Steric Effects Of Alkyl Substituents Research Articles

Steric Effects of Alkyl Substituents at N-Donor Bidentate Amines Direct the Nuclearity, Bonding and Bridging Modes in Isothiocyanato-Copper(II) Coordination Compounds

A series of Cu(II)-isothiocyanato coordination compounds derived from sterically hindered N-donor diamines were synthesized and characterized: catena-[Cu(Me3en)(μ-NCS)(NCS)] (1), catena-[Cu(NEt2Meen)(μ-NCS)(NCS)] (2), catena-[Cu(N,N,2,2-Me4pn)(μ-NCS)(NCS)] (3), the dimeric: [Cu2(N,N′-isp2en)2(µ-NCS)2(NCS)2] (4) and the monomeric compound [Cu(N,N′-t-Bu2en)(NCS)2] (5), where Me3en = N,N,N′-trimethylethylenediamine, NEt2Meen = N,N-diethyl-N′-methylethylenediamine, N,N,2,2-Me4pn = N,N,2,2-tetramethylpropylenediamine, N,N′-isp2en = N,N′-diisopropylethylenediamine and N,N′-t-Bu2en = N,N′-di(tert-butyl)ethylenediamine. The coordination compounds were characterized by elemental microanalyses, IR, and UV–Vis spectroscopy as well as single crystal X-ray crystallography. Density Functional Theory (DFT) was used to evaluate the role of steric effects in compounds 4 and 5 and how this may affect the adaption of a specific geometry, NCS-bonding mode, and the dimensionality of the resulting coordination compound.

Ab initio study of the mechanism of carboxylic acids cross-ketonization on monoclinic zirconia via condensation to beta-keto acids followed by decarboxylation

Catalytic mechanism of acetic and isobutyric acids mixture conversion into two symmetrical and one cross-ketone product on monoclinic zirconia (111) surface was extensively modeled by Density Functional Theory for periodic structures. Several options were evaluated for each mechanistic step by calculating their reaction rate constants. The best option for each kinetically relevant step was chosen by matching calculated rates of reaction with experimental values.Four zirconium surface atoms define each catalytic site. The most favorable pathway includes condensation between surface carboxylates, one of which is enolized through alpha-hydrogen abstraction by lattice oxygen. Condensation of gas phase molecules with the enolized carboxylate on surface is less attainable.The kinetic scheme considers all steps being reversible, except for decarboxylation. The equilibrium constant of the enolization step and the rate constant of the condensation step define the global reaction rate for non-bulky acetic acid. For bulky isobutyric acid, decarboxylation step is added to the kinetic scheme as kinetically significant, while hydrocarbonate departure may also compete with the decarboxylation. Electronic and steric effect of alkyl substituents on the decarboxylation step is disclosed.The cross-selectivity is controlled by both condensation and decarboxylation steps. None of the mechanistic steps require metal oxide to be reducible/oxidizable.

Steric effects of alkyl substituents in silicon chemistry

Reactivity data for a number of reactions taking place at Si atoms in organosilicon compounds containing a variety of alkyl groups have been examined for statistical correlations with steric parameters. Four practically equivalent sets of parameters are now available for quantitative description of steric effects of alkyl substituents in organosilicon compounds. Alkyl substituents contribute to the reactivity exclusively with their steric effects. Steric effects in silicon compounds are additive.

Kinetic study of hydrolysis of benzoates. Part XXIII—Influence of the substituent and temperature on the kinetics of the alkaline hydrolysis of alkyl benzoates in aqueous 2.25 MBu4NBr and 80% DMSO

AbstractThe second‐order rate constantsk2(M−1s−1) for the alkaline hydrolysis of substituted alkyl benzoates, C6H5CO(O)R (R = CH3, CH2Cl, CH2CN, CH2C≡CH, CH2C6H5, CH2CH2Cl, CH2CH2OCH3), were measured in aqueous 2.25 Mn‐Bu4NBr and in 80% (v/v) DMSO solution at several temperatures. The logkvalues were analyzed using the equation logk = logk0 + ρσ + δEsB. TheEsBscale has been proposed for the steric effect of alkyl substituents in the alkyl part of esters:EsB = (logkR − logkCH3), wherekis the rate constant for the acidic hydrolysis of substituted alkyl benzoates or acetates in water. As polar substituent parameters, both Taft σ* and σIconstants were used. The dual parameter treatments of the logkvalues with σ andEsBconstants gave excellent correlations (R = 0.997). For 2.25 Mn‐Bu4NBr, 80% (v/v) DMSO and pure water at 25 °C, calculated susceptibilities to the inductive effect of alkyl substituents ρ* were found to be 2.07, 2.21 and 1.64, respectively. The corresponding ρIvalues were 4.64, 4.94 and 3.64. The dependence of ρIon solvent and temperature in the alkaline hydrolysis of substituted alkyl benzoates was similar to that observed earlier formeta‐ andpara‐substituents in the alkaline hydrolysis of substituted phenyl benzoates and tosylates. The substituent dependence of the activation energy,E, was found to be completely caused by the polar effect. Susceptibility to steric effect in the alkaline hydrolysis of alkyl benzoates (δ ≈ 1) appeared to be independent of the solvent and temperature. Copyright © 2002 John Wiley & Sons, Ltd.

Steric effect of alkyl substituents on propagation rate constants of N-(2,6-dialkylphenyl)maleimides in radical polymerization

Steric effect of alkyl substituents on propagation rate constants of N-(2,6-dialkylphenyl)maleimides in radical polymerization

Polymerization of anilines by the use of copper(II) perchlorate as an oxidative coupling agent

Polyaniline and polyalkylaniline perchlorates were prepared by oxidative coupling polymerization of anilines by the use of copper(II) perchlorate in acetonitrile as an oxidant. The corresponding polymer bases obtained by treating the perchlorates with alkaline solution were soluble in dimethylsulfoxide and partly soluble in tetrahydrofuran. The molecular weights of the THF-soluble components were 2000–6000. The charge transport of the perchlorates is influenced by the chain length and the steric effect of alkyl substituents.

Conformational study of anticholinesterase carbamates in the furylbenzene series by 1H and 13C NMR spectroscopy

AbstractThe structural analysis of carbamates derived from 2‐(α‐furyl)benzaldoximes and 2‐(α‐furyl)benzyl alcohols was carried out by 1H and 13C NMR spectroscopy. The conjugative and steric effects of alkyl substituents introduced on the benzene rings were found to modify the relative orientation of the aromatic and furan rings. The existence of a close relationship between the stereochemistry of the studied compounds and their anticholinesterase activity has been proposed.

Radical reactions in the co‐ordination sphere of transition metals IX—phenoxy and cyclohexadienonyloxy radicals derived from 2,2′‐biphenyldiols and 2,2′‐thiobisphenols

AbstractFrom a series of antioxidants comprising 2,2′‐biphenyldiols, 2,2′‐thiobisphenols and 2,2′‐dithiobisphenol, free phenoxyls were prepared by oxidation with tert‐butylperoxyls co‐ordinated to Co(III) in non‐polar media at ambient temperature. Both the effect of extended conjugation as a consequence of the direct bond between the two aromatic nuclei and, also, the steric effect of alkyl substituents could be observed. In the presence of free tert‐butylperoxyls in excess, primary phenoxyls are transformed into cyclohexadienonyloxyls, which form stable radical complexes with Co(III). A mechanism for the formation of the cyclohexadienonyloxyls is suggested.

Steric Effects of Alkyl Substituents on the N–H Stretching Absorptions and Rotational Isomerism of N,N′-Dialkylthioureas

Abstract The fundamental N–H stretching absorptions of twenty nine N,N′-dialkylthioureas were observed in dilute solutions. The observed frequencies were classified into five groups assignable to the out, three different types of the trans, and the cis forms. The steric hindrance between the bulky t-butyl group and the thiocarbonyl sulfur, which is larger than for N,N′-dialkylureas, was suggested to cause the out form. Dialkylthioureas of RTUtB type seem to exist to a great extent in the trans-out conformation in solutions. It is suggested that it is not the N–H group with the bulky t-butyl group but the N–H group in the opposite part which is out of the skeletal plane. The energy difference between the out and the trans forms was found to be ΔH=430±100 cal mol−1.

Organic Carbonates. Part XIII. Separation of Polar and Steric Effects in the Hydrolyses of Substituted Ethylene and Trimethylene Carbonates

AbstractThe rate constants, the free energies (ΔF#), the energies (ΔH#) and entropies (ΔS#) of activation, and the steric parameter of substituent (Es), for both the acid and the alkaline hydrolyses of four highly branched ethylene carbonates (Ia ‐ d), and 11 trimethylene carbonates (IIa ‐ k) were analysed according to Taft's procedure for a quantitative separation of polar and steric effects of alkyl substituents in the total effect of structure upon reactivity in hydrolysis. Application of Eq. (II) with Σσ* to the most hindered substrates (“α‐axial methyl effect”) (IId ‐ f) and to the moderately hindered substrates (IIa ‐ c) gave well‐separated parallel straight lines for the two groups. Deviations from Eq. (II) were manifested by the rate‐enhanced group of compounds, geminally 2,2‐disubstituted trimethylene carbonates (IIg ‐ k) (“2,2‐gem‐dialkyl effect”), shown to exert a polar “field effect”; the assessment of its substituent constant σF* (= +0.17) is provided by Eq. (III). Analysis of 018 exchange experiments and ΔΔFA# vs ΔΔFB# correlation (Eq. (IV)) show that the acid‐ and base‐catalysed reactions occur in parallel via similar transition states, the attainment of each designating the rate‐determining step in both systems. A value of 0.5 kcal/mole was found for the net steric interaction (ESB ‐ ESA) exerted by an α‐axial methyl substituent on the attacking hydroxide ion in attaining the transition state. It was concluded that in alkaline hydrolysis, the transition states of compounds exhibiting an “α‐axial methyl effect” should closely resemble the tetrahedral intermediate (IX), while the transition states of the 2,2‐gem‐dialkyl series should more closely resemble the ground‐state.

Steric Effects of Alkyl Substituents upon the Opening of the Lactone Ring of Coumaran-2,3-diones

Abstract The stability of the lactone rings of alkyl substituted coumaran-2,3-diones against hydrolysis and alcoholysis is greatly affected by the positions of alkyl substituents. From UV spectral studies it was suggested that methyl substitutions at positions adjacent to the lactone ring contribute to stability for steric reasons.

Cyclic organophosphorus compounds. Part VI. The hydrolysis of some 1,3,2-dioxaphospholans and 1,3,2-dioxaphosphorinans in aqueous alkali–dioxan solution

Stepwise replacement of the hydrogen atoms at positions 4 and 5 in the 2-thiono-1,3,2-dioxaphospholan ring system by methyl groups stabilises the ring by a constant factor of two (approximately) for solutions of the compounds in aqueous sodium hydroxide–dioxan. This effect is thought to be due to electronic rather than steric interactions, and is very much weaker than that produced by exocyclic groups attached to phosphorus. Under the conditions employed, 2-oxo-2-phenoxy- and 2-ethoxy-4,4,5,5-tetramethyl-2-thiono-1,3,2-dioxaphospholan yield ring-opened products predominantly.In contrast, 2-chloro-5,5-dimethyl-2-thiono- and 5,5-dimethyl-2-S-(morpholinocarbonylmethyl)thio-2-thiono-1,3,2-dioxaphosphorinan hydrolyse at the exocyclic bonds attached to phosphorus, the rate of reaction being comparable with that of the analogous acyclic ester. In the six-membered-ring system, steric effects of alkyl substituents are probably also of importance.