Presence Of i-PrOH Research Articles

Potassium complexes supported by monoanionic tetradentate amino-phenolate ligands: synthesis, structure and catalysis in the ring-opening polymerization of rac-lactide.

A series of potassium complexes bearing monoanionic tetradentate amino-phenolate ligands, [LK]2 (L = {(2-R1)C6H4CH2N[(CH2)2R2]CH2(4-R4-6-R3)C6H2O-}, R1 = NMe2, R2 = NEt2, R3 = CPh3, R4 = Me (1); R1 = R2 = NEt2, R3 = CPh3, R4 = Me (2); R1 = NMe2, R2 = NEt2, R3 = R4 = cumyl (4); R1 = R2 = OMe, R3 = tBu, R4 = Me (6); L = (2-NMe2)C6H4CH2N[[CH2-(S)-1-butylpyrrolidinyl]CH2(4-Me-6-CPh3)C6H2O-] (3)), have been synthesized via reactions of KN(SiMe3)2 and 1 equiv. of the corresponding aminophenols. The solid-state structures of typical complexes 4 and 6 are determined via X-ray diffraction studies, which reveal the dinuclear nature of these complexes. By contrast, DOSY measurements of 1, 4 and 6 suggest that these complexes are monomeric in solution. It is noteworthy that the coordination chemistry of these potassium complexes is versatile, which is closely related to the nature of the ortho-substituent of the phenolate ring, as indicated by the results of the corresponding spectroscopic studies. In the presence of iPrOH, 1-4 and 6 could initiate the polymerization of 500 equiv. of rac-lactide to achieve high monomer conversions within several minutes but afford atactic PLAs with slightly isotactic-enriched microstructures (Pm = 0.58-0.60). Experimental results also demonstrated that a bulky trityl substituent at the ortho-position of the phenolate ring of the ligand framework is beneficial for the enhancement of the activities of these potassium complexes.

Gold-catalyzed synthesis of tetrazoles from alkynes by C-C bond cleavage.

Golden duality: Tetrazoles are formed by the reaction of alkynes with TMSN3 (TMS=trimethylsilyl) in the presence of iPrOH and the gold(I) catalyst [JohnPhosAu(MeCN)]SbF6. In this transformation gold plays a dual role, first activating the alkyne and then generating a Brønsted acid in situ.

Synthetic and Mechanistic Aspects of the Immortal Ring‐Opening Polymerization of Lactide and Trimethylene Carbonate with New Homo‐ and Heteroleptic Tin(II)‐Phenolate Catalysts

Several new heteroleptic Sn(II) complexes supported by amino-ether phenolate ligands [Sn{LO(n)}(Nu)] (LO(1)=2-[(1,4,7,10-tetraoxa-13-azacyclopentadecan-13-yl)methyl]-4,6-di-tert-butylphenolate, Nu=NMe(2) (1), N(SiMe(3))(2) (3), OSiPh(3) (6); LO(2)=2,4-di-tert-butyl-6-(morpholinomethyl)phenolate, Nu=N(SiMe(3))(2) (7), OSiPh(3) (8)) and the homoleptic Sn{LO(1)}(2) (2) have been synthesized. The alkoxy derivatives [Sn{LO(1)}(OR)] (OR=OiPr (4), (S)-OCH(CH(3))CO(2)iPr (5)), which were generated by alcoholysis of the parent amido precursor, were stable in solution but could not be isolated. [Sn{LO(1)}](+)[H(2)N{B(C(6)F(5))(3)}(2)](-) (9), a rare well-defined, solvent-free tin cation, was prepared in high yield. The X-ray crystal structures of compounds 3, 6, and 8 were elucidated, and compounds 3, 6, 8, and 9 were further characterized by (119)Sn Mössbauer spectroscopy. In the presence of iPrOH, compounds 1-5, 7, and 9 catalyzed the well-controlled, immortal ring-opening polymerization (iROP) of L-lactide (L-LA) with high activities (ca. 150-550 mol(L-LA) mol(Sn)(-1) h(-1)) for tin(II) complexes. The cationic compound 9 required a higher temperature (100 °C) than the neutral species (60 °C); monodisperse poly(L-LA)s were obtained in all cases. The activities of the heteroleptic pre-catalysts 1, 3, and 7 were virtually independent of the nature of the ancillary ligand, and, most strikingly, the homoleptic complex 2 was equally competent as a pre-catalyst. Polymerization of trimethylene carbonate (TMC) occurs much more slowly, and not at all in the presence of LA; therefore, the generation of PLA-PTMC copolymers is only possible if TMC is polymerized first. Mechanistic studies based on (1)H and (119)Sn{(1)H} NMR spectroscopy showed that the addition of an excess of iPrOH to compound 3 yielded a mixture of compound 4, compound [Sn(OiPr)(2)](n) 10, and free {LO(1)}H in a dynamic temperature-dependent and concentration-dependent equilibrium. Upon further addition of L-LA, two active species were detected, [Sn{LO(1)}(OPLLA)] (12) and [Sn(OPLLA)(2)] (14), which were also in fast equilibrium. Based on assignment of the (119)Sn{(1)H} NMR spectrum, all of the species present in the ROP reaction were identified; starting from either the heteroleptic (1, 3, 7) or homoleptic (2) pre-catalysts, both types of pre-catalysts yielded the same active species. The catalytic inactivity of the siloxy derivative 6 confirmed that ROP catalysts of the type 1-5 could not operate according to an activated-monomer mechanism. These mechanistic studies removed a number of ambiguities regarding the mechanism of the (i)ROPs of L-LA and TMC promoted by industrially relevant homoleptic or heteroleptic Sn(II) species.

Synthesis of Pd nanoparticles on heteropolyacid-supported silica by a photo-assisted deposition method: an active catalyst for the direct synthesis of hydrogen peroxide

A simple and unique methodology to synthesize active Pd nanoparticles using Cs2.5H0.5PW12O40 heteropolyacid-supported silica (CsHPA/SiO2) under UV-light irradiation has been developed. By the photo-assisted deposition (PAD) method, a Pd2+ precursor in aqueous solution can be deposited and easily reduced by the [PW12O40]4− species generated by photolysis of [PW12O40]3− species in the presence of iPrOH as a sacrificial reagent, to afford nanosized Pd0 metal particles (Pd/CsHPA/SiO2). No direct Pd metal deposition was observed on either pure SiO2 without the CsHPA phase under UV-light irradiation or on CsHPA/SiO2 without UV-light irradiation. The catalytic activity for the direct synthesis of hydrogen peroxide (H2O2) using H2 and O2 gases under atmospheric pressure was strongly dependent on the preparation conditions, variation of CsHPA content, and the type of catalyst support. In addition, the Pd/CsHPA/SiO2 catalyst prepared by the PAD method exhibited significantly better activity than the conventionally prepared impregnation catalyst. The Pd surface area from CO adsorption measurements, and also the suppression of undesired side reactions were determined as the crucial factors to achieve efficient catalytic performance.

An efficient one-pot synthesis of highly substituted furans catalyzed by N-bromosuccinimide

N-Bromosuccinimide was found to efficiently catalyze the synthesis of highly functionalized, tetra-substituted furan derivatives in the one-pot reactions of but-2-ene-1,4-diones and acetoacetate esters in the presence of i-PrOH as solvent under mild and neutral conditions at 80-90?C for 3-7 h in high yields (87-94%).

Samarium(II)‐Promoted Radical Spirocyclization onto an Aromatic Ring.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Samarium(II)-promoted radical spirocyclization onto an aromatic ring.

Samarium(II)-mediated spirocyclization onto an aromatic ring was achieved by the reaction of methyl 4-(4-oxoalkyl)benzoates with SmI(2) in the presence of i-PrOH and HMPA, yielding methyl 1-alkyl-1-hydroxyspiro[4.5]dec-6-ene-8-carboxylates in moderate to high yields. Utilizing this chemistry, spiro[3.5] and -[5.5] systems, and sterically congested spiro[4.5] systems, were easily synthesized. For the successful conversion, appropriate activation of the aromatic ring has proven to be extremely important: while an ester or amide functionality on the aromatic ring can promote the spirocyclization, a sulfonamide substituent causes ortho cyclization.

Water-miscible organic cosolvents enhance phosphatidylinositol-specific phospholipase C phosphotransferase as well as phosphodiesterase activity

Phosphatidylinositol-specific phospholipase C (PI-PLC) from Bacillus thuringiensis catalyzes the hydrolysis of phosphatidylinositol (PI) in a Ca 2+-independent two-step mechanism: (i) an intramolecular phosphotransferase reaction to form inositol 1,2-(cyclic)-phosphate (cIP), followed by (ii) a cyclic phosphodiesterase activity that converts cIP to inositol 1-phosphate (I-1-P). Moderate amounts of water-miscible organic solvents have previously been shown to dramatically enhance the cyclic phosphodiesterase activity, that is, hydrolysis of cIP. Cosolvents [isopropanol (iPrOH), dimethylsufoxide (DMSO), and dimethylformamide (DMF)] also enhance the phosphotransferase activity of PI-PLC toward PI initially presented in vesicles, monomers, or micelles. Although these water-miscible organic cosolvents caused large changes in PI particle size and distribution (monitored with pyrene-labeled PI fluorescence, 31P NMR spectroscopy, gel filtration, and electron microscopy) that differed with the activating solvent, the change in PI substrate structure in different cosolvents was not correlated with the enhanced catalytic efficiency of PI-PLC toward its substrates. PI-PLC stability was decreased in water/organic cosolvent mixtures (e.g., the T m for PI-PLC thermal denaturation decreased linearly with added iPrOH). However, the addition of myo-inositol, a water-soluble inhibitor of PI-PLC, helped stabilize the protein. At 30% iPrOH and 4 °C (well below the T m for PI-PLC in the presence of iPrOH), cosolvent-induced changes in protein secondary structure were minimal. iPrOH and diheptanoylphosphatidylcholine, each of which activates PI-PLC for cIP hydrolysis, exhibited a synergistic effect for cIP hydrolysis that was not observed with PI as substrate. This behavior is consistent with a mechanism for cosolvent activation that involves changes in active site polarity along with small conformational changes involving the barrel rim tryptophan side chains that have little effect on protein secondary structure.

The first samarium(II)-mediated stereoselective spirocyclization onto an aromatic ring.

The first samarium(II)-mediated spirocyclisation onto an aromatic ring was achieved by the reaction of methyl 4-(4-oxoalkyl)benzoates with SmI2 in the presence of i-PrOH and HMPA, yielding methyl 1-alkyl-1-hydroxyspiro-[4.5]dec-6-ene-8-carboxylates in moderate to high yields.

In Situ EXAFS Study of the Photocatalytic Reduction and Deposition of Gold on Colloidal Titania

The photocatalytic reduction of gold on a colloidal Ti02 sample in the presence of i-PrOH has been studied by in situ EXAFS. This colloidal sample shows a quantum size effect in its band gap UV-vis absorption and a photocatalytic activity for the formation of Ti3+ species. An experimental setup has been used that consists of a photochemical reactor connected to a peristaltic pump and an EXAFS transmission cell specially designed for liquid samples. The results show that it is possible to follow the two-step mechanism of the photoreduction of noble metals on titania, i.e. the nucleation and the growth of the metallic crystallites. At the same time, a surprisingly low intensity of the threshold feature at the Au LIII edge has been found for low irradiation times, when small gold particles are formed. This effect has been interpreted as a result of a negative charge injection into the metal particles which is favored by the presence of the alcohol acting as a hole trap. For comparison, an in situ XPS analysis has been carried out to follow the photoreduction of gold in a Au3+-Ti02 material in the presence of i-PrOH at the gas-solid interface.