Studies on the Autoxidation of Aryl Alkenes

Autoxidation activity of substituted styrenes (aryl alkenes) and their product formation were studied in the temperature range of 65–125 °C using cumene or chlorobenzene as solvent. It was observed that higher reaction temperatures, inert solvent, electron donating substituents, and bulky sterical groups at α‐, ortho‐ and para‐ positions promoted radical formation of epoxides and led to C=C oxidative cleavage. In contrast, electron accepting substituents and using cumene as the solvent favored the formation of oligomeric peroxides. The relative activity of all investigated styrenes could be correlated to the linear free energy (LFE) relation and the ionization potential. For α‐substituted styrenes and ring substituted α‐methylstyrenes LFE correlation for the reactivity of aryl alkenes and formation of epoxides was found to depend on the polar and steric factors. A general pathway for product formation by autoxidation of aryl alkenes has been proposed.     

Mechanistic Study of the Reaction of Thiol‐Containing Enzymes with α,β‐Unsaturated Carbonyl Substrates by Computation and Chemoassays

We investigated the reactions between substituted α,β‐unsaturated carbonyl compounds (Michael systems) and thiols by computations as well as chemoassays. The results give insight into variations in the underlying mechanisms as a function of the substitution pattern. This is of interest for the mechanisms of inhibition of the SARS coronavirus main protease (SARS‐CoV M^pro) by etacrynic acid derivatives as well as for the excess toxicity of substituted α,β‐unsaturated carbonyl compounds. This study compares possible reaction courses including 1,4‐addition followed by a ketonization step, and underscores the importance of a base‐catalyzed step for the reactivity of thiol groups in enzymes. Phenyl and methyl substituents at the Michael system decrease the reactivity of the electrophilic compound, but chlorophenyl substituents partly recover the reactivity. Computations also indicate that electron‐pushing substituents lead to a change in the reaction mechanism. The conformation of the Michael system is also found to significantly influence reactivity: the s‐cis conformation leads to higher reactivity than the s‐trans conformation. The computed data explain the trends in measured inhibition potencies of substituted α,β‐unsaturated carbonyl compounds and of reaction rates in chemical assays. They also indicate that the reversibility of inhibition does not stand in contrast to the formation of a new covalent bond between inhibitor and protease.     

Enantioselective Carbonyl‐Ene Reactions of Arylglyoxals with a Chiral Palladium(II)‐BINAP Catalyst

The palladium(II)‐BINAP‐catalyzed enantioselective carbonyl‐ene reactions between ten arylglyoxals and five alkenes were systematically investigated and demonstrated good to excellent enantioselectivities with high ee values of up to 93.8 %. The results showed that both arylglyoxals and alkenes exert evident effects on the enantioselectivity. Particularly, the ortho‐methyl substituents of the substrates could increase the enantioselectivity. The achieved excellent enantioselectivities may be due to the corresponding substrate matches well fitting the chiral space created by the chiral palladium(II)‐BINAP catalyst. The ortho‐methyl substituents may improve the fitting of the substrate match to the chiral space created by the chiral catalyst, hence the enantioselectivity is improved. When using dienes (1,4‐diisopropenylbenzene and 1,3‐diisopropenylbenzene) as substrates in this reaction, only one of the two carbon‐carbon double bonds participated into the reaction affording tetrafunctional organic compounds with moderate enantioselectivities of up to 83.8 % ee. The chiral Lewis acid palladium(II) catalyst incorporating (R)‐BINAP, which is a conformationally restricted chiral ligand, is very stable in ionic liquids and could be recycled 21 times with retention of the high enantioselectivity.     

Effects of substituants in stilbene upon its reactivity in radical polymerizations

Summary Polymerizations of methyl methacrylate and styrene have been performed in the presence of thecis andtrans isomers of 4-methoxy-4-chlorostilbene (SSTL) using as initiators benzoyl peroxide and 2-cyano-2-propylazoformamide, both enriched with carbon-13.¹³C-NMR spectra of the polymers show that SSTL units are present at many of the sites adjacent to benzoate end-groups but not to a detectable extent at sites adjacent to 2-cyano-2-propyl end-groups. The presence of the substituents in the molecule of stilbene greatly enhances its reactivity towards the benzoyloxy radical.     

The free radical copolymerization characteristics of α-methylene γ-butyrolactone

The free radical copolymerization of α-methylene γ-butyrolactone (α-MBL, I) has been studied at 60°C, with methyl methacrylate, styrene, acrylamide, acrylonitrile and vinylene carbonate, respectively. The reactivity ratios thus determined generally establish the high reactivity of α-MBL as a comonomer.     

The effect of substituents on the adsorption of alkenes on (111) Pt and Pd surfaces: a theoretical study

The coordination modes of ethylene, propene, 1- and 2-butenes, 2-methyl-2-butene, and styrene on a Pt(111) or a Pd(111) surface have been compared on the basis of extended Hückel calculations. The presence of methyl substituents induces a uniform decrease in the binding energies of the alkenes, both on Pt and on Pd. The phenyl group yields the same result on Pt but, on the contrary, gives a stronger adsorption mode on Pd. The interpretation of these results is based on the balance between attractive and repulsive interactions, the role of the latter being predominant. This trend in binding energy is related to the decrease of the hydrogenation reactivity of olefins upon substitutionin the case of a competitive reaction.     

Recent advances in the use of tri(2-furyl)germane,triphenylgermane and their derivatives in organic synthesis

Tri(2-furyl)germane is a readily prepared, easy-to-handle, and storable triorganogermane. The three 2-furyl groups confer unique reactivity on it in a variety of reactions. Tri(2-furyl)germane is a promising alternative to tributyltin hydride in radical reaction, especially superior in radical addition to alkenes. The hydrogen of tri(2-furyl)germane is so acidic that generation of the corresponding germyl anion is achieved with the aid of weak bases, such as potassium tert-butoxide and cesium carbonate. The anion participates in nucleophilic addition to aldehydes, 1,4-addition to α,β-unsaturated carbonyl compounds, and coupling reaction with aryl and vinyl halides under palladium catalysis. Palladium-catalyzed hydrogermylation with tri(2-furyl)germane create some novel phenomena. Some chemistry of triphenylgermane will be described.     

A new simple procedure to calculate monomer reactivity ratios by using on-line 1H NMR kinetic experiments: Copolymerization system with greater difference between the monomer reactivity ratios

Free radical copolymerization reaction of vinyl acetate (VA) and methyl acrylate (MA) in solution of benzene-d₆ using benzoyl peroxide (BPO) as the initiator was studied with on-line ¹H NMR kinetic experiments at 60 °C. It was observed that composition drifts in the comonomer mixture with reaction progress is significant. Hence, the monomer reactivity ratios of VA/MA system could be calculated by the data collected only from one sample via on-line following the comonomer mixture and copolymer compositions at different reaction time intervals up to medium overall monomer conversions. The results were in good agreement with the literature data reported for this system. The good fitting between theoretical and experimental changes in the comonomer mixture compositions as a function of reaction progress was observed, indicating the accuracy of the monomer reactivity ratios calculated by the new procedure presented here.     

Reactivities towards the benzoyloxy radical of monomers with beta-methyl groups

Summary Polymers of methyl methacrylate have been prepared at 60°C with benzoyl peroxide as initiator in the presence of methyl tiglate (MT), methyl crotonate (MC) or crotononitrile (CNL); they have been examined for end-groups derived from the initiator. The introduction of beta-methyl groups into methyl methacrylate, methyl acrylate and acrylonitrile (to give MT, MC and CNL respectively) increases reactivity towards the benzoyloxy radical. The effects can be explained in terms of the electron-repelling methyl groups activating the double bonds for reaction with the electrophilic benzoyloxy radical.     

High molecular weight poly(methyl alkyl fumarates): radical high polymerization of methyl alkyl fumarates and monomer-isomerization radical polymerization of methyl alkyl maleates

Summary High molecular weight poly(methyl alkyl fumarate) were prepared through radical polymerization of methyl alkyl fumarates(MRF) and monomer-isomerization radical polymerization of methyl alkyl maleates(MRM). The polymerization reactivities(yield and viscosity) of MRFs and MRMs were found to increase with increasing of the bulkiness of the ester alkyl substituents, but MRFs showed generally higher reactivities than MRMs. These polymers were also observed to consist of less- or non-flexible rod-like poly(methoxy-carbonylmethylene-alt-alkoxycarbonylmethylene) structure depending on their bulkiness. The MtBF polymer did not melt, which showed somewhat decreased thermal stability.Polymers from 1,2-Disubstituted Ethylenic Monomers. VIII.     

The penultimate unit effect of radical copolymerization

Summary The effect of polar - and -substituents on the selectivity of alkyl radical additions to alkenes has been measured. Although these substituents don't influence the nucleophilicity of radicals they reduce the rate of addition to diethyl fumarate. It is concluded that the penultimate unit effect is a through space repulsive interaction of polar substituents between the radicals and the alkenes.     

Autoxidative and Photooxidative Reactivity of Highly Branched Isoprenoid (HBI) Alkenes

Autoxidation of several mono-, di-, tri- and tetra-unsaturated highly branched isoprenoid (HBI) alkenes was induced in organic solvents using a radical initiator and enhancer, and their degradation rates were compared to those of classical phytoplanktonic lipids (mono-unsaturated fatty acids, sterols and chlorophyll phytyl side-chain). Autoxidation of two HBI trienes was also investigated in senescent and highly photodegraded diatom cells, collected in the Antarctic, using Fe²⁺ ions as radical inducers. Autoxidation rates of HBI alkenes were found to increase with the number of tri-substituted double bonds, as expected. Further, HBI trienes possessing one bis-allylic position (where hydrogen abstraction is highly favoured) were found to be particularly reactive towards autoxidation and degraded at similar rates compared to polyunsaturated fatty acids in diatom cells. By comparison of the autoxidation products of the most reactive tri-unsaturated HBI with the corresponding photooxidation products, some specific tracers of these two types of abiotic degradation processes were identified. The lack of reactivity of the mono-unsaturated HBI IP₂₅ and a structurally similar di-unsaturated HBI towards autoxidative degradation supports the good preservation of these biomarkers in marine sediments.     

Reversible addition fragmentation chain transfer copolymerization: influence of the RAFT process on the copolymer composition

Reversible addition fragmentation chain transfer (RAFT) mediated and conventional copolymerizations at low monomer conversions have been carried out for the systems methyl methacrylate (MMA)-styrene, methyl acrylate (MA)-styrene and methyl methacrylate-butyl acrylate (BA). The polymer samples have been analyzed via ¹H-NMR spectroscopy to obtain the copolymer composition and the terminal model reactivity ratios. In the RAFT mediated copolymerizations, the polymer mole fraction of the monomer with the larger reactivity ratio is increased compared to the conventional copolymerization. Simulations have been carried out using the program package PREDICI^® to examine possible explanations for the experimental findings. The simulations demonstrate that the RAFT process itself may alter the macroradical populations and the copolymer composition by offering additional reaction pathways. Further, the rate coefficients for the initiation reaction and the pre-equilibrium play an important role in determining the copolymer composition. The rate coefficients governing the main equilibrium of the RAFT process have only a minor impact on the copolymer composition.     

Evaluation of end groups in poly(methyl methacrylate-co-styrene) by 13C NMR

Summary The carbonyls of the various benzoate end groups in poly(methyl methacrylate-co-styrene) prepared with benzoyl-carbonyl-¹³C peroxide give rise to discrete and characteristic signals in the 62.9 MHz ¹³C NMR spectra of the copolymers. Thus, by integration of the spectra, it is possible to estimate the relative reactivity of benzoyloxy radicals towards styrene and methyl methacrylate. In addition, information on the amount of initiator consumed in other processes such as transfer to initiator and primary radical termination can be obtained.     

Limitations of the Reactivity—Selectivity Principle. Application to Free Radical Addition to Alkenes and Nucleophilic Aliphatic Substitution

The limitations of the reactivity—selectivity principle (RSP) are discussed in terms of the qualitative configuration mixing (CM) model. It is concluded that the RSP is expected to break down for cases in which the reaction transition state takes on character largely absent in reactants or products, though not all breakdowns of the RSP are attributed to this cause. This conclusion is illustrated with two model reactions that do not follow the RSP: (a) radical addition to alkenes, and (b) nucleophilic aliphatic substitution. Analysis leads us to agree with previous statements by Jencks and Arnett that the RSP should be abandoned as a predictive tool.     

The hydrogenation of HO-terminated telechelic polybutadienes in the presence of a homogeneous hydrogenation catalyst based on tris(triphenylphosphine)rhodium chloride

The hydrogenation kinetics of multiple bonds in HO-terminated telechelic polybutadienes was investigated using two types of these prepolymers prepared by the anionic and radical polymerization. The rate of addition of hydrogen to the π-bonds of these polydienes in the presence of tris(triphenylphosphine) rhodium chloride as a homogeneous hydrogenation catalyst was determined by the chemical structures of the starting polydienes, their concentration in the solvent, the partial hydrogen pressure, the concentration of the catalyst, and the temperature. The effect of kinetic parameters given above on the rate of hydrogenation reaction can be interpreted in the sense of Wilkinson's reaction mechanism of the hydrogenation of alkenes in the presence of the Rh(I)-complex. Due to the predominant 1,2-structural units, the anionic prepolymer reacted twice as quickly with hydrogen (k = 0.093 mol^?1 dm³ s^?1) compared with the radical prepolymer (k = 0.045 mol^?1 dm³ s^?1). During the hydrogenation of multiple bonds there is a partial loss of hydroxy groups in modified telechelic prepolymers; the extent of this reaction depends on reaction conditions of the hydrogenation reaction.     

Electrochemical oxidation of organic compounds in fluorosulfuric acid—III. Voltammetric investigation of aromatic carboxylic acids and acylium ions

The anodic behavior of a series of substituted benzoic acids $\text{1a}$–$\text{1w}$ and of the corresponding benzoylium ions $\text{5}$ is investigated by cyclic voltammetry in fluorosulfuric acid at ?76°C. The acids are oxidized to cation radicals, the stability of which decreases with decreasing number of alkyl substituents. As a subsequent reaction the functionalization of one of the methyl groups to the fluorosulfonyloxymethyl group is found. After dissolving at room temperature of the acids with one o-alkyl group are between 80 and 100% and the acids with two o-alkyl groups are completely transformed into benzoylium ions $\text{5}$, which are between 450 and 560 mV more positively oxidized than the acid. The main reaction of the dication radical formed is the addition of SO₃F^? to the carbonylium C-atom. As the number of alkyl substituents decreases this reaction competes with the functionalization of a methyl group. The difference between the voltammetric data in HSO₃F and acetonitrile is discussed in terms of a protonation of the benzoic acids and their oxidation products.     

Intermolecular electron transfer reactivity determined from cross-rate studies

Electron-transfer cross-reactions between neutral molecules and their radical cations spanning a wide range of structural type and intrinsic reactivity have been analyzed using classical Marcus theory. The principal factor found to govern intrinsic reactivity is the inner-shell bond reorganization energy. The HOMO-LUMO overlap of alkyl groups on reacting molecules is generally sufficient to provide facile electron transfer; however, a significant steric effect on this overlap is observed for hydrazines with alkyl groups larger than methyl.     

Vilsmeier–Haack–Arnold formylations of aliphatic substrates with N-chloromethylene-N,N-dimethylammonium salts

The classical electrophilic substitution of activated aromatics with the Vilsmeier–Haack reagent N-chloromethylen-N,N-dimethylammonium chloride (Schemes 1 and 2) has been more recently extended to a great variety of aliphatic substrates, mainly due to the work of Arnold. In this review, a collection of representative examples for these henceforth called Vilsmeier–Haack–Arnold (VHA) formylation reaction of aliphatics is given: the reaction of polymethine cyanines, merocyanines, and other vinylogous iminium salts with the VHA reagent gives, after hydrolysis of the primary substitution products, trialdehydes such as triformylmethane (Scheme 3); VHA reaction with ene-diamines and diene-diamines yields N,N-dialkylaminomalonaldehydes and tetraaldehydes such as 1,1,2,2-tetraformylethane, respectively (Schemes 4 and 5); aldehyde acetals, enol ethers, and carboxylic acids deliver with VHA reagents 2-substituted malonaldehydes (Scheme 6), and α-amino acids give derivatives of the unstable aminomalonaldehyde (Scheme 7); alkenes and polyenes react with VHA reagents to give α,β-unsaturated or higher vinylogous aldehydes (Schemes 8 and 9), and alkenes with donor substituents yield alkylidene-malonaldehydes (Scheme 10); enolizable methyl and methylene ketones produce with VHA reagents 3-chlorovinylaldehydes (Scheme 11). Eventually, the VHA reagent can be used for the intermediate preparation of nucleophilic amino-chlorocarbenes (Scheme 12).     

The Synthesis Characterization and Polymerization of Novel Thiophene Substituted Maleimide

Ethyl 2-amino 4,5,6,7-tetrahydrobenzo thiophene-3-carboxylate maleimide (ETTCM) has been synthesized and investigated as a new thio-maleimide derivative. The structure of the prepared compounds has been elucidated by elemental and spectral analyses. The free radical polymerization of (ETTCM) has been conducted in several solvents using azobisisobutyronitrile (AIBN) as an initiator. The kinetic parameters of polymerization of ETTCM were investigated, and it was found that the polymerization reaction follows the conventional free radical scheme. The initial rate of polymerization, the overall activation energy Ea was determined (Ea = 57.013 kJmol^?1) and intrinsic viscosity was measured ([η] = 0.04 dl/g). The prepared polymer is a good chelating agent with some metal ions and a moderate antifungal and antibacterial effect. The monomer reactivity ratios for the copolymerization of ETTCM with methyl methacrylate MMA, vinyl acetate VA, and vinyl ether VE were calculated by two methods, the Kelen-Tüdös and a nonlinear method. Thermal stability of the ETTCM polymers and copolymers were investigated by thermogravimetric analysis.