Neighboring Group Effect Research Articles

Hydrolysis of phenylthio, phenylsulfinyl and phenylsulfonyl acetates, and neighboring group effect

Batch data for hydrolysis of phenylthio, phenylsulfinyl and phenylsulfonyl acetates have been determined at various pH values and temperatures. The dual regression equations of logk vs. pH and 1 T were obtained. Neighboring group participation in the hydrolysis of phenylthio acetates was discovered, and the relationship between anchimeric assistance(A) and pH was established.

Reaction kinetics of polymer substituents: macromolecular steric hindrance effect in quaternization of poly(vinylpyridines)

The retardation in the quaternization of poly(vinylpyridines) is of steric origin. Using very voluminous reagents, a steric effect also induces an important limitation to the extent of quaternization which is not included in theoretical treatments of neighboring group effect. A kinetic expression based on the classical second-order equation is proposed. It includes the rate constant κ 0 for reaction of a pyridyl group and a macromolecular steric factor β lowering the effective pyridine group concentration capable of reacting. β Characteristizes the effect of the size and nature of the substituents on the accessibility of the pyridine groups

Notizen: Synthese und NMR-spektroskopische Eigenschaften von Benzyl-2,3-anhydro-4-desoxy-4-[N]-pyridinium-pyranosiden/Synthesis and NMR Spectroscopic Properties of Several Benzyl 2,3-Anhydro-4-deoxy-4-[N]-pyridinium Pyranosides

Benzyl 2,3-anhydro-4-0-triflyl-β-L-ribopyranoside (1) and benzyl 2,3-anhydro-4-O-triflyl-α-D-lyxopyranoside (3) react with pyridine to give benzyl 2,3-anhydro-4-deoxy-4-[N]-pyridinium - α-D-lyxopyranoside triflate (2) and benzyl 2,3-anhydro-4-deoxy-4-[N]-pyridinium -β-L-ribopyranoside triflate (4). Similar reactions with thiourea and dimethylsulfoxide are mentioned. Compound 2 is stable at room temperature, whereas compound 4 decomposes in a few hours (neighbouring group effect). The 13C NMR assignments of 2 are proved by 13C {1H} single frequency decoupling experiments.

Pyrolysis of Aryl Azides. XI. Enhanced Neighboring Group Effects of Carbonyl in a Locked Conformation

Rates of pyrolysis in nitrobenzene solution have been measured for 1-azido-9H-fluoren-9- one, 1-azido-9H-xanthen-9-one, 1-azidoacridin-9(10H)-one and 1-azidoanthracene-9,1O-dione; relative to azidobenzene at 120� these were respectively 5.68, 1750, 5090 and 18400. The lack of neighbouring group participation for the first azide is related to the large distance between carbonyl oxygen and the inner azido nitrogen atom, and the data argue against a published proposal that the transition state is stabilized by electrostatic attraction. In the remaining azides, the 'locked conformation' leads to much larger neighbouring group assistance than is observed for freely rotating ortho groups such as benzoyl (krel79). Only the last azide yields an isoxazole on pyrolysis , the second and third ones providing the first reported examples of neighbouring -group-assisted pyrolysis in which no cyclic product is obtained. These results are interpreted in terms of an electrocyclic mechanism in which the transition state is early and N---0 bond formation is less advanced than other changes in bonds. Much of the rate enhancement is attributed to an electron distribution which favours nitrogen loss. 1-Aminoanthracen-9(10H)-one, 1-amino-9H-fluoren-9-one and 1-amino-9H-xanthen-9-one do not yield the corresponding isoxazoles when oxidative cyclization is attempted.

The Reaction‐Accelerating Neighboring‐Group Effects in the Reaction of Vinyl Bromides with Alkyl‐Transition Metal Reagents

The neighboring-group effect is not caused by the electron-donor center itself, but probably by the metal atom bound to it in an intermediate such as 1. The result of the effect is that α or β donor substituents (OH, OMe, NC) in vinyl bromides (for example 2) strongly accelerate the alkylating substitution reaction with alkyl transition-metal reagents like Me4MnLi2. The more pronounced effect of α substituents is also in agreement with the proposed intermediate 1 (n = 0), because five-membered rings are known to form more readily than six-membered rings. [M] = Fe or Mn complex fragments.

Ligand field photochemistry of pentacyanoferrate(II) complexes with diamine ligands: influence of the chelating ligands and the role of neighboring group effects on the photoinduced ring closure process

Ligand field photochemistry of pentacyanoferrate(II) complexes with diamine ligands: influence of the chelating ligands and the role of neighboring group effects on the photoinduced ring closure process

Polymeric protecting groups, 4. Amino‐protection with tert‐butoxycarbonyl‐analogous comblike methacrylic polymers by aminolysis of activated carbonates

AbstractThe synthesis of highly amino‐sensitive activated carbonates based on tert‐butoxycarbonyl(BOC)‐modified monomers was performed by reaction of phthalimido chloroformate and phenyl chloroformate with the free OH groups of N‐(2‐hydroxy‐2‐methylpropyl)methacrylamide (1), N‐(3‐hydroxy‐3‐methylbutyl)methacrylamide (2) and N‐(2‐hydroxy‐2‐methylpropyl)‐6‐methacrylamidohexanamide (6), respectively. The monomeric carbonates were radically homo‐ and copolymerized with methyl methacrylate (MMA). The kinetics of aminolysis of these polymers with hexanamide was followed by UV and NMR spectroscopy. Differences between phthalimide‐ and phenyl‐containing systems, neighbouring group effects, influence of different comonomers and spacer groups on the reactivity of the activated carbonates are discussed.

Bromation heterolytique du polystyrene en phase homogene—II. Etude cinetique de la bromation catalysee par SnCl 4, SbCl 5 et TiCl 4

The bromination of polystyrene (PS) by Br 2 has been studied kinetically in the homogeneous liquid phase. The catalysts were the Lewis acids (AL) SnCl 4, SbCl 5 and TiCl 4. The rate is expressed as k 4[AL] α[Br 2] 3[PS] where ∝ = 1.22, 1.25 and 2.0 for SbCl 5, TiCl 4 and SnCl 4 respectively. The rate constant k 4 increases with the proportion of CH 2Cl 2 in the mixture cyclohexane/CH 2Cl 2. The reactivity of PS is very close to that of cumene according to our kinetic measurements. No “neighbouring group effect” has been found. The presence of a brominated unit has no effect on the reactivity of a nearby styrene unit. A mechanism of bromination is proposed.

Hydrolysis of gibberellin 7-methyl esters: anchimeric assistance by a 15-alcohol

The neighbouring group effects of 15α- and 15β-hydroxy groups on the rates of hydrolysis of gibberellin 7-methyl esters are described. The presence of a 15-alcohol increases the rate of ester hydrolysis and a 15α- has a greater influence than a 15β-alcohol. The 15α-alcohol may be effective either through hydrogen bonding between the hydroxy proton and the ester carbonyl thus stabilising the tetrahedral intermediate formed on hydrolysis of the ester, or via 7,15α-lactonisation followed by hydrolysis of the lactone. The 15β-hydroxy may act through hydrogen bonding of a water molecule between the proton of the 15β-alcohol and the 7-carbonyl function. The effect is enhanced by the presence of a 13-acetate.

Search for optimum regimes in the imidization of polyamido acid in solution

-- A mathematical model of the process of imidizing a polyamido acid in the liquid phase has been developed using the theory of neighboring group effects.

Determination of microstructure and composition in butadiene and styrene-butadiene polymers by near-infrared spectroscopy

Transmission spectroscopy in the near-infrared region (1,100-2,500 nm) is used to determine the microstructure and the composition of poly(butadiene) (PBD) polymers and styrene-butadiene (SBR) copolymers in bulk and in carbon tetrachloride solution. The multivariate method of classical least squares (CLS) is used to analyze near-infrared spectra of polymers with NMR-determined microstructures and compositions. Although the near-infrared spectra of the pure analytes (cis-1,4-butadiene, trans-1,4-butadiene, 1,2-butadiene, and styrene) are highly overlapped, the CLS method provides accurate predictions of analyte concentrations, because all available spectral frequencies are used for quantitation. The sensitivity of near-infrared spectroscopy to intermolecular interactions and neighboring-group effects in these polymers is demonstrated.

ChemInform Abstract: Neighbouring Group Effects of the Sulfonamide Function in Amine Oxidations.

AbstractDehydrogenation of the 5‐nitro‐2‐cycloalkylaminobenzenesulfonamides (I) with Hg(II) EDTA produces the cyclic sulfonylamidines (III) via the sulfonylaminals (II) as isolable intermediates.

The 4-methacrylamido-2-methyl-2-butoxy-carbonyl-function as polymerizable amino-protecting group

The synthesis of an highly acidic sensitive polymerizable amino-protecting group is described in the case of the N-protected model compound 4-methacryl-amido-2-methyl-2-butyl-N-p-chlorophenylcarbamate. The new monomer was homopolymerized and copolymerized by radical initiation. Kinetics of acidically induced p-chloroaniline cleavage from the monomer and polymers were followed by NMR spectroscopy. The influence of Calcium-ions and neighbouring group effects are discussed.

Pyrolysis of Aryl Azides.X. Effects of Azide Concentration on Rate Constants and Product Yields

First-order rate constants (k1) have been measured for pyrolysis of azidobenzenes in decalin solution, in the presence of a free-radical chain inhibitor to prevent any induced decomposition. The new values of k1 for the spontaneous unimolecular thermolysis are lower than previously reported ones, and require revision of published neighbouring group effects. Product yields ( azo compound and primary amine) vary with initial concentration of azide in ways which suggest the species responsible for induced decomposition is not triplet arylnitrene , but a solvent-derived free radical. There is no evidence for induced decomposition when nitrobenzene is the solvent. For aryl azides with no neighbouring group effects operating in their pyrolysis, the Arrhenius parameters Eact and ΔSactobey a precise linear relationship.

Dextran structural details from high-field proton NMR spectroscopy

Proton nuclear magnetic resonance at 500 MHz resolves anomeric proton resonances of dextrans well enough to furnish useful structural information. Minor resonances which would take inordinately long accumulation times to detect by carbon-13 spectroscopy are readily accessible. Enzymic hydrolysis of the dextrans to homologous series of oligosaccharides was useful in assigning anomeric resonances to specific structural features in the polysaccharides. Neighbouring group effects are also discussed.

Derivation of fluorine‐containing pyridine dicarboxylates. II. Elaboration at the 4‐position

AbstractIn response to the bio‐activity found in fluorine‐containing 4‐alkyl‐3, 5‐pyridinedicarboxylates, a series of novel 4‐substituted derivatives, not directly available by Hantzsch sequences, were prepared. Starting 4‐alkylpyridines, 1, were converted via enamine 2 to materials 3–8. Derivatives 9–16 in turn were derived from aldehyde 3, while acid derivatives 28–36 were prepared from 14. Addition of oxygen, sulfur, and carbenoids effected conversion of 4‐allylpyridine 16 to epoxy and cyclopropyl derivative 16–22. A number of neighboring group effects were noted, including those forming the fused‐ring systems 23‐27.

Polymere aminoschutzgruppen auf basis von tert‐butoxycarbonyl‐ und benzyloxycarbonylfunktionen enthaltenden polymethacrylamiden

AbstractThe synthesis of tert‐butoxycarbonyl‐ and benzyloxycarbonyl‐modified monomers was performed by the addition of p‐chlorophenyl isocyanate or benzyl isocyanate to the free OH groups of N‐(2‐hydroxy‐2‐methylpropyl)methacrylamide (1), N‐(2‐hydroxy‐2‐methylpropyl)‐6‐methacrylamidohexanamide (2) and N‐β‐hydroxyphenethylmethacrylamide (3). The monomers were homopolymerized and copolymerized with MMA. The kinetics of acidic‐induced amine cleavage from the new amino‐protecting groups was followed by NMR spectroscopy and it became evident that the polymers are suitable amino‐protecting groups. Ca2+ ions enhance the rate of amine cleavage considerably. Neighboring group effects, the influence of comonomers and spacer groups on the reactivity of the urethane groups are also discussed.

The Pagodane Route to Dodecahedranes: 4,9,14,19‐Tetrasubstituted [1.1.1.1]Pagodanes by Intramolecular Functionalization

The highly efficient double intramolecular Barton functionalization of the syn, syn-diamide 1, accessible in gram amounts, to 2 is a decisive step of the dodecahedrane synthesis at Freiburg University. Directed epimerizations, which are probably important for the subsequent CC couplings, are partly possible by exploiting neighboring group effects.

ChemInform Abstract: A Neighboring Group Effect of Carbonyl as Shown in the Mass Fragmentation Patterns of 4‐Oxochroman‐5‐acetic Acids (Ia) and of Some Related Compounds.

ChemInform Abstract: A Neighboring Group Effect of Carbonyl as Shown in the Mass Fragmentation Patterns of 4‐Oxochroman‐5‐acetic Acids (Ia) and of Some Related Compounds.

Oxidation of aryloxyaminoalcohols with activated dimethylsulfoxide; a novel C-N oxidation facilitated by neighboring group effect

Oxidation of aryloxy-β-aminopropanols ( 1 ) with “activated” dimethylsulfoxide (DMSO) was found to provide a convenient “one-pot” method to synthesize aryloxy-β-aminoketooximes ( 3 ), without isolation of the unstable ketones 2 . Assignment of Z and E stereoisomers of oximes 3 was based on 1H-and 13C-NMR studies. Spontaneous isomerization of 3 was also observed and discussed. Dioxime derivatives 6 and 7 were first isolated as by-products in the oxidation of compounds 1 and an improved synthetic method for 6 was developed. The novel C-N oxidation step involved in the formation of 6 from 1 was rationalized on the basis of neighboring group effect.