Modeling of S N Bond Breaking of an Aroma4c Sulfilimine. By Jacob Brunsvold Advisor: Dr Stacey Stoffregen

Transcription

1 Modeling of S N Bond Breaking of an Aroma4c Sulfilimine By Jacob Brunsvold Advisor: Dr Stacey Stoffregen

2 Deoxygena)on of Aroma)c Sulfoxides O S hν S + O( 3 P) Φ < 0.01 upon photolysis, dibenzothiophene S oxide (DBTO) deoxygenates with a low quantum yield chemical trapping studies were consistent with forma4on of O( 3 P), sugges4ng involvement of 3 DBTO an understanding of the reac4on mechanism currently being sought J. Org. Chem., 2005, 70 (9), pp

3 Proposed Mechanism of Deoxygena)on for DBTO heavy atom subs4tu4on resulted in a significant increase in the quantum yield of deoxygena4on, sugges4ng the involvement of an intersystem crossing event while inves4ga4ng the mechanism of DBTO deoxygena4on is computa4onally expensive, smaller analogs are being studied J. Org. Chem., 2004, 69 (24), pp

4 Computa)onal Inves)ga)on of Thiophene S Oxide Deoxygena)on O S thiophene-s-oxide the photochemical deoxygena4on of thiophene S oxide, a DBTO analog, was computa4onally modeled thiophene S oxide could not be studied directly in a laboratory sewng, because of its tendency to self dimerize via a Diels Alder reac4on no dis4nct maximum in charge separa4on was observed beyond the equilibrium S O bond distance, thus, charge separa4on is not the driving mechanism of ISC as previously proposed blue indicates the charge on S red indicates the charge on O Manuscript in prepara4on

5 Proposed Mechanism of Thiophene S Oxide Deoxygena)on CASSCF calcula4ons were used to inves4gate the poten4al energy diagram of thiophene S oxide deoxygena4on the results suggested that ISC to the second excited triplet state (3A ) results in a nearly barrierless pathway to the proposed products O S thiophene-s-oxide Manuscript in prepara4on

6 Photochemistry of Sulfilimine Analogs like aroma4c sulfoxides, laboratory trapping and 4me resolved IR spectroscopy studies have shown that many aroma4c sulfilimines cleave their S N bond upon photolysis to form DBT and nitrene products J. Org. Chem., 2008, 73 (12), pp J. Org. Chem., 2007, 72 (18), pp

7 Photochemistry of Sulfonium Ylide Analogs similar studies have shown the sulfonium ylides photochemically react to form DBT and carbenes J. Org. Chem., 2009, 74 (7), pp

8 Sulfilimine Analog Currently Under Inves)ga)on H N S 1H-1λ4-thiophen-1-imine 1H 1λ4 thiophen 1 imine (a.k.a. NH thiophene) is a small analog of the aroma4c sulfilimines under photochemical inves4ga4on as nitrene sources in the laboratory the poten4al energy diagram of the S N bond breaking of. NH thiophene is currently being generated and has been the focus of this research

9 Types of Calcula)ons Being Conducted all computa4ons were carried out with GAMESS MacMolPlt, a graphical interface for GAMESS, was used to view all of the molecules and orbitals generated NH thiophene was op4mized at the HF/6 31G(d,p) level maintaining Cs symmetry (ground state with RHF and excited state with UHF) a Hessian was run to confirm the op4mized geometry as minimum the poten4al energy of sulfilimine S N bond breaking was generated by performing op4miza4ons in which the S N bond length was constrained but all other coordinates were op4mized

10 Hartree Fock Calcula)ons the geometry of NH thiophene was first op4mized to find its ground state electronic energy using a restricted Hartree Fock calcula4on the geometry of the molecule was then op4mized at incrementally longer and shorter S N distances the energies of NH Thiophene were ploeed against their S N distances this process is currently being repeated for the excited triplet state of NH Thiophene

11 Results of the HF/6 31G(d,p) Op)miza)ons of NH thiophene Energy Rela)ve to Ground State (kcal/mol) S N Bond Length (Angstroms) NH thiophene Ground State NH thiophene Triplet Excited State

12 CASSCF/6 31G(d,p) Constrained Op)miza)ons of NH Thiophene coordinates and orbitals from the HF constrained op4miza4ons were used as ini4al coordinates and orbitals for the CASSCF calcula4ons the ac4ve space only included the orbitals that were most affected by the breaking of the sulfur nitrogen bond. N S σ N H σ C S σ A C S σ A S LP N LP N LP C C π A C C π A N S σ* N H σ* C S σ* A C S σ* A C C π* A C C π* A

13 CASSCF Calcula)ons ajer the ac4ve space was defined, the geometry of NH Thiophene was again op4mized (this 4me using the CASSCF method) in an aeempt to find the structure and energy of the ground state. op4mized structures and energies of NH Thiophene at various fixed N S distances are currently being obtained like the data from the HF calcula4ons, the energies of NH Thiophene will be ploeed against their corresponding S N distances

14 Discussion the poten4al energy plot of NH Thiophene S N bond breaking is being inves4gated computa4onally in an aeempt to beeer understand the mechanism by which aroma4c sulfoxides and sulfilimines photochemically decompose more data on the energe4cs of NH Thiophene s excited states must be obtained, before we can gain an understanding of the mechanism by which S N bond breaking occurs the rela4ve energies of the singlet and triplet excited states will be compared with the energy of decomposed material to see which excited states are reasonable precursors to the products this analysis will indicate whether the mechanism proposed for thiophene oxide is also reasonable for the sulfilimine under inves4ga4on

15 Future CASSCF Calcula)ons the lowest energy NH thiophene singlet and triplet excited states will be op4mized using the CASSCF method the S N distances of these states will then be fixed at various lengths, while the structures are again op4mized finally, the poten4al energy diagram of S N bond lengthening will be prepared and analyzed

16 Acknowledgements Advisor: Dr. Stacey Stoffregen Funding: Na4onal Science Founda4on Midwest Undergraduate Computa4onal Chemistry Cluster The University of Wisconsin River Falls Chemistry Department