Methoxymethylsilylene: (1 + 6)- and (1 + 4)-cycloadditions to heterotrienes

Co-pyrolytic gas phase reactions of 1,2-dimethyl-1,1,2,2-tetra-(methoxy)disilane with conjugated 1-oxatrienes in a flow-reactor furnish, via intermediate methoxymethylsilylene, mixtures of diastereoisomers of 3-phenyl-1-oxa-2-silacyclohepta-4,6-dienes and 3-styryl-1-oxa-2-silacyclopent-4-enes, usually including smaller amounts of the corresponding 1-oxa-2-silacy-clopent-3-ene isomers. In co-pyrolysis with an analogous N-isopropylazatriene, five-membered rings were formed preferentially, and seven-membered isomers could not be detected by NMR spectroscopy. © 1995 John Wiley & Sons, Inc.     

Gold-catalyzed synthesis of substituted 2-aminofurans via formal [4+1]-cycloadditions on 3-en-1-ynamides

Gold-catalyzed formal [4+1]-cycloadditions between 3-en-1-ynamides 1 and 8-methylquinoline oxide are reported. The success of this reaction relies on a hypothetic oxa-Nazarov cyclization on gold-stabilized allylic cations. Preliminary results on a new 1,2-difunctionalization of 3-en-1-ynamide are also reported.     

[2+2]-cycloadditions of cyclopentyne

Cyclopentyne, as generated from dibromomethylenecyclobutane, a formerly unknown cyclopentyne source, undergoes [2+2]-cycloadditions with various substituted olefins yielding bicyclo[3.2.0]hept-1(5)-ene derivatives.     

Complexation of uranium(VI) by salicylate and substituted salicylates: A kinetic study

Summary Kinetic studies on the complexation of uranium(VI) by salicylate and various substituted salicylates have been carried out using the stopped-flow spectrophotometric technique at pH 7.0–8.5 (NH₄OH+NH₄NO₃ buffer). Results are in conformity with a mechanism involving binding of UO₂OH⁺ species through the carboxylate group of the salicylate to form an inner-sphere species in a fast equilibrium (equilibrium constant=K) followed by a slow rate-determining ring closure (rate constant=k) involving loss of a molecule of water between the OH group bound to uranium(VI) and the phenolic group of the salicylate. The value of the equilibrium constant (K) obtained from the kinetic data in the case of 5-sulphosalicylate (log K=3.21 at 25 °C, I=1 M) is compatible with the literature thermodynamic value (log K = 3.89 at 25 °C, I=0.015 M). Increase in pH retards the reaction due to the equilibrium, UO₂OH⁺ + OH^– UO₂(OH)₂, the UO₂(OH)₂ being unreactive. The average value of K (log K=8.58 at 25°C, I=1M) obtained kinetically from the results of investigation with different ligands is also in good agreement with the literature thermodynamic value (log K= 8.8 at 25°C, I=0.1M). Both K and k are sensitive to the nature of the substituent in the benzene ring, decreasing with increasing acidity of the -CO₂H group of the salicyclic acid; the substituent effect is well demonstrated by the plot of log kversus _L (where ), which is linear. H^# and S^# values corresponding to k have been evaluated in each case. S^# values are all negative in conformity with ring closure in the rate-determining step.     

1.3.2.4.5-dioxadithiazin-2.2.4.4-tetraoxides - new reagents for [2+2] - and [2+4]-cycloadditions to olefins

The titled compounds react with olefins under mild conditions to give 1.2-thiazetidin-1.1-dioxides and/or 1.4.5-oxathiazin-4.4-dioxides being formal versatile sources of 1.2- and 1.4-dipoles. Relation of these paths changes widely depending upon structural effects of alkenes and conditions of the reaction.     

A new domino reaction based on [4+2]-cycloadditions of pyrido[1,2-a]pyrazines with azadienophiles

The reaction of pyrido[1,2-a]pyrazines 1 with nitroso compounds 2 provides pyridyl substituted 1,2,4-triazinones 4 via a domino reaction which involves a cycloaddition and a ring transformation reaction. The intermediate and regioselective formed oxadiazines 4′ were trapped by complexation yielding the (CO)₄Mo-complex 5. Derivatives of diazene such as N-phenyltriazolindione 6a , phthalazinedione 6b or esters of azodicarboxylic acid 6c-6f reacted with 1 to give different derivatives of 1,2,4-triazine 7a-f . The use of oxygen gave oxadiazinones 8 .     

[4+2]-cycloadditions of thiones with nitrosoalkenes: synthesis of derivatives of 4H-1,5,2-oxathiazine,a new heterocyclic system.

Conjugated nitrosoalkenes, generated $\text{in situ}$$\text{via}$ dehydrobromination of α-bromoketoximes, undergo an easy [4+2]-cycloaddition with thiocarbonyl compounds to give very high yields of the new heterocycles 4H-1,5,2-oxathiazines. The thermolysis and the behaviour to oxidation of these adducts were tested.     

Ketenacetale als dienophile: Reaktivität und regiospezifität bei (4+2)-cycloadditionen mit inversem elektronenbedarf

Ketene acetales 3a–3e differ in reactivity by 5–6 powers of ten in the reaction with monoaryl tetrazines 1a–1e. While 3a–3d yield “ortho-adducts” 4 almost exclusively, the ketene-N,N-acetale 3e give both regioisomers 4 and 5, the isomer ratio is depending on the polarity of the solvent used.     

Nitrone [2+3]-cycloadditions in stereocontrolled synthesis of a potent proteasome inhibitor: (-)-omuralide

A new stereocontrolled synthetic route to omuralide has been developed from methyl pyroglutamate. This route involves regio- and stereoselective N-methylnitrone 1,3-dipolar cycloadditions to appropriate pyrrolinones, beta-eliminations, and highly selective hydrogenations as the main steps.     

Atmospheric reactions Cl+CH3-(CH2)n-OH (n=0-4): a kinetic and theoretical study

The reactions of Cl with a series of linear alcohols: methanol (k1), ethanol (k2), 1-propanol (k3), 1-butanol (k4), and 1-pentanol (k5) were investigated as a function of temperature in the range of 264-382 K by laser photolysis-resonance fluorescence. The obtained kinetic data were used to derive the following Arrhenius expressions: k1=(3.55+/-0.22)x10(-10) exp[-(559+/-40)T], k2=(5.25+/-0.52)x10(-11) exp[(190+/-68)T], k3=(2.63+/-0.21)x10(-11) exp[(525+/-51)T], k4=(3.12+/-0.31)x10(-11) exp[(548+/-65)T], and k5=(3.97+/-0.48)x10(-11) exp[(533+/-77)T] (in units of cm(3) molecule(-1) s(-1)). To our knowledge, these are the first absolute kinetic data reported for 1-butanol and 1-pentanol and also the first kinetic study as a function of temperature for these two compounds. Results, mechanism, and tropospheric implications are discussed and compared with the reported reactivity with OH radicals. Moreover, a theoretical insight into the mechanisms of these reactions has also been pursued through ab initio M?ller-Plesset second-order perturbation treatment calculations with 6-311G** basis sets. Optimized geometries and vibrational frequencies have been obtained for transition states and molecular complexes appearing along the different reaction pathways. Furthermore, molecular energies have been calculated at quadratic configuration interaction with single, double, and triple excitations level in order to get an estimation of the activation energies.     

A new synthesis of substituted 2(1H)-pyrazinones

The hydrohalides of 2-sec-aminoalkanenitriles on treatment with oxalyl halides in o-dichlorobenzene at 80-100° give 3, 5-dihalo-2(1H)-pyrazinones, of which the 3-halo substituent is easily replaced by nucleophiles. A reaction mechanism for the pyrazinone synthesis is proposed.     

A new synthesis of aryl substituted quinazolin-4(1H)-ones

Treatment of a variety of substituted 2-aminobenzonitriles with formic acid under strong acid catalysis provides the corresponding quinazolin-4(1H)-ones in good yield. A potential reaction pathway is described.     

(Diazoalkyl)Phosphoranes from [4 + 1]-cycloadditions of tetrahalo-orthobenzoquinones to (diazoalkyl)phosphanes [1]

The (diazoalkyl)phosphanes 4a–e undergo [4 + 1]-cycloaddition reactions with the tetrahalo-orthobenzoquinones 5a,b to furnish the previously unknown (diazoalkyl)phosphoranes 6a–h . The structure of product 6h has been investigated by X-ray crystallography.     

A novel type of donor–acceptor cyclopropane with fluorine as the donor: (3 + 2)-cycloadditions with carbonyls

gem-Difluorocyclopropane diester is disclosed as a new type of donor–acceptor cyclopropane, which smoothly participates in (3 + 2)-cycloadditions with various aldehydes and ketones. This work represents the first application of gem-difluorine substituents as an unconventional donor group for activating cyclopropane substrates in catalytic cycloaddition reactions. With this method, a wide variety of densely functionalized gem-difluorotetrahydrofuran skeletons, which are otherwise difficult to prepare, could be readily assembled in high yields under mild reaction conditions. Computational studies show that the cleavage of the C–C bond between the difluorine and diester moieties occurs upon a S_N2-type attack of the carbonyl oxygen.

A new type of donor–acceptor cyclopropane with gem-difluorine as an unconventional donor group undergoes (3 + 2)-cycloadditions with various aldehydes/ketones, affording densely functionalized gem-difluorotetrahydrofurans.     

A temperature dependence kinetic study of O(1D) + CH4: overall rate coefficient and product yields

Using a recently-developed chemiluminescence technique for monitoring O(1D), the rate coefficient, k1, of the important atmospheric reaction O(1D) + CH4 --> products has been determined over a wide temperature range, 227 to 450 K. The rate coefficient was shown to be independent of temperature, having a value of (1.91 +/- 0.08) x 10(-10) cm3 s(-1); the quoted uncertainties are with 95% confidence. This highly precise value, based on an extended set of determinations with very low scatter, is significantly greater, 26%, than current recommended values. Secondly, the fraction of O(1D) quenched to O(3P) by CH4, k(1q)/k1, was precisely determined from chemiluminescence decays over the temperature range 236 to 340 K. A temperature independent value for k(1q)/k1 of 0.002 +/- 0.003 was found. Finally, LIF detection of OH has been applied to accurately determine the product branching fraction to OH of O(1D) + CH4 at room temperature. Our value, k(1a)/k1 = 0.76 +/- 0.08 (95% confidence), is in line with recent determinations by other groups.     

Oxidation of substituted phenethyl alcohols by sodium-N-chloro-p-toluene sulfonamide: A kinetic study

Summary The kinetics of the oxidation of sixp-substituted phenethyl alcohols (PEA, R=–H, –Cl, –Br, –CH₃, –OCH₃, and -NO₂) by sodium-N-chloro-p-toluene sulfonamide (chloramine-T,CAT) in the presence of HCl was studied at 35°C. The rate shows a first order dependence on [CAT]₀ and [H⁺]₀ and a fractional order in [PEA]₀ and [Cl^–]₀. Ionic strength variations, addition of reaction product toluene sulfonamide, and variation of the dielectric constant of the medium have no effect on the rate. The solvent isotope effect amounts to about 0.90. Proton inventory studies have been made in H₂O-D₂O mixtures. The rates correlate satisfactorily withHammett's relationship. The reaction constant was –3.3 for electron releasing substituents and –0.25 for electron withdrawing groups at 35°C. The activation parameters H ^#, S ^#, G ^#, and logA were derived. H ^# and S ^# are linearly related, and an isokinetic relationship is observed with =166.7K, indicating entropy as a controlling factor.

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Die Oxidation substituierter Phenethylalkohole mit Natrium-N-chlor-p-toluolsulfonamid: Kinetische Untersuchungen

Zusammenfassung Die Kinetik der Oxidation von sechsp-substituierten Phenethylalkoholen (PEA), R=–H, –Cl, –Br, –OCH₃ und NO₂) mit Natrium-N-chlor-p-toluolsulfonamid (Chloramin-T,CAT) in Gegenwart von HCl bei 35°C wurde untersucht. Die Reaktionsgeschwindigkeit ist in bezug auf [CAT]₀ und [H⁺]₀ ester und hinsichtlich [PEA]₀ und [Cl]₀ gebrochener Ordnung. Variation der Ionenstärke, Zusatz von Reaktionsprodukt oder Toluolsulfonamid und Variation der Dielektrizitätskonstante des Mediums haben keinen Einfluß auf die Reaktionsgeschwindigkeit Der Lösungsmittel-Isotopeneffekt beläuft sich auf etwa 0.90. Die Protonenbilanz wurde in H₂O-D₂O Mischungen untersucht. Die Geschwindigkeiten korrelieren zufriedenstellend nach derHammettschen Beziehung. Die Reaktionskonstante wurde mit =–3.3 für elektronenabgebende und =–0.25 für elektronenanziehende Substituenten bei 35°C bestimmt. Die Aktivierungsparameter H ^#, G ^#, G ^# und logA wurden abgeleitet; H ^# und S ^# korrelieren linear, und eine isokinetische Beziehung mit =166.7K weist auf die Entropie als kontrollierenden Faktor hin.

     

[2+2]-cycloadditions of alkenes with the triphenylallenyl cation

The triphenylallenyl cation (8), generated from triphenylpropynol (7) and FSO₃H, reacts with alkenes to give the allyl cations 12, which may be deprotonated to yield the methyleneccylobutenes 14. Alternatively, 12 can be converted into the 2-vinyl-indenes 13 $\text{via}$ two subsequent electrocyclic reactions.     

Intramolecular [2+2] photocycloaddition of substituted isoquinolones: enantioselectivity and kinetic resolution induced by a chiral template

From simple to complex: Starting from easily accessible isoquinolones 1 (X=Br, OH), complex cyclobutane photoproducts such as compound 2 can be obtained with high enantioselectivity (88-96?%?ee) through the use of a chiral template. Compound 3, which was isolated in 53?%?ee starting from a racemic substrate, is the product of a unique, unprecedented kinetic resolution process.