RUI: Dynamic Control of Electrophilic Carbene Additions to Strained C-C Bonds

Barnard College, New York NY

Investigators

Abstract

RUI: Dynamic Control of Electrophilic Carbene Additions to Strained C-C Bonds This grant supports Dr. Dina C. Merrer's undergraduate research program at Barnard College. The Merrer group elucidates the mechanisms (pathways) of extremely exothermic organic reactions with low or zero activation energies using experimental and computational techniques. The group has thus far determined that dihalocarbenes add to strained carbon-carbon (C-C) bonds such as those in cyclopropene and benzocyclopropene substrates in a one-step manner, through a single transition state that undergoes post-transition state branching of the pathway to several products. The formation of multiple products in an organic chemical reaction typically signifies the existence of multiple reaction mechanisms. However, when multiple products appear to derive from the same pathway, using traditional transition state theory (TST) to describe the reaction is inadequate. Formation of the products is instead controlled by molecular dynamics. Molecular dynamics include the motions, velocity, acceleration, and momenta of the reacting molecules as well as the way in which these factors change molecular shape and potential energy over the course of the reaction. The substrates in the carbene addition reactions must possess a threshold amount of strain energy to induce dynamic control, and this threshold lies between 30 and 40 kcal/mol. As detailed in this grant, the group plans to: (a) determine the threshold strain energy in C-C multiple-bond systems required to observe dynamic control, and (b) determine the limits of reactivity of halocarbenes with C-C single bonds. These two goals will be examined via mechanistic and dynamic studies of halocarbene reactions with three classes of strained substrates: (1) cyclopropenes, cyclooctyne, and cycloheptyne; (2) trans-bicyclo[n.1.0]alkanes; and (3) norbornadiene and bridgehead olefins with strain energies between 30 and 40 kcal/mol. This research will be conducted at Barnard College, a liberal arts college for women affiliated with Columbia University. A liberal arts environment provides exceptional opportunities to overlap research with the education of undergraduate students. These research projects provide educational opportunities for the PI's student co-workers not afforded them in the formal laboratory curriculum. These include using both classical (product studies) and modern (laser flash photolysis and computational chemistry) physical organic techniques. To date, 14 Barnard undergraduates have participated in the research funded by the PI's past NSF grant, nine of whom have since graduated. Of these nine students, six have begun or will begin advanced-degree programs in chemistry, biochemistry, medicine, or public health. This grant's research will extend the chemistry community's understanding of carbene addition mechanisms and the extent to which these mechanisms are governed by reaction dynamics, as well as provide the PI with increased opportunities to excite and encourage bright and capable women to pursue careers in science.