Tuning Reactivities of tert-Butyllithium by the Addition of Stoichiometric Amounts of Tetrahydrofuran.

In alkyllithium chemistry the highest reactivity has historically been linked to the smallest degree of aggregation possible. Since tert-butyllithium is known to form a monomer in tetrahydrofuran solution, using just stoichiometric amounts of the lewis base to selectively form a dimeric species seemed irrational. In this study, we showed a considerable increase of the reactivity of t-BuLi when using stoichiometric amounts of THF in the non-polar solvent n-pentane in order to enable the deprotonation of simple methyl silanes and other low C-H-acidic substrates. In this context, we were able to obtain the corresponding aggregates of t-BuLi with the ligand THF in suspension and as crystalline solids and investigate them by single crystal X-ray structural analysis, in situ FTIR spectroscopy and quantum chemical calculations. Furthermore, we were able to explain the enhanced reactivity of t-BuLi with stoichiometric amounts of THF on the basis of structural features of the bridged dimer obtained under these conditions. With these findings, we present a new target in the aggregation of alkyllithium reagents: the selectively formed "frustrated" aggregates!

Comprehensive Study of the Enhanced Reactivity of Turbo-Grignard Reagents.

Since its introduction in 2004, Knochel's so called Turbo-Grignard reagents revolutionized the usage of Grignard reagents. Through the simple addition of LiCl to a magnesium alkyl an outstanding increase in reactivity can be achieved. Though the exact composition of the reactive species remained mysterious, the reactive mixture itself is readily used not only in synthesis but also found its way into more distant fields like material science. To unravel this mystery, we combined single-crystal X-ray diffraction with in-solution NMR-spectroscopy and closed our investigations with quantum chemical calculations. Using such a variety of methods, we have gained insight into and an explanation for the extraordinary reactivity of this extremely convenient reagent by determining the structure of the first bimetallic reactive species [t-Bu2 Mg ⋅ LiCl ⋅ 4 thf] with two tert-butyl anions at the magnesium center and incorporated lithium chloride.

Weak yet Decisive: Molecular Halogen Bond and Competing Weak Interactions of Iodobenzene and Quinuclidine.

The halogen bonded adduct between the commonly used constituents quinuclidine and iodobenzene is based on a single weak nitrogen-iodine contact, and the isolation of this adduct was initially unexpected. Iodobenzene does not contain any electron-withdrawing group and therefore represents an unconventional halogen bond donor. Based on excellent diffraction data of high resolution, an electron density study was successfully accomplished and confirmed one of the longest N···I molecular halogen bonds with a distance of 2.9301(4) Å. The topological analysis identified the XB as a directional but weak σ hole interaction and revealed secondary contacts between peripheral regions of opposite charge. These additional contacts and their competition with a nitrogen-based interaction were confirmed by NOESY experiments in solution. Integration enabled us to determine the relative NOE ratios and provided insight regarding the existing interactions.

The Halogen Bond to Ethers - Prototypic Molecules and Experimental Electron Density.

Halogen bonds to dialkyl ether molecules have remained largely unexplored. We here address the synthesis and the structural chemistry of the first halogen-bonded noncyclic alkyl ethers, combining 1,4-diiodotetrafluorobenzene and the prototypic or commonly used ethers dimethyl ether, tetrahydrofuran, and methyl-tert-butyl ether as halogen acceptors. Two different structural motifs based on moderately strong halogen bonds were obtained: Discrete trimolecular aggregates are formed, and unexpected halogen-bonded supramolecular chain adducts feature oxygen-bifurcated halogen bonds with 1:1 donor:acceptor ratio. Both structure types may be selectively obtained even for the same ether by adjusting the stoichiometry in the crystallization experiments. The geometric features of the etheric oxygen center were found to be flexible, in contrast to the almost linear geometry about the halogen donor atom. A high-resolution X-ray diffraction experiment on the extended adduct of dimethyl ether allowed us to study the electronic details of the acceptor-bifurcated I···O···I halogen bonds. The electron density in the bond critical points and derived properties such as the Laplacian indicate essentially electrostatic interactions and explain the geometrical flexibility of ethers in halogen bonds. Our studies demonstrate the great versatility of ethers as halogen bond acceptors, that can occur in many geometrical arrangements and whose contribution to nature's structural designs should not be underestimated.

Gauging the Strength of the Molecular Halogen Bond via Experimental Electron Density and Spectroscopy.

Strong and weak halogen bonds (XBs) in discrete aggregates involving the same acceptor are addressed by experiments in solution and in the solid state. Unsubstituted and perfluorinated iodobenzenes act as halogen donors of tunable strength; in all cases, quinuclidine represents the acceptor. NMR titrations reliably identify the strong intermolecular interactions in solution, with experimental binding energies of approx. 7 kJ/mol. Interaction of the σ hole at the halogen donor iodine leads to a redshift in the symmetric C-I stretching vibration; this shift reflects the interaction energy in the halogen-bonded adducts and may be assessed by Raman spectroscopy in condensed phase even for weak XBs. An experimental picture of the electronic density for the XBs is achieved by high-resolution X-ray diffraction on suitable crystals. Quantum theory of atoms in molecules (QTAIM) analysis affords the electron densities and energy densities in the bond critical points of the halogen bonds and confirms stronger interaction for the shorter contacts. For the first time, the experimental electron density shows a significant effect on the atomic volumes and Bader charges of the quinuclidine N atoms, the halogen-bond acceptor: strong and weak XBs are reflected in the nature of their acceptor atom. Our experimental findings at the acceptor atom match the discussed effects of halogen bonding and thus the proposed concepts in XB activated organocatalysis.

Crystal structures of [(N,N-di-methyl-amino)-meth-yl]ferrocene and (R p,R p)-bis-{2-[(di-methyl-amino)-meth-yl]ferrocen-yl}di-methyl-silane.

The title compound [(N,N-di-methyl-amino)-meth-yl]ferrocene, [Fe(C5H5)(C8H12N)], (1), is an inter-esting starting material for the synthesis of planar chiral 1,2-disubstituted ferrocenes, as demonstrated by the preparation of (R p,R p)-bis-{2-[(di-methyl-amino)-meth-yl]ferrocen-yl}di-methyl-silane, [Fe2(C5H5)2(C18H18N2Si)], (2), from the li-thia-ted derivative of 1. The configuration of the lithium compound is unchanged after the substitution reaction and the chirality is preserved in space group P212121. In both compounds, the Cp rings adopt eclipsed conformations. Hirshfeld surface analysis was used to investigate the inter-molecular inter-actions, and showed that H⋯H (van der Waals) inter-actions dominate in both structures with contact percentages of 83.9 and 88.4% for 1 and 2, respectively.