Laser-induced crosslinking of siloxanes: A semi-empirical molecular orbital study

The semi-empirical molecular orbital method AM1 has been used as a tool for the prediction of radical intermediates and crosslink structures that may be of importance in the direct laser excitation of siloxanes. Special emphasis was put on configuration interaction (Cl) in order to calculate energies and geometries of excited triplet states as well as potential curves of triplet reactions. Heats of formation of intermediates and reaction enthalpies of monomolecular fragmentation reactions of T¹ excited states of 2-phenyl-1,1,1,2,3,3,3-heptamethyltrisiloxane and octamethyltrisiloxane were calculated. In the former case bimolecular reactions were also considered. Dissociation of a methyl group was found to be the most probable monomolecular fragmentation. Consecutive reactions of the silyl radical formed lead to crosslink structures like Si? CH₂? Si and Si? C₆H₄? Si. Biphenyl- and phenylcyclohexadienyls are the most probable crosslink structures of bimolecular reactions with phenyl exciplexes as intermediates. Experimental evidence is given for these crosslink structures.     

Complementary Ways to Diastereomers of Bridged Aminopiperidine Derivatives — A Stereochemical Challenge

Routes to diastereomers of constrained 4‐amino‐piperidines — a common pharmaceutically used diaminic building block — are realized mainly on the basis of CN‐double bond species or their radicalic or anionic analogues. Kinetically controlled reactions on the one hand and thermodynamic control of reactions, reversible introduction of a repulsive group, direction of a reactant by intramolecular complexation, or involvement of radicalic or anionic intermediates with strong isomerization tendency on the other hand are the tools for a complementary accessibility of both diastereomers.     

The Pyridyldiisopropylsilyl Group: A Masked Functionality and Directing Group for Monoselective ortho‐Acyloxylation and ortho‐Halogenation Reactions of Arenes

A novel, easily removable and modifiable silicon‐tethered pyridyldiisopropylsilyl directing group for C H functionalizations of arenes has been developed. The installation of the pyridyldiisopropylsilyl group can efficiently be achieved via two complementary routes using easily available 2‐(diisopropylsilyl)pyridine ( 5 ). The first strategy features a nucleophilic hydride substitution at the silicon atom in 5 with aryllithium reagents generated in situ from the corresponding aryl bromides or iodides. The second milder route exploits a highly efficient room‐temperature rhodium(I)‐catalyzed cross‐coupling reaction between 5 and aryl iodides. The latter approach can be applied to the preparation of a wide range of pyridyldiisopropylsilyl‐substituted arenes possessing a variety of functional groups, including those incompatible with organometallic reagents. The pyridyldiisopropylsilyl directing group allows for a highly efficient, regioselective palladium(II)‐catalyzed mono‐ortho‐acyloxylation and ortho‐halogenation of various aromatic compounds. Most importantly, the silicon‐tethered directing group in both acyloxylated and halogenated products can easily be removed or efficiently converted into an array of other valuable functionalities. These transformations include protio‐, deuterio‐, halo‐, boro‐, and alkynyldesilylations, as well as a conversion of the directing group into the hydroxy functionality. In addition, the construction of aryl‐aryl bonds via the Hiyama–Denmark cross‐coupling reaction is feasible for the acetoxylated products. Moreover, the ortho‐halogenated pyridyldiisopropylsilylarenes, bearing both nucleophilic pyridyldiisopropylsilyl and electrophilic aryl halide moieties, represent synthetically attractive 1,2‐ambiphiles. A unique reactivity of these ambiphiles has been demonstrated in efficient syntheses of arylenediyne and benzosilole derivatives, as well as in a facile generation of benzyne. In addition, preliminary mechanistic studies of the acyloxylation and halogenation reactions have been performed. A trinuclear palladacycle intermediate has been isolated from a stoichiometric reaction between diisopropyl(phenyl)pyrid‐2‐ylsilane ( 3a ) and palladium acetate. Furthermore, both C H functionalization reactions exhibited equally high values of the intramolecular primary kinetic isotope effect (k_H/k_D=6.7). Based on these observations, a general mechanism involving the formation of a palladacycle via a C H activation process as the rate‐determining step has been proposed.     

Simple,Mild, and Practical Esterification,Thioesterification, and Amide Formation Utilizing p‐Toluenesulfonyl Chloride and N‐Methylimidazole

We have developed an efficient method for the esterification or thioesterification of equimolar amounts of carboxylic acids and alcohols or thiols using a novel reagent, p‐toluenesulfonyl chloride (TsCl) together with N‐methylimidazole. The present method is simple, mild, and reactive, uses readily available and economical reagents. The choice of amine is critical for the present method. The amine, N‐methylimidazole, has two roles: (i) as an HCl scavenger for the initial smooth generation of mixed anhydrides between carboxylic acids and TsCl and (ii) successive formation of highly reactive ammonium intermediates from mixed anhydrides. This method could be applied to various types of carboxylic acids, alcohols, and thiols: a) several functionalities were tolerated; b) two N‐Cbz amino acids were smoothly esterified without racemization; and c) the labile 1β‐methylcarbapenem key intermediate and a pyrethroid insecticide, prallethrin, were successfully prepared. The related amide formation between carboxylic acids and primary or secondary amines was also performed. The proposed reaction mechanism involves a novel method for producing the reactive acylammonium intermediates. The production of these intermediates was rationally supported by a careful ¹H NMR monitoring study.     

Caulobacter segnis Dioxygenase CsO2: A Practical Biocatalyst for Stilbenoid Ozonolysis

Ozonolysis is a useful as well as dangerous reaction for performing alkene cleavage. On the other hand, enzymes are considered a more sustainable and safer alternative. Among them, Caulobacter segnis dioxygenase (CsO2) known so far for its ability to catalyze the coenzyme-free oxidation of vinylguaiacol into vanillin, was selected and its substrate scope evaluated towards diverse natural and synthetic stilbenoids. Under optimized conditions, CsO2 catalyzed the oxidative cleavage of the C=C double bonds of various trans-stilbenes, providing that a hydroxyl moiety was necessary in para-position of the phenyl group (e. g., resveratrol and its derivatives) for the reaction to take place, which was confirmed by modelling studies. The reactions occurred rapidly (0.5–3 h) with high conversions (95–99 %) and without formation of by-products. The resveratrol biotransformation was carried out on 50–mL scale thus confirming the feasibility of the biocatalytic system as a preparative method.     

An Electron‐Deficient Diene as Ligand for Palladium‐Catalyzed Cross‐Coupling Reactions: An Efficient Alkylation of Aryl Iodides by Primary and Secondary Alkylzinc Reagents

An electron‐deficient diene, L₁ , was found to be an effective ligand in facilitating palladium‐catalyzed Negishi couplings involving primary and secondary alkylzinc reagents. The reactions took place readily at 60 °C in THF with 5 mol% of a catalyst generated in situ from bis(acetonitrile)palladium dichloride [PdCl₂(MeCN)₂] and L₁ , and functional groups such as chloro, bromo, etc. attached to phenyl ring as well as β‐H atoms adjacent to the reaction site were well tolerated. The problematic isomerizations in secondary alkyzinc reagents involved in the reactions reported in the literature were also observed in our system when isopropylzinc chloride was employed alone as the nucleophile. However, the isomerization was significantly suppressed when i‐Pr₂Zn was utilized in the presence of L₁ .     

Copper(I)‐Mediated Highly Stereoselective syn‐Carbometalation of Secondary or Tertiary Propargylic Alcohols with Primary Grignard Reagents in Toluene with a High Linear Regioselectivity

A highly regio‐ and stereoselective syn‐carbometalation of terminal secondary or tertiary propargylic alcohols with primary alkyl Grignard reagents in toluene or phenylmagnesium bromide in Et₂O was developed, in which the alkyl or phenyl group from the Grignard reagents is introduced into the terminal position of the alcohols. The organometallic intermediate formed may be used directly for the coupling reaction with organic halides. Upon treatment with I₂ after the carbometalation, iodides may be obtained, which may undergo Sonogashira coupling reaction and highly stereoselective Novozym‐435‐catalyzed kinetic resolution to afford the optically active products.     

Synthesis and Reactions of Aroyl Substituted Enone Mannich Salts

The classical Mannich reaction of aromatic methyl ketones with paraformaldehyde and dimethylamine hydrochloride resp. dimethyl(methylene)ammonium chloride has been extended to a few cases of 1‐aryl‐2‐dimethylaminomethyl‐prop‐2‐en‐1‐ones (ADMP reagents). They have gained remarkable attention in medicinal chemistry, but only more recently their properties as valuable building blocks for ring closure reactions to form either aroyl or dimethylaminomethyl substituted heterocyclic compounds has been evaluated. In this review, a collection of representative examples for their preparation (Scheme 2) and biological effects as well as the synthetic potential for the synthesis of heterocycles (Scheme 3—8) is given. The reaction of 4‐hydroxycoumarin delivers with ADMP reagents, via the formation of detectable 3‐(2‐benzoylallyl)‐4‐hydroxycoumarins as secondary substitution products, after ring closure 3‐benzoyl‐3,4‐dihy‐dro‐2H,5H‐1‐benzopyrano[4,3‐b]pyran‐5‐ones (Scheme 4). The reaction of 4‐hydroxy‐6‐methylpyran‐2‐one with ADMP reagents is investigated systematically in order to assess its suitability in carbon—carbon bond forming reactions as well as to provide the conditions for subsequent ring closure or intermolecular addition of further nucleophiles to the enone double bond of the intermediate 3‐(2‐benzoylallyl)‐4‐hy‐droxy‐6‐methylpyran‐2‐ones yielding miscellaneous 3‐substituted 2‐pyrones (Scheme 5 and 8), and their application as building blocks for the combinatorial chemistry. The condensation of ADMP reagents with 2‐aminopyridines gives rise to 3‐benzoyl‐3,4‐dihydro‐2H‐pyrido[1,2‐a]pyrimidines (Scheme 6) and the corresponding reaction using amidines affords 5‐benzoyl‐1,4,5,6‐tetrahydropyrimidines (Scheme 7).     

Convenient synthesis of<Emphasis Type="Italic">n</Emphasis>-methylpyrrolidine-2-thione and some thioamides

The synthesis of thioamides and thiolactams, which are used as important organic intermediates, has attracted great attention. However, expensive reagents, severe reaction conditions and low yields of the target products made conventional methods inconvenient and economically infeasible. To overcome these disadvantages, we investigated a new process for synthesizing thioamides and thiolactams. We examined thermal reactions of CS₂ withN- methyl-2-pyrrolidinone, formylamide, acetamide andN,N-dimethylformylamide, respectively. The results show that under optimum conditionsN-methylpyrrolidine-2-thione and the corresponding thioamides can be obtained in good to excellent yields by the above thionation reactions.     

Optically Pure 1,2‐Bis[(o‐alkylphenyl)phenylphosphino]ethanes and Their Use in Rhodium‐Catalyzed Asymmetric Hydrogenations of α‐(Acylamino)acrylic Derivatives

Optically pure (S,S)‐1,2‐bis[(o‐alkylphenyl)phenylphosphino]ethanes 1a–d were prepared in four steps from phenyldichlorophosphine via phosphine‐boranes as the intermediates. The rhodium complexes 5a–d of these diphosphines were used for the asymmetric hydrogenations of α‐(acylamino)acrylic derivatives including β‐disubstituted derivatives. Markedly high enantioselectivity (78–>99%) was observed for the reduction of β‐monosubstituted derivatives. β‐Disubstituted derivatives were also reduced in considerably high enantioselectivity (up to 90%). The single crystal X‐ray analysis of the rhodium complex 5c of (S,S)‐1,2‐bis[phenyl(5′,6′,7′,8′‐tetrahydronaphthyl)phosphino]ethane ( 1c) revealed its δ‐type structure with face orientation of the two tetrahydronaphthyl groups and edge orientation of the two phenyl groups. This conformation corresponds to that of the rhodium complex of 1,2‐bis[(o‐methoxyphenyl)phenylphosphino]ethane (DIPAMP); the rhodium complex of (R,R)‐DIPAMP, whose chirality at phosphorus is opposite that of 5c , exhibits a λ‐type structure with the face orientation of the two o‐methoxyphenyl groups and the edge orientation of the two phenyl groups. The conformational similarity of these rhodium complexes as well as the stereochemical outcome in the asymmetric hydrogenations means that the coordinative interaction of the methoxy group of DIPAMP with rhodium metal is not the main factor that affects asymmetric induction.     

A convenient access to 1,2-diferrocenyl-substituted ethylenes via [3 + 2]-cycloelimination of 2-silylated 4,4,5,5-tetrasubstituted 1,3-dithiolanes

Ferrocenyl thioketones bearing a hetaryl, phenyl or alkyl group as the second substituent react with (trimethylsilyl)diazomethane at ca. ?30°C in THF solution without formation of a stable [3?+?2]-cycloadduct. After the spontaneous evolution of N₂, the corresponding sterically crowded 4,4,5,5-tetrasubstituted 2-silylated 1,3-dithiolanes are formed as products of the second [3?+?2]-cycloaddition of the intermediate thiocarbonyl S-methanide with the starting thioketone. After desilylation by treatment with TBAF, they are converted into the corresponding carbanions, which display different stability depending on the type of substituent. The presence of hetaryl and phenyl groups results in the exclusive formation of 1,2-diferrocenyl ethylenes. In contrast, the presence of methyl groups significantly enhances the stability of the carbanion, which by protonation yields trans-4,5-diferrocenyl-4,5-dimethyl-1,3-dithiolane.     

Nickel‐catalysed Kumada polycondensation of di‐functionalized Grignard reagent with aryl fluoride

A nickel catalyst promoted the polymerization between various Grignard reagents with 2‐phenyl‐4,6‐bi(4‐fluorophenyl)‐1,3,5‐triazine. The reaction scope was thoroughly investigated and fully characterized. The Sp²‐C ? F bond was successfully activated by the triazine group in Kumada coupling reactions. Also, di‐Grignard reagents showed higher activity than mono‐Grignard reagents. The reaction scope results reveal that a Grignard reagent with strong nucleophilicity and aryl fluoride with strong electrophilicity are necessary to lead to Kumada polycondensation of oligothiophenes with aryl fluoride. In this work, polymerization between di‐functionalized oligothiophene Grignard reagents with aryl fluoride was first conducted. The weight‐average molecular weight is high, up to 14.8 kDa. The π‐conjugated polymers obtained exhibit emission colours of blue, green and yellow with efficient photoluminescence. © 2016 Society of Chemical Industry     

Recent Developments in Enzymatic Asymmetric C?C Bond Formation

Some of the recent developments in enzymatic asymmetric C C bond formation are described in this review. The close relationship of biocatalysis and biosynthesis is highlighted with a special emphasis on diversity and biogenesis. One focus of this review is the creation of tetrasubstituted carbon stereocenters. Members of the supposedly well‐known aldolase and hydroxynitrile lyase enzyme families possess the ability to catalyze the formation of tertiary alcohols. In the case of aldolases, this can occur through intramolecular cyclization or intermolecular asymmetric C C bond formation. Thiamine diphosphate‐dependent YerE has been identified as a potent catalyst for the acyloin condensation with ketones as acceptor substrates. C₁ transformations such as methylation or carboxylation are catalyzed in an asymmetric manner by enzymes from different classes, for example S‐adenosylmethionine‐dependent (radical) enzymes or NADPH‐dependent oxidoreductases. Insights from biosynthetic and mechanistic studies of enzymatic reactions proceeding via radical intermediates give valuable hints towards possible applications in biocatalysis. Still, the oxygen sensitivity of many of these biocatalysts poses a considerable challenge for practical applications.     

Isomerisation of xylenes over a silica–alumina catalyst

The catalytic vapour phase isomerisation of m-xylene into o- and p-xylenes was studied in a flow system in the temperature range 460–700 °C, and over a wide range of space velocities on a silica–alumina catalyst. The isomeric distribution of the products, as determined by a combination of spectroscopic and chromatographic methods, was found to be consistent with calculated equilibrium values. Reduced pressures tended to suppress undesirable side reactions such as disproportionation and favoured the isomerisation. First order kinetics appeared to be applicable and the overall activation energy for both isomerisation reactions (i.e. meta- to para-, and meta- to ortho-) was found to be close to 25 kcal/mol, being of the same order of magnitude in both cases. Isomerisation of o-, m- and p-xylenes failed to occur even at 600 °C under non-catalytic thermal treatment, although formation of polynuclear ring systems (“coke”) in small quantities was observed. The results appear to indicate that the mechanism of isomerisation of xylenes over acidic catalysts involves essentially ionic intermediates. Based on the evidence obtained, it is concluded that the vapour phase isomerisation of xylenes apparently proceeds via the formation of a localised π-complex, leading to a 1,2-intramolecular shift of the methyl group.     

Preparation ofbis-quaternary ammonium salts from epichlorohydrin

A novelbis-quaternary ammonium salt was prepared conveniently and almost quantitatively fromN,N-dimethyldodecylamine, its hydrochloride, and epichlorohydrin. Reaction ofN,N-dimethyldodecylamine with epichlorohydrin (in the presence of the amine hydrochloride) or various dichloro compounds was investigated by using¹H nuclear magnetic resonance. The reaction route was studied by examining the reactivity of reagents with the amine and the effect of reaction temperature. The ease of the reaction with epichlorohydrin was found to be due to the assistance of amine hydrochloride in opening the epoxide ring and to neighboring-group participation by the hydroxyl group of the intermediatemono-ammonium salt in the quaternization step. Neighboring-group participation by the hydroxyl group in these quaternization reactions is also discussed.     

Relationship between photofading and chemical structure of disperse azo dyes on nylon fabric†

The initial rates of photofading for 30 disperse azo dyes on nylon fabric upon exposure to a carbon arc in air have been analysed by formulating a kinetic equation that describes azo scission via the disproportionation reactions and intramolecular H‐transfer by two kinds of hydrazinyl radicals and the conversion of a nitro group to a nitroso group via the disproportionation reaction of nitrosyl hydroxide radicals. The five reaction rate constants are discussed in terms of the heats of reaction by calculating the heats of formation for the reactants, intermediates and products of each reaction using the PM5 method. Phenylazo‐ and thiazoleazo‐N,N‐substituted anilines and phenylazo‐pyridones exhibited large rate constants of multiple terms, while phenylazo‐phenols had the highest light fastness and very small rate constants for the disproportionation reactions of hydrazinyl radicals. Photofading on nylon fabric was primarily controlled by the thermal reactions of photo‐induced monohydrogenated dye radicals, which occurred via one or two primary multiple routes.     

Modification of cyclohexanone-formaldehyde-resin

The reactions of cyclohexanone-formaldehyde-resin via its hydroxy and carbonyl functional groups with a number of different reagents such as acetic anhydride, benzoyl chloride, hydroxylamine, semicarbazide, and phenyl hydrazine were studied. Melting points, IR spectrums, and solubilities in various solvents of the products were determined.     

A convenient synthesis of novel 1,3-phenylene bridged bis-heterocyclic compounds

Reactions of bis-hydrazonoyl bromide 1 with each of phenyl-5-arylidene-2-thioxo-thiazol-4-one, triazinethiones and 4,6-dimethyl-2,6-dioxocyclohexane-1-thiocarboxanilide as sulfur dipolarophilic reagents led to the formation of the hitherto unreported 1,3-phenylene bis-heterocycles 4, 8 and 10, respectively. The structures of the isolated products were established on the basis of their elemental and spectral analyses. The mechanism and the site selectivity in the studied reactions are discussed.     

Insight into the Mechanism and Stereochemistry of the Transformations of Alkyltitanium Ate‐Complexes. An Enhanced Enantioselectivity in the Cyclopropanation of the Carboxylic Esters with Titanacyclopropane Reagents

The dependence of the stereoselectivity of the cyclopropanation reaction of γ,γ‐diphenyl‐γ‐butyrolactone and carboxylic esters with alkylmagnesium bromides in the presence of titanium(IV) TADDOLates on the structure of the reactants has been examined in detail. It has been found that the enantioselectivity in the formation of cis‐1,2‐disubstituted cyclopropanols is a function of the structure of the carboxylic ester alkoxy group and leads, in the case of cyclopropanation of hexafluoroisopropyl alkanoates, to 84% ee. To rationalize these observations, as well as the NMR data on the structures of the TADDOL‐derived alkyltitanium species, a new mechanistic model for the formation of the five‐coordinated chiral titanacyclopropane reactive intermediates and their subsequent reorganization in the reactions with esters is proposed. The essential features of this model consist in the generation of configurationally stable TADDOL derived titanacyclopropane species with an apical or equatorial position of the stereogenic carbanion center which is followed by retention or inversion of absolute configuration in course of the displacement of the carboxylic ester alkoxy group.

     

Structure of an <Emphasis Type="Italic">n</Emphasis>-heptane/toluene flame: Molecular beam mass spectrometry and computer simulation investigations

Molecular beam mass spectrometry was used to measure mole fraction profiles of the reactants, major reaction products and intermediates, including precursors of polycyclic aromatic hydrocarbons, in a premixed fuel-rich (equivalence ratio of 1.75) n-heptane/toluene/O₂/Ar flame stabilized on a flat burner at atmospheric pressure. The ratio of the liquid volumes in the n-heptane/toluene mixture was 7: 3. The chemical structure of the flame was modeled using a detailed mechanism of chemical reactions tested against experimental data of other authors on n-heptane/toluene flames and comprising the reactions of formation of polycyclic aromatic hydrocarbons. The mechanism was extended with cross-reactions involving derivatives of n-heptane and toluene. Overall, the new experimental data are in satisfactory agreement with the numerical simulation results; however, there are differences between the measured and calculated mole fraction profiles of some species. Analysis shows that in the n-heptane/toluene flame, the main reactions leading to the formation of low-aromatic compounds (benzene and phenyl) are reactions typical of the pure toluene flame.