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11. Mechanism of the B(C<INF>6</INF>F<INF>5</INF>)<INF>3</INF>-Catalyzed Reaction of Silyl Hydrides with Alkoxysilanes. Kinetic and Spectroscopic Studies

Author

Chojnowski, J., Rubinsztajn, S., Cella, J. A., Fortuniak, W., Cypryk, M., Kurjata, J., and Kazmierski, K.

Abstract

The coupling reaction of silyl hydrides with alkoxysilanes to produce siloxanes and hydrocarbons catalyzed by tris(pentafluorophenyl)borane was studied by gas chromatography and UV spectroscopy using model reagent systems:  Ph2MeSiH + Ph2MeSiOn-Oct (I) and Ph2MeSiH + Me3SiOn-Oct (II). Detailed kinetic studies performed for system I showed that the reaction is first order in both substrates and the rate is proportional to the catalyst concentration. A highly negative apparent entropy of activation points to a crowded transition state structure, leading to a significant dependence of the rate on steric effects. Studies of system II demonstrated that the exchange of the Si−H and Si−OR functionality accompanies the coupling process and in many cases is the dominating reaction in this system. Ultraviolet spectra recorded during the reaction show a distinct strong absorption band with λmax = 303−306 nm, which is due to an allowed electronic transition in the uncomplexed B(C6F5)3 molecule. This absorption also gives rise to intense fluorescence with a maximum of the emission band at 460 nm. When the borane is complexed by oxygen nucleophiles, such as water, alcohol, or silanol and is not active as a catalyst, it does not show the absorption in the 303−306 nm region. This absorption may serve as a measure of the concentration of the active uncomplexed catalyst in the reaction system. Since complexes of B(C6F5)3 with the alkoxysilane substrates and the disiloxane products are relatively weak, the catalyst appears in the reaction system mostly as an uncomplexed species and its concentration is not significantly changed during the reaction. The mechanism proposed includes the transient formation of a complex between hydrosilane, borane, and alkoxysilane in which H- is transferred from silicon to boron and an oxonium ion moiety is generated by interaction of alkoxysilane with positive silicon. The decomposition of the complex occurs by the H- transfer to one of the three electrophilic centers of the oxonium structure, which explains the competition between the siloxane formation and the Si−H/Si−OR exchange. In the case of alkoxysilanes derived from primary alcohols, H- is preferably transferred to silicon. However, for alkoxysilanes derived from a secondary alcohol, such as isopropyl alcohol, the secondary carbon is more readily attacked than silicon by H-, which leads to a high yield of mixed disiloxane.

Published
2005

42. Radioimmunoassay of methaqualone and its monohydroxy metabolites in urine.

Author

Berman AR, McGrath JP, Permisohn RC, and Cella JA

Subjects
Chromatography, Gas, Humans, Methaqualone metabolism, Radioimmunoassay, Structure-Activity Relationship, Methaqualone urine
Abstract

A commercial radioimmunoassay kit was evaluated for efficacy in detecting methaqualone or its metabolites in urine of persons receiving this drug. The drug and its unconjugated 3'- and 4'-monohydroxy metabolites could be detected equally well. The unconjugated alpha-monohydroxy metabolite was about 80% as reactive and the unconjugated 6-monohydroxy metabolite reacted only very weakly. Quantitation of the conjugated metabolites was less sensitive than of unconjugated. Nineteen urine specimens which reacted positively to radioimmunoassay and which thin-layer chromatography had shown to contain methaqualone and its metabolites were also examined by gas-liquid chromatography. Those specimens that reacted strongly to radioimmunoassay contained high concentrations of the drug or its metabolites. In the specimens examined by gas-liquid chromatography, the apparent concentrations of the metabolites were generally higher than those of the drug itself. Methaqualone in combination with its unconjugated metabolites reacted additively with the radioimmunoassay, resembling the same concentration of parent drug alone. Detection limits were between 10-200 mug/liter.

Published
1975

43. Preparation of spin-labeled opiates: morphine and codeine.

Author

Cella JA and Kelley JA

Subjects
Isomerism, Methods, Codeine analogs & derivatives, Codeine chemical synthesis, Morphine Derivatives chemical synthesis, Spin Labels chemical synthesis
Abstract

The preparation of 6-spin-labeled codeine and morphine is described. Treatment of either 6-chlorocodide or 8-bromocodide with 4-amino-2,2,6,6-tetramethylpiperidino-1-oxyl free radical in dimethylformamide afforded 6-spin-labeled codeine. Similar treatment of 6-chloromorphide afforded 6-spin-labeled morphine. Exclusive formation of the 6-isomer in these reactions is explained by halide-ion-catalyzed isomerization of the 6-halo opiate to the 8-halo isomer followed by a normal SN2' displacement of the halogen. Both spin-labeled compounds displayed weak in vivo analgesic activity and did not bind appreciably to receptors in brain homogenate.

Published
1977
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44. Thin-layer chromatographic detection and identification of methaqualone metabolites in urine.

Author

Sleeman HK, Cella JA, Harvey JL, and Beach DJ

Subjects
Chromatography, Thin Layer, Humans, Hydrolysis, Indicators and Reagents, Methods, Ultraviolet Rays, Methaqualone urine
Abstract

A procedure for detecting methaqualone and identifying methaqualone metabolites in urine by thin-layer chromatography is described and evaluated. Urine is hydrolyzed with HCl or NaIO4, adjusted to pH 9.5, and extracted with chloroform. The chloroform extract is evaporated, reconstituted in methanol, applied to fluorescent silica-gel plates, and developed with ethyl acetate:methanol:ammonium hydroxide (28%) (85:10:5 by vol). Methaqualone use is detected by a pattern of four metabolites, which can be seen under ultraviolet light or are made visible by acidified iodoplatinate reagent. Synthetic methaqualone metabolites are used for identification and to compensate for procedural variables. More than 250 positive urine specimens were correctly identified by this method. Hydrolyzed natural and synthetic metabolites were identical by several criteria.

Published
1975

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