Article pubs.acs.org/Organometallics
Dimethylcuprate-Mediated Transformation of Acetate to Dithioacetate Jiawei Li, George N. Khairallah, 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, Melbourne, Victoria 3010, Australia S Supporting Information *
ABSTRACT: Dithiocarboxylic acids, RCS2H, and their esters, RCS2R′, are useful reagents that can be synthesized by the reaction of carbon disulfide with organometallic reagents. Here the coinage-metal-mediated transformation of acetate to dithioacetate is explored in the gas phase using multistage mass spectrometry experiments in a linear ion trap mass spectrometer in conjunction with density functional theory (DFT) calculations. The ion− molecule reactions between coinage-metal dimethylmetalate anions [CH3MCH3]− (M = Au, Ag, Cu), formed via double decarboxylation of the metal acetate anions, [CH3CO2MO2CCH3]−, and carbon disulfide, were examined. Only [CH3CuCH3]− reacts with CS2 with a reaction efficiency of 0.8% of the collision rate to yield the adduct [CH3CuS2CCH3]− (77.5%) as well as CH3CS2− (22.5%). Collision-induced dissociation (CID) of the adduct [CH3CuS2CCH3]− gives CH3CS2− as the major product, with a small amount of [CuS2CCH2]− being formed via loss of methane. DFT calculations reveal the following. (i) [CH3CuCH3]− reacts via oxidative addition to form a Cu(III) intermediate, followed by reductive elimination of CH3CS2−, which is captured by Cu to form [CH3CuS2CCH3]−. This energetic adduct can fragment via loss of CH3CS2− or can be collisionally cooled by the helium bath gas used in the experiments. (ii) Loss of CH4 from [CH3CuS2CCH3]− also involves a Cu(III) intermediate and results in formation of the metalladithiolactone [Cu(CH2CS2)]−.
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INTRODUCTION
Dithiocarboxylic acids, RCS2H, and their esters, RCS2R′, are useful reagents in applications ranging from their role as ligands in metal complexes1 through to their use in RAFT polymerization reactions.2 A number of methods have been used to synthesize these reagents,3 of which the reaction of carbon disulfide, CS2,4,5 with an organometallic reagent has proven popular.6−8 Thermally induced metal-catalyzed decarboxylation reactions, in which a carboxylic acid is transformed into an organometallic intermediate which reacts with an organic substrate to undergo C−X bond coupling, are emerging as new synthetic protocols in organic chemistry.9 Although the reactions of a wide range of organic substrates have been probed, the simple metal-catalyzed decarboxylative transformation of carboxylic acids to dithiocarboxylic acids (eq 1) has not yet been reported. RCO2 H + CS2 → RCS2 H + CO2
abstraction (eq 4), and formation of HS− (eq 5).10d In contrast, the bare phenyl anion, also formed via decarboxylation of the benzoate anion (eq 2, where R = Ph) reacts to form dithiobenzoate (eq 6).10i As noted above, organometallic reagents (including organocopper) are known to react with CS2 to promote the desired C−C bond formation. Ph− + CS2 → PhCS2−
We have examined the gas-phase formation of organometallic ions via decarboxylation of metal carboxylate ions12,13 and their subsequent unimolecular14 and bimolecular reactions with organic substrates.15 We have found that of all the coinagemetal dimethylmetalate anions, [CH3MCH3]−, dimethylcuprate (M = Cu) is considerably more reactive in C−C bond coupling reactions with methyl iodide and allyl iodide (eq 7). Here we use a combination of multistage mass spectrometry (MSn) experiments on a linear ion trap mass spectrometer16 and DFT calculations17,18 to probe the gas-phase reactions of coinage-
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What is known about the key step in such a transformation, which requires nucleophilic attack of a carbanion/organometallic agent onto CS2? The gas-phase reactions of carbanions with CS2 have been widely studied.10,11 The bare methyl anion, formed via decarboxylation of the acetate anion (eq 2, where R RCO2− → R− + CO2
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= CH3), is highly reactive toward CS2, giving rise to products from three channels: electron transfer (eq 3), sulfur atom © 2015 American Chemical Society
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Received: November 6, 2014 Published: January 7, 2015 488
DOI: 10.1021/om501117p Organometallics 2015, 34, 488−493
Article
Organometallics [CH3MCH3]− + RI → [CH3MI]− + CH3R
from m/z 50 to 300 for [CH3CuCH3]− and from m/z 50 to 500 for both [CH3AgCH3]− and [CH3AuCH3]−. Theoretical rates for the reaction were calculated with the program COLRATE21 using the average dipole orientation (ADO) theory of Su and Bowers.22 Theoretical Methods. Theoretical calculations were carried out to gain insights into possible mechanisms for the ion−molecule reaction of coinage-metal dimethylmetalates with carbon disulfide. Gaussian 09,23 utilizing the M06 functional,24 was used for all geometry optimizations and vibrational frequency calculations. The Stuttgart Dresden (SDD) basis set was used for the copper, silver, and gold atoms, while the 6-31+G(d) all-electron basis set was used for carbon, sulfur, and hydrogen.25 The combination of basis set and DFT functional was chosen because it was proven to be effective at calculating organometallic reactions. 26 For the reactions of [CH3CuCH3]− with CS2, full geometry optimizations and frequency calculations were also carried out using: (i) the same basis set and the B3LYP functional; (ii) the SDD/6-311++G(2df,p) basis set with either the B3LYP or M06 functionals. The results of these studies are compiled in Table S2 in the Supporting Information. All transition state (TS) geometries were characterized by the presence of a single imaginary frequency. Intrinsic reaction coordinate (IRC) calculations were carried out to connect transition states to reactants and products.
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metal dimethylmetalate anions, [CH3MCH3]−, with carbon disulfide (Scheme 1), which represents a key step in the metalcatalyzed transformation of acetic acid to dithioacetic acid (eq 1, R = CH3).
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EXPERIMENTAL SECTION
Materials. All chemicals were used as received: copper(II) acetate and silver(I) acetate from Aldrich; gold(III) acetate from Alfa Aesar; Acetic acid-d4 from Cambridge Isotope Laboratories; carbon disulfide from Aldrich. Methanol was AR grade (Merck). Mass Spectrometry Experiments. Mass spectrometry experiments were conducted on a Thermo Scientific (Bremen, Germany) LTQ FT hybrid mass spectrometer equipped with a Finnigan ESI (electrospray ionization) source. This instrument consists of a linear ion trap (LTQ) coupled to a Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometer19a and modified to allow ion−molecule reaction studies to be undertaken.19b,c A recent study has demonstrated that, under ion−molecule reaction conditions, collisions with the helium bath gas quasi-thermalizes the ions to room temperature.20 Both collision-induced dissociation (CID) and ion− molecule reactions (IMR) experiments were performed on this instrument. Copper(II) acetate, silver(I) acetate, or gold(III) acetate was dissolved in methanol with acetic acid in a 1:10 molar ratio. Methanolic solutions of metal acetate with metal concentrations of 1.0 mM were injected into the ESI source at a flow rate of 5 μL/min. Typical electrospray source conditions involved needle potentials of 2.5−3.5 kV and a heated capillary temperature of 300 °C. Single isotopes were mass-selected from the appropriate precursor ions and were based on the metal isotopes 63Cu, 107Ag, and 197Au. The metalate anions [CH3MCH3]− (M = Cu, Ag, Au) were formed via MSn experiments with CID on metal acetate anions (Scheme 1) as described previously15c−e and their elemental composition confirmed via high-resolution accurate mass measurement (Supporting Information, Table S1). Acetic acid-d4 was used to synthesize the deuteriumlabeled cuprates [CH3CuCD3]− and [CD3CuCD3]− as described previously.14c Kinetic Modeling. The kinetics for the reaction between dimethylcuprate, [CH3CuCH3]−, and carbon disulfide were examined using the LTQ FT hybrid mass spectrometer. Ion−molecule reaction rate measurements were conducted by isolating the reactant ion [CH363CuCH3]− in a MS4 experiment and then allowing it to react with CS2, similar to previously reported ion−molecule reactions.15 The neutral substrate carbon disulfide was introduced at various concentrations into the ion trap via the helium inlet line. Rates were measured by varying the time delay between isolation of the [CH3CuCH3]− reactant ion and its mass analysis (“reaction delay”, RD). The decay of [CH3CuCH3]− was monitored over at least eight values of RD. The intensity of the reactant ion was calculated by integration of its ion count within the mass-selected window. Pseudofirst-order rates were estimated by extrapolation of plots of −ln(([CH3MCH3]− intensity)/(total ions)) vs RD. Rate constants were calculated by dividing the pseudo-first-order rate coefficient by the calculated concentration of carbon disulfide in the ion trap. Three independent measurements were taken over 3 days. The mass selection window was m/z 1.5, and the scan mass ranges were
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RESULTS AND DISCUSSION Dimethylmetalate anions, [CH3MCH3]−, were generated in the gas phase in a series of MS2 and MS3 experiments (Scheme 1) involving sequential decarboxylation of the acetate complexes [CH3CO2MO2CCH3]− under CID conditions. As this process has been described in detail previously,14a,b,15c the focus herein is on establishing how these organometalates, [CH3MCH3]−, react with CS2. Gas-Phase Ion−Molecule Reactions of [CH3MCH3]− with CS2. The organometalates [CH3MCH3]− were massselected in a series of MS4 experiments and allowed to undergo ion−molecule reactions with carbon disulfide introduced into the LTQ mass spectrometer. Product ion assignments were confirmed by high-resolution mass spectrometry experiments (Supporting Information, Table S1) as well as by using the d e ut e r i u m - l a be l e d a n a lo g u e s [ C H 3 C u CD 3 ] − a n d [CD3CuCD3]− (Figure 1). [CH3CuCH3]− reacts with CS2 with a reaction efficiency (Table 1) of 0.8% to give two main products: the adduct [CH3CuS2CCH3]− (eq 8, 77.5%) and the dithioacetate anion
(eq 9, 22.5%). In contrast, both [CH 3 AgCH 3 ] − and [CH3AuCH3]− were found to be unreactive toward carbon disulfide under the conditions used (Supporting Information, Figure S1b,c). Thus, they are at least 2 orders of magnitude less reactive than [CH3CuCH3]− (Table 1). Finally, the reactivity of
Scheme 1. Gas-Phase Synthesis of Coinage Metal Dimethylmetalate Anions, [CH3MCH3]−, and Their Reaction with Carbon Disulfide via Multistage Mass Spectrometry (MSn) Experiments
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DOI: 10.1021/om501117p Organometallics 2015, 34, 488−493
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Organometallics
Figure 2. LTQ-FT-ICR MS5 spectra of mass selected adduct ions formed in Figure 1 undergoing CID at a normalized collision energy of 35 (arbitrary units) in the linear ion trap: (a) [CH363CuS2CCH3]− (m/z 169); (b) [CD363CuS2CCH3]− and [CH363CuS2CCD3]− (m/z 172); (c) [CD363CuS2CCD3]−(m/z 175). The mass-selected organometallic ions are denoted by asterisks.
Figure 1. LTQ-FT-ICR MS4 spectra showing ion−molecule reactions between carbon disulfide (1.6 × 1010 molecules cm−3) and (a) [CH3CuCH3]− (m/z 93), (b) [CH3CuCD3]− (m/z 96), and (c) [CD3CuCD3]− (m/z 99) for a period of 10000 ms in the linear ion trap. The mass-selected organometallic ions are denoted by asterisks.
[CH3CuCH3]− toward electrophiles follows the order CH2 CHCH2I (RE = 6.6%15d) > CH3I (RE = 3%15a) > CS2 (RE = 0.8%, this work) > CH2CHCH2O2CCH3 (RE = 0.03%15e). CID of the Adduct [CH3CuS2CCH3]−. The observation of both the adduct (eq 8) and the dithioacetate anion (eq 9) suggests that the C−C bond forming reaction is sufficiently exothermic to allow a fraction of the adducts to fragment to form the dithioacetate anion before being collisionally stabilized. The phenomenon of an exothermic reaction driving fragmentation of a gas-phase product ion has been observed before. Examples include the reactions of carbanions with CS210a and the loss of CO2 from [CH3CuO2CCH3]−, which was formed in the highly exothermic C−C bond coupling reaction between [CH3CuCH3]− and allyl acetate.15e To provide further evidence for a direct link between adduct (eq 8) and dithioacetate anion formation (eq 9), [CH3CuS2CCH3]− was mass-selected in a MS5 experiment and subjected to CID (Figure 2). The major fragmentation channel observed is dithioacetate anion formation (eq 10). Another minor channel was also observed, corresponding to
the loss of CH4 (eq 11), which was confirmed by the deuterium labeling experiments, where CH3D and CD3H losses are observed from the adducts [CH 3 CuS 2 CCD 3 ] − and [CH 3 CS 2 CuCD 3 ] − (Figure 2b) and CD 4 loss from [CD3CuS2CCD3]− (Figure 2c). Loss of methane (eq 11) results in the formation of a Cu(I) complex possessing a deprotonated thioacetate ligand. Related deprotonated carboxylate or thiocarboxylate ligands have been reported, including the lithium acetate enolate anion, 27 metallalactones [(CH3CO2)M(CH2CO2)]− (M = Ni, Pd),28 and ene-1,1dithiolato complexes.4,5,29 Interestingly, [CH3CuS2CCH3]− does not fragment via loss of CS2,. This contrasts with the previous reports on the loss of CO2 from the related copper(I) anions [CH3CuO2CCH3]− 14a and [CH3SO3CuO2CCH3]− 14g and the loss of SO2 from [CH3SO2CuO2CCH3]− and [CH3SO2CuO3SCH3]−.14g Thus, unlike the case for several reported examples of reversible copper-catalyzed carboxylation/decarboxylation,30 insertion of CS2 appears to be nonreversible under our conditions. DFT Calculations on the Ion−Molecule Reaction of [CH3CuCH3]− with CS2. Previous studies on the reactions of [CH3CuCH3]− with the electrophiles methyl iodide15c and allyl iodide15d have considered approaches of the electrophile to the central copper atom to trigger oxidative addition/reductive elimination (OA/RE) mechanisms versus a side-on approach to
Table 1. Kinetics and Branching Ratios of Ionic Products for the Gas-Phase Ion−Molecule Reactions of CS2 with Dimethylcuprate, Dimethylargentate, and Dimethylaurate [CH3CuCH3]− kmeasd kADO
CH3CS2− (m/z 91) [CH3CuS2CCH3]− (m/z 169)
[CH3AgCH3]−
Rate Data 8.96 × 10−12 a
