Theodore L. Brown

Ligand steric properties

Abstract Methods of estimating the steric requirements of ligands are reviewed. The most widely employed measure is the cone angle, ⊝, first proposed by Tolman. Elaborations on the cone angle concept include mathematical methods for its estimation, estimates based on X-ray structural data, and solid cone angle measures. The ligand repulsive parameter, E R , is based upon molecular mechanics calculations of the structures of Cr(CO) 5 complexes of the various ligands. The ⊝ and E R parameters correlate reasonably well, but significant disparities are found among a large group of phosphorus, arsenic and nitrogen ligands for which both ⊝ and E R , values are available. Consideration of the various approaches to estimating ligand steric requirements indicates that each ligand has a range of steric requirements relative to other ligands, depending on the details of the particular complex or reaction involved. Applications of ligand steric requirements include quantitative linear free energy relationships. Given the relative imprecision with which the steric (and probably also the electronic) parameters can be determined, the use of additional parameters to account for the relative importances of σ and π bonding, or the existence of a steric threshold may not be justified by the number and variety of data available. Nevertheless, despite their lack of high precision, linear free energy relationships can provide important information regarding the electronic and steric demands of the transition state relative to the ground state in chemical reactions.

Interaction of alkyllithium compounds with base : Complex formation between ethyllithium and lithium ethoxide in hydrocarbon solvents

Abstract Freezing point lowering measurements have been carried out upon cyclohexane solutions containing mixtures of ethyllithium and lithium ethoxide. The results demonstrate that lithium ethoxide forms complexes with ethyllithium. The data are explained in terms of an initial coordination of two molecules of lithium ethoxide at two coordination sites of the hexamer. Subsequent attachment of ethoxide is presumed to occur by formation of clusters of lithium ethoxide at each of the two sites of binding to hexamer. The infrared and proton NMR data are consistent with this model. The 7 Li nuclear resonance spectra of solutions containing both ethyllithium and lithium ethoxide exhibit only one absorption, which broadens and shifts to higher field with increasing ethoxide concentration. A rapid exchange of lithium nuclei between ethyllithium and lithium ethoxide is indicated. Interaction of lithium ethoxide with ethyllithium does not appear to promote dissociation of the hexamer. In this respect, it differs from amines and ethers.

The Structures of Organolithium Compounds

Publisher Summary This chapter describes the structures of organolithium and related compounds. The implications of structural considerations for an understanding of the mechanisms of organolithium reactions are also discussed. The organolithium compounds are members of a larger class that is often referred to as electron deficient and includes Group II and III organometallic compounds, boron hydrides, and others. Electron deficiency is characterized by the formation of polymeric species through the delocalization of one or more bonding electron pairs. Organolithium compounds vary widely in reactivity, but all are vigorously reactive with both air and moisture. These compounds may also interact with bases by dissociating. Typical adducts formed in this way are proposed on the basis of kinetic or colligative property data obtained upon the addition of the base to hydrocarbon solutions of the alkyllithium compounds. Both alkyl and aryllithium compounds are capable of reacting with other organometallic compounds to form mixed species of well-defined stoichiometry that may or may not contain coordinated solvent. The chapter discusses the kinetics and mechanisms of the organolithium reactions, where the association of alkyllithium compounds with hydrocarbon solvents and the effect of bases on alkyllithium kinetics are discussed.

Photolysis of dirhenium decacarbonyl in the presence of simple olefins. Thermal reactivity of (.mu.-hydrido)(.mu.-alkenyl)dirhenium octacarbonyl compounds

Photolyse de Re 2 (CO) 10 a 255°C en presence de C 2 H 4 , C 3 H 6 ou de butenes-1 et -2 conduisant aux complexes (μ-H) (μ-alcenyl) Re 2 CO 8 avec de hauts rendements. Mecanisme: processus radicalaire produisant un intermediaire Re 2 (CO) 8 (η 2 -olefine) 2 conduisant par une reaction thermique aux complexes observes

Trimethylamine N-oxide promoted reactions of manganese, molybdenum and tungsten carbonyl complexes

Abstract Substitution reactions of PhMn(CO)5 and η5-C5H5M(CO)3X complexes (M = Mo or W; X = halide) with phosphines and arsines proceed rapidly at room temperature in the presence of (CH3)3NO to give cis-PhMn(CO)4L, or cis-η5-C5H5M(CO)2(L)X, respectively. Stereoselectivity, product yields, and reaction rates are dramatically enhanced by use of the amine oxide reagent.

Structures and reactivities of organolithium compounds

The characteristic species present in organolithium compounds under various conditions are reviewed. The effects of environment on the kinetics and energetics of exchange reactions are discussed. A summary is given of various pathways by which organolithium compounds might react. Kinetic and spectroscopic evidence for these processes under various conditions are reviewed. A discussion is presented of the significance of certain results obtained by chemically induced dynamic nuclear polarization (CIDNP).

Solvent effects on the aggregation of lithium bis(trimethylsilyl)amide

Abstract Concentration and temperature dependent 1H and 7Li NMR spectra have been evaluated for lithium bis(trimethylsilyl)amide, LiN[Si(CH3)]32, in ethers and hydrocarbon solvents. The results are interpreted in terms of a monomer-dimer equilibrium in THF and a dimer-tetramer equilibrium in hydrocarbon solvents. Thermodynamic parameters, ΔH0 = −4.0 kcal/mole, ΔS0 = −17 cal · deg−1 · mole−1, obtained for the equilibrium, dimer ⇄ 2 monomer, in THF solutions indicate that the monomer is preferentially solvated in solution. These conclusions are substantiated by isopiestic molecular weight determinations.

Models for the adsorption and reactions of metal carbonyl compounds on alumina surfaces

Abstract Reaction models are presented to account for the observed behaviors of metal carbonyl compounds on aluminas. Among the major conclusions of the work are: 1. The relatively stable Mo(CO) 3 (ads) species on alumina involves a strong interaction between the terminal CO of a mononuclear metal species and a coordinatively unsaturated Al 3+ site on the surface. 2. A detailed mechanistic scheme is offered to account for oxidation of the metal center accompanied by H 2 formation, via an oxidative addition of surface OH at the metal center. 3. A molybdenum intermediate which serves as an active olefin metahesis catalyst is proposed to be a Mo(IV) alkylidene species.

Reactions of Re2(CO)8[.mu.-(L-L)] (L-L = dppm, dmpm) and Re2(CO)7[.mu.-(L-L)]NCMe) with alkynes

Caracterisation des photoproduits suite a la reaction du compose du titre avec RC≡CH (R=H, Ph). On obtient ainsi: (μ-H)Re 2 (CO) 7 [μ(L-L)] (η 1 -C≡R); (μ-H)Re 2 (CO) 6 [μ-L-L)] (μ-D 1 , η D 2 -C≡CR); Re 2 (CO) 5 (L-L) (μ-η 1 , η 2 -C(R)=CH 2 ) (μ-η 1 , η 2 C≡CR) et Re 2 (CO) 6 [μ-(L-L)] (μ-η 1 , η 2 -C(R)=CH 2 ) (μ-η 1 , η 2 -C≡CR). Rendement. Etude cinetique. Mecanisme. Proprietes thermochimiques. Structure cristalline

The infrared carbonyl absorption in some p-quinones and related substances

Abstract The integrated intensity of the infrared carbonyl absorption in a number of quinones and related compounds has been measured. In some of the compounds the absorption is a single, symmetrical band, while in others it possesses a more complex structure. From a comparison of the integrated intensities it appears that the cause of the major splitting is a Fermi-resonance interaction. A further splitting in benzoquinone is attributed to the fact that at room temperature the low-frequency out-of-plane carbonyl bending mode is excited in an appreciable fraction of the molecules.