Bruce M. Foxman

Catalytic asymmetric cross-coupling of imines constitutes a particularly desirable method for the synthesis of chiral vicinal diamines directly from readily available achiral precursors. The potential of this method lies in the possibility of utilizing a variety of imines as reacting partners. However, the realization of highly stereoselective cross-coupling of two different imines proved to be a formidable challenge. Herein we report an unprecedented catalytic asymmetric cross-coupling reaction that tolerates a variety of ketimines and aldimines as nucleophiles and electrophiles, respectively. The realization of this reaction resulted from the development of a new chiral ammonium catalyst, which was guided by insights from studies of catalyst-substrate interactions. With a 0.5 mol % loading of an organocatalyst, this reaction proceeded in a highly diastereo- and enantioselective manner to afford a diverse range of chiral vicinal diamines as nearly single stereoisomers. This catalytic reaction establishes a new approach for the asymmetric synthesis of chiral vicinal diamines.

Knowledge of space groups and the implications of space group symmetry on the physical and chemical properties of solids are pivotal factors in all areas of crystalline solids. As Jerry Jasinski and I met to bring our ideas in teaching this subject to life, we both felt that `early and often' – teaching the concepts with textual and visual reinforcement, is the key to providing a sound basis for students in this subject. The tutorial contains > 200 PowerPoint `slides', in five modules, arranged by crystal class; a sixth module covers special topics. A `credits' module gives the direct addresses of all embedded links. Space-group diagrams appear in International Tables format. The triclinic and monoclinic groups (2 + 13) are built from `scratch', and are derived from the Hermann–Mauguin symbol. An additional section provides practice on many (but not all) of the orthorhombic groups in crystal class 222. Finally, a `Special Topics' section on enantiomorphous space groups features space groups P4<sub>1</sub> and P4<sub>3</sub>. In the tutorial, lattice points build iteratively and interactively via keyclick, and the coordinates of points `pop up' as the unit cell is filled. We trust that the elements of guidance, inquiry and occasional humor will make the learning process enjoyable.

The crystal structure of the cis isomer of cis-bis(L-DOPA-κ² N,O)copper(II) monohydrate (L-DOPA is 3,4-dihydroxy-L-phenylalanine) (CuLD), [Cu(C₉H₁₀NO₄)₂]·H₂O, is a singular example of a structurally characterized, homoleptic, crystalline metal L-DOPA complex. CuLD crystallizes in the space group P2₁, with Z′ = 2. The two independent molecules are square planar, and are interconnected by a linear hydrogen-bonded chain containing 12 independent hydrogen bonds. The copper ions in both molecules have weak apical intermolecular Cu...O interactions [2.739 (2) and 2.973 (2) Å] with catechol –OH groups. A survey of the Cambridge Structural Database suggested that cis and trans isomers of Cu(NH₂–C–CO₂)₂ amino acid complexes are equally likely to occur. 12 strong O—H...O and N—H...O hydrogen bonds stabilize an unusual linear arrangement of the Cu complexes. The Cu...Cu′ distances along the chain are nearly equal [5.0739 (3) and 5.1107 (3) Å] and the Cu...Cu′...Cu angles are nearly linear [176.75 (1)°]. The MATCH procedure available in the Oxford University Crystals for Windows package was used to carry out a detailed analysis of the relationship between the two independent molecules. MATCH has some particular advantages in studying the details of pseudosymmetry, which include: (i) no atomic-order requirements; (ii) the pseudosymmetry matrix is readily available, which allows quick insight into the symmetry elements involved and their location; and (iii) the differences between molecular centroids, as well as between all atomic positions and torsion angles, are listed. A tutorial presentation is designed to attract new users to the technique. In the present case, a search for a pseudosymmetric relationship between the two independent molecules showed that they are related by a pseudo-4₂ axis along the crystallographic c direction. A detailed analysis shows that the pseudo-4₂ symmetry is disrupted by torsions about the CH₂—C(ipso) bonds, and that there is no supergroup that can be used to describe the crystal structure.

While Δ⁹-tetrahydrocannabinolic acid A (THCA-A) has been reported to be difficult to crystallize and/or amorphous, we have obtained THCA-A in a pure crystalline form by extraction of marijuana and selective fractionation with liquid CO₂. THCA-A (systematic name: 1-hydroxy-6,6,9-trimethyl-3-pentyl-6a,7,8,10a-tetrahydro-6H-benzo[c]isochromene-2-carboxylic acid, C₂₂H₃₀O₄) crystallizes in the orthorhombic space group P2₁2₁2₁, with Z = 8 and Z′ = 2. The two independent molecules are related by a pseudo-twofold axis centered between the two –CO₂H groups, but the conformations of the two –C₅H₁₁ chains are quite different (tgt and ttg; t is trans and g is gauche). The carboxylate groups form an intermolecular R ₂ ²(8) hydrogen-bonded ring; the two C₂O₂ carboxylate planes are twisted out of the planes of the attached arene rings in opposite directions by 13.59 (8) and 18.92 (8)°, respectively, with a resultant interplanar angle of 28.89 (8)°. Each molecule also has an intramolecular S(6) hydrogen-bond motif between the ortho –OH group and the dihydropyran-ring O atom. Other conformational aspects of the two independent molecules are quite similar to those found in the previously determined structure of THCA-B. THCA-A has shown promise in a number of medical applications. Demonstration of the crystallinity and details of the crystal structure are expected to provide a standard point of departure for chemical and medical experiments.

A tridentate [PPP] ligand has been used to construct a series of dimeric cobalt complexes and explore cooperative multi-electron redox processes that are both metal- and ligand-centered. Reduction of ((PPP)-P-Cl)CoCl2 (1) with excess magnesium affords the (CoCoI)-Co-I N-heterocyclic phosphido (NHP-)-bridged symmetric dimer [(mu-PPP)Co](2) (2). Two-electron oxidation of 2 with FcPF(6) generates an asymmetrically bridged dication [(mu-PPP)Co](2)[PF6](2) (3) in which the oxidation has occurred in a delocalized fashion throughout the Co2P2 core. In contrast, [(mu-PPP)Co](2)(+) (5), which can be generated either by one-electron oxidation of 2 with FcPF(6) or comportionation of 2 and 3, features an asymmetric geometry and localized mixed valence. Treatment of 1 with the milder reductants CoCp2 and KBEt3H does not lead to formation of 2, 3, or 5 but instead generates dimeric species [(PPP)CoCl](2) (6) and [(PPP)CoH](2) (7). Unlike 2-5, where the phosphine side arms of the tridentate [PPP] ligand span the two Co centers, complex 6 and 7 are connected solely by NHP- ligands that bridge the two (PPP)Co fragments.

A reduced Zr^IV/Co^−I heterobimetallic complex facilitates the formation of 1,3-enynes from terminal alkynes through a cooperative bimetallic C–H activation process.

Oxygen reduction is a critical half reaction in renewable fuel cell development and a key step in the development of aerobic oxidation reactions. Herein, we report rapid two-electron O₂ reduction by a d⁰ Zr^IV center with an appended redox-active Co^–I site serving as an electron reservoir. The early/late heterobimetallic Zr/Co complex (THF)Zr(MesNPⁱPr₂)₃CoCN^tBu (1) reacts readily with O₂ and O atom transfer reagents to generate reactive oxygenated species including terminal peroxo and oxo complexes, (O₂)Zr(MesNPⁱPr₂)₃CoCN^tBu (2) and O≡Zr(MesNPⁱPr₂)₃CoCN^tBu (3). The bimetallic Zr/Co complex provides a new cooperative synthetic pathway to promote multielectron redox processes such as oxygen reduction, with each metal playing a distinct role as a substrate binding site or redox mediator.