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Research Projects in the Robert G. Bergman GroupUpdated June, 2007 Professor Bergman and his coworkers utilize a range of chemical techniques to discover new chemical reactions and then determine how those reactions work. Much of his research focuses on reactions important in the areas of organometallic chemistry and homogeneous catalysis. In this field, very small amounts of soluble organometallic complexes are used to transform much larger quantities of organic compounds--such as those found in living organisms, or in natural feedstock sources such as petroleum-- into more complicated substances that might ultimately be useful in the synthesis of important chemical compounds. Professor Bergman's group has generated reactive organometallic intermediates capable of undergoing intermolecular oxidative addition with the normally inert C-H bonds in alkanes and other organic molecules. This process holds potential for converting alkanes into functionalized organic molecules such as alkenes and alcohols. Another area of investigation involves the study of organometallic complexes having metal-oxygen, -nitrogen and -sulfur bonds to obtain information about the mechanisms of metal-mediated oxidation, amination and desulfurization processes. Some of the group’s research in organometallic chemistry has moved during the past few years into the development and mechanistic understanding of catalytic reactions, especially in applications to problems in organic synthesis, and the intersection of organometallic chemistry with molecular recognition. Several of the compounds prepared in these projects have been utilized in highly enantioselective asymmetric induction reactions, and others act as homogeneous catalysts for new carbon-hydrogen and carbon-heteroatom bond-forming processes such as hydroamination of alkynes and allenes. The advent of readily available computational methodology, especially for carrying out density functional (DFT) calculations, has been increasingly incorporated into many of the above research activities, and has provided a powerful addition to the arsenal of tools available for understanding the detailed course of organic and organometallic reactions. Much of this work has been facilitated by collaborations with other faculty members in the department of chemistry and elsewhere. Details of selected specific projects are outlined below.
I. Carbon-hydrogen bond activationII. Chemistry controlled by molecular recognition processes: organometallic and organic chemistry in the cavities of water-soluble nanovesselsIII. The chemistry of metal complexes bearing carbon-nitrogen single and multiple bondsIV. Educational innovation and outreach activities
I. Carbon-hydrogen bond activationWe have made substantial progress in understanding the mechanism of the high-valent iridium carbon-hydrogen bond activation process that we discovered in the early 1990’s. It is now clear that these reactions proceed via transient cationic Ir(III) intermediates, and the critical step in the reaction involves oxidative addition of C-H bonds to the iridium centers, leading to the formation of Ir(V) complexes as intermediates (parts of this work were carried out in collaboration with the R. A. Andersen and T. D. Tilley groups). An important process that grew out of these investigations was a method that results in catalytic exchange of deuterium and tritium into common organic molecules. This method is currently being used in pharmacological tracer research by workers at the Pharmacia/Pfizer company. In a related research direction, we have collaborated with the J. Ellman group in developing new methods for the intramolecular addition (cyclization) of aromatic and vinyl C-H bonds across carbon-carbon double bonds, leading to new methods for C-C bond formation that do not require prior functionalization at the C-H position. The methods have been demonstrated to occur with wide scope and in good yields. Preliminary results on the corresponding intermolecular reactions, on enantioselective variants, and on alkylation of alkenyl C-H bonds, have also been recently obtained. Finally, we have initiated a collaborative project with the C. Sukenik group at Bar-Ilan University in Israel to develop C-H activation reactions that can be used to modify organic films bound to surfaces. II. Chemistry controlled by molecular recognition processes: organometallic and organic chemistry in the cavities of water-soluble nanovesselsRecently a collaboration was initiated between this group and the K. N. Raymond group aimed at exploring the possibility of carrying out organometallic reactions in the cavities of water-soluble cluster complexes (so-called “nanovessels”) that had been developed earlier in his group. The first achievement of this collaboration was the demonstration that cationic iridium complexes synthesized in our laboratory, related to those described in the previous section of this report, are successfully encapsulated into the cavities of the Raymond nanovessels. We then demonstrated that these encapsulated complexes are capable of carrying out C-H activation reactions on water-soluble organic molecules. We now have strong evidence that these reactions take place inside the nanovessel cavities, and that this is responsible for dramatic (and as yet not well understood) selectivity differences between the reactions carried out inside and outside of the nanovessels. In a parallel project, we have identified a series of organic rearrangements and hydrolyses that experience acceleration in rate when they take place inside the nanovessel cavities. This has allowed us to demonstrate that small quantities of nanovessel can be used to catalyze the rearrangement of much larger quantities of organic substrate. Several of these reactions have been shown to follow Michaelis-Menten kinetics, emphasizing their relevance to operation of biological catalysts such as enzymes. III. The chemistry of metal complexes bearing carbon-nitrogen single and multiple bondsDuring the past 15 years the Bergman laboratory has played a major role in the synthesis of complexes with metal-nitrogen single and double bonds, and in the understanding and applications of their reactions in organic and inorganic chemistry. We have developed many of the first reactions of these materials with unsaturated organic compounds, and this work has led to the observation of cycloaddition reactions between the M=N bonds in imido (or nitrene) complexes and unsaturated organic compounds, such as alkynes, alkenes, allenes, nitriles and imines. We prepared the first optically active imido complexes, and these materials were used to develop a method for the highly enantioselective kinetic resolution of chiral allenes. Several of these compounds also react with organic allyl compounds, in reactions that take place with SN2 or SN2’ regiochemistry, depending on the M=X bond involved and on the leaving group in the allyl moiety. Catalytic processes useful in alkene hydroamination, imine metathesis and heterocumulene metathesis have recently grown out of this work (some of our more recent results in this area were obtained in collaboration with the J. Arnold group). We have also played a role in developing methods for the synthesis of late transition metal compounds bearing metal-nitrogen and metal-phosphorus single bonds. An important feature of these materials is the exceptionally high basicity exhibited by the nitrogen atom when it is bound to a low-valent transition metal center. In collaboration with the F. D. Toste group, we have obtained preliminary results indicating that metal-phosphido complexes exhibit enhanced nucleophilicity, much like their metal-nitrogen counterparts, and that this property can be used to develop catalytic methods for the enantioselective alkylation of secondary phosphines. A recent outgrowth of this work has been the development of the first well-controlled living polymerization of aziridines to give nitrogen-containing polymers that have exceptionally low polydispersities. IV. Educational innovation and outreach activitiesProf. Bergman has been involved in the following experimental education projects during the past five years: (a) a NSF-supported project designed to restructure the chemistry curriculum, headed by Angelica Stacy; (b) modification of the teaching style in undergraduate and graduate courses, designed to foster more interactive student participation and active learning, and (c) an outreach program called “Chemistry in the Classroom,” developed with the Berkeley-based nonprofit organization Community Resources for Science, that is designed to encourage chemistry graduate students to make science presentations in grade 3-5 classrooms of local elementary schools. For additional information on the Chemistry in the Classroom project, see http://www.crscience.org/. [Main Page]
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