Chemoselective hydrosilylation of hydroxyketones

A chemoselective method for the hydrosilylation of ketones has been developed, using the combination of triphenylsilane and a catalyst prepared from Ni(COD)₂ and the simple N-heterocyclic carbene IMes. The most notable feature of this method is that free hydroxyls are largely unaffected, thus providing a simple one-step procedure for the conversion of hydroxyketones to mono-protected diols, wherein the protecting group is exclusively installed on the ketone-derived hydroxyl. The process is typically high yielding with both simple ketones and more complex hydroxyketone substrates.     

Silylation of hydroxyketones by silicon chlorine derivatives

The reaction of 3-methyl-3-hydroxy-2-butanone (an-hydroxyketone) with dimethyl-dichlorosilane gave 2,2,4,4-tetramethyl-5-methylene-1,3,2-dioxasilolane. The silylation of-hydroxyketones leads to acyclic mono- and bis(-ketoalkoxy)silanes.Deceased.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2153–2155, September, 1990.     

Facile preparation of allenic hydroxyketones via rearrangement of propargylic alcohols

[formula: see text] Treatment of tertiary propargylic alcohols 13 with 3-diazo-2-butanone 6 and catalytic dirhodium tetraacetate in benzene gave good yields of the diastereomeric allenic hydroxyketones 14, with, in some cases, good diastereocontrol. These products are presumably formed via the [2,3]-sigmatropic rearrangement of an alpha-propargyloxy enol derivative. This reaction has been extended to the preparation of homoallylic hydroxyketones from allylic alcohols by reaction with 6 and the rhodium catalyst.     

Chemo- and enantioselective routes to chiral fluorinated hydroxyketones using ketoreductases

Chiral fluorinated hydroxyketones were synthesized with excellent ee (>98%) and yield by a chemo- and stereoselective reduction of prochiral methyl/trifluoromethyl diketones using commercially available ketoreductase enzymes. By using p- and m-trifluoroacetyl substituted acetophenones, we demonstrate that ketoreductases can selectively differentiate between methyl and trifluoromethyl ketones within the same molecule. As a result, useful catalysts were identified that eliminated the need for costly and time-consuming protection/deprotection of the ketone moiety, enabling a more convergent synthesis of hydroxyketones. Further, a route to chiral methyl hydroxyketones is provided where an enzyme selectively reduces the unactivated ketone.     

Photochemical preparation of tricyclic hydroxyketones by transanular cyclization of bridged 4-benzoylcyclohexanones

Summary The bridged 4-benzoyl-cyclohexanones3a–f were synthesized by ,-annelation of cyclic ketones. Irradiation of3a–f revealed a strong dependence of the photochemical behaviour on the ring size and the introduction of a nitrogen atom. Ketones which are able to form 1,6-biradicals (3b,c,e) undergo unselective photolytic decomposition, whereas3a,d,f afforded tricyclic hydroxyketones. The diastereoselectivity of ring closure is remarkably improved by introduction of a protected nitrogen atom (3d,f) in comparison to the carbocyclic diketone3a. Moreover, the N protective group of 4-azatricyclo-4.3.1.0^3,8]decan-7-one (7) could be removed affording the free hydroxy amino ketone8 in good yields. An explanation of the diastereoselective cyclization of3a and of the surprisingly low quantum yield of3d was found by conformational analysis of the corresponding triplet biradicals.

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Photochemische Darstellung tricyclischer Ketone durch transanulare Cyclisierung verbrückter 4-Benzoylcyclohexanone

Zusammenfassung Die verbrückten 4-Benzoyl-cyclohexanone3a–f wurden durch ,-Anellierung cyclischer Ketone synthetisiert. Das photochemische Verhalten von3a–f hängt in starkem Maße von der Ringgröße und von der Einführung eines Stickstoffatoms ab. Ketone, die in der Lage sind, 1,6-Biradikale zu bilden (3b,c,e), unterliegen einer unselektiven photolytischen Zersetzung, während3a,d,f tricyclische Hydroxyketone liefern. Die Diastereoselektivität des Ringschlusses wird durch Einführung eines Stickstoffatoms (3d,f) im Vergleich zum carbocyclischen Analogon3a deutlich gesteigert. Weiterhin gelang es, die N-Schutzgruppe im 4-Azatricyclo-[4.3.1.0^3,8]decan-7-on (7) unter Bildung des freien Hydroxyaminoketons8 in guten Ausbeuten zu entfernen. Eine Erklärung für die diastereoselektive Cyclisierung von3a und für die überraschend geringe Quantenausbeute von3d wurde durch Konformationsanalyse der entsprechenden Triplett-Biradikale gefunden.

     

Comparative reactivity of hydroxyketones and their derivatives in the reactions with N,S-nucleophiles

Russian Chemical Bulletin - The reactions of thiosemicarbazide (TSC), thiocarbohydrazide, and hydrazine thiocarbamate with oxyketones, haloketones, and derivatives of acetylenic alcohols were...     

Copper(II) complexes of schiff bases derived from tryptamine and hydroxyketones or hydroxydiketones

Summary Cu^II complexes of Schiff bases derived from tryptamine/ hydroxyketone or hydroxydiketone condensation have been prepared and characterized by mass spectrometry. i.r., visible-ultraviolet and¹H n.m.r. spectroscopy and by magnetic moments.     

Mechanism of cyclopropane-propene isomerization

A SINDO method was used to investigate the sequence of rearrangements in the cyclopropane-propene isomerization. Geometries of reactant and product were optimized for a complete set of 21 independent internal coordinates. Then the bond angle of the migrating H atom was chosen as reaction coordinate and all other geometrical parameters were energy minimized along this reaction coordinate. In the neighborhood of the transition state a Newton type iteration process is initiated to locate the transition state. The latter is characterized by a CCC bond angle close to 90 ° and an almost finished migration of the H atom. The activation energy was found about 25% lower than the experimental value of 65 kcal/mole. A sequence of rearrangements was established, suggesting the initiation of the reaction by a combination of two normal modes: a torsional vibration of the methylene groups together with an asymmetric stretch of the carbon framework. Our calculations classify the reaction as concerted.     

Nucleophilic isomerization of perfluoropentenes

Conclusions The isomerization of perfluoro-1-pentene by CsF in polar aprotic solvents leads to a mixture of trans- and cis-perfluoro-2-pentene in an isomer ratio of 61.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2646–2647, November, 1981.     

Base-catalyzed isomerization of phenylpropenes

The prototropic rearrangement of 3-phenyl-1-propenes to the corresponding 1-phenyl-1-propenes was investigated in basic media utilizing 0.1M sodium ethoxide in absolute ethanol at 81°C. It was found that the effect of substituents on the rate of such isomerizations follows the order: p-NO₂ > o-Cl > m-Cl ≥ m-F > p-Br > o-CH₃ > m-CH₃ > m-CH(CH₃)₂ > p-CH(CH₃)₂ ≥ p-C(CH₃)₃ > o-OCH₃. This is consistent with first-order kinetics and “BS” mechanism. Quantitative treatment in terms of Hammett's equation showed a straight line, with a slope (p value) of +2.25. An increase in the strength of the base was also found to cause an increase in the rate of isomerization.     

Ultrafast carbene-carbene isomerization

The photochemistry of two isomeric aryl diazo ketones was investigated by fs time-resolved UV-vis and IR spectroscopies. Both diazo ketone excited states decompose in less than 300 fs by multiple pathways. One pathway involves concerted Wolff rearrangement and nitrogen extrusion, most likely in the syn rotomer. In the anti rotomer of one isomer, oxygen migration proceeds in concert with nitrogen extrusion to form rearranged keto carbene. This rotomer excited state also decomposes to form unrearranged carbene, which isomerizes in 5 ps.     

Cis-trans isomerization of glyoxal

The CNDO/2 method with the original parameter set predicts the conformation with the mutually perpendicular CH=O groups to be the most stable isomer of glyoxal; the cis form is favoured over the trans form. The order of stabilities is not changed upon the full optimization of coordinates of the isomers. Calculations fail to reproduce the observed order of isomer stabilities unless allowance is made for limited configuration interaction with the lowest ππ* doubly excited state and for geometry optimization. Doubly excited configurations of the ππ* and σσ* types have negligible effect on the energies of isomers.     

Radiation-induced isomerization of benzene

Summary It was shown that, when liquid benzene saturated with argon is subjected to the action of accelerated electrons, the isomeric hydrocarbon fulvene is formed. The yield of fulvene is 0.76–0.85 molecules/100 ev.Paper read at a session of the Section of Radiochemistry and Chemistry of Isotopes at the Eighth Mendeleev Congress on General and Applied Chemistry.     

The vinylidene-acetylene isomerization barrier

The barrier to rearrangement of vinylidene to acetylene has been determined using Møller-Plesset perturbation theory and an extended polarized basis set. With the inclusion of zero-point vibrational corrections, the barrier is found to be only 0.9 $\text{kcal}\text{mole}$, suggesting that vinylidene will have an extremely short lifetime.     

Skeletal isomerization of 1-pentene

Series of ZSM-5 and Y zeolite catalysts have been prepared. Zeolites were submitted to ion exchange, drying, formation with 50 wt.% of aluminium hydroxide as a binder and then to final thermal treatment. Determination of catalysts activity in 1-pentene isomerization reaction was carried out in the temperature range of 175–325°C. The highest activity in isomerization reaction was attained in the presence of ZSM-5 zeolite catalysts with strong and medium strength acidity.     

Gas-phase thermal isomerization of hexachlorocyclopropane

The gas-phase thermal isomerization of hexachlorocyclopropane to hexachloropropene at 208–283°C is first order and unaffected by changes in the surface-to-volume ratio or by the addition of iodine, tetrachloroethylene, and oxygen. The first-order rate constants fit the Arrhenius equation The reaction was interpreted as an unimolecular process taking place with chlorine atom migration. A comparison of the reactivities of several chlorocyclopropanes is made.