Reactivity of the hydrido/nitrosyl radical MHCl(NO)(CO)(P(i)Pr(3))(2), M = Ru, Os

The reaction of equimolar NO with the 16 electron molecule RuHCl(CO)L(2) (L = P(i)Pr(3)) proceeds, via a radical adduct RuHCl(CO)(NO) L(2), onward to form RuCl(NO)(CO)L(2) (X-ray structure determination) and RuHCl(HNO)(CO)L(2), in a 1:1 mole ratio. The HNO ligand, bound by N and trans to hydride, is rapidly degraded by excess NO. The osmium complex behaves analogously, but the adduct has a higher formation constant, permitting determination of its IR spectrum; both MHCl(CO)(NO)L(2) radicals are characterized by EPR spectroscopy, and DFT calculations on the Ru system show it to have a "half-bent" Ru-N-O unit with the spin density mainly on nitrogen. DFT (PBE) energies rule out certain possible mechanistic steps for forming the two products. A survey of the literature leads to the hypothesis that NO should generally be considered as a (neutral) Lewis base (2-electron donor) when it binds to a 16 electron complex which is resistant to oxidation or reduction, and that the resulting N-centered radical has a M-N-O angle of approximately 140 degrees, which distinguishes it from NO(-) (bent at <140 degrees ) and from NO(+) (>170 degrees ).     

Spirophosphoranylation d'α-aminoacides—I : Preparation et tautomerie

New spirophosphoranes have been prepared by reaction of &alpha-aminoacids with tricoordinate phosphorus substrates. In one case, 1i, an equilibrium chain-cycle P^IIIP^v has been observed.     

Replacement of the Cobalt Center of Vitamin B12 by Nickel: Nibalamin and Nibyric Acid Prepared from Metal-Free B12 Ligands Hydrogenobalamin and Hydrogenobyric Acid

The (formal) replacement of Co in cobalamin ( Cbl ) by Ni^II generates nibalamin ( Nibl ), a new transition-metal analogue of vitamin B₁₂. Described here is Nibl , synthesized by incorporation of a Ni^II ion into the metal-free B₁₂ ligand hydrogenobalamin ( Hbl ), itself prepared from hydrogenobyric acid ( Hby ). The related Ni^II corrin nibyric acid ( Niby ) was similarly synthesized from Hby , the metal-free cobyric acid ligand. The solution structures of Hbl , and Niby and Nibl , were characterized by spectroscopic studies. Hbl features two inner protons bound at N2 and N4 of the corrin ligand, as discovered in Hby . X-ray analysis of Niby shows the structural adaptation of the corrin ligand to Ni^II ions and the coordination behavior of Ni^II. The diamagnetic Niby and Nibl , and corresponding isoelectronic Co^I corrins, were deduced to be isostructural. Nibl is a structural mimic of four-coordinate base-off Cbls , as verified by its ability to act as a strong inhibitor of bacterial adenosyltransferase.     

Synthesis and structure of a nucleoside with pi-conjugated nitroxide spin label forming a one-dimensional ferromagnetic chain

Spin labeled 2'-deoxyuridine, in which a significant fraction of the spin density is delocalized from a nitroxide radical to the DNA base residue, was prepared as a crystalline solid, stable at ambient conditions. The crystal packing, which includes multiple hydrogen bonds, leads to one-dimensional chains of molecules with predominant intrachain ferromagnetic coupling and weaker interchain antiferromagnetic coupling.     

First examples of oxaphosphirane pentacarbonylchromium(0) and -molybdenum(0) complexes: synthesis, structures and reactions

Synthesis of the first oxaphosphirane chromium(0) and molybdenum(0) complexes of the type [{(R(1)PCH(R(2))-O}M(CO)(5)] (R(1) = C(5)Me(5)) (8a-e, 15a-e) and (R(1) = CH(SiMe(3))(2)) (9a-e, 16a-e) via reaction of dichloro(organo)- (1, 2, 10, 11) and chloro(organo)phosphane complexes (3,4,12) with lithium bases and aldehydes 7a-e is reported. Furthermore, bicyclic 1,3-oxaphospholane complexes 17 and 18 have been obtained via O-protonation of oxaphosphirane complexes 8a and 15a with HCl. All complexes were characterized by NMR, IR spectroscopic, mass spectrometric investigations and, in addition, single-crystal X-ray structures of complexes 8a-e, 9a,c, 15a,b,e, 16a-c, 17, 18 are presented and discussed.     

Chemical synthesis, crystal structure and enzymatic evaluation of a dinucleotide spore photoproduct analogue containing a formacetal linker

Spore photoproduct (SP) is the exclusive DNA photodamage product found in bacterial endospores. Its photoformation and repair by a metalloenzyme spore photoproduct lyase (SPL) composes the unique SP biochemistry. Despite the fact that the SP was discovered almost 50 years ago, its crystal structure is still unknown and the lack of structural information greatly hinders the study of SP biochemistry. Employing a formacetal linker and organic synthesis, we successfully prepared a dinucleotide SP isostere 5R-CH(2) SP, which contains a neutral CH(2) moiety between the two thymine residues instead of a phosphate. The neutral linker dramatically facilitates the crystallization process, allowing us to obtain the crystal structure for this intriguing thymine dimer half a century after its discovery. Further ROESY spectroscopic, DFT computational, and enzymatic studies of this 5R-CH(2) SP compound prove that it possesses similar properties with the 5R-SP species, suggesting that the revealed structure truly reflects that of SP generated in Nature.     

Conformational and electronic consequences in crafting extended, π-conjugated, light-harvesting macrocycles

The synthesis of a new series of free‐base, Ni^II and Zn^II 2,3,12,13‐tetra(ethynyl)‐5,10,15,20‐tetraphenyl porphyrins is described. Upon heating, two of the four ethynyl moieties undergo Bergman cyclization to afford the monocyclized 2,3‐diethynyl‐5,20‐diphenylpiceno[10,11,12,13,14,15‐jklmn]porphyrin in 30 %, 10 %, and trace yields, respectively. The structures of all products were investigated by using quantum chemical calculations and the free‐base analogue was isolated and crystallized; all compounds show significant deviation from the idealized planar structure. No fully‐cyclized bispiceno[20,1,2,3,4,5,10,11,12,13,14,15‐fghij]porphyrin was isolated from the reaction mixture. To understand why only two of the four enthynyl groups undergo Bergman cyclization, the reaction coordinates were examined by using DFT at the PWPW91/cc‐pVTZ(‐f) level coupled to a continuum solvation model. The barrier to cyclization of the second pair of ethynyl groups was found to be 5.5 kcal mol^?1 higher than the first, suggesting a negative cooperative effect and significantly slower rate for the second cyclization. Cyclization reactions for model porphyrin–enediynes with ethene‐ and H‐functionality substitutions at the meso‐phenyl rings were also examined, and found to have a similar barrier to diradical formation for the second cyclization event as for the first in these highly planar molecules. By enforcing an artificial 30° cant in two of the pyrrole rings of the porphyrin, the second barrier was increased by 2 kcal mol^?1 in the ethene model system; this suggests that the disruption of the π conjugation of the extended porphyrin structure is the cause of the increased barrier to the second cyclization event.     

Oxidation of annelated diarylamines: analysis of reaction pathways to nitroxide diradical and spirocyclic products

Oxidation of diaryldiamine 2, a tetrahydrodiazapentacene derivative, provides diarylnitroxide diradical 1 accompanied by an intermediate nitroxide monoradical and a multitude of isolable diamagnetic products. DFT-computed tensors for EPR spectra and paramagnetic (1)H NMR isotropic shifts for nitroxide diradical 1 show good agreement with the experimental EPR spectra in rigid matrices and paramagnetic (1)H NMR spectra in solution, respectively. Examination of the diamagnetic products elucidates their formation via distinct pathways involving C-O bond-forming reactions, including Baeyer-Villiger-type oxidations. An unusual diiminoketone structure and two spirocyclic structures of the predominant diamagnetic products are confirmed by either X-ray crystallography or correlations between DFT-computed and experimental spectroscopic data such as (1)H, (13)C, and (15)N NMR chemical shifts and electronic absorption spectra.     

Enzymatic Conversion of Flavonoids using Bacterial Chalcone Isomerase and Enoate Reductase

Flavonoids are a large group of plant secondary metabolites with a variety of biological properties and are therefore of interest to many scientists, as they can lead to industrially interesting intermediates. The anaerobic gut bacterium Eubacterium ramulus can catabolize flavonoids, but until now, the pathway has not been experimentally confirmed. In the present work, a chalcone isomerase (CHI) and an enoate reductase (ERED) could be identified through whole genome sequencing and gene motif search. These two enzymes were successfully cloned and expressed in Escherichia coli in their active form, even under aerobic conditions. The catabolic pathway of E. ramulus was confirmed by biotransformations of flavanones into dihydrochalcones. The engineered E. coli strain that expresses both enzymes was used for the conversion of several flavanones, underlining the applicability of this biocatalytic cascade reaction.