Note pubs.acs.org/Organometallics
Unusual Mechanism for the Reaction of a Niobocene Hydride Complex with Activated Alkynes. Experimental and DFT Studies † ́ Antonio Antiñolo,*,† Santiago Garcıa-Yuste, Antonio Otero,*,† and Arturo Espinosa‡ †
Dpto. de Quı ́mica Inorgánica, Orgánica y Bioquı ́mica, Facultad de Ciencias y Tecnologı ́as Quı ́micas, Universidad de Castilla La Mancha, 13071 Ciudad Real, Spain ‡ Departamento de Quı ́mica Orgánica, Facultad de Ciencias, Universidad de Murcia, Campus de Espinardo, 30100 Espinardo, Murcia, Spain S Supporting Information *
ABSTRACT: The reactivity of the activated alkynes RCCR (R = CO2Me, CO2tBu) with the Nb−H bond of the 18e− niobocene hydride trimethyl phosphite complex [Nb(η5C5H4SiMe3)2(H)(P(OMe)3)] (1) has been studied, and the new (E)-niobocene alkenyl trimethyl phosphite complexes [Nb(η5-C5H4SiMe3)2(C(CO2R)C(CO2R)H)(P(OMe)3)] (R = Me (2a), tBu (2b)) have been obtained. DFT studies were carried out to identify the lowest energy mechanism and the possible intermediates in the process. An unusual nucleophilic d2 Nb(III) center was identified as a σ-donor entity responsible for the nucleophilic attack on a carbon atom of the activated (electrophilic) alkyne molecules. A subsequent hydrogen transfer to the other carbon atom produces the final (E)niobocene alkenyl trimethyl phosphite complexes, the stereochemistries of which were elucidated by NMR spectroscopy.
T
Scheme 1
he elementometalation process, a term that was coined by Negishi several years ago,1 is formally a 1,2-insertion that corresponds to the addition of the two fragments of the E−M system (E = H, C, heteroatom (X), and M = metal) to unsaturated moieties such as those present in alkyne or alkene substrates. The reaction mechanism for the insertion into alkynes or alkenes has been studied by several groups2 and involves a four-center transition state. In broad terms, two types of insertion reactions can be distinguished: namely, migratory and nonmigratory insertions. Migratory insertions, which are better known, require a vacant site at the metal center or a labile ligand that can be easily replaced by the incoming reagent, and these should be cis stereospecific processes.2 In contrast, complexes that are coordinatively saturated or are inert toward substitution cannot undergo migratory insertion, and these produce new complexes of varying stereochemistry through cis or trans stereospecific processes. These reactions have been called nonmigratory insertion reactions3 (Scheme 1). Some of us previously reported the nonmigratory insertion reaction of [Nb(η5-C5H4SiMe3)2(H)(L)] (L= CO, CNR) with activated alkynes4 in a process that allowed the preparation of new niobocene isocyanide or carbonyl (E)-alkenyl and (Z)alkenyl complexes (Scheme 1). However, an additional route that involves nucleophilic attack on alkenes or alkynes is well-known to produce the same final product, albeit through a different mechanism. In fact, a few years ago it was shown that, when an electron-rich ligand with one lone pair is present in some niobocene complexes, this type of attack was responsible for the interaction with the © XXXX American Chemical Society
activated alkyne species, thus allowing the isolation of phosphorus alkenyl complexes (Scheme 2).5 As a continuation of our studies on the complexes [Nb(η5C5H4SiMe3)2(H)(L)] (L = π-acid ligand), we report here the reaction of [Nb(η5-C5H4SiMe3)2(H)(P(OMe)3)] with two activated alkynes. The results of DFT studies established that Scheme 2
Special Issue: Mike Lappert Memorial Issue Received: October 31, 2014
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DOI: 10.1021/om5010989 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
Theoretical Studies and Reaction Pathways. In an effort to gain a deeper insight into the mechanistic aspects of the formation of niobocene alkenyl phosphites 2, as well as the electronic and structural features of the intermediates involved, quantum chemical calculations were carried out at the DFT level (see Computational Details). First, a detailed exploration of the (zero point energy corrected) potential energy surface (PES) was performed for the reaction of complex 1 with acetylenedicarboxylic acid (ADCA) as a model for the activated alkynes used in the experiments. After the formation of the initial van der Waals complex, the only intermediate found corresponds to the nucleophilic attack of the electron-rich Nb atom at the electrophilic alkyne, affording a zwitterionic niobocenium hydride phosphite alkenylide intermediate (3′) (the prime numbering indicates the model R = H species) (Scheme 5).
the nucleophilic attack of the niobium center on the CC bond promotes the formation of the final alkenyl-containing niobocenes 2.
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RESULTS AND DISCUSSION Reaction of Activated Alkynes with [Nb(η 5 C 5 H 4 SiMe 3 ) 2 (H)(P(OMe) 3 )]. The complex [Nb(η 5 C5H4SiMe3)2(H)(P(OMe)3)]6 (1) reacts with the electrophilic alkynes RO2CCCCO2R (R = Me, tBu) to afford the corresponding alkenyl-containing species [Nb(η 5 C5H4SiMe3)2(C(CO2R)C(CO2R)H)(P(OMe)3)] (R = Me (2a), tBu (2b)) (Scheme 3). Scheme 3
Scheme 5
The structural characterization of the alkenyl niobocene complexes was carried out by IR and NMR spectroscopy. The IR spectra show the characteristic bands of the alkenyl ligands (see the Experimental Section). The NMR data indicate the formation of the (E)-alkenyl isomers. In fact, in the 13C NMR spectra the value of the vicinal coupling constant of the alkenyl protons with the carbonyl carbon of the CO2Me group is a useful tool to establish E or Z stereochemistry for the alkenyl group.4 In complexes 2a,b, the 3J1H−13C coupling constant values are ca. 15 Hz and this is consistent with a relative disposition of the two R-ester groups in the E isomer. Complex 2b appears to be a stable species, and the NMR spectrum remains unchanged with time. However, 2a partially evolves (30%) to a new complex, which gives rise to a new set of signals in the NMR spectrum that are consistent with a new (E)-alkenyl conformer. In fact, two conformers of 2a can be envisaged, namely a parallel synperiplanar (2asp) and a parallel antiperiplanar (2aap) stereoisomer (Scheme 4) with regard to
In comparison to 1 and ADCA, complex 3′ shows an electron density depletion at Nb (ΔqN = +0.305 au) and an increase at (mainly) alkenyl C(β) (ΔqN = −0.266 au), with the hydride H ligand attached to both the Nb (dNb−H = 1.791 Å; WBI = 0.595) and the phosphite P atom (dP···H = 2.063 Å; WBI = 0.171), as unambiguously characterized by location of the corresponding bond critical points (BCP) (ρ(r)Nb−H = 10.44 × 10−2 e/a03; ρ(r)P···H = 6.50 × 10−2 e/a03). The particular s-cis conformation of the H−Nb−CC moiety (dihedral 7.6°) allows the subsequent niobium to carbon [1,3]H shift through a low-lying transition state (Figure 1) that transforms
Scheme 4
Figure 1. Calculated (COSMOTHF/PBE-D3/def2-TZVPPecp//COSMOTHF/PBE-D3/def2-TZVPecp) minimum energy for the model transformation 1 → 2′sp. All energy minima for the actual system derived from DMAD and leading to 2asp are represented in gray.
the relative orientation of the alkenyl plane with respect to the C−Nb−P plane (i.e., the P−Nb−C(α)−C(β) dihedral), with these two conformational stereoisomers interconverted by a rotation of the Nb−C bond.7 Steric effects may play a role in the aforementioned conversion, as rotation of the alkenyl group around the M−C bond would become more difficult on increasing the size of the ester O-alkyl group from methyl (2a) to tert-butyl (2b). Indeed, this conformational equilibrium leading to a kind of atropisomerism7,8 was observed for 2a but not for 2b.
intermediate 3′ into the niobocene alkenyl model complex 2′ in a markedly exergonic process. Complex 2′ has a cis arrangement for Nb and H at the olefinic group and an s-cis conformation (an almost parallel sp conformation, according to Scheme 4) for the P−Nb−C(α)C(β) moiety (dihedral −21.1°), which is stabilized by a moderate CH···O hydrogen bond (dCH···O = 2.231 Å; WBI = 0.006; ρ(r)CH···O = 2.51 × 10−2 B
DOI: 10.1021/om5010989 Organometallics XXXX, XXX, XXX−XXX
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Organometallics e/a03). The antiperiplanar conformer 2′ap is formed by rotation of the Nb−C(α) bond, presumably through a low-lying transition state (not computed), and this is 1.56 kcal/mol more stable. In the case of the real system, i.e., the reaction of 1 with dimethyl acetylenedicarboxylate (DMAD), the initial polar addition step (1·DMAD → 2a) is more endergonic (Figure 1), but the subsequent highly exergonic [1,3]H shift affords a niobocene phosphite alkenyl complex (2a; Figure 2b) with a
example of a niobocene complex bearing a sterically demanding ligand that hampers the free rotation around of the Nb−C bond of the alkenyl fragment. The results of the DFT study allowed us to propose a new mechanistic pathway in the reaction of activated (electrophilic) alkynes toward 18-electron niobocene hydride complexes. The process involves nucleophilic attack by the electron-rich niobocene metal center to the unsaturated organic molecule. This constitutes the first case in which a niobocene complex behaves as σ-donor reagent, i.e., a Lewis base, through the use of the lone electron pair at the Nb(III) metal center.
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EXPERIMENTAL SECTION
General Procedures. All reactions were carried out using standard Schlenk techniques. Oxygen and water were excluded through the use of vacuum lines supplied with purified N2. Hexane was distilled from sodium/potassium alloy. THF was distilled from sodium benzophenone. All solvents were deoxygenated prior to use. Deuterated solvents were dried over 4 Å molecular sieves and degassed prior to use. [Nb(η5-C5H4SiMe3)2(H)(P(OMe)3)]6 (1) was prepared as described in the literature. Dimethyl acetylenedicarboxylate (MeO2CC CCO2Me) and di-tert-butyl acetylenedicarboxylate (tBuO2CC CCO2tBu) were used as supplied by Aldrich. 1H and 13C{1H} NMR spectra were recorded on a Varian Unity 300 MHz spectrometer at room temperature unless stated otherwise. 1H and 13C{1H} NMR chemical shifts (δ values) are given in ppm relative to the solvent signal (1H, 13C) or standard resonances. IR spectra were recorded on a PerkinElmer 883 spectrophotometer as Nujol/polyethylene mulls. Microanalyses were carried out with a PerkinElmer 2400 microanalyzer. [Nb(η5-C5H4SiMe3)2(C(R)C(R)H)(P(OCH3)3)] (R = CO2Me (2a), CO2tBu (2b)). Complex 2a. The activated alkyne MeO2CC CCO2Me (DMAD; 0.062 mL, 0.50 mmol) was added to a solution of [Nb(η5-C5H4SiMe3)2(H)(P(OMe)3)] (1; 0.21 g, 0.50 mmol) in THF (ca. 30 mL). The resulting mixture was stirred for 15 min at room temperature, and the solvent was removed in vacuo to give a red oily solid, which was washed with cold hexane (20 mL at −30 °C) to yield the red solid 2a (0.26 g, 81%). IR (Nujol/polyethylene): ν (cm − 1 ) 1741 (CO 2 Me), 1624 (CC). Anal. Calcd for C25H48NbO7PSi2: C, 46.87; H, 7.55. Found: C, 46.78; H, 7.50. Complex 2asp. 1H NMR (300 MHz, C6D6): δ 0.06 (s, 18H, SiMe3), 3.20, (d, 9H, 3J31P−1H = 10.1 Hz, P(OMe)3), 3.47, 3.78 (s, 3H, CO2Me), 4.71, 5.15, 5.24, 5.54 (m, 2H, exact assignment not possible, C5H4SiMe3), 6.60 (s, 1H, CCH). 13C{1H} NMR (75 MHz, C6D6): δ −1.0 (SiMe3), 51.9, 52.1 (CO2Me); 90.8 (C1), 98.4, 100.9, 100.9, 105.1 (C2−5, exact assignment not possible, C5H4SiMe3), 126.7 (CCH), 162.0, 180.3 (CO2Me), 215.0 (CCH). 13C NMR (75 MHz, C6D6): 126.7 (d, 3J13C−1H = 125.20 Hz, CCH), 162.0 (q, 3 13 1 J C− H = 4.00 Hz, CO2Me), 180.3 (dq, 3J13C−1H = 14.28 Hz, 3J13C−1H = 4.00 Hz, CO2Me). Complex 2aap. 1H NMR (300 MHz, C6D6): δ 0.06 (s, 18H, SiMe3), 3.20 (d, 9H, 3J31P−1H = 10.1 Hz, P(OMe)3), 3.58, 3.87 (s, 3H, CO2Me), 4.78, 5.30, 5.41, 5.64 (m, 2H, exact assignment not possible, C5H4SiMe3), 6.95 (s, 1H, CCH). 13C{1H} NMR (75 MHz, C6D6): δ −1.0 (SiMe3), 51.9, 52.1 (CO2Me); 94.6 (C1), 97.4, 98.0, 99.9, 104.2 (C2−5, exact assignment not possible, C5H4SiMe3), 124.6 (CCH), 162.0, 180.3 (CO2Me), 220.0 (CCH). 13C NMR (75 MHz, C6D6): 124.6 (d, 1J13C−1H = 125.5 Hz, CCH), 162.0 (q, 3J13C−1H = 4.00 Hz, CO2Me), 180.3 (dq, 3J13C−1H = 14.28 Hz, 3J13C−1H = 4.00 Hz, CO2Me). Complex 2b. Complex 2b was prepared in a manner similar to that for 2a, to give 75% yield. IR (Nujol/polyethylene): ν (cm−1) 1735 (CO2tBu), 1620 (CC). 1H NMR (300 MHz, C6D6): δ 0.13 (s, 18H, SiMe3), 1.45, 1.75 (s, 9H, CO2tBu), 3.26, 3.31 (d, 9H, 3J31P−1H = 10.1 Hz, P(OCH3)3), 4.70, 5.15, 5.43, 5.54 (m, 2H, exact assignment not possible, C5H4SiMe3), 6.38 (s, 1H, CCH). 13C{1H} NMR (75 MHz, C6D6): δ −1.0 (SiMe3), 27.8, 27.6 (CO2tBu); 90.8, 94.6 (C1), 98.4, 100.9, 105.1 (C2−5, exact assignment not possible, C5H4SiMe3),
Figure 2. Calculated (COSMOTHF/PBE-D3/def2-TZVPecp) structures for (a) 3a, (b) 2asp, and (c) 2aap.
relative energy similar to that found in model systems (2′). The particular isomer 2asp has a weak CH···O hydrogen bond (dCH···O = 2.231 Å; WBI = 0.005; ρ(r)CH···O = 1.40 × 10−2 e/ a03) that forces a parallel synperiplanar alignment of the alkenyl substituent (dihedral C(β)C(α)−Nb−P −24.6°) and several other conformational minima are also found in the PES of the system, with the antiperiplanar arrangement (dihedral C(β) C(α)−Nb−P 150.1°) 2aap (Figure 2c) being only 0.36 kcal/ mol less stable at the working level of theory. The 2aap conformer has several noncovalent interactions, one of which stabilizes the electrophilic C atom of the carboxylate at C(α) by pulling electron density from one O of the other carboxylate (dC···O = 2.677 Å; WBI = 0.008; ρ(r)C···O = 1.38 × 10−2 e/a03). It is likely that the various conformers arising from Nb−Cp′ and Nb−alkenyl rotations are responsible for the complexes observed in the NMR experiments, as mentioned previously. Finally, taking into account the concerted nature of the Nb to C [1,3]H shift in the conversion of 3 into 2 (as observed in the transition state for the model reaction 3′ → 2′), the alkenyl ligand is formed with a relative cis arrangement for the two carboxy groups and this stereochemistry is retained in all conformers due to the genuine double-bond character of the C(α)−C(β) linkage (2asp, d = 1.356 Å; WBI = 1.770, ρ(r) = 33.33 × 10−2 e/a03; 2aap, d = 1.358 Å; WBI = 1.757, ρ(r) = 33.08 × 10−2 e/a03).9 Subsequent nucleophilic attack by the d2 Nb center opens up a new model in the understanding of the “insertion reaction” process involving activated alkynes by low-valent coordinatively saturated metal complexes. It is worth noting that, to our knowledge, the behavior described above has never been reported in a niobocene complex; the reaction of a transition-metal center as a σ-donor reagent (Lewis base) has usually only been reported for later transition metal complexes.10 Concluding Remarks. The synthesis of new phosphitecontaining niobocene (E)-alkenyl complexes [Nb(η 5 C5H4SiMe3)2(C(CO2R)C(CO2R)H)(P(OMe)3)] (2) is described, where two parallel synperiplanar (2asp) or parallel antiperiplanar (2aap) conformers are possible. In the particular case of 2b (R = tBu) interconversion between the two conformers is not observed. This complex represents a rare C
DOI: 10.1021/om5010989 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
(4) Antiñolo, A.; Carrillo-Hermosilla, F.; Fajardo, M.; Garcı ́a-Yuste, S.; Lanfranchi, M.; Otero, A.; Pellinghelli, M. A.; Prashar, S.; Villaseñor, E. Organometallics 1996, 15, 5507−5513. (5) (a) Antiñolo, A.; Garcı ́a-Yuste, S.; López-Solera, M. I.; Otero, A.; Pérez-Flores, J. C.; Reguillo-Carmona, R.; Villaseñor, E. Dalton Trans. 2006, 35, 1495−1496. (b) Antiñolo, A.; Garcı ́a-Yuste, S.; López Solera, I.; Otero, A.; Pérez-Flores, J. C.; Reguillo-Carmona, R.; Villaseñor, E.; Santos, E.; Zuidema, E.; Bo, C. Dalton Trans. 2010, 39, 1962−1971. (6) Antiñolo, A.; Carrillo-Hermosilla, F.; Fajardo, M.; Garcı ́a-Yuste, S.; Otero, A. J. Organomet. Chem. 1994, 482, 93−98. (7) Amaudrut, J.; Leblanc, J.-C.; Möise, C.; Sala-Pala, J. J. Organomet. Chem. 1985, 295, 167−174. (8) Bringmann, G.; Mortimer, A. J. P.; Keller, P. A.; Gresser, M. J.; Garner, J.; Breuning, M. Angew. Chem., Int. Ed. 2005, 44, 5384−5427. (9) For comparison, dimethyl maleate: d = 1.342 Å; WBI = 1.889, ρ(r) = 34.29 × 10−2 e/a03. (10) (a) Bauer, J.; Braunschweig, H.; Brenner, P.; Kraft, K.; Radacki, K.; Schwab, K. Chem. Eur. J. 2010, 16, 11985−11992. (b) Amgoune, A.; Bourissou, D. Chem. Commun. 2011, 859−871. (11) Neese. F. ORCA-An ab initio, DFT and semiempirical SCF-MO package. Version 2.9.1; Max Planck Institute for Bioinorganic Chemistry, D-45470 Mülheim/Ruhr, Germany, 2012; http://www. mpibac.mpg.de/bac/logins/neese/description.php. Neese, F. WIREs Comput. Mol. Sci. 2012, 2, 73−78. (12) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865−3868. (13) Neese, F.; Wennmohs, F.; Hansen, A.; Becker, U. Chem. Phys. 2009, 356, 98−109. (14) Weigend, F.; Ahlrichs, R. Chem. Phys. 2005, 7, 3297−3305. (15) Andrae, D.; Häussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor. Chim. Acta 1990, 77, 123−141. (16) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. J. Chem. Phys. 2010, 132, 154104. (17) Schäfer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100, 5829−5835. Basis sets may be obtained from the Basis Set Exchange (BSE) software and the EMSL Basis Set Library: https://bse.pnl.gov/ bse/portal Feller, D. J. Comput. Chem. 1996, 17, 1571−1586. (18) (a) Klamt, A.; Schüürmann, G. J. Chem. Soc., Perkin Trans. 2 1993, 220, 799−805. (b) Klamt, A. J. Phys. Chem. 1995, 99, 2224− 2235. (19) (a) Bader, R. F. W. In Atoms in Molecules: A Quantum Theory; Oxford University Press: Oxford, U.K., 1990. (b) Bader, R. F. W. Chem. Rev. 1991, 91, 893−928. (c) Matta, C. F.; Boyd, R. J. In The Quantum Theory of Atoms in Molecules; Matta, C. F., Boyd, R. J., Eds., Wiley-VCH: New York, 2007; pp 1−34. (20) (a) Biegler-König, F; Schönbohm, J. AIM2000 v. 2.0; 2002; home page http://www.aim2000.de/. Biegler-König, F.; Schönbohm, J.; Bayles, D. J. Comput. Chem. 2001, 22, 545−559. (b) Biegler-König, F.; Schönbohm, J. J. Comput. Chem. 2002, 23, 1489−1494. (21) Wiberg, K. Tetrahedron 1968, 24, 1083−1096. (22) (a) Reed, A. E.; Weinhold, F. J. Chem. Phys. 1983, 78, 4066− 4073. (b) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys. 1985, 83, 735−746. Using the NBO 5.0 code: Glendening, E. D.; Badenhoop, J. K.; Reed, A. E.; Carpenter, J. E.; Bohmann, J. A.; Morales, C. M.; Weinhold, F. NBO 5.0; Theoretical Chemistry Institute, University of Wisconsin, Madison, WI, 2001. (23) VMD-Visual Molecular Dynamics: Humphrey, W.; Dalke, A.; Schulten, K. J. Mol. Graphics 1996, 14, 33−38 http://www.ks.uiuc. edu/Research/vmd/, (home page http://www.ks.uiuc.edu/Research/ vmd/).
126.3 (CCH), 177.1, 178.2 (CO2tBu), 210.0 (CCH). 13C NMR (75 MHz, C6D6): 126.3 (d, 3J13C−1H = 125.00 Hz, CCH), 178.2 (q, 3 13 1 J C− H = 4.00 Hz, CO2Me), 177.1 (dq, 3J13C−1H = 14.28 Hz, 3J13C−1H = 4.00 Hz, CO2Me). Anal. Calcd for C31H66NbO7PSi2: C, 50.94; H, 9.10. Found: C, 50.85; H, 9.40. Computational Details. Quantum chemical calculations were performed with the ORCA electronic structure program package.11 All geometry optimizations were run with tight convergence criteria using the PBE12 functional together with the new efficient RIJCOSX algorithm,13 the def2-TZVP basis set,14 and the [SD(28,MWB)] effective core potential (ECP) for Nb atoms.15 In all optimizations and energy evaluations, the latest Grimme’s semiempirical atom-pairwise correction, accounting for the major part of the contribution of dispersion forces to the energy, was included.16 From these geometries all energy data were obtained by means of single-point (SP) calculations using the same functional as well as the more polarized def2-TZVPP basis set.14,17 Data are corrected for the zero-point vibrational (ZPV) term at the optimization level. All electronic properties were obtained at the same level. Solvent effects (THF) were taken into account by using the COSMO solvation model.18 Bond strengths were characterized by electron densities at bond critical points, ρ(r), using Bader’s AIM (atoms in molecules) methodology19 and the AIM2000 software,20 as well as by Wiberg bond indices (WBI)21 obtained from the NBO (natural bond orbital) partition scheme.22 The latter was also used to obtain natural charges (qN). Figure 2 was drawn using the VMD software.23
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ASSOCIATED CONTENT
S Supporting Information *
Figure S1 giving the HOMO in complex 1 (top (a) and side (b) views) and tables and xyz files giving Cartesian coordinates (in Å) and energies (in hartrees) for all computed species. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail for A.A.:
[email protected]. *E-mail for A.O.:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the Ministerio de Economı ́a y Competividad (MINECO), of Spain (Grant Nos. Consolider-Ingenio 2010 ORFEO CSD 200700006, CTQ2009-09214, CTQ2011-22578/BQU), and European Union (COST actions CM0802 “PhoSciNet” and CM1302 “SIPs”).
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DEDICATION Dedicated to the memory of Professor Michael F. Lappert. REFERENCES
(1) (a) Negishi, E.-i. Angew. Chem., Int. Ed. 2011, 50, 6738−6764. (b) Negishi, E.-i.; Wang, G.; Rao, H.; Xu, Z. J. Org. Chem. 2010, 75, 3151−3182. (2) (a) Roe, D. C. J. Am. Chem. Soc. 1983, 105, 7770−7779. (b) Green, M. L. H.; Wong, L.-L. J. Chem. Soc., Dalton Trans. 1987, 411−416. (c) Doherty, N. M.; Bercaw, J. E. J. Am. Chem. Soc. 1985, 107, 2670−2682. (3) (a) Antiñolo, A.; Garcı ́a-Yuste, S.; Otero, A.; Villaseñor, E. J. Organomet. Chem. 2007, 698, 4436−4447. (b) Herberich, G. E.; Mayer, H. Organometallics 1990, 9, 2655−2661. (c) Otsuka, S.; Nakamura, A. Adv. Organomet. Chem. 1976, 1, 245−283. D
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