Andreas Mix

Reversible Base Coordination to a Disilene

During the last few decades, alkene and alkyne analogues of the heavier Group 14 elements have attracted considerable interest. Their isolation as stable derivatives has become possible by the use of carefully designed bulky substituents that provide kinetic (and to some extent thermodynamic) stabilization. The considerable differences in structure, bonding, and reactivity of such compounds in comparison to the carbon-based species have prompted various experimental and theoretical studies. By and large, the lower electronegativity of heavier elements and the increasing spatial extension of their valence electron shells are responsible for many of these differences. One of several rationalizations for structure and reactivity of such doubly and triply bonded species is based on zwitterionic (Ib and IIb in Scheme 1) and

Reversible Transformation of a Stable Monomeric Silicon(II) Compound into a Stable Disilene by Phase Transfer: Experimental and Theoretical Studies of the System {[(Me3Si)2N](Me5C5)Si }n with n = 1,2

The salt (eta(5)-pentamethylcyclopentadienyl)silicon(II) tetrakis(pentafluorophenyl)borate (5) reacts at -78 degrees C with lithium bis(trimethylsilyl)amide in dimethoxyethane (DME) as solvent to give quantitatively the compound [bis(trimethylsilyl)amino][pentamethylcyclopentadienyl]silicon(II) 6A in the form of a colorless viscous oil. The reaction performed at -40 degrees C leads to the silicon(IV) compound 7, the formal oxidative addition product of 6A with DME. Cycloaddition is observed in the reaction of 6A with 2,3-dimethylbutadiene to give the silicon(IV) compound 8. Upon attempts to crystallize 6A from organic solvents such as hexane, THF, or toluene, the deep yellow compound trans-1,2-bis[bis(trimethylsilyl)amino]-1,2-bis(pentamethylcyclopentadienyl)disilene (6B), the formal dimer of 6A, crystallizes from the colorless solution, but only after several days or even weeks. Upon attempts to dissolve the disilene 6B in the described organic solvents, a colorless solution is obtained after prolonged vigorous shaking or ultrasound treatment. From this solution, pure 6A can be recovered after solvent evaporation. This transformation process can be repeated several times. In a mass spectroscopic investigation of 6B, Si=Si bond cleavage is observed to give the molecular ion with the composition of 6A as the fragment with the highest mass. The X-ray crystal structure analysis of the disilene 6B supports a molecule with a short Si=Si bond (2.168 A) with efficiently packed, rigid sigma-bonded cyclopentadienyl substituents and silylamino groups. The conformation of the latter does not allow electron donation to the central silicon atom. Theoretical calculations at the density functional level (RI-BP86 and B3LYP, TZVP basis set) confirm the structure of 6B and reveal for silylene 6A the presence of an eta(2)-bonded cyclopentadienyl ligand and of a silylamino group in a conformation that prevents electron back-donation. Further theoretical calculations for the silicon(II) compound 6A, the disilene 6B, and the two species 11 and 11* derived from 6A (which derive from Si=Si bond cleavage) support the experimental findings. The reversible phase-dependent transformation between 6A and 6B is caused by (a) different stereoelectronic and steric effects exerted by the pentamethylcyclopentadienyl group in 6A and 6B, (b) some energy storage in the solid state structure of 6B (molecular jack in the box), (c) a small energy difference between 6A and 6B, (d) a low activation barrier for the equilibration process, and (e) the gain in entropy upon monomer formation.

Role of the ssu and seu Genes of Corynebacterium glutamicum ATCC 13032 in Utilization of Sulfonates and Sulfonate Esters as Sulfur Sources

ABSTRACT Corynebacterium glutamicum ATCC 13032 was found to be able to utilize a broad range of sulfonates and sulfonate esters as sulfur sources. The two gene clusters potentially involved in sulfonate utilization, ssuD1CBA and ssuI-seuABC-ssuD2, were identified in the genome of C. glutamicum ATCC 13032 by similarity searches. While the ssu genes encode proteins resembling Ssu proteins from Escherichia coli or Bacillus subtilis, the seu gene products exhibited similarity to the dibenzothiophene-degrading Dsz monooxygenases of Rhodococcus strain IGTS8. Growth tests with the C. glutamicum wild-type and appropriate mutant strains showed that the clustered genes ssuC, ssuB, and ssuA, putatively encoding the components of an ABC-type transporter system, are required for the utilization of aliphatic sulfonates. In C. glutamicum sulfonates are apparently degraded by sulfonatases encoded by ssuD1 and ssuD2. It was also found that the seu genes seuA, seuB, and seuC can effectively replace ssuD1 and ssuD2 for the degradation of sulfonate esters. The utilization of all sulfonates and sulfonate esters tested is dependent on a novel putative reductase encoded by ssuI. Obviously, all monooxygenases encoded by the ssu and seu genes, including SsuD1, SsuD2, SeuA, SeuB, and SeuC, which are reduced flavin mononucleotide dependent according to sequence similarity, have SsuI as an essential component. Using real-time reverse transcription-PCR, the ssu and seu gene cluster was found to be expressed considerably more strongly during growth on sulfonates and sulfonate esters than during growth on sulfate.

Solid‐State Structure of a Li/F Carbenoid: Pentafluoroethyllithium

Lithium carbenoids are versatile compounds for synthesis owing to their intriguing ambiphilic behavior. Although this class of compounds has been known for several years, few solid-state structures exist because of their high reactivity and often low thermal stability. Using cryo X-ray techniques, we were now able to elucidate the first solid-state structure of a Li/F alkyl carbenoid, pentafluoroethyllithium (LiC2F5), finally yielding a prototype for investigating structure-reactivity relationships for this class of molecules. The compound forms a diethyl ether-solvated dimer bridged by a rare C-F-Li link. Complementary NMR spectroscopy studies in solution show dynamic processes and indicate rapid exchange of starting material and product. Theoretical investigations help to understand the formation of the observed unusual structural motif.

Stabilization of Organosilicenium Ions by means of Intramolecular Coordination of O, S, or P Ligands

For the intramolecular stabilization of silicenium ions (R3Si+), O, S, and P donors as well as the known nitrogen-containing systems (as in 1 a) are suitable. The silyl cations in 1 a–d show a trigonal-bipyramidal structure; dynamic processes can be proved by NMR spectroscopy for 1 c, d. Calculations on model compounds document substantial differences in the bonding relationships and support the structural findings. Furthermore, preliminary experiments with 1 b–d indicate significant differences in reactivity.

The Pentamethylcyclopentadienylsilicon(II) Cation as a Catalyst for the Specific Degradation of Oligo(ethyleneglycol) Diethers

Covalent silicon(IV) compounds of the type R3SiX (R= organic substituent, X= electronegative group) have long been used as catalysts for several organic reactions, mainly in carbon–carbon bond-forming processes (such as Diels–Alder reactions, aldol condensations). Some ionic silicon(IV) compounds containing R3Si + cations have also been applied as catalysts in C C bond formation reactions and in some other transformations. The most important examples are collected in Scheme 1.

CH Activation versus Yttrium–Methyl Cation Formation from [Y(AlMe4)3] Induced by Cyclic Polynitrogen Bases: Solvent and Substituent‐Size Effects

The reaction of 1,3,5-triisopropyl-1,3,5-triazacyclohexane (TiPTAC) with [Y(AlMe(4))(3)] resulted in the formation of [(TiPTAC)Y(Me(3)AlCH(2)AlMe(3))(μ-MeAlMe(3))] by C-H activation and methane extrusion. In contrast, the presence of bulkier cyclohexyl groups on the nitrogen atoms in 1,3,5-tricyclohexyl-1,3,5-triazacyclohexane (TCyTAC) led to the formation of the cationic dimethyl complex [(TCyTAC)(2)YMe(2)][AlMe(4)]. The investigations reveal a dependency of the reaction mechanism on the steric bulk of the N-alkyl entity and the solvent employed. In toluene C-H activation was observed in reactions of [Y(AlMe(4))(3)] with 1,3,5-trimethyl-1,3,5-triazacyclohexane (TMTAC) and TiPTAC. In THF molecular dimethyl cations, such as [(TCyTAC)(2)YMe(2)][AlMe(4)], [(TMTAC)(2)YMe(2)][AlMe(4)] and [(TiPTAC)(2)YMe(2)][AlMe(4)], could be synthesised by addition of the triazacyclohexane at a later stage. The THF-solvated complex [YMe(2)(thf)(5)][AlMe(4)] could be isolated and represents an intermediate in these reactions. It shows that cationic methyl complexes of the rare-earth metals can be formed by donor-induced cleavage of the rare-earth-metal tetramethylaluminates. The compounds were characterised by single-crystal X-ray diffraction or multinuclear and variable-temperature NMR spectroscopy, as well as elemental analyses. Variable-temperature NMR spectroscopy illustrates the methyl group exchange processes between the cations and anions in solution.

Mechanism of Host–Guest Complex Formation and Identification of Intermediates through NMR Titration and Diffusion NMR Spectroscopy

The formation of host-guest (H-G) complexes between 1,8-bis[(diethylgallanyl)ethynyl]anthracene (H) and the N-heterocycles pyridine and pyrimidine (G) was studied in solution using a combination of NMR titration and diffusion NMR experiments. For the latter, diffusion coefficients of potential host-guest structures in solution were compared with those of tailor-made reference compounds of similar shape (synthesized and characterized by NMR, HRMS, and in part XRD). Highly dynamic behavior was observed in both cases, but with different host-guest species and equilibria. With increasing concentrations of the pyridine guest, the equilibrium H2⇄H2κ(1)-G1⇄HG2 is observed (in the second step a host dimer coordinates one guest molecule); for pyrimidine the equilibrium H2→H1κ(2)-G1⇄HG2 is observed (the formation of a 1:1 aggregate is the second step).

A Neutral Silicon/Phosphorus Frustrated Lewis Pair.

Frustrated Lewis pairs (FLPs) have a great potential for activation of small molecules. Most known FLP systems are based on boron or aluminum atoms as acid functions, few on zinc, and only two on boron-isoelectronic silicenium cation systems. The first FLP system based on a neutral silane, (C2F5)3SiCH2P(tBu)2 (1), was prepared from (C2F5)3SiCl with C2F5 groups of very high electronegativity and LiCH2P(tBu)2. 1 is capable of cleaving hydrogen, and adds CO2 and SO2. Hydrogen splitting was confirmed by H/D scrambling reactions. The structures of 1, its CO2 and SO2 adducts, and a decomposition product with CO2 were elucidated by X-ray diffraction.

Desulfation Followed by Sulfation: Metabolism of Benzylglucosinolate in Athalia rosae (Hymenoptera: Tenthredinidae)

The sawfly species Athalia rosae (L.) (Hymenoptera: Tenthredinidae) is phytophagous on plants of the family Brassicaceae and thus needs to cope with the plant defence, the glucosinolate–myrosinase system. The larvae sequester glucosinolates in their haemolymph. We investigated how these compounds are metabolized by this specialist. When larvae were fed with ([14C]‐labelled) benzylglucosinolate, one major degradation metabolite, with the same sum formula as benzylglucosinolate, was defecated. This metabolite was also found in the haemolymph along with desulfobenzylglucosinolate, which continuously increased in concentration. NMR spectroscopy in conjunction with LC‐TOF‐MS measurements revealed the major degradation metabolite to be desulfobenzylglucosinolate‐3‐sulfate, probably converted from desulfobenzylglucosinolate after sulfation at the sugar moiety. The enzymes responsible must be located in the haemolymph. Additionally, a putative sulfotransferase forms benzylglucosinolate sulfate in the gut from intact, non‐sequestered glucosinolate. The corresponding desulfoglucosinolate sulfates were also detected in faeces after feeding experiments with phenylethylglucosinolate and prop‐2‐enylglucosinolate, which indicates a similar degradation mechanism for various glucosinolates in the larvae. This is the first report on glucosinolate metabolism of a glucosinolate‐sequestering insect species.