Beyond Stereoselectivity,Switchable Catalysis: Some of the Last Frontier Challenges in Ring‐Opening Polymerization of Cyclic Esters

Metal‐based catalysts and initiators have played a pivotal role in the ring‐opening polymerization (ROP) of cyclic esters, thanks to their high activity and remarkable ability to control precisely the architectures of the resulting polyesters in terms of molar mass, dispersity, microstructure, or tacticity. Today, after two decades of extensive research, the field is slowly reaching maturity. However, several challenges remain, while original concepts have emerged around new types or new applications of catalysis. This Review is not intended to comprehensively cover all of these aspects. Rather, it provides a personal overview of the very recent progress achieved in some selected, important aspects of ROP catalysis—stereocontrol and switchable catalysis. Hence, the first part addresses the development of new metal‐based catalysts for the isoselective ROP of racemic lactide towards stereoblock copolymers, and the use of syndioselective ROP metal catalysts to control the monomer sequence in copolymers. A second part covers the development of ROP catalysts—primarily metal‐based catalysts, but also organocatalysts—that can be externally regulated by the use of chemical or photo stimuli to switch them between two states with different catalytic abilities. Current challenges and opportunities are highlighted.     

On covering bridged plane triangulations with balls

We show that every plane graph of diameter 2r in which all inner faces are triangles and all inner vertices have degree larger than 5 can be covered with two balls of radius r. © 2003 Wiley Periodicals, Inc. J Graph Theory 44: 65–80, 2003     

Nombres de Liouville et nombres normauxLiouville numbers and normal numbers

We prove that there exist Liouville numbers which are normal, as well as Liouville numbers which are non-normal to any base. To cite this article: Y. Bugeaud, C. R. Acad. Sci. Paris, Ser. I 335 (2002) 117–120.     

Cation-controlled self-assembly of a hexameric europium wheel

The simple asymmetric tetradentate ligand 2,2':6',2' '-terpyridine-6-carboxylic acid leads to the self-assembly of the first europium nanowheel containing europium ions in two different coordination environments. Moreover the self-assembly of bis(terpyridinecarboxylate) europium species to form a hexameric wheel capable of strongly binding LnIII cations is controlled by the ligand/cation ratio.     

A convergent approach to huperzine A and analogues

We describe a concise and convergent synthesis of (rac)-5-methoxy-6-azatricyclco[7.3.1.0(2,7)]trideca-2(7),3,5,11-tetraen-13-ol, which has the basic ring system of huperzine A, a potent inhibitor of acetylcholinesterase. We also describe the synthesis of the novel system 5-methoxy-6-azatricyclo[7.2.2.0(2,7)]trideca-2(7),3,5-trien-10-one and a series of related systems.     

Poly(9,9'-spirobifluorene-manganese porphyrin): a new catalytic material for oxidation of alkenes by iodobenzene diacetate and iodosylbenzene

Anodic oxidation of tetraspirobifluorene-manganese porphyrin lead to the coating of the working electrode by insoluble electroactive poly(9,9'-spirobifluorene-manganese porphyrin) films for which electrochemical behaviour and physicochemical properties are described; these polymeric materials are able to catalyze the heterogeneous epoxidation of styrene with iodobenzene diacetate and iodosylbenzene.     

Total synthesis of anachelin H

[structure: see text] The first total synthesis of anachelin H is reported. Starting from L-Ser, a stereodivergent synthesis of the polyketide fragment resulting in all possible diastereoisomers is described. The alkaloid peptide fragment is prepared via a tellurium-mediated oxidative aza annulation as the key step. Coupling of the fragments gave synthetic anachelin H, which was found to be identical to a sample of the natural product, thus confirming the configuration by total synthesis.     

Aromatic foldamers as scaffolds for metal second coordination sphere design

As metalloproteins exemplify, the chemical and physical properties of metal centers depend not only on their first but also on their second coordination sphere. Installing arrays of functional groups around the first coordination sphere of synthetic metal complexes is thus highly desirable, but it remains a challenging objective. Here we introduce a novel approach to produce tailored second coordination spheres. We used bioinspired artificial architectures based on aromatic oligoamide foldamers to construct a rigid, modular and well-defined environment around a metal complex. Specifically, aza-aromatic monomers having a tethered [2Fe–2S] cluster have been synthesized and incorporated in conical helical foldamer sequences. Exploiting the modularity and predictability of aromatic oligoamide structures allowed for the straightforward design of a conical architecture able to sequester the metal complex in a confined environment. Even though no direct metal complex–foldamer interactions were purposely designed in this first generation model, crystallography, NMR and IR spectroscopy concurred to show that the aromatic oligoamide backbone alters the structure and fluxional processes of the metal cluster.

Wrapping a [2Fe–2S] metal complex in an aromatic foldamer helix is introduced as a new approach to tailor a second coordination sphere.     

Surface organometallic chemistry of main group elements: selective synthesis of silica supported [triple bond Si-OB(C(6)F(5))(3)](-)[HNEt(2)Ph](+)

The reaction of the Lewis acid B(C(6)F(5))(3) with silanol groups of silica surfaces, dehydroxylated at different temperatures (300, 500, 700, and 800 degrees C), has been investigated in presence of the Br?nsted base NEt(2)Ph. The structure of the resulting modified silica supports [triple bond Si-OB(C(6)F(5))(3)](-)[HNEt(2)Ph](+) (1) has been carefully identified by IR and multinuclear solid-state NMR spectroscopies, isotopic (2)H and (18)O labeling, elemental analysis, molecular modeling, and comparison with synthesized molecular models. Highly dehydroxylated silica surfaces were required to transform selectively each silanol group into unique [triple bond Si-OB(C(6)F(5))(3)](-)[HNEt(2)Ph](+) fragments. For lower dehydroxylation temperatures, two sorts of surface sites were coexisting on silica: the free silanol groups [triple bond SiOH] and the ionic species 1.     

The Structure of Bis[N‐6‐aminopyridyl‐N‐(1S)‐(+)‐10‐camphorsulfonylamino]palladium in the Solid State and in Solution

The complex, bis[N‐6‐aminopyridyl‐N‐(1S)‐(+)‐10‐camphorsulfonylamino]palladium, Pd[(S)‐APCS]₂, 1 , was prepared by reaction of 2‐[(1S)‐(+)‐10‐camphorsulfonamino]‐6‐aminopyridine with PdCl₂ in THF. Complex 1 has been characterized by spectroscopic methods and its structure has been determined by X‐ray crystallography. Crystal data: space group C2, a= 16.082 (2), b = 17.104 (2), c = 13.051 (2)Å, β = 99.95 (1)°, V = 3535.9 (8) ų, Z = 2 with final residuals R1 = 0.0491 and wR2 = 0.0944. Two independent molecules, (S,S)‐Pd[(S)‐APCS]2, 1a , and (R,R)‐Pd[(S)‐APCS]2, 1b , were found in each asymmetric unit, which exchange to each other via a series of nitrogen inversion and C‐C bond rotation. The inversion energy (ΔGc₁^≠) and the energy barrier (δGc₂^≠) were 11.5 ± 0.1 Kcal mol^?1 at 246 K and 9.8 ± 0.1 Kcal mol^?1 at 199 K, respectively, calculated by dynamic NMR data.