With the support of the Chemical Synthesis Program of the Chemistry Division, Professor Guy Bertrand of the Chemistry Department at the University of California - Riverside, will prepare several novel classes of carbenes, the isolation of which have been impeded by the belief that they are inherently unstable, or have eluded the synthetic skills of investigators. These species have not only been chosen because they represent exciting synthetic challenges but more importantly because they should feature interesting electronic and steric properties, which can lead to important applications. These carbenes will be used for the activation of small molecules and enthalpically strong bonds, tasks previously exclusive to transition metal complexes. They will also be utilized for the stabilization of radicals and triplet diradicals based on main group elements, especially phosphorus and boron. Imines derived from these carbenes will allow for the preparation of very strong neutral bases and hydride donors, compounds that are of great interest for organic synthesis. These carbenes are original ligands for transition metal catalysts, and over the years the success of homogeneous catalysis can be attributed largely to the development of a diverse range of ligand frameworks that have been used to tune the behavior of the various systems.
It is of paramount importance for the future to find catalysts able to transform abundant and cheap molecules into useful compounds ranging from bulk chemicals to therapeutics. The activation of small molecules without transition metal centers offers an alternative paradigm in the continuing search for efficient catalytic systems, and should allow for considerably reducing the cost of important chemical processes. Additionally, this project is at the interface between physical, organic and organometallic chemistry and is therefore well suited to the education of scientists at all levels. The University of California- Riverside, classified as a minority serving institution, is well positioned to provide the highest quality educational experience for students underrepresented in science. Because of this fact, a diverse group of students will be involved in the project. This research will also promote collaborative research and the exchange of undergraduates, and graduates students between UC Riverside and several French universities.
Efficient and selective preparation of organic molecules is critical for mankind. It is of paramount importance for the future to find catalysts able to transform abundant and cheap molecules into useful compounds ranging from bulk chemicals to therapeutics. We have found that carbenes can activate small molecules and enthalpically strong bonds, processes that were believed to be only achievable by transition metal centers. The activation of small molecules without metals offers an alternative paradigm in the continuing search for efficient catalytic systems, and should allow considerable reductions in the cost of important chemical processes. As an example, the activation of P4 by carbenes might help to meet the growing demand in phosphorus derivatives, and the increasingly stringent environmental regulations. Indeed, new processes using white phosphorus but avoiding chlorine are highly desirable. We have also discovered that stable electrophilic singlet carbenes can stabilize highly reactive diamagnetic and paramagnetic organic, inorganic and even transition metal species. Among the highlights in this field, is the isolation of the first complexes featuring one and two atoms of gold in the formal zero oxidation state. The preparation of these compounds constitutes the first step towards the synthesis of ultra-small gold nanoparticles, which have applications ranging from care diagnostics to advanced methods of controlled intracellular delivery. Also noteworthy is the synthesis of a tricoordinated neutral boron derivative, which is isoelectronic with amines. This compound is especially interesting because in contrast to the usual boranes, which are prototypical Lewis acids, it is a Lewis base, and a potential L ligand for transition metals. We have reported the preparation of the first nitrene, a nitrogen analogue of carbenes, which is stable at room temperature. This compound, namely a phosphinonitrene, is related to transition metal nitrido complexes (metallonitrenes, LnMN), which have extensively been studied due to their implications in biological nitrogen fixation by the nitrogenase enzymes, and the industrial Haber-Bosch hydrogenation process of N2 into NH3. We have shown that the phosphinonitrene can as well act as a nitrogen atom transfer agent. Six postdoctoral fellows who participated in this project are Assistant Professors in US and foreign research universities. A Hispanic student, who is still working on our NSF project, is the first of his family to earn an undergraduate degree. Because of his ethnic minority and working class background, he was accepted into UC Riverside’s Mentoring Research Intership Program (MSRIP) in 2009, and now in the Graduate Program of UCSD where he has been selected to receive a GAANN fellowship (Graduate Assistance in Areas of National Need). This project has favored exchanges of students between the United States and European countries. Two undergraduates from Toulouse and Munich, and one graduate student from Graz have spent 9 months at UCR and then UCSD, working on this NSF project. Three UCSD graduate students have spent 2 months in Germany to improve their knowledge in computational chemistry.
