Double Metalation of Acetone by a Nickel–NHC Complex: Trapping of

Note pubs.acs.org/Organometallics

Double Metalation of Acetone by a Nickel−NHC Complex: Trapping of an Oxyallyl Ligand at a Dinickel Center Anna Magdalena Oertel,† Vincent Ritleng,† Asleche Busiah,† Luis F. Veiros,‡ and Michael J. Chetcuti*,† †

Laboratoire de Chimie Organométallique Appliquée, UMR CNRS 7509, Ecole Européenne de Chimie, Polymères et Matériaux, Université de Strasbourg, 25 rue Becquerel, 67087 Strasbourg, France ‡ Centro de Química Estrutural, Complexo I, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal S Supporting Information *

ABSTRACT: We report the first example of a structurally characterized oxyallyl −CH2C(O)CH2− group, trapped as a bridging ligand that spans two nickel centers. The ligand results from the double metalation of acetone on a nickel− NHC platform. A competing reaction leads to a single metalation of acetone.

R

Scheme 2. Formation of the Acetone Complex 1 and the Subsequent Metalation of Acetone to Give the Acetonyl Complex 2 and the Oxyallyl Complex 3

ecent research from our group has targeted the chemistry of nickel complexes bearing NHC ligands.1 We recently reported that nickel−NHC complexes are able to activate one of the C−H bonds of a labile acetonitrile ligand in the presence of a strong base. The reaction results in the formal loss of a hydrogen atom from the ligand, which does a sharp flip to give a neutral cyanomethyl−nickel complex (Scheme 1).2 Here, we

Scheme 1. Base-Promoted C−H Activation of Acetonitrile2

describe the synthesis of the dinickel complex [{Cp(Mes2NHC)Ni}2{μ-CH2C(O)CH2}], in which an oxyallyl group is trapped as a bridging ligand spanning the two metal centers. The complex results from the deprotonation of two C−H bonds from the two different methyl groups of an acetone molecule on a nickel−NHC platform. While trimethylenemethane has been trapped as a ligand in many stable organometallic complexes,3 the isoelectronic oxyallyl group has never been isolated free or even structurally characterized as a ligand.4 Treatment of [Ni(Mes2NHC)ClCp] with AgBF4 in acetone affords the red cationic η1-acetone complex [Ni(Mes2NHC){(O)C(CH3)2}Cp]+BF4−, 1, in high yield (Scheme 2). The 1H and 13C{1H} NMR spectra in acetone-d6 show that the (CH3)2C(O) ligand is labile in solution. The acetone ν(CO) stretch is observed at 1656 cm−1 in the IR spectrum. An X-ray diffraction study of the cation of 1, which is presented here (Figure 1), reveals a C2−O−Ni angle of 132°. The rest of the structure is typical of Cp-Ni−NHC fragments in [CpNi(NHC)(X)] complexes1,2 and deserves no particular comment. © 2011 American Chemical Society

When 1 is treated with a suspension of KOtBu in toluene, two new species are formed. One of these is the brown nickel acetonyl complex [Ni(Mes2NHC){CH2C(O)CH3}Cp], 2 (Scheme 2). In contrast to the heavier group 10 elements, Pd and Pt, where metal-acetonyl or so-called C-bound enolate complexes are prevalent,5 the latter have rarely been reported for nickel:6,7 similar reactions with this metal usually generate O-bound enolate nickel complexes.8 Spectroscopic data for the CH2C(O)CH3 ligand in 2 are consistent with spectral signals observed for other reported acetonyl ligands: the CH2 protons appear at δ = 1.07 ppm in the 1H NMR spectrum (in CDCl3), Received: September 1, 2011 Published: November 14, 2011 6495

dx.doi.org/10.1021/om200819c | Organometallics 2011, 30, 6495−6498

Organometallics

Note

molecule are quite similar, being related by a pseudo (noncrystallographically imposed) C2 axis along the CO bond. The ketonic carbon atom C2 is trigonal [angles around C2 are 120 ± 1.5°; Σ (angles around C2) = 360°]. The two nickel atoms lie in a planar, but distorted, trigonal environment. Thus, in both halves, the oxyallyl carbon atoms (C1) and the carbenoid carbons (C3) subtend angles of ≈93° at their respective nickel atoms. The Ni−CCp carbon distances exhibit significant variations from each other and range from 2.108(2) to 2.206(3) Å. The aromatic C−C distances in the Cp rings also exhibit discrepancies and range from 1.386(4) to 1.432(4) Å. These deviations arise from electronic effects and have been previously observed in pseudotrigonal CpNi systems.10 There are intermolecular hydrogen-bonded interactions in the crystal between the oxygen atom and one of the imidazolylidene CHCH hydrogen atoms (O···H = 2.134 Å; C−H···O = 163.8°) of an adjacent molecule. The mechanism leading to the acetonyl complex 2 from 1 has been explored by DFT calculations with a simplified model consisting of the cationic dimethyl NHC complex [Ni(Me2NHC){(O)C(CH3)2}Cp]+, 1′, and MeO− as the base.11 The screening of possible intermediates suggests that C−H oxidative addition to the nickel to give a Ni(IV) [Ni(Me2NHC)(H)(CH2COCH3)Cp]+ three-legged piano stool complex is highly unlikely as such species could not be optimized. A more probable pathway is H+ transfer to the base to give an initially O-bound enolate species E (Scheme 3) in a very favorable reaction (ΔE = −23.6 kcal·mol−1). In a second step, enolate E isomerizes to the C-bonded acetonyl complex 2′ in another exergonic transformation (ΔE = −7.7 kcal·mol−1). The mechanistic investigations unveiled two possible pathways for this rearrangement. The lower-energy pathway is isomerization in a single step from the enolate to the acetonyl complex (Scheme 3). In an alternative, but less favorable, three-step pathway, formation of 2′ involves a η 1 -Cp/η 3 -enolate intermediate (see Scheme S1, Supporting Information). Among the very short list of molecules containing a bridging oxyallyl ligand,4 the complex [{Fe(H)(dmpe)}2(μ-CD2C(O)CD2)] was obtained via C−D oxidative additions of acetone-d6 onto two iron centers and was observed spectroscopically, but was not isolated.4a In accord with this early work, where the oxyallyl complex was not observed in the presence of excess acetone, formation of complex 3 was suppressed when the reaction was conducted in acetone, even in the presence of excess KOtBu. Nevertheless, the reaction is not clean, as at least one other not fully characterized minor side-product is formed. We suspect it to be a Cp−Ni(NHC) complex bearing an aldol condensation product of acetone as a ligand, but the small quantities of this species precluded its isolation and purification. In contrast, when the reaction was run in toluene with excess base present, 1H NMR analysis of the crude reaction mixture showed that the oxyallyl complex 3 was the major compound formed. Moreover, a pure sample of 3 in acetone-d6 that contained traces of water slowly converted to 2. This suggests that the oxyallyl complex 3 may be formed via a pathway that is related to the acid−base mechanism leading to 2. Nevertheless, another type of mechanism cannot be ruled out, and studies are currently under way to gain a better understanding of its formation. This work and the related activation of alkylnitriles,2 together with the recent disclosure of Ni(II)-catalyzed C−H arylations of arenes and heteroarenes, in the presence of tBuO− as a

Figure 1. Molecular structure of 1 showing all non-H atoms. Key atoms are labeled. The asymmetric unit contains two molecules; only one is shown. Selected distances (Å) and angles (°) for both molecules: Ni−C1, 1.8972(19), 1.8912(19); Ni−O, 1.9249(14), 1.9205(15); O−C2, 1.239(3), 1.229(3); Ni−Cpcent, 1.757, 1.749; C2−O−Ni, 131.45(15), 133.47(15); C1−Ni−O, 96.68(7), 99.36(7); C1−Ni−Cpcent, 132.5, 130.4; O−Ni−Cpcent, 130.0, 129.8.

and the ν(CO) stretch in the IR spectrum now appears at 1618 cm−1. The second species was identified as the complex [{Cp(Mes2NHC)Ni}2{μ-CH2C(O)CH2}], 3 (Scheme 2). This molecule contains a bridging oxyallyl ligand that is derived from the double base-promoted nickelation of acetone. Spectrosopic data are consistent with this structure: a singlet integrating for four protons is observed for the two equivalent sets of the oxyallyl CH2 protons at δ = 0.67 ppm in the 1H NMR spectrum (CDCl3), and the ν(CO) stretch appears at 1564 cm−1 in the IR spectra. The structure of this molecule was established via a single-crystal X-ray diffraction study; its central core is shown in Figure 2.

Figure 2. Molecular structure of 3. Key atoms are labeled. Only the ipso-carbon atom of each Mes group is shown, for clarity. Only H atoms of the oxyallyl are shown (as isotropic black spheres). Selected distances (Å) and angles (°) for both halves of the molecule: Ni−C1, 1.977(2), 1.976(2); Ni−C3, 1.888(2), 1.878(2); C1−C2, 1.483(3), 1.477(3); C2−O, 1.235(3); Ni−Cpcent, 1.794, 1.790; C1−C2−C1′, 118.5; Ni−C1−C2, 111.27(17), 112.85(16); C1−Ni−C3, 94.38(10), 92.53(10); C1−Ni−Cpcent, 128.6, 130.6; C3−Ni−Cpcent, 137.1, 136.9.

The structure consists of two Cp(Mes2NHC)Ni groups, linked by a bridging −CH2−C(O)−CH2− ligand that spans the two nickel centers. The −CH2−C(O)−CH2− group has been invoked as an intermediate species that could form on a metal surface during a syngas conversion process in which two methylene units have coupled with CO.9 The two nickel− carbon bonds to the oxyallyl ligand are close to identical [1.977(2) and 1.976(2) Å], and indeed, the two halves of the 6496

dx.doi.org/10.1021/om200819c | Organometallics 2011, 30, 6495−6498

Organometallics

Note

Scheme 3. DFT Calculated Thermodynamics of the Transformation of the η1-Acetone Complex 1′ to the Enolate Intermediate E, and Energetics of the Direct Pathway of E to the Acetonyl Complex 2′ (Energies in kcal·mol−1)

base,12 hints at the growing potential of nickel in transitionmetal-catalyzed C−H bond functionalization.

■

139.1 (p-CAr), 137.0 (ipso-CAr), 135.5 (o-CAr), 129.4 (m-CAr), 123.9 (NCH), 91.3 (η5-C5H5), 29.0 (COCH3), 21.3 (p-Me), 18.5 (o-Me), 3.6 (CH2). IR [ATR]: ν(Csp2−H) 3156 (w), 3120 (w), 3092 (w); ν(Csp3−H) 2915 (w), 2860 (w); ν(CO) 1618 (m). [Ni2(Mes2NHC)2{(CH2)2CO}Cp2] (3). To a suspension of 1 (573 mg, 1.0 mmol) in toluene (3 mL) was added KOtBu (224 mg, 2.0 mmol) under vigorous stirring. The suspension was stirred at room temperature for 4 h. The solvent was removed in vacuo, and the residue was extracted in CH2Cl2 and filtered through alumina, which was rinsed with THF until the washings were colorless. Volatiles were evaporated under vacuum, and the resulting brown residue was crystallized in a THF/pentane mixture to afford 3 (93 mg, 0.10 mmol, 20% yield based on the amount of Ni) as brown crystals. Despite repeated attempts, we were unable to obtain satisfactory elemental analyses. 1H NMR (acetone-d6, 300.13 MHz): δ = 7.23 (s, 4H, NCH), 7.06 (s, 8H, m-H), 4.27 (s, 10H, η5-C5H5), 2.39 (s, 12H, p-Me), 2.19 (s, 24H, o-Me), 0.58 (s, 4H, CH2). 1H NMR (CDCl3, 300.13 MHz): δ = 7.00 (s, 8H, m-H), 6.88 (s, 4H, NCH), 4.34 (s, 10H, η5-C5H5), 2.39 (s, 12H, p-Me), 2.18 (s, 24H, o-Me), 0.67 (s, 4H, CH2). 13C{1H} NMR (CDCl3, 75.47 MHz): δ = 230.9 (CO), 180.5 (NCN), 138.5 (pCAr), 137.2 (ipso-CAr), 135.7 (o-CAr), 129.2 (m-CAr), 123.3 (NCH), 90.7 (η5-C5H5), 21.3 (p-Me), 19.0 (o-Me), 1.3 (CH2). IR [ATR]: ν(Csp2−H) 3165 (w), 3099 (w); ν(Csp3−H) 2953 (m), 2915 (m), 2856 (m); ν(CO) 1564 (m).

EXPERIMENTAL SECTION

General Comments. All reactions were carried out using standard Schlenk or glovebox techniques under an atmosphere of dry argon. Acetone was degassed three times using a freeze−pump−thaw method prior to use. All other solvents were distilled from appropriate drying agents under argon. Solution NMR spectra were recorded at 298 K on an FT-Bruker Ultra Shield 300 spectrometer operating at 300.13 MHz for 1H and at 75.47 MHz for 13C {1H}. DEPT 13C spectra were recorded for all compounds to help in the 13C signal assignments. The chemical shifts are referenced to the residual deuterated solvent peaks. Chemical shifts (δ) and coupling constants (J) are expressed in parts per million and hertz, respectively. IR spectra of solid samples of complexes 1, 2, and 3 were recorded on an FT-IR Nicolet 380 spectrometer equipped with a diamond SMART-iTR or SMARTORBIT ATR. Vibrational frequencies are expressed in cm −1. Elemental analyses were performed by the Service d’Analyses, de Mesures Physiques et de Spectroscopie Optique, UMR CNRS 7177, Institut de Chimie, Université de Strasbourg. Commercial compounds were used as received. [Ni(Mes2NHC)ClCp] was prepared according to published methods.13 [Ni(Mes2NHC){(CH3)2CO}Cp]+BF4− (1). AgBF4 (195 mg, 1.00 mmol) was added to a solution of [Ni(Mes2NHC)ClCp] (464 mg, 1.00 mmol) in acetone (10 mL). The resulting suspension immediately turned bright red and was stirred at room temperature for 15 min. The mixture was filtered through Celite, which was rinsed with acetone (3 × 3 mL), and the solvent was removed under vacuum. Crystallization from an acetone/diethylether solution (1:3) at room temperature afforded 1 (410 mg, 0.72 mmol, 72%) as red crystals. Anal. Calcd for C29H35BF4N2NiO: C, 60.78; H, 6.16; N, 4.89. Found: C, 60.73; H, 6.36; N, 4.79. 1H NMR (acetone-d6, 300.13 MHz):14 δ = 7.69 (s, 2H, NCH), 7.27 (s, 4H, m-H), 4.68 (s, 5H, η5-C5H5), 2.44 (s, 6H, p-Me), 2.19 (s, 12H, o-Me). 13C{1H} NMR (acetone-d6, 75.47 MHz):15 δ = 160.8 (NCN), 140.8 (p-CAr), 137.1 (ipso-CAr), 136.4 (oCAr), 130.5 (m-CAr), 126.1 (NCH), 94.0 (η5-C5H5), 21.1 (p-Me), 18.2 (o-Me). IR [ATR]: ν(Csp2−H) 3174 (w), 3142 (w), 3105 (w); ν(Csp3−H) 2950 (w), 2919 (w), 2861 (w); ν(CO) 1656 (m); ν(B−F) 1049 (s), 1032 (s). [Ni(Mes2NHC){CH2(CO)CH3}Cp] (2). To a red solution of 1 (100 mg, 0.174 mmol) in acetone (3 mL) was added KO tBu (21 mg, 0.187 mmol) under vigorous stirring. The resulting suspension immediately turned brown and was stirred at room temperature for 1.5 h. The reaction medium was then concentrated under vacuum, extracted in THF (5 mL), and filtered through Celite, which was rinsed with THF until the washings became colorless. The solvent was removed under vacuum, and crystallization from a THF/pentane (1:4) solution at room temperature gave 2 (30 mg, 0.062 mmol, 36%) as brown crystals. Anal. Calcd for C29H34N2NiO: C, 71.77; H, 7.06; N, 5.77. Found: C, 70.91; H, 7.13; N, 5.57. 1H NMR (acetone-d6, 300.13 MHz): δ = 7.37 (s, 2H, NCH), 7.14 (s, 4H, m-H), 4.37 (s, 5H, η5C5H5), 2.40 (s, 6H, p-Me), 2.16 (s, 12H, o-Me), 1.48 (s, 3H, COCH3), 0.93 (s, 2H, CH2). 1H NMR (CDCl3, 300.13 MHz): δ = 7.07 (s, 4H, m-H), 6.99 (s, 2H, NCH), 4.45 (s, 5H, η5-C5H5), 2.40 (s, 6H, p-Me), 2.13 (s, 12H, o-Me), 1.60 (s, 3H, COCH3), 1.07 (s, 2H, CH2). 13 C{1H} NMR (CDCl3, 75.47 MHz): δ = 217.1 (CO), 177.8 (NCN),

■

ASSOCIATED CONTENT S Supporting Information * NMR spectra of complexes 2 and 3, X-ray data in CIF format for complexes 1 and 3, and full DFT computational details. This material is available free of charge via the Internet at http://pubs.acs.org.

■ ■

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel: + 33 3 68 85 26 31. ACKNOWLEDGMENTS A.M.O. thanks the Ministère de l’Enseignement Supérieur et de la Recherche for a doctoral fellowship. We are grateful to the CNRS and the University of Strasbourg for their financial help. Dr. Lydia Brelot is gratefully acknowledged for her work in the X-ray structure resolutions. L.F.V. is thankful to U.T.L./ Santander for financial support.

■

REFERENCES

(1) (a) Ritleng, V.; Barth, C.; Brenner, E.; Milosevic, S.; Chetcuti, M. J. Organometallics 2008, 27, 4223. (b) Milosevic, S.; Brenner, E.; Ritleng, V.; Chetcuti, M. J. Dalton Trans. 2008, 1973. (c) Ritleng, V.; Oertel, A. M.; Chetcuti, M. J. Dalton Trans. 2010, 39, 8153. (2) (a) Oertel, A. M.; Ritleng, V.; Chetcuti, M. J.; Veiros, L. F. J. Am. Chem. Soc. 2010, 132, 13588. (b) Oertel, A. M.; Freudenreich, J.; Gein, J.; Ritleng, V.; Veiros, L. F.; Chetcuti, M. J. Organometallics 2011, 30, 3400. (3) Representative examples include: (a) Pettit, R.; Ward, J. S. J. Chem. Soc. D 1970, 1419. (b) Ehrlich, K.; Emerson, G. F. J. Am. Chem. 6497

dx.doi.org/10.1021/om200819c | Organometallics 2011, 30, 6495−6498

Organometallics

Note

S.; Gonidec, M.; Ritleng, V.; Braunstein, P.; Welter, R.; Chetcuti, M. J. Eur. J. Inorg. Chem. 2010, 403. (11) DFT calculations were perfomed with the PBE1PBE functional using the Gaussian 03 package. The energy values include solvent effects obtained with the PCM model. See the Supporting Information for full details. (12) (a) Canivet, J.; Yamaguchi, J.; Ban, I.; Itami, K. Org. Lett. 2009, 11, 1733. (b) Hachiya, H.; Hirano, K.; Satoh, T.; Miura, M. Org. Lett. 2009, 11, 1737. (c) Kobayashi, O.; Uraguchi, D.; Yamakawa, T. Org. Lett. 2009, 11, 2679. (13) (a) Ritleng, V.; Brenner, E.; Chetcuti, M. J. J. Chem. Educ. 2008, 85, 1646. (b) Abernethy, C. D.; Cowley, A. H.; Jones, R. A. J. Organomet. Chem. 2000, 596, 3. (14) Free acetone that results from exchange with acetone-d 6 is seen as a singlet (at 2.09 ppm) on the downfield side of the multiplet due to residual (CHD2)2CO observed at 2.05 ppm. (15) Free acetone that results from exchange with acetone-d6 is seen as two singlets (at 30.60 and 205.9 ppm) overlapping with the multiplet due to residual (CHD2)2CO at 29.84 ppm and with the singlet due to (CD3)2CO at 206.3 ppm.

Soc. 1972, 94, 2464. (c) Jones, M. D.; Kemmitt, R. D. W. J. Chem. Soc., Chem. Commun. 1985, 811. (d) Grosselin, M. J.; Le Bozec, H.; Moinet, C.; Toupet, L.; Dixneuf, P. H. J. Am. Chem. Soc. 1985, 107, 2809. (e) Fildes, M. J.; Knox, S. A. R.; Orpen, A. G.; Turner, M. L.; Yates, M. I. J. Chem. Soc., Chem. Commun. 1989, 1680. (f) Lokey, R. S.; Mills, N. S.; Rheingold, A. L. Organometallics 1989, 8, 1803. (g) Chetcuti, M. J.; Fanwick, P. E.; Grant, B. E. Organometallics 1991, 10, 3003. (h) Bazan, G. C.; Rodriguez, G.; Cleary, B. P. J. Am. Chem. Soc. 1994, 116, 2177. (i) Plantevin, V.; Blosser, P. W.; Gallucci, J. C.; Wojcicki, A. Organometallics 1994, 13, 3651. (j) Herberich, G. E.; Kreuder, C.; Englert, U. Angew. Chem., Int. Ed. Engl. 1994, 33, 2465. (k) Girard, L.; Baird, M. C.; Chetcuti, M. J.; McGlinchey, M. J. J. Organomet. Chem. 1994, 478, 179. (l) Kissounko, D. A.; Sita, L. R. J. Am. Chem. Soc. 2004, 126, 5946. (4) (a) Ittel, S. D.; Tolman, C. A.; English, A. D.; Jesson, J. P. J. Am. Chem. Soc. 1978, 100, 7577. (b) Bullock, R. M.; Hembre, R. T.; Norton, J. R. J. Am. Chem. Soc. 1988, 110, 7868. (c) Waters, W. L.; Kiefer, E. F. J. Am. Chem. Soc. 1967, 89, 6261. (d) Johnson, F. A.; Perry, W. D. Organometallics 1989, 8, 2646. (5) (a) Vicente, J.; Arcas, A.; Fernández-Hernández, J. M.; Bautista, D. Organometallics 2008, 27, 3978. (b) Berenguer, J. R.; Lalinde, E.; Torroba, J. Inorg. Chem. 2007, 46, 9919. (c) Vicente, J.; Arcas, A.; Fernández-Hernández, J. M.; Aullón, G.; Bautista, D. Organometallics 2007, 26, 6155. (d) Arai, S.; Ochiai, M.; Ishihara, K.; Matsumoto, K. Eur. J. Inorg. Chem. 2007, 2031. (e) Vicente, J.; Arcas, A.; FernándezHernández, J. M.; Bautista, D. Organometallics 2006, 25, 4404. (f) Suzaki, Y.; Osakada, K. Organometallics 2006, 25, 3251. (g) Vicente, J.; Arcas, A.; Fernández-Hernández, J. M.; Sironi, A.; Masciocchi, N. Chem. Commun. 2005, 1267. (h) Matsumoto, K.; Arai, S.; Ochiai, M.; Chen, W.; Nakata, A.; Nakai, H.; Kinoshita, S. Inorg. Chem. 2005, 44, 8552. (i) Culkin, D. A.; Hartwig, J. F. Organometallics 2004, 23, 3398. (j) Vicente, J.; Arcas, A.; Fernández-Hernández, J. M.; Bautista, D. Organometallics 2001, 20, 2767. (k) Lin, Y.-S.; Misawa, H.; Yamada, J.; Matsumoto, K. J. Am. Chem. Soc. 2001, 123, 569. (l) Albéniz, A. C.; Catalina, N. M.; Espinet, P.; Redón, R. Organometallics 1999, 18, 5571. (m) Vicente, J.; Abad, J. A.; Chicote, M.-T.; Abrisqueta, M.-D.; Lorca, J.-A.; Ramírez de Arellano, M. C. Organometallics 1998, 17, 1564. (n) Henderson, W.; Fawcett, J.; Kemmitt, R. D. W.; McKenna, P.; Russell, D. R. Polyhedron 1997, 16, 2455. (o) Falvello, L. R.; Garde, R.; Miqueleiz, E. M.; Tomás, M.; Urriolabeitia, J. P. Inorg. Chim. Acta 1997, 264, 297. (p) Vicente, J.; Abad, J. A.; Bergs, R.; Jones, P. G.; Bautista, D. J. Chem. Soc., Dalton Trans. 1995, 3093. (q) Suzuki, K.; Jindo, A.; Hanaki, K. Inorg. Chim. Acta 1993, 210, 57. (r) Suzuki, K.; Yamamoto, H. Inorg. Chim. Acta 1993, 208, 225. (s) Bennett, M. A.; Robertson, G. B.; Whimp, P. O.; Yoshida, T. J. Am. Chem. Soc. 1973, 95, 3028. (6) Acetonyl complexes have been obtained by halide displacement from a nickel complex with potassium or lithium enolates: Burkhardt, E. R.; Bergman, R. G.; Heathcock, C. H. Organometallics 1990, 9, 30. (7) For an example of equilibrium between nickelacyclic C- and Obound enolates: (a) Cámpora, J.; Maya, C. M.; Palma, P.; Carmona, E.; Gutiérrez, E.; Ruiz, C.; Graiff, C.; Tiripicchio, A. Chem.Eur. J. 2005, 11, 6889. (b) Cámpora, J.; Maya, C. M.; Palma, P.; Carmona, E.; Gutiérrez-Puebla, E.; Ruiz, C. J. Am. Chem. Soc. 2003, 125, 1482. (8) For examples of O-bound nickel−enolate complexes obtained via similar acid−base reactions: (a) Shanmuganathan, S.; Kühl, O.; Jones, P. G.; Heinicke, J. Cent. Eur. J. Chem. 2010, 8, 992. (b) Cámpora, J.; Matas, I.; Palma, P.; Á lvarez, E.; Graiff, C.; Tiripicchio, A. Organometallics 2007, 26, 5712. (c) Ketz, B. E.; Ottenwaelder, X. G.; Waymouth, R. M. Chem. Commun. 2005, 5693. (d) Cámpora, J.; Maya, C. M.; Palma, P.; Carmona, E.; Graiff, C.; Tiripicchio, A. Chem. Commun. 2003, 1742. (e) Matt, D.; Huhn, M.; Fischer, J.; De Cian, A.; Kläui, W.; Tkatchenko, I.; Bonnet, M. C. J. Chem. Soc., Dalton Trans. 1993, 1173. (9) Khodakov, A. Y.; Chu, W.; Fongarland, P. Chem. Rev. 2007, 107, 1692. (10) (a) Smith, M. E.; Andersen, R. A. J. Am. Chem. Soc. 1996, 118, 11119. (b) Holland, P. L.; Smith, M. E.; Andersen, R. A.; Bergman, R. G. J. Am. Chem. Soc. 1997, 119, 12815. (c) Carmona, C. C.; Clapham, 6498

dx.doi.org/10.1021/om200819c | Organometallics 2011, 30, 6495−6498