CO Double Bond Research Articles

Mechanistic Insights into the Rhodium-Catalyzed Intramolecular Ketone Hydroacylation

[Rh((R)-DTBM-SEGPHOS)]BF(4) catalyzes the intramolecular hydroacylation of ketones to afford seven-membered lactones in large enantiomeric excess. Herein, we present a combined experimental and theoretical study to elucidate the mechanism and origin of selectivity in this C-H bond activation process. Evidence is presented for a mechanistic pathway involving three key steps: (1) rhodium(I) oxidative addition into the aldehyde C-H bond, (2) insertion of the ketone CO double bond into the rhodium hydride, and (3) C-O bond-forming reductive elimination. Kinetic isotope effects and Hammett plot studies support that ketone insertion is the turnover-limiting step. Detailed kinetic experiments were performed using both 1,3-bis(diphenylphosphino)propane (dppp) and (R)-DTBM-SEGPHOS as ligands. With dppp, the keto-aldehyde substrate assists in dissociating a dimeric precatalyst 8 and binds an active monomeric catalyst 9. With [Rh((R)-DTBM-SEGPHOS)]BF(4), there is no induction period and both substrate and product inhibition are observed. In addition, competitive decarbonylation produces a catalytically inactive rhodium carbonyl species that accumulates over the course of the reaction. Both mechanisms were modeled with a kinetics simulation program, and the models were consistent with the experimental data. Density functional theory calculations were performed to understand more elusive details of this transformation. These simulations support that the ketone insertion step has the highest energy transition state and reveal an unexpected interaction between the carbonyl-oxygen lone pair and a Rh d-orbital in this transition state structure. Finally, a model based on the calculated transition-state geometry is proposed to rationalize the absolute sense of enantioinduction observed using (R)-DTBM-SEGPHOS as the chiral ligand.

Highly Enantioselective Diels−Alder Reactions of Danishefsky Type Dienes with Electron-Deficient Alkenes Catalyzed by Yb(III)-BINAMIDE Complexes

1-Methoxy-3-trimethylsiloxy-1,3-butadiene (Danishefsky's diene) is recognized as a synthetically useful diene due to its high reactivity in the Diels-Alder reaction with electron-deficient alkenes to give oxygen-functionalyzed cyclohexenes and substituted cyclohexenones, which are important building blocks for the total synthesis of natural products. However, the development of catalytic enantioselective versions of Diels-Alder reactions using Danishefsky type dienes with electron-deficient alkenes has been difficult because of the instability of the dienes under Lewis acidic conditions. Only highly reactive CO and CN double bonds are employed in a hetero-Diels-Alder reaction which proceeds under catalysis of chiral Lewis acids. We have developed a new chiral ligand, BINAMIDE, which is easily prepared from 1,1'-binaphtyl-2,2'-diamine by acylation. The highly diastereo- and enantioselective Diels-Alder reaction of Danishefsky type dienes with electron-deficient alkenes in the presence of an Yb(III)-BINAMIDE complex has been developed. The reaction proceeded in an exoselective mode and gave chiral highly functionalized cyclohexene derivatives in good yields.

Synthesis, Spectroscopic Characterization, and Conformational Properties of Trichloromethanesulfenyl Acetate, CCl3SOC(O)CH3

Trichloromethanesulfenyl acetate, CCl 3SOC(O)CH 3, belongs to the family of sulfenic esters. This molecule has been characterized by vibrational spectroscopy. The conformational and geometrical properties of this species have been determined by IR and Raman spectroscopy, X-ray diffraction, and quantum chemical calculations. Geometry optimizations of the most stable forms were performed with ab initio (HF, MP2) and density functional theory (B3LYP) methods. According to our data, this compound results in a gauche-syn conformer with C 1 symmetry (gauche orientation around the S-O bond and syn orientation of the CO double bond with respect to the S-O single bond) for the most stable geometry, and trans-syn conformer with C s symmetry (trans orientation around the S-O bond and syn orientation of the CO double bond with respect to the S-O single bond) for the second stable conformer (1.1 and 0.53 kcal/mol higher in energy than the most stable C 1 form according to the matrix FTIR spectroscopy and MP2/6-31G* level of the theory, respectively). The crystalline solid (monoclinic, P2 1/ n, a = 8.0152(17) A, b = 5.7922(13) A, c = 17.429(4) A, alpha = gamma = 90 degrees , beta = 100.341(3) degrees ) consists exclusively of the main form. The geometrical parameters (X-ray diffraction) are d C-Cl = 1.767(19) A, d C-S = 1.797(2) A, d S-O = 1.663(14) A, d CO = 1.189(2) A, d O-C = 1.389(3) A, d C-C = 1.483(3) A, angles Cl-C-Cl = 110.3(11) degrees , Cl-C-S = 111.8(12) degrees , C-S-O = 97.4(8) degrees , S-O-C = 116.7(11) degrees , O-CO = 122.8(19) degrees , OC-C = 127.1(2) degrees , and the main torsion angles are delta(CSOC) = 105.9(15) degrees and delta(SOC(O)) = 7.6(3) degrees . The geometrical data calculated with B3LYP/6-31G++(3df,3pd), B3LYP/6-311G++(3df,3pd), B3LYP/aug-cc-pVTZ, and MP2/6-31G* are in good agreement with diffraction data.

Preparation and Properties of Methoxycarbonylsulfenyl Isocyanate, CH3OC(O)SNCO

Pure methoxycarbonylsulfenyl isocyanate, CH3OC(O)SNCO, is quantitatively prepared by the metathesis reaction between CH3OC(O)SCl and AgNCO. This novel species has been obtained in its pure form and characterized by 1H and 13C NMR, UV-vis, FTIR, and FT-Raman spectroscopy. The conformational properties of the gaseous molecule have been studied by vibrational spectroscopy and quantum chemical calculations (B3LYP and MP2 methods). The compound exhibits a conformational equilibrium at room temperature having the most stable form CS symmetry with the C=O double bond synperiplanar with respect to the S-N single bond. A second form was observed in the IR spectrum and corresponds to a conformer possessing the C-S bond antiperiplanar with respect to the N=C double bond of the isocyanate group. The structure of a single crystal of CH3OC(O)SNCO was determined by X-ray diffraction analysis at low temperature using a miniature zone melting procedure. The crystalline solid (triclinic, P1, a = 8.292(6) A, b = 9.839(7) A, c = 11.865(8) A, alpha = 67.290(2) degrees , beta = 71.5570(10) degrees , gamma = 83.4850(10) degrees and Z = 6) shows the presence of molecules having exclusively a synperiplanar conformation with respect to the three phi(CO-C=O), phi(O=C-SN), and phi(CS-N=C) dihedral angles.

Transfer Hydrogenation of Imines and Alkenes and Direct Reductive Amination of Aldehydes Catalyzed by Triazole-Derived Iridium(I) Carbene Complexes

A series of new iridium(I) triazole-based NHC complexes [(cod)Ir(NHC)L]BF4 (L = PPh3, pyridine) were prepared and showed good activity for transfer hydrogenation on CO, CN, and CC double bonds in 2-propanol with K2CO3. The phosphine series was shown to be more active than the pyridine series in the case of imine transfer hydrogenation. A neopentyl wingtip substituent on the NHC gave the best catalytic activity with the following competitive order: aldehyde > ketone > imine. In a substrate containing both aldehyde and ketone functionalities, only the aldehyde was reduced. Of great interest, the transfer hydrogenation of polarized and nonpolarized CC bonds was also proved possible. In a useful organic synthetic application, direct, one-pot reductive amination of RCHO with R‘NH2 to give RCH2NHR‘ was shown for a variety of cases.

Hydrogenation of Aldehydes, Esters, Imines, and Ketones Catalyzed by a Ruthenium Complex of a Chiral Tridentate Ligand

A novel ruthenium complex prepared from a chiral tridentate amine functionalized phosphine has been characterized by X-ray crystallography and been found to be active in the hydrogenation of an unprecedented range of CO and CN double bonds, including the enantioselective hydrogenation and transfer hydrogenation of normally unreactive bulky ketones with up to 90% ee.

The Active Role of the Water Solvent in the Regioselective CO Hydrogenation of Unsaturated Aldehydes by [RuH2(mtppms)x] in Basic Media

The regioselective hydrogenation of cinnamaldehyde in basic aqueous media catalyzed by [RuH2(mtppms)x] [x = 3,4; mtppms = meta-sulfonatophenyl-diphenylphosphine] is analyzed by means of theoretical calculations. The water solvent is modeled by the inclusion of a (H2O)3 cluster in addition to a continuum model. Two Ru complexes are evaluated as active species for the reduction process: the major identified species [RuH2P4] and the related phosphine dissociated complex [RuH2P3]. Several reaction mechanisms are computationally evaluated, and their analysis suggests that the reaction takes place in several steps. The first hydrogenation process takes place by means of a hydrogen transfer from the metal catalyst, whereas the second hydrogenation process is performed by a water solvent molecule. This mechanism can account for the selectivity of the CO versus CC double bond reduction experimentally observed under basic reaction conditions and can be directly related to the mechanism found in acidic media, where...

Global Reaction Route Mapping on Potential Energy Surfaces of Formaldehyde, Formic Acid, and Their Metal-Substituted Analogues

Global reaction route mapping of equilibrium structures, transition structures, and their connections on potential energy surface (PES) has been done for MCHO (M = H, Li, Na, Al, Cu) and HCO2M (M = H, Li). A one-after-another technique based on the scaled hypersphere search method has been successfully applied to exploring unknown chemical structures, transition structures, and reaction pathways for organometallic systems. Upon metal substitution, considerable changes of stable structures, reaction pathways, and relative heights of transition structures have been discovered, though some features are similar among the analogues. Al and Cu atoms were found to behave as very strong scissors to cut the CO double bond in MCHO. Energy profiles of the CO insertion into Li-H and Li-CH3 bonds were found to be very similar, especially around the structures where the Li atom is not directly connected with the methyl group, which indicates little effects of alkyl substitution on the reaction route topology.

CN − Secondary Ions Form by Recombination as Demonstrated Using Multi-Isotope Mass Spectrometry of 13C- and 15N-Labeled Polyglycine

We have studied the mechanism of formation CN- secondary ions under Cs+ primary ion bombardment. We have synthesized 13C and 15N labeled polyglycine samples with the distance between the two labels and the local atomic environment of the 13C label systematically varied. We have measured four masses in parallel: 12C, 13C, and two of 12C14N, 13C14N, 12C15N, and 13C15N. We have calculated the 13C/12C isotope ratio, and the different combinations of the CN isotope ratios (27CN/26CN, 28CN/27CN, and 28CN/26CN). We have measured a high 13C15N- secondary ion current from the 13C and 15N labeled polyglycines, even when the 13C and 15N labels are separated. By comparing the magnitude of the varied combinations of isotope ratios among the samples with different labeling positions, we conclude the following: CN- formation is in large fraction due to recombination of C and N; the CO double bond decreases the extent of CN- formation compared to the case where carbon is singly bonded to two hydrogen atoms; and double-labeling with 13C and 15N allows us to detect with high sensitivity the molecular ion 13C15N-.

Computational Study of Carbon Atom (3P and 1D) Reaction with CH2O. Theoretical Evaluation of 1B1 Methylene Production by C (1D)

Singlet and triplet free energy surfaces for the reactions of C atom ((3)P and (1)D) with CH(2)O are studied computationally to evaluate the excited singlet ((1)B(1)) methylene formation from deoxygenation of CH(2)O by C ((1)D) atom as suggested by Shevlin et al. Carbon atoms can react by addition to the oxygen lone pair or to the C=O double bond on both the triplet and singlet surfaces. Triplet C ((3)P) atoms will deoxygenate to give CO plus CH(2) ((3)B(1)) as the major products, while singlet C ((1)D) reactions will form ketene and CO plus CH(2) ((1)A(1)). No definitive evidence of the formation of excited singlet ((1)B(1)) methylene was found on the singlet free energy surface. A conical intersection between the (1)A' and (1)A' ' surfaces located near an exit channel may play a role in product formation. The suggested (1)B(1) state of methylene may form via the (1)A' ' surface only if dynamic effects are important. In an effort to interpret experimental observation of products trapped by (Z)-2-butene, formation of cis- and trans-1,2-dimethylcyclopropane is studied computationally. The results suggests that "hot" ketene may react with (Z)-2-butene nonstereospecifically.

Empty Level Structure in Phenyl and Benzyl Isocyanates

The energies of the lowest-lying anion states of phenyl (C6H5N=C=O) and benzyl (C6H5CH2N=C=O) isocyanates have been determined experimentally in the gas phase for the first time using electron transmission spectroscopy (ETS), and their localization properties have been evaluated using HF/6-31G, MP2/6-31G*, and B3LYP/6-31G* calculations. The lowest-lying anion state of phenyl isocyanate, mainly of benzene ring character but with some contribution also from the N=C=O pi-system, lies at significantly higher energy than that of other benzenes substituted by pi-functionals, such as benzaldehyde or styrene. The scaling with the use of suitable empirical equations of the virtual orbital energies (VOEs) for orbitals with predominantly pi*(ring) character calculated for the neutral-state molecules leads to vertical attachment energies (VAEs) which closely correspond to those determined experimentally, whereas those calculated for the predominantly pi*(CO) and pi*(NC) orbitals (3rd and 4th LUMO, respectively) are significantly different from the corresponding measured values notwithstanding the fact that the calculations reproduce the shortening of the N=C and C=O double bonds.

Reactivity of Germene Mes2GeCR2 with Cumulative Unsaturation-Containing Compounds: Synthesis of New Germanium Heterocycles

Germene 1 reacts with the CO and CS double bonds respectively of phenylisocyanate and methylisothiocyanate by formal [2+2] cycloadditions, leading to four-membered ring derivatives 2 and 3. With CS 2 , dithiagermolane 11 and dithiadigermetane 12 are obtained; the first step of this reaction is probably a formal [2+2] cycloaddition between the Ge=C and C=S double bonds. With nitromethane, oxazadigermolidine 19 and fluorenone are obtained, the first step likely being a [2+3] cycloaddition between the Ge=C double bond and the NO 2 moiety. Compounds 11 and 19 were characterized by X-ray diffraction analysis.

Computational Study of the Reaction of Fluorine Atom with Acetone

The reaction of F(2P) with acetone has been studied theoretically using ab initio quantum chemistry methods and transition state theory. The potential energy surface was calculated at the G3MP2 level using the MP2/6-311G(d,p) optimized structures. Additionally, to ensure the accuracy of the calculations, optimizations with either larger basis set (e.g., MP2/G3MP2Large) or higher level electron correlation [e.g., CCSD/ 6-311G(d,p)] were also performed. It has been revealed that the F + CH3C(O)CH3 reaction proceeds via two pathways: (1) the direct hydrogen abstraction of acetone by F gives the major products HF + CH3C(O)CH2; (2) the addition of F atom to the >C=O double bond of acetone and the subsequent C-C bond cleavage gives the minor products CH3 + CH3C(O)F. All other product channels are of no importance due to the occurrence of significant barriers. Both abstraction and addition appear to be barrierless processes. Variational transition state model and multichannel RRKM theory were employed to calculate the temperature- and pressure-dependent rate constants and branching ratios. The predicted rate constants for the abstraction channel and the yields of HF + CH3C(O)CH3 and CH3 + CH3C(O)F are both in good agreement with the experimental data at 295 K and 700 Torr. A negative temperature dependence of the overall rate constants was predicted at temperatures below 500 K.

Reversible Beta-Hydrogen Elimination of Three-Coordinate Iron(II) Alkyl Complexes: Mechanistic and Thermodynamic Studies

High-spin organometallic complexes have not received extensive mechanistic study, despite their potential importance as unsaturated intermediates in catalytic transformations. We have found that, with a suitably bulky bidentate ligand, three-coordinate, high-spin alkyl complexes of iron(II) are stable. They undergo isomerization and exchange reactions of the alkyl group through β-hydride elimination and reinsertion, and the β-hydride elimination step is rate-limiting. The alkyl complexes transfer a β-hydrogen atom to CC, CN, and CO double bonds and undergo deprotonation by Bronsted acids. The reversible β-hydride elimination reactions can be used to explore relative M−C bond energies. Competition experiments and density functional calculations demonstrate an enthalpic preference for alkyl isomers with iron bound to the terminal carbon of the alkyl fragment. This preference arises from steric and electronic effects. The steric preference could be overcome with a phenyl substituent, which steers iron to the...

Terminal titanium-ligand multiple bonds. Cleavages of C=O and C=S double bonds with Ti imido complexes.

Treatment of (t-)BuN=TiCl(2)Py(3) with 2 equiv lithium ketiminate compound, Li[OCMeCHCMeN(Ar)] (where Ar = 2,6-diisopropylphenyl), in toluene at room temperature gave (t-)BuN=Ti[OCMeCHCMeN(Ar)](2) (1) in high yield. The reaction of 1 with phenyl isocyanate at room-temperature resulted in imido ligand exchange producing PhN=Ti[OCMeCHCMeN(Ar)](2) (2). Compound 1 decomposed at 90 degrees C to form a terminal titanium oxo compound O=Ti[OCMeCHCMeN(Ar)](2) (3) and (t-)BuNHCMeCHCMeNAr (4). Also, the compound 3 could be obtained by reacting 1 with CO(2) under mild condition. Similarly, while 1 reacts with an excess of carbon disulfide, a novel terminal titanium sulfido compound S=Ti[OCMeCHCMeN(Ar)](2) (5) was formed via a C=S bond breaking reaction. A novel titanium isocyanate compound Ti[OCMeCHCMeN(Ar)](2)(NCO)(OEt) (6) was formed on heating 1 with 1 equiv of urethane, H(2)NCOOEt. Compounds 1-6 have been characterized by (1)H and (13)C NMR spectroscopies. The molecular structures of 1, 3, 5, and 6 were determined by single-crystal X-ray diffraction. A theoretical calculation predicted that the cleavage of the C-S double bonds for carbon disulfide with the Ti=N bond of compound 1 was estimated at ca. 21.8 kcal.mol(-1) exothermic.

Trifluoromethyl Chloroformate, ClC(O)OCF3: Structure, Conformation, and Vibrational Analysis Studied by Experimental and Theoretical Methods

The gas phase conformational properties and geometric structure of trifluoromethyl chloroformate, ClC(O)OCF3, have been studied by vibrational spectroscopy (IR (gas), IR (matrix), and Raman (liquid)), gas electron diffraction (GED), and quantum chemical calculation (HF, B3LYP and MP2 methods with 6-311G* basis sets). The molecule exhibits only one form having Cs symmetry with synperiplanar orientation of the O−C single bond relative to the CO double bond. If heated Ar:ClC(O)OCF3 mixtures are deposited as a matrix at 14 K, bands appear in the IR spectra which are assigned to the anti form. At room temperature, the contribution of the anti rotamer is estimated to be less than 1%. This high energy conformer is not observed in the GED experiment. The structure of solid ClC(O)OCF3 was determined by X-ray diffraction analysis from crystals obtained at low temperature, using a miniature zone melting procedure. The molecule crystallizes forming a dimeric structure belonging to the monoclinic crystal system and adopts the P21/n spatial group. Furthermore, we report the structure of the similar molecule trifluoroacetyl chloride, CF3C(O)Cl, in its crystalline phase by using the same method.

Experimental and Computational Studies of the Kinetics and Mechanisms for C6H5 Reactions with Acetone-h6 and -d6

Kinetics and mechanisms for the C6H5 + CH3C(O)CH3 and CD3C(O)CD3 reactions have been investigated by cavity ring-down spectrometry (CRDS) and hybrid density functional theory (DFT) calculations. The rate constants measured for the two reactions at the constant pressure of 45 Torr using Ar as a carrier gas can be represented by the following Arrhenius expressions in units of cm3 mol-1 s-1: kH = (4.2 ± 0.4) × 1011 exp[-(955 ± 30)/T] and kD = (5.1 ± 0.6) × 1011 exp[-(1114 ± 43)/T] in the temperature ranges of 299−451 and 328−455 K, respectively. The significant kinetic isotope effect observed suggests that H-abstraction is the major path, according to their activation energies, with the C6H5 + CD3C(O)CD3 reaction being higher by 0.3 kcal/mol. DFT calculations at the B3LYP/aug-cc-PVTZ//B3LYP/cc-PVDZ level of theory indicate the reaction can in principle take place by three reaction paths, one H-abstraction and two addition reactions to the CO double bond at both C and O atom sites, with the latter two processes having significantly higher reaction barriers. The rate constants predicted by canonical variational transition state theory (CVT) with small curvature tunneling (SCT) corrections for the direct H- and D-abstraction reactions are in reasonable agreement with the experimental data after slightly decreasing the calculated barriers from 3.9 kcal/mol to 3.3 kcal/mol and from 4.7 kcal/mol to 4.1 kcal/mol, respectively. The predicted rate constants for C6H5 + CH3COCH3 in the temperature range of 298−1200 K can be represented reasonably by the expression, kH = (1.7 ± 0.6) × 10-1 T (4.2 ± 0.1) exp[-(466 ± 26)/T] cm3 mol-1 s-1.

New insights into the palladium-mediated selective hydrolysis of the His18–Thr19 peptide bond in cytochrome c: 1H NMR and density functional theory investigation for model compounds

The peptides, CH3CO-Met-His-GlyH, CH3CO-CysMe-His-GlyH, CH3CO-CysMe-His-Gly-OEt and its imidazole derivatives, Nτ-benzyl, Nτ-tosyl, Nτ-benzyl-Nπ-phenacyl, have been synthesized and used as model compounds for the mechanistic study of the selective cleavage of cytochrome c promoted by Pd(II) complexes. The peptide bond cleavage of these substrates by cis-[Pd(en)(solvent)2]2+ (solvent: D2O or CD3OD) was monitored by 1H NMR spectroscopy. The results showed that the methionine-containing tripeptide differs from the S-methylcysteine-containing tripeptides in the mode of coordination to Pd(II). The former coordinates to Pd(II) through a sulfur atom, an amide nitrogen of methionine and an Nπ atom of imidazole, forming a polycyclic chelate, and is resistant to hydrolysis. The latter, as a model compound for cleavage of the His18–Thr19 bond in cytochrome c with Pd(II) complexes, coordinates to Pd(II) via a sulfur atom, an amide nitrogen and a carbonyl oxygen of histidine to form a polycyclic chelate in which the His–Gly peptide bond is cleaved. Kinetic studies showed that protonation of the Nπ atom of imidazole in the S-methylcysteine-containing tripeptides is one of the key factors in controlling the cleavage of the His–Gly bond. In order to obtain theoretical guidance on the cleavage reaction, the geometries of a representative Nπ protonated tripeptide cation of CH3CO-CysMe-His-GlyNMe and its Pd(II) complex with and without ancillary water molecules are optimized at the B3LYP density functional theory level using 3-21G, 6-31G(d) and LanL2DZ basis sets. Based on the experimental and theoretical results obtained from the model compounds, a mechanism is proposed for the first time to explain the nature of selective cleavage of the His18–Thr19 bond in cytochrome c promoted by Pd(II) complexes. Coordination of Pd(II) to the carbonyl oxygen of histidine and hydrogen bond formed between the CO and ancillary dimer water weaken and polarize the CO double bond of histidine, giving rise to cleavage of the peptide bond.

Ruthenium(II)‐Catalyzed [2 + 2 + 2] Cycloaddition of 1,6‐Diynes with Tricarbonyl Compounds.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Reactions of Silylene with Unreactive Molecules. I: Carbon Dioxide; Gas-Phase Kinetic and Theoretical Studies

Time-resolved studies of the reaction of silylene, SiH2, with CO2 have been carried out over the temperature range 298−681 K, using laser flash photolysis to generate and monitor SiH2. Between 339 and 681 K the reaction obeys second-order kinetics and the derived rate constants gave the following Arrhenius parameters: log(A/cm3 molecule-1 s-1) = −11.89 ± 0.13, Ea = +16.36 ± 1.23 kJ mol-1, where the uncertainties are single standard deviations. The reaction shows no overall pressure dependence. This reaction is unusual for SiH2 in being extremely slow and having a positive activation energy. Ab-initio calculations at the G2 level suggest a mechanism occurring via the intermediacy of siloxiranone (formed via addition of SiH2 to one of the CO double bonds) leading to overall formation of H2SiO + CO, consistent with kinetic findings. Direct abstraction of an O atom appears to be ruled out, as are other potential pathways. The mechanism resembles that of CH2(1A1) + CO2.