Frustrated Lewis Pairs-2 - Franklin Chm Colostate

Welch, G. C.; Stephan, D. W. J. Am. Chem. Soc. 2007, 129, 1880.
grrrr…

• Background
Outline
• The Beginning: H 2 AcCvaCon
• Metal-­‐Free HydrogenaCon
• AddiCons to Unsaturated Compounds
• CO 2 AcCvaCon

Lewis Acids and Bases
“... we may say that a basic substance is one which has a
lone pair of electrons which may be used to complete
the stable group of another atom, and that an acid
substance is one which can employ a lone pair from
another molecule in comple@ng the stable group of one
of its own atoms.”
Gilbert N. Lewis
base = e -­‐ pair donor
acid = e -­‐ pair acceptor
G. N. Lewis, Valence and the Structure of Atoms and Molecules, Chemical Catalogue Company, New York, 1923.

Lewis Acids and Bases
base = e -­‐ pair donor
acid = e -­‐ pair acceptor

Brown:
WiMg:
Tochtermann:
FLPs in History
Brown, H. C.; Schlesinger, H. I.; Cardon, S. Z. J. Am. Chem. Soc. 1942, 64, 325.
WiWg, G.; Benz, E. Chem. Ber. 1959, 92, 1999.
Tochtermann, W. Angew. Chem. Int. Ed. Engl. 1966, 5, 351.

New ReacCvity
first reversible H 2 ac@va@on by a non-­‐transi@on metal system
Welch, G. C.; San Juan, R. R.; Masuda, J. D.; Stephan, D. W. Science 2006, 314, 1124.

H 2 AcCvaCon by Main Group Compounds
Spikes, G. H.; FeWnger, J. C.; Power, P. P. J. Am. Chem. Soc. 2005, 127, 12232.
Frey, G. D.; Lavallo, V.; Donnadieu, B.; Schoeller, W.; Bertrand, G. Science 2007, 316, 439.
Welch, G. C.; San Juan, R. R.; Masuda, J. D.; Stephan, D. W. Science 2006, 314, 1124.

H 2 Cleavage by Phosphines and Boranes
strict steric and electronic requirements for H 2 ac@va@on
Welch, G. C.; Stephan, D. W. J. Am. Chem. Soc. 2007, 129, 1880.

Mechanism of H 2 AcCvaCon
termolecular collision = entropically disfavored
H 2 adduct formaCon disfavored due to Pauli repulsion
weak associaCon of LB and LA generates a ‘frustrated complex’
Rokob, T. A.; Hamza, A.; SCrling, A.; Soós, T.; Pápai, I. Angew. Chem. Int. Ed. 2008, 47, 2435.

H 2 Cleavage by the Frustrated Complex
• stabilizaCon energy of -­‐11.5 kcal/mol
• dispersion interacCons
• mulCple C-­‐H F hydrogen bonds
• planar BC 3 unit = no charge transfer
• structural flexibility allows H 2 access
• H-­‐H polarizaCon in TS
• simultaneous tBu 3 P σ*(H 2 ) and σ
(H 2 ) B(C 6 F 5 ) 3 leads to H-­‐H cleavage
Rokob, T. A.; Hamza, A.; SCrling, A.; Soós, T.; Pápai, I. Angew. Chem. Int. Ed. 2008, 47, 2435.

Bridged Phosphine-­‐Borane Adduct
Organometallics 2008, 27, 1135.
reversible adduct forma@on allows for H 2 ac@va@on
Stephan, D. W. Chem. Commun. 2007, 5072. Erker, G. Dalton Trans. 2009, 1534.

Reversible H 2 AcCvaCon
Organometallics
1999, 18, 1724
2.08Å
proton lability leads to reversible ac@va@on?
Wang, H.; Fröhlich, R.; Kehr, G.; Erker, G. Chem. Comm. 2008, 5966.

Reversibility: H 2 EliminaCon

Reversible H 2 AcCvaCon by Design
balance of acidity and basicity
Ullrich, M.; Lough, A. J.; Stephan, D. W. J. Am. Chem. Soc. 2009, 131, 52.
Dalton Trans. 2007, 3407.

H 2 AcCvaCon: FLPs as TransiCon Metals
small energy separa@on between fron@er orbitals
Power, P. P. Nature 2010, 463, 171; Kubas, G. J. Chem. Rev. 2007, 107, 4152; Pápai, I. Int. J. Quant. Chem. 2009, 109, 2416.

Frustrated Lewis Pairs Today
R 3 X / ER’ 3
PR 3 , NR 3 , BR’ 3 , AlR’ 3
NHR 2 , :CR 2

• Background
Outline
• The Beginning: H 2 AcCvaCon
• Metal-­‐Free HydrogenaCon
• AddiCons to Unsaturated Compounds
• CO 2 AcCvaCon

First Metal-­‐free CatalyCc HydrogenaCon
Chase, P. A.; Welch, G. C.; Jurca, T.; Stephan, D. W. Angew. Chem. Int. Ed. 2007, 46, 8050.

Imine HydrogenaCon
Sumerin, V.; Schulz, F.; Atsumi, M.; Wang, C.; Nieger, M.; Leskelä, M.; Repo, T.; Pyykkö, P.; Rieger, B. J. Am. Chem. Soc. 2008, 130, 14117.

Silyl Enol Ether HydrogenaCon
Wang, H.; Frölich, R.; Kehr, G.; Erker, G. Chem. Comm. 2008, 5966.

HydrogenaCon by the Bridged FLP
Spies, P.; Schwendemann, S.; Lange, S.; Kehr, G.; Frölich, R.; Erker, G. Angew. Chem. Int. Ed. 2008, 47,7543.

Organometallic Substrates
Schwendemann, S.; Asli Tumay, T.; Axenov, K. V.; Peuser, I.; Kehr, G.; Frölich, R.; Erker, G. Organometallics 2010, 29, 1067.

Frustrated Substrates
Chase, P. A.; Jurca, T.; Stephan, D. W. Chem. Commun. 2008, 1701. Chen, D.; Klankermayer, J. Chem. Commun. 2008, 2130.

• Background
Outline
• The Beginning: H 2 AcCvaCon
• Metal-­‐Free HydrogenaCon
• AddiCons to Unsaturated Compounds
• CO 2 AcCvaCon

1,2 AddiCon to Carbonyl Groups
Mömming, C. M.; Frömel, S.; Kehr, G.; Fröhlich, R.; Grimme, S.; Erker, G. J. Am. Chem. Soc. 2009, 131, 12280.

AddiCons to Olefins
McCahill, J. S. J.; Welch, G. C.; Stephan, D. W. Angew. Chem. Int. Ed. 2007, 46, 4968.
Voss, T.; Chen, C.; Kehr, G.; Nauha, E.; Erker, G.; Stephan, D. W. Chem. Eur. J. 2010, 16, 3005.

1,4 AddiCon to Conjugated Systems
Ullrich, M.; Seto, K. S. H., Lough, A. J.; Stephan, D. W. Chem. Comm. 2009, 2335.
Mömming, C.; Kehr, G.; Wibbeling, B.; Fröhlich, R.; Schirmer, B.; Grimme, S.; Erker, G. Angew. Chem. Int. Ed. 2010, 49, 2414.

Terminal Alkyne AcCvaCon
Dureen, M. A.; Stephan, D. W. J. Am. Chem. Soc. 2009, 131, 8396. Jiang et al. Organometallics 2010, 29, 125.

• Background
Outline
• The Beginning: H 2 AcCvaCon
• Metal-­‐Free HydrogenaCon
• AddiCons to Unsaturated Compounds
• CO 2 AcCvaCon

CO 2 AcCvaCon and ReducCon
Mömming, C. M.; Oten, E.; Kehr, G.; Fröhlich, R.; Grimme, S.; Stephan, D. W.; Erker, G. Angew. Chem. Int. Ed. 2009, 48, 6643.
Ashley, A. E.; Thompson, A. L.; O’Hare, D. Angew. Chem. Int. Ed. 2009, 48, 9839.

Conclusion
• CombinaCon of unquenched acidity and
basicity lends unique reacCvity to frustrated
Lewis pairs
• FLP small molecule acCvaCon and reacCons
with π systems suggest a parallel with
transiCon metal chemistry
• FLP catalysis provides a mild, metal-­‐free
alternaCve for hydrogenaCon