Omar W. Steward

Lithiation of tris(trimethylsilyl)methane and tetrakis-(trimethylsilyl)methane

Abstract Lithiation of (Me 3 Si) 3 CH by methyllithium (ether-THF) yields (Me 3 Si) 3 CLi and by t-butyllithium (C 5 H 12 -TMEDA) yields (Me 3 Si) 2 CHSiMe 2 CH 2 Li. Only starting material is recovered when (Me 3 Si) 3 CH is allowed to react with n-butyl- lithium (ether-THF and C 5 H 12 -TMEDA) and t-butyllithium (C 5 H 12 and C 5 H 12 - THF). (Me 3 Si) 4 C is lithiated by t-butyllithium (C 5 H 12 -TMEDA) to give (Me 3 Si) 3 - CSiMe 2 CH 2 Li, but not by methyllithium (ether-THF and ether-THF-TMEDA). The structures of the lithiated compounds are based on the carbonation products. The above results are explained in terms of carbanion stability and steric effects. Spectral data are reported on the α-silylacetic acids.

Rearrangements of organosilicon compounds I. The migratory aptitudes of various organic groups in the aluminum chloride-catalyzed rearrangement of (chloromethyl)triorganosilanes

Abstract The migratory aptitudes of various organic groups in the aluminum chloride- catalyzed rearrangement of (chloromethyl)triorganosilanes have been determined: Me, 1.00; Et, 2.07; n-Pr, 2.97; iso-Pr, ⪡ 1.00;ClCH2, ⪡ 1.00. Migration of the organic group from silicon to carbon is facilitated by electron-release and inhibited, relative to methyl, by branching at the α-carbon. The migratory aptitudes determined in this study and obtained by other workers are discussed in terms of a mechanism involving synchronous intrmolecular migration of the organic group and the chloride ion.

Metalated nitriles: internal 1,2-asymmetric induction.

Alkylations of conformationally constrained acyclic nitriles containing vicinal dimethyl groups and an adjacent phenyl group or trisubstituted alkene are exceptionally diastereoselective. Probing the alkylation stereoselectivity with a series of C- and N-metalated nitriles implicates a reactive conformation in which an sp2-hybridized substituent projects over the metalated nitrile to avoid allylic strain. Steric screening thereby directs the electrophilic attack to the face of the metalated nitrile opposite the projecting substituent. Excellent stereoselectively is maintained in a diverse range of alkylations that efficiently install quaternary centers, even with isopropyliodide in which a contiguous array of tertiary-quaternary-tertiary stereocenters is created! Screening the conformational requirements with a series of acyclic nitriles and esters reveals the key structural requirements for high selectivity while providing a robust, predictive model that accounts for comparable ester alkylations affording the opposite diastereomer! The intensive survey of metalated nitrile alkylations identifies the key structural features required for high 1,2-asymmetric induction, addresses the long-standing challenge of asymmetric alkylations with acylic metalated nitriles, and provides a versatile method for installing hindered quaternary centers with excellent stereocontrol.

Carboxy-silanes and -germanes: IV. Synthesis and properties of triorganosilyl derivatives☆

Abstract β-Triorganosilyl derivatives of silane- and germane-carboxylic acids are reported for the first time. Studies are reported on their spectral properties (IR, UV, PMR), acidity and thermolysis. No evidence has been found in support of extended d -orbital participation, (π → d → d )π bonding, in the ground state of the polysilanecarboxylic acids and their conjugate base forms. ( d → d )π bonding may be an important factor in lowering the activation energy for the thermal decarbonylation of pentaphenyldisilanecarboxylic acid.

Ionization constants of hydroxy compounds of carbon, silicon and germanium. The novel acidity of the compounds, Ph3MM′Ph2OH

Abstract The hydroxy compounds, Ph 3 MM′Ph 2 OH, (where M and M′  C, Si or Ge) are more acidic by 1.5 to 2.7 pK a units than the compounds, Ph 3 MOH, in dimethyl sulfoxide. This enhanced acidity which is not reflected in the hydrogen bonding studies is explained in terms of an n o → LUMO MM ′ interaction in the conjugate base form.

Magneto-Structural Correlations in Dimeric Copper(II) Carboxylates

Abstract For dimeric copper(II) carboxylates, the most important factor determining the strength of the spin superexchange interactions is the electronic structure of the carboxylato bridge. It also has been revealed that the trigonal bipyramidal deformation of the coordination geometry around the copper atoms, and the bending of the carboxylato bridge decrease the antiferromag-netic interactions.

Alkynenitriles: stereoselective chelation controlled conjugate addition–alkylations

Abstract Chelation-controlled conjugate addition of Grignard reagents to γ-hydroxyalkynenitriles stereoselectively generates tri- and tetra-substituted alkenenitriles. t -BuMgCl-initiated deprotonation of hydroxyalkynenitriles followed by addition of a second Grignard reagents triggers a facile conjugate addition leading to a cyclic magnesium chelate. Protonation of the chelate stereoselectively generates trisubstituted nitriles whereas the addition of t -BuLi causes conversion to an ‘ate’ complex that allows alkylation with aldehyde electrophiles. The chelation-controlled conjugate addition–alkylation generates tri- and tetra-substituted alkenenitriles that are otherwise difficult to synthesize.

β-Siloxy unsaturated nitriles: stereodivergent cyclizations to cis- and trans-decalins

Abstract Nitrile and enolate anions exhibit divergent intramolecular cyclization stereoselectivities. Enolates cyclize to cis-decalones, whereas nitrile anions are predisposed to pyramidalize, cyclizing instead through a less-congested conformation to trans-decalins. Conjugation of nitrile anions with adjacent sp2 centers prevents pyramidalization, and redirects cyclization through a planar-delocalized anion to cis-decalins. Collectively these cyclizations allow conversion of a single β-siloxynitrile to either cis- or trans-decalins that are ideally suited for elaboration into terpenoid natural products.

New mechanisms for the base-catalyzed cleavage of SiSi bonds in organopolysilanes: the base-catalyzed solvolysis of pentaphenyldisilanecarboxylic acid and pentaphenyldisilanol in ethanol/water media

A kinetic investigation of the base-catalyzed decomposition of pentaphenyldisilanecarboxylic acid (1) and pentaphenyldisilanol (2) in ethanol/water media is reported. The solvolysis of the SiSi bond in 2, which also is formed on the base-catalyzed decarbonylation of 1, proceeds by concurrent first-order and second-order processes. At low base concentrations where the first-order process predominates, the intermediate, triphenylsilane (3), has been isolated. Solvent isotope effects and activation parameters have been determined. Mechanisms are proposed for the two kinetically distinguishable processes for SiSi bond cleavage in which the pentaphenyldisilanolate ion undergoes either an internal nucleophilic displacement reaction or nucleophilic attack at Si by base in the rate-determining step. A general mechanistic approach for the cleavage of Si bonds in polysilanes by aqueous-alcoholic base is proposed in which polysilanolate ions are formed by nucleophilic attack by base at Si which undergo internal nucleophilic attack resulting in cleavage of the SiSi bond adjacent to the anionic termini. Subsequently, polysilanolate ions are regenerated in which the number of Si atoms is reduced by one.

A kinetic investigation of the role of solvent in the alkali-catalyzed hydrolysis of organosilicon hydrides

Abstract The role of solvent in the alkali-catalyzed hydrolysis of organosilicon hydrides, specifically, 3-[(2-methoxy)ethoxy]propyldimethylsilane, has been investigated kinetically in dioxane/water media by varying the solvent composition. Statistical analysis of the kinetic data indicates that approximately three water molecules are involved in the transition state of the rate-determining step. The mechanistic implications are discussed.