Madeleine Helliwell

Ultra-remote stereocontrol by conformational communication of information along a carbon chain.

Many receptors and allosteric proteins function through binding of a molecule to induce a conformational change, which then influences a remote active site. In synthetic systems, comparable intramolecular information transfer can be effected by using the shape of one part of a molecule to control the stereoselectivity of reactions occurring some distance away. However, the need for direct communication with the reaction site usually limits such remote stereocontrol to distances of not more than about five bond lengths. Cyclic structures overcome this problem by allowing the controlling centre and the reaction site to approach each other, but the information transfer spans only short absolute distances. Truly remote stereocontrol can, however, be achieved with rigid compounds containing amide groups: the conformation of the amides can be controlled by stereogenic centres and responds to that of neighbouring amide groups and in turn influences stereoselective reactions. This strategy has allowed remote stereocontrol spanning 8 (ref. 11) or 9 (ref. 12) bonds. Here we demonstrate stereocontrol over a reaction taking place more than 20 bond lengths from the controlling centre, corresponding to a linear distance of over 2.5 nm. This transmission of information, achieved by conformational changes relayed through the molecule, provides a chemical model of allostery and might serve as a molecular mechanism for communicating and processing information.

Heme-like coordination chemistry within nanoporous molecular crystals

Iron Exposure The macrocyclic heme motif coordinates iron ions in proteins and plays a widespread role in biochemical oxidative catalysis. Bezzu et al. (p. 1627) prepared crystals in which analogous iron-centered macrocycles were aligned in pairs. The outer faces of the pairs exposed the iron ions to vacant cavities, where ligand exchange could take place; the inner faces were bound together by rigid bridging ligands lending the crystals structural integrity. The stability and high porosity of these crystals lend themselves to potential catalytic applications. Metal-organic framework compounds expose iron atoms for reactions in a manner analogous to heme sites in proteins. Crystal engineering of nanoporous structures has not yet exploited the heme motif so widely found in proteins. Here, we report that a derivative of iron phthalocyanine, a close analog of heme, forms millimeter-scale molecular crystals that contain large interconnected voids (8 cubic nanometers), defined by a cubic assembly of six phthalocyanines. Rapid ligand exchange is achieved within these phthalocyanine nanoporous crystals by single-crystal–to–single-crystal (SCSC) transformations. Differentiation of the binding sites, similar to that which occurs in hemoproteins, is achieved so that monodentate ligands add preferentially to the axial binding site within the cubic assembly, whereas bidentate ligands selectively bind to the opposite axial site to link the cubic assemblies. These bidentate ligands act as molecular wall ties to prevent the collapse of the molecular crystal during the removal of solvent. The resulting crystals possess high surface areas (850 to 1000 square meters per gram) and bind N2 at the equivalent of the heme distal site through a SCSC process characterized by x-ray crystallography.

3d–4f Clusters with large spin ground states and SMM behaviour

Three new Cu-Ln cage complexes bridged by phosphonates and acetates are reported; one--a Cu(24)Dy(8) cage--is a new single molecule magnet (SMM).

Enantioselective Biocatalytic Oxidative Desymmetrization of Substituted Pyrrolidines

Direct activation of sp C H bonds a to a nitrogen atom represents an attractive strategy for functionalization of amines, especially those found in 5and 6-membered ring heterocycles. In particular, C H activation by oxidation, followed by nucleophilic addition, generates products of an overall oxidative Strecker or Mannich process. Recent reports have described the use of various metal-based oxidants to achieve this transformation although there are few examples of catalytic and/or enantioselective processes. The enantioselective oxidation of N-protected pyrrolidines has been reported using manganese–salen catalysts and iodosobenzene as the stoichiometric oxidant. Herein we report the enantioselective enzyme-catalyzed desymmetrization of a range of unprotected pyrrolidines to the corresponding Dpyrrolines (Scheme 1), which serve as useful building blocks for the synthesis of l-proline analogues of high enantiomeric purity.

The Chemical Vapor Deposition of Nickel Phosphide or Selenide Thin Films from a Single Precursor

Nickel phosphide, nickel selenide thin films and their heterostructure (Ni0.85Se/Ni2P) were deposited from a newly synthesized single source precursor {Ni[iPr2P(S)NP(Se)iPr2]2} just by altering the deposition temperature using CVD.

Anion induced formation of supramolecular associations involving lone pair-π and anion-π interactions in Co(II) malonate complexes: experimental observations, Hirshfeld surface analyses and DFT studies.

Three Co(II)-malonate complexes, namely, (C(5)H(7)N(2))(4)[Co(C(3)H(2)O(4))(2)(H(2)O)(2)](NO(3))(2) (1), (C(5)H(7)N(2))(4)[Co(C(3)H(2)O(4))(2)(H(2)O)(2)](ClO(4))(2) (2), and (C(5)H(7)N(2))(4)[Co(C(3)H(2)O(4))(2)(H(2)O)(2)](PF(6))(2) (3) [C(5)H(7)N(2) = protonated 2-aminopyridine, C(3)H(4)O(4) = malonic acid, NO(3)(-) = nitrate, ClO(4)(-) = perchlorate, PF(6)(-) = hexafluorophosphate], have been synthesized from purely aqueous media, and their crystal structures have been determined by single crystal X-ray diffraction. A thorough analysis of Hirshfeld surfaces and fingerprint plots facilitates a comparison of intermolecular interactions in 1-3, which are crucial in building supramolecular architectures. When these complexes are structurally compared with their previously reported analogous Ni(II) or Mg(II) compounds, a very interesting feature regarding the role of counteranions has emerged. This phenomenon can be best described as anion-induced formation of extended supramolecular networks of the type lone pair-π/π-π/π-anion-π/π-lone pair and lone pair-π/π-π/π-anion involving various weak forces like lone pair-π, π-π, and anion-π interactions. The strength of these π contacts has been estimated using DFT calculations (M06/6-31+G*), and the formation energy of the supramolecular networks has been also evaluated. The influence of the anion (NO(3)(-), ClO(4)(-), and PF(6)(-)) on the total interaction energy of the assembly is also studied.

Magnetic anisotropy of the antiferromagnetic ring [Cr8F8Piv16].

A new tetragonal (P42(1)2) crystalline form of [Cr8F8Piv16] (HPiv = pivalic acid, trimethyl acetic acid) is reported. The ring-shaped molecules, which are aligned in a parallel fashion in the unit cell, form almost perfectly planar, regular octagons. The interaction between the CrIII ions is antiferromagnetic (J = 12 cm(-1)) which results in a S = 0 spin ground state. The low-lying spin excited states were investigated by cantilever torque magnetometry (CTM) and high-frequency EPR (HFEPR). The compound shows hard-axis anisotropy. The axial zero-field splitting (ZFS) parameters of the first two spin excited states (S = 1 and S = 2, respectively) are D1 = 1.59(3) cm(-1) or 1.63 cm(-1) (from CTM and HFEPR, respectively) and D2 = 0.37 cm(-1) (from HFEPR). The dipolar contributions to the ZFS of the S = 1 and S = 2 spin states were calculated with the point dipolar approximation. These contributions proved to be less than the combined single-ion contributions. Angular overlap model calculations that used parameters obtained from the electronic absorption spectrum, showed that the unique axis of the single-ion ZFS is at an angle of 19.3(1) degrees with respect to the ring axis. The excellent agreement between the experimental and the theoretical results show the validity of the used methods for the analysis of the magnetic anisotropy in antiferromagnetic CrIII rings.

Mn(II)-Gd(III) Phosphonate Cages with a Large Magnetocaloric Effect

) is the ideal spin center for MCEas it has a high isotropic spin; however, the internal natureof the 4f electrons hampers the magnetic communicationbetween them. A mixed-metal 3d–4f approach should havethe advantage of a better magnetic interaction and thus,a larger ground spin in the preferred ferromagnetic case. Wehave used this approach to make cobalt(II) (s=3/2) andnickel(II) (s=1) gadolinium phosphonate complexes withlarge MCEs.

Highly Stereoselective Synthesis of Substituted Prolyl Peptides Using a Combination of Biocatalytic Desymmetrization and Multicomponent Reactions

Time and pep-tide wait for no man: Optically pure 3,4disubstituted 1-pyrrolines, generated from the corresponding meso-pyrrolidines by biocatalytic desymmetrization (MAO-N=monoamine oxidase N), react with carboxylic acids and isocyanides in a highly diastereoselective Ugi-type multicomponent reaction to give substituted prolyl peptides of high pharmaceutical relevance. (Figure Presented)

Cyclic Imine Nitro-Mannich/Lactamization Cascades: A Direct Stereoselective Synthesis of Multicyclic Piperidinone Derivatives

An efficient nitro-Mannich/lactamization cascade of gamma-nitro esters with cyclic imines for the preparation of architecturally complex multicyclic piperidinone ring-containing structures has been developed. The reaction is broad in scope and stereoselective and may be coupled to an enantioselective nitroolefin Michael addition reaction as part of a highly enantio- and diastereoselective multicomponent process.