Asymmetric transformation of N-nitrosamines by inclusion crystallization with optically active hosts.

Several N-nitrosopiperidines with chirality solely due to a hindered rotation about the N-N bond were resolved to enantiomers by inclusion crystallization with optically active diols (TADDOLs). The absolute configuration of the guest nitrosamines was deduced from the X-ray crystal structures of the inclusion complexes. The enclathrated nitrosamines were liberated by a competitive complexation of the host diols with piperazine. The optical activity of the resolved nitrosamines is manifested by their CD spectra. A simple chirality rule was proposed for a rationalization of the observed Cotton effect sign corresponding to the n-pi* electronic transition. The optically active nitrosamines are configurationally labile compounds and gradually racemize in solution but they are indefinitely stable in the solid state. The first-order kinetics of the racemization in solution allowed us to assign the N-N rotation barriers by simple polarimetric measurements.     

Synthesis, X-ray crystal structures, spectroscopic and cyclic voltammetric studies of Cu(II) Schiff base complexes of pyridoxal

Two mononuclear Cu(II) complexes, [Cu(L¹H₂)](ClO₄)_1.25Cl_0.75·1.25H₂O (1) and [Cu(L²H₂)](ClO₄)₂ (2), of the pyridoxal Schiff base ligands N,N′-dipyridoxylethylenediimine (L¹H₂) and N,N′-dipyridoxyl-1,3-propanediimine (L²H₂) are reported. X-ray crystal structures of both complexes are also reported. In both complexes the pyridoxal nitrogen atoms remain protonated. In the solid state, the tetradentate Schiff base ligand is virtually planar in 1, while in 2 the ligand conformation is like an inverted umbrella. In cyclic voltammetry experiments it is found that in these complexes the Cu(III) and Cu(I) states are more easily accessible than in their salen type analogs. The pyridoxal Schiff base complexes are also found to be resistant to oxidative electro-polymerization, unlike their corresponding salicyl aldehyde Schiff base complexes.     

Highly enantioselective hydrogenation of exocyclic double bond of N-tosyloxazolidinones catalyzed by a neutral rhodium complex and its synthetic applications

A highly enantioselective synthesis of optically active N-tosyl-4-alkyl-1,3-oxazolidin-2-ones based on the asymmetric hydrogenation of the trisubstituted exocyclic double bond of N-tosyl-4-alkylidene-1,3-oxazolidin-2-ones under the catalysis of neutral [Rh(COD)Cl]₂ (COD=1,5-cyclooctadiene) and (S)-(+)-DTBM-SEGPHOS was developed. The utility of this highly enantioselective reaction was exemplified by the synthesis of optically active amino acids, amino alcohols, and piperidine derivatives.     

Mannich reaction of pyrrole and dimethylpyrrole with monoamines and diamines

The Mannich reaction of pyrrole with ethylenediamine dihydrochloride and formaldehyde gave two compounds: N¹,N¹,N²,N²-tetrakis(pyrrol-2-ylmethyl)ethane-1,2-diamine 1 and 1,3-bis(pyrrol-2-ylmethyl)imidazolidine 2 in poor yields. Conversely, the reaction of pyrrole with propylenediamine dihydrochloride afforded macrocyclic compound 3 containing two pyrrole rings bridged by two dimethylenehexahydropyrimidine moieties in 50% yield. The reaction of 2,5-dimethylpyrrole with methylamine hydrochloride and formaldehyde gave novel bicycle 4 consisting of a dimethyltetrahydro-1,3-diazepine ring fused with a 2,5-dimethylpyrrole ring. The reaction of 2,5-dimethylpyrrole with ammonium chloride gave the tripodal ligand tris(2,5-dimethylpyrrol-3-yl)methylamine 5 in 62% yield. The structures of 2–4 were determined by single crystal X-ray diffraction studies.     

Generation of chirality in 4,4′-azopyridine by co-crystallization with optically active dicarboxylic acids

The co-crystals of anti-4,4′-azopyridine apy with optically active 9,10-dihydro-9,10-ethanoanthracene-11,12-dicarboxylic acid 1 and trans-1,2-cyclohexanedicarboxylic acid 2 were prepared and their solid state CD spectra were measured. The positive Cotton effect sign, corresponding to the lowest energy n–π¹ transition, was correlated with the M helicity of the twisted Ar–NN chromophore. The absolute sense of the twist of the guest apy molecule was deduced from the X-ray structures of the (S,S)-1·apy and (S,S)-2·apy complexes.     

Titanocene complexes of the N-methyl and N-phenyl-1,3-thiazoline-2-thione-4,5-dithiolates and 4,5-diselenolate ligands

Cp₂Ti(dithiolene) and Cp₂Ti(diselenolene) complexes containing the N-methyl-1,3-thiazoline-2-thione-4,5-dithiolate ligand (Me-thiazdt), the N-phenyl-1,3-thiazoline-2-thione-4,5-dithiolate ligand (Ph-thiazdt) and the N-methyl-1,3-thiazoline-2-thione-4,5-diselenolate ligand (Me-thiazds) have been synthesized. Three approaches have been developed in order to generate the dithiolene or the diselenolene ligands which were reacted with Cp₂TiCl₂ to form the corresponding heteroleptic complexes. Their X-ray crystal structures, UV-Vis absorption spectra as well as their redox properties, determined by cyclic voltammetry have been investigated and discussed. Variable-temperature ¹H NMR experiments have been performed in order to determine the activation energies of the chelate ring inversion.     

4,6‐Dimethyl‐2‐[(4‐phenylpiperidin‐1‐yl)methyl]isothiazolo[5,4‐b]pyridin‐3(2H)‐one and 4,6‐dimethyl‐2‐[(4‐phenyl‐1,2,3,6‐tetrahydropyridin‐1‐yl)methyl]isothiazolo[5,4‐b]pyridin‐3(2H)‐one

In the crystal structures of the title compounds, C₂₀H₂₃N₃OS, (II), and C₂₀H₂₁N₃OS, (III), significant differences occur in the conformation of, respectively, the phenylpiperidine and phenyltetrahydropyridine substituents at the 2‐position of the isothiazolopyridine system. The piperidine ring adopts a chair conformation, while the tetrahydropyridine ring assumes a half‐chair form. The phenylpiperidine and phenyltetrahydropyridine fragments exhibit different conformations resulting from the steric and conjugation effects in the phenyl ring, respectively. Theoretical calculations show that both conformations are energetically stable and correspond to a minimum of energy for the analyzed systems. The molecular packing in (II) is influenced by π–π interactions of the isothiazolopyridine systems, with a shortest centroid‐to‐centroid separation of 3.5843 (11) Å between pyridine rings. In the crystal structure of (III), the molecules are linked by C—H...O hydrogen bonds and C—H...π interactions.     

Chiral and supramolecular model complexes for vanadium haloperoxidases: Host–guest systems and hydrogen bonding relays for vanadate species

Supramolecular interactions and hydrogen bonding play a fundamental role in determining both structure and function of vanadate in enzymatic systems and in particular for the active site of vanadium haloperoxidases. Vanadium complexes with N-salicylidene hydrazide ligands provide a versatile approach towards molecular model systems with hydrogen bonding interactions. The variation of the side chains within these hydrazone ligands provides the ability to introduce chirality in molecular model complexes by the utilization of appropriate carbohydrate fragments. Moreover, the synthetic potential and the transformation reactions found for dioxidovanadium(V) complexes with N-salicylidene hydrazide ligands are reminiscent of what is usually observed for carboxylates and can therefore be regarded as their inorganic counterpart. The anisotropy effect of the oxido groups in vanadium complexes is a valuable tool that allows for the configurational and conformational analysis of structures with corresponding chelate rings. Utilizing appropriate vanadium complexes it is possible to generate inclusion compounds with cyclodextrins. The dependence of solid state and solution structures on the ring size of the cyclodextrin is discussed.     

Structural basis of solid solution formation during chiral resolution

A systematic study of crystal packing in a series of structures is presented: an isostructural triplet of an optically active compound {(R)-2-(6,7-diethoxy-1,2,3,4-tetrahydro-1-isoquinolidene)-2-[2-hydroxy-3-(4-morpholinyl)propyl]mercaptoacetonitrile hydrochloride, C₂₂H₃₂ClN₃O₄S} 1, its racemate 2 and their achiral dehydroxy parent compound {2-(6,7-diethoxy-1,2,3,4-tetrahydro-1-isoquinolidene)-2-[3-(4-morpholinyl)propyl]mercaptoacetonitrile hydrochloride, C₂₂H₃₂ClN₃O₃S} 3. Based on the structures we suggest some requirements necessary for an optically active compound and its racemate to be isostructural. A generally used resolving agent d-champhor sulphonic acid was unable to provide full separation of the racemate. The crystal structure of a partially resolved product (C₃₂H₄₇N₃O₈S₂) 4 sheds light on the possible reasons of this failure. We suggest a few criteria for solid solution formation of diastereomeric salts.     

Synthesis,structural and conformational study of some esters derived from 8-phenethyl-8-azabicyclo[4.3.1]decan-10α-ol

A series of 10α-acyloxy-N-phenethyl-8-azabicyclo[4.3.1]decane derivatives have been synthesized and studied by ¹H and ¹³C nmr spectroscopy, and the crystal structure of N-phenethyl-10aL-(xanten-9″-carbonyloxy)bicyclo[4.3.1]decane 2 has been determined by X-ray diffraction. The compounds studied display in deuteriochloroform the same preferred conformation adopted by the piperidine ring as a distorted chair conformation flattened at N-8 with both the phenethyl and acyloxy groups in the equatorial position with respect to the piperidine ring. These results are in close agreement with that found for compound 2 in the crystalline state.     

Synthesis and structure of pseudotetrahedral Co(II) zwitterionic complexes: trichloro(1-methylpiperazin-1-ium-N4)cobalt(II) and trichloro(1,4-dimethylpiperazin-1-ium-N4)cobalt(II)

The pseudotetrahedral cobalt(II) zwitterionic complexes, [CoCl₃(H₂Meppz)] (1) [H₂Meppz⁺=1-methylpiperazin-1-ium cation] and [CoCl₃(HMe₂ppz)] (2), [HMe₂ppz⁺=1,4-dimethylpiperazin-1-ium cation] have been synthesized and characterized in the solid state by X-ray single crystal analysis, IR spectra, magnetic measurements and electronic spectra. In both the compounds the cobalt(II) center is coordinated in a distorted tetrahedral fashion by the three chloride ions and by one nitrogen of the piperazine ring that retains the more stable chair conformation. The distorted coordination polyhedron in complex 1 preserves the C_3v symmetry while in complex 2 it retains only the m symmetry. In complex 1, the (H₂Meppz)⁺ cation binds the Co(II) ion in the equatorial position of the piperazine ring using the unmethylated N1–H nitrogen atom that is less hindered than the methylated one. Complex 2, on the contrary, is a novelty being the first example of a Co(II) ion bound in the axial position of a piperazine ring, this produces a long Co(II)–N bond, 2.108(4) Å. Electronic spectra in the solid state are in perfect accordance with the X-ray crystallographic results indicating a C_3v symmetry for complex 1 and a C_s(m) symmetry for complex 2. These complexes present strong two-center and three-center hydrogen bonds of N⁺–H⋯Cl type.     

Synthesis,structure, and conformation of terphenylene-derived azacalix[4]aromatics

Several novel azacalix[4]aromatics constituting terphenylene units have been synthesized via sequential nucleophilic aromatic substitution reactions of 5′-t-butyl-(1,1′:3′,1″-terphenyl)-3,3″-diamine 9 and 5′-t-butyl-(1,1′:3′,1″-terphenyl)– 4,4″-diamine 11 with 1,5-difluoro-2,4-dinitrobenzene and cyanuric chloride, respectively. The bridging –NH– functions of the tetra-nitro substituted azacalix[2]arene[2]terphenylenes 1 and 2 have been transformed to the corresponding –N(CH₃)– bridged azacalix[2]arene[2]terphenylenes 3 and 4 via N-alkylation. Single crystal X-ray analysis revealed that the terphenyl-3,3″-diamine derived azacalix[2]terphenylene[2]triazine 5 adopts a distorted chair conformation in the solid state, and the terphenyl-4,4″-diamine derived azacalix[2]terphenylene[2]triazine 6 was found to adopt a 1,3-alternate conformation.     

Optical activity of the guest azobenzene molecule generated by inclusion complexation with steroidal bile acids

The crystalline inclusion complexes of azobenzene 1 with cholic CA and deoxycholic DCA acid were prepared and their solid state CD spectra measured. The positive Cotton effect sign, corresponding to the lowest energy n–π* transition, was correlated with the M,M helicity of the twisted guest molecule. The absolute sense of twist of the enclathrated guest molecule 1 was deduced from the X-ray structure of the 1·CA complex. In addition, the solid state CD spectrum of the enantiomorphous crystals of methyl yellow 2 was measured. The source of the optical activity is a slight twisting of the constituent molecules and their helical arrangement in the crystal.     

Asymmetric synthesis of 1,3-thiazolidine-derived spiro-β-lactams via a Staudinger reaction between chiral ketenes and imines

Enantiomerically pure 1,3-thiazolidine-derived spiro-β-lactams were stereoselectively synthesised by means of a Staudinger ketene–imine reaction starting from optically active N-Boc-1,3-thiazolidine-2-carboxylic acid derivatives and imines. The reactions were stereoselective and afforded spiro-β-lactams with a relative trans-configuration. The absolute configuration of the new stereocentres was assigned on the basis of the well-accepted mechanism and confirmed by means of the X-ray crystal structure analysis. The spiro-β-lactams were transformed into enantiomerically pure chiral monocyclic β-lactams by opening the thiazolidine ring and recovering the chiral auxiliary.     

Heterocalix[8]arene inclusion complexes: Crystal structure and mobility of the host shape

A supramolecular host-guest system based on heterocalix[8]arene 1, incorporating benzimidazol-2-one and 2-methoxy-5-tert-butyl-1,3-phenylene subunits in an alternate cyclic arrangement, has been studied. The crystal structures of two solvent endo/exo inclusion complexes formed from the host molecule 1 and 3-methylpyridine in hydrated and unhydrated forms demonstrate the influence of crystal packing on the shape of the host molecule.     

Intermolecular C—H...O and C—H...X interactions in substituted spiroacenaphthylene structures

In the three spiroacenaphthylene structures 5′′‐[(E)‐2,3‐dichlorobenzylidene]‐7′‐(2,3‐dichlorophenyl)‐1′′‐methyldispiro[acenaphthylene‐1,5′‐pyrrolo[1,2‐c][1,3]thiazole‐6′,3′′‐piperidine]‐2,4′′‐dione, C₃₅H₂₆Cl₄N₂O₂S, (I), 5′′‐[(E)‐4‐fluorobenzylidene]‐7′‐(4‐fluorophenyl)‐1′′‐methyldispiro[acenaphthylene‐1,5′‐pyrrolo[1,2‐c][1,3]thiazole‐6′,3′′‐piperidine]‐2,4′′‐dione, C₃₅H₂₈F₂N₂O₂S, (II), and 5′′‐[(E)‐4‐bromobenzylidene]‐7′‐(4‐bromophenyl)‐1′′‐methyldispiro[acenaphthylene‐1,5′‐pyrrolo[1,2‐c][1,3]thiazole‐6′,3′′‐piperidine]‐2,4′′‐dione, C₃₅H₂₈Br₂N₂O₂S, (III), the substituted aryl groups are 2,3‐dichloro‐, 4‐fluoro‐ and 4‐bromophenyl, respectively. The six‐membered piperidine ring in all three structures adopts a half‐chair conformation, the thiazolidine ring adopts a slightly twisted envelope and the pyrrolidine ring an envelope conformation; in each case, the C atom linking the rings is the flap atom. In all three structures, weak intramolecular C—H...O interactions are present. The crystal packing is stabilized through a number of intermolecular C—H...O and C—H...X interactions, where X = Cl in (I) and F or S in (II), and C—H...O interactions are observed predominantly in (III). In all three structures, molecules are linked through centrosymmetric ring motifs, further tailored through a relay of C—H...X [Cl in (I), Br in (II) and O in (III)] interactions.     

Monomeric and polymeric structures of oxovanadium(IV) complexes with Schiff base ligands derived from (R,R)-2,4-pentanediamine: Synthesis and crystal structure

New tetradentate Schiff base–oxovanadium(IV) complexes which have electron donating or withdrawing groups at the 5-position of the salicylaldehyde moieties, [VO{Xsal-(R,R)-2,4-ptn}] (H₂{Xsal-(R,R)-2,4-ptn}: N,N′-di-Xsalicylidene-(R,R)-2,4-pentanediamine; X=5-MeO (methoxy), 5-Br, and 5-NO₂) were prepared. The structures and redox potentials for the V(V)/V(IV) couple of the complexes were compared with those of other [VO{Xsal-(R,R)-2,4-ptn}] (X=3-EtO (ethoxy), 3-MeO, and H). The 5-MeO substituted complex which has electron donating groups at the 5-position of the salicylaldehyde moieties forms a monomeric structure in the solid state. The 3-EtO substituted complex has both monomeric and polymeric structures. On the other hand, the other [VO{Xsal-(R,R)-2,4-ptn}] (X=H, 3-MeO, 5-Br, 5-NO₂) complexes have only polymeric structures. X-ray crystal structure analysis of [VO{5-MeOsal-(R,R)-2,4-ptn}]⋅CH₃OH (1) was carried out. Complex 1 has a monomeric five-coordinate square–pyramidal structure. The six-membered N–N chelate ring forms a distorted flattened boat form with two methyl groups in the axial positions.     

Effect of Through‐Bond Interaction on Conformation and Structure in Rod‐Shaped Donor–Acceptor Systems. Part 2.

The crystal structures of seven N‐aryltropan‐3‐one (=8‐aryl‐8‐azabicyclo[3.2.1]octan‐3‐one) derivatives 1T1, 2T1, 2T2, 3T2, 5T2, 2T3 , and 3T3 are presented (Fig. 2 and Tables 1–5) and discussed together with the derivatives 1T2 and 4T2 published previously. The piperidine ring adopts a chair conformation. In all structures, the aryl group is in the axial position, with the plane through the aryl C‐atoms nearly perpendicular to the mirror plane of the piperidine ring. The through‐bond interaction between the piperidine ring N‐atom (one‐electron donor) and the substituted exocyclic C?C bond (acceptor) not only elongates the central C? C bonds of the piperidine ring but also increases the pyrimidalization at C(4) of the piperidine ring. Flattening of the C(2)–C(6) part of the piperidine ring decreases the through‐bond interaction.     

Molecular tectonics: design and structural analysis of 1-D and 2-D self-inclusion molecular networks in the crystalline state

The synthesis of the new symmetrical koiland 2 based on the double fusion of two p-ethylcalix〚4〛arene in cone conformation by two silicon atoms was achieved and its structure confirmed by X-ray diffraction on a single crystal. Koiland 2, a concave building block possessing two calix-type pre-organised cavities disposed in a divergent fashion as well as ethyl groups as potential connector moieties, affords either a discrete exo-binuclear inclusion complex in the solid state in the presence of p-xylene acting as a stopper or infinite 1-D or 2-D self-inclusion networks or koilates, mainly based on van der Waals interactions and resulting from the inclusion processes taking place between concave and convex moieties of consecutive koilands.     

Cadmium coordination polymers based on biimidazole and bibenzimidazole: Syntheses,crystal structures and fluorescent properties

Three new Cd^II complexes with two analogous ligands of 2,2′-biimidazole (H₂biim) and 2,2′-bibenzimidazole (H₂bbim), have been synthesized and characterized by elemental analyses, IR spectroscopy as well as single crystal X-ray diffraction. The results reveal that the structures of the complexes range from one-dimensional (1D) chains in [Cd(H₂biim)(N₃)₂]_n (1) to two-dimensional (2D) layer in [Cd(bbim)_0.5(Hbbim)]_n (2) and three-dimensional (3D) network in [Cd₃(biim)₂(Hbiim)(N₃)]_n (3), with various modes of these ligands, which is tuned by different polarity of solvents and pH values. The fluorescent spectra of these complexes and ligands have been also investigated in the solid state at room temperature.