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Effects of Electronic and Steric Modification on Carbene Reactivity

Effects of Electronic and Steric Modification on Carbene Reactivity . Leigh Abrams Landis Group Organic Student Seminar 11/11/10. Carbenes : a prolific field. Divalent carbon atom Uncharged 6 electrons. History of carbenes in organocatalysis.

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Effects of Electronic and Steric Modification on Carbene Reactivity

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  1. Effects of Electronic and Steric Modification on Carbene Reactivity Leigh Abrams Landis Group Organic Student Seminar 11/11/10

  2. Carbenes: a prolific field Divalent carbon atom Uncharged 6 electrons

  3. History of carbenes in organocatalysis Arduengo, A. J.; Harlow, R. L.; Kline, M. J. Am. Chem. Soc. 1991, 113, 361-363. Enders, D.; Niemeier, O.; Henseler, A. Chem. Rev. 2007, 107, 5606-5655. Often used in their salt form for organocatalysis 1991: Arduengo; first isolated free carbene

  4. Singlet vs. triplet carbenes Triplet Singlet Arduengo, A. J. Acc. Chem. Res. 1999, 32, 913-921. Focusing on singlet carbenes, also on stable (mostly isolable) carbenes Singlet carbenes are excellent σ-donating ligands

  5. Electronic structure of carbenes LUMO LUMO LUMO HOMO HOMO HOMO Lower LUMO: more electrophilic Higher HOMO: more nucleophilic

  6. Classes of carbenes Kato, T.; Gornitzka, H.; Baceiredo, A.; Savin, A.; Bertrand, G. J. Am. Chem. Soc.2000, 122, 998-999. Herrmann, W. A.; Kocher, C. Angew. Chem. Int. Ed Engl. 1997, 36, 2162-2187. Gillette, G. R.; Baceiredo, A.; Bertrand, G. Angew. Chem. Int. Ed Engl. 1990,29, 1429-1431. • push-push • N-heterocyclic carbenes (NHCs) • push-spectator • (alkyl)(amino) carbenes • push-pull • (phosphino)(silyl) carbenes

  7. N-Heterocyclic carbenes in organocatalysis Benzoin Condensation: Stetter Reaction: Enders, D.; Niemeier, O.; Henseler, A. Chem. Rev. 2007, 107, 5606-5655. Moore, J. L.; Kerr, M. S.; Rovis, T. Tetrahedron 2006, 62,11477-11482. powerful for generation of d1 and d3 synthons

  8. NHC homoenolate reactions Marion, N.; Diez-Gonzalez, S.; Nolan, S. P. Angew. Chem. Int. Ed. 2007, 46, 2988-3000.

  9. Mechanism of Stetter/Benzoin reaction de Alaniz, J.R.; Rovis, T. Synlett 2009, 2009, 1189-1207.

  10. Steric effects in organocatalysis Ar= Ph, p-MeOC6H4, p-ClC6H4, p-CF3C6H4, C6F5 Stetter, H.; Kuhlmann, H.; Organic Reactions; John Wiley & Sons, Inc: 2004; . Enders, D.; Breuer, K.; Runsink, J.; Teles, J. H. HCA 1996, 79, 1899-1902. Rovis, T. Chem. Lett. 2008, 37, 2-7. Early catalysts for the Stetter reaction were thiazolium salts: Current catalysts are triazolium salts:

  11. Steric model for catalyst design Rovis, T. Chem. Lett. 2008, 37, 2-7.

  12. Enantioselectivity in the Stetter reaction Ar=p-OMeC6H4 Enders, D.; Niemeier, O.; Henseler, A. Chem. Rev. 2007, 107, 5606-5655. Kerr, M. S.; Read, d. A.; Rovis, T. J. Am. Chem. Soc. 2002, 124, 10298-10299.

  13. Abnormal NHCs Crabtree: Bertrand: Normal NHC Abnormal NHC Crabtree, R. H. et. al.Organometallics 2004, 23, 2461-2468. ldeco-Perez, E.; Rosenthal, A. J.; Donnadieu, B.; Parameswaran, P.; Frenking, G.; Bertrand, G. Science 2009, 326, 556-559.

  14. New substitution patterns of NHCs νav(CO) (cm-1) 2030 2045 2040 2035 for [(NHC)RhCl(CO)2]complexes Benhamou, L.; Vujkovic, N.; Cesar, V.; Gornitzka, H.; Lugan, N.; Lavigne, G. Organometallics 2010, 29, 2616-2630.

  15. Backbone modification effects vs. Ad=adamantyl Dipp=2,6-diisopropylphenyl Mes=2,4,6-trimethylphenyl Isolable NHCs generally unreactive with H2 or CO • Small molecule activation requires carbenes with different electronic properties • Can structural modifications make this activation possible?

  16. N,N-Diamidocarbenes (DACs) Hudnall, T. W.; Bielawski, C. W. J. Am. Chem. Soc. 2009, 131, 16039-16041. Cesar, V.; Lugan, N.; Lavigne, G. Eur. J. Inorg. Chem. 2010, 2010, 361-365. Bielawski: Lavigne:

  17. Decreased stability in DACs DAC-1 NHC-1 NHC-2 DFT calculated singlet-triplet gaps performed at the PBE0/6-311+G(d,p)level Hudnall, T. W.; Moorhead, E. J.; Gusev, D. G.; Bielawski, C. W. J. Org. Chem. 2010, 75, 2763-2766. • C-H insertion product resulted from mild heating

  18. Electronic basis for reactivity difference HOMO values calculated to be similar for all systems LUMO energies responsible for changes in ΔHS-T molecular orbitals optimized at the HF/STO-3G level Hudnall, T. W.; Moorhead, E. J.; Gusev, D. G.; Bielawski, C. W. J. Org. Chem. 2010, 75, 2763-2766.

  19. N,N-Diamidocarbene reactivity Ammonia activation: No reaction with dihydrogen Hudnall, T. W.; Moorhead, E. J.; Gusev, D. G.; Bielawski, C. W. J. Org. Chem. 2010, 75, 2763-2766. RNC Hudnall, T. W.; Moerdyk, J. P.; Bielawski, C. W. Chem. Commun. 2010, 46, 4288-4290. (NH3) Hudnall, T. W.; Bielawski, C. W. J. Am. Chem. Soc. 2009, 131, 16039-16041. (CO) Carbon monoxide addition: Isocyanide addition: ketenesketimines

  20. Another approach to small-molecule activation A: Push-Pull Carbene B: Push-Spectator Carbene In B, phenyl group is not acting as a π-acceptor Sole, S.; Gornitzka, H.; Schoeller, W. W.; Bourissou, D.; Bertrand, G. Science 2001, 292, 1901-1903. Buron, C.; Gornitzka, H.; Romanenko, V.; Bertrand, G. Science 2000, 288, 834-836. (aryl)(amino) carbenes Early efforts:

  21. (alkyl)(amino) carbenes (AACs) Selected angles: N1−C1−C2 120.50° sum of bond angles around N = 360.0° (planar) Δ(s-t)=26.7 kcal/mol; HOMO = -4.3 eV acyclic diaminocarbene: Δ(s-t)=58 kcal/mol, HOMO = -5.2 eV Bond angle: N1-C1-C2 106.54° • Calculations at (U)B3LYP/6-31g* level Lavallo, V.; Mafhouz, J.; Canac, Y.; Donnadieu, B.; Schoeller, W. W.; Bertrand, G. J. Am. Chem. Soc. 2004, 126, 8670-8671. Lavallo, V.; Canac, Y.; Prasang, C.; Donnadieu, B.; Bertrand, G. Angew Chem. Int. Ed. 2005, 44, 5705-5709. aAAC: cAAC:

  22. aAAC and cAAC activation of H2 and NH3 • First H2 activation by this • type of system • NH3 activation difficult for • transition-metal complexes Frey, G. D.; Lavallo, V.; Donnadieu, B.; Schoeller, W. W.; Bertrand, G. Science 2007, 316, 439-441.

  23. Proposed mode of H2 activation by carbenes Frey, G. D.; Lavallo, V.; Donnadieu, B.; Schoeller, W. W.; Bertrand, G. Science 2007, 316, 439-441. Analogous transition metal activation: Proposed carbene activation:

  24. Calculated model for H2 activation B3LYP/6-311 g** Frey, G. D.; Lavallo, V.; Donnadieu, B.; Schoeller, W. W.; Bertrand, G. Science 2007, 316, 439-441.

  25. Calculated thermodynamic parameters B3LYP/6-311 g** Frey, G. D.; Lavallo, V.; Donnadieu, B.; Schoeller, W. W.; Bertrand, G. Science 2007, 316, 439-441. Model carbenes used for the calculated values shown:

  26. Activation of hydrogen-heteroatom bonds Frey, G. D.; Masuda, J. D.; Donnadieu, B.; Bertrand, G. Angew. Chem. Int. Ed. 2010asap

  27. Reaction with CO to form stable ketenes Allows for the probing of structural characteristics of amino ketenes Forced planarization induces biradical character in cyclic ketene Lavallo, V.; Canac, Y.; Donnadieu, B.; Schoeller, W. W.; Bertrand, G. Angewandte Chemie 2006, 118, 3568-3571.

  28. A ferrocene-based NHC 2-Ad = 2-adamantyl Siemeling, U.; Farber, C.; Bruhn, C. Chem. Commun. 2009, 98-100. Bond length increases from 1.331 Å to 1.39 Å upon deprotonation Fe-C1 distance increases from 3.31 Å to 3.44 Å upon deprotonation, because of decrease in N-C-N angle

  29. Reactions of ferrocenediaminocarbenes Reactivity similar to (alkyl)(amino)carbenes Siemeling, U.; Farber, C.; Bruhn, C.; Leibold, M.; Selent, D.; Baumann, W.; von Hopffgarten, M.; Goedecke, C.; Frenking, G. Chem. Sci. 2010, 697-704.

  30. Calculated energies of various carbenes • BP86/def2-SVP Siemeling, U.; Farber, C.; Bruhn, C.; Leibold, M.; Selent, D.; Baumann, W.; von Hopffgarten, M.; Goedecke, C.; Frenking, G. Chem. Sci. 2010, 697-704.

  31. Summary The isolation of stable carbenes has allowed for more rational design of catalyst systems and better control of their reactivity Improvements in the design of NHC catalysts should lead to improved selectivity and versatility in umpolung chemistry Small molecule insertion is the first step in using carbenes as transition metal analogues The electronic structure of non-N-heterocyclic carbenes allows them to be isolable like NHCs, yet reactive enough to insert small molecules

  32. Acknowledgements Kat Myhre Practice talk attendees: Brad Ryland Corinne Lipscomb Joanne Redford Jackie Brown Vivian Trang HongNgoc Pham Clark Landis Landis Group: Luke Nelsen Gene Wong Tyler Adint Andrew Yan Helen Dauer Eleanor Rolfe NuruStracey SucheewinChotchatchawankul

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