Gas-Phase Formation and Fragmentation Reactions of the

Article pubs.acs.org/Organometallics

Gas-Phase Formation and Fragmentation Reactions of the Organomagnesates [RMgX2]− George N. Khairallah,†,‡,§ Charlene C. L. Thum,†,‡ Denis Lesage,∥ Jean-Claude Tabet,∥ 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 ∥ Institut Parisien de Chimie Moléculaire, Université Pierre et Marie Curie-Paris 6 (UPMC), UMR 7201- FR2769, Place Jussieu, 75252 Paris Cedex 05, France S Supporting Information *

ABSTRACT: A range of mononuclear organomagnesates [RMgX2]− were generated in the gas phase by decarboxylation of the magnesium carboxylate precursors [RCO2MgX2]− (R = Me, Et, Pr, iPr, tBu, vinyl, allyl, HCC, Ph, PhCH2, PhCH2CH2; X = Cl, Br, I). The gas-phase formation and unimolecular reactivity of these organomagnesates were examined using a combination of experiments carried out in linear ion trap and triple-quadrupole mass spectrometers and DFT calculations. Halide loss was found to directly compete with decarboxylation in the formation of mononuclear [RMgX2]−. However, sterically unhindered, stable R− substituents and strong Mg−Cl bonds can be employed to facilitate the decarboxylation reaction at the expense of the halide loss channel. Thus, in the case of R = HCC, PhCH2, decarboxylation is the main fragmentation pathway. The resultant mononuclear organomagnesates [RMgX2]− were mass-selected, and their unimolecular chemistry was examined. Four competing fragmentations were observed: bond homolysis, bond heterolysis, halide loss, and β-hydride transfer. Which of these competing reactions dominates depends on the nature of R and X. A conjugatively stabilized R• allows the observation of [MgX2]•−, whereas the presence of a β-hydride generates [HMgX2]−. Weaker Mg−X bonds (e.g., Br and I) promote the formation of X− upon CID.

■

INTRODUCTION Grignard reagents are among the oldest and most widely known and used organometallic reagents in organic synthesis.1,2 Despite more than 100 years of use, research into the structure, reactivity, and applications of organomagnesiums continues on at least two fronts. The first is driven by the desire to develop new synthetic methods and includes enhancing reactivity via the use of organomagnesates3 and “Turbo Grignards”4,5 and the continued development of transition-metal-catalyzed reactions involving Grignard reagents.6 The second is using organomagnesiums as models of complexity in understanding the mechanisms of organometallic reactions. This complexity arises from solvent-dependent processes, including the Schlenk equilibrium, dimerization, and ionization.7 One way of removing this complexity is to study the formation and reactivity of organomagnesium ions in the solvent-free environment of a mass spectrometer.8,9 These © 2013 American Chemical Society

organomagnesium ions are uniquely defined by their m/z values, allowing them to be manipulated via mass selection, thereby providing confident product assignment by establishing a direct link between the reactant ion and the unimolecular or bimolecular product ion(s).9 Indeed, by a combination of the multistage mass spectrometry (MSn) capabilities of ion trap mass spectrometers10 with theoretical calculations11 detailed mechanistic insights into gas-phase organomagnesium chemistry can be gleaned.9 Rather than face the major challenge of carrying out electrospray ionization (ESI/MS) on organomagnesiums,12−14 we have been performing ESI on solutions of carboxylic acids, magnesium salts, and other additives (e.g., auxiliary ligands) to generate stable gas-phase magnesium Received: December 10, 2012 Published: April 5, 2013 2319

dx.doi.org/10.1021/om3011917 | Organometallics 2013, 32, 2319−2328

Organometallics

Article

Scheme 2. Magnesium Carboxylates [RCO2MgX2]− (1) Surveyed in This Study To Form Organomagnesates [RMgX2]− (2) via Decarboxylation

carboxylate ions, which are mass-selected and then subjected to collision-induced dissociation (CID) to “synthesize” the desired organomagnesium via decarboxylation.15,16 A potential challenge for this approach is that other fragmentation reactions can potentially compete with the formation of the organometallate. This is illustrated for the case of the magnesium carboxylate [RCO2MgX2]− (1) in Scheme 1. Scheme 1. Competing Pathways for the Formation and Fragmentation Reactions of Organomagnesates [RMgX2]−

Thus loss of the carboxylate RCO2− (3; eq 1b of Scheme 1) or halide X− (4; eq 1c of Scheme 1) can be in competition with the desired decarboxylation reaction to form the organomagnesate (2; eq 1a of Scheme 1). To date, we have successfully formed and studied the reactivity of the following organomagnesium ions: [CH 3 MgCl 2 ] − , 9 a [CH 3 Mg(O2CCH3)2]−,9a,b [PhCH2MgCl2]−,9c [RCCMgCl2]− and [RCCMg2Cl4]− (where R = H, Ph),9d and [CH3MgL]+ (where L = betaine,9e crown ethers9f). Interestingly, the dimers [RCCMg2Cl4]− displayed an increased reactivity toward hydrolysis in comparison to the monomers [RCCMgCl2]−, highlighting the role of the second metal in enhancing reactivity.9d Organomagnesium compounds are generally more thermally robust and less light sensitive than other organometallic compounds; however, they can undergo thermal and photochemical decomposition.17 The condensed-phase thermal decomposition reactions of organomagnesiums can proceed via bond homolysis with free radical formation,17a,b alkene elimination through β-hydride transfer to form magnesium hydrides,17c−g alkane loss to form magnesium carbenes,17h and precipitation of magnesium.17i We have been examining the gas-phase unimolecular decomposition reactions of organometallic ions.9c,d,18 While the gas-phase unimolecular chemistry of the organocuprates and organoargentates [RMCH3]− (M = Cu, Ag) have been studied in detail,18a,b reports on the gasphase unimolecular chemistry of organomagnesate anions have been limited to [PhCH2MgCl2]− 9c and [PhCCMgCl2]− and [PhCCMg2Cl4]−.9d The first species fragments via bond homolysis to form the novel magnesium(I) radical anion [MgCl2]•− (6; eq 2b of Scheme 1), while [PhCCMgCl2]− fragments via formation of the acetylide PhCC− (5; eq 2a of Scheme 1). Fragmentation of the dinuclear organomagnesate [PhCCMg2Cl4]− affords the formation of MgCl3− and [PhCCMgCl2]− via the losses of the neutrals PhC CMgCl and MgCl2, respectively. Here we (i) examine whether a range of organomagnesates can be synthesized in the gas phase via decarboxylation of the magnesium carboxylates shown in Scheme 2 and (ii) provide the first comprehensive gas-phase survey of the unimolecular decomposition reactions of these organomagnesates to establish how the nature of the R group and halide influences

the types of fragmentation channels observed (eqs 2a−2d of Scheme 1). This is achieved via the use of multistage lowenergy CID experiments in a quadrupole linear ion trap (LTQ) mass spectrometer, variable energy CID experiments on a triple-quadrupole mass spectrometer (TQ), and DFT calculations to shed light on the mechanisms and estimate the energetics associated with the competing reaction pathways shown in Scheme 1.

■

EXPERIMENTAL SECTION

Reagents. Trimethylacetic acid, acrylic acid, vinylacetic acid, propiolic acid, and hydrocinnamic acid were obtained from Aldrich. MgCl2, acetic acid, propionic acid, n-butyric acid, and phenylacetic acid were obtained from BDH Laboratory Supplies. MgBr2 and MgI2 were obtained from Aldrich. Isobutyric acid and methanol were obtained from Ajax. Benzoic acid was obtained from May Baker. All chemicals were used without further purification. Mass Spectrometry. Experiments were conducted using two mass spectrometers: (i) a Thermo LTQ-FT hybrid quadrupole ion trap FTICR mass spectrometer equipped with a Finnigan electrospray ionization source19 and (ii) a modified Micromass triple-quadrupole mass spectrometer (Quattro II) equipped with a standard Z-spray electrospray ionization source.20 The low-energy CID process is different in ion trap (LTQ) and beam (TQ) mass spectrometers in a number of respects.21 In triple-quadrupole mass spectrometers cascade fragmentation can occur and thus secondary fragmentation products can be observed. In contrast, in the LTQ the helium bath gas not only serves as the collision target but also cools the primary product ion(s) to the bath gas temperature (298 K). Thus, primary product ions formed in the LTQ do not generally undergo further fragmentation. In addition, the use of TQ mass spectrometers allows the observation of ions with m/z values below 50, which is typically not possible with an ion-trap instrument due to the low-mass cutoff. Furthermore, the TQ allows access to higher energy processes due to the fact that a larger amount of energy may be deposited during a single collision. The maximum energy transferred is the center of mass energy Ecm given by Ecm = E(mt/(mt + mi)), where E is the kinetic energy of the precursor ion in the laboratory frame and mt and mi are the mass of the target gas (argon) and the mass of the precursor ion, respectively. Thus, the Ecm value is smaller for heavier ions. This energy is in addition to that initially deposited into the ion as it travels through the ion source and the transfer hexapole. The final difference concerns the use of multistage mass spectrometry experiments (MSn), which reflects the 2320


Organometallics

Article

Figure 1. CID spectra for the [MeCO2MgX2]− systems: (a) in the LTQ, X = Cl; (b) in the TQ, X = Cl; (c) in the LTQ, X = Br; (d) in the TQ, X = Br (note that the isotopic peak selected here corresponds to [MeCO2Mg79Br81Br]−); (e) in the LTQ, X = I; (f) in the TQ, X = I. The CID energy in the TQ experiments is 10 V. An asterisk (*) denotes the mass-selected ion, while a pound sign (#) denotes the hydrolysis product [HOMgX2]− from background reactions of [MeMgX2]− with water (eq 3). different nature of these experiments: “tandem in time” for the LTQ and “tandem in space” for the TQ.21 Thus, in the LTQ, the organomagnesate decarboxylation product ion [RMgX 2]− (2) (Scheme 1) can be mass-selected and its fragmentation reactions can be studied using CID in a MS3 experiment. In contrast, in the TQ, a further stage of mass spectrometry is not possible, and thus the organomagnesate [RMgX2]− must be formed via “in source” CID22 and then mass-selected and subjected to CID, in what is termed a “pseudo-MS3” experiment. The carboxylic acids and magnesium salts were dissolved in methanol in a 1:2 molar ratio, with typical concentrations of ca. 1.0 mM. These solutions were delivered into the electrospray sources via syringe pumps operating at rates of 5 and 10 μL/min for the LTQ and the TQ, respectively. Typical electrospray source conditions on LTQ involved needle potentials of 2.5−3.5 kV and heated capillary temperatures of 200−300 °C. Tuning of the electrospray conditions and various transfer voltages was carried out to optimize the signal of the desired magnesium carboxylate ion [RCO2MgX2]−. Mass selection and CID were carried out by standard isolation and excitation procedures using the LTQ software. In the CID reactions, the activation time was kept constant for all systems (30 ms), while the normalised CID energy was varied. For the magnesium carboxylates, [RCO2MgCl2]−, it varied between 35 (for R = HCC and vinyl) and 44 (for R = trimethyl acetic), while for the organomagnesates, [RMgCl2]−, it varied between 30 (for R = Pr) and 45 (for R = Me). Typical conditions for the triple quadrupole are as follows: the ESI source involved a capillary potential of 3 kV and counter electrode of −300 V. The “sampling cone” (or cone) voltage was varied from −20 to −35 V and from −35 to −55 V (for [RCO2MgCl2]− and [RMgCl2]−, respectively). The “extractor” was set 5 V higher than the cone voltage. Nitrogen was used as the drying and nebulizing gas with flow rates of 400 and 60 L/h, respectively. The source temperature was set at 80 °C. CID spectra were recorded at 5,10, 20, and 40 V collision voltages and at 4 eV center of mass energy (Ecm). Argon (ca. 2 × 10−4 mbar; 0.15 mTorr) was used as the target gas for CID processes.

Automatic data acquisition was processed using the software Masslynx 4.0. Halide-containing species were identified by their respective halide isotope patterns (Cl, 35Cl 75.8%, 37Cl 24.2%; Br, 79Br 50.7%, 81Br 49.3%; I, 127I 100%). However, single peaks are selected in the reported mass spectra. These are chosen generally according to the most intense isotopic peak in the cluster unless otherwise indicated. Theoretical Calculations. Density functional theory (DFT) calculations were carried out using the Gaussian 03 package23 with the hybrid B3LYP functional.24 Calculations were performed using two methods: (i) the 6-31+G(d) basis sets25 for all atoms (H, C, O, Mg, and Cl) and (ii) a combination of the Stuttgart effective core potential (SDD)26 for halides (Br and I) and the 6-31+G(d) basis set for all other atoms (H, C, O, and Mg). Reaction thermodynamics and kinetics calculations were corrected for zero-point energies scaled by 0.9806.27 Energies are reported throughout in eV (1 eV = 96.49 kJ mol−1 = 23.06 kcal mol−1). Although the theoretical benchmarking of organomagnesiums has not been extensively studied, the B3LYP/6-31+G(d) combination has been used in several studies.9,28 A challenge of assessing the performance of this combination of method and basis set is that comparisons between theoretical and experimental bond lengths are not possible, as the X-ray crystal structures of the types of magnesium carboxylates and organomagnesates studied here have not been experimentally determined. Nonetheless, in Table S1 (Supporting Information) we have included typical ranges for R−Mg, Mg−O (carboxylate), Mg−Cl, Mg−Br, and Mg−I bonds found in the literature,29 and the structures we have found fall into these ranges. As a further check of the theoretical method used, the theoretical bond lengths were compared at different levels of theory. Comparisons of calculated Mg−O and Mg−C bond lengths in the typical magnesium carboxylate [MeCO2MgCl2]− and organomagnesate [MeMgCl2]− are presented in Table S2 (Supporting Information). In order to evaluate the reliability of the theoretical energies obtained using the B3LYP functional, further MP230 optimizations were carried out on the decarboxylation reaction of the simplest carboxylate, [MeCO2MgCl2]− 2321


Organometallics

Article

in Table 1 for all [MeCO2MgX2]−. Each system is now discussed in detail.

(Figure S1, Supporting Information). Reasonable agreement between MP2 and B3LYP optimized relative energies is found. Thus, since the B3LYP level of theory was found to yield bond lengths and relative energies in reasonable agreement with higher levels of theory, it was used to calculate the unimolecular chemistry of all magnesium carboxylates [RCO2MgX2]− and organomagnesates [RMgX2]−. This approach provides a compromise between accuracy and computational expense for calculations on larger systems and also allows a direct comparison with previously published work.9

■

RESULTS AND DISCUSSION Negative ion electrospray ionization of carboxylic acid (RCO2H) and magnesium(II) halide (MgX2) solutions generated complex mass spectra, which provided access, among others, to the mononuclear and binuclear magnesium carboxylates [RCO2MgX2]− and [RCO2Mg2X4]−, respectively. All mononuclear magnesium carboxylates [RCO2MgX2]− (1) with the various combinations of R and X shown in Scheme 2 could be formed in this way and were subsequently massselected and subjected to CID using both types of mass spectrometers. The resultant MS2 spectra are discussed in detail in section 1, which also includes the results of DFT calculations on the energetics associated with eqs 1a-1c in Scheme 1. Those organomagnesates, [RMgX2]−, 2, that could be formed via decarboxylation (eq 1a of Scheme 1), were mass selected and subjected to a further stage of CID in MS3 (or pseudo-MS3 for the TQ) experiments. The types of fragmentation reactions observed and the DFT predicted energetics associated with eqs 2a-2d in Scheme 1 are discussed in section 2. (1). Fragmentation of the Magnesium Carboxylates [RCO2MgX 2]−. All 33 of the magnesium carboxylates [RCO2MgX2]− (1a1−1k1, 1a2−1k2, 1a3−1k3) were subjected to low-energy CID in the linear quadrupole ion trap and variable-energy CID in the triple-quadrupole mass spectrometer. Over 100 different CID spectra were collected; these data are given in Figure 1 and the Figures S2-S8 (Supporting Information), and correlations between these mass spectrometry based experiments and the DFT calculated energetics associated with eqs 1a−1c are given in Tables 1 and 2. The formation of [RMgX2]− (R = Me and X = Cl, Br, I) is discussed first, followed by the formation of [RMgX2]− for all other R groups (Scheme 2) and X = Cl, Br, I. (a). Formation of Mononuclear Organomagnesates [MeMgX2]− (X = Cl, Br, I). Single-isotope experiments are used throughout this work to avoid overlap of isotopic peaks arising from ion−molecule reactions of the organomagnesate with background water (eq 39). Mass spectra with full isotopes are presented in Figure S6 (Supporting Information). The magnesium hydroxide products [HOMgX2]− of these hydrolysis reactions are designated with a # in all spectra. [RMgX 2]− + H 2O → [HOMgX 2]− + RH

Figure 2. B3LYP/6-31+G(d) calculated (a) relative energies (eV) for the fragmentation pathways of [MeCO2MgCl2]− and (b) structures, including bond lengths (Å), for decarboxylation of [MeCO2MgCl2]− as well as for the neutrals arising from ligand loss, [MeCO2MgCl] and [MgCl2].

(i). X = Cl. While the low-energy CID spectrum of [MeCO2MgCl2]− in a 3D ion trap has been previously reported,9a we were interested in re-examining its fragmentation reactions in both a linear ion trap and a triple-quadrupole mass spectrometer. CID of mass-selected [MeCO2MgCl2]− (m/z 153) in the LTQ yielded the desired organomagnesate [MeMgCl2]− at m/z 109, arising from the loss of neutral CO2 (eq 1a, Figure 1a). No other product ions, apart from a weak peak at m/z 111 due to the hydrolysis reaction (eq 3),9a were visible in the mass spectrum, indicating that loss of acetate anion (m/z 59, cf. eq 1b) does not occur for this system. The low mass cutoff of the ion trap (m/z 50) meant that the possibility of Cl− loss could not be detected (cf. eq 1c). However, experiments on the triple quadrupole instrument (TQ) permitted detection of ions with low m/z (m/z ≥10) and also allowed a qualitative determination of how the collision energy influenced the relative yields of the different product ions. Indeed, Cl− loss was observed (m/z 35, eq 1c, Figure 1b), although at the lowest CID energy examined (5 V; Figure S7 (Supporting Information)) the abundance of Cl− was less than that of the organomagnesate [MeMgCl2]− (m/z 109, eq 1a). Loss of MeCO2− (eq 1b) was observed at a higher collision voltage (40 V; Figure S7), in addition to [MgCl2]•− (m/z 94). The latter most likely arises via loss of Me• from [MeMgCl2]−.

(3)

The lower energy CID spectra of each of the magnesium acetates [MeCO2MgX2]− in both the linear ion trap and triplequadrupole mass spectrometer are shown in Figure 1, while higher energy CID spectra from the triple-quadrupole mass spectrometer are given in Figure S7 (Supporting Information). In all cases the desired organomagnesates [MeMgX2]− (1a1− 1a3) were formed, although the yields varied considerably and competing halide loss (eq 1c, Scheme 1) was observed to different extents. The DFT calculated decarboxylation reaction and the energetics for this and the other competing pathways are shown for [MeCO2MgCl2]− in Figure 2 and are tabulated 2322


Organometallics

Article

Table 1. Experimental Observations (LTQ and Low Energy, 10 V, TQ) and Relative Calculated Energies (eV) for Competing Fragmentation Reactions of [MeCO2MgX2]− [MeMgX2]− + CO2a

[MeCO2MgX] + X−b

MeCO2− + MgX2b

X

exptl observation

energy (theory)

exptl observation

energy (theory)

exptl observation

energy (theory)

Clc Br I

major minor not obsd

2.22 2.22 2.22

major major major

2.40 2.12 1.81

not obsd not obsd not obsd

3.42 3.42 3.44

a

Activation energy (eV) for decarboxylation (Scheme 1, eq 1a). bReaction endothermicity (eV) for carboxylate anion and halide loss (assumed as barrierless; Scheme 1, eqs 1b and 1c). Experimental observations are taken from the TQ results. cPredicted energies for the Cl system using the 631+G(d) basis set are presented. The SDD basis set was used for the halides (Br and I) for concordant comparisons; however, since this basis set is not optimized for X = Cl, the results obtained were not used here. The basis set used for Cl was 6-31+G(d).

Table 2. Experimental Observations (LTQ and Low Energy, 10 V, TQ) and Relative Calculated Energies (eV) for the Competing Fragmentation Reactions of [RCO2MgCl2]− a [RMgCl2]− + CO2b

[RCO2MgCl]− + Cl−c,d

RCO2− + MgCl2e

R

exptl observation

energy (theory)

exptl observation

energy (theory)

Me Et Pr iPr tBu vinyl allyl HCC Ph PhCH2 PhCH2CH2

major minor minor minor very minor major major major major major major

2.22 2.42 2.44 2.53 2.64 2.20 2.05 1.34 2.27 2.08 2.32

major major major major major minor minor very minor minor very minor major

2.40 2.40 2.41 2.40 2.41 2.40 2.46 2.57 2.42 2.44 2.46

exptl observation not not not not not not not not not not not

obsd obsd obsd obsd obsd obsd obsd obsd obsd obsd obsd

energy (theory) 3.42 3.40 3.38 3.37 3.35 3.31 3.26 3.03 3.22 3.19 3.29

a

The relative abundances given were extracted from the TQ spectra. bActivation energy (eV) for decarboxylation (Scheme1, eq 1a). cReaction endothermicity (eV) for carboxylate anion and Cl. loss (assumed as barrierless; Scheme1, eqs 1b and 1c). dTQ experiments with 10 V collision energy were used. eThese ions were observed with the CID in the TQ at 40 V energy.

Figure 1c) together with ions arising from Br− loss (m/z 79, eq 1b) and the formation of the magnesium hydroxide [HOMgBr2]− via a hydrolysis reaction (m/z 199, eq 3). We were not able to establish whether there is any loss of the acetate anion due to the low-mass cutoff of the LTQ ion trap. Experiments on the triple-quadrupole (TQ) mass spectrometer allowed us to look for the formation of acetate and to examine product formation at a range of collision energies. At 10 V collision energy (Figure 1d) only Br− and [MeMgBr2]− are formed, with the latter being less abundant. At high collision energies (40 V; Figure S7 (Supporting Information)) a minor loss of MeCO2− is also observed. The DFT calculations (Table 1) support these experimental results with the barrierless Br− loss (2.12 eV) being predicted to be slightly lower in energy than decarboxylation (2.22 eV) while barrierless loss of the acetate anion is calculated to be considerably more endothermic at 3.42 eV (Table S2 (Supporting Information)) and thus is only observed in the high-energy CID experiments (40 V). (iii). X = I. CID of [MeCO2MgI2]− (m/z 337) in the ion trap only results in the loss of I− (Figure 1e), consistent with the DFT predicted energetics. Thus, the barrierless loss of I− is the least endothermic pathway, 0.41 eV lower than the decarboxylation transition state and 1.63 eV lower than the loss of acetate (Table S3 (Supporting Information)). Similar results were obtained in the TQ experiments at both low (10 V; Figure 1f) and high (40 V; Figure S7 (Supporting Information)) collision energies, with a weak peak corresponding to the decarboxylation channel observed at 40 V.

Although DFT calculations have previously addressed the energetics associated with decarboxylation (eq 1a) and acetate loss (eq 1b) for [MeCO2MgCl2]−,9a the observation of a competitive Cl− loss has prompted us to recalculate all three competing pathways, and the results are given in Figure 2 and Table 1. The DFT calculated competing reaction pathways (Figure 2a) are consistent with the low-energy CID results (Figure 1b, Figure S7 (Supporting Information)). The activation energy for decarboxylation (2.22 eV) is predicted to be slightly lower in energy than the barrierless Cl− loss (2.40 eV), consistent with the slight experimental preference for the formation of [MeMgCl2]− over Cl−. Loss of MeCO2− is much higher in energy (3.42 eV) and is only observed experimentally at much higher collision energies (40 V; Figure S7). Figure 2b shows the key species associated with the decarboxylation reaction. The global minimum of [MeCO2MgCl2]− (1a1) involves coordination of the acetate in a bidentate fashion.31 Decarboxylation proceeds via the fourcentered transition state, involving the transfer of the Me group from the carboxyl moiety to the electrophilic MgCl2 center (TS1a1-9), to yield the weakly bound product complex 9, which upon loss of CO2 gives the trigonal-planar organomagnesate [MeMgCl2]− (2a1). The thermodynamically favored ligand loss channel (Cl− rather than MeCO2−) may be due to the preference for Mg to maximize its coordination environment.32 Thus, [MeCO2MgCl] formed via Cl− loss is trigonal planar due to the bidentate acetate, while loss of MeCO2− gives linear [MgCl2] (Figure 2b). (ii). X = Br. CID of [MeCO2MgBr2]− (m/z 241) in the LTQ yields the organomagnesate [MeMgBr2]− (m/z 197, eq 1a, 2323


Organometallics

Article

Figure 3. Illustration of the role of R and X on the fragmentation reactions of [RMgX2]− in low-energy LTQ CID spectra: (a) R = PhCH2CH2 and X = Cl; (b) R = PhCH2 and X = Cl; (c) R = PhCH2 and X = Br; (d) R = PhCH2 and X = I. An asterisk (*) denotes the mass-selected ion, while a pound sign (#) denotes the hydrolysis product [HOMgX2]− from background reactions of [RMgX2]− with water (eq 3).

(b). Formation of Other Organomagnesates [RMgX2]− (X = Cl, Br, I). Table 1 highlights two important trends: (i) the activation energy for decarboxylation of the acetate ligand in [MeCO2MgX2]− is virtually identical for X = Cl, Br, I but (ii) the energy barrier for halide loss significantly decreases in the order Cl > Br > I (2.40, 2.12, and 1.81 eV, respectively), consistent with a decrease in the Mg−X bond strengths. These results emphasize the importance of the ancillary halide ligand in the formation of gas-phase mononuclear organomagnesates via decarboxylation. Given the weaker Mg−Br and Mg−I bond strengths, we first use mass spectrometry experiments and DFT calculations to examine how the R group influences the fragmentation reactions of [RCO2MgCl2]− (1b1−1k1 of Scheme 2). We then use triple-quadrupole mass spectrometry experiments to probe whether decarboxylation can occur in a range of bromides [RCO2MgBr2]− (1b2−1k2 of Scheme 2) and iodides [RCO2MgI2]− (1b3−1k3 of Scheme 2). (i). X = Cl. Given that the types of reaction pathways in the CID spectra and the general features of the DFT calculated potential energy diagrams for [RCO2MgCl2]− are similar to those of [MeCO2MgCl2]−, these are not presented individually here. Rather, Table 2 summarizes key experimental and DFT calculated data for the fragmentation reactions of all [RCO2MgCl2]−, while the Supporting Information contains the low-energy linear ion trap CID spectra (Figures S2) and the CID experiments on the triple quadrupole instrument (10 V and 20 V: Figure S3a,b). A comparison of the experiment and theory for each of the three potential fragmentation channels (eqs 1a−1c of Scheme 1) reveals good agreement. Loss of the carboxylate anion is not observed, consistent with it being the highest predicted fragmentation channel. The DFT calculations highlight the role of R in several trends: (i) the activation energy for decarboxylation increases in the order Me < Et < iPr < tBu (2.22, 2.42, 2.53, and 2.64 eV, respectively), consistent with an increase in the steric bulk of the alkyl group, (ii) the

activation energy for decarboxylation decreases in the order sp3 > sp2 > sp (CH3CH2, H2CCH, HCC; 2.42, 2.20, and 1.34 eV, respectively) consistent with an increase in the stabilizing effect of the respective carbanions, R−, on the polar-covalent organomagnesates,33 (iii) the energy barrier for Cl− loss vary little across the R groups, with an exception of the electronwithdrawing sp hybridized acetylide (HCC) (2.57 eV), and (iv) although carboxylate anion loss is not observed, the DFT calculations suggest that both the size and hybridization of the R group have a stabilizing effect on the energetics for this channel (size effects Me > Et > iPr > tBu, 3.42, 3.40, 3.37, and 3.35 eV, respectively; hybridization CH3CH2 > H2CCH > HCC, 3.40, 3.31, and 3.03 eV, respectively). Furthermore, where known, the DFT predicted energetics for loss of RCO2− (eq 1b) track the experimentally determined gas-phase acidities of RCO2H.34 The data from Table 2 highlight that both the steric bulk and hybridization of the R substituent influence the relative yields of [RMgCl2]−. This is apparent, for example, in the very small amount of [tBuMgCl2]− (m/z 151) generated from sterically hindered [tBuCO2MgCl2]− (m/z 195) (Figure S8 (Supporting Information)). In contrast, [HCCCO2MgCl2]− (m/z 163) undergoes clean decarboxylation to give high yields of [HC CMgCl2]− (m/z 119) under relatively low energy CID conditions in the LTQ and TQ (cf. Figures S2 and S3 (Supporting Information)). Decarboxylation of [HC CCO2MgCl2]− is the most favored out of all R groups, a consequence of the stability of HCC− and the sterically unhindered linear R group. It is interesting to note that facile metal-mediated decarboxylation of propiolic acids has been observed in both the condensed and gas phases.35 (ii). X = Br, I. Similarly to the case of Cl, the low-energy CID reactions of all the [RCO2MgBr2]− and [RCO2MgI2]− ions shown in Scheme 2 were examined in the LTQ and TQ (10 and 20 V) instruments. However, given the issues of the low2324


Organometallics

Article

Table 3. Experimental Observations and Relative Calculated Energies (eV) for the Competing Fragmentation Reactions of [RMgCl2]− (a) R− + MgCl2b

R• + [MgCl2]•−b

RMgCl + Cl−c

[HMgCl2]− + RHc,d

R

exptl observation

energy (theory)

exptl observation

energy (theory)

exptl observation

energy (theory)

exptl observation

energy (theory)

Me Et Pr iPr tBu Vinyl Allyl HCC Ph PhCH2 PhCH2CH2

N.O. N.O. N.O. N.O. N.O. N.O. N.O. N.O. N.O. very minor N.O.

4.01 4.01 3.86 3.81 3.52 3.81 3.06 3.37 3.59 2.84 3.65

minor N.O. N.O. N.O. N.O. N.O. major N.O. N.O. major very minor

2.71 2.38 2.43 2.16 2.04 3.18 2.20 5.13 3.34 2.38 2.62

major minor very minor N.O. N.O. major very minor major major N.O. N.O.

2.24 2.26 2.26 2.30 2.36 2.41 2.47 2.56 2.54 2.53 2.41

N.P. major major major major major minor N.P. N.P. N.P. major

N.A. 1.50 1.42 1.50 1.56 2.36 2.11 N.A. N.A. N.A. 1.50

a The results are a combination of TQ and LTQ observations. N.O. = not observed; N.P. = not possible. bReaction endothermicity (eV) for bond heterolysis, bond homolysis, and halide loss (assumed as barrierless) (cf. eqs 2a−2c, Scheme 1). cActivation energy (eV) for β-hydride transfer (c.f. eq 2d, Scheme 1). dSystems without β-hydrogens were not calculated (denoted N.A.).

observed in the linear ion trap. For X = Cl, β-hydride transfer (eq 2d) and β-phenide transfer (eq 4) occur when R = PhCH2CH2 (Figure 3a), while bond homolysis (eq 2b) is the dominant reaction for R = PhCH2 (Figure 3b). For R = PhCH2 halide loss (eq 2c) becomes more important on moving from X = Br (Figure 3c) to X = I (Figure 3d), highlighting the effect of the Mg−X bond strength on the competition between bond homolysis and halide loss.

mass cutoff of the LTQ, whereby peaks with m/z values less than ca. one-third of the parent cannot be observed, only the 10 and 20 V CID reactions using the TQ are given for all the [RCO2MgBr2]− and [RCO2MgI2]− species shown in Scheme 2. An examination of these CID spectra (Figures S4 and S5 (Supporting Information)) reveals that the halide loss channel is favored in most cases. Exceptions for the bromides include R = allyl, HCC, PhCH2, while for the iodides the only exception is for R = HCC. In these cases, formation of the organomagnesates [RMgX2]− was favored over the halide loss channel. In addition, several other organomagnesates could be formed to a lesser extent in both cases (Figures S4 and S5) . Thus, in the case of X = Br, [RMgX2]− was observed for all R groups. However, [EtMgBr2]−, [PrMgBr2]−, [iPrMgBr2]−, and [tBuMgBr2]− were only accessed upon high-energy CID (40 V) (Figure S4c). In the case of X = I and due to the fact that the Mg−I bond is the weakest among the Mg−X bonds studied, only the organomagnesates [RMgI2]− with R = allyl, vinyl, Ph, PhCH2 were formed, with the last three only being formed at high CID energy (40 V, Figure S5c). While we have not used DFT calculations to explore the energetics for all the fragmentation channels for the bromides and iodides, these experimental results are entirely consistent with the related systems calculated for [MeCO2MgX2]− (Table 1) and [RCO2MgCl2]− (Table 2). Since the DFT calculations on [RCO2MgCl2]− reveal little influence of R on the energetics for chloride loss, the energy for halide loss is expected to be around 2.1 eV for [RCO2MgBr2]− and 1.8 eV for [RCO2MgI2]− (cf. Table 1). Thus, facile organomagnesate formation is most likely for systems that have activation energies around or below the energies for the halide loss channels. Therefore, on the basis of the activation energies calculated for decarboxylation of the chloride systems, the following organomagnesates are expected to be readily formed: (i) bromides with R = allyl (∼2.0 eV, cf. Table 2), HCC (∼1.3 eV, cf. Table 2), PhCH2 (∼2.1 eV, cf Table 2) and (ii) only the iodide [HCCMgI2]−. (2). Fragmentation of the Organomagnesate [RMgX2]−. A range of organomagnesates [RMgX2]− (2a1− 2k1, 2a2−2k2, 2f3−2j3) were subjected to CID to examine which of the four possible fragmentation pathways shown in Scheme 1 operate for a given system. Figure 3 illustrates how both R and X influence the types of fragmentation pathways

[PhCH 2CH 2MgCl2]− → [PhMgCl2]− + CH 2CH 2

(4)

We have used linear ion trap and triple-quadrupole mass spectrometry experiments together with DFT calculations to further consider these effects for other combinations of R and X. The halide effect is discussed first for [MeMgX2]−, followed by a discussion on the fragmention reactions of the other [RMgX2]− species, focusing on the case of X = Cl. (a). Fragmentation of [MeMgX2]−. (i). X = Cl. [MeMgCl2]− loses Me• upon CID in the ion trap to generate [MgCl2]−• (Figure S9 (Supporting Information), eq 2b). In order to establish whether Cl− loss also occurs, TQ experiments were performed. These showed that loss of Cl− (eq 2c) is more prominent than loss of Me• (Figure S10 (Supporting Information)). This result is in agreement with the DFT calculated reaction endothermicities, where the lowest energy pathway was the loss of Cl− (2.24 eV) followed by the homolytic cleavage to form [MgCl2]•− (2.71 eV) (cf. Table 3). Loss of CH3− was not observed, consistent with its much higher calculated endothermicity (4.01 eV). (ii). X = Br. Upon CID of [MeMgBr2]−, loss of Br− (eq 2c) prevails with a small loss of Me• to yield [MgBr2]•− (2b), observed in both the LTQ and TQ experiments (Figure S11 and S12 (Supporting Information)). This is also in accordance with the DFT calculations results, where the loss of Br− was the lowest energy pathway (eq 2c, 2.16 eV) followed by loss of Me• (eq 2b, 2.73 eV, Table S5 (Supporting Information)). (b). Fragmentation of Other [RMgCl2]−. A survey of the fragmentation pathways of all [RMgCl2]− formed revealed that potentially all channels (eqs 2a−d) can be observed experimentally (cf. Figure S10 (Supporting Information)). Table 3 summarizes the experimental and DFT data for all systems, while the DFT predicted potential energy diagram for β-hydride transfer in competition with other pathways is 2325


Organometallics

Article

expelled. Second, the formation of [MgCl2]•− is no longer favored for substituents containing a β-hydrogen, a consequence of β-hydride transfer being the lowest energy pathway. For example, [PhCH2CH2MgCl2]− mainly fragments via βhydride transfer in the linear ion trap (Figure 3a), consistent with the DFT calculations, which predict this to be the lowest energy pathway (1.50 eV). Finally, CID of [PhMgCl2]− (m/z 171) in the linear ion trap resulted in a loss of ion signal with no ionic reaction products from bond homolysis or β-hydride transfer being observed. This is presumably because the former expels a destabilized σ radical, Ph•, while the latter yields a highenergy species: i.e., benzyne. Low-energy CID experiments on the triple-quadrupole mass spectrometer show a signal corresponding to loss of Cl− (Figure S10a (Supporting Information)), which is predicted by the DFT calculations to be the lowest energy pathway for the [PhMgCl2]− system (2.54 eV). In summarizing the results for all [RMgX2]− systems, one can find that the formation of [MgX2]•− requires a combination of: (i) conjugatively stabilized R•, (ii) R substituents without βhydrogens, and (iii) strong Mg−X bonds. However, the formation of [MgI2]•− from [PhCH2MgI2 ]− via bond homolysis (c.f. eq 2b) demonstrates that the stabilizing effect of the R• substituent in facilitating the bond homolysis reaction takes precedence over the effect of the Mg−X bond strength in promoting other competing channels. (c). [PhCH2CH2MgCl2]−: The Special Case of β-Hydride versus β-Phenide Transfer. CID of mass-selected [PhCH2CH2MgCl2]− (m/z 199) in the ion trap yielded two products. The major product was [HMgCl2]− (m/z 95, eq 2d, Figure 3a)36 from β-hydride transfer with concomitant elimination of PhCHCH2.17c−g A minor product corresponding to the formation of the phenylmagnesate [PhMgCl2]− (m/z 171) arises from expulsion of ethylene via a β-phenide transfer reaction (eq 4).37,38 The β-hydride fragmentation channel dominated the low-energy (10 V) CID spectrum in the triplequadrupole mass spectrometer (Figure S10a (Supporting Information)). At higher CID energies (20 and 40 V), Cl− loss (eq 2c) and bond homolysis to form [MgCl2]•− (eq 2b) are also observed in competition with β-hydride transfer (Figure S10b,c (Supporting Information)). DFT calculations were carried out to predict the energetics associated with the various possible fragmentation channels of [PhCH2CH2MgCl2]− (eqs 2a−2d of Scheme 1 and eq 4). The resultant potential energy diagram shown in Figure 4a supports these experimental results. Thus, β-hydride transfer (TS(2k110); 1.50 eV) is predicted to be favored over β-phenide transfer (TS(2k1-11); 1.73 eV) on kinetic rather than thermodynamic grounds, and both of these channels are observed in the linear ion trap (Figure 3a), with the β-hydride pathway dominating. The DFT calculations highlight that Cl− loss (7k1; 2.41 eV) and bond homolysis (6k1,; 2.62 eV) are the next most likely pathways to occur, and these are indeed observed, albeit as minor peaks in the higher energy CID TQ experiments (Figures S10b,c (Supporting Information)). Loss of the carbanion PhCH2CH2− (5k1) is much higher in energy, with a predicted energy barrier of 3.65 eV and is not observed, even at the highest CID energy used (40 V; Figure S10c (Supporting Information)).

illustrated for R = PhCH2CH2 in Figure 4 and discussed separately below.

Figure 4. B3LYP/6-31+G(d) calculated (a) relative energies (eV) for the fragmentation of [PhCH2CH2MgCl2]− and (b) structures, including bond lengths (Ǻ ), for β-hydride transfer and β-phenide transfer of [PhCH2CH2MgCl2]−.

An examination of Table 3 reveals the effects of R in several trends: (i) loss of R− is consistently the highest energy pathway, with the exception of the acetylide group, where bond homolysis takes the highest energy barrier due to the conjugatively destabilized HCC•, (ii) both steric bulk and increase in s character of the alkyl group have a stabilizing effect on the energy barrier for R− loss (Me > Et > iPr > tBu, 4.01, 4.01, 3.81, and 3.52 eV, respectively; hybridization CH3CH2 > H2CCH > HCC, 4.01, 3.81, and 3.37 eV, respectively), (iii) the energy barrier for bond homolysis is predicted to increase in the order sp3 < sp2 < sp (2.38, 3.18, and 5.13 eV, respectively), consistent with a decrease in the conjugative stabilization of the alkyl radicals expelled upon bond homolysis, and (iv) β-hydride transfer is predicted to always be the lowest energy pathway for systems containing a β-hydrogen atom, consistent with it being the experimentally favored reaction channel. The theoretical results described in Table 3 indicate that the R substituent affects the competition between bond homolysis and other pathways in two aspects which are best illustrated by observing the fragmentation of the aryl systems: Ph, PhCH2, and PhCH2CH2. First, the stability of the alkyl radical expelled can either promote or prevent the bond homolysis channel dominating. For instance, [PhCH2MgCl2]− fragments via bond homolysis to generate high yields of [MgCl2]•− (Figure 3a). This is proposed to arise from the stability of the benzyl radical

■

CONCLUSIONS This study is the first to have examined the role of both the R group and the halide on the gas-phase formation and 2326


Organometallics

Article

for a Europe travel fellowship and JCT for a visiting scientist award at UPMC. We thank Dr. Tom Waters for useful discussions and Ann Jacob for carrying out preliminary calculations.

fragmentation reactions of mononuclear organomagnesates [RMgX2]− using a combination of experiments carried out on different types of mass spectrometers and DFT calculations. First the competition between decarboxylation (eq 1a of Scheme 1) and ligand loss channels (eqs 1b and 1c) was examined for the 33 different magnesium carboxylate precursors [RCO2MgX2]− shown in Scheme 2. The DFT calculations predict that loss of the carboxylate RCO2− (eq 1b) is substantially higher in energy than either decarboxylation (eq 1a) or halide loss (eq 1c), consistent with it not being observed in the low-energy CID experiments for both types of mass spectrometers (Tables 1 and 2). In contrast, halide loss (eq 1c) was found to directly compete with decarboxylation (eq 1a), although the nature of both R and X influence the relative yields of these pathways. The stronger Mg−Cl bond, which varies little (2.40−2.57 eV) for the 11 carboxylates [RCO2MgCl2]− studied, allowed the decarboxylation reaction to be a major pathway for 10 of the carboxylates. Both the sterics and hybridization of the R substituent play a role in tuning the barrier height for the decarboxylation channel, which varies substantially (1.34−2.64 eV). The sterically bulky tBu group has the highest calculated decarboxylation barrier (2.64 eV), consistent with [tBuMgCl2]− being a minor product. The lowest predicted barrier (1.34 eV) is for the sp-hybridized acetylide, consistent with [HCCMgX2]− being a major product for all halides studied. A total of 27 out of the possible 33 mononuclear organomagnesates [RMgX2]− were formed, and their fragmentation reactions were also studied. Bond homolysis only occurs for [RMgX2]− containing a stabilized R• group (e.g., R = PhCH2) and provides access to all three novel Mg(I) halides, [MgX2]•−. Bond heterolysis to form the carbanion is rarely observed, and only then for stabilized carbanions (e.g., R = PhCH2). For R groups possessing βhydrogens, β-hydride transfer is often observed. Halide loss is a common pathway, which can directly compete with β-hydride transfer. Now that access to a wide range of organomagnesates [RMgX2]− is available, there is the opportunity to examine their bimolecular reactivity toward a range of substrates. Such studies are underway and will be reported in due course.

■

■

(1) For the presentation speech of Francois Auguste Victor Grignard’s 1912 Nobel prize, see: http://www.nobel.se/chemistry/ laureates/1912/press.html. For an essay on Grignard reagents see: Seyferth, D. Organometallics 2009, 28, 1598. (2) (a) Wakefield, B. J. Organomagnesium Methods in Organic Synthesis; Academic Press: London, 1995. (b) Handbook of Grignard Reagents; Silverman, G. S., Rakita, P.E., Eds.; Dekker: New York, 1996. (c) Grignard Reagents: New Developments; Richey, H. G., Jr., Ed.; Wiley: Chichester, U.K., 2000. (d) The Chemistry of Organomagnesium Compounds; Rappoport, Z., Marek, I., Eds.; Wiley: Chichester, U.K., 2008. (3) Yorimitsu, H.; Oshima, K. In The Chemistry of Organomagnesium Compounds; Rappoport, Z., Marek, I., Eds.; Wiley: Chichester, U.K., 2008; Chapter 15, p 681. (4) (a) Krasovskiy, A.; Knochel, P. Angew. Chem., Int. Ed. 2004, 43, 3333. (b) Krasovskiy, A.; Straub, B. F.; Knochel, P. Angew. Chem., Int. Ed. 2006, 45, 159. (5) The Turbo Grignard, formulated as iPrMgCl·LiCl, was awarded the EROS Best Reagent Award 2011 and is part of a fascinating group of heterometallic complexes that continue to attract considerable attention. For recent reviews see: (a) Mulvey, R. E. Organometallics 2006, 25, 1060. (b) Mulvey, R. E.; Mongin, F.; Uchiyama, M.; Kondo, Y. Angew. Chem., Int. Ed. 2007, 46, 3802. (c) Mulvey, R. E. Acc. Chem. Res. 2009, 42, 743. (d) Hevia, E.; Mulvey, R. E. Angew. Chem., Int. Ed. 2011, 50, 6448. (e) O’Hara, C. Synergistic effects in the activation of small molecules by s-block elements. In: Organometallic Chemistry; Royal Society of Chemistry: London, 2011; p 1. For ongoing discussions on the structure and reactivity of Turbo Grignards see: (f) Garcia-Alvarez, P.; Mulvey, R. E.; Parkinson, J. A. Angew. Chem., Int. Ed. 2011, 50, 9668. (g) Blasberg, F.; Bolte, M.; Wagner, M.; Lerner, H.-W. Organometallics 2012, 31, 1001. (6) Cahiez, G.; Duplais, C. in The Chemistry of Organomagnesium Compounds; Rappoport, Z., Marek, I., Eds.; Wiley: Chichester, U.K., 2008; Chapter 13, p 595. (7) (a) Ertel, T.S.; Bertagnolli, H. in Grignard Reagents: New Developments, (Ed.: Richey, H. G.; , Jr.), Wiley, Chichester, 2000, Chapter 10; (b) Lioe, H.; White, J. M.; O’Hair, R. A. J. J. Mol. Mod. 2011, 17, 1325. (8) For a review on the formation, chemistry, and structure of organomagnesiums in solvent-free environments see: O’Hair, R. A. J. In The Chemistry of Organomagnesium Compounds; Rappoport, Z., Marek, I., Eds.; Wiley: Chichester, U.K., 2008,; Chapter 4, p 155. (9) (a) O’Hair, R. A. J.; Vrkic, A. K.; James, P. F. J. Am. Chem. Soc. 2004, 126, 12173. (b) Jacob, A. P.; James, P. F.; O’Hair, R. A. J. Int. J. Mass Spectrom. 2006, 255−256, 45. (c) Thum, C. C. L.; Khairallah, G. N.; O’Hair, R. A. J. Angew. Chem., Int. Ed. 2008, 47, 9118. (d) Khairallah, G. N.; Thum, C.; O’Hair, R. A. J. Organometallics 2009, 28, 5002. (e) Khairallah, G. N.; Yoo, E. J. H.; O’Hair, R. A. J. Organometallics 2010, 29, 1238. (f) Leeming, M. G.; Khairallah, G. N.; da Silva, G.; O’Hair, R. A. J. Organometallics 2011, 30, 4297. (10) O’Hair, R. A. J. Chem. Commun. 2006, 1469. (11) For a review on the use of theory to examine the reactivity of organomagnesiums see: Yamabe, S., Yamazaki, S. In The Chemistry of Organomagnesium Compounds; Rappoport, Z., Marek, I., Eds.; Wiley: Chichester, U.K., 2008; Chapter 9, p 369. (12) For reviews on ESI/MS of organometallics see: (a) Traeger, J. C. Int. J. Mass Spectrom. 2000, 200, 387. (b) Henderson, W.; McIndoe, J. S. Mass Spectrometry of Inorganic and Organometallic Compounds: Tools - Techniques - Tips; Wiley: Chichester, U.K., 2005. (13) For reports on the use of MS to study the composition of Grignard reagents see: (a) Sakamoto, S.; Imamoto, T.; Yamaguchi, K. Org. Lett. 2001, 3, 1793. (b) Tjurina, L. A.; Smirnov, V. V.; Barkovskii,

ASSOCIATED CONTENT

S Supporting Information *

Text, tables, and figures giving LTQ and TQ CID mass spectra, Cartesian coordinates, energies, and vibrational frequencies for reactants, intermediates, products, and transition states, and the full citation for ref 23. This material is available free of charge via the Internet at http://pubs.acs.org.

■

REFERENCES

AUTHOR INFORMATION

Corresponding Author

*R.A.J.O.: tel, +61 3 8344-2452; fax, +61 3 9347-5180; e-mail, [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank the ARC for financial support via Grant Nos. DP0558430 (to R.A.J.O.), DP1096134 (to G.N.K.) and DP110103844 (to R.A.J.O. and G.N.K.) and through the Centres of Excellence Program. VICS is acknowledged for the Chemical Sciences High Performance Computing Facility (Gomberg). G.N.K. thanks the Australian Academy of Science 2327


Organometallics

Article

G. B.; Nikolaev, E. N.; Esipov, S. E.; Beletskaya, I. P. Organometallics 2001, 20, 2449. (14) Koszinowski has successfully examined organometallates via ESI/MS: (a) Koszinowski, K.; Boehrer, P. Organometallics 2009, 28, 100. (b) Koszinowski, K.; Boehrer, P. Organometallics 2009, 28, 771. (c) Putau, A.; Koszinowski, K. Organometallics 2010, 29, 3593. (d) Putau, A.; Koszinowski, K. Organometallics 2011, 30, 4771. (e) Fleckenstein, J. E.; Koszinowski, K. Organometallics 2011, 30, 5018. (f) Putau, A.; Brand, H.; Koszinowski, K. J. Am. Chem. Soc. 2012, 134, 613. (g) Koszinowski, K.; Müller, C.; Brand, H. J. E. Fleckenstein. Organometallics 2012, 31, 7165. (15) O’Hair, R. A. J. In MS investigations of reactive intermediates in solution; Santos, L. S., Ed.; Wiley-VCH: Weinheim, Germany, 2010; Chapter 6, p 199. (16) Decarboxylation has been used to form organometallics both stoichiometrically and catalytically. For reviews 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. (c) Goossen, L. J.; Goossen, K.; Rodriguez, N.; Blanchot, M.; Linder, C.; Zimmermann, B. Pure Appl. Chem. 2008, 80, 1725. (d) Goossen, Rodriguez, N.; Goossen, K. Angew. Chem., Int. Ed. 2008, 47, 3100. (17) (a) Bryce-Smith, D.; Cox, G. F. J. Chem. Soc. 1958, 1050. (b) Holm, T. J. Chem. Soc., Perkin Trans. 2 1981, 464. (c) Wiberg, E.; Bauer, R. Z. Naturforsch. 1950, 56, 396. (d) Wiberg, E.; Bauer, R. Chem. Ber. 1952, 85, 593. (e) Zakharkin, L. I.; Okhlobystin, O. Y.; Strunin, B. N. J. Organomet. Chem. 1965, 4, 349. (f) Lefrancois, M.; Gault, Y. J. Organomet. Chem. 1969, 16, 7. (g) Ashby, E. C.; Kovar, R. A.; Kawakami, K. Inorg. Chem. 1970, 9, 317. (h) Ziegler, K.; Nagel, K.; Patheiger, M. Z. Anorg. Allg. Chem. 1955, 282, 345. (i) Maercker, A.; Weber, K. Justus Liebigs Ann. Chem. 1972, 756, 33. (j) Wagner, B. O.; Hammond, G. S. J. Organomet. Chem. 1975, 185, 1. (18) (a) Rijs, N. J.; Yates, B. F.; O’Hair, R. A. J. Chem. Eur. J. 2010, 16, 2674. (b) Rijs, N. J.; O’Hair, R. A. J. Organometallics 2010, 29, 2282. (c) Rijs, N. J.; Sanvido, G.; Khairallah, G. N.; O’Hair, R. A. J. Dalton Trans. 2010, 39, 8655. (d) 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. (19) Feketeova, L.; Khairallah, G. N.; O’Hair, R. A. J. Eur. J. Mass. Spectrom. 2008, 14, 107. (20) Dossmann, H.; Afonso, C.; Lesage, D.; Tabet, J.-C.; Uggerud, E. Angew. Chem., Int. Ed. 2012, 51, 6938. (21) For an excellent monograph on tandem mass spectrometry that discusses the differences between “tandem in time” and “tandem in space” mass spectrometers see: Busch, K. L.; Glish, G. L.; McLuckey, S. A. Mass Spectrometry/Mass Spectrometry: Techniques and Applications of Tandem Mass Spectrometry; VCH: New York, 1988. (22) Weinmann, W.; Stoertzel, M.; Vogt, S.; Svoboda, M.; Schreiber, A. J. Mass Spectrom. 2001, 36, 1013 and references cited therein. (23) Frisch, M. J., et al. Gaussian_03; Gaussian, Inc., Pittsburgh, PA, 2003. (24) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623. (25) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54, 724. (26) (a) Petersson, G. A.; Allaham, M. A. J. Chem. Phys. 1991, 94, 6081. (b) Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; Schleyer, P. V. J. Comput. Chem. 1983, 4, 294. (27) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502. (28) Ye, J. L.; Huang, P. Q.; Lu, X. J. Org. Chem. 2007, 72, 35. (29) (a) Markies, P. R.; Akkerman, O. S.; Bickelhaupt, F.; Smeets, W. J. J.; Spek, A. L. 1991, 32, 147. (b) Weiss, E. Angew. Chem., Int. Ed. 1993, 32, 1501. (c) Holloway, C. E.; Melnik, M. Coord. Chem. Rev. 1994, 135, 287. (d) Holloway, C. E.; Melnik, M. J. Organomet. Chem. 1994, 465, 1. (e) Uhm, H. L. In Handbook of Grignard Reagents; Silverman, G. S., Rakita, P. E., Eds.; Dekker: New York, 1994; p 117. (f) Bickelhaupt, F. In Grignard Reagents: New Developments; Richey, H. G., Ed.; Wiley: Chichester, U.K., 2000; p 299. (g) Fromm, K. M.; Gueneau, E. D. Polyhedron 2004, 23, 1479. (h) Jastrzebski, J. T. B. H.; Boersma, J.; van Koten, G. In The chemistry of Organomagnesium

Compounds; Rappoport, Z., Marek, I., Eds.; Wiley: Chichester, U.K., 2008; p 1. (i) Hanusa, T. P. In Comprehensive Organometallic Chemistry III; Mingos, D. M., Crabtree, R. H., Eds.; Elsevier: Amsterdam, 2007; Vol. 2, pp 78−114. (j) Hill, E. A. In Encyclopedia of Inorganic Chemistry; King, R. B., Ed.; Wiley: Chichester, U.K., 1994; Vol. 1, pp 245−267. (30) Møller, C.; Plesset, M. S. Phys. Rev. 1934, 46, 618. (31) Bidentate binding of carboxylates to Mg(II) centers is generally favored over monodentate binding in complexes with low coordination numbers. See: (a) Dudev, T.; Lim, C. Acc. Chem. Res. 2007, 40, 85. (b) Dudev, T.; Lim, C. Annu. Rev. Biophys. 2008, 37, 97. (c) Paterová, J.; Heyda, J.; Jungwirth, P.; Shaffer, C. J.; Révész, A.; Zins, E. L.; Schröder, D. J. Phys. Chem. A 2011, 115, 6813. (32) DFT calculations on model systems suggest that carboxylates in proteins help protect Mg from being dislodged by Cl−: Dudev, T.; Lim, C. J. Am. Chem. Soc. 2006, 127, 10541. (33) Smith, M. B.; March, J. March’s Advanced Organic Chemistry, 5th ed.; Wiley: New York, 2001; Chapter 5. (34) (a) Caldwell, G.; Renneboog, R.; Kebarle, P. Can. J. Chem. 1989, 67, 611. (b) by Bartmess, J.E. Negative Ion Energetics Data. In NIST Chemistry WebBook; Eds. Linstrom, P. J., Mallard, W. G., Eds.; National Institute of Standards and Technology: Gaithersburg, MD, NIST Standard Reference Database Number 69, 20899, http:// webbook.nist.gov (retrieved Nov 30, 2012). (35) For Cu: (a) Kolarovič, A.; Fáberová, Z. J. Org. Chem. 2009, 74, 7199. (b) Kolarovič, A.; Schnürch, M.; Mihovilovic, M. D. J. Org. Chem. 2011, 76, 2613. For Ag: (c) Ruaudel-Teixier, A. Mol. Cryst. Liq. Cryst. 1983, 96, 365. (d) Khairallah, G. N.; Williams, C. M.; Chow, S.; O’Hair, R. A. J. sp-sp3 Coupling Reactions of Alkynylsilver Cations, RCCAg2+ (R = Me and Ph) with Allyliodide. Dalton Trans. 2013, DOI: 10.1039/C2DT32143B. (36) For related magnesium hydrides see: (a) Berlin, K. D.; Snider, T. E.; Dermer, O. C. Tetrahedron Lett. 1970, 46, 3991. (b) Ashby, E. C.; Kovar, R. A.; Kawakami, K. J. Am. Chem. Soc. 1977, 99, 310. (c) Khairallah, G. N.; O’Hair, R. A. J. Int. J. Mass Spectrom. 2006, 254, 145. (d) Mohajeri, A.; Alipour, M.; Mousaee, M. J. Phys. Chem. A 2011, 115, 4457. (37) β-phenide(aryl) transfer reactions are uncommon in organometallic chemistry. For some recent examples see: (a) Zhao, P.; Hartwig, J. F. J. Am. Chem. Soc. 2005, 127, 11618. (b) Zhao, P.; Incarvito, C. D.; Hartwig, J. F. J. Am. Chem. Soc. 2006, 128, 3124. (38) The reverse of the β-phenide transfer reaction is known as carbomagnesiation. For a review see: Itami, K.; Yoshida, J.-I. In The Chemistry of Organomagnesium Compounds; Rappoport, Z., Marek, I., Eds.; Wiley: Chichester, U.K., 2008; Chapter 14, p 631.

2328


Gas-Phase Formation and Fragmentation Reactions of the

Recommend Documents