Meso Configuration Research Articles

Capturing a metastable chiral polymorph of an achiral molecule—hexa-O-benzoyl-myo-inositol

myo-Inositol hexabenzoate having meso configuration produces chiral polymorph (form I) when crystallized rapidly but yields achiral polymorph (form II) when allowed to crystallize slowly; in the mother liquor form I slowly but completely disappears to give form II.

ADDITION OF CYCLOHEXYLISOTHIOCYANATE TO O,O-DIETHYL-N,N′-ETHYLENE-BIS-PHENYL METHYL PHOSPHONATE. X-RAY STRUCTURE OF THE CONDENSATION PRODUCT

The synthesis and the molecular structure of the title compound has been determined by single crystal X-ray analysis. C38H60N4O6P2S2 has a meso configuration, its crystal is monoclinic space group P21/c with a = 11.656(2), b = 18.480(4), c = 10.210(2), α = γ = 90°, β = 94.91(3)° and Z = 2. The refinement had a R-value 0.0537 for 2045 reflections. Bond distances and angles are quite normal and were within the expected range for all of the atoms. The torsion angle, NI-C8-C8A-N1A, is 180° so the molecule takes on a rather layered structure form one half to the other. There are a number of weak intermolecular interactions, but the strongest is between S1 and the hydrogens in C18 of the nearest neighbour molecule.

Synthesis of ultra‐high molecular weight polyacrylonitrile with highly isotactic content (mm > 0.60) using dialkylmagnesium/polyhydric alcohol system as catalyst

AbstractAn attempt was made to synthesize a catalyst having high activity for synthesis of highly isotactic polyacrylonitrile (PAN) with high molecular weight by anionic polymerization. The reaction products of di‐n‐hexylmagnesium [(n‐C6H13)2Mg] with alicyclic alcohols such as 1,4‐cyclohexanediol or 1,5‐decalindiol were found to be effective as polymerization catalysts for preparation of PAN samples whose triad isotacticity [the content of mm (m, meso configuration)] is about 0.64 and viscosity‐average molecular weight (Mv) exceeds 1 × 105. The triad tacticities [mm, mr, and rr contents (r, racemo configuration)] of the polymers obtained were analysed by 13C NMR and their Mvs were determined by viscometry in dimethylsulphoxide at 25°C using Kamide et al.'s Mark–Houwink–Sakurada equation.Mv of PAN increased significantly by after‐treatment of the catalysts, prepared from (n‐C6H13)2Mg and alicyclic alcohols, with triethylalyminium (Et3Al). Ultra‐high molecular weight PAN samples ranging in Mv from 2 × 106 to 6 × 106 [(mm) = 0.57–0.66] were obtained in anisole at 135°C by using the above catalyst system. A main role of Et3Al in these catalyst systems is considered to reduce, in advance, OH groups on the catalyst, resulting in a significant suppression of termination reactions of propagating PAN chains during the polymerization.

Synthesis of stereoisomers of antithrombotic nipecotamides.

The stereoisomers of alpha,alpha'-bis[3-(N,N-diethylcarbamoyl)-piperidino]-p-xylene (1) were synthesized. Rac ethyl nipecotate was resolved by diastereomeric (-)-D- and (+)-L-tartrate salt formation. The enantiomeric esters were hydrolyzed to the corresponding nipecotic acids, which were then converted into t-BOC derivatives. Treatment of the latter with diethylamine/isobutyl chloroformate and removal of the t-BOC protecting group afforded (R)- and (S)-N,N-diethylnipecotamides. Condensation of the latter with alpha,alpha'-dibromo-p-xylene gave (R,R)- and (S,S)-1. The meso-diastereomer was obtained by stereospecific synthesis in addition to our earlier procedure involving fractional crystallization of the diastereomeric mixture obtained by synthesis. The latter was resolved earlier into 1A, 1B, and 1C using chiral high-performance liquid chromatography (HPLC). Based on the stereospecific synthesis now achieved, 1A and 1B are assigned the configurations, (R,R) and (S,S) respectively, and 1C is assigned the meso configuration. The (R,S) structure of the latter is also confirmed by X-ray crystallography.

Transition metal complexes with sulfur ligands Part CII. Changes in configuration and reactivity caused by alkyl substituents R in [Fe(L)(‘N RS 4’)] complexes (LCO, NO +, NO, PMe 3, N 2H 4; RCH 3, CH 2CH 2CO 2CH 3, CH 2CH 2CO 2−)

In search for complexes revealing functional and structural characteristics of the active sites of nitrogenases the iron(II) complexes [Fe(L)(‘N ES 4’)] (E = CH 2CH 2COOCH 3; L = CO ( 2), PMe 3 ( 11), N 2H 4 ( 9)) and [Fe(‘N ES 4’)] x ( 7), [Fe(L)(‘N MeS 4’)] (L = CO ( 1), PMe 3 ( 10), N 2H 4 ( 8), NO ( 13), NO + ( 12)) and [Fe(‘N MeS 4’)] x ( 6), Na[Fe(CO)(‘N AS 4’)] ( 16) (A = CH 2CH 2COO −) and {Na[Fe(‘N AS 4’)} x ( 15) as well as the free ligands ‘N ES 4’-H 2·HCl ( 5), ‘N MeS 4’-H 2·HCl ( 4) and Na 3-‘N AS 4’ ( 14) were synthesized. 1 was characterized by X-ray structure analysis. It crystallizes in the space group P2 1/ c with a = 753.9(4), b = 1283.7(8), c = 1931.9(10) pm, β = 95.86(4)°, Z = 4. Unexpectedly, the [Fe(L)(‘N RS 4’)] complexes (R = CH 3, CH 2CH 2CO 2CH 3, CH 2CH 2CO 2Na) show different coordination geometries and considerably higher reactivities in comparison to the corresponding [Fe(L)(‘N HS 4’)] complexes with the parent ligand ‘N HS 4’ 2− (2,2′-bis(2-mercapto-phenylthio)diethylamine(2-)). These differences can be traced back to the fact that the N atom is a tertiary amine in ‘N RS 4’ 2− and a secondary amine in the ‘N HS 4’ 2− ligand. Repulsive interactions among the three N substituents in the ‘N RS 4’ 2− ligands lead to a weakening of the FeN bond, to a meso configuration of the [Fe(‘NS 4’)] core and ultimately to higher reactivities of the [Fe(L)(‘N RS 4’)] complexes in comparison to the [Fe(L)(‘N HS 4’)] patent compounds. The low-spin complexes [Fe(CO)(‘N MeS 4’)] ( 1) and [Fe(CO)(‘N HS 4’)] ( 3) may serve as examples: [Fe(CO)(‘N HS 4’)] ( 3) has C 1 symmetry and a short FeN distance (207.2(8) pm), whereas [Fe(CO)(‘N MeS 4’)] ( 1) reveals C 5 symmetry and a long FeN bond (218.7(4) pm). Such long FeN bonds can only be observed in the patent [Fe(L)(‘N HS 4’)] complexes with σ ligands when electrons occupy antibonding σ* molecular orbitals.

Crystal and molecular structures of d- glycero- l- gulo-heptitol, meso- glycero- allo-heptitol, and meso- glycero- allo-heptitol heptaacetate

The molecular structures of the title compounds were determined by X-ray crystallography using direct methods, and refined to final residual parameters of R = 0.040, 0.063, and 0.044, respectively. All the compounds investigated adopt bent conformations, the molecules of meso configuration thus becoming chiral conformers. For meso- glycero- allo-heptitol, this involves a 1,3-parallel interaction between C-5 and O-2. The derived heptaacetate is found in a different conformation with O-3/C-6 and O-4/C-7 aligned 1,3-parallel. The reported solution for meso- glycero- allo-heptitol is conventional, but with regard to the optimum in crystallography has to be considered as preliminary.

STRUCTURAL INVESTIGATION ON 1-AMINO-ARYL-METHYLPHOSPHONATE ESTERS. X-RAY STRUCTURE OF O,O-DIETHYL-N-N′-ETHYLENE-BIS-PHENYLMETHYLPHOSPHONATE

Abstract The crystal and molecular structure of the title compound has been determined by single crystal x-ray analysis. C24H38N2O6P2 has a meso configuration, its crystal is triclinic, space group P1, with a = 11.795(2), b = 8.616(2), c = 7.694(2), α = 112.72(2), β = 94.42(2), γ = 104.91(2), and Z = 1. The refinement had a R-value of 0.036 for 1580 reflections.

Silver macrocyclic complexes: synthesis, crystal structures and redox properties of [Ag([18]aneN2S4)]PF6and [Ag(Me2[18]aneN2S4)]BPh4([18]aneN2S4= 1,4,10,13-tetrathia-7,16-diazacyclooctadecane, Me2[18]aneN2S4= 7,16-dimethyl-1,4,10,13-tetrathia-7,16-diazacyclooctadecane)

Reaction of AgNO3 with one molar equivalent of [18]aneN2S4(1,4,10,13-tetrathia-7,16-diazacyclooctadecane) or Me2[18]aneN2S4(7,16-dimethyl analogue) in refluxing MeOH–H2O affords the complex cations [Ag([18]aneN2S4)]+ and [Ag(Me2[18]aneN2S4)]+ respectively. [Ag([18]aneN2S4)]PF6 crystallises in the orthorhombic space group Pcab with a= 16.7327(25), b= 17.5023(11), c= 14.7031(6)Å and Z= 8. The metal ion is bound to a highly distorted octahedral arrangement of the six macrocyclic donor atoms [Ag–S 2.630(4), 2.664(4), 2.719(4), 2.774(4)Å; Ag–N 2.533(10), 2.817(15)Å]. The complex cation adopts a rac configuration in which the two S–N–S linkages are bound meridionally to the metal ion. [Ag(Me2[18]aneN2S4)]BPh4 crystallises in the monoclinic space group P21/n with a= 10.7529(15), b= 19.0571(28), c= 18.9360(22)Å, β= 106.035(10)° and Z= 4. The cation is disordered and shows two different configurations. The major [70.8(8)%] component shows an unusual kite-based pyramidal geometry with AgI bound via a periplanar arrangement of four S-donor atoms [Ag–S(1) 2.583(4), Ag–S(4), Ag–S(4) 2.819(3), Ag–S(10) 2.663(4), Ag–S(13) 2.673(4)Å] and one apical N-donor atom [Ag–N(7) 2.517(11)Å]. The second N-donor atom does not interact with the metal centre [Ag ⋯ N(16) 3.684(11)Å]. In view of the asymmetric bond length distribution, the overall stereochemistry may alternatively be described as tetrahedral through S(1), N(7), S(10) and S(13), with S(4) interacting more weakly at long range. Although this complex is severely distorted, it is clear that unlike in [Ag([18]aneN2S4)]+, the S(4)–N(7)–S(10) linkage in [Ag(Me2[18]aneN2S4)]+ binds facilally to the AgI centre to give a distorted meso isomer. The minor [29.2(8)%] component differs only in the orientation of the S(1), N(16′) and S(13) linkage. Although the C–C and C–S bond lengths in this region were constrained, it is clear that N(16′) is directed towards, and is co-ordinated to the Ag1 centre [Ag–N(16′) 2.778(10)Å], giving an octahedral complex in a genuine meso configuration. [Ag([18]aneN2S4)]+ shows a reversible AgI–AgII couple at E½=+0.65 V and a quasi-reversible AgI–Ag0 couple at E½=–0.74 V while [Ag(Me2[18]aneN2S4)]+ shows an irreversible AgI–AgII couple at Epa=+0.78 V and an irreversible AgI–Ag0 couple at Epc=–0.78 V (all potentials vs. ferrocene–ferrocenium).

Asymmetric induction of four chiral centers by hetero Diels‐Alder reaction of a chiral nitroso dienophile

AbstractThe Diels‐Alder reaction of cis‐5,6‐diacetoxy‐1,3‐cyclohexadiene (1), which has a meso configuration, with the chiral nitroso dienophile 2 occurs with induction of four asymmetric centers in very high optical yield (ee = 94%) and leads to the dihydro oxazine 3 (89%). The influence of the reaction temperature on the asymmetric induction was investigated and the absolute configuration of the product determined. Reductive cleavage of the N  O bond of 3 presents a simple route to enantiomerically pure derivatives of (1R)‐conduramine A1 (82%).

Ruthenium thioether chemistry: the synthesis and structure of a host–guest complex [Ru([9]aneS3)2][BPh4]2·2Me2SO, and of [Ru([9]aneS3)2][BPh4]2·2MeNO2and [Ru([18]aneS6)][BPh4]2([9]aneS3=1,4,7-trithiacyclononane, [18]aneS6= 1,4,7,10,13,16-hexathiacyclo-octadecane)

Reaction of [{RuCl2(arene)}2](arene = C6H6, C6Me6, or 4-MeC6H4Pri) with four molar equivalents of 1,4,7-trithiacyclononane ([9]aneS3) in MeOH at 293 K affords a colourless or pale yellow solution. Addition of NaBPh4 gives [Ru([9]aneS3)2][BPh4]2 which can be recrystallised from Me2SO, MeNO2, or MeCN. The complex [Ru([9]aneS3)2][BPh4]2·2Me2SO crystallises in the monoclinic space group P21/c with a= 14.134(3), b= 11.096 3(19), c= 19.743(4)A, β= 94.497(16)°, and Z= 2. The single crystal X-ray structure of the complex shows a centrosymmetric cation with homoleptic thioether co-ordination to an octahedral RuII ion. Each macrocycle is bound facially to the metal centre, Ru–S(1) 2.327 2(14), Ru–S(4) 2.335 7(14), Ru–S(7) 2.333 1 (14)A. Two molecules of Me2SO are associated with each [Ru([9]aneS3)2]2+ cation; these interact with the rear cavities of the co-ordinated trithia macrocycles to form a host/guest-type interaction, with the oxygen atoms involved in hydrogen bonding with the methylene protons of [9]aneS3, O ⋯ H 2.201(8), 2.419(8), 2.790(8), and 3.291(8)A. The Me2SO molecules in [Ru([9]aneS3)2][BPh4]2·2Me2SO can be replaced by MeCN or MeNO2 by recrystallisation of the complex from these solvents. The complex [Ru([9]aneS3)2][BPh4]2·2MeNO2 crystallises in the monoclinic space group P21/n with a= 11.200(5), b= 18.247(13), c= 14.323(17)A, β= 90.78(4)°, and Z= 2. The single crystal X-ray structure of this species, however, shows that there is no secondary interaction between the two MeNO2 molecules and the complex cation, the closest contact between the methylene protons and the oxygen atoms of MeNO2 being over 3.5 A. The complex cation [Ru([9]aneS3)2]2+ shows a quasi-reversible oxidation at platinum electrodes in MeCN at E½=+1.41 V vs. ferrocene–ferrocenium indicating that RuII is highly stabilised by RuS6 co-ordination. Reaction of RuCl3·3H2O with one molar equivalent of 1,4,7,10,13,16-hexathiacyclo-octadecane ([18]aneS6) in refluxing dimethylformamide (dmf)–methanol affords [Ru([18]aneS6)]Cl2as a white precipitate. Dissolution of this product in hot water followed by addition of an excess of NaBPh4 yields [Ru([18]aneS6)][BPh4]2 which crystallises in the monoclinic space group P21/c, with a= 12.801(4), b= 10.663(3), c= 20.036(9)A, β= 109.09(3)°, and Z= 2. The single crystal X-ray structure of the complex shows a centrosymmetric cation with hexathia co-ordination of the macrocycle to RuII in a meso configuration, Ru–S(1) 2.322 2(23), Ru–S(4) 2.332 6(25), Ru–S(7) 2.337 2(23)A, and confirms that [18]aneS6 can readily fully encapsulate a relatively large cation such as RuII. The oxidative chemistry of [Ru([9]aneS3)2]2+ and [Ru([18]aneS6)]2+ is discussed.

Synthesis of α-, γ-phosphorus functionalized alkyl lithium species; X-ray structures of [{Li(L)(Ch2PMeR)}2][L =NNN′N′-tetramethylethylenediamine (tmen), R = Me or Ph; L =(–)sparteine, R = Ph] and [Li(tmen){CH(SiMe3)C6H4PPh2-o}

[{Li(L)(CH2PMeR)}2][L = tetramethylethylenediamine (tmen), R = Me (1) or Ph (2); L =(–)-sparteine (sp), R = Ph (3)], prepared by treating the appropriate phosphine with LiBun(L) in hexane or diethyl ether, are dimeric in the solid. The phosphinomethyl ligands bridge the two lithium atoms as part of six-atom heterocycles; Li–C, P 2.145, 2.604; 2.14, 2.64; 2.20, 2.72A, respectively; (2) and (3) are homochiral and thus have a meso configuration. In benzene (1), (2), and [Li(tmen)(CH2PPh2)](5), are monomeric (cryoscopy), and 7Li–31P coupling only below ca. –70 °C for (1)–(3) and (5) in toluene (1 : 1 doublet, 7Li; 1 : 1 : 1 : 1 quartet, 31P, JLip 44.0–53.4 Hz) is consistent with the presence of symmetrical dimers of the type found in the solid for (1)–(3). [[graphic omitted]Ph2-o}], (4), similarly prepared, has the lithium as part of a chelate ring in the solid, binding through the ipso carbon and PIII centre; Li–C,P 2.25(1), 2.65(1)A. Trilithio species based on P(C6H4CHR–)3(R = H or SiMe3) are generated via metallation using LiBun(tmen). Treating the monolithio species derived from P(O)Me2Ph and LiBun(sp) with Etl yields P(O)MePhPrn of 14% estimated optical purity.

Sequence Structure of Styrene-Butadiene Copolymer Determined by Ozonolysis-HPLC Method

Abstract The configurational sequence of styrene units and the arrangement of styrene and 1,2-butadiene units in styrene-butadiene copolymers were characterized by 1H- and 13C-NMR analysis of the ozonolysis products which were separated by a combination of GPC and HPLC. The ozonolysis products from diad and triad styrene sequences flanked by 1,4-butadiene units showed two and three peaks in HPLC, respectively, reflecting the diad and triad tacticity. The probability of racemic addition was found to be 0.56 and 0.58 for radical and anionic SBR, respectively. The ozonolysis products from styrene-1,2 sequences were separated into three fractions by HPLC. The first and second fractions were assigned to a 1,4-1,2 styrene-1,4 (VS) structure differed only in cotacticity. The third fraction was considered to be a mixture of the meso and racemic forms of the l,4-styrene-l,2-l,4 (SV) sequence. The GPC fraction corresponding to a sequence consisting of two styrenes and one 1,2 units was separated into four peaks by HPLC. Both large peaks contained SSV + VSS structures, where one peak consisted of meso configurations with respect to the two styrene units, and the second peak contained racemic styrene alignments. The two small peaks were SVS with the separation due to cotacticity. Based on the intensity of HPLC peaks, it was deduced that the addition of a 1,2 unit after the styrene terminal predominated the addition of a styrene unit after the 1,2 terminal.

Models for illustrating chirality at two centers

This note suggests common objects that can be used as models to illustrate chirality at two centers, and to introduce the concepts of diastereomers and the meso configuration.

Synthesis (via reductive elimination or metathesis) and x-ray crystal structure of a tetrameric meso-tin(II) metallocycle [formula omitted], and characterisation of meso, [formula omitted

A yellow, diamagnetic tin(II) metallocycle, m e s o − S n ( C H S i M e 3 C 6 H 4 ︹ C H S i M e 3 − o ) (I) showing a monomeric parent ion in its mass spectrum and a first ionisation energy at 7.6 eV in its photoelectron spectrum, is tetrameric in the solid with an Sn 4 ring (Sn—Sn 2.852(3)Å), as determined by X-ray structure analysis, and was isolated as a minor product on treating SnCl 4 with [{ o-C 6H 4 (CHSiMe 3) 2}{Li(tmeda)} 2] (II) at −78° C (but not +30° C) [tmeda = Me 2NCH 2CH 2NMe 2] in OEt 2. The major product is the spirocyclic complex S n ( C H S i M e 3 C 6 H 4 ︹ C H S i M e 3 − o ) 2 in which both ligands are in the meso configuration; compound I was more conveniently obtained from II and Sn(OC 6H 2Me-4-Bu t 2-2,6) 2

Polymer conformation and NMR chemical shifts, 1. 1H NMR spectrum of alternating methyl methacrylate‐styrene copolymer

AbstractNMR chemical shifts of the methoxyl and α‐methyl protons in the alternating methyl methacrylate‐styrene copolymer are calculated by taking into account the contributions of the diamagnetic shielding and the magnetic anisotropy effect of the benzene rings in styrene units. The conformational probabilities of the copolymer chain are calculated according to Flory's matrix method. It is deduced from the calculation that NMR should occur at higher field for the methoxyl and at lower field for the α‐methyl protons of the methyl methacrylate unit in meso configuration with an adjacent styrene unit (regarding the methoxycarbonyl and phenyl groups) than for the corresponding protons in the unit of racemic configuration. Further, the differences between the chemical shifts for these protons in meso and racemic dyads are nearly equivalent to the differences in chemical shifts between two adjacent peaks of the observed triplet spectra. This result indicates that the spectral assignment based on a theoretical calculation is quite consistent with the one made by a conventional simulation method: from the lower magnetic field, peaks of cosyndiotactic, coheterotactic, and coisotactic triads appear in this order for the methoxyl protons and in the opposite order for the α‐methyl protons. Magnetic anisotropy effect contributes to these tacticity‐dependent chemical shifts to a far greater extent than diamagnetic shielding. The variation of the chemical shifts due to measuring temperature, probability of isotactic addition, and interactions between groups more than five bonds apart is also discussed.

Détermination par diffraction de rayons x de la structure de: I ‐ bis (ethoxycarbonyl‐1 oxocyclopéntyl‐2)‐1, 2 éthane C18H26O6 II ‐ bis(ethoxycarbonyl‐1 oxocyclopéntyl‐2)‐1, 3 propane C19H28O6 III ‐ bis (ethoxycarbonyl‐1 oxocyclopéntyl‐2)‐1, 6 hexane C22H34O6

AbstractThe crystal and molecular structure of the title compounds have been established from X‐ray diffractometer data. All possess molecular symmetry: I and III with centrosymmetrical molecules are in the meso configuration; II exhibits non crystallographic twofold axial symmetry. Molecular dimensions and conformations are very similar in the three compounds.

Ni(II) Complexes with bis(β-Acylvinyl)amines and (8-Quinolyl-β-acylvinyl)amines

The title complexes have been obtained and shown by elemental analysis, IR and PMR spectra to be octahedral. The trans structure has been assigned to bis(β-acylvinyl)amine complexes based on magnetic equivalence of geminal methyl groups in appropriately substituted representatives of this group of compounds. The (8-quinolyl-β-acylvinyl)-amine complexes may have either trans or s-cis (meso) configuration.

Observation of pentads in deuterated polyacrylonitrile by NMR spectroscopy

AbstractThe deuterium‐decoupled methine proton spectra of polyacrylonitrile‐β,β‐d2 were measured at 156°C in dimethylsulfoxide‐d6 solution. Splittings of triad peaks caused by pentad sequences were observed. The isotactic triad resonance was resolved into three peaks and the heterotactic resonance into two peaks, while the syndiotactic resonance was unresolved or resolved into three peaks. The splittings were assigned by comparing the probability of each pentad sequence with observed intensities. The pentad signal shifted to the higher magnetic field with increasing number of meso configurations as neighbors of the central triad. It was observed that the pentad of atactic polyacrylonitrile‐β,β‐d2 obeyed Bernoullian statistics but that of isotactic polymer obtained by γ‐ray irradiation of the canal complex seemingly obeyed first‐order Markov statistics.

Solvent Effects on the Proton Magnetic Resonance Spectra of 2,3-Dichlorobutane and the Problem of Rotational Isomers

Abstract Effect of solvent on the coupling and magnetic shielding constants of meso and racemic 2,3-dichlorobutanes has been examined using proton magnetic resonance (PMR). The iterative method of spectrum analysis is applied to the simple sub-state in order to improve the computation speed and the computer capacity. The variations in the coupling constants for the meso configuration are to some extent correlated with the population as a function of dielectric constant ε of solvent using an Onsager model. The coupling constants of the racemic configuration were almost independent of the solvent dielectric constant. The estimated chemical shift dependence on ε of the reaction field does not adequately describe the experimental data.

Studies of C-Substituted Tartaric Acid. I. The Preparation of Optically Active 2-Methyltartaric and 2, 3-Dimethyltartaric Acid

Abstract Two racemic forms of 2-methyltartaric acid were prepared, and their optical resolutions were performed. Four optically active isomers were obtained. 2, 3-Dimethyltartaric acid was also prepared and separated into two isomeric optically inactive forms. The isomer with the higher melting point could be resolved into two optically active isomers, but the other, with the lower melting point, could not. The former was, therefore, assigned to the racemic configuration, and the latter, to the meso configuration.