Jun-Chul Choi

Selective and high yield synthesis of dimethyl carbonate directly from carbon dioxide and methanol

Supercritical carbon dioxide is efficiently converted to dimethyl carbonate (DMC) via the reaction with methanol in the presence of a catalytic amount of dialkyltin oxide or its derivatives. The removal of water is the key to accomplishing the high conversion by shifting the equilibrium to dimethyl carbonate. Dehydration is successfully carried out by circulating the reaction mixture through a dehydrating tube packed with molecular sieve 3A. Under the effective dehydration conditions, the DMC yield is almost linearly dependent on the reaction time, catalyst amount, methanol concentration, and CO2 pressure.

Synthesis of dimethyl carbonate from carbon dioxide: catalysis and mechanism

Abstract The turnover number of the catalytic production of dimethyl carbonate from CO2 and methanol is restricted by thermodynamics as well as catalyst decomposition. The catalytic efficiency is remarkably improved using an acetal as the starting material in methanol. The catalytic activity and selectivity also depend strongly on the CO2 pressure. A possible catalytic cycle involving transformation of CO2 to dimethyl carbonate is postulated based on mechanistic studies at a molecular level.

Fe(OTf)(3)-catalyzed addition of sp C-H bonds to olefins.

We have developed a novel acid-catalyzed addition of acetylenes to olefins in the presence of catalytic triflic acid or its metal salts. Among the various triflates, the catalytic activities depend on the cation and decrease in the order Fe(3+) > Al(3+) >> H(+), In(3+), Sc(3+) >> Cu(2+), Ag(+). In general, hard acids gave higher yields than soft acids such as copper and silver triflates. Among relatively hard acids, Fe(OTf)(3) was the best catalyst, which is also the case for ester formation from carboxylic acids and olefins. Our procedure is unique and attractive for the following reasons: (i) The reaction proceeds even for isolated C=C double bonds, as in norbornene. (ii) The reaction is promoted by acid catalysts and does not include an oxidation-reduction cycle for transition metals. (iii) Moreover, these catalysts are inexpensive, abundant, and less toxic than precious-metal-based catalysts. The reaction proceeds even under air and does not require precious metals.

Selective synthesis of N-aryl hydroxylamines by the hydrogenation of nitroaromatics using supported platinum catalysts

Various substituted nitroaromatics were successfully hydrogenated to the corresponding N-aryl hydroxylamines in excellent yields (up to 99%) using supported platinum catalysts such as Pt/SiO2 under a hydrogen atmosphere (1 bar) at room temperature. The key to the fast and highly selective formation of hydroxylamines is the addition of small amounts of amines such as triethylamine and dimethyl sulfoxide; amines promote the conversion of nitroaromatics, while dimethyl sulfoxide inhibits further hydrogenation of hydroxylamines to anilines. The promotive effect depends on which type of amine and primary amine was most effective. The hydrogenation efficiently proceeded in common organic solvents, including isopropanol, diethyl ether, and acetone. This methodology should extend the application range of conventional solid catalysts to fine chemicals synthesis.

Iron-catalysed green synthesis of carboxylic esters by the intermolecular addition of carboxylic acids to alkenes

Iron triflate, in situ-formed from FeCl3 and triflic acid, or FeCl3 and silver triflate efficiently catalyse the intermolecular addition of carboxylic acids to various alkenes to yield carboxylic esters; the reaction is applicable to the synthesis of unstable esters, such as acrylates.

Nickel-catalyzed dehydrative transformation of CO2 to urethanes

Ni(OAc)2-bipyridine or phenanthroline complexes catalyze the environmentally benign synthesis of urethane from amine, alcohol, and CO2 without toxic and corrosive phosgene; electron-donating and rigid ligands on Ni promote the reaction.

Synthesis and reactivity of phenoxycarbonyl palladium complex: relevant to the mechanism of oxidative carbonylation of phenol

Phenoxycarbonyl palladium complex was synthesized and its reactivity was investigated relevant to the mechanism of the palladium-catalyzed oxidative carbonylation of phenol to produce diphenyl carbonate (DPC). The phenoxycarbonyl palladium complex PdCl(CO 2 Ph)(PPh 3 ) 2 ( 1 ) was synthesized by oxidative addition of phenyl chloroformate to Pd(PPh 3 ) 4 . Complex 1 could be isolated as single crystals and characterized by X-ray crystallography. The thermolysis of 1 resulted in DPC formation, although degradation of the PPh 3 ligand to PhCl and PhCO 2 Ph simultaneously occurred. PdCl 2 (PPh 3 ) 2 was a major newly formed palladium species. An efficient DPC formation was observed for the reaction of 1 with phenyl chloroformate. On the other hand, the reaction of 1 with sodium phenoxide (one equivalent) proceeded at −20xa0°C causing the instant formation of a new species assignable to Pd(OPh)(CO 2 Ph)(PPh 3 ) 2 ( 2 ) as judged by NMR ( 1 H, 13 C{ 1 H}, and 31 P{ 1 H}) spectroscopy; the nucleophilic attack by phenoxide preferentially took place on the palladium center rather than on the carbonyl group. When the reaction mixture was heated, DPC was produced probably via the reductive elimination from 2 . These results as well as the previous finding that diaryl carbonate is formed from palladium diaryloxide by carbonylation and subsequent reductive elimination suggest that Pd(OPh)(CO 2 Ph)L 2 is the final intermediate toward DPC: the reductive elimination requires a relatively high temperature.

SYNTHESIS AND PROPERTIES OF AMIDO- AND ALKOXOPALLADIUM(II) COMPLEXES WITH TMEDA (N,N,N',N'-TETRAMETHYLETHYLENEDIAMINE) LIGAND

Abstract PdCl 2 (tmeda) reacts with NaN(SiMe 3 ) 2 to give PdCl[N(SiMe 3 ) 2 ](tmeda) ( 1 ). Single crystal X-ray analysis shows the structure of 1 which has a slightly distorted square planar coordination around the Pd center with Pdue5f8N(amido) bond distance of 2.043(6) A and Pdue5f8N(amine) bond distances of 2.104(7) and 2.102(7) A, respectively. Reactions of complex 1 with 1,1,1,3,3,3-hexafluoro-2-propanol and with phenol cause substitution of the amido ligand to give the corresponding alkoxide and phenoxide palladium complexes, PdCI(OR)(tmeda) ( 2 : R ue5fb CH(CF 3 ) 2 , 3 : R ue5f8 C 6 H 5 , respectively). Reactions of dimethylpalladium complex, PdMe 2 (tmeda), with the fluoro alcohol and with phenol give PdMe(OCH(CF 3 ) 2 )(tmeda) ( 4 ) and PdMe(OC 6 H 5 )(tmeda) ( 5 ), respectively. Complex 5 reacts further with HOPh to give PdMe(OC 6 H 5 )(tmeda) · (HOPh) ( 6 ) whose 1 H NMR spectrum shows the OH hydrogen peak at extremely low magnetic field position (10.3 ppm) due to strong Oue5f8H … O hydrogen bonding between the phenoxide ligand and phenol. The tmeda ligand in complex 4 is easily displaced by addition of phosphine ligands such as dppm (bis(diphenylphosphino)methane), dppe (1,2-bis(diphenylphosphino)ethane), and dppp (1, 3-bis(diphenylphosphino)propane) to give the corresponding palladium alkoxide complexes with the phosphine ligand, PdMe(OCH(CF 3 ) 2 )(L) ( 7 : L = dppm; 8 : L = dppe; 9 : L = dppp).

Layered Hybrid Perovskites with Micropores Created by Alkylammonium Functional Silsesquioxane Interlayers

Layered organic-inorganic hybrid perovskites that consist of metal halides and organic interlayers are a class of low-dimensional materials. Here, we report the fabrication of layered hybrid perovskites using metal halides and silsesquioxane with a cage-like structure. We used a silsesquioxane as an interlayer to produce a rigid structure and improve the functionality of perovskite layers. Propylammonium-functionalized silsesquioxane and metal halide salts (CuCl2, PdCl2, PbCl2, and MnCl2) were self-assembled to form rigid layered perovskite structures with high crystallinity. The rigid silsesquioxane structure produces micropores between the perovskite layers that can potentially be filled with different molecules to tune the dielectric constants of the interlayers. The obtained silsesquioxane-metal halide hybrid perovskites exhibit some characteristic properties of layered perovskites including magnetic ordering (CuCl4(2-) and MnCl4(2-)) and excitonic absorption/emission (PbCl4(2-)). Our results indicate that inserting silsesquioxane interlayers into hybrid perovskites retains and enhances the low-dimensional properties of the materials.

Structure of dialkyltin diaryloxides and their reactivity toward carbon dioxide and isocyanate

Abstract The synthesis, structure, and reactivity of dialkyltin diaryloxides, R2Sn(OAr)2 (R=Me, Bu; Ar=Ph, p-F-C6H4, p-t-Bu-C6H4), were investigated focusing on the differences between aryloxides and methoxides. Me2Sn(OAr)2 in the solid state formed a dimer. The aryloxo-bridged structure was confirmed by single crystal X-ray analysis. On the other hand, R2Sn(OAr)2 in solution easily dissociated upon dilution. The 119Sn-NMR spectra, especially for Me2Sn(OAr)2, exhibited two distinctly resolved signals assignable to the presence of isomeric five-coordinate environments. The reactivity of R2Sn(OAr)2 was much lower than that of R2Sn(OMe)2. Thus, R2Sn(OAr)2 did not react with carbon dioxide even at high pressure and excess amount of carbon disulfide. On the other hand, the reaction with p-methoxyphenyl isocyanate led to the corresponding aryl carbamates, suggesting the occurrence of an isocyanate insertion into the tin–oxygen bond of R2Sn(OAr)2.