A γ-amino acid that favors 12/10-helical secondary structure in α/γ-peptides.

H-bonded helices in conventional peptides (containing exclusively homochiral α-amino acid residues) feature a uniform H-bonding directionality, N-terminal side C═O to C-terminal side NH. In contrast, heterochiral α-peptides can form helices in which the H-bond directionality alternates along the backbone because neighboring amide groups are oriented in opposite directions. Alternating H-bond directions are seen also in helices formed by unnatural peptidic backbones, e.g., those containing ß- or γ-amino acid residues. In the present study, we used NMR spectroscopy and crystallography to evaluate the conformational preferences of the novel γ-amino acid (1R,2R,3S)-2-(1-aminopropyl)-cyclohexanecarboxylic acid (APCH), which is constrained by a six-membered ring across its Cα-Cß bond. These studies were made possible by the development of a stereoselective synthesis of N-protected APCH. APCH strongly enforces the α/γ-peptide 12/10-helical secondary structure, which features alternating H-bond directionality. Thus, APCH residues appear to have a conformational propensity distinct from those of other cyclically constrained γ-amino acid residues.

Dichlorido{2-[(3,5-diphenyl-1H-pyrazol-1-yl-κN2)methyl]pyridine-κN}palladium(II).

The title compound, [PdCl(2)(C(21)H(17)N(3))], is a member of a sequence of Pd, Pt and Co dichloride complexes bearing polysubstituted (pyrazol-1-ylmethyl)pyridine ligands. It is shown that there is a correlation between the steric bulkiness of the bidentate (pyrazol-1-ylmethyl)pyridine ligands and the Pd-N(pyrazole) distances, i.e. the larger the ligand, the longer the bond. In contrast, no trend is observed between the steric properties of the ligand and the Pd-N(pyridine) bond lengths.

An example of the refinement of positional disorder modeled as compositional disorder in [5-(2-formylphenyl)-10,15,20-triphenylporphyrinato]nickel(II).

The title compound, [Ni(C(45)H(28)N(4)O)], crystallizes in the space group I ̅42d and resides on a crystallographic fourfold rotoinversion axis with only a quarter of the complex in the asymmetric unit. The complex displays positional disorder as the one aldehyde group on the ligand can be located at four different positions. It was necessary to model this as compositional disorder to obtain a correct model and refinement. The practical approach to the refinement is explained.

Ethyl 3-ferrocenyl-1-(pyridin-2-ylmeth-yl)-1H-pyrazole-5-carboxyl-ate.

The title compound, [Fe(C(5)H(5))(C(17)H(16)N(3)O(2))], crystallizes with an essentially eclipsed conformation of the cyclo-penta-dienyl (Cp) rings. The unsubstituted ring is disordered over two positions with the major component being present 90 (1)% of the time. The substituted Cp ring, the pyrazole ring and three atoms of the eth-oxy-carbonyl group form a conjugated π-system. These 13 atoms are coplanar within 0.09 Å.

5',11'-Dihydro-dispiro-[cyclo-hexane-1,6'-indolo[3,2-b]carbazole-12',1''-cyclo-hexa-ne].

The title compound, C(28)H(30)N(2), is a symmetrical 2:2 product from the condensation of indole and cyclo-hexa-none. It is the only reported 5,11-dihydro-indolo[3,2-b]carbazole compound in which the spiro atoms are quaternary C atoms. Crystals were grown by vapor diffusion in a three-zone electric furnace. The mol-ecule resides on a crystallographic inversion center. The cyclo-hexyl rings are in a slightly distorted chair conformation, whereas the indole units and the spiro-carbons are coplanar within 0.014 Å.

(3,5-Di-tert-butyl-2-eth-oxy-benzyl-idene)[2-(3,5-di-tert-butyl-1H-pyrazol-1-yl)eth-yl]amine.

The angles within the benzene ring in the title compound, C(30)H(49)N(3)O, ranging from 116.34 (16) to 124.18 (16)°, reflect the presence of electron-donating and electron-withdrawing substituents. The angles at the two electron-donating tert-butyl substituents are smaller than 120°, at the electron-withdrawing eth-oxy substituent larger than 120°, and at the imine substituent equal to 119.59 (16)°. The latter does not reflect the electron-donating nature of the imine group due to the presence of other substituents.

1,2-Bis[(3,5-diphenyl-1H-pyrazol-1-yl)meth-yl]benzene.

The title compound, C(38)H(30)N(4), a potentially mono- and bidentate ligand, does not seem to form palladium complexes similar to other poly(pyrazol-1-ylmeth-yl)benzenes due to the large steric size of the phenyl substituents on the pyrazole rings. The pyrazole rings have a 21.09 (5)° angle between their mean planes and exhibit a trans-like geometry in which the in-plane lone pairs of electrons on the 2-N nitrogen atoms point in opposite directions.

Dichlorido[2-(3,5-dimethyl-1H-pyrazol-1-yl-κN)ethanamine-κN]zinc(II).

The amine title complex, [ZnCl(2)(C(7)H(13)N(3))], resulted from imine hydrolysis in a Schiff base compound. The Zn metal atom has a distorted tetra-hedral geometry with the most significant deviation identified in the magnitude of the N-Zn-N angle. This deviation stems from the participation of the Zn and N atoms in a six-membered metallocyclic ring. The latter is in an approximate screw-boat conformation. Two strong N-H⋯Cl hydrogen bonds link the mol-ecules into ribbons propagating along the b-axis direction. The ribbons contain two second-order hydrogen-bonded motifs: a chain and a ring. The chain described by the graph set notation C(2) (2)(6) is formed by one hydrogen bond going in the forward direction (donor to acceptor) and the other in the backward direction (acceptor to donor). In the ring motif R(2) (2)(8), both hydrogen bonds propagate in the forward direction.

(µ-Piperazine-1,4-dicarbodithio-ato-κS,S:S,S)bis-[bis-(triphenyl-phos-phane-κP)gold(I)] chloro-form disolvate.

In the title compound, [Au(2)(C(6)H(8)N(2)S(4))(C(18)H(15)P)(4)]·2CHCl(3), the digold complex resides on a crystallographic inversion center and co-crystallizes with two mol-ecules of chloro-form solvent. The piperazine-1,4-dicarbodithio-ate linker has an almost ideal chair conformation. The geometry about the gold atoms is severely distorted tetra-hedral punctuated by a very acute S-Au-S bite angle.

{1,2-Bis[(3,5-dimethyl-1H-pyrazol-1-yl-κN)meth-yl]benzene}-dichloridozinc(II).

The title zinc complex, [ZnCl(2)(C(18)H(22)N(4))], contains a bidentate 1,2-bis(3,5-dimethyl-1H-pyrazol-1-ylmeth-yl)benz-ene ligand that binds to the zinc atom, forming a nine-membered metallocyclic ring. The geometry about the Zn atom is distorted tetra-hedral, with the largest deviation observed in the magnitude of the Cl-Zn-Cl angle. Similar distortions are observed in the cobalt analogue and related zinc compounds containing metallocyclic rings with more than six members. The copper analogue exhibits a more severe distortion of the metal coordination sphere than is observed in the title compound.

Ring opening polymerization of d,l-lactide and ε-caprolactone catalysed by (pyrazol-1-yl)copper(ii) carboxylate complexes.

1,2-Bis{(3,5-dimethylpyrazol-1-yl)methyl}benzene (L) reacts with [Cu(OAc)2] and C6H5COOH, 4-OH-C6H4COOH, 2-Cl-C6H4COOH and (3,5-NO2)2-C6H3COOH to afford the copper complexes [Cu2(C6H5COO)4(L)2] (1), [Cu2(4-OH-C6H4COO)4(L)2] (2), [Cu2(2-Cl-C6H4COO)4(L)2] n (3) and [Cu{(3,5-NO2)2-C6H3COO}2L] n (4) which are characterised by IR, mass spectrometry, elemental analyses, and X-ray crystallography. The structural data revealed two geometries that are adopted by the complexes: (i) paddle wheel in 1, 2·7H2O, 3 and (ii) regular chains in 3 and 4. Magnetic studies show strong antiferromagnetic couplings in the paddle wheel complexes and a weak antiferromagnetic coupling in the monometallic chain one. Catalysis studies performed with these complexes (1-4) showed that they initiate ring opening polymerization (ROP) of ε-caprolactone (ε-CL) under solvent-free conditions and d,l-lactide in toluene at elevated temperatures. Polycaprolactone (PCL) and poly(d,l-lactide) (PLA) obtained from the polymerization reactions are of low molecular weights (858 for PCL and 602 Da for PLA for initiator 1) and polydispersity indices (typically 2.16 for PCL and 1.64 for PLA with 1 as the initiator). End group analysis of the polymers, determined by MALDI-ToF MS, indicates that the polymers have benzoate, hydroxyl, methoxy and cyclic end groups.

Crystallographic characterization of 12-helical secondary structure in ß-peptides containing side chain groups.

Helices are the most extensively studied secondary structures formed by ß-peptide foldamers. Among the five known ß-peptide helices, the 12-helix is particularly interesting because the internal hydrogen bond orientation and macrodipole are analogous to those of α-peptide helices (α-helix and 3(10)-helix). The ß-peptide 12-helix is defined by i, i+3 C═O···H-N backbone hydrogen bonds and promoted by ß-residues with a five-membered ring constraint. The 12-helical scaffold has been used to generate ß-peptides with specific biological functions, for which diverse side chains must be properly placed along the backbone and, upon folding, properly arranged in space. Only two crystal structures of 12-helical ß-peptides have previously been reported, both for homooligomers of trans-2-aminocyclopentanecarboxylic acid (ACPC). Here we report five additional crystal structures of 12-helical ß-peptides, all containing residues that bear side chains. Four of the crystallized ß-peptides include trans-4,4-dimethyl-2-aminocyclopentanecarboxylic acid (dm-ACPC) residues, and the fifth contains a ß(3)-hPhe residue. These five ß-peptides adopt fully folded 12-helical conformations in the solid state. The new crystal structures, along with previously reported data, allow a detailed characterization of the 12-helical conformation; average backbone torsion angles of ß-residues and helical parameters are derived. These structural parameters are found to be similar to those for i, i+3 C═O···H-N hydrogen-bonded helices formed by other peptide backbones generated from α- and/or ß-amino acids. The similarity between the conformational behavior of dm-ACPC and ACPC is consistent with previous NMR-based conclusions that 4,4-disubstituted ACPC derivatives are compatible with 12-helical folding. In addition, our data show how a ß(3)-residue is accommodated in the 12-helix, thus enhancing understanding of the diverse conformational behavior of this flexible class of ß-amino acids.

Constructor graph description of the hydrogen-bonding supramolecular assembly in two ionic compounds: 2-(pyrazol-1-yl)ethylammonium chloride and diaquadichloridobis(2-hydroxyethylammonium)cobalt(II) dichloride.

Covalent bond tables are used to generate hydrogen-bond pattern designator symbols for the crystallographically characterized title compounds. 2-(Pyrazol-1-yl)ethylammonium chloride, C(5)H(10)N(3)(+).Cl(-), (I), has three unique, strong, charge-assisted hydrogen bonds of the types N-H...Cl and N-H...N that form unary through ternary levels of graph-set interactions. Diaquadichloridobis(2-hydroxyethylammonium)cobalt(II) dichloride, [CoCl(2)(C(2)H(8)NO)(2)(H(2)O)(2)]Cl(2), (II), forms five unique charge-assisted hydrogen bonds of the types O-H...Cl and N-H...Cl. These form graph-set patterns up to the quinary level. The Co complex in (II) resides at a crystallographic inversion center; thus the number of hydrogen bonds to consider doubles due to their G-equivalence, and the handling of such a case is demonstrated.

Intramolecular hydrogen bonding in dichloridobis(3,5-di-tert-butyl-1H-pyrazole-κN2)cobalt(II) as a consequence of ligand steric bulk.

The title compound, [CoCl(2)(C(11)H(20)ClN(2))(2)], forms two intramolecular hydrogen bonds [graph set S(5)] between the N atoms of the pyrazole ligands and the chloride ligands. This hydrogen-bonding motif is uncommon among related compounds but occurs here because of the bulk of tert-butyl substituents on the pyrazole ligands which shield the central metal atom to a significantly larger extent than pyrazole ligands with smaller 3,5-substituents.

{Bis[2-(3,5-dimethyl-pyrazol-1-yl-κN)eth-yl]amine-κN}chloridopalladium(II) chloride 0.25-hydrate.

The title compound, [PdCl(C(14)H(23)N(5))]Cl·0.25H(2)O, is a pseudopolymorph of the previously reported compound [PdCl(C(14)H(23)N(5))]Cl·2H(2)O [de Mendoza et al. (2006 ▶). Acta Cryst. E62, m2934-m2936]. The cationic complex and chloride anion are disordered over two positions each in a 0.584 (4):0.416 (4) ratio. The geometry about the Pd atom is distorted square-planar. The pyrazole rings are almost perpendicular, forming a dihedral angle of 86.6 (6)° to each other, to mitigate steric conflict between their methyl groups.

Bis(2-hy-droxy-ethanaminium) tetra-chloridopalladate(II).

In the title compound, (C(2)H(8)NO)(2)[PdCl(4)], 2-hy-droxy-ethanaminium cations and tetra-chloridopalladate(II) dianions crystallize in a 2:1 ratio with the anion residing on a crystallographic inversion center. The cations and anions are linked in a complex three-dimensional framework by three types of strong hydrogen bonds (N-H⋯O, N-H⋯Cl, and O-H⋯Cl), which form various ring and chain patterns of up to the ternary graph-set level.

(µ-1,4,7,10-Tetra-oxacyclo-dodeca-ne)bis-[(1,4,7,10-tetra-oxacyclo-dodeca-ne)lithium] bis-(perchlorate).

12-Crown-4 ether (12C4) and LiClO(4) combine to form the ionic title compound, [Li(2)(C(8)H(16)O(4))(3)](ClO(4))(2), which is com-posed of discrete Li/12C4 cations and perchlorate anions. In the [Li(2)(12C4)(3)](2+) cation there are two peripheral 12C4 ligands, which each form four Li-O bonds with only one Li(+) atom. Additionally there is a central 12C4 in which diagonal O atoms form one Li-O bond each with both Li(+) atoms. Therefore each Li(+) atom is penta-coordinated in a distorted square-pyramidal geometry, forming four longer bonds to the O atoms on the peripheral 12C4 and one shorter bond to an O atom of the central 12C4. The cation occupies a crystallographic inversion centre located at the center of the ring of the central 12C4 ligand. The Li(+) atom lies above the cavity of the peripheral 12C4 by 0.815 (2) Šbecause it is too large to fit in the cavity.

(1,4,7,10-Tetra-oxacyclo-dodeca-ne)(trideuteroacetonitrile)lithium perchlorate.

In the title compound, [Li(C(8)H(16)O(4))(CD(3)CN)]ClO(4), the Li atom is penta-coordinate. The O atoms of the 12-crown-4 ether form the basal plane, whereas the N atom of the trideutero-aceto-nitrile occupies the apical position. The Li(+) atom is displaced by 0.794 (6) Štoward the apical position from the plane formed by the O atoms because the Li(+) atom is too large to fit in the cavity of the 12-crown-4 ether, resulting in a distorted square-pyramidal geometry about the Li(+) atom.

Crystallographic characterization of helical secondary structures in 2:1 and 1:2 alpha/beta-peptides.

Oligomers containing both alpha- and beta-amino acid residues ("alpha/beta-peptides") are intriguing as potential foldamers. A large set of alpha/beta-peptide backbones can be generated by combining alpha- and beta-amino acid residues in different patterns; however, most research to date has focused on the simplest pattern, 1:1 alpha:beta. We have begun to explore the range of variation that can be achieved with alpha-residue/beta-residue combinations by examining the folding behavior of oligomers that contain 2:1 and 1:2 alpha:beta patterns. The beta-residues in our systems have a five-membered-ring constraint (trans-2-aminocyclopentanecarboxylic acid (ACPC) residues), because these preorganized subunits strongly promote helical folding for 1:1 alpha:beta backbones and pure beta backbones. Previously we concluded that two helical conformations are available to 2:1 and 1:2 alpha/beta-peptides containing ACPC or analogously constrained beta-residues, one helix defined by i,i+3 CO...H-N backbone hydrogen bonds and the other defined by i,i+4 CO...H-N hydrogen bonds. These deductions were based on 2D NMR analysis of a 2:1 heptamer and a 1:2 hexamer in methanol. Crystallographic analysis of a pair of analogous nonpolar alpha/beta-peptides showed only the i,i+3 hydrogen-bonded helical conformations. We now report four new crystal structures of 2:1 alpha/beta-peptides, ranging in length from 5 to 11 residues, and six new crystal structures of 1:2 alpha/beta-peptides, ranging in length from 6 to 10 residues. All 10 of these new structures are fully helical, and all helices display the i,i+3 CO...H-N hydrogen bonding pattern. These crystallographic data sets, collectively, provide high structural definition for the i,i+3 hydrogen-bonded helical secondary structures available to these foldamer backbones.

Synthesis of Pd Complexes Bearing an Enantiomerically-Resolved Seven-Membered N-Heterocyclic Carbene Ligands and Initial Studies of their Use in Asymmetric Wacker-Type Oxidative Cyclization Reactions.

The development of enantiomerically-resolved, axially-chiral seven-membered N-heterocyclic carbene ((7)NHC) ligands for palladium is described. These (7)NHC ligands are derived from enatiomerically pure 2,2'-diamino-6,6'-dimethylbiphenyl, which is transformed via a synthetic sequence consisting of ortho-arylation, N-alkylation, and cyclization to afford seven-membered-ring amidinium salts. Synthesis of the 7-membered amidinium salts benefits from microwave irradiation, and in-situ metalation of the amidinium salts yields (7)NHC-Pd(II) complexes. The chiral (7)NHC-Pd complexes were examined as chiral catalysts under aerobic conditions in two intramolecular oxidative amination reactions of alkenes. In one case, enantioselectivities up to 63% e.e. were obtained, while the other substrate underwent cyclization to afford essentially racemic products. The catalytic data compare favorably to results obtained with a Pd(II) catalyst bearing a chiral five-membered-ring NHC ligand and, thereby, highlight the potential significance of this new class of chiral NHC ligands.