Transition Metal-Catalyzed Carbon-Carbon Bond Cleavage (C-C Activation) Group Meeting 01-15-2008 Timothy Chang
Outlines - Fundamental considerations, C-H versus C-C activation - Orbital interactions - Evidence of M C-C agostic interaction, C-C activation - Thermodynamics - Stoichiometric reactions - Catalytic reactions - Activation of strained molecules - Direct cleavage - Utilization of a carbonyl group - b-alkyl elimination - Activation of unstrained molecules - b-alkyl elimination, utilization of a p-allyl intermediate - Chelation assistance - Conclusion
Orbital Interactions Kinetic barriers of C-C activation due to: 1. High directionality of C-C s bond 2. Substituents on both carbons (steric congestion) 3. Statistical abundance of C-H bonds Murakami, M., Ito, Y. Topics in Organometallic Chemistry 1999, 97-129
Evidence of M C-C Agostic Interaction C25-H25 0.97(3) Å C21-H21 0.95(3) Å No M C-H agostic interaction from Binor-S 1 Weller, A. S., et. al. ACIEE, 2006, 45, 452-456. Weller, A. S., et. al. PNAS, 2007, 104, 6921-6926.
Supports for M C-C Agostic Interaction, C-C Activation Fluxional at RT 1 H NMR C11, C15, C21 and C25 are all equivalent at 298 K (d( 1 H) = 3.23ppm, d( 13 C) = 25.5 ppm) 5 signals (298 K) 10 signals (200 K)
Thermodynamic Considerations C-C bond 90 kcal/mol M-C bond x2 40-60 kcal/mol Microscopic reversibility M-H bonds are generally stronger than M-C bonds by 15 25 kcal/mol even though Me-H and H-H have almost the same BDE. Labinger, J. A., Bercaw, J. E. Organometallics 1988, 7, 926-928. Crabtree, R. H. The Organometallic Chemistry of the Transition Metals 4 th Ed. P80. Unlike in the gas phase, M-H bonds are generally stronger than M-C bonds in the solution. Simões, J. A. M., et al. Chem. Rev. 1990, 90, 629-688. Examples: (η 5 -C 5 Me 5 )(PMe 3 ) 2 Ru H 167.4 kcal/mol (η 5 -C 5 Me 5 )(PMe 3 ) 2 Ru CH 3 142.3 kcal/mol CRC 88 th Ed.
Thermodynamic Considerations Strategies to facilitate C-C bond activation: 1. Employing strained starting materials E(C-C) 61 kcal/mol Tipper (1955) Chatt (1961) Jun, C.-H Chem. Soc. Rev., 2004, 33, 610-618. Milstein, D., Rybtchinski B. ACIEE 1999, 38, 870-883.
Thermodynamic Considerations Strategies to facilitate C-C bond activation (c ontd): 2. Use high energy metal complexes 16 e Coordinatively unsaturated Jun, C.-H Chem. Soc. Rev., 2004, 33, 610-618. Milstein, D., Rybtchinski B. ACIEE 1999, 38, 870-883.
Thermodynamic Considerations Strategies to facilitate C-C bond activation (contd): 3. Extrusion of gas, aromatization 4. Formation of a stable metallacycle and chelation assistance Five-membered metallacyclic complexes are relatively stable. Rh-C aryl and Ir-C aryl bonds are sometimes stronger than the corresponding M-H bonds. Jun, C.-H Chem. Soc. Rev., 2004, 33, 610-618
Outlines - Fundamental considerations, C-H versus C-C activation - Orbital interactions - Evidence of M C-C agostic interaction, C-C activation - Thermodynamics - Stoichiometric reactions - Catalytic reactions - Activation of strained molecules - Direct cleavage - Utilization of a carbonyl group - b-alkyl elimination - Activation of unstrained molecules - b-alkyl elimination, utilization of a p-allyl intermediate - Chelation assistance - Conclusion
Direct Cleavage a 2.75 mm (PPh 3 ) 3 RhCl and 0.138 M substrate in anhydrous toluene. b Turnover frequency determined at 18 h. Chirik, P. J., et. al. JACS 2003, 125, 886.
Direct Cleavage Murakami, M. et. al. JACS 2007, 129, 12596.
Utilization of a Carbonyl Group Stoichiometric reactions showed that the insertion of Rh(I) into the a C-C bond is feasible 5 mol% 87 % yield Murakami, M. et. al. Nature 1994, 370, 540. Murakami, M. et. al. JACS 1996, 370, 8259.
Predict Products and Propose Mechanisms Both reactions were carried out under argon. Murakami, M. et. al. ACIEE 2000, 39, 2484.
Utilization of a Carbonyl Group Murakami, M. et. al. JACS 2002, 124, 13976.
b-alkyl Elimination (1) Oxidative Addition b-alkyl elimination is the reverse of insertion Uemura, S. et. al. JACS, 2000, 122, 12049.
b-alkyl Elimination (1)
b-alkyl Elimination (1) + (S) 93 % yield 91 % ee 93 % yield 73 % ee 79 % yield 78 % ee Uemura, S. et. al. JACS, 2003, 125, 8862.
b-alkyl Elimination (2) Uemura, S. et. al. JACS, 2003, 125, 8862. C-C bond activation by 1. Insertion of Rh(I) cat into a C(O)-C bond 2. b-alkyl elimination Murakami, M. et. al. JACS, 1997, 119, 9307.
b-alkyl Elimination (3) Murakami, M. et. al. JACS, 2005, 127, 6932.
Other Strained Molecules for C-C Activation O O O O O O Mitsudo, T. et. al. Chem. Lett, 2005, 34, 1462.
Mechanism of the Pyranone Formation Mitsudo, T. et. al. ACIEE, 2004, 43, 5369.
Outlines - Fundamental considerations, C-H versus C-C activation - Orbital interactions - Evidence of M C-C agostic interaction, C-C activation - Thermodynamics - Stoichiometric reactions - Catalytic reactions - Activation of strained molecules - Direct cleavage - Utilization of a carbonyl group - b-alkyl elimination - Activation of unstrained molecules - b-alkyl elimination, utilization of a p-allyl intermediate - Chelation assistance - Conclusion
b-alkyl Elimination, Utilization of a p-allyl Intermediate 5 mol% Mitsudo, T. et. al. JACS, 1998, 120, 5587.
b-alkyl Elimination, Formation of a Pd-C(Sp 2 ) Bond Miura, M. et. al. JACS, 2001, 123, 10407.
b-alkyl Elimination, Formation of a Pd-C(Sp) Bond 33-61 % yield Uemura. S. et. al. JACS, 2003, 5, 2997.
Sequential b-alkyl Elimination, Conjugate Addition Hayashi. T. et. al. JACS, 2007, 129, 14158.
Sequential b-alkyl Elimination, Conjugate Addition Hayashi. T. et. al. JACS, 2007, 129, 14158.
Chelation Assistance Jun, C.-H Chem. Soc. Rev., 1999, 121, 880.
Nickel-Catalyzed Arylcyanation of Alkynes time (h) yield(%) Hiyama, T. JACS, 2004, 126, 13904.
Conclusion - Different strategies toward C-C single bond activation have emerged - Mild reaction conditions need to be developed - Substrate scopes are limited in many cases - Only a few enantioselective reactions have been reported - This technology is still at the exploration stage