Нydrolytic polycondensation of organochlorosilanes (OCS) RR´SiCl
2
, where R,R´= Me, Et, C
2
H
3
, and H, in an acetone—carbamide non-aqueous system was studied. It was found by GLC, GPC,
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Si NMR ...spectroscopy, mass spectrometry, and elemental analysis that linear and cyclic oligoorganosiloxanes were formed in a quantitative yield as a result of hydrolysis. The mechanism of the process includes conjugate mutually dependent reactions: (i) generation of water
via
dehydrocondensation of acetone and carbamide involving HCl and (ii) hydrolysis of OCS to form HCl. This determines the cascade character of the overall process. The hydrolysis of OCS in the system studied develops with autoacceleration like branched cascade processes because of peculiarities of stoichiometry of the dehydrocondensation of acetone and carbamide.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
A bimetallic titanium(IV)salen complex has been developed as an exceptionally active catalyst for the asymmetric addition of trimethylsilyl cyanide to ketones. For the corresponding addition to ...aldehydes, a vanadium(IV)salen complex was found to give higher levels of asymmetric induction. In both cases, additional evidence in support of the proposed catalytic cycle is presented.
For aldehydes, a vanadium(salen) catalyst is optimal, whilst ketones require a more reactive titanium based catalyst. Kinetic studies suggest a common mechanism for these reactions.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
The use of chiral (salen)TiCl2 complexes to induce the asymmetric addition of trimethylsilyl cyanide to aldehydes has been investigated. The complexes are catalytically active at ...substrate-to-catalyst ratios as high as 1000:1, and the optimal catalyst (2e) which is derived from (R,R)-1,2-diaminocyclohexane and 3,5-di-tert-butyl-2-hydroxybenzaldehyde produces trimethylsilyl ethers of cyanohydrins with up to 90% enantiomeric excess at ambient temperature. Water plays a key role in these reactions since under strictly anhydrous conditions much lower enantiomeric excesses are produced. The role of water has been shown to be to generate dimeric complexes of the form (salen)Ti(μ-O)2 (4) which are the real catalyst precursors. A structure for one of these complexes (4a derived from (R,R)-1,2-diaminocyclohexane and 2-hydroxybenzaldehyde) has been determined by X-ray crystallography. The dimeric complexes are more active than the dichloride precursors, and at substrate-to-catalyst ratios between 100 and 1000:1 give cyanohydrin trimethylsilyl ethers with up to 92% enantiomeric excess in less than 1 h at ambient temperature.
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IJS, KILJ, NUK, PNG, UL, UM
A new type of efficient chiral catalyst has been elaborated for asymmetric C-alkylation of CH acids under PTC conditions. Sodium alkoxides formed from chiral derivatives of tartaric acid and ...aminophenols (TADDOL's 2a−e and NOBIN's 3a−h) can be used as chiral catalysts in the enantioselective alkylation, as exemplified by the reaction of Schiff's bases 1a−e derived from alanine esters and benzaldehydes with active alkyl halides. Acid-catalyzed hydrolysis of the products formed in the reaction afforded (R)-α-methylphenylalanine, (R)-α-naphthylmethylalanine, and (R)-α-allylalanine in 61−93% yields and with ee 69−93%. The procedure could be successfully scaled up to 6 g of substrate 1b. When (S,S)-TADDOL or (R)-NOBIN are used, the (S)-amino acids are formed. A mechanism rationalizing the observed features of the reaction has been suggested.
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IJS, KILJ, NUK, PNG, UL, UM
The PHENIX Electromagnetic Calorimeter (EMCal) is used to measure the spatial position and energy of electrons and photons produced in heavy ion collisions. It covers the full central spectrometer ...acceptance of 70°⩽
θ⩽110° with two walls, each subtending 90° in azimuth. One wall comprises four sectors of a Pb-scintillator sampling calorimeter and the other has two sectors of Pb-scintillator and two of a Pb-glass Cherenkov calorimeter. Both detectors have very good energy, spatial and timing resolution, while the Pb-scintillator excels in timing and the Pb-glass in energy measurements. Also, having two detectors with different systematics increases the confidence level of the physics results. Design and operational parameters of the Pb-scintillator, Pb-glass and special readout electronics for EMCal are presented and running experience during the first year of data taking with PHENIX is discussed. Some examples of data taken during the first run are shown.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The utility of the chiral Ti(μ‐O)(salen)2 complexes (R)‐ and (S)‐1 (H2salen was prepared from (R,R)‐ or (S,S)‐cyclohexane‐1,2‐diamine and 3,5‐di(tert‐butyl)‐2‐hydroxybenzaldehyde) as catalysts for ...the asymmetric addition of KCN and Ac2O to aldehydes to produce O‐acetylcyanohydrins was investigated. It was shown that the complexes were active at a substrate/catalyst ratio of 100 : 1 and produced the O‐protected cyanohydrins with ee in the range of 60–92% at −40°. Other complexes, Ti2(AcO)2(μ‐O)(salen)2 ((R)‐4) and Ti(CF3COO)2(salen) ((R)‐5), were prepared from (R)‐1 by treatment with different amounts of Ac2O and (CF3CO)2O, and their catalytic activities were tested under the same conditions. The efficiency of (R)‐4 was found to be even greater than that of (R)‐1, whereas (R)‐5 was inactive. The synthesis of the corresponding salen complexes of VIV and VV, V(O)(salen) ((R)‐2) and V(O)(salen)(H2O) S(O)3OEt ((R)‐3), was elaborated, and their X‐ray crystal structures were determined. The efficiency of (R)‐3 was sufficient to produce O‐acetyl derivatives of aromatic cyanohydrins with ee in the range of 80–91% at −40°.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
A (salen)titanium catalyst has been found to induce the asymmetric addition of potassium cyanide and acetic anhydride to aldehydes, giving enantiomerically enriched cyanohydrin esters with up to 92% ...enantiomeric excess using just 1 mol% of the catalyst. This is the first report of the asymmetric synthesis of cyanohydrin derivatives using a cyanide source which is non-volatile and inexpensive.
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