Werner Kaminsky

WinXMorph: a computer program to draw crystal morphology, growth sectors and cross sections with export files in VRML V2.0 utf8-virtual reality format

A Microsoft Windows application for PC-users, WinXMorph, is introduced with which near reality crystal morphologies are made and exported as virtual reality files in the VRML V2.0 format.

AN AUTOMATIC OPTICAL IMAGING SYSTEM FOR BIREFRINGENT MEDIA

A new optical microscope imaging system is described. The instrument is capable of producing coded colour images of an optically birefringent material, where the colour represents either |sin δ|, a function of the optical retardation; ɸ, the orientation of a section of the optical indicatrix; or Io, the transmittance, at any place within the image. Thus the contrast normally seen in a standard crossed-polars experiment is separated out into its components. The technique is both qualitative and quantitative. Examples of the application of this system to crystals, a rock section and a biological specimen are given.

Oxidatively Induced Reductive Elimination from (tBu2bpy)Pd(Me)2: Palladium(IV) Intermediates in a One-Electron Oxidation Reaction

This communication describes studies of oxidatively induced C-C bond-forming reductive elimination from ((t)Bu(2)bpy)Pd(II)(Me)(2). With the outer-sphere oxidant ferrocenium, the data are consistent with a mechanism involving Pd(III) and Pd(IV) intermediates, with C-C bond formation occurring from the latter. The reaction with Ag(+) appears to proceed via a Pd-Ag(+) adduct, which then undergoes inner sphere electron transfer to generate Pd(III). In contrast, the slower benzoquinone reaction forms ethane by a different pathway that does not involve methyl group scrambling and generates Pd(0) products.

Enantioselective Palladium-Catalyzed Diamination of Alkenes Using N-Fluorobenzenesulfonimide

An enantioselective Pd-catalyzed vicinal diamination of unactivated alkenes using N-fluorobenzenesulfonimide as both an oxidant and a source of nitrogen is reported. The use of Ph-pybox and Ph-quinox ligands afforded differentially protected vicinal diamines in good yields with high enantioselectivities. Mechanistic experiments revealed that the high enantioselectivity arises from selective formation of only one of four possible diastereomeric aminopalladation products of the chiral Pd complex. The aminopalladation complex was characterized by X-ray crystallography.

Structural Plasticity of Malaria Dihydroorotate Dehydrogenase Allows Selective Binding of Diverse Chemical Scaffolds

Malaria remains a major global health burden and current drug therapies are compromised by resistance. Plasmodium falciparum dihydroorotate dehydrogenase (PfDHODH) was validated as a new drug target through the identification of potent and selective triazolopyrimidine-based DHODH inhibitors with anti-malarial activity in vivo. Here we report x-ray structure determination of PfDHODH bound to three inhibitors from this series, representing the first of the enzyme bound to malaria specific inhibitors. We demonstrate that conformational flexibility results in an unexpected binding mode identifying a new hydrophobic pocket on the enzyme. Importantly this plasticity allows PfDHODH to bind inhibitors from different chemical classes and to accommodate inhibitor modifications during lead optimization, increasing the value of PfDHODH as a drug target. A second discovery, based on small molecule crystallography, is that the triazolopyrimidines populate a resonance form that promotes charge separation. These intrinsic dipoles allow formation of energetically favorable H-bond interactions with the enzyme. The importance of delocalization to binding affinity was supported by site-directed mutagenesis and the demonstration that triazolopyrimidine analogs that lack this intrinsic dipole are inactive. Finally, the PfDHODH-triazolopyrimidine bound structures provide considerable new insight into species-selective inhibitor binding in this enzyme family. Together, these studies will directly impact efforts to exploit PfDHODH for the development of anti-malarial chemotherapy.

σ-Borane Complexes of Iridium : Synthesis and Structural Characterization

Reaction of NaBH4 with (tBuPOCOP)IrHCl affords the previously reported complex (tBuPOCOP)IrH2(BH3) (1) (tBuPOCOP = kappa(3)-C6H3-1,3-[OP(tBu)2]2). The structure of 1 determined from neutron diffraction data contains a B-H sigma-bond to iridium with an elongated B-H bond distance of 1.45(5) A. Compound 1 crystallizes in the space group P1 (Z = 2) with a = 8.262 (5) A, b = 12.264 (5) A, c = 13.394 (4) A, and V = 1256.2 (1) A(3) (30 K). Complex 1 can also be prepared by reaction of BH3 x THF with (tBuPOCOP)IrH2. Reaction of (tBuPOCOP)IrH2 with pinacol borane gave initially complex 2, which is assigned a structure analogous to that of 1 based on spectroscopic measurements. Complex 2 evolves H2 at room temperature leading to the borane complex 3, which is formed cleanly when 2 is subjected to dynamic vacuum. The structure of 3 has been determined by X-ray diffraction and consists of the (tBuPOCOP)Ir core with a sigma-bound pinacol borane ligand in an approximately square planar complex. Compound 3 crystallizes in the space group C2/c (Z = 4) with a = 41.2238 (2) A, b = 11.1233 (2) A, c = 14.6122 (3) A, and V = 6700.21 (19) A(3) (130 K). Reaction of (tBuPOCOP)IrH2 with 9-borobicyclononane (9-BBN) affords complex 4. Complex 4 displays (1)H NMR resonances analogous to 1 and exists in equilibrium with (tBuPOCOP)IrH2 in THF solutions.

Experimental and phenomenological aspects of circular birefringence and related properties in transparent crystals

Here, we review the history, theory, measurement technique and experimental results on gyrotropic phenomena including optical rotation (optical activity), electrogyration, the Faraday effect and magneto-electrogyration in transparent crystals, including examples of structural phase transitions. Relations to the absolute structure are discussed and model calculations are performed on the basis of electronic polarizability and crystal structure.

Preparation of a Dihydrogen Complex of Cobalt

Ammonia borane (AB) is an attractive candidate for the chemical storage of hydrogen. Recently, our research group reported that the complex [(pocop)IrH2] (pocop= k -C6H31,3-[OP(tBu)2]2) has facilitated the rapid release of H2 from AB under mild conditions. While this result is promising, iridium is too expensive for widespread application. Our efforts to extend the catalytic chemistry of [(pocop)IrH2] to Co have led to several intriguing complexes. Direct analogues of the Ir catalyst were obtained, thereby demonstrating the unusual ligation of H2 on a Co center. Herein we report the first cobalt–dihydrogen complex, which was characterized by NMR spectroscopy and studied by means of theoretical calculations. The metalation of pocop–H with Co, achieved with a method similar to procedures developed for Ir or Ni, did not proceed in satisfactory yield. An alternative approach that makes use of an iodinated pocop ligand was pursued, as described for a similar synthesis. Complex 1 was prepared in good yield by activation of the ligand with nBuLi and the addition of CoI2·THF (Scheme 1).

Polarimetric imaging of crystals

Classical crystal optics has recently undergone a renaissance as developments in optical microscopy and polarimetry, enabled in part by sensitive imaging CCD cameras and personal computers, now permit the analytical separation of various optical effects that are otherwise convolved in polarized light micrographs. In this tutorial review, we review recent developments in the measurement of the principal crystallo-optical quantities including linear birefringence, linear dichroism, circular birefringence, and circular dichroism, as well as new effects in crystal optics encountered in unusual mixed crystals. The new microscopies and polarimetries are applied to problems of crystallographic twinning, phase transformations, stress birefringence, symmetry reduction, and the design of new crystalline materials.

Mechanism of Copper-Catalyzed Hydroalkylation of Alkynes: An Unexpected Role of Dinuclear Copper Complexes

This article describes a mechanistic study of copper-catalyzed hydroalkylation of terminal alkynes. Relying on the established chemistry of N-heterocyclic carbene copper hydride (NHCCuH) complexes, we previously proposed that the hydroalkylation reaction proceeds by hydrocupration of an alkyne by NHCCuH followed by alkylation of the resulting alkenylcopper intermediate by an alkyl triflate. NHCCuH is regenerated from NHCCuOTf through substitution with CsF followed by transmetalation with silane. According to this proposal, NHCCuH must react with an alkyne faster than with an alkyl triflate to avoid reduction of the alkyl triflate. However, we have determined that NHCCuH reacts with alkyl triflates significantly faster than with terminal alkynes, strongly suggesting that the previously proposed mechanism is incorrect. Additionally, we have found that NHCCuOTf rapidly traps NHCCuX (X = F, H, alkenyl) complexes to produce (NHCCu)2(μ-X)(OTf) (X = F, H, alkenyl) complexes. In this article, we propose a new mechanism for hydroalkylation of alkynes that features dinuclear (NHCCu)2(μ-H)(OTf) (X = F, H, alkenyl) complexes as key catalytic intermediates. The results of our study establish feasible pathways for the formation of these intermediates, their ability to participate in the elementary steps of the proposed catalytic cycle, and their ability to serve as competent catalysts in the hydroalkylation reaction. We also provide evidence that the unusual reactivity of the dinuclear complexes is responsible for efficient hydroalkylation of alkynes without concomitant side reactions of the highly reactive alkyl triflates.