Unimolecular Reactions of Organocuprates and Organoargentates

Apr 19, 2010 - Jiawei Li , George N. Khairallah , and Richard A. J. O'Hair. Organometallics 2015 .... Han-Gook Cho and Lester Andrews. Inorganic Chemi...
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Organometallics 2010, 29, 2282–2291 DOI: 10.1021/om1000875

Unimolecular Reactions of Organocuprates and Organoargentates Nicole J. Rijs and Richard A. J. O’Hair* School of Chemistry, Bio21 Institute of Molecular Science and Biotechnology, and ARC Centre of Excellence for Free Radical Chemistry and Biotechnology, The University of Melbourne, Victoria 3010, Australia Received February 4, 2010

The gas-phase fragmentation reactions of the group 11 organometallates [CH3MR]- (R = CH3CH2, CH3CH2CH2, (CH3)2CH, (CH3)3C, CH2CHCH2, PhCH2, Ph, M = Cu, Ag) and [CH3CH2CuCH2CH3]- were studied by density functional theory (DFT) calculations and, for those accessible in the gas phase, via collision-induced dissociation (CID) and selected deuterium labeling experiments. The mixed metallates [CH3MR]- were found to fragment via a diverse set of pathways, including bond homolysis, bond heterolysis, and β-hydride elimination. A 1,2-dyotropic rearrangement was observed for R = Ph. DFT calculations suggest that the M-C bond energy, the availability of metal orbitals for π-bonding, and the nature of the ligand R group substituents are the main factors that control the observed reactivity. Comparisons with the known solution-phase reactivity of organocopper and organosilver species are made.

Introduction Organocopper reagents are among the most widely used organometallic reagents in organic synthesis,1 and there is growing interest in the use of organosilver reagents in C-X bond coupling applications.2,3 A key challenge in the use of these organometallic reagents is that decomposition reactions can compete with the desired coupling reaction, thereby limiting the temperature range at which coupling reactions can be carried out.4 For example, the recommended temperature range for C-C bond coupling reactions of Gilman reagents is from -100 to 0 °C, with most lithium *To whom correspondence should be addressed. Fax: þ61 3 9347 5180. Tel: þ61 3 8344 2452. E-mail: [email protected]. (1) For reviews and monographs on organocopper species see: (a) Modern Organocopper Chemistry; Krause, N., Ed.; Wiley-VCH: Weinheim, Germany, 2002. (b) Lipshutz, B. H. Organometallics in Synthesis; Schlosser, M., Ed.; Wiley: Chichester, U.K., 1994; pp 283-382. (c) Organocopper Reagents: A Practical Approach; Taylor, R. J. K., Ed.; Oxford University Press: Oxford, U.K., 1994. (2) For X = C see: (a) Halbes-Letinois, U.; Weibel, J.-M.; Pale, P. Chem. Soc. Rev. 2007, 36, 759. (b) Pouwer, R. H.; Williams, C. M.; Raine, A. L.; Harper, J. B. Org. Lett. 2005, 7, 1323. (c) Pouwer, R. H.; Harper, J. B.; Vyakaranam, K.; Michl, J.; Williams, C. M.; Jessen, C. H.; Bernhardt, P. V. Eur. J. Org. Chem. 2007, 2, 241. (d) Someyaa, H.; Yorimitsu, H.; Oshima, K. Tetrahedron Lett. 2009, 50, 3270. For X = Si see: (e) Murakami, K.; Hirano, K.; Yorimitsu, H.; Oshima, K. Angew. Chem., Int. Ed. 2008, 47, 5833. (3) For an entire issue of Chemical Reviews devoted to the theme of coinage metals in organic synthesis see: Chem. Rev. 2008, 108, issue 8, Lipshutz, B. H., Yamamoto, Y. Eds. (4) (a) Organocopper Reagents: A Practical Approach; Taylor, R. J. K., Ed.; Oxford University Press: Oxford, U.K., 1994; pp 60-61. Fast decomposition of n-propylsilver(I) occurs at -60 °C: (b) Westmijze, H.; Kleijn, H.; Vermeer, P. J. Organomet. Chem. 1979, 172, 377. (5) For reviews and key references on the general mechanistic features of decomposition reactions of organometallics see: (a) Baird, M. C. J. Organomet. Chem. 1974, 64, 289. (b) Davidson, P. J.; Lappert, M. F.; Pearce, R. Chem. Rev. 1976, 76, 219. (c) Akermark, B.; Ljungqvist, A. J. Organomet. Chem. 1979, 182, 59. (d) Barker, P. J.; Winter, J. N. in The Chemistry of the Metal-Carbon Bond; Hartley, F. R., Patai, S., Eds.; Wiley: Chichester, U.K., 1985; Vol. 2, Chapter 3. (e) Hager, E.; Sivaramakrishna, A.; Clayton, H. S.; Mogorosi, M. M.; Moss, J. R. Coord. Chem. Rev. 2008, 252, 1668. pubs.acs.org/Organometallics

Published on Web 04/19/2010

dialkylcuprates in ethereal solutions decomposing above -20 °C.4a The condensed-phase decomposition reactions of organocopper and organosilver reagents are complex and can give rise to a range of products that arise from different pathways, including5 β-hydride transfer,6 radical reactions,7 and coupling reactions between ligands on adjacent metal centers,8,9 including multiple CF2 insertion reactions of CF3Cu.10 Metallic nanoparticles can be formed, which can autocatalytically induce further reactions.11 The mechanistic features of these decompositions are complicated (6) (a) Whitesides, G. M.; Stedronsky, E. R.; Casey, C. P.; San Filippo, J. J. Am. Chem. Soc. 1970, 92, 1426. (b) Whitesides, G. M.; Bergbreiter, D. E.; Kendall, P. E. J. Am. Chem. Soc. 1974, 96, 2806. (c) Miyashita, A.; Yamamoto, T.; Yamamoto, A. Bull. Chem. Soc. Jpn. 1977, 50, 1109. (d) House, H. O.; DuBose, J. C. J. Org. Chem. 1975, 40, 788. (7) (a) Whitesides, G. M.; Panek, E. J.; Stedronsky, E. R. J. Am. Chem. Soc. 1972, 94, 232. (b) Hashimoto, H.; Nakano, T. J. Org. Chem. 1966, 31, 891. (c) Bergbreiter, D. E.; Lynch, T. J. J. Org. Chem. 1981, 46, 727. (d) Yamamoto, K.; Nakanishi, K.; Kumada, M. J. Organomet. Chem. 1967, 7, 197. (e) Shutova, T. G.; Butovskaya, G. V.; Agabekov, V. E.; Moiseichuk, K. L. Kinet. Catal. 1996, 37, 29. (f) Akermark, B.; Ljungqvist, A. J. Organomet. Chem. 1979, 182, 47. (8) (a) Whitesides, G. M.; Casey, C. P. J. Am. Chem. Soc. 1966, 88, 4541. (b) Kauffmann, T.; Sahm, W. Angew. Chem., Int. Ed. 1967, 6, 85. (c) Whitesides, G. M.; Casey, C. P.; Krieger, J. K. J. Am. Chem. Soc. 1971, 93, 1379. (d) Pasynkiewicz, S.; Poplawska, J. J. Organomet. Chem. 1985, 282, 427. (e) Pasynkiewicz, S.; Pikul, S.; Poplawska, J. J. Organomet. Chem. 1985, 293, 125. (f) Pasynkiewicz, S. J. Organomet. Chem. 1990, 387, 1. (g) Akermark, B.; Ljungqvist, A. J. Organomet. Chem. 1979, 182, 47. (9) For examples of reductive elimination reactions of well-defined copper cluster compounds see: (a) Cairncross, A.; Sheppard, W. A. J. Am. Chem. Soc. 1971, 93, 247. (b) van Koten, G.; Noltes, J. G. J. Chem. Soc., Chem. Commun. 1974, 575. (c) Cohen, T.; Treblow, M. D. J. Org. Chem. 1976, 41, 1986. (d) Janssen, M. D.; Corsten, M. A.; Spek, A. L.; Grove, D. M.; van Koten, G. Organometallics 1996, 15, 2810. (10) (a) Wiemers, D. M.; Burton, D. J. J. Am. Chem. Soc. 1986, 108, 832–4. (b) Yang, Z. Y.; Wiemers, D. M.; Burton, D. J. J. Am. Chem. Soc. 1992, 114, 4402–3. (c) Yang, Z.-Y.; Burton, D. J. J. Fluorine Chem. 2000, 102, 89–103. (11) (a) Wada, K.; Tamura, M.; Kochi, J. J. Am. Chem. Soc. 1970, 92, 6656. (b) Kochi, J. Acc. Chem. Res. 1974, 7, 351. (c) Tamura, M.; Kochi, J. K. Bull. Chem. Soc. Jpn. 1972, 45, 1120. r 2010 American Chemical Society

Article

by solvent effects, availability of coligands, and aggregation12 and are thus not generally well understood. Despite this rather gloomy mechanistic view, there are a number of examples in which decomposition reactions of organocuprates have been harnessed for synthetic applications. These include reductive elimination reactions,9 oxidative coupling of ligands,13 and 1,2-ligand migration in C-X bond formation reactions.14 An alternative approach to understanding the formation, stability, and reactivity of organocopper and organosilver species is to study the gas-phase chemistry of well-defined organometallic ions, where the additional effects of solvent and counterions are absent and where the aggregation state (or cluster size) can be simply controlled by mass selection.15,16 Combining the multistage mass spectrometry (MSn) capabilities of ion trap mass spectrometers17 with theoretical calculations 18 provides opportunities to gain detailed mechanistic insights into gas-phase metal-mediated chemistry.19 Stable precursor ions can be transferred to the gas phase by electrospray ionization (ESI) and then subjected to collision-induced dissociation (CID) to “synthesize” novel and reactive gas-phase ions.17,20 Thus, rather than face the challenges of carrying out ESI/MS on organometallics,21 we have shown that decarboxylation of metal carboxylate ions can be used to readily synthesize organometallic (12) The nature of both the organic ligand and the coligand can have a profound effect on the thermal stability of organocuprates: (a) van Koten, G.; James, S. L.; Jastrzebski, J. T. B. H. Comprehensive Organometallic Chemistry II; Pergamon Press: Oxford, U.K., 1995; Vol. 3, pp 75-77. (b) Bertz, S. H.; Dabbagh, G. J. Chem. Soc., Chem. Commun. 1982, 1030. (13) Surry, D. S.; Spring, D. R. Chem. Soc. Rev. 2006, 35, 218. (14) (a) Kocienski, P.; Barber, C. Pure Appl. Chem. 1990, 62, 1933. (b) Inoue, A.; Kondo, J.; Shinokubo, H.; Oshima, K. J. Am. Chem. Soc. 2001, 123, 11109. (c) Kondo, J.; Ito, Y.; Shinokubo, H.; Oshima, K. Angew. Chem., Int. Ed. 2004, 43, 106–108. (d) Kondo, J.; Someya, H.; Kinoshita, H.; Shinokubo, H.; Yorimitsu, H.; Oshima, K. Org. Lett. 2005, 7, 5713–5715. (15) For a review on gas-phase metal cluster ion chemistry see: O’Hair, R. A. J.; Khairallah, G. N. J. Cluster Sci. 2004, 15, 331. (16) For a recent example on the role of cluster size in the gas-phase decomposition reactions of silver alkynyl cluster cations, [(C4H9CCAg)nAg]þ, see: Wang, F. Q.; Khairallah, G. N.; Koutsantonis, G. A.; Williams, C. M.; Callahan, D. L.; O’Hair, R. A. J. Phys. Chem. Chem. Phys. 2009, 11, 4132–4135. (17) O’Hair, R. A. J. Chem. Commun. 2006, 1469. (18) For reviews of DFT methods to calculate potential energy surfaces for organometallic reactions see: (a) Ziegler, T.; Autschbach, J. Chem. Rev. 2005, 105, 2695–2722. (b) Niu, S.; Hall, M. B. Chem. Rev. 2000, 100, 353–406. (19) For a review on the marriage of theory and experiment for coinage metal ions see: Roithova, J.; Schr€ oder, D. Coord. Chem. Rev. 2009, 253, 666. (20) O’Hair, R. A. J. Gas Phase Ligand Fragmentation to Unmask Reactive Metallic Species. In MS Investigations of Reactive Intermediates in Solution; Santos, L. S., Ed.; Wiley-VCH: Weinheim, Germany, 2010; Chapter 6, pp 199-227 (ISBN: 978-3-527-32351-7). (21) For examples on the direct use of ESI/MS on solutions of organometallates see: (a) Lipshutz, B. H.; Keith, J.; Buzard, D. J. Organometallics 1999, 18, 1571. (b) Koszinowski, K.; Boehrer, P. Organometallics 2009, 28, 100. (c) Koszinowski, K.; Boehrer, P. Organometallics 2009, 28, 771. (22) (a) O’Hair, R. A. J. Chem. Commun. 2002, 20. (b) O'Hair, R. A. J.; Vrkic, A. K.; James, P. F. J. Am. Chem. Soc. 2004, 126, 12173. (c) James, P. F.; O'Hair, R. A. J. Org. Lett. 2004, 6, 2761. (d) Jacob, A. P.; James, P. F.; O'Hair, R. A. J. Int. J. Mass Spectrom. 2006, 255-256, 45. (e) O'Hair, R. A. J.; Waters, T.; Cao, B. Angew. Chem., Int. Ed. 2007, 46, 7048. (f) Rijs, N.; Waters, T.; Khairallah, G. N.; O'Hair, R. A. J. J. Am. Chem. Soc. 2008, 130, 1069. (g) Thum, C. C. L.; Khairallah, G. N.; O'Hair, R. A. J. Angew. Chem., Int. Ed. 2008, 47, 9118. (h) Khairallah, G. N.; Waters, T.; O'Hair, R. A. J. Dalton Trans. 2009, 2832–2836. (i) Rijs, N. J.; O'Hair, R. A. J. Organometallics 2009, 28, 2684–2692. (j) Khairallah, G. N.; Thum, C.; O'Hair, R. A. J. Organometallics 2009, 28, 5002–5011. (k) Rijs, N. J.; Yates, B. F.; O'Hair, R. A. J. Chem. Eur. J. 2010, 16, 2674–2678. (l) Khairallah, G. N.; Yoo, E. J. H.; O'Hair, R. A. J. Organometallics 2010, 29, 1238–1245.

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Scheme 1

species.22 We note that decarboxylation reactions have long been used to generate organometallics in the condensed phase23 and that there has been a recent renaissance in their use for catalytic applications in cross-coupling reactions.24,25 With regard to organometallates of the coinage metals, we have used a double-decarboxylation procedure to form the range of organocuprates22f and organoargentates22i shown in Scheme 1. Gas-phase synthesis of these organometallates allows a direct comparison of the chemistry of organocuprates and organoargentates. Thus, we have shown that both the bimolecular and unimolecular reactions of the parent dimethylmetallates are sensitive to the nature of the metal. For example, dimethyl cuprate, [CH3CuCH3]- (1a), reacts with methyl iodide via the Corey-Posner reaction while dimethyl argentate, [CH3AgCH3]- (1b), does not.22c More recently, we have shown that while dimethyl cuprate, [CH3CuCH3]- (1a), and dimethyl argentate, [CH3AgCH3]- (1b), both undergo bond homolysis (eq 1), only dimethyl cuprate undergoes a 1,2-dyotropic rearrangement26 to form [CH3CH2CuH]- (eq 2), which then undergoes β-hydride fragmentation (eq 3) 22k. Since these and other studies have shown that the ligand plays an important role in organocopper chemistry,27 here we examine via mass spectrometry experiments employing low-energy CID the role of both the organic group R (aryl and alkyl) and the metal center (copper versus silver) on the gas-phase unimolecular fragmentation reactions of the organometallates shown in Scheme 1, and further explore the reactivity with supporting DFT (23) For older reviews focusing on the condensed-phase formation of organometallics via decarboxylation reactions see: (a) Deacon, G. B. Organomet. Chem. Rev. A 1970, 5, 355. (b) Deacon, G. B.; Faulks, S. J.; Pain, G. N. Adv. Organomet. Chem. 1986, 25, 237. (24) For recent reviews on metal-catalyzed transformations of carboxylic acids see: (a) Goossen, L. J.; Goossen, K.; Rodriguez, N.; Blanchot, M.; Linder, C.; Zimmermann, B. Pure Appl. Chem. 2008, 80, 1725. (b) Goossen, L. J.; Rodriguez, N.; Goossen, K. Angew. Chem., Int. Ed. 2008, 47, 3100. (25) Copper and silver carboxylates are also used in solution as precursors for generation of reactive organometallic species in catalytic applications. For recent examples see: (a) Yin, L.; Kanai, M.; Shibasaki, M. J. Am. Chem. Soc. 2009, 131, 9610. (b) Lu, P. F.; Sanchez, C.; Cornella, J.; Larrosa, I. Org. Lett. 2009, 11, 5710. (c) Goossen, L. J.; Linder, C.; Rodriguez, N.; Lange, P. P.; Fromm, A. Chem. Commun. 2009, 7173. (d) Shang, R.; Fu, Y.; Wang, Y.; Xu, Q.; Yu, H. Z.; Liu, L. Angew. Chem., Int. Ed. 2009, 48, 9350. (26) For reviews on dyotropic rearrangements see: (a) Reetz, M. T. Adv. Organomet. Chem. 1977, 16, 33. (b) Fernandez, I.; Cossio, F. P.; Sierra, M. A. Chem. Rev. 2009, 109, 6687. (c) Gridnev, I. D. Coord. Chem. Rev. 2008, 252, 1798. (27) (a) Lipshutz, B. H. Synlett 2009, 04, 509. (b) Deutsch, C.; Lipshutz, B. H.; Krause, N. Org. Lett. 2009, 11, 5010. (c) Pelss, A.; Kumpulainen, E. T. T.; Koskinen, A. M. P. J. Org. Chem. 2009, 74, 7598. (28) Frisch, M. J. et al. Gaussian_03; Gaussian, Inc., Pittsburgh, PA, 2003. (29) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785.

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calculations.28,29

½CH3 MCH3  - f ½CH3 M• - þ CH3 •

ð1Þ

½CH3 MCH3  - f ½CH3 CH2 MH -

ð2Þ

½CH3 CH2 MH - f ½HMH - þ CH2 dCH2

ð3Þ

Experimental Section Reagents. Copper(II) acetate, silver(I) acetate, vinylacetic acid, and trimethylacetic acid were obtained from Aldrich. Propionic acid, phenylacetic acid, and n-butyric acid were obtained from BDH Laboratory Supplies. Isobutyric acid and methanol were obtained from Ajax. Benzoic acid was obtained form May Baker. Propionic-d3 acid and benzoic-d5 acid were obtained from Isotec. Acetic-d4 acid was obtained from Cambridge Isotopes. All chemicals were used without further purification. Mass Spectrometry. Mass spectra were recorded using a Finnigan LTQ FT hybrid linear ion trap (Finnigan, Bremen, Germany) fitted with the standard factory Finnigan electrospray ionization source, as described previously.22i Either silver(I) acetate or copper(II) acetate was dissolved in methanol together with the appropriate carboxylic acid in a 1:2 molar ratio, with typical concentrations of 0.5-1 mM. These solutions were transferred to the electrospray source via a syringe pump, at a rate of 5 μL/min. Typical electrospray source conditions with needle potentials of 4.0-5.0 kV and a heated capillary temperature of 180-200 °C were used. The desired precursor ion was mass-selected and subjected to CID using standard isolation and excitation procedures. The silver isotope pattern (107Ag, 51.8%; 109Ag, 48.2%) and the copper isotope pattern (63Cu, 69.2%; 65Cu, 30.8%) were used to identify silver- and copper-containing species, respectively. For the deuterium labeling experiments performed, the anions were mass selected with a window of 1 Da to avoid complications arising from adjacent isotopes. DFT Calculations. Theoretical calculations were carried out to provide insights into the fragmentation mechanism of the organometallates studied. DFT calculations were carried out with Gaussian 0328 using the B3LYP hybrid functional.29 The Stuttgart Dresden (SDD) basis set with effective core potential was used for both copper and silver, and the 6-31þG(d) allelectron basis set was used for carbon and hydrogen.30 Calculated geometries of the organometallates were compared with those determined from X-ray crystallography,31 while the calculated structure of methylcopper was compared to that determined experimentally via gas-phase millimeter-wave rotational spectroscopy.32 There was good structural agreement between (30) (a) Dolg, M.; Wedig, U.; Stoll, H.; Preuss, H. J. Chem. Phys. 1987, 86, 866. (b) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213. (c) Clark, T.; Chandrasekhar, J.; Schleyer, P. v. R. J. Comput. Chem. 1983, 4, 294. (d) Krishnam, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys. 1980, 72, 650. (e) Gill, P. M. W.; Johnson, B. G.; Pople, J. A.; Frisch, M. J. Chem. Phys. Lett. 1992, 197, 499. (31) For the structure of the simplest model system, the dimethylcuprate ion [MeCuMe]-, see: (a) Hope, H.; Olmstead, M. M.; Power, P. P.; Sandell, J.; Xu, X. J. Am. Chem. Soc. 1985, 107, 4337. (b) Dempsey, D. F.; Girolami, G. S. Organometallics 1988, 7, 1208. (32) Grotjahn, D. B.; Halfen, D. T.; Ziurys, L. M.; Cooksy, A. L. J. Am. Chem. Soc. 2004, 126, 12621–12627. (33) DFT calculations utilizing similar levels of theory and basis sets yield vibrational frequencies that compare well with those determined experimentally for the metal hydride anions [CuH2]- and [AgH2]-. See: (a) Andrews, L.; Wang, X. J. Am. Chem. Soc. 2003, 125, 11759. (b) Wang, X.; Andrews, L. Angew. Chem., Int. Ed. 2003, 42, 5201.

the calculated geometries and those determined experimentally. Properties of copper and silver hydrides have previously been successfully compared to B3LYP calculated structures.33 Adiabatic electron affinities for some related copper(I) species were calculated to test the reliability of energy levels and were in fair agreement (within about 0.1 eV) with experimental results.34 Basis set superposition error was not corrected for, as weak ion neutral complexes were not pertinent to understanding reactive pathways. No difference was found when calculating energies of the organometallates using unrestricted versus restricted B3LYP. We have compared the performance of the B3LYP/6-31þ G(d) þ SDD ECP(Cu/Ag) level of theory with other methods to predict the relative energies associated with the activation barrier for 1,2-dyotropic rearrangement, bond homolysis and bond heterolysis endothermicities for the dimethylcuprate anion, 1a (Supporting Information, Figure S1). The use of an ECP versus an all-electron method was not found to alter the performance. The use of ab initio (MP2) methods provides homolytic bond dissociation energies that appear closer to chemical accuracy than the chosen DFT method. Although it is generally accepted that DFT methods underestimate bond dissociation energies, they can provide useful information on relative energies for a series of related systems.35a For this reason, despite the shortcomings, the B3LYP/6-31þG(d) þ SDD ECP(Cu/Ag) method and basis sets were chosen: (i) because they allow direct comparison with our previous work22f,i and (ii) as they are less computationally demanding than the higher levels of ab initio theory. A detailed theoretical study aimed at benchmarking the homolytic bond dissociation energies (BDE) of organocopper and organosilver species has been conducted and is the subject of a forthcoming manuscript.35b

Results and Discussion The gas-phase unimolecular chemistry of the organometallates shown in Scheme 1 has been studied using a combination of mass spectrometry experiments and DFT calculations. Since the behavior of the dimethylmetallates 1a,b has been previously described,22k here we focus on the unimolecular reactivity of the homocuprate anion diethylcuprate (Scheme 1; 2a) by first outlining the experimentally observed fragmentation channels followed by supporting DFT calculations. In subsequent sections the unimolecular reactivity of the mixed metallates [CH3MR]- is discussed, initially in light of the DFT predicted energetics associated with the various competing pathways, followed by an outline of the experimentally observed gas-phase chemistry. The correspondence between experiment and theory is then considered for each mixed metallate system, [CH3MR]-, and finally the overall trends in reactivity are described. 1. Unimolecular Fragmentation Reactions of Diethylcuprate. The dominant fragmentation pathway for diethylcuprate 2a involves formation of the hydride [CH3CH2CuH](Figure 1a, eq 4a) via β-hydride transfer, as confirmed via CID (34) There are currently no experimental data on the electron affinities of organocopper and organosilver compounds. Our combination of method and basis provides reasonable relative electron affinities compared to experimentally determined electron affinities for the related copper(I) species: EAexp(CuH) = 0.44 eV, EAcalc(CuH) = 0.51 eV; EAexp(CuCN) = 1.44 eV, EAcalc(CuCN) = 1.58 eV. Experimental data are taken from: (a) Calvi, R. M. D.; Andrews, D. H.; Lineberger, W. C. Chem. Phys. Lett. 2007, 442, 12–16. (b) Yuichi, N.; Tomokazu, Y.; Fumitaka, H.; Miki, K.; Satoshi, Y.; Atsushi, N.; Koji, K. J. Chem. Phys. 2000, 113, 1725–1731. (35) (a) Feng, Y.; Liu, L.; Wang, J. T.; Huang, H.; Guo, Q. X. J. Chem. Inf. Comput. Sci. 2003, 43, 2005. (b) Rijs, N. J; Brookes, N. J; O'Hair, R. A. J.; Yates, B. F., Manuscript in preparation.

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Figure 1. LTQ mass spectra showing collision-induced dissociation (CID) of the diethylcuprates: (a) [CH3CH263CuCH2CH3]-, m/z 121; (b) [CD3CH263CuCH2CD3]-, m/z 127. The mass-selected precursor ion is marked with an asterisk in each case.

on the deuterium-labeled analogue [CD3CH2CuCH2CD3](Figure 1b, eq 5). CID on another deuterium-labeled analogue, [CD3CH2CuCH2CH3]- (Supporting Information, Figure S2a; eq 6), provided an isotope effect (kH/kD) of 2.85 for the β-hydride elimination reaction (eq 6a versus eq 6b). Isotope effects of similar magnitude have been observed in β-hydride elimination reactions of other transition-metal alkyl complexes.36 β-Hydride elimination was also observed in the CID spectra of [CH3CO2CuCH2CH3]- and [CH3CH2CO2CuCH2CH3]-.22f There is no evidence for bond homolysis (eq 4b), while we are unable to observe the ethyl anion (eq 4c) due to the low mass cutoff of the ion trap. Finally, a small amount of [HCuH]- is formed as a result of the secondary loss of ethene (eq 4d) and confirmed by an additional stage of CID on [CH3CH2CuH](Supporting Information, Figure S3f). Note that [CH3CH2CuH]- is the product of the isomerization reaction of [CH3CuCH3]- (eq 2) and its sole fragmentation channel is via β hydride elimination (eq 3, M = Cu) which is entirely consistent with the previously calculated potential energy surface 22k.

½CH3 CH2 CuCH2 CH3  - f ½CH3 CH2 CuH - þ CH2 dCH2

ð4aÞ f ½CH3 CH2 Cu• - þ CH3 CH2 • ð4bÞ f CH3 CH2 - þ ½CH3 CH2 Cu ð4cÞ ½CH3 CH2 CuH - f ½HCuH - þ CH2 dCH2 ð4dÞ ½CD3 CH2 CuCH2 CD3  - f ½CD3 CH2 CuD - þ CH2 dCD2

ð5Þ ½CD3 CH2 CuCH2 CH3 

-

energy for β -hydride transfer (TS1, 1.70 eV; Figure 2) is substantially lower than the energy required for bond homolysis (2.42 eV). The DFT calculations predict that bond heterolysis to ethyl anion formation (eq 4c) is also a much higher energy process (3.16 eV). Since the calculated structures and fragmentation pathway for the β-hydride transfer reaction are very similar to those described in detail for [CH3CH2CuO2CR]-,22f they are not discussed here. 2. Unimolecular Fragmentation Reactions of the Mixed Organometallates [CH3MR]-. The predictions of DFT calculations describing the energetics associated with a range of different fragmentation pathways of the mixed organometallates are presented first and are then followed by a description of the experimental observed fragment ions formed under conditions of low-energy CID. Finally, the experimental and theoretical data are compared for each individual pair of metallates [CH3CuR]- and [CH3AgR]-. 2.1. Potential Fragmentation Pathways Based on DFT Calculations. Here we use DFT calculations to explore a number of potential fragmentation channels for the mixed metallates [CH3MR]- (Scheme 1). Although it is possible to calculate a large number of fragmentation reactions, informed by the different types of reactions examined for the homometallates [CH3CuCH3]-, [CH3AgCH3]-,22k and [CH3CH2CuCH2CH3]- (2a) (section 1), here we limit our DFT survey37 to include the following processes: homolytic cleavage of the M-CH3 bond (eq 7a),38 heterolytic cleavage of the M-CH3 bond (eq 7b), homolytic cleavage of the M-R bond (eq 7c), heterolytic cleavage of the M-R bond (eq 7d), β-hydride transfer for suitable R groups possessing hydrogen at the β-position (eq 7e), β-methide transfer for R = CH2CH2CH3 (eq 7f), and (g) 1,2-dyotropic rearrangement to give the isomer [HMCH2R]- (eq 7g).

½CH3 MR - f ½RM• - þ CH3 •

ð7aÞ

f CH3 - þ ½RM

ð7bÞ

f ½CH3 M• - þ R•

ð7cÞ

f R - þ ½CH3 M

ð7dÞ

-

f ½CD3 CH2 CuH þ CH2 dCH2 ð6aÞ f ½CH3 CH2 CuD - þ CH2 dCD2 ð6bÞ

The experimentally observed preference for β-hydride transfer (eq 4a) over bond homolysis (eq 4b) is borne out by DFT calculations shown in Figure 2. Thus, the activation (36) For a review on the isotope effects associated with the formation and reactions of metal hydrides see: Bullock, R. M. In Transition Metal Hydrides; Dedieu, A., Ed.; VCH: New York, 1992; Chapter 8.

(37) For example, we have not calculated the R-hydride elimination reaction [CH3MR]- f [HMR]- þ CH2, since it is not observed experimentally. (38) Some related coinage metal radical anions, [RM]•-, have been previously observed experimentally and/or studied via theory: for R = Ph and M = Cu, Ag, see: (a) Liu, X.-J.; Zhang, X.; Han, K.-L.; Xing, X.-P.; Sun, S.-T.; Tang, Z.-C. J. Phys. Chem. A 2007, 111, 3248–3255. (b) Liu, X.-J.; Yang, C.-L.; Zhang, X.; Han, K.-L.; Tang, Z.-C. J. Comput. Chem. 2008, 29, 1674–1667. For R = CN and M = Cu, Ag, see: (c) Boldyrev, A. I.; Li, X.; Wang, L.-S. J. Chem. Phys. 2000, 112, 3627–3632.

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Figure 2. (a) DFT calculated energies for fragmentation of [CH3CH2CuCH2CH3]-. (b) Structures of minima and transition states relevant to fragmentation via β-hydride transfer (eq 4a).

f ½CH3 MH - þ ½R- H

ð7eÞ

f ½CH3 MCH3  - þ ½R- CH3  ð7fÞ f ½HMCH2 R ð7gÞ The results of these calculations are summarized in Table 1, while all optimized geometries of reactants, intermediates, transition states, and products are available in the Supporting Information (Figures S5-S15). Although we have previously shown that it is not possible to synthesize [CH3CuC(CH3)3]-, [CH3AgCH(CH3)2]-, and [CH3AgC(CH3)3]via double decarboxylation of suitable carboxylate precursors,22f,i for completeness we have explored theoretical potential fragmentation pathways for these organometallates. We note that our DFT methods employed herein were effective in predicting the reactivity of the other systems studied.22 Finally, the formation of stable, bound radical anions [CH3M]•- (eq 7c) and [RM]•- (eq 7a) requires that CH3M and RM have positive adiabatic electron affinities (EA, eq 8,38). The DFT predicted adiabatic electron affinities of CH3Cu and CH3Ag are 0.47 and 0.61 eV, confirming them to be bound radical anions in the ion trap. A table containing the DFT calculated electron affinities for all other RM is available in the Supporting Information (Figure S4) and shows that these are all bound anions and therefore should be observable anions in an ion trap.38 Finally, an examination of the SOMOs of [CH3Cu]•- and [CH3Ag]-• reveals that they are metal-based (Supporting Information, Figure S16), while orbital analysis of the organometallic radical anions RM•- reveals that the SOMOs are predominately metal based in the case of copper but vary depending on the ligand (R) and can be predominately carbon based for

silver (Figure S16).

RM þ e - f RM• - ðΔH ¼ - EAÞ

ð8Þ

Although we will discuss the DFT data (Table 1) in the context of comparing experiment with theory for each pair of metallates [CH3CuR]- and [CH3AgR]-, it is worth highlighting that even a cursory glance reveals that both the ligand, R, and the nature of the metal, M, can have a profound influence on the thermochemistry associated with the different fragmentation channels (eqs 7a-7g). For example, the DFT predicted energetics associated with loss of the R group as an anion (eq 7d) tracks the known gasphase basicities of R-.39 Thus, the benzyl and allyl anions, which are the weakest bases and are thus the best leaving groups, correspond to the lowest DFT predicted energetics for loss of R- from [CH3MR]-. On the other hand, the activation barrier for a 1,2-dyotropic rearrangement is high in all cases and is not greatly affected by the nature of the R group present. The nature of the metal center M is noted to make an overall difference in reactivity, with silver centers predicted to have lower bond dissociation energies and copper centers predicted to have lower activation energies for β-hydride transfer, owing to higher lying d-orbitals available for π interaction.40 2.2. Overview of the Fragmentation Pathways Observed in CID Experiments. An examination of Figure 3 and Figure S3 in the Supporting Information highlights the range of (39) Gas-phase thermochemical data and definitions (e.g., anion proton affinity) are taken from the NIST Web site: http://webbook. nist.gov/chemistry/ (40) (a) Mori, S.; Nakamura, E. J. Mol. Struct. (THEOCHEM) 1999, 461, 167–175. (b) Nakanishi, W.; Yamanaka, M.; Nakamura, E. J. Am. Chem. Soc. 2005, 127, 1446.

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Table 1. DFT Predicted Energy Barriers for Competing Homolytic Bond Dissociation (Eqs 7a and 7c), Heterolytic Bond Dissociation (Eqs 7b and 7d), β-Hydride Transfer Reaction (Eq 7e), Methide Transfer (Eq 7f), and 1,2-Dyotropic Rearrangement (Eq 7g) for [CH3MR]-, Where R = CH3CH2, CH3CH2CH2, (CH3)2CH, (CH3)3C, CH2CHCH2, PhCH2, Ph and M = Cu, Aga R 3a 3b 4a 4b 5a 5bf 9af 9bf 6a 6b 7a 7b 8a 8b

CH3CH2 CH3CH2CH2 (CH3)2CH (CH3)3C CH2CHCH2 PhCH2 Ph

M

eq 7a,b

eq 7b,b

eq 7c,b

eq 7d,b

eq 7e,c

eq 7f,d

eq 7g,e

Cu Ag Cu Ag Cu Ag Cu Ag Cu Ag Cu Ag Cu Ag

2.71 2.40 2.72 2.41 2.73 2.39 2.75 2.38 2.76 2.37 2.77 2.38 2.87* 2.53*

3.14 2.97 3.20 3.03 3.16 2.98 3.25 3.06 3.47 3.26 3.58 3.38 3.67 3.49

2.42 2.11* 2.52 2.22 2.25 1.93 2.19 1.87 2.32* 2.02* 2.56 2.25 3.54 3.17

3.21 3.04 3.07 2.91 3.06 2.90 2.84 2.66 2.35‡ 2.19‡ 2.20* 2.03* 2.95* 2.73*

1.75 (0.83)* 1.91 (0.66)* 1.71 (0.75)* 1.86 (0.58)* 1.79 (0.67)* 1.91 (0.49) 1.81 (0.61) 1.96 (0.43) 2.15 (1.57)* 1.97 (0.43)* n/a n/a n/a (4.03)g n/a (3.81)g

n/a n/a 2.26 2.44 n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a

3.34 3.48 3.36 3.50 3.32 3.48 3.37 3.52 3.36 3.52 3.43 3.59 3.49 3.63

a The experimentally observed channels are denoted by an asterisk (*). Channels that might reasonably be predicted to occur but cannot be experimentally observed, due to the low mass cutoff of the linear ion trap, are denoted by a double dagger (‡). b Reaction endothermicity (eV), assumed as barrierless. c Activation energy (eV) for β-hydride transfer reactions. Data in parentheses refer to the overall endothermicity of the reaction. d Activation energy (eV) for β-methide transfer reactions. In cases where this reaction is not possible for an organometallate, the entry n/a (not applicable) is used. e Activation energy (eV) for 1,2-dyotropic rearrangement reactions. f Not studied experimentally, as the precursor organometallate cannot be formed.22f,i g No transition state for benzyne elimination was found. The total endothermicity is given if benzyne elimination were able to occur.

different fragmentation reactions that occur for the mixed organometallates under low-energy CID conditions. These experiments reinforce the DFT predictions that both the ligand and the metal can control reactivity. For example, β-hydride elimination (eq 7e) is the sole product ion for [CH3CuCH2CH3]- (3a; Figure 3a) but both the β-hydride elimination pathway (eq 7e) and bond homolysis to yield [CH3Ag]•- and an ethyl radical (eq 7c) occur for [CH3AgCH2CH3]- (3b; Figure 3b and Figure S2c in the Supporting Information). When R = PhCH2, the sole fragmentation channel for both metals (7a,b) involves benzyl anion formation (Figure 3c,d, eq 7d). In contrast, M-CH3 bond homolysis (eq 7a) dominates over R- formation (eq 7d) for R = Ph (8a,b; Figure 3e,f). The different behavior of [CH3MCH2Ph]- and [CH3MPh]- most likely arises from the differences in key thermodynamic properties of the bare R groups: PhCH2- is a weaker base than both Ph- and CH3(their anion proton affinities are 1587, 1680, and 1749 kJ mol-1, respectively39) and is thus a better leaving group; CH3• has a lower electron affinity than both Ph• and PhCH2• (their EAs are 0.080, 1.096, and 0.912 eV, respectively39) and thus CH3- is more likely to undergo electron transfer. Finally, a comparison of parts g and h of Figure 3 highlights how the metal center can dictate the reactivity. Thus, while methylallylcuprate (6a) readily undergoes β-hydride elimination (eq 7e), methylallylargentate (6b) preferentially undergoes bond homolysis (eq 7c). While we will compare the experimental and DFT data for each pair of metallates [CH3CuR]- and [CH3AgR]- in detail below, some comments on the correspondence between experiment and theory are warranted. Low-energy reaction channels that have barriers which are predicted by the DFT calculations to differ by 0.2 eV or less are expected to be competitive with each other under conditions of low-energy CID. However, similar energetics will not necessarily lead to similar product intensities. For example, when bond homolysis (or heterolysis) competes with an activation barrier associated with a rearrangement reaction (e.g., a β-H elimination reaction), the latter is entropically disfavored because of the tight transition state involved and thus the intensity of these ions might be lower than that of ions arising from direct

bond cleavage. Generally we find that there is good agreement between the major fragment ion(s) observed in the CID spectra and the DFT predicted lowest energy pathway(s) (experimentally observed products are highlighted by an asterisk in Table 1). The only exceptions, highlighted by a double dagger in Table 1, can be rationalized as not being observable under the experimental conditions due to the low mass cutoff, which is an inherent limitation of ion traps. Finally, in our discussion of the experimentally observed fragmentation reactions of each homometallate, [CH3MR]-, we will restrict ourselves to low-energy processes. Thus, pathways that are predicted by the DFT calculations to be higher energy processes, such as the 1,2-dyotropic rearrangement for all species (except R = Ph) or dissociation of the M-CH3 bond, are not discussed. 2.2.1. Fragmentation Reactions of [CH3CuCH2CH3]- and [CH3AgCH2CH3]-. The DFT calculations predict that β-hydride elimination (eq 7e) should be the overwhelmingly favored pathway for both [CH3CuCH2CH3]- and [CH3AgCH2CH3]- (Table 1: 3a, 1.75 eV; 3b, 1.91 eV). The M-R homolysis pathway (eq 7c) is found to be the next lowest in energy for both metals (M = Cu, 2.42 eV; M = Ag, 2.11 eV). For 3a (M = Cu), there is a significant energy difference between these two lowest energy pathways (0.67 eV), such that the higher energy homolysis pathway is anticipated to occur. However, for 3b (M = Ag), both an increase in the activation energy of the β-hydride pathway and a decrease in the endothermicity of the bond homolysis pathway in comparison to the copper species lead to an energy difference of only 0.20 eV, suggesting that these pathways may be competitive. M-CH3 bond homolysis (eq 7a) is considerably higher in energy (2.71 and 2.40 eV, respectively) and is thus predicted to not occur. For both 3a and 3b the M-C heterolysis pathways (eqs 7b and 7d) are predicted to be much higher in energy (>1 eV) than the lowest energy pathway and therefore are not expected to occur under conditions of low-energy CID (Table 1; 3a,b). Finally, the dyotropic rearrangement (eq 7g) is not predicted to occur, due to the high activation barriers (3.34 and 3.48 eV for Cu and Ag metal centers, respectively).

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Figure 3. LTQ mass spectra showing low-energy collision-induced dissociation (CID) of the organometallates: (a) [CH363CuCH2CH3]-, m/z 106; (b) [CH3107AgCH2CH3]-, m/z 151; (c) [CH363CuCH2Ph]-, m/z 169; (d) [CH3107AgCH2Ph]-, m/z 213; (e) [CH363CuPh]-, m/z 155; (f) [CH3107AgPh]-, m/z 199; the ion at m/z 216 arises from the addition of a neutral mass of 32 (either O2 or CH3OH) to [PhAg]•-; (g) [CH363CuCH2CHCH2]-, m/z 119; (h) [CH3107AgCH2CHCH2]-, m/z 163. The mass-selected precursor ion is marked with an asterisk in each case.

These DFT predictions are entirely consistent with the types of fragment ions observed under low-energy CID conditions: the β-hydride elimination product [CH3CuH](eq 7e), as confirmed by deuterium labeling experiments (Supporting Information, Figure S2b), is the sole product observed in the CID spectrum of [CH3CuCH2CH3](Figure 3a). For [CH3AgCH2CH3]- the β-hydride product [CH3AgH]- is also the dominant product ion (Figure 3b), although a small amount of the Ag-R bond homolysis product CH3Ag•- (eq 7c) is also observed. The even smaller amount of Ag- observed is likely due to subsequent fragmentation of CH3Ag•-, as noted previously22k and supported by an additional CID experiment (Supporting Information, Figure S3h).

2.2.2. Fragmentation Reactions of [CH3CuCH2CH2CH3]and [CH3AgCH2CH2CH3]-. The trends previously discussed for R = CH3CH2 are continued when the ligand alkyl chain is lengthened to R = CH3CH2CH2 (4a,b; Table 1). β-Hydride elimination is again predicted to be the significantly preferred pathway for both organometallates (4a, M = Cu, 1.71 eV; 4b, M = Ag, 1.86 eV). Of the series of R examined in this study (Scheme 1), the β-methide transfer pathway (eq 7f) is a unique possibility for the propyl ligand. This process is predicted to have a moderate activation barrier for both metals (M = Cu, 2.26 eV; M = Ag, 2.44 eV). The difference separating the activation barrier for β-hydride versus β-methide transfer is, however, relatively large for both 4a and 4b (0.55 and 0.58 eV, respectively), and thus it is reasonable to suggest that β-methide

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transfer will not be competitive with the β-hydride transfer pathway. It is noteworthy that both the β-hydride and β-methide transfer activation energies are predicted to be lower for M = Cu than for the M = Ag congener. This reflects the relative lack of π-bonding ability in the organoargentate, which DFT calculations indicate is due to the lower lying d-orbitals.40 When M = Cu (4a), bond homolysis (eqs 7a and 7c) is predicted to require >0.8 eV more energy than the lowest energy pathway (β-hydride elimination) and therefore is not expected to take place experimentally. When M = Ag (4b), M-R homolysis (eq 7c) is predicted to be the second lowest energy process (2.22 eV). Though the energy difference here is found to be smaller than that of the cuprate 4a, the difference is still >0.3 eV and thus might not be expected to be observed experimentally in the CID spectrum of 4b. In agreement with these DFT predictions, β-hydride elimination is observed upon CID of both 4a and 4b (Supporting Information, Figures S3a and S3b). Also consistent with theory, products arising from β-methide transfer, bond homolysis, or dyotropic rearrangement are not observed for either metal. Bond heterolysis products are not observed, as previously noted, due to the low mass cutoff of the linear ion trap. 2.2.3. Fragmentation Reactions of [CH3CuCH(CH3)2]-, [CH3AgCH(CH3)2]-, [CH3CuC(CH3)3]-, and [CH3AgC(CH3)3]-. The low-energy CID fragmentation of experimentally formed 5a, along with only the theoretical examination of 5b and 9a,b, will now be discussed in light of the DFT predictions (Table 1, 5a,b and 9a,b) and previous CID results. When M = Cu and R = (CH3)2CH, (CH3)3C (Table 1, 5a and 9a), the ever-present β-hydride elimination pathway (eq 7e) is predicted to be the most energetically favorable pathway (5a, 1.79 eV; 9a, 1.81 eV). When M = Ag and R = (CH3)2CH, (CH3)3C (Table 1 5b and 9b), β-hydride elimination is predicted to be the lowest energy path for 5b (1.91 eV) and a competitive pathway for 9b (1.96 eV). M-R bond homolysis (eq 7c) is competitive with these reactions for M = Ag. It is the lowest energy pathway for 9b (1.87 eV) and second lowest for 5b (1.93 eV). Therefore, it is predicted that for M = Cu (5a and 9a), β-hydride elimination (eq 7e) should be the sole observed pathway, while for M = Ag (5b and 9b), M-R homolysis (eq 7c) and -H elimination (eq 7e) should be in competition. Since 5a,b and 9b cannot be formed via double decarboxylation,22 only the CID spectrum of 5a is available (Supporting Information, Figure S3c). An examination of this spectrum reveals that β-hydride elimination is the only pathway operating, consistent with the aforementioned DFT predictions. 2.2.4. Fragmentation Reactions of [CH3CuCH2CHCH2]and [CH3AgCH2CHCH2]-. While β-hydride transfer is the lowest energy path for the organometallates 6a,b, the hybridization of the allyl ligand and the geometry of the allene product appear to exert a unique influence on this reaction. First, there is a noteworthy increase in the overall endothermicities (activation energies of 2.15 and 1.97 eV, respectively) of these reactions in comparison to alkyl β-hydride elimination products (Table 1). Second, this is the only occasion where β-hydride transfer is predicted to be lower for the organoargentate than for the organocuprate. It appears that the nature of allyl ligand hybridization allows a transition state of different geometry around the Ag center to take place during β-hydride transfer (Supporting Information, Figure S9).

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The next lowest energy pathway for both metals is M-R bond homolysis (eq 7c). For 6b the barrier is essentially equal to the activation energy for β-hydride transfer, while for 6a it is only 0.17 eV higher in energy. Therefore, M-R is anticipated to be in competition with β-hydride transfer. M-R bond heterolysis (eq 7d) is the next lowest energy pathway for 6a,b (2.35 and 2.19 eV, respectively) and is also expected to be a viable fragmentation pathway for both metals. CID of 6a yields fragmentation products corresponding to β-hydride elimination (eq 7e, Figure 3g). The CID spectrum of 6b (Figure 3h) also shows products of β-hydride elimination (eq 7e) and Ag-R homolysis (eq 7c). Noteworthy product ions that are absent in these CID spectra are MCH2CHCH2•-, derived from M-CH3 homolysis, and [HCuH]-, arising from a 1,2-dyotropic rearrangement (eq 7g); these results are also consistent with DFT predictions. Heterolysis (eqs 7b and 7d) is not observable in either of these experiments. This is the only example of which the low mass cutoff value of the ion trap may be limiting the experimental observation of a potentially competitive pathway, namely the heterolysis of M-R (eq 7d, Table 1: 6a, 2.35 eV; 6b, 2.19 eV). Overall, there is good agreement between the DFTpredicted endothermicities for [CH3MCH2CHCH2]- and the types of product ions observed under low-energy CID conditions (Figure 3g,h). 2.2.5. Fragmentation Reactions of [CH3CuCH2Ph]- and [CH3AgCH2Ph]-. [CH3MCH2Ph]- is the first mixed metalate not possessing a hydrogen in the β-position (Table 1, 7a,b). As there is no hydride or other transferable group in the β-position, β-transfer cannot dominate the observed fragmentation. Thus, the DFT predicted order of reactivity defaults to the heterolysis of the M-R bond (eq 7d) as the most energetically favorable for both metals (7a, 2.20 eV; 7b, 2.03 eV). This is the only example herein where a bond heterolysis is predicted as the lowest relative energy pathway. For 7a, the next lowest energy pathways are predicted to be those of Cu-R and Cu-CH3 homolysis (eqs 7a and 7c), being 0.36 and 0.57 eV more endothermic than Cu-R heterolysis, respectively. Cu-CH3 heterolysis (eq 7b) is found to be significantly higher in energy (3.58 eV). In the case of 7b, the next closest bond dissociations are predicted to be homolysis of Ag-R and Ag-CH3: 0.22 and 0.35 eV higher in energy, respectively. Indeed, fully supported by these DFT predictions, the only visible product in the CID spectra of 7a (Figure 3c) and 7b (Figure 3d) corresponds to M-CH2Ph heterolysis (m/z 91, PhCH2-). 2.2.6. Fragmentation Reactions of [CH3CuPh]- and [CH3AgPh]-. The lowest energy pathway for both 8a and 8b is predicted to be M-CH3 bond homolysis (eq 7a) (Table 1: 8a, 2.87 eV; 8b, 2.53 eV), the only example examined in this study where M-CH3 bond homolysis is favored over other pathways. The next lowest energy dissociation, predicted to be competitive with this reaction for both metals, is the heterolysis of M-Ph (eq 7d: 8a, 2.95 eV; 8b, 2.73 eV). No transition state to benzyne elimination (eq 7e) from the π-bound intermediate was found, and this dissociation reaction is predicted to be highly endothermic for both metals (8a, 4.03 eV; 8b, 3.81 eV); thus, these two factors suggest that this process is not viable in either case. The other bond dissociation pathways (eqs 7b and 7c) and the 1,2-dyotropic rearrangement process (eq 7g) with either metal center are high in energy (>0.6 eV above lowest energy pathway) and therefore are not expected to be observed in the low-energy CID spectrum.

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The formation of the organometallic radical anion [PhCu]•- (m/z 140) as the major product in the low-energy CID spectrum of 8a (Figure 3e) is consistent with the DFT predictions that Cu-CH3 homolysis (eq 7a) is the lowest energy pathway. A smaller fragment (m/z 77) is observed, in line with the DFT calculations that heterolysis of Ph-Cu (eq 7d) is the pathway with the second lowest energy requirement. Finally, the presence of minor products in the CID spectrum of [CH3CuPh]- reveals some experimental surprises. The observation of PhCH2- in the spectrum given in Figure 3e is consistent with a dyotropic rearrangement (eq 7g) occurring, followed by loss of CuH from the rearranged [PhCH2CuH]- isomer (eq 9).

½PhCH2 CuH - f PhCH2 - þ CuH

ð9Þ

Deuterium labeling experiments confirm this is indeed a rearrangement, the origin of the methene inserted into the Cu-Ph group being the methyl group (Supporting Information, Figure S2d; eq 7g). This is the only example where there is a conflict with the DFT calculations and, consistent with the related theoretical data for [CH3CuCH3]- (Supporting Information, Figure S1), is likely to be a result of the level of theory underestimating the bond dissociation energies and overestimating the barrier for dyotropic rearrangement. [CH3CuPh]- produces minor fragment ions at m/z 126 and 144. The masses of these fragments are unchanged in the CID spectrum of [CD3CuPh]- (Supporting Information, Figure S2d) but shift to m/z 129 and 149 for [CH3CuC6D5]- (Supporting Information, Figure S2e). Although the precise origin of these ions remains to be established, the deuterium labeling experiments suggest fragmentation of the phenyl ring.41 The low-energy CID of 8b (Figure 3f) yields the organometallic radical anion [PhAg]•- (m/z 184) as the major product, with Ph- (m/z 77) being a minor product. The relative abundances of these product ions are in good agreement with the DFT calculations, which predict that homolysis of the Ag-CH3 bond (eq 7a) requires less energy than heterolysis of the Ag-Ph bond (eq 7a). 2.3. Overall Trends in Gas-Phase Reactivity and Comparison with Condensed-Phase Decomposition Pathways for Organocopper and Organosilver Species. The organocuprate anions were found to readily and preferentially undergo β-hydride elimination in all systems where hydrogen is present in the β-position. This is entirely consistent with observations of organocopper decomposition in the condensed phase.6 β-Hydride transfer is a low-energy process due to the availability of higher lying copper d-orbitals for π-interaction in the transition state. Unfortunately these same d-orbitals also confer organocuprates their superior nucleophilic properties 40 and this highlights the key challenge for solution phases syntheses that employ organocuprates with alkyl ligands possessing β-hydrogen. Where there (41) A reviewer has suggested that these minor products may be the result of rearrangements that occur either in solution or in the electrospray process, leading to isomeric ions having the same mass as the expected species. Although we cannot fully exclude these possibilities, the deuterium labeling experiments and additional experiments in which [PhCu]•- are mass selected (data not shown) are consistent with these minor products arising from gas-phase ion-molecule reactions between [PhCu]•- and background oxygen, whereby O2 addition is followed by loss of CO and [CH2O2] to yield the ions at m/z 144 and 126, respectively. Future studies will examine the ion-molecule reactions of [RM]•- in detail.

is no β-hydrogen available in the organocuprate, other pathways arise. Depending on the nature of the R group, bond heterolysis, homolysis, and a 1,2-dyotropic rearrangement with subsequent bond dissociation are observed to occur to different extents for the various mixed cuprates, [CH3CuR]-. Indeed, the preferred dissociation channel of these organocuprates is dependent on the properties of ligands present (e.g., radical stabilities; gas phase anion proton affinity), which dictates the leaving group ability. The fragmentation of the organoargentates is dominated by Ag-C bond homolysis, with concomitant formation of silver radical anions. This is entirely consistent with the wellknown radical decomposition reactions of organosilver species in solution.7 The β-hydride pathway is higher in energy for the silver systems, and the DFT calculations indicate it occurs via a mechanism slightly different from that for copper. In particular, no π-bonded alkene product is formed due to lower lying d-orbitals of the silver. However, β-hydride elimination still represents a major decomposition pathway in these organoargentate anions.

Conclusions The combined use of gas-phase experiments and DFT theory provides powerful mechanistic insights into the formation, unimolecular and bimolecular reactivity of organometallic ions, and this information can potentially be used to shed light on related solution-phase systems. For example, our past work on the preparation of the organometallates [R2M]- and [CH3MR]- via double-decarboxylation reactions can be used in conjunction with the current work on their unimolecular reactions to provide a firm foundation for a better understanding of key intermediates and potentially competing side reactions that might occur in metal-catalyzed transformations of carboxylic acids.24 Although the reactivity of these organometallates with electrophiles in the gas phase, which is the subject of ongoing work in our laboratory, is a key missing piece of the puzzle, some comments on the use of silver and copper salts in the condensed phase to catalyze protodecarboxylation and C-C bond coupling reactions initiated by decarboxylation are warranted here. The Achilles heel of solution-phase thermal decarboxylation reactions was recognized by Deacon 40 years ago: “The preparation of organometallics by thermal decarboxylation is of necessity limited to compounds that are thermally stable at the decarboxylation temperature.”23a Considerable progress has been made in the field: where previously perfluoro organometallics were the main types of organometallics accessible via decarboxylation,23 by using appropriate ligands, solvents, and other additives, a range of other transient organocopper and organosilver species have been implicated in catalytic decarboxylation reactions.25 Nonetheless, the scope of these silver- and copper-catalyzed transformations has been limited to aromatic carboxylic acids25b-d and carboxylic acids that are substituted at the R-position with stabilizing groups such as Ph and CN.25a Our results suggest the reasons that these types of carboxylic acids have been successfully used are twofold: (i) the barriers for decarboxylation of [CH3MO2CR]- are among the lowest when R = Ph, PhCH222f,i and (ii) the resultant organometallates, [CH3MR]-, do not suffer from low-energy β-hydride fragmentation reactions and are thus thermally more robust than those [CH3MR]- species that contain a simple

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alkyl group (see Table 1). Our recent work on the metalmediated protodecarboxylation of propiolic acid and its derivatives22j suggest that metal acetylides might prove to be useful targets as intermediates for condensed-phase metal-catalyzed decarboxylation reactions.42 Further studies are underway to (i) establish the scope of metal-mediated decarboxylation reactions for a wide range of metals and carboxylic acids and (ii) study the subsequent unimolecular and bimolecular reactivity of the resultant organometallics.

Acknowledgment. We thank the ARC for financial support via Grant DP0558430 (to R.A.J.O.) and through (42) Indeed, Pd-catalyzed C-C bond coupling of acetylide ligands formed by decarboxylation has been recently described: (a) Rayabarapu, D. K.; Tunge, J. A. J. Am. Chem. Soc. 2005, 127, 13510. (b) Kolarovic, A.; Faberova, Z. J. Org. Chem. 2009, 74, 7199.

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the ARC CoE program. N.J.R. thanks the Faculty of Science for a Science Faculty Scholarship. The VPAC is acknowledged for generous provision of computational resources. The VICS is acknowledged for the Chemical Sciences High Performance Computing Facility. We thank Prof. Brian F. Yates for discussions on the DFT calculations, Dr. Phillip J. Barker for discussions on bond homolysis and for a reprint of ref 5d, and Dr. George N. Khairallah for commenting on the manuscript. Supporting Information Available: Text giving the complete citation for ref 28 and tables and figures giving Cartesian coordinates and energies for all DFT calculated species, additional mass spectra, and SOMO analysis. This material is available free of charge via the Internet at http:// pubs.acs.org.