Conformational analysis of a TADDOL-based phosphoramidite P,N ligand in a palladium(II) η3-π-allyl complex

Highlights Two lowest energy conformers differ in allyl conformation, conformer with exo allyl having lower energy. Lowest energy conformer has “chair” conformation of 7-membered ring and “edge-on/face-on/face-on/edge-on” array of pH groups. Next lowest energy conformer with exo allyl has alternating “edge-on/face-on” arrangement and “twist” 7-membered ring. Changes in “chair/twist” conformations with different substrates is possible due to 2.4 kcal/mol energy difference. These results explain stereochemical outcome of the reaction, and will guide design of new catalysts. Abstract The most stable conformations of a TADDOL-based phosphoramidite P,N ligand coordinated to a palladium(II) η3-π-allyl fragment have been investigated using molecular mechanical and quantum mechanical calculations. The conformational analysis initially generated 53 unique structures within 5 kcal/mol and subsequent geometry optimization narrowed the number of low-energy conformers down to 13. The two lowest energy conformers differ mainly in the conformation of the allyl group. The conformer with an endo allyl group has a slightly higher relative energy than the conformer with an exo allyl group. Comparison of the main geometric parameters around the Pd(II) metal center in the two lowest energy conformers with the available X-ray single crystal structures of Pd(II) η3-π-allyl complexes of P,N ligands shows a good agreement in both the bond lengths and angles. The lowest energy structure has a “chair” conformation of the seven-membered phospha-dioxa-cycloheptane ring and “edge-on/face-on/face-on/edge-on” arrangement of the phenyl rings. The next lowest energy conformer with an exo allyl group has a “twist” conformation of the seven-membered ring and alternating “edge-on” and “face-on” arrangement of the phenyl rings as anticipated from the Knowles “edge-on/face-on” concept. The results of this study support published hypotheses regarding the origin of the chiral induction in the enantioselective Pd(0) catalyzed intramolecular allylic alkylation reaction by the repulsive interactions between one of the phenyl groups in the seven-membered ring in the lowest energy conformer of the ligand with the substrate. As such, the results of this research can be used to guide the synthesis of new and improved variants of this important catalyst family

Ortho-Fluoro Effect on the C–C Bond Activation of Benzonitrile Using Zerovalent Nickel

The effect of fluoro substitution on the C–C bond activation of aromatic nitriles has been studied by reacting a variety of fluorinated benzonitriles with the nickel(0) fragment [Ni(dippe)] and by locating the reaction intermediates and transition-state structures on the potential energy surface by using density functional theory calculations with the [Ni(dmpe)] fragment (dippe = 1,2-bis(diisopropylphosphino)ethane, dmpe = 1,2-bis(dimethylphosphino)ethane). As in the previous reports, the reaction of fluorinated benzonitriles with the [Ni(dippe)] fragment initially formed an η2-nitrile complex, which then converted to the C–CN bond activation product. Thermodynamic parameters for the equilibrium between these complexes have been determined experimentally in both a polar (tetrahydrofuran) and a nonpolar (toluene) solvent for 3-fluoro- and 4-fluorobenzonitrile. The stability of the C–C bond activation products is shown to be strongly dependent on the number of ortho-F substituents (−6.6 kcal/mol per o-F) and only slightly dependent on the number of meta-F substituents (−1.8 kcal/mol per m-F)

Uptake of Ba2+ ions by natural bentonite and CaCO3: A radiotracer, EDXRF and PXRD study

Ba2+ uptake by natural bentonite, CaCO3 in addition to a number of bentonite-CaCO3 mixtures with variable compositions as a function of pH and Ba2+ concentration was studied. Radiotracer method, EDXRF, and PXRD were used. The results of radiotracer experiments showed that the uptake of Ba2+ by CaCO3 was larger than its uptake by natural bentonite samples, particularly at low initial concentrations of Ba2+ and higher pH values. This finding was supported by the EDXRF results, According to the sorption data, the apparent ΔG° values of sorption were in the range -9±1 to -13±3 kJ/mol. The PXRD studies revealed the formation of BaCO3 upon sorption of Ba2+ on pure CaCO3 and on some of the bentonite-CaCO3 mixtures

Investigation on the Synthesis, Application and Structural Features of Heteroaryl 1,2-Diketones

A set of unsymmetrical heteroaryl 1,2-diketones were synthesized by a heteroarylation/oxidation sequence with up to 65% isolated yields. Palladium catalyst XPhos Pd G4 and SeO2 were the key reagents used in this methodology, and microwave irradiation was utilized to facilitate an efficient and ecofriendly process. The application of heteroaryl 1,2-diketones is demonstrated through the synthesis of an unsymmetrical 2-phenyl-3-(pyridin-3-yl)quinoxaline (5a) from 1-phenyl-2-(pyridin-3-yl)ethane-1,2-dione (4a). The lowest energy conformations of 4a and 5a were located using Density Functional Theory (DFT) at the M06-2X/def2-TZVP level of theory. Two lowest energy conformations of 4a differ with respect to the position of the N atom in the pyridyl ring and 0.27 kcal/mol energy difference between them corresponds to 60.4 and 39.6% at 50 °C in toluene. Four lowest energy conformations for 5a have the energy differences of 0.01, 0.03 and 0.07 kcal/mol that corresponds to 26.0, 25.7, 24.9 and 23.4%, respectively. A comparison of 4a and 5a to the less hindered analogs (oxalyl chloride and oxalic acid) is used to investigate the structural features and bonding using Natural Bond Orbital (NBO) analysis

Investigation on the Synthesis, Application and Structural Features of Heteroaryl 1,2-Diketones

A set of unsymmetrical heteroaryl 1,2-diketones were synthesized by a heteroarylation/oxidation sequence with up to 65% isolated yields. Palladium catalyst XPhos Pd G4 and SeO2 were the key reagents used in this methodology, and microwave irradiation was utilized to facilitate an efficient and ecofriendly process. The application of heteroaryl 1,2-diketones is demonstrated through the synthesis of an unsymmetrical 2-phenyl-3-(pyridin-3-yl)quinoxaline (5a) from 1-phenyl-2-(pyridin-3-yl)ethane-1,2-dione (4a). The lowest energy conformations of 4a and 5a were located using Density Functional Theory (DFT) at the M06-2X/def2-TZVP level of theory. Two lowest energy conformations of 4a differ with respect to the position of the N atom in the pyridyl ring and 0.27 kcal/mol energy difference between them corresponds to 60.4 and 39.6% at 50 °C in toluene. Four lowest energy conformations for 5a have the energy differences of 0.01, 0.03 and 0.07 kcal/mol that corresponds to 26.0, 25.7, 24.9 and 23.4%, respectively. A comparison of 4a and 5a to the less hindered analogs (oxalyl chloride and oxalic acid) is used to investigate the structural features and bonding using Natural Bond Orbital (NBO) analysis

Palladium-Catalyzed Direct α‑C(sp3) Heteroarylation of Ketones 2 under Microwave Irradiation

Heteroaryl compounds are valuable building blocks in medicinal chemistry and chemical industry. A palladium-catalyzed direct α-C(sp3) heteroarylation of ketones under microwave irradiation is developed and reported in this study. Under optimized conditions, twentyeight (28) heteroarylated ketones were prepared in this study to demonstrate the substrate scope of this reaction. The ground-state optimized structure of Pd(0) active catalyst with 2-dicyclohexylphosphino-2′,4′,6′-triisopropylbiphenyl (XPhos) in toluene, and the products of its reaction with 3-bromopyridine and acetophone were studied using all-atom density functional theory. This study provided insightful information for palladium catalytic system design to generate heteroaryl compounds

Methyl Complexes of the Transition Metals

Organometallic chemistry can be considered as a wide area of knowledge that combines concepts of classic organic chemistry, that is, based essentially on carbon, with molecular inorganic chemistry, especially with coordination compounds. Transition-metal methyl complexes probably represent the simplest and most fundamental way to view how these two major areas of chemistry combine and merge into novel species with intriguing features in terms of reactivity, structure, and bonding. Citing more than 500 bibliographic references, this review aims to offer a concise view of recent advances in the field of transition-metal complexes containing M-CH fragments. Taking into account the impressive amount of data that are continuously provided by organometallic chemists in this area, this review is mainly focused on results of the last five years. After a panoramic overview on M-CH compounds of Groups 3 to 11, which includes the most recent landmark findings in this area, two further sections are dedicated to methyl-bridged complexes and reactivity.Ministerio de Ciencia e Innovación Projects CTQ2010–15833, CTQ2013-45011 - P and Consolider - Ingenio 2010 CSD2007 - 00006Junta de Andalucía FQM - 119, Projects P09 - FQM - 5117 and FQM - 2126EU 7th Framework Program, Marie Skłodowska - Curie actions C OFUND – Agreement nº 26722