Electrochemical, Spectroelectrochemical, Mößbauer, and EPR Spectroscopic Studies on Ferrocenyl-Substituted Tritylium Dyes.

Four monoferrocenyl tritylium derivatives with donor-substituted (OMe, NMe2 ) aryl rings are reported, along with their spectroscopic and electrochemical properties. All the complexes show a one-electron reduction and a quasi-reversible ferrocene oxidation at a very positive potential. Small quadrupole splittings, ΔEQ , in Mößbauer spectra agree with highly electron-deficient ferrocenes. Comparison of the experimental half-wave potentials for ferrocene oxidation, E1/2 (Fc/Fc+ ), with those estimated from established correlations of E1/2 (Fc/Fc+ ) with ΔEQ indicates that the E1/2 values of the anisyl-substituted congeners FcOMe+ and FcMeOMe+ are affected by Coulombic repulsion between the positive charges at the Fe ion and the neighboring methylium site. Electronic spectra are recorded and interpreted with the aid of quantum chemical calculations. UV/Vis spectroelectrochemical measurements as well as chemical reduction provide insight into the redox-induced color changes upon ferrocene oxidation or upon reduction to the neutral trityl radicals. The neutral radicals reversibly form EPR-silent dimers. This process is studied by temperature-dependent EPR spectroscopy, and thermodynamic data for their dimerization are determined. Experimental and quantum chemical data suggest that the dimers assume classical hexaarylethane structures as opposed to normal or "offset" Jacobson-Nauta-type structures.

Electronically Strongly Coupled Divinylheterocyclic-Bridged Diruthenium Complexes.

Complexes [{Ru(CO)Cl(PiPr3 )2 }2 (µ-2,5-(CH-CH)2 -(c) C4 H2 E] (E=NR; R=C6 H4 -4-NMe2 (10 a), C6 H4 -4-OMe (10 b), C6 H4 -4-Me (10 c), C6 H5 (10 d), C6 H4 -4-CO2 Et (10 e), C6 H4 -4-NO2 (10 f), C6 H3 -3,5-(CF3 )2 (10 g), CH3 (11); E=O (12), S (13)) are discussed. The solid state structures of four alkynes and two complexes are reported. (Spectro)electrochemical studies show a moderate influence of the nature of the heteroatom and the electron-donating or -withdrawing substituents R in 10 a-g on the electrochemical and spectroscopic properties. The CVs display two consecutive one-electron redox events with ΔE°'=350-495 mV. A linear relationship between ΔE°' and the σp Hammett constant for 10 a-f was found. IR, UV/Vis/NIR and EPR studies for 10(+) -13(+) confirm full charge delocalization over the {Ru}CH-CH-heterocycle-CH-CH{Ru} backbone, classifying them as Class III systems according to the Robin and Day classification. DFT-optimized structures of the neutral complexes agree well with the experimental ones and provide insight into the structural consequences of stepwise oxidations.

A Stable Planar-Chiral N-Heterocyclic Carbene with a 1,1'-Ferrocenediyl Backbone.

This paper focuses on the stable, ferrocene-based N-heterocyclic carbene (NHC) rac-[Fe{(η(5)-t-BuC5H3)NpN}2C:] (A'-Np, Np = neopentyl), which is planar-chiral due to the two tert-butyl substituents in 3,3'-positions. A'-Np was synthesized in nine steps starting from 1,1'-di-tert-butylferrocene (1), the first step being its 3,3'-dilithiation to afford rac-[Fe(η(5)-t-BuC5H3Li)2] (rac-fc'Li2, 2). The structures of rac-fc'(SiMe3)2 (3), rac-fc'Br2 (4), rac-fc'(N3)2 (5), and the immediate carbene precursor [A'-NpH]BF4 were determined by single-crystal X-ray diffraction (XRD). The chemical properties of A'-Np were found to be very similar to those of its tert-butyl-free congener A-Np, both being ambiphilic NHCs with rather high calculated HOMO energies (ca. -4.0 eV) and low singlet-triplet gaps (ca. 35 kcal/mol). A Tolman electronic parameter value of 2050 cm(-1) was derived from IR data of cis-[RhCl(A'-Np)(CO)2], indicating the high donicity of A'-Np as a ligand. Consistent with its ambiphilic nature, A'-Np was found to react readily with carbon monoxide, affording the betainic enolate (A'-Np)2CO as four stereoisomers, viz. (RpRp-A'-Np)═C(O(-))(RpRp-A'-Np(+)), (SpSp-A'-Np)═C(O(-))(SpSp-A'-Np(+)), (RpRp-A'-Np)═C(O(-))(SpSp-A'-Np(+)), and (SpSp-A'-Np)═C(O(-))(RpRp-A'-Np(+)). The former two isomers were structurally characterized as a racemic compound by single-crystal XRD. A'-Np was found to react swiftly with dichloromethane, affording the addition product A'-NpH-CHCl2 in a reaction that is unprecedented for diaminocarbenes. A-NpH-CHCl2 was obtained analogously. Both compounds were structurally characterized by single-crystal XRD. An electrochemical investigation of A'-Np by cyclic and square wave voltammetry revealed a reversible oxidation of the carbene at a half-wave potential of -0.310 vs ferrocene/ferrocenium (THF/NBu4PF6). The electrochemical data previously published for A-Np were identified to be incorrect, since unnoticed hydrolysis of the NHC had taken place, affording A-Np(H2O). The hydrolysis products of A-Np and A'-Np were found to be reversibly oxidized at half-wave potentials of -0.418 and -0.437 V, respectively.

Kinetic resolution of aliphatic beta-amino acid amides by beta-aminopeptidases.

The growing demand for enantiomerically pure beta-amino acids to be used in the pharmaceutical industry and as fine chemicals requires the development of new strategies for their synthesis. The beta-aminopeptidases BapA from Sphingosinicella xenopeptidilytica 3-2W4, BapA from Sphingosinicella microcystinivorans Y2, and DmpA from Ochrobactrum anthropi LMG7991 are hydrolases that possess the unique ability of cleaving N-terminal beta-amino acids from peptides and amides. Hydrolysis of racemic beta(3)-amino acid amides catalyzed by these enzymes displays enantioselectivity with a strong preference for substrates with the L-configuration and gives access to various aliphatic beta(3)-amino acids of high enantiopurity. This approach presents a new access to enantiopure beta(3)-amino acids under mild reaction conditions and complements chemical asymmetric synthesis strategies.