Crystal-packing-induced antiferromagnetic interactions of metallocenes: cyanonickelocenes, -cobaltocenes, and -ferrocenes

The cyano-substituted metallocenes [M(C5H4CN)2] (M=Fe, 1; Co, 2; Ni 3) and [M(C5Me5)(C5H4CN)] (M=Fe, 4; Co, 5; Ni, 6) were synthesized in yields up to 58 % by treating K(C5H4CN) or Tl(C5H4CN) with suitable transition-metal precursors. Cyclic voltammetry indicated that the oxidation and reduction potentials of all the cyanometallocenes were shifted to positive values by up to 0.8 V. Single-crystal X-ray structure analysis showed that 1 had eclipsed ligands, formed planes in the lattice, and--unlike usual metallocenes--lined up in stacks perpendicular to these planes. Powder X-ray studies established that 1 and 2 are isotypic. The 1H and 13C NMR spectra were recorded for all the new compounds. Signal shifts of up to delta=1500 ppm were recorded for the paramagnetic molecules 2 and 3 and were, at a given temperature, strikingly different for solution and solid-state spectra. These results pointed to antiferromagnetic interactions as a consequence of molecular ordering in the lattice, as confirmed by magnetic measurements. The temperature-dependent susceptibilities were reproduced by Heisenberg spin-chain models (H=-J sum n- 1 i=1 SiSi+1), thus yielding J=-28.3 and -10.3 cm(-1) for 2 and 3, respectively, whereas J=-11.8 cm(-1) was obtained for 3 from the Ising spin-chain model. In accordance with molecular orbital (MO) considerations, much spin density was found to be delocalized not only on the cyclopentadienyl ligand but also the cyano substituents. The magnetic interaction was interpreted as a Heitler-London spin exchange and was analyzed based on how the interaction depends on the singly occupied MOs and the shift of parallel metallocenes relative to each other.     

Evidence for concerted proton-electron transfer in the electrochemical oxidation of phenols with water as proton acceptor. Tri-tert-butylphenol

Proton-coupled electron transfer oxidation of phenols play a prominent role in several natural processes, and one may wonder if their high efficiency is related to the possibility that the electron and proton transfer steps are concerted. The cyclic voltammetric observation of the electrochemical oxidation and reverse reaction has allowed, with the example of 2,4,6-tri-tert-butylphenol in nonbuffered aqueous media, the clear identification of a pathway in which a phenol is directly and reversibly converted into the phenoxyl radical while the generated proton is accepted by a water molecule in a concerted manner. In very basic media, a stepwise mechanism takes place in which the phenol is deprotonated by OH- and the resulting phenoxide ion rapidly oxidized into the phenoxyl radical. As the pH decreases, this pathway progressively shuts down to the advantage of the concerted pathway. The latter assignment is confirmed by the observation of a substantial H/D kinetic isotope effect. At moderately basic pH 10.5, the contributions of the two pathways are about equal and the occurrence of the two competing routes is directly visualized in the cyclic voltammetry response.     

Polymerization of a Photocleavable Monomer Using Visible Light

The polymerization of the photocleavable monomer, o‐nitrobenzyl methacrylate (NBMA), is investigated using photoinduced electron/energy transfer reversible addition‐fragmentation chain transfer polymerization. The polymerizations under visible red (λ _max = 635 nm, 0.7 mW cm⁻²) and yellow (λ _max = 560 nm, 9.7 mW cm⁻²) light are performed and demonstrate rational evidence of a controlled/living radical polymerization process. Well‐defined poly(o‐nitrobenzyl methacrylate) (PNBMA) homopolymers with good control over the molecular weight and polymer dispersity are successfully synthesized by varying the irradiation time and/or targeted degree of polymerization. Chain extension of a poly(oligo(ethylene glycol) methyl ether methacrylate) macro‐chain transfer agent with NBMA is carried out to fabricate photocleavable amphiphilic block copolymers (BCP). Finally, these self‐assembled BCP rapidly dissemble under UV light suggesting the photoresponsive character of NBMA is not altered during the polymerization under yellow or red light. Such photoresponsive polymers can be potentially used for the remote‐controlled delivery of therapeutic compounds.

     

On the preparation of carbohydrate-protein conjugates using the traceless Staudinger ligation

[reaction: see text] The nature of a linker used for preparing glycoconjugate vaccines is of utmost importance as it may lead to immunogenic biomolecules. We report the conjugation of carbohydrate haptens to protein carriers leading to potential vaccines using the traceless Staudinger ligation. The ligation relies on the selective transfer of a phosphane substituent to an azide to form a native amide bond in the final product upon release of an oxidized phosphane byproduct. We designed new phosphino-functionalized cross-linkers suitable for protein carrier derivatization. We evaluated their utility in preparing conjugates using both synthetic and purified bacterial carbohydrates. The use of a borane-protected phosphane which is deprotected at the time of the ligation reaction led to the best results observed thus far in terms of stability toward oxidation and reactivity.     

Pyridine as proton acceptor in the concerted proton electron transfer oxidation of phenol

Taking pyridine as a prototypal example of biologically important nitrogen bases involved in proton-coupled electron transfers, it is shown with the example of the photochemically triggered oxidation of phenol by Ru(III)(bpy)(3) that this proton acceptor partakes in a concerted pathway whose kinetic characteristics can be extracted from the overall kinetic response. The treatment of these data, implemented by the results of a parallel study carried out in heavy water, allowed the determination of the intrinsic kinetic characteristics of this proton acceptor. Comparison of the reorganization energies and of the pre-exponential factors previously derived for hydrogen phosphate and water (in water) as proton acceptors suggests that, in the case of pyridine, the proton charge is delocalized over a primary shell of water molecules firmly bound to the pyridinium cation.     

Bioinspired Oxidative Cyclization of the Geissoschizine Skeleton for Enantioselective Total Synthesis of Mavacuran Alkaloids

Reported is the enantioselective total syntheses of mavacuran alkaloids, (+)‐taberdivarine H, (+)‐16‐hydroxymethyl‐pleiocarpamine, and (+)‐16‐epi‐pleiocarpamine, and their postulated biosynthetic precursor 16‐formyl‐pleiocarpamine. This family of monoterpene indole alkaloids is a target of choice since some of its members are subunits of intricate bisindole alkaloids such as bipleiophylline. Inspired by the biosynthetic hypothesis, an oxidative coupling approach from the geissoschizine framework to form the N1?C16 bond was explored. Quaternization of the aliphatic nitrogen center was key to achieving the oxidative coupling induced by KHMDS/I₂ as it masks the nucleophilicity of the aliphatic nitrogen center and locks in the required cis conformation.     

An Oxygen Paradox: Catalytic Use of Oxygen in Radical Photopolymerization

A peculiar radical polymerization reaction is presented in which oxygen serves as a cocatalyst, alongside triethylamine, to provide activation with light in the far‐red (690 nm, 3 mW cm^?2) of the PET‐RAFT process in the presence of zinc(II) (2,3,7,8,12,13,17,18‐octaethyl‐5,10,15,20‐tetraphenylporphyrin) as photocatalyst. Apart from the ability to exert temporal control by switching the light on or off, this system possesses the exciting capability of inducing temporal control by removal or reintroduction of oxygen. Furthermore, this multicomponent catalytic system was typified by controlled polymerizations of various acrylate and acrylamide monomers, which all resulted in well‐defined polymers with low dispersity (<1.2). The process displayed excellent living characteristics that were demonstrated through chain extensions and a range of degrees of polymerization (200–1600).