The Organic and Macromolecular Chemistry Program in the Chemistry Division at the National Science Foundation supports Professor Robert A. Moss of Rutgers University who will study carbenes, diazirines, and other reactive species by fast and ultrafast spectroscopic methods. These studies are intended to correlate molecular structure with the reaction channels of highly reactive, short-lived organic intermediates. Novel syntheses will also be developed for diazirine precursors of carbenes. Target areas will include the measurement of activation parameters for carbene-alkene additions and the interplay of enthalpy and entropy in these additions; spectroscopic studies of carbene solvation; and the chemistry of incarcerated carbenes. Difluorocarbene will be generated from difluorodiazirine, and the absolute kinetics of its additions to alkenes will be determined. Activation parameters for alkene additions will be measured by nanosecond laser flash photolysis (LFP) for phenylchlorocarbene, dichlorocarbene, fluorochlorocarbene, methoxychlorocarbene, phenylfluorocarbene, difluorocarbene, methoxyfluorocarbene, methylmethoxycarbene, phenylmethoxycarbene, and dimethoxycarbene. A fundamental and synoptic picture of carbene-alkene addition reactions will be constructed and parallel computational studies will be conducted.
Research by Professor Moss in the previous award period has contributed to the training of five undergraduates (2 females, 3 males), three graduate students (2 females and one African American), and six postdoctoral students (one female, 5 males). In addition to the training of a diverse group of students, Professor Moss has produced scholarly reviews that permit amplification of his NSF-sponsored research and thus reach a wider segment of the chemical community. During his most recent grant period, he published two well-received, co-edited books on the chemistry of reactive intermediates. In addition, he authored four reviews dealing with the chemistry of singlet carbenes; the synthesis of diazirines; SNi fragmentation reactions; and new mechanisms in carbene and diazirine chemistry. The publication of scholarly reviews will continue, with future topics to include the fragmentation of carbenes, the interplay of enthalpy and entropy in carbene-alkene additions, and the synthesis of dihalodiazirines.
We set out to examine the reactions and reactivity of various highly unstable, very short-lived derivatives of divalent carbon; i.e., carbenes. We were particularly interested in correlating the chemical structure of the carbenes with their reaction channels. Along the way, we also developed new syntheses for the diazirine precursors of the carbenes. Our results were reported in 23 publications in peer-reviewed journals. In addition, with M. P. Doyle, we co-edited a new book, Contemporary Carbene Chemistry (Wiley, 2014), that will be of value to mechanistic and synthetic organic chemists around the world. The bulk of our work dealt with halocarbenes, especially arylchloro- and dichlorocarbenes. We studied their reactions by nanosecond laser flash photolysis of their diazirine precursors, coupled with UV spectroscopy to monitor the carbenes. We were able to characterize the solvation/complexation of these short-lived species by ethers, aromatic solvents, and halide ions. In some cases, equilibria were demonstrated between the carbenes and their complexes. A particularly interesting finding was an apparent compensation between the energies and entropies of activation for the additions of the carbenes to olefins: the more favorable the energy of these processes, the more they were opposed by unfavorable probability factors. We extended these studies to the highly reactive chlorotrifluoromethylcarbene with similar results. Our work has elucidated the fundamental linkage between the structures of carbenes and their reactivities and stabilities. Our results will aid synthetic chemists in their use of carbenes, and will also serve a pedagogic function in the teaching of physical organic and mechanistic chemistry to advanced undergraduate and graduate students.
