No abstract

Methyldioxorhenium (MDO)-catalyzed C-O bond cleavage of a variety of lignin β-O-4-model compounds yields phenolic and aldehydic compounds in homogeneous phase under mild reaction conditions. MDO is in situ generated by reduction of methyltrioxorhenium (MTO) and is remarkably stable under the applied reaction conditions allowing its reuse for least five times without significant activity loss. Based on the observed and isolated intermediates, 17 O- and 2 H-isotope labeling experiments, DFT calculations, and several spectroscopic studies, a reaction mechanism is proposed.

Reaction of the dichloro complexes [M(N(2) (TBS)N(py))Cl(2)] (M=Zr: 1, Hf: 2; TBS: tBuMe(2)Si; py: pyridine) with one molar equivalent of LiNHNPh(2) gave mixtures of the two diastereomeric chlorohydrazido(1-) complexes [M(N(2) (TBS)N(py))(NHNPh(2))Cl] (M=Zr: 3 a,b, Hf: 4 a,b) in which the diphenylhydrazido(1-) ligand adopts a bent kappa(1) coordination. This mixture of isomers could be cleanly converted into the deep green diphenylhydrazido(2-) complexes [Zr(N(2) (TBS)N(py))(NNPh(2))(py)] (5) and [Hf(N(2) (TBS)N(py))(NNPh(2))(py)] (6), respectively, by dehydrohalogenation with lithium hexamethyldisilazide (LiHMDS) in the presence of one molar equivalent of pyridine. Both complexes contain a linearly coordinated hydrazinediide for which a DFT-based frontier orbital analysis established bonding through one sigma and two pi orbitals. A high polarity of the M=N bond was found, in accordance with the description of hydrazinediide(2-) acting as a six-electron donor ligand. The pyridine ligand in [M(N(2) (TBS)N(py))(NNPh(2))(py)] (M=Zr: 5, Hf: 6) is substitutionally labile as established by line-shape analysis of the dynamic spectra (DeltaG(not equal)=19 kcal mol(-1)). A change in denticity of the hydrazido unit from kappa(1) to kappa(2) was studied by DFT methods. Both forms are calculated to be very close in energy and are only separated by shallow activation barriers, which supports the notion of a rapid kappa(1) to kappa(2) interconversion. This process is believed to happen early on in the N-N scission in the presence of coupling reagents. Frontier orbital and natural population analyses suggest that a primarily charge-controlled nucleophilic attack at N(alpha) is unlikely whereas interaction with an electrophile could play an important role. This hypothesis was tested by the reaction of 5 and 6 with one molar equivalent of B(C(6)F(5))(3) to give [Zr(N(2) (TBS)N(py))(NNPh(2)){B(C(6)F(5))(3)}] (7) and [Hf(N(2) (TBS)N(py))(NNPh(2)){B(C(6)F(5))(3)}] (8). In these products, B(C(6)F(5))(3) becomes attached to the N(alpha) atom of the side-on bound hydrazinediide and there is an additional interaction of an ortho-F atom of a C(6)F(5) ring with the metal centre.

Inhibition of photosynthesis after exposure to solar radiation was investigated in the marine green alga Dunaliella salina by monitoring photosynthetic optimal quantum yield FylFj^ and efficiency of oxygen production. Samples were exposed to solar radiation in Ancient Korinth, Greece (37°58' N, 23°0' E) in August 1994. Within 30 min, F^F^^ and efficiency of oxygen production decreased with similar kinetics with increasing exposure time. The inhibition, however, diminished when ultraviolet radiation was progressively excluded by means of colour filter glasses. Samples exposed for 3 h showed complete or partial recovery of photosynthesis, with almost the same rate under all irradition conditions. The fit of the experimental data with an analytical model describing inhibition of photosynthesis as a function of a linear combination of the photon fiuence in the UV-B, UV-A and PAR allows one to estimate the relative mean effectiveness for inhibition by the tbree spectral ranges [about 2 X 10~^, 4 x 10*' and 2 x 10"^ (|Limol photons m"^r' for UV-B, UV-A and PAR, respectively].