Oxidation State, Aggregation, and Heterolytic Dissociation of Allyl Indium Reagents

Anionic Dimers of Fluorinated Alcohols

Negative-ion mode electrospray ionization of solutions of ethanol (R^F0OH), 2-fluoroethanol (R^F1OH), 2,2-difluoroethanol (R^F2OH), and/or 2,2,2-trifluoroethanol (R^F3OH) produces anionic dimers of the types (R^FnO)₂H^- and (R^FnO)(R^Fn+1O)H^-. The exchange reactions of these anionic dimers with the neutral alcohols are examined in a quadrupole-ion trap to extract kinetic data, from which the reaction Gibbs energies are obtained. In all cases, the formation of anionic dimers containing the more highly fluorinated alcohols is favored. Quantum chemical calculations confirm this trend and, besides affording structural data, also determine the dissociation energies of the anionic dimers. These dissociation energies are much higher than those of the corresponding neutral dimers and increase further for the more highly fluorinated alcohols due to the stronger hydrogen-bond donor ability of the latter. The present results on the interaction of individual alkoxide anions and neutral alcohol molecules contribute to a better understanding of the association of the fluorinated alcohols in solution.

Solvent Effects on Heterogeneous Rate Constants for Indium Mediated Allylations

Indium mediated allylation is a highly selective tool for synthetic chemists to create carbon-carbon bonds, but the first step, heterogeneous reaction of allyl halides at solid indium surfaces, is still poorly understood. For example, the nature of the solvent dramatically affects the rate of reaction, but solvent choice is often based on empirical experiments. Fundamental kinetic studies are the best way to study this effect, but the determination of heterogeneous rate constants is challenging. In an effort to better understand solvent effects, we use optical microscopy to determine heterogeneous rate constants for IMA in aqueous acetonitrile, methanol, ethanol, and 2-propanol. We fit the reaction rate data over a range of mass transport rates using only two adjustable parameters, the heterogeneous rate constant and the mass transport rate. The results emphasize the critical importance of water in determining the rate of reaction. Surprisingly, the polarity of the organic solvent in the mix does not have a major effect on the rate. It is hypothesized that the oxygen atom in water and alcohols is an especially effective Lewis base to stabilize the transition state and the organoindium intermediates, similar to the importance of the oxygen in ethers for the formation of Grignard reagents. This study again demonstrates the power of microscopy for the study of heterogeneous reactions.

Direct Detection of Free and Counterion-Bound Carbanions by Electrospray-Ionization Mass Spectrometry

We propose electrospray-ionization (ESI) mass spectrometry as a robust and powerful method for the in situ analysis of carbanions. ESI mass spectrometry selectively probes the charged components of the sampled solution and, thus, is ideally suited for the detection of free carbanions. We demonstrate the potential of this method by analyzing acetonitrile solutions of 15 different carbon acids AH, whose acidities cover a range of 11.1 ≤ pK_a(DMSO) ≤ 29.5. After treatment with KO^tBu as a strong base, all but the two least acidic compounds were successfully detected as free carbanions A^- and/or as potassium-bound aggregates [K_n-1A_n]^-. The association equilibria can be shifted toward smaller aggregates and free carbanions by the addition of the crown ether 18-crown-6, which facilitates the evaluation of the mass spectra. When KO^tBu was replaced by other bases (LiOH, LiNⁱPr₂, NaH, NaOH, KOH, NBu₄OH) or when tetrahydrofuran or methanol was used as a solvent, carbanions were also successfully observed. For further demonstrating the utility of the proposed method, we applied it to the analysis of the Michael addition of deprotonated dimedone to butenone. ESI mass spectrometry allowed us to follow the decrease of the reactant carbanion and the buildup of the product carbanion in time.

Modulation of Gas-Phase Lithium Cation Basicities by Microsolvation

In contrast to the extensive knowledge of lithium cation affinities and basicities, the thermochemistry of microsolvated lithium cations is much less explored. Here, we determine the relative stabilities of Li(A,B)_n⁺ complexes, n = 2 and 3, by monitoring their gas-phase reactions with A and B substrate molecules, A/B = Me₂O, Et₂O, tetrahydrofuran, and MeCN, in a three-dimensional quadrupole-ion trap mass spectrometer. Kinetic analysis of the observed ligand displacement reactions affords equilibrium constants, which are then converted into Gibbs reaction energies. In addition, we use high-level quantum chemical calculations to predict the structures and sequential ligand dissociation energies of the homoleptic Li(A)_n⁺ complexes, n = 1-3. As expected, the ligands dissociate more easily from complexes in higher coordination states. However, the very nature of the ligand also matters. Ligands with different steric demands can, thus, invert their relative Li⁺ affinities depending on the coordination state of the metal center. This finding shows that microsolvation of Li⁺ can result in specific effects, which are not recognized if the analysis takes into account only simple lithium cation affinities and basicities.

Mechanisms of Cobalt/Phosphine-Catalyzed Cross-Coupling Reactions

>The combination of CoCl₂ with bidentate phosphines is known to catalyze challenging cross-coupling and Heck-type reactions, but the mechanisms of these valuable transformations have not been established. Here, we use electrospray-ionization mass spectrometry to intercept the species formed in these reactions. Our results indicate that a sequence of transmetalation, reductive elimination, and redox disproportionation convert the cobalt(II) precatalyst into low-valent cobalt complexes. These species readily transfer single electrons to alkyl bromides, which thereupon dissociate into alkyl radicals and Br^- . In cross-coupling reactions, the alkyl radicals add to the cobalt catalyst to form observable heteroleptic complexes, which release the coupling products through reductive eliminations. In the Heck-type reactions, the low abundance of newly formed ionic species renders the analysis more difficult. Nonetheless, our results also point to the occurrence of single-electron transfer processes and the involvement of radicals in these transformations.

Bi-mediated allylation of aldehydes in [bmim][Br]: a mechanistic investigation

The inexpensive room temperature ionic liquid (RTIL), [bmim][Br] has been found to be a superior medium for the Bi-mediated Barbier-type allylation of aldehydes compared to other conventional solvents. It plays the dual role of a solvent and a metal activator enabling higher yields of the products in a shorter reaction time using stoichiometric/near-stoichiometric amounts of reagents. Plausibly, [bmim][Br] activates Bi metal by a charge transfer mechanism. The ¹H VT-NMR studies suggested that both the allylating species, allylbismuth dibromide and diallylbismuth bromide, are generated in situ.

Oxidation States, Stability, and Reactivity of Organoferrate Complexes

We have applied a combination of electrospray-ionization mass spectrometry, electrical conductivity measurements, and Mössbauer spectroscopy to identify and characterize the organoferrate species R _nFe _m^- formed upon the transmetalation of iron precursors (Fe(acac)₃, FeCl₃, FeCl₂, Fe(OAc)₂) with Grignard reagents RMgX (R = Me, Et, Bu, Hex, Oct, Dec, Me₃SiCH₂, Bn, Ph, Mes, 3,5-(CF₃)₂-C₆H₃; X = Cl, Br) in tetrahydrofuran. The observed organoferrates show a large variety in their aggregation (1 ≤ m ≤ 8) and oxidation states (I to IV), which are chiefly determined by the nature of their organyl groups R. In numerous cases, the addition of a bidentate amine or phosphine changes the distributions of organoferrates and affects their stability. Besides undergoing efficient intermolecular exchange processes, several of the probed organoferrates react with organyl (pseudo)halides R'X (R' = Et, ⁱPr, Bu, Ph, p-Tol; X = Cl, Br, I, OTf) to afford heteroleptic complexes of the type R₃FeR'^-. Gas-phase fragmentation of most of these complexes results in reductive eliminations of the coupling products RR' (or, alternatively, of R₂). This finding indicates that iron-catalyzed cross-coupling reactions may proceed via such heteroleptic organoferrates R₃FeR'^- as intermediates. Gas-phase fragmentation of other organoferrate complexes leads to β-hydrogen eliminations, the loss of arenes, and the expulsion of organyl radicals. The operation of both one- and two-electron processes is consistent with previous observations and contributes to the formidable complexity of organoiron chemistry.

Unimolecular reactivity of organotrifluoroborate anions, RBF3- , and their alkali metal cluster ions, M(RBF3 )2- (M = Na, K; R = CH3 , CH3 CH2 , CH3 (CH2 )3 , CH3 (CH2 )5 , c-C3 H5 , C6 H5 , C6 H5 CH2 , CH2 CHCH2 , CH2 CH, C6 H5 CO)

RATIONALE

Potassium organotrifluoroborates (RBF₃ K) are important reagents used in organic synthesis. Although mass spectrometry is commonly used to confirm their molecular formulae, the gas-phase fragmentation reactions of organotrifluoroborates and their alkali metal cluster ions have not been previously reported.

METHODS

Negative-ion mode electrospray ionization (ESI) together with collision-induced dissociation (CID) using a triple quadrupole mass spectrometer were used to examine the fragmentation pathways for RBF₃^- (where R = CH₃ , CH₃ CH₂ , CH₃ (CH₂ )₃ , CH₃ (CH₂ )₅ , c-C₃ H₅ , C₆ H₅ , C₆ H₅ CH₂ , CH₂ CHCH₂ , CH₂ CH, C₆ H₅ CO) and M(RBF₃ )₂^- (M = Na, K), while density functional theory (DFT) calculations at the M06/def2-TZVP level were used to examine the structures and energies associated with fragmentation reactions for R = Me and Ph.

RESULTS

Upon CID, preferentially elimination of HF occurs for RBF₃^- ions for systems where R = an alkyl anion, whereas R^- formation is favoured when R = a stabilized anion. At higher collision energies loss of F^- and additional HF losses are sometimes observed. Upon CID of M(RBF₃ )₂^- , formation of RBF₃^- is the preferred pathway with some fluoride transfer observed only when M = Na. The DFT-calculated relative thermochemistry for competing fragmentation pathways is consistent with the experiments.

CONCLUSIONS

The main fragmentation pathways of RBF₃^- are HF elimination and/or R^- loss. This contrasts with the fragmentation reactions of other organometallate anions, where reductive elimination, beta hydride transfer and bond homolysis are often observed. The presence of fluoride transfer upon CID of Na(RBF₃ )₂^- but not K(RBF₃ )₂^- is in agreement with the known fluoride affinities of Na⁺ and K⁺ and can be rationalized by Pearson's HSAB theory.

Low-Valent Ate Complexes Formed in Cobalt-Catalyzed Cross-Coupling Reactions with 1,3-Dienes as Additives

The combination of CoCl₂ and 1,3-dienes is known to catalyze challenging alkyl-alkyl cross-coupling reactions between Grignard reagents and alkyl halides, but the mechanism of these valuable transformations remains speculative. Herein, electrospray-ionization mass spectrometry is used to identify and characterize the elusive intermediates of these and related reactions. The vast majority of detected species contain low-valent cobalt(I) centers and diene molecules. Charge tagging, deuterium labeling, and gas-phase fragmentation experiments elucidate the likely origin of these species and show that the diene not only binds to Co as a π ligand, but also undergoes migratory insertion reactions into Co-H and Co-R bonds. The resulting species have a strong tendency to form anionic cobalt(I) ate complexes, the superior nucleophilicity of which should render them highly reactive toward electrophilic substrates and, thus, presumably is the key to the high catalytic efficiency of the system under investigation. Upon the reaction of the in situ formed cobalt(I) ate complexes with organyl halides, only the final cross-coupling product could be detected, but no cobalt(III) species. This finding implies that this reaction step proceeds in a direct manner without any intermediate or, alternatively, that it involves an intermediate with a very short lifetime.

Enantioselective Barbier-type allylation of ketones using allyl halide and indium in water

We disclose herein an efficient enantioselective Barbier-type allylation of ketones using allyl halide and indium metal in water.

Stability and Unimolecular Reactivity of Palladate(II) Complexes [Ln PdR3 ]- (L=Phosphine, R=Organyl, n=0 and 1)

The reduction of Pd^II precatalysts to catalytically active Pd⁰ species is a key step in many palladium-mediated cross-coupling reactions. Besides phosphines, the stoichiometrically used organometallic reagents can afford this reduction, but do so in a poorly understood way. To elucidate the mechanism of this reaction, we have treated solutions of Pd(OAc)₂ and a phosphine ligand L in tetrahydrofuran with RMgCl (R=Ph, Bn, Bu) as well as other organometallic reagents. Analysis of these model systems by electrospray- ionization mass spectrometry found palladate(II) complexes [L_n PdR₃ ]^- (n=0 and 1), thus pointing to the occurrence of transmetallation reactions. Upon gas-phase fragmentation, the [L_n PdR₃ ]^- anions preferentially underwent a reductive elimination to yield Pd⁰ species. The sequence of the transmetallation and reductive elimination, thus, constitutes a feasible mechanism for the reduction of the Pd(OAc)₂ precatalyst. Other species of interest observed include the Pd^IV complex [PdBn₅ ]^- , which did not fragment via a reductive elimination but lost BnH instead.

Intermediates Formed in the Reactions of Organocuprates with α,β-Unsaturated Nitriles

Conjugate additions of organocuprates are of outstanding importance for organic synthesis. To improve our mechanistic understanding of these reactions, we have used electrospray ionization mass spectrometry for the identification of the ionic intermediates formed upon the treatment of LiCuR2 ⋅LiCN (R=Me, Bu, Ph) with a series of α,β-unsaturated nitriles. Acrylonitrile, the weakest Michael acceptor included, did not afford any detectable intermediates. Fumaronitrile (FN) yielded adducts of the type Lin-1 Cun R2n (FN)n (-) , n=1-3. When subjected to fragmentation in the gas phase, these adducts were not converted into the conjugate addition products, but re-dissociated into the reactants. In contrast, the reaction with 1,1-dicyanoethylene furnished the products of the conjugate addition without any observable intermediates. Tri- and tetracyanoethylene proved to be quite reactive as well. The presence of several cyano groups in these substrates opened up reaction pathways different from simple conjugate additions, however, and led to dimerization and substitution reactions. Moreover, the gas-phase fragmentation behavior of the species formed from these substrates indicated the occurrence of single-electron transfer processes. Additional quantum-chemical calculations provided insight into the structures and stabilities of the observed intermediates and their consecutive reactions.

The Role of Ate Complexes in the Copper-Mediated Trifluoromethylation of Alkynes

Trifluoromethylation reactions have recently received increased attention because of the beneficial effect of the trifluoromethyl group on the pharmacological properties of numerous substances. A common method to introduce the trifluoromethyl group employs the Ruppert-Prakash reagent, that is, Si(CH3 )3 CF3 , together with a copper(I) halide. We have applied this method to the trifluoromethylation of aromatic alkynes and used electrospray-ionization mass spectrometry to investigate the mechanism of these reactions in tetrahydrofuran, dichloromethane, and acetonitrile as well as with and without added 1,10-phenanthroline. In the absence of the alkyne component, the homoleptic ate complexes [Cu(CF3 )2 ](-) and [Cu(CF3 )4 ](-) were observed. In the presence of the alkynes RH, the heteroleptic complexes [Cu(CF3 )3 R](-) were detected as well. Upon gas-phase fragmentation, these key intermediates released the cross-coupling products R-CF3 with perfect selectivity. Apparently, the [Cu(CF3 )3 R](-) complexes did not originate from homoleptic cuprate anions, but from unobservable neutral precursors. The present results moreover point to the involvement of oxygen as the oxidizing agent.

Exposing elusive cationic magnesium–chloro aggregates in aluminate complexes through donor control

Judicious choice of Lewis donor provides control over the aggregation state of the [Mg_xCl_2x−1]⁺ cation in a series of Mg battery relevant magnesium aluminates.

Indium-assisted aluminium-based stereoselective allylation of prostereogenic α,α-disubstituted cycloalkanones and imines

Stereoselective allylation of prostereogenic cycloalkanone and imine systems using allylic halides, aluminium powder and a catalytic amount of InCl₃ is reported.

Tetraalkylcuprates(III): formation, association, and intrinsic reactivity

Tetraalkylcuprates are prototypical examples of organocopper(III) species, which remained elusive until their recent detection by NMR spectroscopy. In agreement with the NMR studies, the present electrospray ionization mass spectrometric experiments, as well as supporting electrical conductivity measurements, indicate that LiCuMe(2)·LiCN reacts with a series of alkyl halides RX. The resulting Li(+)Me(2)CuR(CN)(-) intermediates then afford the observable Me(3)CuR(-) tetraalkylcuprate anions upon Me/CN exchanges with added MeLi. In contrast, the reactions of LiCuMe(2)·LiCN with neopentyl iodide and various aryl halides give rise to halogen-copper exchanges. Concentration- and solvent-dependent studies suggest that lithium tetraalkylcuprates are not fully dissociated in ethereal solvents, but partly form Li(+)Me(3)CuR(-) contact ion pairs and presumably also triple ions LiMe(6)Cu(2)R(2)(-). According to theoretical calculations, these triple ions consist of two square-planar Me(3)CuR(-) subunits binding to a central Li(+) ion. Upon fragmentation in the gas phase, the mass-selected Me(3)CuR(-) anions undergo reductive elimination, yielding both the cross-coupling products MeR and the homocoupling product Me(2). The branching between these two fragmentation channels markedly depends on the nature of the alkyl substituent R. The triple ions LiMe(6)Cu(2)R(2)(-) (as well as their mixed analogues LiMe(6)Cu(2)R(R')(-)) also afford both cross-coupling and homocoupling products upon fragmentation, but strongly favor the former. On the basis of theoretical calculations, we rationalize this prevalence of cross-coupling by the preferential interaction of the central Li(+) ion of the triple ions with two Me groups of each Me(3)CuR(-) subunit, which thereby effectively blocks the homocoupling channel. Our results thus show how a Li(+) counterion can alter the reactivity of an organocopper species at the molecular level.