Formation and Reactivity of Nitrenes with Silver Catalysts for C-H H Bond Amination. Joseph Scanlon Ripon College

Transcription

1 Formation and Reactivity of Nitrenes with Silver Catalysts for C-H H Bond Amination Prasoon Saurabh, Kelcey Anderson, Joseph Scanlon Ripon College

2 Why we want C-N C N bonds More chemically reactive than C-H C bonds Important in pharmacology and synthesizing natural products HO O H HO H H Morphine N CH 3 Penicillin

3 Catalysts for the Formation of C-N C Bonds [Ag 2 (tbu 3 tpy) 2 (NO 3 )] + Only intramolecular amination reactions H H 2 N O CH 2 mol% AgNO3 and tbu3tpy O CH3CN 82 C PhI(OAc) 2 Y. Cui, C He, Angew. Chem. Int. Ed. 2004, 43,

4 [(Agbp) 2 OTf 2 H 2 O] Intermolecular Reactions 2 mol% Cat PhI=NNs CH 2 Cl 2 50 C + PhI Alkane Reactions 2 mol% Cat PhI=NNs CH 2 Cl 2 50 C CH NHNs + PhI L. Zigang, D. Capretto, R. Rahaman, C. He, Angew. Chem. Int. Ed. 2007, 46,

5 Benefits of using Ag catalysts for amination reactions Relatively cheap by comparison to other potential metals Ligands used are available commercially Able to react at a relatively low temperature Reacts with relatively inert C-H C bonds in alkanes

6 Research Goals Part 1 Generation of a model system of the disilver catalysts to determine the mechanism of formation of nitrene Ag mediated generation of a nitrene Singlet-triplet triplet gaps for intermediate molecules and the nitrene Calculation of enthalpy of formation of intermediates and nitrene

7 Research Goals Part 2 Determine mechanism for uncatalyzed C-H H bond amination Singlet-triplet triplet splitting Determine effect of catalyst on mechanism and rate It is not certain that the Ag 2 Phen 2 dimer is the reactive species, so we will also study the AgPhen form of the catalyst

8 Theoretical Methods: Density Functional: B3LYP Basis sets used on all non-metal atoms: midi! and 6-31G(d) 6 Stuttgart basis set and effective core potential for Ag In computational models, Truncated tbu 3 tpy to tpy Truncated bathophenanthroline to phenanthroline

9 Formation of Nitrene: An organic compound containing nitrogen atom with 6 valence electron with general formula: For studying formation of nitrene, ethenediamine (L) used as a model ligand for phenanthroline. The similar ligand was used for a nickel complex as studied computationally by Cundari and Morello 1. L Cundari T. R. ; Morello G. R. J. Org. Chem., 2009, 74 (15), pp

10 Singlet-Triplet Gap For NTs,, triplet is favored energetically over singlet by -9.6 kcal/mol. NTs Optimizing a triplet PhI-NTs (nitrene precursor) leads to I-N I N bond breaking.

11 Model Ligand Vs Actual Ligand: Uncoordinated Ag-N/Å AgL AgPhen N2 N1 N-Ag-N/ Coordinated Nitrene LAg-NTs Agphen-NTs Agphen Ag-N1/Ǻ Ag-N2/Ǻ Ag-N3/Ǻ N2 N1-Ag-N2/ N1-Ag-N3/ N3 N1 N2-Ag-N3/ LAg-NTs

12 Energy Diagram of Intermediates of Nitrene Formation Atoms in parenthesis are coordinated to silver Many possible intermediates found Similar structures and relative energies as found by Cundari s nickel system

13 Uncatalyzed Intramolecular Reaction Nitrene (Triplet) Nitrene (Singlet) TS (Singlet) Product Rel. E. (kcal/mol)

14 Uncatalyzed Intermolecular Reaction Reactant Triplet Reactant Singlet TS (Singlet) Product Rel. E. (kcal/mol)

15 [AgPhen] + Catalyzed Reaction [Ag(Phen)(nitrene)] + Singlet [Ag(Phen)(nitrene)] + Triplet TS Singlet [AgPhen] + and Product Rel. E. (kcal/mol)

16 [Ag 2 (tpy) 2 ] +2 Catalyzed Reaction [Ag 2 (tpy) 2 (nitrene)] +2 (triplet) [Ag 2 (tpy) 2 (nitrene)] +2 (singlet) TS (singlet) [Ag 2 (tpy) 2 ] +2 + Product Rel. E. (kcal/mol)

17 Conclusion Ethenediamine does a good job as a model ligand for phenanthroline. Desired product LAg 3 NTs is not the lowest energy species so perhaps the reaction goes through an intermediate. For uncatalyzed reactions, there are barrier heights of kcal/mol. [AgPhen] + lowered the barrier to kcal/mol. [Ag 2 (tpy) 2 ] +2 had a barrier of approximately 4 kcal/mol. Spin crossing could be a possibility for these systems.

18 Future research Determine full reaction pathway for nitrene formation by locating transition states connecting intermediates to nitrene complex Replace model ligands with experimental ligands for studying nitrene formation Model the full [(AgPhen) 2 ] +2 dimer system for C-C H bond amination Explore different methods (M06L) and larger basis sets Investigate singlet reaction surfaces that have a transition state lower in energy than reactant minimum

19 Acknowledgements Dr. Joseph Scanlon Ripon College Chemistry Department Midwest Undergraduate Computational Chemistry Consortium (MU3C) Minnesota Supercomputing Institute Rachel Vanden Berg