP)NiH and benzaldehyde was the insertion product 4, we independently prepared such a species from NaOCH2Ph and (PNP)NiCl in diethyl ether at room temperature in 90% isolated yield. The 1 H NMR spectrum for complex 4 reveals salient spectroscopic features for the methylene protons at 4.55 ppm (2 H) as well as (75) Liang, L.-C.; Chien, P.-S.; Lee, P.-Y. Organometallics 2008, 27, 3082–3093. (76) Pfirrmann, S.; Limberg, C.; Ziemer, B. Dalton Trans. 2008, Advance Articles.

2240 Organometallics, Vol. 28, No. 7, 2009 Scheme 3. Proposed Catalytic Cycle for the Hydrosilylation of the Carbonyl Functionality Mediated by [(PNiPr3)Ni(µ2-Br)]2 (1) with KOtBu and Et3SiH

Tran et al. 1

H and 31P NMR spectra for hydride 3 suggests this species to retain a dimeric form in solution (unless equilibration to a monomer results in a paramagnetic species that we cannot detect), therefore hinting that the substrate (carbonyl moiety) might be responsible for breaking of the dimer.

3. Conclusions

a singlet resonance at 25.80 ppm in the 31P{1H} NMR spectrum. Therefore, the spectroscopic characterization data for 4 exclude this species as the minor resonance observed at 33.74 ppm (vide supra). Surprisingly, it was found that treatment of complex 4 with Et3SiH at 100 °C over 12 h afforded the compound (PNP)NiH quantitatively. Formation of (PNP)NiH from 4 may result from alkoxide for hydride metathesis; however, a β-hydride elimination pathway cannot be ruled out, since thermolysis of 4 at 100 °C over 12 h revealed the presence of (PNP)NiH and 4. Moreover, quantitative conversion 4 to (PNP)NiH was accomplished after extended periods (72 h at 100 °C). These combination of results suggest that both σ-bond metathesis as well as β-hydride elimination, to generate (PNP)NiH, can occur in this type of system. On the basis of these results, we propose that the migratory insertion step is likely forbidden in a complex such as (PNP)NiH, due to the lack of a vacant site for binding of the carbonyl group in a substrate such as benzaldehyde (i.e., filled d(z2)). However, Liang and co-workers have observed the insertion of unactivated olefins (ethylene, 1-hexene, and norbornene) into an analogous (PNPPh)NiH complex (PNPPh ) N[2PPh2-phenyl]2-) is influenced by the substituents on the phosphine.71 For example, all unactivated olefins reacted with (PNPPh)NiH, but no reaction was detected with the more sterically congested and electron-rich PNP derivative (PNPiPr)NiH (PNPiPr ) N[2-PiPr2-phenyl]2), even at elevated temperatures. Therefore, the lack of reactivity between (PNP)NiH and benzaldehyde is not simply due to the availability of a vacant site but also a combination of steric congestion and/or lability of the Ni-P bond. On the basis of all these results, the most plausible catalytic cycle for the Ni(II)-mediated hydrosilylation of aldehydes and ketones using 1 as a precatalyst, and in the presence of KOtBu and Et3SiH, involves first the in situ generation of a nickel alkoxide intermediate, A. Regardless of whether A is a dimer or monomer, this complex converts to the hydride 3 by σ-bond metathesis with Et3SiH concurrent with elimination of the silyl ether Et3SiOtBu, which has been observed via a stoichiometric reaction. Our 1H NMR spectroscopic data are in accord with 3 being a dimer in solution, and we currently do not know if such a species is necessarily the active state of the catalyst. Coordination of the carbonyl group of OdCRR′ into the nickel-hydride bond in 3 forms the transient adduct (PNiPr3)Ni(H)(OdCRR′), which then undergoes migratory insertion to produce a hypothetical alkoxide intermediate complex (PNiPr3)Ni(OCHRR′). The latter would then undergo a σ-bond metathesis reaction with triethylsilane to close the catalytic hydrosilylation cycle by reforming 3 and the silyl ether product (Scheme 3). Our solution

In addition to developing a convenient route to sterically demanding amidophosphine ligands, we have shown that a nickel(II) complex supported by the monoanionic bidentate PNiPr3 scaffold is capable of mediating catalytic hydrosilylation on a variety of carbonyl-containing substrates, when combined with common reagents such as KOtBu and Et3SiH. Our studies indicate that a nickel(II) hydride is likely generated, which then undergoes migratory insertion of the carbonyl group to generate the corresponding alkoxide. The metathesis of the alkoxide group for the hydride of the silane results in the expulsion of the silyl ether product, thereby closing the hydrosilylation cycle. Reactivity studies indicated that aromatic aldehydes are more efficiently reduced than the aliphatic analogues or aliphatic and aromatic ketones. Our catalyst is also tolerant of the aryl halide containing functionality; however, donor groups such as (dimethylamino)aryl reduce the rate of conversion, while pyridineand thiophene-containing substrates completely inhibit catalytic turnover. Interestingly, when the square-planar complex (PNP)NiH was employed as the catalyst, no turnover was observed, even under forcing conditions. Through a series of control experiments we demonstrated that the migratory insertion step of the carbonyl functionality can be inhibited when the nickel(II) hydride is confined to a chelate-enforced square-planar environment. We are currently exploring the chemistry of the reactive intermediate 3, since such a species might undergo other insertion chemistry at the hydride ligand as well as reductive elimination reactions to generate transient “(PNiPr3)Ni” fragments.

4. Experimental Section 4.1. General Considerations. Unless otherwise stated, all operations were performed in an M. Braun Laboratory Master double-drybox under an atmosphere of purified nitrogen or using high-vacuum standard Schlenk techniques under a nitrogen atmosphere. Anhydrous nhexane, npentane, toluene, and benzene were purchased from Aldrich in Sure-Seal reservoirs (18 L) and dried by passage through two columns of activated alumina and Q-5. Diethyl ether and CH2Cl2 were dried by passage through a column of activated alumina. THF was distilled, under nitrogen, from purple sodium benzophenone ketyl and stored under sodium metal. Distilled THF was transferred into an inert atmosphere using vacuum reservoirs. C6D6 and CDCl3 were purchased from Cambridge Isotope Laboratory (CIL), degassed, and vacuum-transferred to 4 Å molecular sieves. Celite, alumina, and 4 Å molecular sieves were activated under vacuum overnight at 200 °C. All other chemicals were used as received unless otherwise stated. 1H, 13C, and 31P NMR spectra were recorded on Varian 300 and 400 MHz NMR spectrometers. 1H and 13C NMR chemical shifts are reported with reference to solvent resonances of C6D6 at 7.15 and 128.0 ppm, respectively. 31P NMR chemical shifts are reported with respect to external H3PO4 (aqueous solution, δ 0.0 ppm). Mass spectrometry experiments were performed on an Agilent 6890N gas chromatograph equipped with a 5973 inert mass selective detector under PC control with ChemStation software at the Indiana University Chemistry Department Mass Spectrometry Facility. X-ray diffraction data were collected on an APEX II Kappa Duo (Bruker) system under a stream of N2(g) at low temperatures.

Hydrosilylation Via a Transient Ni Hydride Complex NiCl2(THF)1.5,77 N-(2,4,6-trimethylphenyl)-4-methylaniline, N-(2,4,6trimethylphenyl)-2-bromo-4-methylaniline, and 1-bromo-2,4,6-triisopropylbenzene were prepared according to published procedures.78,79 The syntheses of (PNP)NiCl and (PNP)NiH were reported elsewhere.60,74 NaOCH2Ph was prepared from sodium metal and benzyl alcohol.80 4.2. N-(2,4,6-Triisopropylphenyl)-4-methylaniline. Inside the glovebox, a mixture of Pd(OAc)2 (80.0 mg, 0.36 mmol) and DPPF (391 mg, 0.71 mmol) were stirred in 300 mL of toluene in a 500 mL two-neck round-bottom flask for 0.5 h, generating an orange solution. The round-bottom flask was removed from the glovebox and capped with a rubber septum. Under a high-pressure flow of nitrogen, to this flask was added 1-bromo-2,4,6-triisopropylbenzene (10.0 g, 35.3 mmol), p-toluidine (4.54 g, 42.4 mmol) in toluene via syringe, and solid sodium t-pentoxide (6.54 g, 59.4 mmol), respectively. The reaction flask was connected to a reflux condenser, and the mixture was heated to reflux at 120 °C for 4 days under a nitrogen atmosphere. After the reaction mixture was cooled to room temperature, the mixture was filtered through a tall silica pad (4 cm) in a medium-porosity frit. The solvent from the filtrate was removed under reduced pressure to obtain a crude dark brown solid. The resulting crude product was extracted into 200 mL of dichloromethane and washed three times with 250 mL of water and once with concentrated brine solution. The CH2Cl2 solution was dried with anhydrous sodium sulfate, the solution was filtered, and the filtrate was dried under reduced pressure to form an orange oil. To the oil was added 75 mL of hexane, and the solution was filtered through a tall silica pad (4 cm) in a medium-porosity frit and the silica was washed with copious amounts of hexane. All volatiles were removed under reduced pressure to afford beige crystalline solids. Yield: 72% (7.87 g, 25.4 mol). 1H NMR (25 °C, 300 MHz, CDCl3): δ 7.04 (s, 2H, Ar-H), 6.95 (d, 2H, JH-H ) 8 Hz, Ar-H), 6.41 (d, JH-H ) 8 Hz, 2H, Ar-H), 4.93 (br s, 1H, NH), 3.17 (m, 2H, ArCH(CH3)2), 2.92 (m, 1H, ArCH(CH3)2), 2.23 (s, 3H, ArCH3), 1.29 (d, JH-H ) 7 Hz, 6H, ArCH(CH3)2), 1.14 (d, JH-H ) 7 Hz, 12H, ArCH(CH3)2). 13C{1H} NMR (25 °C, 100 MHz, CDCl3): δ 147.5 (Ar-C), 147.3 (Ar-C), 146.3 (Ar-C), 133.4 (ArC), 129.9 (Ar-C), 126.7 (Ar-C), 121.9 (Ar-C), 113.17 (Ar-C), 34.5 (ArCH(CH3)2), 28.5 (ArCH(CH3)2), 24.4 (ArCH(CH3)2), 24.2 (ArCH(CH3)2), 20.7 (ArCH3). GC-MS (m/z): calcd for C22H31N 309.25, found 309.00. 4.3. N-(2,4,6-Triisopropylphenyl)-2-bromo-4-methylaniline. N-(2,4,6-triisopropylphenyl)-4-methylaniline (10.0 g, 32.6 mmol) was dissolved in 150 mL of acetonitrile in a 0 °C ice bath. To the cold solution was added N-bromosuccinimide (NBS) (5.08 g, 32.6 mmol) in a solid portion over 5 min in air. The yellow solution turned orange immediately upon addition of NBS. The mixture was warmed to room temperature after stirring for 12 h. The reaction was quenched with 10 mL of a saturated aqueous solution containing sodium bisulfite. After 0.5 h of stirring, all volatiles were removed, giving yellow solids. The crude product was extracted into 200 mL of dichloromethane and washed three times with 200 mL of water and once with 200 mL of concentrated brine solution. The dichloromethane layer was removed in vacuo, the resulting orange solids were extracted into 100 mL of hexane and dried with anhydrous sodium sulfate, and the extract was filtered through a tall silica pad (4 cm) in a medium-porosity frit. The silica pad was washed with copious amounts of hexane to obtain a colorless filtrate. All volatiles were removed from the filtrate, affording pure white solid product. Yield: 92% (11.5 g, 29.7 mmol). 1H NMR (25 °C, (77) Eckert, N. A.; Bones, E. M.; Lachicotte, R. J.; Holland, P. L. Inorg. Chem. 2003, 42, 1720. (78) MacLachlan, E. A.; Fryzuk, M. D. Organometallics 2005, 24, 1112– 1118. (79) Miller, A. R.; Curtin, D. Y. J. Am. Chem. Soc. 1976, 98, 1860– 1865. (80) Caine, D. In e-EROS Encyclopedia of Reagents for Organic Synthesis; Wiley: London, 2001.

Organometallics, Vol. 28, No. 7, 2009 2241 300 MHz, CDCl3): δ 7.28 (s, 1H, Ar-H), 7.04 (s, 2H, Ar-H), 6.80 (d, JH-H ) 8 Hz, 1H, Ar-H), 6.07 (d, JH-H ) 8 Hz, 1H, Ar-H), 5.46 (br s, 1H, NH), 3.05 (m, 2H, ArCH(CH3)2), 2.91 (m, 1H, ArCH(CH3)2), 2.20 (s, 3H, ArCH3), 1.28 (d, JH-H ) 7 Hz, 6H, ArCH(CH3)2)), 1.17 (d, JH-H ) 7 Hz, 6H, ArCH(CH3)2), 1.10 (d, JH-H ) 7 Hz, 6H, ArCH(CH3)2). 13C{1H} NMR (25 °C, 100 MHz, CDCl3): δ 148.4 (Ar-C), 147.8 (Ar-C), 143.4 (Ar-C), 133.2 (ArC), 129.4 (Ar-C), 128.0 (Ar-C), 122.4 (Ar-C), 113.12 (Ar-C), 109.1 (Ar-C), 34.9 (ArCH(CH3)2), 28.9 (ArCH(CH3)2), 25.3 (ArCH(CH3)2), 24.7 (ArCH(CH3)2), 23.8 (ArCH(CH3)2), 20.6 (ArCH3). GC-MS (m/z): calcd for C22H30BrN 387.17, found 387.00. 4.4. Lithium N-(2-(Diisopropylphosphino)-4-methylphenyl)2,4,6-trimethylaniline (Li(PNMe3)). To a solution of N-(2,4,6trimethylphenyl)-2-bromo-4-methylaniline (10.0 g, 32.8 mmol) in 150 mL of diethyl ether in a 250 mL round-bottom flask was added n BuLi (41.0 mL, 65.7 mmol) dropwise via syringe at -190 °C. The mixture was warmed to room temperature over a period of 3 h. After this time, the solution was frozen at -190 °C, and to the thawing solution was added chlorodiisopropylphosphine (5.20 mL, 32.8 mmol) via syringe, resulting in a red solution that was stirred for 48 h. Upon completion, the flask was sealed with a rubber stopper and removed from the glovebox, whereupon the mixture was quenched with 5 mL of carefully degassed water via syringe and stirred at room temperature for 0.5 h, generating a bright yellow solution. The solution was diluted with 50 mL of diethyl ether and washed twice with water and once with concentrated brine solution. The ethereal layer was removed under reduced pressure to afford an orange oil, which was dissolved in 100 mL of hexane and dried with Na2SO4 for 0.5 h. The hexane solution was filtered through a tall silica bed (4 cm) in a medium-porosity frit to obtain a faint yellow filtrate. Removal of all volatiles produced a yellow oil, which was taken into the glovebox for subsequent deprotonation. Accordingly, treating the yellow oil with nBuLi (20.5 mL, 2.8 mmol) at -37 °C in 100 mL of pentane and stirring the solution at room temperature over 8 h resulted in formation of a yellow precipitate. The product was collected via a medium-porosity frit and washed with 25 mL of cold pentane to isolate Li(PNMe3). Yield: 65% (7.43 g, 21.3 mmol). 1H NMR (25 °C, 300 MHz, C6D6): δ 6.98 (dd, JH-H ) 8 Hz, 1H, Ar-H), 6.90 (s, 2H, Ar-H), 6.78 (dd, JH-H ) 8 Hz, 1H, Ar-H), 5.58 (dd, JP-H ) 8 Hz 1H, Ar-H), 2.24 (s, 3H, ArCH3), 2.22 (s, 3H, ArCH3), 2.09 (s, 6H, ArCH3), 1.98 (m, 2H, PCH(CH3)2), 1.12 (dd, JP-H ) 7 Hz, 6H, PCH(CH3)2), 1.03 (dd, JP-H ) 7 Hz, 6H, PCH(CH3)2). 31P{1H} NMR (25 °C, 121 MHz, C6D6): δ -13.1 (s). GC-MS (m/z): calcd for C22H32NP 341.23, found for C22H32NP 341.00. 4.5. N-(2-(Diisopropylphosphino)-4-methylphenyl)-2,4,6-triisopropylaniline ((THF)2LiPNiPr3). To a chilled 100 mL ethereal solution containing N-(2,4,6-triisopropylphenyl)-2-bromo-4-methylaniline (5.03 g, 12.9 mmol) in a 250 mL round-bottom flask was added n-BuLi (16.2 mL, 25.9 mmol) with a syringe and the mixture was stirred at room temperature. After 3 h, the resulting solution was again chilled in the cold well and chlorodiisopropylphosphine (2.06 mL, 12.9 mmol) was added, generating an orange suspension that was stirred for 48 h. Upon completion of the reaction, the round-bottom flask was sealed with a rubber stopper and removed from the glovebox, and the reaction mixture was quenched with 0.50 mL of rigorously degassed water and stirred at room temperature for 0.5 h. The orange suspension gradually became a soluble yellow solution with a white precipitate attributed to LiOH. All volatiles were removed, and the resulting orange oil was dissolved in hexane or ether and sodium sulfate or magnesium sulfate was added, respectively, to remove trace water. The mixture was filtered through a silica pad (4 cm), and the solvent from the filtrate was removed by reduced pressure and dried under vacuum. The crude oil was taken into the glovebox, dissolved in 50 mL of THF in a 250 mL round-bottom flask, and treated with nBuLi (8.10 mL, 12.9 mmol). Removal of the THF and addition of cold hexane generated the product (THF)2LiPNiPr3, which was stored at -37

2242 Organometallics, Vol. 28, No. 7, 2009 °C for 8 h and collected by vacuum filtration with a mediumporosity frit. Yield: 50% (3.71 g, 6.45 mmol). 1H NMR (25 °C, 400 MHz, C6D6): δ 7.25 (s, 2H, Ar-H), 6.97 (d, JH-H ) 6 Hz, 1H, Ar-H), 6.85 (d, JH-H ) 8 Hz, 1H, Ar-H), 6.16 (dd, JP-H ) 14 Hz, 1H, Ar-H), 3.63 (m, 2H, ArCH(CH3)2), 3.19 (br s, 8H, OCH2CH2) 2.97 (m, 1H, ArCH(CH3)2), 2.28 (s, 3H, ArCH3), 2.10 (m, 2H, PCH(CH3)2), 1.37 (dd, JH-H ) 7 Hz, 12H, ArCH(CH3)2)), 1.25 (d, JH-H ) 7 Hz, 6H, ArCH(CH3)2)), 1.20 (br s, 8H, OCH2CH2) 1.06 (m, 12H, PCH(CH3)2). 31P{1H} NMR (25 °C, 121 MHz, C6D6): δ -4.52 (s). 13C{1H} NMR (25 °C, 75 MHz, C6D6): δ 165.0 (d, JP-C ) 21 Hz, Ar-C), 150.5 (Ar-C), 144.1 (Ar-C), 140.4 (Ar-C), 133.0 (Ar-C), 132.3 (Ar-C), 121.2 (Ar-C), 115.5 (Ar-C), 113.1 (Ar-C), 111.1 (d, JP-C ) 12 Hz, Ar-C), 68.3 (OCH2CH2), 34.7 (ArCH(CH3)2), 28.2 (ArCH(CH3)2), 25.4 (OCH2CH2), 25.0 (PCH(CH3)2), 24.9 (ArCH(CH3)2), 23.5 (ArCH(CH3)2), 21.0 (ArCH3), 20.5 (d, JP-C ) 14 Hz, PCH(CH3)2), 20.2 (d, JP-C ) 14 Hz, PCH(CH3)2). EI-HRMS (m/z): calcd for C28H44NP 425.321 12, found 426.3268 [M + H]+. 4.6. [(PNiPr3)Ni(µ2-Br)]2 (1). In a 100 mL round-bottom flask, to a cold slurry of NiBr2 (0.042 g, 0.19 mmol) in THF (10 mL) at -37 °C was added (THF)2Li(PNiPr3) (0.100 g, 0.17 mmol) dissolved in 5 mL of THF at -37 °C. The yellow suspension gradually turned to dark red-brown. After the mixture was stirred for 12 h at 70 °C, all volatiles were removed under reduced pressure. The crude product was extracted into pentane and the extract filtered through a fiberglass plug to remove salt residues. The filtrate was removed of solvent to generate dark red solids. Crystalline materials were obtained from slow evaporation of a concentrated hexane solution in a 20 mL vial at room temperature. Yield: 70% (0.067 g, 0.059 mmol). 1H NMR (25 °C, 300 MHz, C6D6): δ 7.11 (s, 4H, Ar-H), 6.60 (s, 2H, Ar-H), 6.46 (d, JH-H ) 9 Hz, 2H, Ar-H), 5.69 (d, JH-H ) 9 Hz, 2H, Ar-H), 4.10 (m, 4H, ArCH(CH3)2), 2.83 (m, 2H, ArCH(CH3)2), 1.97 (s, 6H, ArCH3), 1.92 (d, JH-H ) 7 Hz, 12H, ArCH(CH3)2), 1.83 (m, 4H, PCH(CH3)2), 1.48 (m, 24H, ArCH(CH3)2), 1.26 (m, 24H, PCH(CH3)2). 31P{1H} NMR (25 °C, 121 MHz, C6D6): δ 63.91 (br s). 13C{1H} NMR (25 °C, 75 MHz, C6D6): δ 168.6 (Ar-C), 147.5 (Ar-C), 145.2 (d, JP-C ) 18 Hz, Ar-C), 134.4 (Ar-C), 130.5 (Ar-C), 121.6 (Ar-C), 120.4 (Ar-C), 115.3 (Ar-C), 34.9 (ArCH(CH3)2), 29.0 (ArCH(CH3)2), 26.1 (ArCH(CH3)2), 24.9 (ArCH(CH3)2), 24.5 (d, JP-C ) 9 Hz, PCH(CH3)2), 20.0 (ArCH3), 18.4 (PCH(CH3)2), 17.5 (PCH(CH3)2). Mp: 196-200 °C. Anal. Calcd for C56H86Br2N2P2Ni2: C, 59.71; H, 7.70; N, 2.49. Found: C, 59.45; H, 7.55; N, 2.30. 4.7. [(PNMe3)Ni(µ2-Cl)]2 (2). In a 20 mL vial, a suspension of NiCl2(THF)1.5 (0.202 g, 0.76 mmol) in 5 mL of THF was added dropwise to Li(PNMe3) (0.264 g, 0.76 mmol) in 5 mL of THF at -37 °C. The yellow slurry gradually transformed into a homogeneous dark reddish brown solution within 2 h. After the mixture was stirred for 12 h, all volatiles were removed under reduced pressure. The crude product was extracted into toluene and filtered through a Celite-filled medium-porosity frit to remove unreactive NiCl2(THF)1.5 and salt side product. The filtrate was evaporated under reduced pressure, triturated with 10 mL of hexane, and stored at -37 °C for 12 h, and subsequent vacuum filtration with a medium-porosity frit afforded red solids. Yield: 75% (0.331 g, 0.380 mmol). 1H NMR (25 °C, 300 MHz, C6D6): δ 6.85 (s, 4H, Ar-H), 6.64 (s, 2H, Ar-H), 6.56 (d, JH-H ) 9 Hz, 2H, Ar-H), 5.70 (d, JH-H ) 9 Hz, 2H, Ar-H), 2.61 (s, 12H, ArCH3), 2.17 (s, 6H, ArCH3), 2.09 (s, 6H, ArCH3), 1.67 (m, 4H, PCH(CH3)2), 1.41 (m, 12H, PCH(CH3)2), 1.27 (m, 12H, PCH(CH3)2). 31P{1H} NMR (25 °C, 121 MHz, C6D6): δ 63.4 (br s, ∆ν1/2 ) 57 Hz). 13C{1H} NMR (25 °C, 75 MHz, C6D6): δ 150.3 (Ar-C), 141.7 (Ar-C), 133.6 (ArC), 131.8 (Ar-C), 131.0 (Ar-C), 129.3 (Ar-C), 121.0 (Ar-C), 117.1 (Ar-C), 109.5 (Ar-C), 30.8 (Ar-CH3), 22.5 (PCH(CH3)2), 20.7 (d, JP-C ) 4 Hz, PCH(CH3)2), 19.8 (Ar-CH3), 19.6 (Ar-CH3). Mp: 222-225 °C. Anal. Calcd for C44H62Cl2N2P2Ni2: C, 60.80; H, 7.19; N, 3.22. Found: C, 60.65; H, 7.25; N, 3.15.

Tran et al.

4.8. (PNP)Ni(OCH2Ph) (4). In a 20 mL vial, (PNP)NiCl (0.15 g, 0.28 mmol) was dissolved in 10 mL of diethyl ether to give a green solution. To the vial was added dropwise Na(OCH2Ph) (0.037 mg, 0.28 mmol) in an 3 mL ethereal solution at room temperature. The reaction mixture was stirred for 12 h, affording a red solution. All volatiles were removed under reduced pressure, and the crude product was extracted into pentane and filtered through a fiberglass plug containing Celite. The resulting dark red filtrate was reduced in volume and placed in the freezer at -30 °C for 12 h to afford red crystals. The crystalline product was collected on a medium-porosity frit by vacuum filtration. Yield: 90% (0.15 g, 0.25 mmol). 1H NMR (25 °C, 300 MHz, C6D6): δ 7.55 (d, JH-H ) 7 Hz, 2H, Ar-H), 7.42 (d, JH-H ) 9 Hz, 2H, Ar-H), 7.03-7.27 (m, 5H, Ar-H), 6.89 (s, 2H, Ar-H), 4.55 (s, 2H, OCH2Ar), 2.17 (m, 4H, PCH(CH3)2), 2.13 (s, 6H, ArCH3), 1.46 (m, 12H, PCH(CH3)2), 1.25 (m, 12H, PCH(CH3)2). 31 P{1H} NMR (25 °C, 121 MHz, C6D6): δ 25.82 (s). 13C{1H} NMR (25 °C, 75 MHz, C6D6): δ 162.2 (Ar-C), 147.4 (Ar-C), 132.1 (Ar-C), 127.1 (Ar-C), 127.0 (Ar-C), 126.7 (Ar-C), 126.0 (Ar-C), 120.1 (Ar-C), 116.9 (Ar-C), 71.3 (OCH2Ar), 24.1 (PCH(CH3)2), 20.6 (ArCH3), 18.7 (PCH(CH3)2), 17.8 (PCH(CH3)2). CI-HRMS: Anal. Calcd for C33H47NNiOP2: 593.2486. Found: 593.2471. Anal. Calcd. for C33H47NNiOP2 · C13H32O2: C, 67.81; H, 9.77; N, 1.72. Found: C, 67.97; H, 9.40; N, 1.54. 4.9. General Conditions for Nickel-Complex-Mediated Hydrosilylation of Carbonyl-Containing Substrates. In a glovebox, a J-Young NMR tube equipped with a Teflon screw cap was charged with 1.0 mL of C6D6 solution containing 2 mol % of 1 (30.8 mg, 0.027 mmol), 4 mol % of KOtBu (6.15 mg, 0.055 mmol), and triethylsilane (262 µL, 1.64 mmol). The J-Young tube was removed from the glovebox, and the carbonyl compound (1.37 mmol) was added to the J-Young via syringe. The sample was placed into a preheated 100 °C oil bath. The reaction was monitored by 1H NMR spectroscopy. Upon completion, all volatiles were removed under reduced pressure and the crude product was subjected to flash chromatography on a thin, long silica gel column (Mallinckrodt Baker 60-200 mesh) with pentane as eluent. The oily product was obtained upon removal of all pentane via a rotary evaporator. 4.10. X-ray Crystallography: General Parameters for Data Collection and Refinement. Single crystals of 1 were grown at room temperature from concentrated solutions of toluene via a slow -evaporation technique. Inert-atmosphere techniques were used to place the crystal onto the tip of a glass capillary (0.06-0.20 mm diameter) mounted on a SMART6000 instrument (Bruker) at 113(2) K. A preliminary set of cell constants was calculated from reflections obtained from three nearly orthogonal sets of 20-30 frames. The data collection was carried out using graphite-monochromated Mo KR radiation with a frame time of 3 s and a detector distance of 5.0 cm. A randomly oriented region of a sphere in reciprocal space was surveyed. Three sections of 606 frames were collected with 0.30° steps in ω at different φ settings with the detector set at -43° in 2θ. Final cell constants were calculated from the xyz centroids of strong reflections from the actual data collection after integration (SAINT).81 The structure was solved using SHELXS-97 and refined with SHELXL-97.82 A direct-methods solution was calculated that provided most non-hydrogen atoms from the E map. Full-matrix least-squares/difference Fourier cycles were performed that located the remaining non-hydrogen atoms. All non-hydrogen atoms were refined with anisotropic displacement parameters, and all hydrogen atoms were refined with (81) SAINT, version 6.1; Bruker Analytical X-ray Systems, Madison, WI, 1999. (82) SHELXL-Plus, version 5.10; Bruker Analytical X-ray Systems, Madison, WI, 1998.

Hydrosilylation Via a Transient Ni Hydride Complex isotropic displacement parameters (unless otherwise specified). Some intensity data were corrected for absorption (SADABS).83 Crystal data for [(PNiPr3)Ni(µ2-Br)]2 (1): C56H86Br2N2P2Ni2, Mw ) 1126.44, monoclinic, space group P21/c, a ) 11.0631(4) Å, b ) 14.6234(5) Å, c ) 18.2594(7) Å, β ) 94.1570(10)°, Z ) 2, µ ) 2.084 mm-1, λ(Mo KR) ) 0.710 73 Å, V ) 2946.24(19) Å3, Fcalcd ) 1.270 Mg/m3, R(F) ) 0.0356, Rw(F2) ) 0.0856, and GOF on F2 ) 1.048 (I > 2σ(I)), dark red platelike crystals with dimensions of 0.20 × 0.20 × 0.10 mm3, 27.48° g θ g 1.79°. Out of a total of 29 529 reflections collected, 6756 were unique and 5206 were observed (Rint ) 2.99%). (83) Blessing, R. Acta Crystallogr. 1995, A51, 33.

Organometallics, Vol. 28, No. 7, 2009 2243

Acknowledgment. We thank Indiana UniversitysBloomington and the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Science, Office of Science, U.S. Department of Energy (No. DE-FG0207ER15893), for support of this research. Ms. Meghan R. Mulcrone is thanked for crystallographic assistance. Supporting Information Available: A CIF file giving crystal data for complex1, text detailing control experiments, and figures giving 1H NMR and GC-MS data of the hydrosilylated products. This material is available free of charge via the Internet at http://pubs.acs.org. OM801160J