CuPd Nanoparticles as a Robust Catalyst for Electrochemical Allylic Alkylation.

An efficient CuPd nanoparticle (NP) catalyst (3 nm CuPd NPs deposited on carbon support) is designed for catalyzing electrochemical allylic alkylation in water/isopropanol (1:1 v/v) and 0.2 m KHCO3 solution at room temperature. The Pd catalysis was Pd/Cu composition-dependent, and CuPd NPs with a Pd/Cu ratio close to one are the most efficient catalyst for the selective cross-coupling of alkyl halides and allylic halides to form C-C hydrocarbons with product yields reaching up to 99 %. This NP-catalyzed electrochemical allylic alkylation expands the synthetic scope of cross-coupling reactions and can be further extended to other organic reaction systems for developing green chemistry electrosynthesis methods.

Reactivity of Lithium ß-Ketocarboxylates: The Role of Lithium Salts.

Lithium ß-ketocarboxylates 1(COOLi), prepared by the reaction of lithium enolates 2(Li+) with carbon dioxide, readily undergo decarboxylative disproportionation in THF solution unless in the presence of lithium salts, in which case they are indefinitely stable at room temperature in inert atmosphere. The availability of stable THF solutions of lithium ß-ketocarboxylates 1(COOLi) in the absence of carbon dioxide allowed reactions to take place with nitrogen bases and alkyl halides 3 to give α-alkyl ketones 1(R) after acidic hydrolysis. The sequence thus represents the use of carbon dioxide as a removable directing group for the selective monoalkylation of lithium enolates 2(Li+). The roles of lithium salts in preventing the disproportionation of lithium ß-ketocarboxylates 1(COOLi) and in determining the course of the reaction with bases and alkyl halides 3 are discussed.

Ligand Binding Constants to Lithium Hexamethyldisilazide Determined by Diffusion-Ordered NMR Spectroscopy.

We report the direct measurement of ligand-binding constants of organolithium complexes using a 1H NMR/diffusion-ordered NMR spectroscopy (DOSY) titration technique. Lithium hexamethyldisilazide complexes with ethereal and ester donor ligands (THF, diethyl ether, MTBE, THP, tert-butyl acetate) are characterized using 1H NMR and X-ray crystallography. Their aggregation and solvation states are confirmed using diffusion coefficient-formula weight correlation analysis, and the 1H NMR/DOSY titration technique is applied to obtain their binding constants. Our work suggests that steric hindrance of ethereal ligands plays an important role in the aggregation, solvation, and reactivity of these complexes. It is noteworthy that diffusion methodology is utilized to obtain binding constants.

Conformational Polymorphism of Lithium Pinacolone Enolate.

A metastable, polymorphic hexameric crystal structure of lithium pinacolone enolate (LiOPin) is reported along with three preparation methods. NMR-based structural characterization implies that the lithium pinacolate hexamer deaggregates to a tetramer in toluene but retains mainly the hexameric structure in nonaromatic hydrocarbon solvents such as cyclohexane. Moreover, the presence of a small amount of lithium aldolate (LiOA) dramatically influences the aggregation state of LiOPin by forming a mixed aggregate with a 3:1 ratio (LiOPin3·LiOA).

Polymorphism of Phosphine-Protected Gold Nanoclusters: Synthesis and Characterization of a New 22-Gold-Atom Cluster.

A new Au22 nanocluster, protected by bis(2-diphenyl-phosphino)ethyl ether (dppee or C28 H28 OP2 ) ligand, has been synthsized and purified with high yield. Electrospray mass spectrometry shows that the new cluster has a formula of Au22 (dppee)7 , containing 22 gold atoms and seven dppee ligands. The cluster is found to be stable as a solid, but metastable in solution. The new cluster has been characterized by UV-Vis-NIR absorption spectroscopy, collision-induced dissociation, and (31) P-NMR. The properties of the new cluster have been compared with the previous Au22 (dppo)6 nanocluster (dppo = 1,8-bis(diphenyl-phosphino)octane or C32 H36 P2 ), which contains two fused Au11 units. All the experimental data indicate that the new Au22 (dppee)7 cluster is different from the previously known Au22 (dppo)6 cluster and represents a new Au22 core, which contains most likely one Au11 motif with several Au2 (dppee) or Au(dppee) units. The Au22 (dppee)7 cluster provides a new example of the ligand effects on the nuclearity and structural polymorphism of phosphine-protected atom-precise gold nanoclusters.

Perfluoroalkyl Grignard Reagents: NMR Study of 1-Heptafluoropropylmagnesium Chloride in Solution.

We report on the generation of a perfluoroalkyl Grignard reagent ((F)RMgX) by exchange reaction between a perfluoroalkyl iodide ((F)R-I) and a Grignard reagent (RMgX). (19)F NMR was applied to monitor the generation of n-C3F7MgCl. Additional NMR techniques, including (19)F COSY, NOESY, and pulsed gradient spin-echo (PGSE) diffusion NMR, were invoked to assign peaks observed in (19)F spectrum. Schlenk equilibrium was observed and was significantly influenced by solvent, diethyl ether, or THF.

Lithium pinacolone enolate solvated by hexamethylphosphoramide.

We report the crystal structure of a substoichiometric, HMPA-trisolvated lithium pinacolone enolate tetramer (LiOPin)4·HMPA3 abbreviated as T3. In this tetramer one HMPA binds to lithium more strongly than the other two causing a reduction in spatial symmetry with corresponding loss of C3 symmetry. A variety of NMR experiments, including HMPA titration, diffusion coefficient-formula weight (D-FW) analysis, and other multinuclear one- and two-dimensional NMR techniques reveal that T3 is the major species in hydrocarbon solution when more than 0.6 equiv of HMPA is present. Due to a small amount of moisture from HMPA or air leaking into the solution, a minor complex was identified and confirmed by X-ray diffraction analysis as a mixed aggregate containing enolate, lithium hydroxide, and HMPA in a 4:2:4 ratio, [(LiOPin)4·(LiOH)2·HMPA4], that we refer to as pseudo-T4. A tetra-HMPA-solvated lithium cyclopentanone enolate tetramer was also prepared and characterized by X-ray diffraction, leading to the conclusion that steric effects dominate the formation and solvation of the pinacolone aggregates. An unusual mixed aggregate consisting of pinacolone enolate, lithium diisopropyl amide, lithium oxide, and HMPA in the ratio 5:1:1:2 is also described.

Diffusion Coefficient-Formula Weight (D-FW) Analysis of (2)H Diffusion-Ordered NMR Spectroscopy (DOSY).

We report extension of the D-FW analysis using referenced (2)H DOSY. This technique was developed in response to limitations due to peak overlay in (1)H DOSY spectra. We find a corresponding linear relationship (R(2) > 0.99) between log D and log FW as the basis of the D-FW analysis. The solution-state structure of THF solvated lithium diisopropyl amide (LDA) in hydrocarbon solvent was chosen to demonstrate the reliability of the methodology. We observe an equilibrium between monosolvated and disolvated dimeric LDA complexes at room temperature. Additionally we demonstrate the application of the (2)H D-FW analysis using a compound with an exchangeable proton that is readily labeled with (2)H. Hence, the (2)H DOSY D-FW analysis is shown to provide results consistent with the (1)H DOSY method, thereby greatly extending the applicability of the D-FW analysis.

Nitric oxide reactivity of [2Fe-2S] clusters leading to H2S generation.

The crosstalk between two biologically important signaling molecules, nitric oxide (NO) and hydrogen sulfide (H2S), proceeds via elusive mechanism(s). Herein we report the formation of H2S by the action of NO on synthetic [2Fe-2S] clusters when the reaction environment is capable of providing a formal H(•) (e(-)/H(+)). Nitrosylation of (NEt4)2[Fe2S2(SPh)4] (1) in the presence of PhSH or (t)Bu3PhOH results in the formation of (NEt4)[Fe(NO)2(SPh)2] (2) and H2S with the concomitant generation of PhSSPh or (t)Bu3PhO(•). The amount of H2S generated is dependent on the electronic environment of the [2Fe-2S] cluster as well as the type of H(•) donor. Employment of clusters with electron-donating groups or H(•) donors from thiols leads to a larger amount of H2S evolution. The 1/NO reaction in the presence of PhSH exhibits biphasic decay kinetics with no deuterium kinetic isotope effect upon PhSD substitution. However, the rates of decay increase significantly with the use of 4-MeO-PhSH or 4-Me-PhSH in place of PhSH. These results provide the first chemical evidence to suggest that [Fe-S] clusters are likely to be a site for the crosstalk between NO and H2S in biology.

Influence of steric factors on chiral lithium amide aggregates.

The solution structures of three mixed aggregates dissolved in toluene-d8 consisting of the lithiated amides derived from (S)-N-isopropyl-1-((triisopropylsilyl)oxy)propan-2-amine, (R)-N-(1-phenyl-2-((triisopropylsilyl)oxy)ethyl)propan-2-amine, or (S)-N-isobutyl-3-methyl-1-((triisopropylsilyl)oxy)butan-2-amine and n-butyllithium are characterized by various NMR experiments including diffusion-ordered NMR spectroscopy with diffusion coefficient-formula weight correlation analyses (D-FW) and other one- and two-dimensional NMR techniques. We report that steric hindrance of R1 and R2 groups of the chiral lithium amide controls the aggregation state of the mixed aggregates. With a less hindered R2 group, lithium (S)-N-isopropyl-1-((triisopropylsilyl)oxy)propan-2-amide forms mostly a 2:2 ladder-type mixed aggregate with n-butyllithium. Increase of steric hindrance of the R1 and R2 groups suppresses the formation of the 2:2 mixed aggregate and promotes formation of a 2:1 mixed aggregate. We observe that lithium (S)-N-isobutyl-3-methyl-1-((triisopropylsilyl)oxy)butan-2-amide forms both a 2:2 mixed aggregate and a 2:1 mixed trimer with n-butyllithium. Further increase in the steric hindrance of R1 and R2 groups results in the formation of only 2:1 mixed aggregate as observed with lithium (R)-N-(1-phenyl-2-((triisopropylsilyl)oxy)ethyl)propan-2-amide.

Chiral lithium diamides derived from linked N-isopropyl valinol or alaninol.

Four different chiral diamino diethers synthesized from N-isopropyl valinol or N-isopropyl alaninol were lithiated with n-butyllithium in tetrahydrofuran or diethyl ether. Crystal structures of the dilithiated diamino diethers were determined by X-ray diffraction. Three dilithiated diamino diethers including (2S,2'S)-1,1'-(butane-1,4-diylbis(oxy))bis(N-isopropylpropan-2-amine) 7, (2S,2'S)-1,1'-(pentane-1,5-diylbis(oxy))bis(N-isopropylpropan-2-amine) 8, and (2S,2'S)-1,1'-(heptane-1,7-diylbis(oxy))bis(N-isopropyl-3-methylbutan-2-amine) 9 are dimers, whereas dilithiated (2S,2'S)-1,1'-(pentane-1,5-diylbis(oxy))bis(N-isopropyl-3-methylbutan-2-amine) 10 is a monomer. The lithium atoms in all crystal structures adopt a nonequivalent coordination protocol and exist in two different environments in which one of the lithium atoms is tetra-coordinated while the other one is tri-coordinated. The solution structures of the dilithiated diamino diethers are also characterized by a variety of NMR experiments including diffusion-ordered NMR spectroscopy (DOSY) with diffusion coefficient-formula (D-FW) weight correlation analyses and other one- and two-dimensional NMR techniques.

Controlling gold nanoclusters by diphospine ligands.

We report the synthesis and structure determination of a new Au22 nanocluster coordinated by six bidentate diphosphine ligands: 1,8-bis(diphenylphosphino) octane (L(8) for short). Single crystal X-ray crystallography and electrospray ionization mass spectrometry show that the cluster assembly is neutral and can be formulated as Au22(L(8))6. The Au22 core consists of two Au11 units clipped together by four L(8) ligands, while the additional two ligands coordinate to each Au11 unit in a bidentate fashion. Eight gold atoms at the interface of the two Au11 units are not coordinated by any ligands. Four short gold-gold distances (2.64-2.65 Å) are observed at the interface of the two Au11 clusters as a result of the clamping force of the four clipping ligands and strong electronic interactions. The eight uncoordinated surface gold atoms in the Au22(L(8))6 nanocluster are unprecedented in atom-precise gold nanoparticles and can be considered as potential in situ active sites for catalysis.

Effect of sodium cation on metallacycle ß-hydride elimination in CO2-ethylene coupling to acrylates.

The catalytic conversion of carbon dioxide and olefins into acrylates has been a long standing target, because society attempts to synthesize commodity chemicals in a more economical and sustainable fashion. Although nickel complexes have been known to successfully couple CO2 and ethylene for decades, a key ß-hydride elimination step has proven a major obstacle to the development of a catalytic process. Recent studies have shown that Lewis acid additives can be used to create a lower-energy pathway for ß-hydride elimination and facilitate a low number of catalytic turnovers. However, the exact manner, in which the Lewis acid promotes ß-hydride elimination remains to be elucidated. Herein, we describe the kinetic and thermodynamic role that commercially relevant and weakly Lewis acidic sodium salts play in promoting ß-hydride elimination from nickelalactones synthesized from CO2 and ethylene. This process is compared to a non-Lewis acid promoted pathway, and DFT calculations were used to identify differences between the two systems. The sodium-free isomerization reaction gave a rare CO2 -derived ß-nickelalactone complex, which was structurally characterized.

Structures of lithium N-monosubstituted anilides: trisolvated monomer to tetrasolvated dimer.

Crystal structure determination of lithiated N-methylaniline with a variety of ligands, including tetrahydrofuran, methyltetrahydrofuran, trans-2,5-dimethyltetrahydrofuran, dimethoxyethane, tetrahydropyran and N,N-diethylpropionamide, reveals a common Li­N­Li­N four-membered-ring dimeric structure motif. A progression of solvation from tetrasolvated dimer (PhNMeLi·S2)2 through trisolvated dimer to disolvated dimer (PhNMeLi·S)2 was observed by increasing the steric hindrance of the ligand. Solid-state structures of several other lithium N-alkylanilides solvated by tetrahydrofuan are also reported. When the methyl group of N-methylaniline is replaced by an isopropyl or a phenyl group, trisolvated monomers are formed instead of dimers. Interestingly, the solid-state structure of lithiated N-isobutylaniline in tetrahydrofuran is a trisolvated dimer while that of lithium N-neopentylanilide is a disolvated dimer.

Synthesis and structure determination of a new Au(20) nanocluster protected by tripodal tetraphosphine ligands.

We report the synthesis and structure determination of a new Au20 nanocluster coordinated by four tripodal tetraphosphine (PP3) ligands {PP3 = tris[2-(diphenylphosphino)ethyl]phosphine}. Single-crystal X-ray crystallography and electrospray ionization mass spectrometry show that the cluster assembly can be formulated as [Au20(PP3)4]Cl4. The Au20 cluster consists of an icosahedral Au13 core and a seven-Au-atom partial outer shell arranged in a local C3 symmetry. One PP3 ligand coordinates to four Au atoms in the outer shell, while the other three PP3 ligands coordinate to one Au atom from the outer shell and three Au atoms from the surface of the Au13 core, giving rise to an overall chiral 16-electron Au cluster core with C3 symmetry.

Characterization of cyclopentyllithium and cyclopentyllithium tetrahydrofuran complex.

The solid-state structures of unsolvated, hexameric cyclopentyllithium and tetrameric cyclopentyllithium tetrahydrofuran solvate were determined by single-crystal X-ray diffraction. Cyclopentyllithium easily crystallized in hydrocarbon solvents. Solution-state structural analyses of cyclopentyllithium and cyclopentyllithium-tetrahydrofuran complexes in toluene-d8 were also carried out by diffusion-ordered NMR spectroscopy with diffusion coefficient-formula weight correlation analyses and other one- and two-dimensional NMR techniques. The solution-state studies suggest that unsolvated cyclopentyllithium exists as hexamer and tetramer equilibrating with each other. Upon solvation with tetrahydrofuran, cyclopentyllithium exists mostly as a tetrahydrofuran tetrasolvated tetramer.

Mixed aggregates of an alkyl lithium reagent and a chiral lithium amide derived from N-ethyl-O-triisopropylsilyl valinol.

The crystal structure of a mixed aggregate containing lithiated (S)-N-ethyl-3-methyl-1-(triisopropylsilyloxy)butan-2-amine derived from (S)-valinol and cyclopentyllithium is determined by X-ray diffraction. The mixed aggregate adopts a ladder structure in the solid state. The ladder-type mixed aggregate is also the major species in a toluene-d8 solution containing an approximately 1:1 molar ratio of the lithiated chiral amide to cyclopentyllithium. A variety of NMR experiments including diffusion-ordered NMR spectroscopy (DOSY) with diffusion coefficient-formula (D-FW) weight correlation analyses and other one- and two-dimensional NMR techniques allowed us to characterize the complex in solution. Solution state structures of the mixed aggregates of n-butyl, sec-butyllithium, isopropyllithium with lithiated (S)-N-ethyl-3-methyl-1-(triisopropylsilyloxy)butan-2-amine are also reported. Identical dimeric, ladder-type, mixed aggregates are the major species at a stoichiometric ratio of 1:1 lithium chiral amide to alkyllithium in toluene-d8 solution for all of the different alkyllithium reagents.

Isotopically enriched 13C diffusion-ordered NMR spectroscopy: analysis of methyllithium.

We report the development of isotopic-labeled (13)C diffusion-ordered NMR spectroscopy (DOSY) NMR with diffusion coefficient-formula weight (D-FW) analysis and its application in characterizing the aggregation state of methyllithium aggregates and complexes with several widely used diamines. Commercially available (13)C-labeled benzene and several easily synthesized (13)C-labeled compounds using (13)C-labeled iodomethane as the isotopic source are developed as internal references for diffusion-formula weight analysis (D-FW). The technique greatly expands the applicability of DOSY D-FW analysis to a much wider variety of compounds because of isotopic labeling. These results reveal that methyllithium exists as a tetrasolvated tetramer in diethyl ether and exclusively as bis-solvated dimers with chelating diamines.

Crystal structure and solution state characterization of lithium (S)-(1-(bis(2-methoxyethyl)amino)-3-methylbutan-2-yl)(methyl)amide.

The solid state structure of lithiated (S)-N(1),N(1)-bis(2-methoxyethyl)-N(2),3-dimethylbutane-1,2-diamine, which is a chiral amide base synthesized from (S)-valine was determined by single-crystal X-ray diffraction. The complex in solution state is also characterized by a variety of NMR experiments including diffusion-ordered NMR spectroscopy (DOSY) with diffusion coefficient-formula weight correlation analyses and other one- and two-dimensional NMR techniques by dissolving the crystal in toluene-d8. The crystallography and NMR results suggest that the chiral amide is dimeric in both solid and solution states.

Deoxygenation of mono-oxo bis(dithiolene) Mo and W complexes by protonation.

Protonation-assisted deoxygenation of a mono-oxo molybdenum center has been observed in many oxotransferases when the enzyme removes an oxo group to regenerate a substrate binding site. Such a reaction is reported here with discrete synthetic mono-oxo bis(dithiolene) molybdenum and tungsten complexes, the chemistry of which had been rarely studied because of the instability of the resulting deoxygenated products. An addition of tosylic acid to an acetonitrile solution of [Mo(IV)O(S2C2Ph2)2](2-) (1) and [W(IV)O(S2C2Ph2)2](2-) (2) results in the loss of oxide with a concomitant formation of novel deoxygenated complexes, [M(MeCN)2(S2C2Ph2)2] (M = Mo (3), W (4)), that have been isolated and characterized. Whereas protonation of 1 exclusively produces 3, two different reaction products can be generated from 2; an oxidized product, [WO(S2C2Ph2)2](-), is produced with 1 equiv of acid while a deoxygenated product, [W(MeCN)2(S2C2Ph2)2] (4), is generated with an excess amount of proton. Alternatively, complexes 3 and 4 can be obtained from photolysis of [Mo(CO)2(S2C2Ph2)2] (5) and [W(CO)2(S2C2Ph2)2] (6) in acetonitrile. A di- and a monosubstituted adducts of 3, [Mo(CO)2(S2C2Ph2)2] (5) and [Mo(PPh3)(MeCN)(S2C2Ph2)2] (7) are also reported.