Controlling the Coordination Sphere of Alkyllithiums Results in Selective Reactions with Allylic Amines.

Described herein is a selective way to control the reaction of allylic amines with metalorganic bases depending on the amine handle as well as the metalorganic base is used. Depending on the number of coordinating groups within the amine handle either a selective carbometalation or deprotonation reaction can be performed. By changing the alkali metal within the base from lithium to either sodium or potassium, a change of chemoselectivity takes place and the reaction of piperidinoallylamine can be controlled.

A diastereomerically enriched, dimeric organolithium compound and the stereochemical course of its transformations.

A dimeric α-silylated ethyllithium compound is presented featuring stereogenic metalated carbon centres with defined configurations. It can be generated by diastereoselective deprotonation of the corresponding ethylsilane. Reaction with Me(3)SnCl proceeds under inversion and the transfer of the stereoinformation is possible with dr's of up to 97:3.

Structure/reactivity studies on an alpha-lithiated benzylsilane: chemical interpretation of experimental charge density.

Modern organic synthesis (e.g., of natural products) is virtually impossible without employment of enantiomerically enriched compounds. In many cases, alkyllithium compounds are key intermediates for the generation of these stereogenic substances. In recent years, the lithiated carbon atom in silicon-substituted benzyllithium compounds has become a focus of interest because it is possible to maintain its stereogenic information. Starting from a highly enantiomerically enriched benzylsilane, (R,S)-2 x quinuclidine could be obtained, and the absolute configuration at the metalated carbon atom was determined by X-ray diffraction analysis. In solution, a quartet was found in the (13)C NMR spectrum for the metalated carbon atom because of coupling between carbon and lithium, indicating a fixed lithium carbon contact at room temperature. After reaction of (R,S)-2 x quinuclidine with trimethylchlorostannane, the trapped product (S,S)-4 was obtained with a dr > or = 98:2 with inversion of the configuration at the metalated carbon. Multipole refinement against high-resolution diffraction data and subsequent topological analysis of the benchmark system (R,S)-2 x quinuclidine provide insight in the electronic situation and thus the observed stereochemical course of the transformations. Surprisingly, the negative charge generated at the carbanion hardly couples into the phenyl ring. The neighboring silicon atom counterbalances this charge by a pronounced positive charge. Therefore, the alpha-effect of the silicon atom is caused not just by a polarization of the electron density but also by an electrostatic bond reinforcement. Furthermore, the experimentally determined electrostatic potential unequivocally explains the observed back side attack of an electrophile under inversion of the stereogenic center with high diastereomeric ratios.

Selective Si--C bond cleavage as synthetic entry to a functionalized lithiosilane.

Treatment of the phenyl-substituted silane 4 with lithium metal afforded the functionalized lithiosilane rac-2 by selective cleavage of one Si-C bond between silicon and a phenyl group. The resulting lithiosilane rac-2 crystallizes as the dimer (2.THF)2, which represents the first example of a dimeric organyl-substituted lithiosilane in the presence of THF.

The crystal structures of a chiral aminoalkoxide cluster and its adduct with benzyllithium.

The crystal structures of the chiral aminoalkoxide cluster (S)-2 and its unexpected adduct with benzyllithium (S)-3 have been determined. For compound (S)-2, a hexameric cyclic ring-ladder is observed in the solid state, which leads to an unshielded coordination site in a chiral pocket that is only accessible from one direction. The presence of (S)-2 leads to the deprotonation of toluene by n-butyllithium, giving benzyllithium. In contrast to earlier studies on lithium alkoxides, the resulting adduct (S)-3 between benzyllithium and (S)-2 is not formed by the exchange of alkoxy groups for alkanide units, as has been observed for a multitude of examples. Compound (S)-3 therefore represents a thus far unconsidered type of structure: the unshielded "top side" of the aminoalkoxide cluster (S)-2 capped by the lithium center of benzyllithium via three Li-O interactions.

The crystal structures of the chiral alkyllithium bases [n-BuLi.(-)-sparteine]2 and [Et2O.(i-PrLi)2.(-)-sparteine].

The crystal structures of the two chiral alkyllithium bases [n-BuLi.(-)-sparteine]2 (1) and [Et2O.(i-PrLi)2.(-)-sparteine] (2) have been determined. For compound 1, a symmetric dimer is observed in the solid state, with two (-)-sparteine ligands coordinating to the lithium centers. Because of steric reasons, compound 2 crystallizes as an unsymmetric dimer with the four methyl groups pointing away from the sterically demanding (-)-sparteine ligand. Compound 2 contains one four-coordinate lithium center [coordinated to (-)-sparteine] and one three-coordinate lithium center (coordinated to Et2O). As a result of this arrangement, significantly different Li-C distances are found in the central four-membered ring of compound 2.

Bis[[diphenyl(piperidinomethyl)silyl]methyl]cadmium and -magnesium.

The homoleptic and monomeric metal alkyls bis[[diphenyl(piperidinomethyl)silyl]methyl]cadmium, [Cd(C(19)H(24)NSi)(2)] or [Cd[CH(2)SiPh(2)(CH(2)NC(5)H(10))](2)], (I), and bis[[diphenyl(piperidinomethyl)silyl]methyl]magnesium, [Mg(C(19)H(24)NSi)(2)] or [Mg[CH(2)SiPh(2)(CH(2)NC(5)H(10))](2)], (II), (CH(2)NC(5)H(10) is piperidinomethyl) are isostructural, and the molecules exhibit crystallographically imposed C(2) symmetry. The metal centres are located on special positions and, for each structure, half of the molecule is located in the asymmetric unit. The metal centres are intramolecularly coordinated and stabilized by two piperidinomethyl groups (side-arm donation). The Si-C and M-C bonds (M is Cd or Mg) are shortened compared with the corresponding non-metallated compounds, indicating stabilization by the diffuse polarizable Si centres (alpha effect). The C-M-C angle is 140.53 (12) degrees in (I) and 123.39 (11) degrees in (II).