Ortho-alkyl Substituents Research Articles

Ortho-substituent effect on metal coordination geometry, chain microstructure and polymer properties in α-diimine nickel-catalyzed long chain α-Olefin polymerization

The tuning of polyolefin microstructure, including the branching density and branching distribution, through the ligand modification in transition metal-based catalysts has received considerable interest. In this work, we present the synthesis and characterization of a series of α-diimine Ni(II) catalysts containing bulky diarylmethyl moiety and varied steric substituents, and a systematic investigation on the catalytic behavior of these catalyst for 1-octene and 1-decene polymerization. Ni3 with ortho-diisopropyl substituents adopts a common distorted tetrahedral geometry, while a rare square planar geometry is observed for Ni4 with ortho-phenyl substituents. The ortho-substituent effect on the preference of insertion fashion (1,2 or 2,1-insertion) and chain-walking in long chain α-olefin polymerization is also established. Highly branched amorphous polyolefin can be obtained by the unsymmetrical catalyst with ortho-alkyl substituents (H, Me, iPr) in which the major 1,2-insetion pathway was observed, whereas moderate branched semicrystalline polyolefin can be produced by the unsymmetrical catalyst with ortho-phenyl substituents, which showed a significant increase in 2,1-insertion. Most interestingly, the branching distribution analysis through 13C NMR indicated that the polymer obtained by Ni3 gave the higher percentage of long-chain branches relative to methyl branches, while Ni4 afforded the polymer with methyl branches being predominant. These significant differences in coordination geometry and catalytic behavior between Ni3 and Ni4 may be associated with the unique steric congestion of the ortho-phenyl substituents. In particular, the comparative investigation of the mechanical properties of the polymers obtained by Ni3 and Ni4 was also performed.

C―H···O hydrogen bonding interactions for sterically hindered phenols and their phenoxyl radicals

AbstractSterically hindered phenols have been widely used as chain‐breaking antioxidants in chemical and polymeric industries, and the remarkable stabilities of their phenoxyl radicals are generally attributed to the steric effects and resonance stabilization of the unpaired electron into the aromatic ring. Non‐covalent interaction (NCI) analysis shows that there are four intramolecular C―H···O hydrogen bonds between oxygen atom and the ortho t‐butyl C―H bonds for the sterically hindered phenoxyl radicals and their phenolic precursors. The C―H···O hydrogen bonds can be directly visualized and quantified from the reduced density gradient NCI isosurfaces and atoms‐in‐molecules analysis, respectively. Furthermore, the phenolic O―H bond strength decreases gradually with increasing volume of the ortho alkyl substituents in three different phenol model systems (Table 5). The nice linear correlation of the O―H bond dissociation enthalpies with the Taft's steric parameters (Figure 8) provides additional evidences to support formation of the intramolecular C―H···O hydrogen bonds between oxygen atom and the ortho t‐alkyl C―H bonds.

Why is cis/trans stereoinversion with Li+(THF)4 migration across the phenyl ring of α-lithiostyrene accelerated by two ortho-methyl groups?

Common wisdom might anticipate that two methyl groups placed on a molecular migration route should act as an impediment. However, the “conducted tour” migration of Li+(THF)4 across the aryl ring (“π-route”) during the cis/trans stereoinversion of α-arylvinyllithiums had been found to occur with practically equal velocities in the presence of either one or two ortho-alkyl substituents. We now report that the omission of both ortho-methyl groups retards the stereoinversion process. In order to arrive at an answer to the title question, we investigate the aggregation equilibria and microsolvation states of ortho, ortho′-unsubstituted α-lithiostyrenes by means of approved secondary NMR criteria. Beyond such necessary knowledge about the ground-state properties, we provide kinetic evidence showing that the retarded cis/trans stereoinversion of α-lithiostyrene proceeds by the pseudomonomolecular, ionic mechanism with Li+(THF)4 migration.

The Mechanism of Acceptorless Amine Double Dehydrogenation by N,N,N-Amide Ruthenium(II) Hydrides: A Combined Experimental and Computational Study

A detailed mechanistic analysis of the acceptorless double dehydrogenation of primary amines to form nitriles by HRu(bMepi)(PPh3)2 (1, bMepi = 1,3-bis(6′-methyl-2′-pyridylimino)isoindolate) is presented. The presence of the ortho-CH3 substituents on bMepi is critical for amine dehydrogenation, and no catalysis was observed with HRu(bpi)(PPh3)2 (1-bpi, bpi = 1,3-bis(2′-pyridylimino)isoindolate). Outer-sphere, inner-sphere, and hemilabile pathways were evaluated through ligand substitution and kinetic studies, catalyst modifications, and computational analysis. We propose an inner-sphere mechanism in which a Ru–hydride is protonated by coordinated amine followed by H2 release, which forms a Ru–amido intermediate. The stability of Ru–amido species was evaluated through NBO, AIM, and NCI analyses, revealing steric pressure as well as weak noncovalent interactions between the coordinated amido nitrogen atom and the ortho-alkyl substituents, and these interactions impact the overall thermodynamic profile for am...

Unusual Dinuclear and Mononuclear Cyclometalated Iridium Complexes of 2,5-Diaryl-1,3,4-oxadiazole Derivatives

A family of new 2,5-diphenyl-1,3,4-oxadiazole (OXD) derivatives 8-11 bearing ortho-alkyl substituents on one of the phenyl rings is reported. The reactions of these OXDs with IrCl(3) under standard cyclometalating conditions did not give the usual μ-dichloro bridged diiridium OXD complexes. Instead, the novel diiridium complexes 12-14 and the monoiridium complex 15 were isolated and characterized by X-ray crystallography. It is proposed that the unusual structures arise because of the ortho-alkyl substituents leading to a substantial twisting of part of the OXD system which, for steric reasons, changes the normal course of the metal-ligand coordination reactions. Subsequent reactions of 13 and 15 gave the mononuclear complexes 16-18 with acac and picolinate anciliary ligands. The crystal structures of 16 and 18 are reported. Photoluminescence is observed in the green (16) and blue-green regions (17 and 18) at room temperature. Complexes 16-18 are phosphorescent at low temperature, with triplet lifetimes of 4.2-5.7 μs at 77 K.

Comparison of photoenolization and alcohol release from alkyl-substituted benzoyl benzoic esters

Photolysis of 1B in argon-saturated solutions yields 4B and releases methanol. Laser flash photolysis of 1B shows formation of biradical 2B, which has a lifetime of ~50 ns and a λmax at 330 nm. Biradical 2B undergoes an intersystem crossing to form photoenols E-3B and Z-3B with a λmax at 390 nm. Laser flash photolysis shows that the lifetimes of E-3B and Z-3B are affected by the solvent. Density functional theory calculations demonstrate that the transition-state barrier for a 1,5-H atom shift from Z-3B to regenerate 1B is affected by the ortho-alkyl substituents, whereas the stereoelectronics of the alkyl substituent affect the transition-state barrier of E-3B as it undergoes electrocyclic ring closure to form 4B. The photoreactivity of 1B was compared with its analogous methyl and isopropyl derivatives 1A and 1C, respectively, to better estimate the effect of the alkyl substituent on reactivity.

Tampering with Molecular Cohesion in Crystals of Hexaphenylbenzenes

Hexaphenylbenzene (HPB) and analogous compounds have properties of broad utility in science and technology, including conformationally well-defined molecular structures, high thermal stability, high HOMO-LUMO gaps, little self-association, inefficient packing, and high solubilities. Previous structural studies of HPB and its analogues have revealed persistent involvement of the central aromatic ring in strong C-H...pi interactions. These interactions can be blocked by adding simple ortho alkyl substituents to the peripheral phenyl groups. Comparison of the structures of HPB and a series of ortho-substituted derivatives has shown systematic changes in molecular cohesion and packing, as measured by packing indices, densities, solubilities, temperatures of sublimation, melting points, and ratios of H...H, C...H, and C...C contacts. These results illustrate how crystal engineering can guide the search for improved materials by identifying small but telling molecular alterations that thwart established patterns of association.

Salicylaldiminato-based cobalt(III)-, iron(III)-, and chromium(III)/methyl aluminoxane catalyst systems for polymerization of t-butyl acrylate

A new series of penta-coordinated Co(III)-, Fe(III)-, and Cr(III)- complexes (6–10) bearing N-salicylidineisopropylaniline and sodium N-(4-sulfonitsalicylidineisopropyl-aniline) ligands has been synthesized and utilized, after activation with methyl aluminoxane, as catalysts for the polymerization of tert-butylacrylate (t-BA). High molar mass P(t-BA) polymers with very low molecular weight distributions were produced (M w/M n = 1.06–1.09). Cobalt- and chromium-based precatalysts showed higher activity towards the polymerization reaction than those of the iron complexes. The presence of sulfonated groups on the para position of the aryl group in the backbone of the ligand decreases the catalytic activity of the complexes. The ortho alkyl substituents on the aryl groups of the ligand have a favorable influence on the polymerization activity compared to the alkyl-free analogue (11).

Diaryl Tellurium Dihalides: From Te-Tetracoordinated Compounds to Te-Tricoordinated Molecular Complexes

X-ray structural studies demonstrate that diaryl tellurium diiodides can be constrained by steric factors (ortho-alkyl substituents) to adopt a trigonal pyramidal structure in preference to the expected bisphenoidal structure. DFT calculations are shown to reproduce successfully structural details of the title compounds; they allow one to predict which coordination number should be energetically favored. All compounds are chiral, but the barrier to stereomutation by intra- and intermolecular pathways (60–85 kJmol−1) is rather low.

New Chromium Complexes for Ethylene Oligomerization: Extended Use of Tridentate Ligands in Metal-Catalyzed Olefin Polymerization

A family of chromium complexes bearing tridentate pyridine-based ligands are disclosed as highly active precatalysts for the oligomerization of ethylene. The ligands are comprised of two distinct types: Type 1, in which both ketone groups of 2,6-diacetylpyridine are converted to imines to produce pyridine bisimine NNN ligands; and Type 2, in which only one ketone group of 2,6-diacetylpyridine is condensed with an aniline derivative to give monoimine NNO coordination sets. Ligands of either type are coordinated to chromium(II) or chromium(III) chlorides, and activation of the resultant complexes with methylaluminoxane (MAO) produces highly active ethylene oligomerization and polymerization catalysts. Catalysts of Type 1 (NNN set) generally produce 1-butene when only two ortho alkyl substituents are present but switch to making waxes or polyethylene when the size and/or number of ortho substituents are increased. Catalysts of Type 2 (NNO set) produce waxes and polyethylene under all of the substitution pat...

Synthesis and Reactivity of Bi-, Tri-, and Hexametallic and Zwitterionic Methyl Aluminum Complexes Containing a Phenoxide/Imine Ligand System

Addition of 3 equiv of trimethylaluminum to tris(3,5-di-tert-butyl-2-hydroxyphenyl)methane, 1H3, affords the simple, C3-symmetric, trimetallic species [1(AlMe2)3], 2. To disrupt the Al3O3 bridging unit, one of the ortho-alkyl substituents from the tris(2-hydroxyphenyl)methane platform of 1H3 was substituted with a reactive aldehyde group to afford the ligands 3-(2,2‘-methylenebis(4-methyl-6-tert-butylphenol)-5-methyl-2-hydroxybenzaldehyde, 3H3, and 3-(2,2‘-methylenebis(4,6-di-tert-butylphenol)-5-methyl-2-hydroxybenzaldehyde, 4H3, respectively. Benzylamine reacts independently with both 3H3 and 4H3 specifically at the aldehyde functionality, affording the mixed donor ligands 3-(2,2‘-methylenebis(4-methyl-6-tert-butylphenol)-5-methylsalicylidenebenzylimine, 5H3, and 3-(2,2‘-methylenebis(4,6-di-tert-butylphenol)-5-methylsalicylidenebenzylimine, 6H3, in high isolable yields. Furthermore, the Salen-type ligand, 7H6, can be prepared from the reaction of 1,2-ethylenediamine and 4H3. Two equivalents of trimethylaluminum will react with 5H3 and 6H3 to produce the binuclear aluminum compounds [(5)Al2Me3], 8, and [(6)Al2Me3], 9, respectively. If an additional equivalent of trimethylaluminum is added to 9, the trimetallic species [(6)Al3Me6], 10, albeit as a slightly impure material, is observed, while 7H6 reacts in an analogous manner to afford [(7)Al6Me12], 11. Treatment of 8 with 1 equiv of DMSO produces the new complex [(5){AlMe2}{AlMe(OSMe2)}], 12, wherein the substrate is coordinated to only one of a possible two aluminum centers. In contrast to this simple transformation, the separate reactions of 8 and 9 with either cyclopentanone or cyclohexanone afford the zwitterionic mononuclear complexes [(5H)+AlMe-], 13, and [(6H)+AlMe-], 14. Solid-state structures are reported for 2, 6H3, 7H6, and 9−13.

Anionic Polymerization of Monomers Containing Functional Groups. 10. Anionic Polymerizations of N-Aryl-N-(4-vinylbenzylidene)amines

Anionic polymerizations of seven styrenes para-substituted with N-arylimino groups were carried out in THF at −78 °C with oligo(α-methylstyryl)dipotassium and -dilithium as the initiators. The N-aryl functionalities contained phenyl (1), 2-(tert-butyl)phenyl (2), 2,6-dimethylphenyl (3), 2,6-diethylphenyl (4), 2,6-diisopropylphenyl (5), 4-cyanophenyl (6), and 2,3,4,5,6-pentafluorophenyl (7) groups. The monomers 3−5 underwent anionic polymerization quantitatively to produce the polymers having predicted molecular weights based on molar ratios of monomers to initiators and narrow molecular weight distributions, the Mw/Mn values being around 1.1. On the other hand, no polymeric products were obtained from the polymerization mixture of 1, 2, 6, and 7 under identical conditions. The bulkiness of the two ortho alkyl substituents on the N-aryl moiety was necessary to achieve the anionic living polymerization of styrenes bearing N-arylimino groups. Well-defined new block copolymers having poly(5) segments were synthesized by the sequential addition of 5 and styrene or tert-butyl methacrylate. The resulting poly(3−5) could be quantitatively transformed into the poly(4-formylstyrene) having tailored chain structures by the acid hydrolysis of N-arylimino moieties.

Arylamine–DNA adduct conformation in relation to mutagenesis

A considerable body of evidence has indicated that local conformational alterations induced by DNA adducts may provide the molecular basis for differences in mutational specificities exhibited by structurally similar adducts. To elucidate the relationships between adduct structure and mutation induction, the ability of several single-ring arylamines present in tobacco smoke (i.e., methylanilines, dimethylanilines, and ethylanilines) to form DNA adducts was investigated. In all cases, the major adducts were C8-substituted deoxyguanosine derivatives, which is consistent with what has been observed with more carcinogenic arylamines, such as 2-aminofluorene and 4-aminobiphenyl. Spectroscopic and theoretical data on the adducts indicated conformational differences depending upon the location of the alkyl substituents. Adducts containing alkyl groups ortho to the amine function (e.g., 2-methylaniline) had a greater percentage of syn conformers about the glycosyl bond than those not bearing such groups. Arylamines with ortho alkyl substituents tend to be more mutagenic and tumorigenic than analogues not containing an ortho alkyl substituent. This increase in biological activity may be due in part to the greater propensity of ortho alkylated adducts to adopt a syn conformation.

Photochemical reactions of phenylphosphinic azides having ortho alkyl substituents: diminution of Curtius-like rearrangement; intramolecular nitrene insertion into C–H bonds

Two types of reaction are important for the photolysis of the phosphinic azides Ar2P(O)N3(Ar = mesityl or 2,4,6-triisopropylphenyl) in MeCN–MeOH: Curtius-like rearrangement, giving ArP(O)(OMe)NHAr, and nitrene insertion into a C–H bond of an ortho substituent (Me or CHMe2), giving a dihydrobenzazaphosphole oxide 11 or 12. The behaviour of ButArP(O)N3(Ar = mesityl) is similar except that two rearrangement products are formed (But or Ar migration; ratio 6:1). Extensive intramolecular insertion is unprecedented for photochemically-generated phosphorus(V) nitrenes. The exceptional behaviour of the present azides probably owes much to steric congestion. It may discourage rearrangement (Ar migration), relative to other phosphinic azides, so more nitrene is made available for insertion reactions; and it may encourage intramolecular reaction, relative to phosphoryl azides, by shielding the nitrene from external molecules (solvent) while ensuring that an internal C–H bond is held in close proximity.

Photolysis of phenylphosphinic azides having ortho alkyl substituents: intramolecular nitrene insertion into C–H bonds

Dihydrobenzazaphosphole oxides (e.g.6) are important products in the photochemical reactions of phosphinic azides having P-mesityl or P-(2,4,6,-triisopropylphenyl) groups, suggesting that PV nitrenes are not necessarily averse to intramolecular insertion.

Steric Effects in Forward and Reverse Menschutkin Reactions of Some Pyridines, Quinolines and Thiazoles

The effects of ortho alkyl substituents on the rates of methylation of pyridine and demethylation of N-methylpyridinium iodides are compared (some quinolines are included as 'substituted pyridines'). For monosubstituted compounds, substituent steric effects are twice as important in the methylation (forward) reaction as in the demethylation reaction. In contrast, the ratio is 3 : 1 for disubstituted compounds, because the substituent effects are additive in the reverse reaction but greater than additive in the forward reaction. Some thiazole results are also reported and, here, steric effects on the forward reaction are small while no steric acceleration of the reverse reaction is evident.

Iminyls. Part 4. Intramolecular abstraction of benzylic hydrogen by diaryliminyls

Diaryliminyls, generated in aqueous solution by oxidation of imino-oxyacetic acids with persulphate abstract hydrogen from primary and secondary ortho-alkyl substituents to give benzylic radicals. These mainly cyclise (before or after oxidation to the corresponding carbonium ions) to oxoanthryl and/or isoindoleninyl products which are further oxidised by the persuiphate. o-Methyl- and o-ethyl-diaryliminyls, generated by thermolysis of t-butyl peracetates in benzene react to give mainly the corresponding imine (and/or ketone by hydrolysis). Intramolecular hydrogen transfer occurs with o-isopropyl- and o-benzyl-diaryliminyls, the resulting benzyl radicals reacting further to give 10,10-dimethylanthrone and 1,3-diphenyl-2-(10-phenyl-9-anthryl)isoindole (and products derived therefrom), respectively. The e.s.r. spectra of several of the iminyls have been detected.

Studies of inter- and intramolecular donor-acceptor interactions: Conformational population study of some substituted N-aralkyl-4-cyanopyridinium ions by NMR spectroscopy

The relative orientation is studied (by NMR spectroscopy) of the substituted phenyl group (D) and the 4-cyanopyridinium group (A) in compounds of the type: DCH 2A and DCH(R)CH(R′)A with R,R′ = H or alkyl. For DCH 2A it is shown that ortho-alkyl substituents in D probably cause the molecule to adopt a conformation in which the ring-planes of D and A make a dihedral angle of 90°. From the measurement of vicinal coupling constants between protons attached to the central CC bond of DCH(R)CH(R′)A, the contribution of rotamers in which D and A are mutually gauche or trans has been calculated. The nature of R and R′ strongly influences the rotameric distribution, and consequently the average distance between the centers of D and A. It is suggested that for D = p-anisyl the repulsion between D and A in a gauche position is smaller than between two phenyl groups in such a position.

THE INHIBITED AUTOXIDATION OF STYREN: PART IV. SOLVENT EFFECTS

The influence of solvents on the rate of the uninhibited oxidation of styrene can be roughly correlated with the dielectric constants of the solvents. It is suggested that the propagation reaction involves a dipolar transition state and that the magnitude of the solvent effects might be used to identify dipolar transition states in other oxidation reactions. The effect of solvents on the rate of oxidation of styrene inhibited by phenolic antioxidants is attributed to hydrogen bonding interactions between the phenolic hydroxyl group and the solvent. The magnitude of these interactions depends on the acidity of the phenol and on the degree of steric protection afforded the hydroxyl group by ortho-alkyl substituents.

THE EFFECT OF SUBSTITUENTS ON THE RELATIVE RATES OF REACTION OF tert-BUTOXY RADICALS WITH PHENOLS IN CARBON TETRACHLORIDE AND IN CHLOROBENZENE

The relative rates of hydrogen abstraction from substituted phenols by t-butoxy radicals can be correlated with the σ+ constants of the substituents. The effects of an aromatic solvent (chlorobenzene) and of ortho-alkyl substituents on the reactivities of the phenols and on the transition state of the reaction are discussed.