Para-substituted Phenylacetylenes Research Articles

Experimental and Computational Evidence for 1,4-Diradical Intermediates in Reactions of Cobalt Fluorocarbene Complexes with Terminal Aryl-alkynes to give Metallacyclobutenes

Cobalt fluorocarbene complex CpCo(═CF(CF3))(PPh2Me) (Cp = η5-cyclopentadienyl) reacts with para-substituted phenylacetylenes to furnish partially fluorinated cobaltacyclobutene complexes [Cp(PPh2Me)Co{κ2-C(Ar)═CHCF(CF3)}], which were isolated and characterized by elemental analysis, multinuclear NMR and UV–vis spectroscopy, and X-ray crystallography. The scope of reactivity between CpCo(═CFRF)(L) and various alkynes was explored. The detailed pathway for the [2 + 2] cycloaddition reaction was investigated using a combination of kinetic studies and DFT computational chemistry (M06/def2-TZVP), with a 1,4 diradical species identified as the key intermediate.

Monomer-induced switching of stereoselectivity and limitation of chain growth in the polymerization of amine-containing para-substituted phenylacetylenes by [Rh(norbornadiene)Cl]2

In the presence of [Rh(nbd)Cl]2 alone, amine-containing phenylacetylenes can serve as both a monomer and cocatalyst in the polymerization. Moreover, the stereo-selectivity of the products was controlled by pendant groups.

Cu-modified hydroxy-apatite as catalyst for Glaser–Hay C[sbnd]C homo- coupling reaction of terminal alkynes

The oxidative Glaser–Hay coupling reaction of terminal alkynes is a very important reaction in organic chemistry to achieve the synthesis of diyne compounds. In general the reaction is performed under homogeneous conditions using Cu(I) or Cu(II) salts in the presence of a reagent such as tetramethylethylenediamine (TMEDA), which can bind to copper ions, an organic base and dioxygen. Although this reaction is known for a long time, the mechanism is still under discussion. Pursuing our investigations on the use of apatites as catalysts for organic reactions we highlight in this work how it is possible to catalyze the Glaser–Hay reaction under heterogeneous conditions using a Cu-modified hydroxyapatite (Cu–HAp). With several para-substituted phenyl-acetylenes and alkynols we show that Cu–HAp acts as catalyst for CC coupling reactions leading to diyne derivatives in high yields without using auxiliary chelating molecules and organic bases. These heterogeneous conditions allow an easy recovery of the catalyst and simplify the purification work-up.

Preparation of conjugated polyphenylenes from maleimide-based enediynes through thermal-triggered Bergman cyclization polymerization

Several maleimide-based enediynes are synthesized through Sonogashira coupling reactions between 3,4-diiodo-N-benzylmaleimide (3) and para-substituted phenylacetylenes (5). All these enediynes undergo thermal-triggered Bergman cyclization at elevated temperature (130–160° C) as revealed by DSC analysis. Conjugated polyphenylenes (7) are obtained by simply heating these enediynes (6) under vacuum. The occurrence of Bergman cyclization is evidenced by IR and NMR spectroscopy. The conjugated polyphenylenes are further characterized by UV-vis and MALDI-TOF mass spectroscopy (MS). Isotope pattern analysis on the MS spectra of the conjugated polyphenylenes shows that the chain ends of the polymers consist of a considerable amount of unmasked free radicals. EPR spectroscopy of the conjugated polyphenylenes confirms the existence of free radicals, which slowly fade away probably by removing active atoms (like hydrogen or halogen) from the environment.

Synthesis and cyclooxygenase inhibition of various (aryl-1,2,3-triazole-1-yl)-methanesulfonylphenyl derivatives

A series of 1,4- and 1,5-diaryl substituted 1,2,3-triazoles was synthesized by either Cu(I)-catalyzed or Ru(II)-catalyzed 1,3-dipolar cycloaddition reactions between 1-azido-4-methane-sulfonylbenzene 9 and a panel of various para-substituted phenyl acetylenes (4-H, 4-Me, 4-OMe, 4-NMe 2, 4-Cl, 4-F). All compounds were used in in vitro cyclooxygenase (COX) assays to determine the combined electronic and steric effects upon COX-1 and COX-2 inhibitory potency and selectivity. Structure-activity relationship studies showed that compounds having a vicinal diaryl substitution pattern showed more potent COX-2 inhibition (IC 50 = 0.03–0.36 μM) compared to their corresponding 1,3-diaryl-substituted counterparts (IC 50 = 0.15 to >10.0 μM). In both series, compounds possessing an electron-withdrawing group (Cl and F) at the para-position of one of the aryl rings displayed higher COX-2 inhibition potency and selectivity as determined for compounds containing electron-donating groups (Me, OMe, NMe 2). The obtained data show, that the central carbocyclic or heterocyclic ring system as found in many COX-2 inhibitors can be replaced by a central 1,2,3-triazole unit without losing COX-2 inhibition potency and selectivity. The high COX-2 inhibition potency of some 1,2,3-triazoles having a vicinal diaryl substitution pattern along with their ease in synthesis through versatile Ru(II)-catalyzed click chemistry make this class of compounds interesting candidates for further design and synthesis of highly selective and potent COX-2 inhibitors.

Two Competing Mechanisms for the Copper-Free Sonogashira Cross-Coupling Reaction

The mechanism of the copper-free Sonogashira cross-coupling was investigated using a model reaction with differently para-substituted phenylacetylenes and 4-iodobenzotrifluoride as coupling partners and a Pd2(dba)3·CHCl3−AsPh3 catalyst system in methanol. A carbopalladation mechanism was ruled out through a series of experiments in which the equivalent of a carbopalladation reaction intermediate was synthesized by an alternate route, and its conversion to product was monitored. A Hammett correlation study revealed a possible mechanistic changeover when going from electron-rich to electron-poor alkynes in the model reaction. It is advocated that the reaction mechanism changes from a pathway involving a fast proton transfer from a slowly forming cationic Pd complex to a pathway involving a slow proton transfer from a neutral Pd complex on going from electron-rich to electron-poor alkynes. The amine base is believed to act as a base in both pathways and as a nucleophile promoting the formation of the cationi...

Substituent effects in the ruthenium catalyzed hydrosilylation of para-substituted phenylacetylenes

The RuCl 2(PPh 3) 3 catalyzed hydrosilylation of various phenylacetylenes ( p-XC 6H 4C CH, X = H, Cl, Me, OMe, CF 3) with HSiCl 2Me was investigated to determine the effect of para-substitution on phenylacetylene reactivity. A series of competition experiments were performed to determine the reactivity of the substituted phenylacetylenes which followed the trend (relative reactivity): CF 3(2.21) > Cl(1.99) > F(1.85) > OMe(1.04) > H(1.00) > Me(0.85). A plot of log (phenylacetylene relative reactivity) versus Hammett σ p exhibited two separate, but parallel, trendlines. The positive slopes ( ρ = 0.60) of these lines indicated that electronegative substituents increased phenylacetylene reactivity. In addition to the electronic effect, a conjugation effect was present that resulted in the second trendline for X = OMe, Cl and F.

Substituent Effects on the Gas Phase Acidity of Phenylacetylenes and Benzyl Alcohols

Abstract The gas phase acidities of meta- and para-substituted phenylacetylenes and benzyl alcohols were determined by measuring equilibrium constants of proton transfer reactions using a FT-ICR spectrometer. The gas phase acidities of phenylacetylenes and benzyl alcohols are related linearly to those of benzoic acids with a slope of unity. Since the CH2 group of benzyl alcohol prevents a direct π-electron interaction between substituents and the negative charge at the oxygen atom, the existence of these linear relationships suggests that σo parameters best describe the substituent effects in all three systems. Indeed, the acidities of phenylacetylenes and benzoic acids correlate with σo values determined in the gas phase. The theoretical calculations of these acidities at MP2/6-311++G**//B3LYP/6-311++G** level of theory could reproduce the substituent effects observed experimentally. The optimized geometries and the elctrostatic charges computed using the CHELPG scheme of Breneman for the conjugate anions of all three acid systems also indicate that there is no π-delocalization of the negative charge into the benzene ring.

Arylethynyl Substituted 9,10-Anthraquinones: Tunable Stokes Shifts by Substitution and Solvent Polarity

2-Arylethynyl- and 2,6- and 2,7-diarylethynyl-substituted 9,10-anthraquinones were synthesized via Sonogashira coupling reactions of 2-bromo-, 2,6-dibromo-, and 2,7-dibromo-9,10-anthraquinone with para-substituted phenylacetylenes. While the redox properties of those compounds are almost insensitive to substitution, their absorption maxima are linearly related to the Hammett constants with different slopes for electron donors and electron acceptors. All compounds are photoluminescent both in solution (quantum yields of emission ≤6%), and as solids. The emission spectra have the characteristics of charge-transfer bands with large Stokes shifts (100−250 nm). The charge-transfer character of the emitting state is supported by large dipole moment differences between the ground and the excited state as concluded on the basis of molecular modeling and Lippert−Mataga correlations of the Stokes shifts with solvent polarity. Maximum Stokes shifts are attained by both electron-donating and -withdrawing groups. This...

Variable Electronic Coupling in Phenylacetylene Dendrimers: The Role of Förster, Dexter, and Charge-Transfer Interactions

A combination of theory and experiment is used to identify a novel variable excitonic coupling in a series of building blocks for small phenylacetylene dendrons. Systematic changes in the experimental emission spectra, radiative lifetimes, and polarization anisotropies as the number of meta-conjugated branches increases provide evidence for a qualitative change in the electronic structure in the relaxed excited state. The excited state electronic structure is investigated theoretically using ab initio CASSCF and CASPT2 calculations, which indicate the presence of large electronic coupling in the emitting geometry that is not seen for the absorbing geometry of the same molecules. The changes in electronic structure that occur upon excited-state relaxation can be understood in terms of a variable excitonic coupling between the phenylactylene branches, which takes these molecules from the weak coupling to the strong coupling regime as they relax on the excited state. The origin of this geometry-dependent coupling is investigated through the interpretation of ab initio calculations in terms of Förster, Dexter, and through-bond charge-transfer interactions. We find that the change in the coupling arises primarily from an increase in the through-bond or charge-transfer component of the coupling, despite the absence of large changes in charge distribution. A theoretical comparison of meta- versus para-substituted phenylacetylenes clarifies why this effect is so pronounced in the meta-substituted molecules.

A theoretical investigation of the CH ··· O interaction between substituted phenylacetylenes and water

The CH ··· O H-bonding interactions between water and various para-substituted phenylacetylenes have been calculated using the AM1 semiempirical molecular orbital method which had been previously shown to be accurate for CH ··· O interactions. Both the interaction energy and the H···O distances vary linearly with the deprotonation energies calculated by the same method.

Degradation thermique de poly-α-acetoxystyrenes para-substitues

A kinetic study of the overall thermal decomposition of two poly para-substituted α-acetoxystyrenes (R = CH 3, Cl) has been carried out. The reaction order is one, over a wide interval of mass loss. The activation energy has been calculated: 42 kcal mol −1 for R = CH 3 and 48 kcal mol −1 for R = Cl. The solid residues have been previously identified as poly para-substituted phenyl-acetylenes with a relatively low molecular mass. The volatile compounds, identified by gas chromatography coupled with mass spectrometry are chiefly acetic acid, acetic anhydride, para-substituted acetophenone and the initial monomer. The reaction of the thermal decomposition is probably a superposition of two reactions: loss of acetic acid and partial depolymerization. A mechanism is proposed.

Equilibrium CH acidity of arylacetylenes in dimethyl sulfoxide

1. We measured the equilibrium CH acidity in DMSO of a series of para-substituted phenylacetylenes, 1- and 2-ethynylnaphthalene, 3- and 4-ethynylpyridines, and 2-ethynylpyrimidine. 2. A linear correlation between the pK and the Hammett σ constants of the substituents is fulfilled in the series of para-substituted phenylacetylenes and ethynylazines.

Polymers of ortho- and para-substituted phenylacetylenes. The relationship between results obtained by Hückel's molecular-orbitals method of calculation and experimental results

The relationship between experimental and theoretical results on the polimerizability of phenylacetylene and its derivatives has been found, and the order of decreasing reactivity of phenylacetylene monomers established. The effect of the position of the substituent on the reactivity of the monomer has been calculated. The absorption bands in the ultraviolet spectra of the monomers have been identified by calculation by Hückel's MO-LCAO method. The most probable sequence of the structural units in phenylacetylene polymers has been established. The factors preventing the production of phenylacetylene polymers of high molecular weight are outlined.

Relative rates of the concurrent reactions in the addition of a substituted benzonitrile oxide to arylacetylenes

The reaction of 3,5-dichloro-2,4,6-trimethylbenzonitrile oxide with some arylacetylenes gives acetylenic oximes, besides 3,5-diarylisoxazoles. Relative rates of concurrent formation of the two kinds of product were measured for some para-substituted phenylacetylenes (substituent, fraction of oxime: NMe2, 0·62; OMe, 0·29; Me, 0·24; H, 0·15; Cl, 0·09).The effect of solvent on the relative rates appears to be small. In the case of phenylacetylene, reaction-temperature variation and deuterium isotope substitution were also employed. There is evidence for a two-step mechanism, with a labile intermediate, for the 1,3-addition. It seems possible that the same intermediate is also on the pathway of the 1,3-cycloaddition, and that the product distribution depends on the conformational distribution of the intermediate.

Kinetics and mechanism of 1,3-cycloaddition of a substituted benzonitrile oxide to a series of arylacetylenes

3,5-Dichloro-2,4,6-trimethylbenzonitrile oxide, which is stable and does not dimerise, was made to react with some para-substituted phenylacetylenes, in carbon tetrachloride and in chloroform, in kinetic experiments followed by infrared analysis. Products were shown to be 3,5-diarylisoxazoles by their n.m.r. spectra. Rate constants are correlated by a V-shaped Hammett plot, with ρ-values –0·34 and +0·61. Change of solvent has a small effect.Also ethylacetylene gives 3,5-disubstituted isoxazoles, both with 3,5-dichloro-2,4,6-trimethylbenzonitrile oxide and with benzonitrile oxide. Molecular orbital calculations were done on some arylacetylenes and on benzonitrile oxide, the Huckel approximation and the ω-technique being applied. On the basis of theoretical and experimental results an interpretation of the cycloaddition is advanced.