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Formation and Reactivity of Nitrenes with Silver Catalysts for C-H Bond Amination

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Formation and Reactivity of Nitrenes with Silver Catalysts for C-H Bond Amination Formation and Reactivity of Nitrenes with Silver Catalysts for C-H Bond Amination Prasoon Saurabh Dr. Joe Scanlon Ripon College Experimental background: Why C-N bonds? • Important in pharmacology and synthesizing natural products • Synthetically very challenging • Reaction of interest Morphine H N C C Penicillin Catalysts for the Formation of C-N Bonds + Catalyst 1 [Ag2(tBu3tpy)2(NO3)] • Only intramolecular amination reactions H 2 mol% H2N O CH AgNO3 and tBu3tpy CH3CN 82°C PhI(OAc) O 2 Y. Cui, C He, Angew. Chem. Int. Ed. 2004, 43, 4210-4212. Catalyst 2 • Intermolecular Reactions 2 mol% Cat PhI=NNs [(Agbp)2OTf2H2O] CH2Cl2 50°C + PhI • Cyclo-alkane Reactions 2 mol% Cat PhI=NNs + PhI CH2Cl2 50°C L. Zigang, D. Capretto, R. Rahaman, C. He, Angew. Chem. Int. Ed. 2007, 46, 5184-5186. Benefits of using Ag catalysts for amination reactions • Ag is readily available • Ligands used are available commercially • Able to react at a relatively low temperature H 2 mol% H2N O CH AgNO3 and tBu3tpy CH3CN 82°C PhI(OAc) O 2 • Reacts with relatively inert C-H bonds in cyclo-alkanes Lower than boiling point of water [50°C for phenanthroline (phen)] My research goals: • 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 gaps for intermediate molecules and the nitrene • Calculation of energy of formation of intermediates and nitrene • Monomer Vs Dimer form of catalysts Agtpy and Agphen • Validation of truncation of ligands + • [Ag2(tBu3tpy)2(NO3)] to Ag2tpy2(catalyst 1) • [(Agbathophen)2OTf2H2O] to Ag2phen2(catalyst 2) Formation of Nitrene: • An organic compound containing nitrogen atom with 6 valence electron around the nitrogen with general formula: • Nitrenes are important reactive electrophilic intermediate in amination reaction • For studying formation of nitrene (NTs), ethenediamine (L) is used as a model ligand for phenanthroline (phen). • The similar ligand was used for a nickel complex as studied computationally by Cundari and Morello1. L=ethenediamine Ag dhpe=1,2-bis-(dihydrophosphino)ethane Ag-(L) Cundari T. R. ; Morello G. R. J. Org. Chem., 2009, 74 (15), pp 5711–5714 Theoretical Methods: • Density Functional: B3LYP, M06L • Basis sets used on all non-metal atoms: midi! and 6-31G(d) • Stuttgart Dresden Dunning (SDD) basis set and effective core potential for Ag Motivation: Cundari* Paper... • Model ligand used was (dhpe=1,2-bis- (dihydrophosphino)ethane) which is similar system as Ag2phen2 • B3LYP/CEP-121G where CEP-121G being combined basis set for both core potential and valence electron dhpe Lowest energy intermediate*: (dhpe)Ni-PhI=NTs *Cundari T. R. ; Morello G. R. J. Org. Chem., 2009, 74 (15), pp 5711–5714 Cundari Paper* •Cundari found that the lowest energy intermediate found have iodine and oxygen coordinated to the nickel and the iodine-nitrogen bond intact Intermediates Products *Cundari T. R. ; Morello G. R. J. Org. Chem., 2009, 74 (15), pp 5711–5714 Energy Diagram of Intermediates of Nitrene Formation for Ag(ethenediamine)+1 • Atoms in parenthesis are coordinated to silver • Solvation might show slightly different results (work in progress) • Similar structures and relative energies as found by Cundari’s nickel system Intermediates Atoms in parenthesis are coordinated to silver (bond length in Å and bond angles in degrees ) LAg(N)NTs LAg(3N)NTsa Ag-N(1) 2.16 2.21 Ag-N(2) 2.32 2.30 Ag-O NA NA Ag-I NA NA LAg(N)NTs N(1)-Ag-N(2) 146.2 140.2 [RE = -46.45 Kcal/mol] N-Ag-O NA NA O-Ag-I NA NA * Ethene diamine (L) (1) N from Nitrene LAg(3N)NTs N(2) from Ligand [RE = -58.22 Kcal/mol] a the product of the silver catalyst mediated nitrene formation Intermediates • Atoms in parenthesis are coordinated to silver (Bond length in Å and bond angles in degrees ) LAg(O)(N)PhI=NTs LAg(I)IPh(O)3NTsb Ag-N(1) 2.28 2.38 Ag-N(2) NA 2.36 3 Ag-O 2.13 2.30 Ag-I NA 3.15 N(1)-Ag-N(2) NA NA N-Ag-O 167.1 102.2 LAg(I)IPh(O)3NTs O-Ag-I NA 91.3 [RE = -63.18 Kcal/mol] * Ethene diamine (L) N(1) from Nitrene N(2) from Ligand b the lowest relative energy intermediate of the silver catalystLAg(O)(N)PhI=NTs [RE = -58.22 Kcal/mol] mediated nitrene formation reaction Results: Singlet-Triplet Gap • In experimental study of Ag catalyst reaction pathways, phenyl iodide nitrene [PhI-NTs] is one of the important precursors. • For NTs, triplet is favored energetically over singlet by 9.6 kcal/mol NTs • However, optimizing a triplet PhI-NTs (nitrene precursor) leads to I-N bond breaking suggesting that it may not be the stable precursor Model Ligand Vs Actual •The calculations validate ethenedLigand:iamine as a model ligand for phen Uncoordinated AgL AgPhen N2 Ag-N/Å 2.32 2.27 N1 N-Ag-N/° 78.2 77.0 Coordinated Nitrene LAg-NTs Agphen-NTs Ag-N1/Ǻ 2.31 2.12 Agphen Ag-N2/Ǻ 2.37 2.19 Ag-N3/Ǻ 2.16 2.02 N2 N1-Ag-N2/° 74.4 78.2 N3 N1 N1-Ag-N3/° 146.2 98.2 N2-Ag-N3/° 138.9 176.4 LAg-NTs Truncating the substituent Used in order to increase the speed of computational process Truncated: Removed tert-butyl from tBu3tpy to form tpy tpy tBu3tpy Removed phenyl groups from bathophenanthroline (Bathphen) to form phenanthroline (phen) phenanthroline bathophenanthroline Proper truncation of substituent To see if the truncating process is proper: Compare the desired bond lengths Decide if the model system is good Optimized geometry for both monomers and dimers of Ag- coordinated with tBu3tpy and tpy ; bathophen and phen using B3LYP/midi! with SDD as effective core potential for Ag Bond distances and bond angles compared Geometry: Results N(4) Table: Comparing Bond lengths in Å for Batho-phen and phen N(3) Ag(2) (1) Ag2Bathophen2 Ag2Phen2 Ag (2) N N(1) Ag(1)-Ag(2) 2.69 3.06 Ag(1)-N(1) 2.19 2.27 Ag2Bathaphen2 Ag(1)-N(2) 2.19 2.27 N(2) Ag(2) N(3) Ag(2)-N(3) 2.19 2.27 Ag(1) Ag(2)-N(4) 2.19 2.27 N(1) •It was found that Ag2Phen2 had stacked geometry while N(4) in Ag2Bathophen2 ligands were on the opposite side of the metals perhaps due to steric hindrance Ag2Phen2 Geometry: Results Table: Comparing bond length in Å between experimental and theoretical values + [Ag2(tpy)2NO3] Experimental M06-L/midi! Ag(1)-Ag(2) 2.84 2.95 Ag(1)-N(1) 2.29 2.32 Ag(1)-N(2) 2.45 2.46 Ag(1)-N(3) 2.24 2.35 Ag(1)-N(5) 2.45 2.37 Ag(2)-N(4) 2.27 2.31 Ag(2)-N(6) 2.27 2.56 (2) (1) - Ag -O 2.33 2.12 N and O from NO3 Ag(2)-O(2) 2.72 2.28 Further characterization • From earlier geometry calculations we find that Ag-Ag bond is shorter in Ag2tpy2 than Ag2phen2 • To see if formation of Ag-Ag is possible in Ag2phen2 compared to Ag2tpy2, bond order (BO) calculations were performed in both the monomers and dimers of the Agtpy and Agphen. • To compare the strength of disilver and Ag-N bonds, similar BO calculations were performed for the Ag- + bathaphenalthroline and [Ag2tpy2(NO3)] Bond Orders Bond orders for Ag-Ag and Ag-N in monomers and dimers of AgPhen and AgBathphen Ag1 Ag2phen2 Ag2bathophen2 Agphen Agbathphen N2 Ag1-Ag2 0.72 0.82 NA NA N1 Ag1-N1 0.39 0.35 0.35 0.28 Ag1-N2 0.39 0.35 0.35 0.28 Ag2-N3 0.39 0.35 NA NA Ag2-N4 0.38 0.35 NA NA Agphen N N2 3 Ag N 1 4 Ag Ag 2 1 N N N 4 1 1 N N Ag 3 2 2 N 2 N Ag 1 1 Agbathphen Ag2phen2 Ag2bathophen2 Bond Orders + Bond orders for Ag-Ag and Ag-N in monomers and dimers of Agtpy; [Agtpy2(NO3)] + Agtpy Ag2tpy2 [Ag2tpy2(NO3)] 5 N1 N Ag1‐Ag2 NA 0.75 0.69 Ag1‐N1 0.28 0.29 0.30 Ag1‐N2 0.28 0.29 0.23 Ag2 1 N6 Ag Ag1‐N3 0.27 NA 0.14 2 2 4 N Ag ‐N NA 0.29 0.27 N4 Ag2‐N5 NA 0.29 0.25 Ag2‐N6 NA 0.18 0.29 Ag2‐N3 NA NA 0.28 Ag2‐N2 NA NA 0.12 Ag1‐O1 NA NA 0.42 Ag2tpy2 Ag1‐O2 NA NA 0.53 O1 N3 O2 N1 N2 Ag 1 N1 N 2 Ag1 N6 N3 N 4 Ag 2 N 5 Agtpy + [Ag2tpy2(NO3)] Conclusion • Ethenediamine does a good job as a model ligand for phenanthroline. • Desired product LAg3NTs is not the lowest energy species so perhaps the reaction goes through an intermediate. • Three intermediates were found with LAg(I)IPh(O)3NTs being the lowest with RE = -63.18 Kcal/mol • Truncated ligands could be used instead of actual ligands for reducing computation time with similar results • Disilver bond length was found to be longer Ag2phen2 in than in Ag2tpy2 • Disilver bond order was 0.75 Ag2tpy2 in compared to 0.72 in Ag2phen2 Next Steps Find transition states between the intermediate and nitrene reactants Perform the actual amination step Solvation calculations Molecular Orbital Analysis Natural Bond Order calculations Acknowledgement • Dr.
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