Chemistry and Biochemistry
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Dr. David M. Birney

Title:

Professor

Education:

Ph.D., Yale University, 1987; Postdoctoral Study, University of California, Los Angeles; Guest Professor, ETH - Zurich, Fall 2008

Research Area:

Physical Organic & Mechanistic Chemistry

Office:

Phone:

Email:

Chemistry 232-C

806-742-3063

David.Birney@ttu.edu

 

Research Group

Principal Research Interests

In our group, we use high level ab initio and density functional calculations to provide fundamental insights into two classes of reactions, pseudopericyclic and pericyclic. We then design and conduct experiments to test the predictions of these calculations. The synergy between theory and experiment has provided insights and research directions that would not be possible from either alone.

Pseudopericyclic Reactions

The "conventional wisdom" of the orbital symmetry rules tells an organic chemist what to expect from a pericyclic reaction. Pseudopericyclic reactions violate all of these expectations of a pericyclic reaction, yet strictly speaking they are orbital symmetry allowed. The fundamental difference between the two is that in a pseudopericyclic reaction, there is not orbital overlap around the ring of breaking and forming bonds. This allows their transition states to have a planar geometry, and, often, very low activation barriers.The difference between a planar pseudopericyclic transition state and a non-planar pseudopericyclic one is illustrated in two animations of these reaction pathways. The dramatic differences in geometries between them are summarized below.

Familiar Pericyclic Reactions

  

Novel Pseudopericyclic Predictions

Cyclic orbital overlap

  

Disconnections in orbital overlap

Non-planar, non-least motion transition states

  

Planar (or nearly planar) transition states

Pericyclic reactions can be allowed or forbidden, depending on the number of electrons

  

All pseudopericyclic reactions are allowed; there are no anti-aromatic transition states

Concerted pericyclic reactions have lower barriers than stepwise alternatives. Barriers are due to enforced electron-electron repulsion

  

Pseudopericyclic reactions can have lower barriers than pericyclic alternative. There is no enforced electron-electron repulsion

Thermal Eliminations of esters. The nature of pseudopericyclic reactions is illustrated in the thermal elimination reactions of esters. The six-centered thermal b-elimination of an ester is a common method for the preparation of sensitive alkenes. An eight-centered d-elimination of cis-1 to trans-2 would be allowed if it were pseudopericyclic. Testing this hypothesis requires some care in the experimental design, as rearrangement of cis-1 to trans-1 followed by six-centered elimination would give the same products, as shown below. Gas-phase multiphoton infrared absorption was used to decipher these possible reactions and demonstrate that approximately 23% of the reaction of cis-1 proceeds directly to give trans-2.

Multiphoton Infrared (MP-IR) Photolysis/Thermolysis

Multiphoton infrared (MP-IR) irradiation can be used to carry out gas-phase pyrolysis of molecules while avoiding subsequent thermal reactions of the product(s). We refer to this as MP-IR photolysis/thermolysis; it is a photolysis because of the absorption of IR photons, but it is a thermolysis because the chemistry occurs via thermally excited ground state molecules. MP-IR photolysis/thermolysis is initiated by the sequential absorption of IR photons from a pulsed IR laser. Thus, molecules that absorb the IR get hot. However, this process can be extremely selective and almost gentle. By tuning the IR laser and by judicious choice of reactants, only one component of a gas mixture may absorb the light and be heated. This molecule can either react or be cooled by collisions with cold molecules. The important point for this work is that these collisions can also cool hot product molecules before they can react further. This amounts to a pyrolysis in a room temperature environment.

Transition state calculations.

Although b-eliminations have been suggested to be pericyclic, B3LYP/6-31G(d,p), MP2 and MP4 calculations suggest that both b- and d-eliminations, as well as [3,3]-sigmatropic rearrangements of esters are primarily pseudopericyclic in character, as judged by both geometrical, energetic and transition state aromaticity (NICS) criteria. Small distortions from the ideal pseudopericyclic geometries are argued to reflect small pericyclic contributions. It is further argued that when both pericyclic and pseudopericyclic orbital topologies are allowed and geometrically feasible, the calculated transition state may be the result of proportional mixing of the two states; this offers an explanation of the range of pseudopericyclic and pericyclic characters found in related reactions. Figure A shows the qualitative orbitals involved in a pericyclic hydrocarbon ene reaction of 1-pentene, while Figure B shows the qualitative orbitals involved in the b-elimination of ethyl formate. Figure C and D show the HOMO and HOMO-1 orbitals respectively in the planar, pseudopericyclic transition state for the b-elimination of ethyl formate. Note the disconnections in the orbital overlap around the ring.

Stereochemical studies have provided among the most detailed, albeit indirect insights into transition state geometries. Unfortunately, there are few stereochemical markers at the orbital orthogonalities in pseudopericyclic reactions. We have used a different approach to verify the predicted planar geometries of pseudopericyclic transition states, namely X-ray crystallography. The ground states of several molecules that can undergo pseudopericyclic reactions have been shown to be distorted towards fragmentation via a planar pathway. For example, the Figure below summarizes the results of an analysis of the X-ray structure of the dimer of camphorketene. Figure A shows the observed bond distances, Figure B shows the deviations in the bond angles as compared to model systems, and Figure C shows deviations in the bond lengths as compared to model systems. Note that the deviations in bond lengths and angles are not consistent with the effects of strain, but instead prefigure the fragmentation.

Sequential Transition States

We have recently begun studying reactions in which two distinct transition states are directly connected on the reaction potential energy surface (PES) without intervening intermediates. This is illustrated in a generic fashion in the figure below. From transition state 1, it is downhill to transition state 2, following the intrinsic reaction coordinate (IRC, in red). However, the IRC leads to a second transition state, not a product. One can envision leaving transition state 1 in a valley on a three-dimensional PES. But following the IRC to transition state 2 clearly leads down along a ridge. The point where the valley opens into a ridge is the valley-ridge inflection point (VRI). From the point of view of chemical dynamics, the VRI is a bifurcation point, past which the reaction paths can lead to either of two products.

We recently reported an example of sequential transition states in the deazetization of nitrosimines. [Bartsch, R. A.; Chae, Y. M.; Ham, S.; Birney, D. M., J. Am. Chem. Soc. 2001, 123, 7479-7486] In this case, the transition state (TS1) for rotation around the C=N bond connects to the transition state (TS2) for pseudopericyclic electrocyclization of 1 to the spiro intermediate (2, Figure and eq 1). In principle, ring-opening of oxetene (3) could proceed via an analogous two-stage process, with a planar, pseudopericyclic transition state connecting to a transition state for rotation around the C4-C3 bond. However, it is calculated to be concerted (eq 2). The difference between the two systems is that the barriers for rotation around the C4=C3 bond in acrolein (4) is much higher than for 1; thus there is an energetic benefit to concerted twisting in the ring-opening of oxetene (3), which allows for partial p-overlap in the transition state. Thus the ring closure of 1 to 2 may be considered to have separated the two fundamental processes that contribute to the concerted, pericyclic electrocyclic ring closure of butadiene, namely rotation and bond formation. Studies of additional potential energy surfaces that may have sequential transition states are currently underway in our group.

 

Representative Publications