A 1H and 13C NMR study of intramolecular rotations in syn- and anti-o-tolyl-di-tert-butyl-carbinols

The syn and anti rotamers of o-tolyl-di-tert-butylcarbinol, 2a and 2b, respectively, have been studied by ¹H NMR at 200 MHz and by natural abundance ¹³C NMR at 50 MHz. ¹H-{¹H} NOE enhancement factors are consistent with the known structures and the calculated geometries of these compounds. The relaxation time, T₁, of the 2-Me protons in 2b is unexpectedly higher than that for 2a. The ¹³C relaxation times of the 2-Me and the quaternary carbon of the tert-butyl group are also both higher in 2b than in 2a, suggesting that the rotation of these groups is faster in the less stable isomer. The activation energies for t-Bu rotation, measured by ¹H DNMR, agree with this conclusion. Further confirmation is provided by theoretical calculation of the 2-Me and t-Bu rotation barriers based on Allinger's MM2 force field. Comparison of measured ΔG? values from this work and from the literature with MM2-calculated ΔH? values indicates that this force field systematically underestimates rotation barriers in open-chain systems by a factor of approximately 0.64.     

Tautomeric and Conformational Isomerism of Mercaptoacetylhydrazones of Methyl Alkyl Ketones

Mercaptoacetylhydrazones of methyl alkyl ketones (Alk = Me, Et, Pr, i-Pr, i-Bu, s-Bu, t-Bu) exist in solutions as a tautomeric mixture of linear and cyclic 1,3,4-thiadiazine forms.The linear hydrazone form exists as a set of conformers caused by restricted rotation of the amide group relative to the C–N bond. It is shown that tautomeric equilibrium constants correlate with the steric constants of the alkyl substituents, E_S.     

I. Etude de l'Influence de la Nature et de l'Orientation d'un Substituant sur les Constantes de Couplage en Série Tertiobutyl-4 Cyclohexanonique par RMN à Haut Champ

The 250 MHz proton spectra of six α-halogeno-4-tert-butylcyclohexanones have been recorded and the dependence of the coupling constants upon the nature, the position, the orientation and the number of substituents (C?O, Cl, t-Bu) has been examined. The results obtained are interpreted in terms of geometrical deformation or substituent effects.     

Déplacements chimiques induits en RMN par un réactif lanthanidique: I—Application á l'analyse conformationnelle de sulfites cycliques

The chemical shifts induced by Eu(fod)₃ in several series of 6-membered cyclic sulfites give the parameters K_c and Δ_SR of the complexation equilibrium for an assumed 1:1 stoicheiometry. The equilibrium constant K_c decreases with increasing bulk of the C-4 and C-6 substituents and polarity of the C-5 substituent, which corresponds to the increase of the i.r. stretching frequency vS?O. Thus axial S?O will be more tightly complexed than equatorial S?O. It can be predicted that when a conformational equilibrium exists without shift reagent, displacement towards an axial S?O form will occur with the reagent. Use of the Δ_SR pseudocontact equation confirms the following: (i) ax S?O chair forms are stabilized; (ii) eq S?O chairs with two eq C-4 and C-6 substituents show an equilibrium with a few percent of the ax S?O flexible conformation, particularly in the absence of an ax C-5 substituent; (iii) twist forms with a 2–5 axis, intermediate S?O and trans-4, 6-di-tert-Bu substituents give a boat form with O at the prow and ax S?O; (iv) the conformational equilibrium of trans-5-tert-butyl-2-oxo-1, 3, 2-dioxathiane (chair with ax tert-Bu and S?O ? 70%) is completely displaced towards that form; (v) cis-4,4,6-trimethyl-2-oxo-1,3,2-dioxathiane, which exists as an equilibrium in which the three types of S?O occur, is complexed essentially in the twist form with a 1–4 axis and ax S?O. Most of these results are supported by the coupling constants analysis for the ratio R₀/S₀ = 1.     

Beiträge zur Chemie des Phosphors. 128. Synthese des Diphosphastanna-cyclopropans (t-BuP)2Sn(t-Bu)2

Contributions to the Chemistry of Phosphorus. 128. Synthesis of the Diphosphastanna-cyclopropane (t-BuP)₂Sn(t-Bu)₂ The first three-membered P₂Sn heterocycle, 1,2,3,3-tetra-tert-butyl-1,2,3-diphosphastanna-cyclopropane (1,2,3,3-tetra-tert-butyl-1,2,3-diphosphastannirane) ( 1 ), has been synthesized by [2+1] cyclocondensation of K(t-Bu)P—P(t-Bu)K with (t-Bu)₂SnCl₂. 1 is stable at room temperature. Besides, (t-BuP)₂[Sn(t-Bu)₂]₂ ( 2 ), (t-BuP)₄Sn(t-Bu)₂ ( 3 ), and (t-BuP)₄ are formed. In the reaction with Et₂SnCl₂, the six-membered ring compound [(t-BuP)₂SnEt₂]₂ ( 4 ) is the main-product; the four- and five-membered cyclostannaphosphanes (t-BuP)₃SnEt₂ ( 5 ) and (t-BuP)₃(SnEt₂)₂ ( 6 ) are also formed. 1 could be isolated in the pure state and has been unambiguously characterized as a three-membered heterocycle with a P₂Sn skeleton. The ³¹P-NMR parameters of the other new cyclostannaphosphanes 2–6 are reported.     

Beiträge zur Chemie des Phosphors. 224. Zur Thermolyse von 1,2-Di-tert-butyldiphosphan, 1,2,3-Tri-tert-butyltriphosphan und Tetra-tert-butylcyclotetraphosphan

Contributions to the Chemistry of Phosphorus. 224. On the Thermolysis of 1,2-Di-tert-butyldiphosphane, 1,2,3-Tri-tert-butyltriphosphane, and Tetra-tert-butylcyclotetraphosphane On disproportionation of 1,2-di-tert-butyldiphosphane, H(t-Bu)P? P(t-Bu)H (1) , 1,2,3-tri-tert-butyltriphosphane, H₂(t-BuP)₃ (2) , is formed which reacts further at temperatures above 100°C to give 1-(tert-butylphosphino)-2,3,4-tri-tert-butylcyclotetraphosphan, P₅(t-Bu)₄H (4) . Compound 4 reacts with 1 or 2 with lengthening of the P-sidechain to furnish the corresponding 1-(1,2-di-tert-butyldiphosphino)-2,3,4-tri-tert-butylcyclotetraphosphane, P₆(t-Bu)₅H (5) . At temperatures above 170°C, 5 disproportionates into the tetra-tert-butylcyclotetraphosphane, (t-BuP)₄ (3) which is stable up to about 200°C, and the bicyclo[3.1.0]hexaphosphane P₆(t-Bu)₄ from which the polycyclophosphanes P₉(t-Bu)₃ and P₈(t-Bu)₆ arise during the further course of the thermolysis. These products are finally converted through even more phosphorus-rich and more highly condensed t-butylcyclophosphanes into elemental phosphorus. In each reaction step, varying amounts of the monophosphane derivatives t-BuPH₂, (t-Bu)₂PH, and (t-Bu)₃P are formed. The proposed course of the reaction is further substantiated by the pyrolysis products of pure 2 and 3 .     

Synthesis and X-ray single crystal structure of first aromatic ortho-di-tert-butyl azolo[1,2,4]triazine

Oxidation of 3,4-di-tert-butyl-8-methyl-1,4-dihydropyrazolo[5,1-c][1,2,4]triazine with NBS/K₂CO₃ furnished 3,4-di-tert-butyl-8-methylpyrazolo[5,1-c][1,2,4]triazine, a mildly strained heteroaromatic compound. X-Ray single crystal diffraction analysis indicated that the conjugated 1,2,4-triazine ring adopts a twist-boat configuration, while the two t-Bu substituents are located on the opposite sides of the azole plane. The related 3-tert-butyl-4-(o-C-carboranyl)-8-methyl-1,4-dihydropyrazolo[5,1-c][1,2,4]triazine was also synthesized, however, its oxidative aromatization was unsuccessful.     

Beiträge zur Chemie des Phosphors. 138. P5(t-Bu)4H – das erste Derivat von iso-P5H5

Contributions to the Chemistry of Phosphorus. 138. P₅(t-Bu)₄H — the First Derivative of iso-P₅H₅ The thermolysis of 1,2-di-tert-butyldiphosphane, H(t-Bu)P? P(t-Bu)H, yields under suitable conditions the compound P₅(t-Bu)₄H ( 1 ) as the main product. Besides, the tert-butylphosphanes t-BuPH₂, P₆(t-Bu)₅H ( 2 ), H₂(t-BuP)₃, and (t-BuP)₄ are formed. 1 has been isolated in the pure state and structurally characterized as 1-(tert-butylphosphino)-2,3,4-tri-tert-butyl-cyclotetraphosphane. Hence, compound 1 is a derivative of iso-P₅H₅ with a branched phosphorus skeleton built up by a four-membered ring and a phosphorus side chain.     

Design of Thermally Stable Amine–Imine Nickel Catalyst Precursors for Living Polymerization of Ethylene: Effect of Ligand Substituents on Catalytic Behavior and Polymer Properties

Nickel complexes bearing amine–imine ligands with various backbone substituents were synthesized and employed as ethylene polymerization catalysts on activation with Et₂AlCl. The substituent on the backbone carbon atom of the amine moiety is decisive for the living nature of ethylene polymerization. A bulky amine–imine nickel precursor with a tert‐butyl group on the carbon atom of the amine group can polymerize ethylene in a living fashion at an elevated temperature of 65 °C, which is the highest temperature of living polymerization of ethylene with late transition‐metal catalysts. The wide applicable temperature range for living polymerization and sensitivity of the branch structure of the polyethylene to temperature enable precise synthesis of di‐ and triblock polyethylenes featuring different branched segments by sequential tuning of the polymerization temperature.     

Catalytic action of metallic salts in autoxidation and polymerization. IV. Polymerization of methyl methacrylate by cobalt(II) or (III) acetylacetonate–tert-butyl hydroperoxide or dioxane hydroperoxide

Polymerization of methyl methacrylate was carried out by four initiating systems, namely, cobalt(II) or (III) acetylacetonate–tert-butyl hydroperoxide (t-Bu HPO) or dioxane hydroperoxide (DOX HPO). Dioxane hydroperoxide systems were much more effective for the polymerization of methyl methacrylate than tert-butyl hydroperoxide systems, and cobaltous acetylacetonate was more effective than cobaltic acetylacetonate in both hydroperoxides. The initiating activity order and activation energy for the polymerization were as follows: Co(acac)₂–DOX HPO (E_a-9.3 kcal/mole) > Co (acac)₃–DOX HPO (E_a = 12.4 kcal/mole) > Co(acac)₂–t-Bu HPO (E_a = 15.1 kcal/mole) > Co(acac)₃–t-Bu HPO (E_a-18.5 kcal/mole). The effects of conversion and hydroperoxide concentration on the degree of polymerization were also examined. The kinetic data on the decomposition of hydroperoxides catalyzed by cobalt salts gave a little information for the interpretation of polymerization process.     

Remarkably Slow Rotation about a Single Bond between an sp3‐Hybridised Carbon Atom and an Aromatic Ring without ortho Substituents

Look, no ortho substituents! A series of polycycles were prepared by using a three‐component Joullié–Ugi reaction. The rate of rotation about the bond between a highly hindered bridgehead and a phenyl ring with no ortho substituents was measured, and was highly dependent on the substitution. Rotamer half‐lives of up to 21 h at 298 K were observed (see figure). Rotamers resulting from this restricted rotation were isolated for the first time.

     

POLYMERIZATION OF ETHYLENE AND PROPYLENE WITH VCL4-BUTYLLITHIUM CATALYSTS

Polymerization of ethylene and propylene with VCl₄-BuLi (Bu = n-Bu, sec-Bu, tert-Bu) catalysts was investigated. The VCl₄-BuLi catalysts were found to initiate the polymerization of ethylene and propylene. The VCl₄-BuLi catalysts gave an ultra high molecular polyethylene. The effect of the Li /V mole ratio on the polymerization of ethylene with the VCl₄-BuLi catalysts was observed, an the catalyst gave an optimum rate at the Li/V ratio of about 3.0. The polyethylene obtained with the VCl₄-BuLi catalyst was found to be a linear structure. In the polymerization of propylene with the VCl₄-BuLi catalyst, the polymers contain mm contents of 56–66% were produced.     

NMR investigation of stereochemical nonrigidity of five-coordinate phosphorus fluorides

Phenomena associated with stereochemical nonrigidity of five-coordinate phosphorus fluorides (Berry pseudorotation and restricted rotation of different functional groups about the bond with the central phosphorus atom) have been analyzed. The main cause of restricted rotation isp d bonding of unsharedp-electron pairs of substituents with vacantd-orbitals of the phosphorus. Additional stabilization of the molecule occurs when it is possible to form an H...F hydrogen bond between the liquid and an axial fluorine atom in the phosphorane. The relationship between pseudorotation and restricted rotation is considered.A. N. Nesmeyanov Institute of Organoelemental Compounds, Russian Academy of Sciences, 117813 Moscow. N. N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, 117977 Moscow. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 8, pp. 1784–1793, August, 1992.     

Chiral furanoside phosphite–phosphoroamidites: new ligands for asymmetric catalytic hydroformylation

We have designed a new series of phosphite–phosphoroamidites ligands 1–4 based on a furanoside backbone. These ligands were screened in the Rh-catalyzed asymmetric hydroformylation of styrene, inducing high regioselectivities with 2-phenylpropanal and moderate enantioselectivities (up to 65% e.e.). The results showed that the configuration of the stereogenic carbon atom C(3) at the ligand backbone had remarkable effects on the activity and enantioselectivity. Replacing the tert-butyl substituents with methoxy substituents at the para positions of the biphenyl moieties improved the enantioselectivities. We have also studied the solution structures of HRh(PP)(CO)₂ complexes.     

Arjunolic Acid in Molecular Recognition: First Synthesis and Cation Binding Studies of a novel Arjuna-18-crown-6

Arjunolic acid, a functionally rich chiral triterpenoid with a rigid pentacyclic backbone, has the potential to be used as a structural framework for the design of molecular receptors and supramolecular architectures. The design and synthesis of the first arjunolic acid-derived 18-crown-6 and its binding studies with metal and tert-butylammonium ions are reported.     

Beiträge zur Chemie des Phosphors. 142. P6(t-Bu)5H – das erste Cyclotetraphosphan mit P2-Seitenkette

Contributions to the Chemistry of Phosphorus. 142. P₆(t-Bu)₅H – the First Cyclotetraphosphane with a P₂ Side Chain The thermolysis of 1, 2-di-tert-butyldiphosphane, H(t-Bu)P? P(t-Bu)H, leads to formation of the hitherto unknown hexaphosphane P₆(t-Bu)₅H ( 1 ). In the first instance the iso-P₅H₅ derivative P₅(t-Bu)₄H [3] is formed, which reacts further with H₂(t-BuP)₂ or H₂(t-BuP)₃ yielding 1 . Compound 1 has been isolated in the pure state and structurally characterized as 1-(1,2-di-tert-butyldiphosphino)-2, 3, 4-tri-tert-butyl-cyclotetraphosphane, i. e. as a four-membered ring compound with a P₂ side chain. Due to the chirality of the P atoms in the side chain, 1 exists as a mixture of two configurational isomers, the threo-and the erythro-form.     

Synthesis and structures of helical polycyclic aromatic hydrocarbons bearing aryl substituents at the most sterically hindered positions

A three-step synthetic sequence starting from condensation between a benzannulated enediyne and an aryl tert-butyl ketone was established to provide easy access to angularly fused polycyclic aromatic hydrocarbons bearing one or two aryl substituents at the most sterically hindered positions to cause helical twists. The dynamic behaviors involving the helix inversion and the restricted rotation of the aryl substituents were investigated by temperature-dependent NMR studies. The X-ray structure of an indeno-fused 1-phenylpentahelicene derivative showed severe distortion of the [5]helicene system from planarity.     

Dynamic stereochemistry at sp2?sp3 bonds part 1—the use of heterocyclic models for quantitative studies of steric effects of alkyl groups

The barriers to rotation of the 3-substituent in six 3-RCH₂-4-Me-δ⁴-thiazoline-2-thiones (R ? Me, Et, Pr, Isobu, Isopr,t-Bu) and in two analogous methiodides have been measured by the ¹H DNMR technique. The barriers are discussed in relation to the possible conformations of the transition states, and comparisons are made with barriers to rotation of the same substituents in other frameworks. It is concluded that the relative steric effects are strongly dependent on the framework and that, in general, a conformational analysis of the individual cases is necessary to understand the steric effects of alkyl groups.     

Graft copolymers prepared by ethoxylation of polyamide 12 and poly(ethylene-co-vinyl alcohol)

Graft copolymers consisting of polyamide 12 or poly(ethylene-co-vinyl alcohol) as backbone polymers and side chains of poly(ethylene oxide) have been synthesized. The amide and hydroxyl groups of the backbone polymers were used as initiation sites for the polymerization of ethylene oxide (EO). Potassium tert-butoxide was used for ionization of the active groups, and the polymerization of EO was carried out in dimethyl sulfoxide. The graft copolymers were characterized with respect to molecular weight and composition using elemental analysis, ¹H-NMR, gel permeation chromatography, and FTIR. The size of the side chains varied between 300 and 1000 g/mol. Thermal properties were examined by DSC. The graft copolymers showed increasing crystallinity and increasing melt temperature with increasing molecular weight of the side chains. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 803–811, 1998     

Arigid pentacoordinate silicon structure generalization of pseudorotation

Examination of an organosilicon compound with a structure very favorable for pentacoordination shows the generality of the pseudorotation at silicon for bi- and tri-functional compounds. In contrast monofunctional compounds do not undergo pseudorotation, the ΔG* being estimated to be > 20 kcal M⁻¹.