Daniel Kost

Hydrazide-Based Hypercoordinate Silicon Compounds

Publisher Summary Hypercoordinate silicon complexes also referred as “hypervalent compounds.” This chapter reviews the introduction and use of a new class of chelating agents, based on the hydrazide functional group. Hydrazide-based silicon complexes are remarkably versatile, forming a variety of novel complexes, penta and hexacoordinate, neutral, cationic, or anionic, as well as zwitterionic compounds. Many of these complexes have intriguing stereochemical properties, show chemical or stereochemical ligand-exchange phenomena, which have been studied extensively using nuclear magnetic resonance (NMR) spectroscopy. Hydrazide-based silicon complexes have been synthesized utilizing a silicon-ligand exchange reaction also known as “transsilylation.” The reaction consists of exchange of ligands between an N- or O-(trimethylsilyl) hydrazide (TMS-hydrazide) and a polyhalosilane, such that the only byproducts are volatile TMS halides, which are readily boiled off. Moreover, the ease and cleanliness of this reaction allow real-time NMR monitoring.

Low-frequency electromagnetic fields induce a stress effect upon higher plants, as evident by the universal stress signal, alanine

15N NMR analysis reveals alanine production in Duckweed plants exposed to low intensity sinusoidally varying magnetic fields (SVMF) at 60 and 100Hz, and fed by 15N-labeled ammonium chloride. Alanine does not accumulate in the absence of SVMF. Addition of vitamin C, a radical scavenger, reduced alanine production by 82%, indicating the roll of free radicals in the process. Alanine accumulation in plants and animals in response to exposure to a variety of stress conditions, including SVMF, is a general phenomenon. It is proposed that alanine is a universal first stress signal expressed by cells.

Different ammonium-ion uptake, metabolism and detoxification efficiencies in two Lemnaceae

Abstract15N-Nuclear magnetic resonance spectroscopy was used to follow nitrogen assimilation and amino-acid production in Wolffia arrhiza (L.) Hork. ex. Wimmer, clone Golan exposed to 4.0 mM 15NH4Cl solutions for 24 h. The main 15N-labelled metabolites were asparagine and glutamine, as well as substantial amounts of unreacted, intracellular NH4+. These results were compared with those of a previous study on Lemna gibba L. clone Hurfeish (Monselise et al., 1987, New Phytol. 10, 341–345) with regard to NH4+uptake, assimilation and detoxification efficiencies. Both species, grown under continuous white light, were capable of preferential uptake of NH4+in the presence of nitrate. Relative growth rates indicate that both species tolerate increased levels of NH4+, up to 10−2 mol · 1−1, with L. gibba showing a slightly greater tolerance. No 15N-labelled free NH4+was detectable in L. gibba, while in W. arrhiza excess NH4+was found within the cells. This fact indicates that L. gibba is more efficient in detoxification than W. arrhiza, presumably because of inability of W. arrhiza to regenerate the “NH4+traps”, glutamate and aspartate, rapidly enough. This is also evident from the observation that addition of α-ketoglutarate to the medium caused nearly complete assimilation of intracellular NH4+in W. arrhiza. In both plants, addition of α-ketoglutarate increased both NH4+uptake and assimilation. Addition of l-methionine dl-sulfoximine, an inhibitor of glutamine synthetase inhibited NH4+assimilation, while addition of azaserine, an inhibitor of glutamate synthase, resulted in 15N incorporation into the glutamine-amide position only. These results are consistent with the glutamine synthetase-glutamate synthase pathway being the major route of NH4+assimilation in the two plants under the conditions used.

The Schmidt Reaction of Dialkyl Acylphosphonates

The scope of the Schmidt rearrangement of ketones has been extended to dialkyl acylphosphonates (11a-111). Surprisingly, it was found that 11a-11dand 11g, in which the acyl moiety was benzoyl alone or benzoyl bearing an electron-attracting or mildly electron-releasing substituent, yielded an overwhelming portion of products resulting from C-to-N migration of the aryl group (N-arylcarbamoylphosphonates, 12, and N-arylformamides, 15). Contrariwise, the arenecarbonylphosphonates, which carry a powerful electron-releasing p-alkoxy group, yielded products resulting from phosphonate group migration from C to N or elimination (dialkyl N-arenecarbonylphosphoramidates, 13, and arenecarbonitriles, 17, respectively)

Donor-stabilized silyl cations ☆: Part 7: Neutral hexacoordinate and ionic pentacoordinate silicon chelates with N-isopropylideneimino-acylimidato ligands

Abstract Silicon complexes with a new chelating donor group, the isopropylideneimino-acylimidato(N,O) moiety [OC(R)NNCMe2], have been prepared (9–12, 15), and their properties are compared with previously described silicon chelates (1, 2, and 16, respectively). The new ligand acts as a more powerful donor than the NNMe2 ligand, based on three criteria: (a) the new complexes form directly as pentacoordinate siliconium salts 9–12, i.e., the expected neutral hexacoordinate precursors ionize spontaneously; (b) comparison of crystallographic bond lengths with those of the NNMe2 complexes shows consistently shorter NSi coordination bonds and longer Sihalogen bonds in equally substituted new relative to the previously studied complexes; (c) while in the previous series 1, the dihalo complexes 20–22, 25, resisted ionization at any temperature or solvent, the dibromo-isopropylideneimino complexes 19a and 19b ionize reversibly upon decrease of temperature in chloroform solution. Steric congestion forces the trans-dihalo configuration on the dichloro and bibromo complexes 18, 19. The ionization-resistant complexes 17, 18 and the partly ionized 19 form stable ionic siliconium salts when their counterions are replaced by I−, BPh4−, or by reaction with strong Lewis acids, AlCl3 and AlBr3.

Conformationally controlled intramolecular charge transfer complexes

Trans-1-acceptor-2-donor-substituted cyclohexanes (1), as well as their 4- (or 5-)methyl-substituted homologues (2), have been prepared and are shown to form intramolecular charge-transfer (donor-acceptor) complexes. These weak complexes are turned on and off by the chair-chair interconversion of the cyclohexane ring. The CT absorptions have been measured and the equilibrium constants for the ring reversal have been determined by UV/vis spectroscopy at 298 K, as well as by NMR spectroscopy at two temperatures: at 183 K, by direct comparison of signals due to the two chair conformations, and at 300 K, by comparison of calculated and measured widths of the alpha-proton signals. The Gibbs free energies assigned to the donor-acceptor interactions range between 0 and -1 kcal mol(-1). A crystal structure of one of the complexes (1b) confirms the intramolecular donor-acceptor alignment and interaction. The regioisomers of the methyl-substituted complexes were characterized by NOE interaction between the methyl and an alpha-proton cis to it.

α-Substituent effects and non-bonding electron interactions atthe SN2 transition state, an ab-initio study.

Abstract The trend in reactivity observed for methyl-, methylene-, and methoxymethyl-chlorides is rationalized in terms of turning on and off the four electron repulsion, at the transition state, between a lone pair on the substituent and a π orbital along the reaction coordinate.

Silicon rehybridization and molecular rearrangements in hypercoordinate silicon dichelates

Hydrazide-based hypercoordinate silicon dichelates are remarkably flexible in terms of geometry and reactivity: this paper demonstrates how rather subtle constitutional changes result in dramatic geometrical and reactivity changes. A change of ligand-donor group from NMe2 to N=CMe2 changes the solid-state geometry from bicapped tetrahedral to octahedral. These two geometries are shown to coexist in solution in dynamic equilibrium. Hexacoordinate complexes are shown to dissociate to pentacoordinate compounds in two distinct ways: ionic or neutral, depending on substitution. Hexacoordinate dichelates with imino-donor groups undergo a skeletal rearrangement (intramolecular aldol-type condensation of imines), catalyzed by their dissociated halide counterions. However, even in the absence of counterions, silacyclobutane dichelates undergo a similar rearrangement.

Ultraviolet radiation induces stress in etiolated Landoltia punctata, as evidenced by the presence of alanine, a universal stress signal: a 15N NMR study

Analysis with (15) N NMR revealed that alanine, a universal cellular stress signal, accumulates in etiolated duckweed plants exposed to 15-min pulsed UV light, but not in the absence of UV irradiation. The addition of 10 mm vitamin C, a radical scavenger, reduced alanine levels to zero, indicating the involvement of free radicals. Free D-alanine was detected in (15) N NMR analysis of the chiral amino acid content, using D-tartaric acid as solvent. The accumulation of D-alanine under stress conditions presents a new perspective on the biochemical processes taking place in prokaryote and eukaryote cells.

Some Observations Regarding the SAFT-VR-Mie Equation of State

This study demonstrates that the advanced theoretical basis and the consequential numerical complexity do not always guarantee the success of EOS models in predicting the experimental thermodynamic property data. Although one of the best versions of SAFT, namely SAFT-VR-Mie might have doubtless advantages in predicting the data of non- spherical molecules, once again it is shown that there is a price to pay for the excessive models complexity. In particular, the present study reveals a previously unnoticed kind of numerical pitfalls, yet generated by the chain term of the SAFT- VR-Mie EOS. A possible way of avoiding the numerical pitfall under consideration is proposed.