Andrew Streitwieser, Jr. - US grants

Carbanions, generally involved as ion pairs or aggregates, are important intermediates in the organic synthesis of many biologically active natural products, pharmaceuticals, drugs, etc. A common synthetic procedure is the metallation of a substrate with an alkyl lithium or lithium amide in an ether followed by reaction with a suitable electrophile. We propose to put this largely empirical technique on quantitative scientific basis by continuing our determination of equilibrium constants for the reactions, R M + R'H = R'M + RH, for indicator hydrocarbons (e.g., fluorenes, arylmethanes, etc.) in tetrahydrofuran (THF) with cesium and lithium as gegenions. These indicator systems will be used to determine equilibria for carbon acids with various functional groups important in synthesis: vinyl, allyl, benzyl and aryl compounds, sulfur derivatives, silicon and phosphorus compounds, amides, ethers, heterocyclic compounds dianions and enolate ions. Determinations as a function of temperature will provide thermodynamic quantities and comparison of the cesium equilibria, which are known to involve contact ion pairs, with the corresponding lithium equilibria, in which a variety of types of ion pairs are possible, will facilitate understanding of the more complex lithium cases. Conductivity studies will relate the ion pairs to dissociated ions. The results will be interpreted with the help of related kinetic acidity measurements, studies of aggregated systems, theoretical quantum chemical calculations and computer graphics analysis of electron density distributions.

Carbanions, generally involved as ion pairs or aggregates, are important intermediates in the organic synthesis of many biologically active natural products, pharmaceuticals, drugs, etc. A common synthetic procedure is the metallation of a substrate with an alkyllithium or lithium amide in an ether followed by reaction with a suitable electrophile. We propose to put this largely empirical technique on a quantitative scientific basis by continuing our determination of equilibrium constants for the reactions, R-M+ + R'H = R'-M+ + RH, for indicator hydrocarbons (e.g., fluorenes, arylmethanes, etc.) in tetrahydrofuran (THF) with cesium and lithium as gegenions. These indicator systems will be used to determine equilibria for carbon acids with various functional groups important in synthesis. Important emphasis will be placed on functional groups used to control regio- and stereochemistry in synthesis of biologically active compounds and natural products, such as oxime ethers and dimethylhydrazone derivatives of carbonyl compounds. The study will include other carbonyl compounds and derivatives such as amides and oxazolines as well as sulfur, selenium and silicon derivatives, vinyl, allyl, benzyl and aryl compounds heterocyclic compounds, and related dianions such as oxime dianions. Determinations as a function of concentration will provide quantitative estimates of the degree of aggregation. For those systems shown to exist as monomer and dimer ion pairs, kinetic studies of alkylation and condensation will be accomplished as a function of concentration to show the role of both species in such reactions. Equilibrium measurements as a function of temperature will provide thermodynamic quantities and comparison of the cesium equilibria, which are known to involve contact ion pairs, with the corresponding lithium equilibria, in which a variety of types of ion pairs are possible, will facilitate understanding of the more complex lithium cases. Conductivity studies will relate the ion pairs to dissociated ions. The results will be interpreted with the help of related kinetic acidity measurements, studies of aggregated systems, theoretical quantum chemical calculations and computer graphics analysis of electron density distributions. The theoretical studies include ab initio computations of structure and reaction paths.

This proposal is concerned with the selectivity problems encountered in the synthetic application of hydrogen reactions like face- and cis-selective hydrogenation, cis-selective semi-hydrogenation, selective hydrogenation of polyenes, and selective hydrogenation of functionalized olefins. The surface structure sensitivity and demanding nature of these hydrogenation reactions and their associated side reactions will be identified. A Pd-membrane reactor will be used to differentiate between surface hydrogen and sub-surface hydrogen as active species in hydrogenation reactions. The mechanistic insight obtained will guide the development of new experimental techniques for more selective hydrogenation reactions. Thin film materials of ordered surface structure will be prepared, characterized, and tested as new prototypes of selective hydrogenation catalysts.

This grant in the Organic and Macromolecular Chemistry Program supports experimental and theoretical research by Prof. Andrew Streitwieser, University of California at Berkeley, aimed at analyzing the chemical structure and reactivity of carbanions. Carbanions are negatively charged fragments of organic molecules such as hydrocarbons and occur as transient intermediates during a wide variety of chemical reactions in the laboratory, in the body, and in industrial chemical processes. Thus, the chemical reactivity of carbanions may be said to determine the path of many chemical changes. Dr. Streitwieser's research will include measurements of kinetic acidities of weak carbon acids by the study of hydrogen isotope exchange rates, including benzene in aqueous sodium hydroxide and cyclic hydrocarbons with lithium and cesium cyclohexylamide in cyclohexylamine. Equilibrium acidity studies will be continued, particularly for dianions and for carbanions not stable in THF. Examples of the latter include pentafluorobenzene and other halogenated hydrocarbons. Following observations of low second pKa's for arylacetic acids, measurements of concentration effects will be made to analyze effects of aggregation of both mono- and di-salts. Theoretical work will focus on ab initio computations of electronic structures and geometries of carbanions and their lithium and sodium salts. Calculations will also be used to estimate the transition structures of carbanion reactions, with particular emphasis on the previously noted systems, and on metallated hydrazones, oximes, and imines whose important stereochemical properties are not yet well understood.

A new series of polymers has been discovered based on polyvinylpyridine containing pendent allyl radicals and anions fully substituted with pyridinium cation groups. These "densely charged" polymers show interesting properties. They are remarkably stable. The dry polyallyl anions are not conducting, but the polyradicals have electrical conductivity in the semiconductor range. A new polymeric structure is proposed, which will be prepared and may have interesting conduction and photoconduction properties. The polymers may also have specific ion-exchange properties. The research deals with the preparation and characterization of a number of such polymers with varying degrees of crosslinking. The electrical properties of these polymers will be tested, and an important part of these tests will be their behavior as electrode-coating materials. This grant is being funded under the Materials Chemistry Initiative.

Experimental and computational methods will be applied to determine the kinetic acidities of a variety of hydrocarbons. Experimental methods will include the determination of exchange rates from cyclohexylamine-N-t catalyzed by cesium and lithium cyclohexylamides by tritium NMR. Some rates will be determined for deuterium exchange with cyclohexylamine-N-d2 using mass spectroscopy and deuterium NMR. Theoretical calculations will be carried out for proton transfer reaction between various hydrocarbons and bases, and compared to experimental results. Calculations of ion-pair SN2 displacement reactions will be continued. These reactions show unusual highly bent transition states and may give relative reactivities different from the corresponding ionic reactions. %%% This grant from the Organic Dynamics Program supports the continuing work of Professor Andrew Streitwieser at the University of California, Berkeley. The acidity of very weakly acidic hydrocarbons will be determined by recently developed sensitive techniques. This will extend an important body of knowledge. Theoretical calculations will be made that relate to the acidity of hydrocarbons and the results will be compared to the experimental findings. This work will provide a fundamental understanding of weakly acidic hydrocarbons and the consequences to chemical reactivity of the carbon-hydrogen bond in hydrocarbons. Additional calculations will be carried out on a fundamental displacement reaction process in organic chemistry. Preliminary calculations have shown an unusual geometry in the course of this reaction and this work will be continued.

DESCRIPTION: The principal investigator notes that the Aldol addition reaction is one of the most important carbon-carbon bond forming reactions in synthetic organic chemistry, that it is widely used in the synthesis of drugs and other biologically active molecules and that the reaction is now most often run in non-polar solvents, such as tetrahydrofuran (THF), frequently with lithium salts of carbonyl compounds (lithium enolates). He reports that the aggregation of these compounds has been characterized by solution properties, NMR and by X-ray crystallography and that such aggregates have been proposed as the reactive species in the Aldol-type additions with other carbonyl derivatives but there have been very few studies on the actual role of such aggregates in reactions. He goes on to note that by a combination of UV-vis spectroscopic and proton transfer equilibrium studies of some lithium enolates, aggregation constants have been obtained even in dilute solution and that spectra as a function of concentration are analyzed by "Singular Value Decomposition" to determine the number of different species present and to permit deconvolution to give the spectrum of each component. For this purpose a glovebox-spectrometer apparatus has been developed in which the sample compartment built into the glovebox is connected with a spectrometer with fiber-optic cables. It is noted that the apparatus permits spectroscopy of solutions prepared and studied under the inert atmosphere of the glovebox. The results thus far are said to suggest that the monomeric ion pairs might play an important kinetic role in reactions. It is proposed to extend such studies to additional enolates of interest and to measure the reaction kinetics of their Aldol additions to aldehydes and ketones and of Michael addition reactions with unsaturated carbonyl compounds. It is indicated that the kinetic studies will show the state of aggregation of the enolate reactive species and that knowledge of the relative roles of ion pair monomers and aggregates will lead to more complete reaction mechanisms and to the better understanding required for sophisticated synthesis design. The principal investigator notes that in particular, the roles of solvent addends, such as lithium salts, hexamethylphosphoric triamide, and di- and triamines will be studied under carefully controlled conditions to determine the role of coordination of lithium in the stereochemistry of the addition reactions. It is also proposed to apply the same techniques of spectroscopic study, proton transfer equilibria and reaction kinetics of Aldol and Michael addition reactions to the dilithium salts of carboxylic acids and beta- diketones. It is noted that these dilithium salts are also being used increasingly in organic synthesis but that the reaction mechanisms are virtually unstudied. It is reported that these salts are also aggregated but nothing is known about the relative reactivities of monomers and aggregates. The proposed studies are to provide unique information about these reactions, which would be difficult to obtain in any other way. It is suggested that subsequent extension to other salts of alkali and alkaline earth metals, early and late transition metals and lanthanides is also proposed since many of these salts have found use in some stereospecific syntheses.

With this renewal award the Organic and Macromolecular Chemistry Program is continuing its support for the work of Dr. Andrew Streitwieser of the Department of Chemistry of the University of California at Berkeley. The research aims to extend earlier work on the equilibria and kinetics involved in the aggregation of lithium enolates and their reactions with organic halides, sulfonates, and carbonyl compounds containing activated double bonds. Variables to be explored include steric effects, conjugation, added functionality which can coordinate with the lithium ions, solvent effects, and the effects of additives such as HMPA and amines.

Research, which involves dilute air-sensitive solutions, will be done in a unique glove box facility, with a thermostated sample holder connected by quartz fiber-optic cables to an external computer-controlled uv/vis spectrometer. Experiments will be complemented by theoretical computations. The increased understanding expected to result from this work will aid synthetic chemists to devise improved ways to form organic molecules containing new carbon-carbon bonds.

With this renewal proposal the Organic and Macromolecular Chemistry Program supports the on- going work of Dr. Andrew Streitwieser in the Department of Chemistry at the University of California, Berkeley. The work is aimed at understanding chemistry by computations rather than the development of new computational methods or algorithms. The reactions of lithium enolates with alkyl halides and carbonyl compounds, for example, are important in synthetic chemistry and the proposed modeling of experimental results, many of which derive from Professor Streitwieser's laboratory, will provide significant predictive power and new understanding of this chemistry. Computational chemistry has reached the point where much of this chemistry can be modeled quantitatively, and it is the thrust of the present proposal using ab initio quantum calculations with standard programs. Such modeling allows a wider range of the effect of structure on reactivity of these and related reactions, and therefore, leads to new chemical principles and a better understanding of ion pair chemistry. The results of this project are expected to serve as a focus for several review articles to summarize Professor Streitwieser's experimental and computational results.

With this Award, the Organic and Macromolecular Chemistry Program of the Division of Chemistry supports the research activities of Professor Andrew Streitwieser of the University of California- Berkeley. Professor Streitwieser continues to be a world leader in understanding reactions of ionic species. In addition to answering some fundamental questions in physical organic chemistry, post-doctoral and undergraduate students will receive unique training in computational methods and ion pair chemistry. And, to underscore the broader impact of this work, Professor Streitwieser had eight undergraduate students work with him during the last two years.