Insertion Of Heteroatoms Research Articles

Effect of synthesis conditions on the pore structure and degree of heteroatom insertion in Zr-doped SBA-15 silica-based materials prepared by classical or microwave-assisted hydrothermal treatment

Zirconium doped mesoporous SBA-15 materials with initial Si/Zr ratios of 10 and 20 have been prepared under microwave-hydrothermal conditions using a non-ionic triblock copolymer surfactant as structure-directing agent, as well as zirconyl chloride and zirconium propoxide as two zirconium sources. The effect of a short (5 min) and long (1.5 h) sol ageing time on the degree of heteroatom incorporation was tested. Changes in the mesopore size and arrangement were monitored with the aid of powder X-ray diffraction (XRD) and gas nitrogen adsorption/desorption at 77 K. The degree of Zr incorporation into the silica framework of the final product was inferred from the analysis by energy-dispersive X-ray spectroscopy and compared with the starting value based on the amount of zirconium added to the reaction mixture. In the case of samples with the lowest initial theoretical molar ratio, the synthesis conditions did not affect the degree of heteroatom incorporation into the silica framework and all zirconium could be introduced under the conditions applied. The structure of materials was strongly dependent on the conditions used, and a more regular pore arrangement was obtained for longer maturation stages. The classical hydrothermal synthesis did not result in a significantly higher efficiency in the heteroatom insertion, though microwave-assisted treatment yielded materials with thicker pore walls. Further increase in the amount of zirconium added to the initial gel did not contribute to a greater heteroatom loading in the final product under microwave conditions.

Unequivocal Heteroatom Insertion into a 24‐Ring Channel Gallophosphate and Its Photoluminescence.

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Unequivocal Heteroatom Insertion into a 24-Ring Channel Gallophosphate and Its Photoluminescence

Five different transition metal elements have been successfully inserted into a unique 24-ring (24R) channel structure, NTHU-1, which is the first neutral framework gallophosphate displaying visible photoluminescence without added metal activators. Incorporation of heteroatoms into the intraframework sites has intricately tuned luminescence properties and made subtle changes in channel size as well as imposed magnetism to NTHU-1. The resultant phases, designated as NTHU-1M with M = Cr, Mn, Fe, Co, CrCo, and Zn, were respectively characterized by X-ray diffraction, thermal analyses, photoluminescence, UV−vis spectra, and/or magnetic studies. Our reaction conditions enabled the formation of large crystals with higher quality and a more definite degree of incorporation, which allowed unambiguous elucidation of the location, population, and environment of each of the heteroatoms in one 24R channel structure from single crystals, than ever reported before. In addition, the successful insertion of Cr produced unprecedented Cr and Ga mixed-metal phosphates with different optical properties from Cr-doped aluminophosphates or zeolites. Two distinct metal sites in different geometries were observed in NTHU-1, where the tetrahedral site was occupied by divalent heteroatoms and the octahedral site by the trivalent. The two distinct sites could even be occupied simultaneously, resulting in a unique CrCo dual insertion. Factors concerning the incorporation and the influence of heteroatoms on the luminescence property are discussed.

Effect of Heteroatom Insertion at the Side Chain of 5-Alkyl-1H-tetrazoles on Their Properties as Catalysts for Ester Hydrolysis at Neutral pH

[reaction: see text] Herein we introduce tetrazole and its suitably designed derivatives as powerful ester-cleaving reagents. By first performing a detailed ab initio computational study, we found that, in the side chain of 5-alkyl-1H-tetrazoles, introduction of a heteroatom (e.g., N, O, or S at the alpha-position of the tetrazole ring) raises the charge on the tetrazole nucleus significantly. All calculations have been performed using restricted Hartree-Fock (RHF) and hybrid ab initio/DFT (B3LYP) methods employing 6-31G* and 6-31+G* basis sets. To estimate the nucleophilicity of these reagents, the charges on conjugate bases of various tetrazole derivatives have been calculated using natural population (NBO) analysis in gas phase and in water. Free energy of protonation (fep) of the 1H-tetrazole derivatives (1-7), free energy of solvation, deltaG(aq), and the corresponding pKa values have been calculated by self-consistent reaction field (SCRF) methods applying the polarized continuum model (PCM). Since the calculation indicates that incorporation of heteroatom leads to enhanced nucleophilicity in their deprotonated anionic tetrazole forms, a series of 5-substituted 1H-tetrazole derivatives have been synthesized. These compounds indeed catalyze the hydrolysis of p-nitrophenyl diphenyl phosphate (PNPDPP) and p-nitrophenyl hexanoate (PNPH) efficiently in cationic cetyl trimethylammonium bromide (CTABr) micelles at pH 7.0 and 25 degrees C. The pseudo-first-order rate constants (k(obs)) were determined for each catalyst against both substrates. The experimental and theoretical results show that, to achieve better k(obs) values for the cleavage of PNPDPP and PNPH under micellar conditions, charge on the N- atom (nucleophile) of conjugate base is important. Replacing the alpha-CH2 in alkyl substituent with S (3), NH (4), or O (5) enhances the accumulation of charge on N- in conjugate bases of tetrazoles and subsequently increases their intrinsic nucleophilic reactivity toward hydrolytic reactions. Significantly large rate enhancements were observed for the cleavage of PNPDPP and PNPH at pH 7.0 in the presence of catalytic system 5/CTABr over background (only CTABr). Tetrazole 4 (alpha-isomer) showed 4-5-fold superior reactivity over 6 (beta-isomer) under identical conditions. Natural charges obtained from NBO analysis (B3LYP/6-31+G*) are -0.94 and -0.852 on N- in the conjugate bases of 4 and 6, respectively. This also predicts that 4 is a better nucleophile than 6. All the newly synthesized tetrazole derivatives in micellar media display true catalytic properties by cleaving several fold excess of substrates.

Melanocyte-Directed enzyme prodrug therapy (MDEPT): Development of second generation prodrugs for targeted treatment of malignant melanoma

Evaluation of second generation prodrugs for MDEPT, by oximetry, has highlighted structural properties that are advantageous and disadvantageous for efficient oxidation using mushroom tyrosinase. In particular, a sterically undemanding prodrug bis-(2-chloroethyl)amino-4-hydroxyphenylaminomethanone 28 was synthesised and found to be oxidised by mushroom tyrosinase at a superior rate to tyrosine methyl ester, the carboxylic acid of which is the natural substrate for tyrosinase. The more sterically demanding phenyl mustard prodrugs 9 and 10 were oxidised by mushroom tyrosinase at a similar rate to tyrosine methyl ester. In contrast, tyramine chain elongation via heteroatom insertion was detrimental and the rate of mushroom tyrosinase oxidation of phenyl mustard prodrugs 21 and 22 decreased by 10 nanomol/min.

Spin–orbital coupling effects on the photophysical properties and photocytotoxicity of merocyanine dyes

A series of merocyanine dyes possessing different heteroatoms in the terminal aromatic and barbiturate units has been synthesized. The nature of the heteroatom does not affect the all-trans alignment of the central polyenic bridge, as demonstrated by 1 H NMR spectroscopy. There are pronounced changes, however, in the photophysical properties of the individual molecules due to structural, electronic and spin–orbital coupling effects induced by the heteroatoms. In particular, it has been possible to vary the quantum yield for formation of the lowest-energy triplet excited state by more than two orders of magnitude. Quantum yields and lifetimes for fluourescence and photoisomerization are also affected by the choice of heteroatom. Despite the major changes in triplet yield that take place upon insertion of heteroatoms displaying strong spin–orbital coupling effects (e.g., Se) the efficacy for light-induced cytotoxicity towards cancer cells shows but a modest dependence on the structure of the dye. There is a reasonably good correlation between triplet yield and the rate of light-induced cell killing as might be expected for a mechanism based on intermediate generation of singlet molecular oxygen. However, equally good correlations exist between cell-killing performance and both relative lipophilicity and the rate at which the dye penetrates into intact biological cells.

Metallaborane Heteroatom Incorporation Reactions: Metallacarboranes, Metallathiaboranes, and an Iridaazaborane from Iridanonaborane Precursors

Formation of metallaheteroboranes by insertion of heteroatoms into an existing metallaborane cluster, rather than adding the metal moiety to the heteroborane which is normally the case, has been achieved in the cases of carbon, nitrogen, and sulfur to produce nine-, ten-, and eleven-vertex metallaheteroborane clusters. Thus passage of acetylene through refluxing p-xylene solutions of either arachno-[(PMe3)2(CO)HIrB8H12] (1a) or nido-[(PMe3)2(CO)IrB8H11] (2a) afforded the 11-vertex cluster nido-[9,9,9-(PMe3)2(CO)-9,7,8-IrC2B8H11] (5a) in up to 44% yield. With the cage substituted species arachno-[(PMe3)2(CO)HIrB8H11Cl] (1b), the analogous product is nido-[5-Cl-9,9,9-(PMe3)2(CO)-9,7,8-IrC2B8H10] (5b) and the position of the Cl substituent in the product suggests that the acetylene moiety attacks across the open face of the intermediate nido-iridanonaborane species and that cage rearrangement does not occur during the course of the reaction. Similarly, reaction of 1a with H2S under identical conditions resul...

Synthesis and applications of olefin‐contaning polymers

AbstractThe problems of ionic coordination polymerization of higher α‐olefins are summarized briefly. During the polymerization such monomers as 3‐methyl‐butene‐1, vinylcyclohexane and others isomerize into unreactive isomers, and the rate of polymerization and final yield of polymer are sharply decreased as compared with the polymerization of lower olefins. The modification of polypropylene by ionic coordination copolymerization of propylene with higher α‐olefins and polar monomers is considered. Significant modification of carbochain polymers may be obtained by the insertion of heteroatoms in the backbone. New possibilities for chemical “modification” of olefins by direct interaction with sulphur and the unusual aspects of the polymerization of olefin sulfides are also considered. The subsequent transformation of polyolefin sulfide gives polymers which have properties drastically differing from those of polyolefins. In conclusion, some new application areas are described.

INSERTION OF OXYGEN ATOM INTO FUSED CYCLOBUTANE RINGS VIA AN ALKOXYL RADICAL REARRANGEMENT

Abstract A new heteroatom insertion into cyclobutane rings via β-scission of alkoxyl radicals generated from photocycloadducts between 4-hydroxycoumarin and cycloalkenes is described.

Angiotensin converting enzyme inhibitors: modifications of a tripeptide analogue.

Modified nonhydrolyzable tripeptide analogues of (S)-1-[5-(benzoylamino)-1,4-dioxo-6-phenylhexyl]-L-proline (1), designed to impart oral angiotensin converting enzyme (ACE) inhibitory activity, were made and evaluated in vivo and in vitro. The N-methyl and C5-methyl analogues of 1 were inactive. Insertion of heteroatoms (O, S, NH) into the C--C chain of 1 gave a series of compounds with high in vitro activity in the guinea pig serum ACE assay. The O-analogue was the most potent with an IC50 = 4.4 x 10(-9) M compared to 1 with an IC50 = 3.2 x 10(-9) M. The structure-activity relationships in this series of compounds lead one to speculate that the heteroatom provides an additional binding site to the surface of the enzyme; however, these compounds were inactive when tested for antihypertensive activity in the renal hypertensive rat at 30 mg/kg by the oral route (captopril is active at 1.0 mg/kg po).

Hypoiodite reaction of N-acetyljervine: The formations of oxepane and oxetane rings by hetero atom insertion from alkoxyl radical

Hypoiodite reaction of N-acetyljervine: The formations of oxepane and oxetane rings by hetero atom insertion from alkoxyl radical

Studies of selenides of thiophene and furan series—V: Mass spectra of some thiophene, furan and selenophene derivatives

AbstractMass spectra of some selenides, sulphides and ethers of furan, thiophene and selenophene series are described. A new fragmentation reaction, consisting in splitting off alkyl, insertion of heteroatom in the cycle and loss of one of heteroatoms as CX, is discussed in detail.

Thermal oxidative degradation of linear polydimethylsiloxanes and polydimethylelementosiloxanes

1. A study was made of the thermal oxidative degradation of linear polydimethylsiloxanes and polydimethylelementosiloxanes. The degradation process is composed of two reactions: oxidation of the organic groups and degradation of the main chains of the molecule. 2. The insertion of hetero atoms or groups in the dimethylsiloxane chain can sharply increase the thermal oxidative stability of the main chain of the molecule. 3. The mechanism of the action of heteroatoms and other groups on the thermal oxidative stability of polydimethylsiloxanes was discussed.

The density matrix in self-consistent field theory II. Applications in the molecular orbital theory of conjugated systems

The methods of a previous paper (McWeeny 1956) are applied in the theory of conjugated systems. The density matrix—which in this case is the array of 'charges’ and 'bond orders'—may be calculated for a general π-electron system, in self-consistent field (s. c. f.) approxima­tion, by iterative refinement of an initial estimate. In choosing an initial estimate it is possible to make use of known Hückel theory solutions for simple hydrocarbons, building up approximations for the more complicated systems by changing parameters (describing, for example, the insertion of hetero-atoms or the inter-connexion of different fragments). This may be done by a density matrix perturbation method which avoids all reference to the wave function itself (§3). In this way, Hückel approximations for rather general systems can be written down with very little calculation (§4), and at the same time, there are interesting chemical applications along the lines of Coulson & Longuet-Higgins (1947 a , b ; 1948 a , b ). Finally, self-consistency can be introduced (§5) and the perturbation method can be carried over into the s. c. f. theory; this permits a rather more realistic discussion of chemical properties and explains the success, in certain cases, of the simple Hückel approach.