13C and 15N spin–lattice relaxation and chemical shift studies of pentane-2,4-diamine

¹³C and ¹⁵N NMR chemical shift and spin–lattice relaxation data have been measured for both meso- and racemic-pentane-2,4-diamine. At high pH (12), relaxation is consistent with hindered rotation of the NH₂ group due, in part, to the formation of intramolecular hydrogen bonds. At low pH (2), relaxation is consistent with relatively unhindered rotation of the NH₃⁺ group. Rotational jump rates and barriers are reported, determined from the NT₁ ratios between ¹⁵N and ¹³C nuclei. In all cases, the ratios for the racemic diastereomer are higher than those of the meso compounds; this is interpreted in terms of conformationally more stable intramolecular hydrogen bond formation in the meso compound. Chemical shifts for the diastereomeric amines show that ¹⁵N shifts move downfield on protonation along with methyl and methylene carbons, while the methine carbon resonances move upfield.     

Ethylene/1‐hexene copolymerization with tetramethyldisiloxane‐bridged bis(indenyl) metallocenes

Ethylene/1‐hexene copolymerizations with disiloxane‐bridged metallocenes, rac‐ and meso‐1,1,3,3‐tetramethyldisiloxanediyl‐bis(1‐indenyl)zirconium dichloride (rac‐ 1 , meso‐ 1 ) activated by modified methylaluminoxane were performed to investigate the influence of conformational dynamics on comonomer selectivity. Although ¹H NOESY (nuclear Overhauser and exchange spectroscopy) analysis indicated that the most stable conformation for the meso isomer in solution was that in which both indenes project over the metal coordination site, this isomer showed higher 1‐hexene selectivity in copolymerization (r_e = 140 ± 30, r_h = 0.024 ± 0.004) than the rac isomer with only one indene over the coordination site (r_e = 240 ± 20, r_h = 0.005 ± 0.001). The meso isomer showed high 1‐hexene selectivity, a high product of reactivity ratios (r_er_h = 3.3 ± 0.5) and produced copolymers that could be separated into fractions with different ethylene content suggesting that the active species exhibited multisite behavior and populated conformations with different comonomer selectivities during the copolymerization. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3323–3331, 2004     

Zur NMR–Spektroskopie der meso- und racem-Formen einiger Verbindungen mit zwei gleichwertigen asymmetrischen Kohlenstoffatomen in einer offenen Kette

The chemical shifts of the CH and OCH₃ protons are obtained for the following compounds (meso and racemic forms) in several solvents: in (CD₃)₂CO; (RCHCO₂H)₂ (R = Cl, Br, C₆H₅S, CH₃COS), (C₆H₅CHCO₂H)₂S and (C₆H₅CHCO₂H)₂S₂; in (CD₃)₂CO, CDCl₃, CCl₄, C₆D₆, CS₂; (RCHCO₂CH₃)₂ (R = Cl, Br, C₆H₅, C₆H₅S, CH₃COS), (C₆H₅CHCO₂CH₃)₂S and (C₆H₅CHCO₂CH₃)₂S₂. It was shown that the chemical shift of a proton in the meso form could be than in the racemic form. The difference between them cannot be used for an assignment of relative configurations of the meso and racemic forms. The difference between the NMR spectra of a meso and a racemic form is often sufficient for their determination in a mixture.     

Über 3,4-Bis-(p-hydroxyphenyl)-hexandiole-(2,5) und ihre 2,5-Dimethylhomologen

Zusammenfassung Von den in der vorigen Mitteilung beschriebenen 3,4-Bis-(p-methoxyphenylhexandionen-(2,5) [meso undracem.] wurde diemeso-Verbindung durch Reduktion in zwei (der drei möglichen) entsprechende sekundäre Diole übergeführt und das höherschmelzende Diol durch Entmethylierung in ein 3,4-Bis-(p-hydroxyphenyl)-hexandiol-(2,5) (Dihydroxyhexoestrol) umgewandelt. Versuche, durch Reduktion eine Überführung in ein Hexoestrol zu erreichen und die sterische Anordnung festzulegen, schlugen sämtlich fehl.Eingehender wurde die Gewinnung homologer tertiärer Diole durch Umsetzung der beiden Hexandionderivate (meso undracem.) mit CH₃MgX bearbeitet, wobei unter Ätherspaltung die beiden Bis-homologen-dihydroxy-hexoestrole resultierten. Ammesoiden Hexandionderivat wurde auch die Grignardreaktion allein (ohne Entmethylierung) durchgeführt.

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Of the two (meso andracemic) 3,4-di(p-methoxyphenyl)-hexanediones-(2,5) described in the previous communication, themeso compound has been converted by reduction into two (of the three possible) corresponding secondary diols. Demethylation of the higher melting isomer yielded 3,4-di(p-hydroxyphenyl)-hexanediol-(2,5) (dihydroxyhexestrol). Attempts to obtain the corresponding hexestrol by reduction and hence to determine its configuration remained unsuccessful.The preparation of homologous tertiary diols by reaction of the two hexanedione derivatives (meso andracemic) with CH₃MgX has been investigated more closely; on ether cleavage the two di-homologous dihydroxyhexestrols were formed. The Grignard reaction was also carried out on themeso hexanedione without subsequent demethylation.

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6. Mitt.:H. Bretschneider undR. Lutz, Mh. Chem.95, 1702 (1964).     

Mechanism of cyclopolymerization. Conformational analysis of cis-1,3-diisocyanatocyclohexane

In 1,3-disubstitued cyclohexanes, in general, the diaxial conformation of the cis isomer is, energetically, the least favored conformation. An interspacial electronic interaction in the ground state of a cis-1,3-disubstituted cyclohexane would be expected to increase the proportion of this conformer in the equilibrium mixture. Such an interaction would provide an energetically favorable pathway for cyclopolymerization. From nuclear magnetic resonance studies on cis-and trans-1,3-diisocyanatocyclohexane the conformational equilibrium in the cis isomer was determined. It is shown that cis-1,3-diisocyanatocyclohexane exists in the diequatorial conformation; this is taken as evidence that a ground-state interaction between isocyanato groups in this monomer, which readily cyclopolymerizes, is not a significant factor in the cyclopolymerization mechanism. The value of the free energy barrier, ΔG?, for trans-1,3-diisocyanatocyclohexane was calculated as 11.1 kcal/mole.     

Conformational characteristics of poly(vinylpyrrolidone). solvent-dependence of the chain dimensions

Conformational energies as function of rotational angles over two consecutive skeletal bonds for both meso and racemic diads of poly(Nvinyl-2-pyrrolidone) have been computed. The results of these calculations were used to formulate a statistical model that was then employed to calculate the unperturbed dimensions of this polymer. The conformational energies are sensitive to the Coulombic interactions, which are governed by the dielectric constant, of the solvent, and to the size of the solvent molecules. Consequently, the calculated values of the polymeric chain dimensions are strongly dependent on the nature of the solvent, as it was experimentally found before.     

A computational study on the stability–aromaticity correlation of triply N-confused porphyrins: CMMSE-09

The B3LYP density functional theory methodology in conjunction with the 6-31G(d,p) basis set has been used to characterize triply N-confused meso-tetraphenylporphyrins. According to our computations, there is no a direct correlation between stability and aromaticity as already found for non-substituted confused porphyrins. The inclusion of these substituents in the calculations provokes a decrease of the planarity and aromaticity of these macrocycles along with a notable rise of their relative stability with respect to the non-substituted case. Steric repulsions, both among phenyl rings and β atoms in the pyrrolic rings, and among H atoms in the core of the macrocycles, dominate over aromaticity in the establishment of the most stable conformation of each isomer.     

Energy-Minimized Structures and Calculated and Experimental Isomer Distributions in the Hexaaminecobalt(III) system [Co(trap)2]3+ (trap = 1,2,3-propanetriamine)

The two isomers of [Co(trap)₂]³⁺ (meso-[Co(trap)₂]³⁺ and rac-[Co(trap)₂]³⁺; trap = 1,2,3-propanetriamine) have been studied by strain-energy minimization. The two isomers have been separated preparatively by fractional crystallization, and fully characterized by ¹³C-NMR and electronic spectroscopy, and microanalyses. The calculated isomer distribution (rac/meso = 60%: 40%) is in good agreement with HPLC analysis of thermodynamic equilibrium mixtures at 298 K and 353 K (rac/meso = 55% : 45%). These results are discussed in relation to the approach of calculating isomer distributions of hexaaminecobalt(III) systems by strain-energy minimization neglecting the differences in environmental effects.     

Polylactones. XXV. Polymerizations of racemic- and meso-D,L-Lactide with Zn,Pb, Sb,and Bi Salts—Stereochemical Aspects

Abstract

Polymerizations of racemic- and meso-d,l-lactide were conducted at 120°C in xylylene or in bulk. Lead oxide (PbO), zinc stearate, antimony(III) 2-ethylhexanoate, and bismuth(III) 2-ethylhexanoate were used as initiators. With PbO and Zn stearate, bulk polymerizations were also conducted at 150°C. High yields (?90%) were only obtained with PbO and Bi(III) 2-ethylhexanoate, but the molecular weights were low in all cases. The stereosequences were analyzed by ¹H- and ¹³C-NMR spectroscopy. A significant stereospecificity was never detected. At higher reaction temperatures the resulting stereosequences show an increasing tendency toward randomness.     

The bidentate complexes of (R,R)- and meso-α,α′-bis(trifluoromethyl)-9,10-anthracenedimethanol with cis-1-amino-2-indanol

The enantiorecognition of 1-aminoindane 3 and cis-1-amino-2-indanol 2 by (R,R)-α,α′-bis(trifluoromethyl)-9,10-anthracenedimethanol 1 is reported. The examination of the bidentate associations between 1 and 2 revealed that the cisoid conformation of 1 is responsible for the separation of the NMR signals. Two types of bimodal associations resulted from a cisoid conformation when meso-1 isomer was tested. Molecular mechanics modelling studies gave the possible structures of the associate species.     

Synthesis and conformational analysis of isomeric 3-propananilidotropanes

The isomeric 3-anilino and 3-propananilidotropanes have been synthesized and obtained in isomerically pure form. The configurations and solute conformations of these isomers were studied via glc and nrar analysis. The 3β-isomers have been shown to exist in the normal piper-idine chair conformation whereas the 3α-anilino tropane exists in a flattened piperidine chair conformation and the 3α-propananilidotropane isomer preferentially exists in a conformation in which the piperidine ring system is a boat.     

Conformational characteristics of polystyrene

Conformational energies, computed with a forcefield including coulombic interactions and a simple accounting for the effects of solvent, of meso and racemic 2,4-diphenylpentane as model substances of polystyrene have been computed as functions of the skeletal torsion angles and the phenyl torsion angles. The relatively high energies of the ḡ conformations rendered these states negligible and no minimum was found in the meso-tt domain. Three minima for the meso diad (gt, tg, gg) and four minima for the racemo diad (tt, tg, gt, gg) are relevant. A two-state rotational isomeric state model is applicable with states at φ_t = 5°C and φ_g = 110° for both meso and racemo diads. Statistical weight matrices have been derived and values predicted for the characteristic ratio, the fractions of 2,4-diphenylpentane and 2,4,6-triphenylheptane at stereo-chemical equilibrium and the vicinal NMR coupling constants are found to be consistent with experimental results.     

Conformations of the Nine‐Membered Ring in the B‐Nor‐5,10‐secosteroids. X‐Ray and NMR Analysis

The conformations of (Z)‐ and (E)‐5‐oxo‐B‐nor‐5,10‐secocholest‐1(10)‐en‐3β‐yl acetates ( 2 and 3 , resp.) were examined by a combination of X‐ray crystallographic analysis and NMR spectroscopy, with emphasis on the geometry of the cyclononenone moiety. The ¹H‐ and ¹³C‐NMR spectra showed that the unsaturated nine‐membered ring of (E)‐isomer 3 in C₆D₆ and (D₆)acetone solution exists in a sole conformation of type B ₁ , which is similar to its solid‐state conformation. The (Z)‐isomer 2 in C₆D₆, CDCl₃, and (D₆)acetone solution, however, exists in two conformational forms of different families, with different orientation of the carbonyl group, the predominant form (85%) corresponding to the conformation of type A ₁ and the minor (15%) to the conformation A ₂ present also in the crystalline state. In this solid‐state conformations of the nine‐membered ring of both compounds, the 19‐Me and 5‐oxo groups are ‘β’‐oriented. The NMR analysis suggests that the nine‐membered ring of 4 has a conformation of type C ₁ in CDCl₃ solution.     

Dimers from α-alkoxybenzyl radicals. An NMR study

meso and dl Dimers (ArCHOR)₂ where R is Me, Et, ⁱPr, ^tBu, cyclohexyl and 1-adamantyl may readily be differentiated by their NMR spectra; the benzylic protons of the meso isomer always absorb at a slightly higher field than those of the dl isomer in each of the solvents used. Differences in chemical shift are discussed in terms of preferences in conformer distribution. The formation of equal amounts of both dimers from the corresponding radical ArCHOR shows that steric and polar factors are not important in influencing the dimerization. Magnetic non-equivalence due to the presence of asymmetric centres was found in some of the compounds discussed above.     

Porphyrins.

The synthesis of octaalkyl esters of 1,2-(meso-coproporphyrin-I-yl)ethane has been carried out from the corresponding copper complexes ofmeso-hydroxymethyl- and the nickel complexes ofmeso-dimethylaminomethylporphyrins. Their conversion into the correspondingtrans- andcis-ethylenebisporphyrins has been investigated. On boiling in AcOH or xylene, ethylenebisporphyrins form an equilibrium mixture ofcis andtrans isomers. The cis and trans isomers of 1,2-di(octaethylporphyrin-I-yl)ethylene and the octaethyl ester of 1,2-di(coproporphyrin-I-yl)ethylene have been isolated and their interconversions investigated. It was demonstrated by PMR that thecis isomers of ethylenebisporphyrins have a rigid structure in which there is no free rotation of the porphyrin rings. The presence of two atropisomers is also a characteristic of thecis isomer of the coproporphyrin-I derivative. The formation of metal complexes in combination with the preparation of complexes with additional ligands enables control of the conformation of ethanebisporphyrins in solution. A mechanism has been proposed for the oxidation of ethanebisporphyrins totrans-ethylenebisporphyrins in acetic and other lower aliphatic acids.For part 35 see [1]: for preliminary communications see [2, 3].Institute of Biomedicinal Chemistry, Russian Academy of Medical Sciences, Moscow 119832.Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki-shi, Osaka, 567, Japan. Queensland University of Technology. GPO Box 2434, Brisbane 4001, Australia. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1627–1645, December, 1997.     

Unbridged bivalent lanthanidocenes for isotactic and stereomultiblock polymerization of methyl methacrylate

Bivalent organolanthanides with unbridged substituted indenyl or fluorenyl ligands (1-SiMe₃Ind)₂YTHF ( I ), (9-SiMe₃Flu)₂YTHF ( II ), were found to efficiently catalyze the stereoregular polymerization of methyl methacrylate. The microstructure of resultant polymers was shown to be dependent of a conformation that the ligands adopt at the polymerization temperature. The formation of isotactic rich PMMAs from complex II was proposed to be associated with the fluctuation of the 9-trimethylsilylfluorenyls around a C₂ symmetric twisted-conformation. The formation of the multi(syndioPMMA-block-iso-PMMA) polymers from the mixture of rac- and meso-isomers of I was rationalized on the basis of competing conjugate addition and inversion of the metallocene conformation. Surprisingly, both rac- and meso-isomers of I were found to operate with similar activity and stereospecifity, although the stereospecific operation of the meso-form was not completely understandable. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1599–1606, 1998     

Enantioselective Syntheses and Absolute Configurations of Viridiene and Aucantene,Two Constitutents of Algae Pheromone Bouquets

Viridiene ((+)- 6 ; (+)-(3R,4S)-3-((1Z)-1,3-butadienyl)-4-vinylcyclopentene) and aucantene ((+)- 18 ; (+)-(4R,5R)-4-((1E)-1-propenyl)-5-vinylcyclohexene) are constituents of the pheromone bouquets of several brown algae species. Key synthons to the title compounds are optically active γ-lactones with known or experimentally determined absolute configurations. Horse liver alcohol dehydrogenase, which catalyses the oxidation of meso- and racemic non-meso diols to chiral lactones, and pig-liver esterase, which catalyzes the saponification of meso-diesters to chiral half-esters, were utilized for the asymmetric synthesis of such precursors. The racemic non-meso diol rac- 1 is converted to the two stereoisomeric γ-lactones (+)- 2 and (+)- 3 which are readily separated. meso-Diol 12 is oxidized to the chiral γ-lactone (?)- 11 . Its enantiomer (+)- 11 is obtained by enantioselective saponification of the meso-diester 9 with pig-liver esterase. Appropriately designed syntheses lead from these chiral intermediates to both enantiomers (+)- and (?)- 6 of viridiene and (+)- and (?)- 18 of aucantene. In addition, kinetically controlled reduction of the racemic aldehydes rac- 5a and rac- 15 with horse liver alcohol dehydrogenase offers a convenient alternative to the enantioselective preparation of the enantiomers of the two hydrocarbons 6 and 18 . Chromatography of 6 on triacetylated cellulose as a stationary chiral phase confirms the enantiospecificity of the synthetic routes designed.     

Conformational Studies of a Six-Membered Phostone

Abstract

Although 1,3,2-dioxaphosphorinanes generally assume chair conformations,¹ there are examples in which the ring adopts the boat or twist-boat form.¹ Recent studies on the synthesis, stereochemistry, and reactivity of 2-alkoxy-2-oxo-1,2-oxaphosphorinanes (phostones) have revealed both cis and trans isomers of 3-(diphenylhydroxymethyl)-2-ethoxy-2-oxo-1,2-axaphosphorinane² to assume a chair conformation in the solid state. In the present work, the conformational properties of cis and trans-3-methoxycarbonyl-2-methoxy-2-oxo-1,2-oxaphosphorinanes were investigated by X-ray analysis, variable temperature ³¹P, ¹H and ¹H{³¹P} NMR spectroscopy, molecular mechanics, and semiempirical calculations. The X-ray crystal structure of the trans isomer revealed a chair dormation with equatorial phosphoryl and carbomethoxy groups. No changes were observed in the ³¹P NMR spectra of either isomer in the temperature range of 183–333 K. A complete set of vicinal J_HH coupling constants was extracted from the ¹H{³¹P} spectra of each isomer taken at five temperatures over the range of 213–293 K and refined by simulation of the spectra. The best-fit analysis of this data using a generalized Karplus equation³ revealed that the conformation of the trans isomer in solution was close to that found in the solid state. This conformation corresponded to the global energy minimum calculated by both molecular mechanics and PM3 semiempirical method. A substantial contribution from an inverted chair conformation of the cis isomer had to be assumed to achieve a reasonable fit of the coupling constants calculated from the generalized Karplus equation.     

(E,E)-1,1′-[1,2-Bis(4-chlorophenyl)ethane-1,2-diyl]bis(phenyldiazene) revisited: threefold configurational disorder of (S,S), (R,R) and (S,R) isomers,a detailed critique

Crystal structures described as concomitant triclinic ( I ) and monoclinic ( II ) polymorphs of meso-(E,E)-1,1′-[1,2-bis(4-chlorophenyl)ethane-1,2-diyl]bis(phenyldiazene) [Mohamed et al. (2016). Acta Cryst. C 72 , 57–62] have been re-investigated. The published model for II was distorted due to forcing the symmetry of space group C2/c on an incomplete structure model. It is shown here to be a likely three-component superposition of S,S and R,R enantiomers with a lesser amount of the meso form. A detailed analysis of how the improbable distortion in the published model aroused suspicion and the subsequent construction of undistorted chemically and crystallographically plausible alternatives having the symmetry of Cc and C2/c is presented. For the sake of completeness, an improved model for the triclinic P structure of the meso isomer I , revised to include a minor disorder component, is also given.     

Shaping a Porphyrinoid Frame by Heteroatoms Extrusion: Formation of an Expanded [22]Triphyrin(6.6.0)

An aromatic expanded triphyrin, [22]triphyrin(6.6.0) 2 , containing a pyrrole unit, a bipyrrole moiety, and annulene links, was obtained from a tellurium-containing precursor meso-tetraaryl-26,28-ditellurasapphyrin 1 . The reaction path proceeds through an acid-promoted tellurium extrusion from 1 yielding directly 2 , characterized in a dicationic form by X-ray crystallography. In solution the neutral macrocycle 2 reveals flexibility typical for annulenes and it exists as a mixture of conformers that differ by the configuration of the annulene fragments, as proven by ¹H NMR studies and analyzed by DFT methods. The conformation is controlled by protonation state, the nature of an interacting anion, solvent identity, and by interaction with water.