3 first tautomerizes to form complex 3’. Conjugated unsaturated nitrile I’ reacts with
complex 3’ to form ketimido complex C which is in equilibrium with an enamido
complex D formed by the reaction of an unconjugated unsaturated nitrile II’ with
complex 3’. Nucleophilic addition of a nucleophile (alcohol or amine) to the
ketimido complex C results in the formation of a new enamido complex E that
can tautomerize to form an imine complex F. Complex F undergoes C–C cleavage
reaction to release the product and regenerate the dearomatized complex 3’.
5 Summary and Outlook
In conclusion, metal-ligand cooperation operated by the dearomatization/aromatization mode of pincer complexes has allowed facile reversible activation of strong
bonds and ultimately led to the development of new catalytic transformations that are
atom-economic and environmentally benign. As the pincer ligands in these transformations are non-innocent and the bond activation or catalysis is mediated by a
synergy between metal and ligand and not only by the metal, without overall change
in the metal oxidation state, it has been possible to utilize several transition metals,
especially earth-abundant metals, in place of noble metals making the catalytic
processes more sustainable. Using the same concept new ligand designs may achieve
appropriate synergy with the main group elements and perform reversible bond
activation offering new platform to discover new catalytic reactions free from
transition metals. Development of bimetallic pincer complexes with the ability to
perform metal-ligand cooperation can also bring new modes of reactivity due to
additional possibility of metal-metal cooperation.
References
1. van Koten G (1989) Pure Appl Chem 61:1681–1694
2. van der Vlugt JI, Reek JNH (2009) Angew Chem Int Ed 48:8832–8846
3. Selander N, Szabó KJ (2011) Chem Rev 111:2048–2076
4. Peris E, Crabtree RH (2018) Chem Soc Rev 47:1959–1968
5. Choi J, MacArthur AHR, Brookhart M, Goldman AS (2011) Chem Rev 111:1761–1779
6. Albrecht M, van Koten G (2001) Angew Chem Int Ed 40:3750–3781
7. van der Boom ME, Milstein D (2003) Chem Rev 103:1759–1792
8. Gunanathan C, Milstein D (2014) Chem Rev 114:12024–12087
9. Gunanathan C, Milstein D (2011) Bond activation by metal-ligand cooperation: design of
“Green” catalytic reactions based on aromatization-dearomatization of pincer complexes. In:
Ikariya T, Shibasaki M (eds) Bifunctional molecular catalysis. Springer, Berlin, pp 55–84
10. Khusnutdinova JR, Milstein D (2015) Angew Chem Int Ed 54:12236–12273
11. Milstein D (2015) PhilosTrans Royal Soc A 373:20140189
12. Gunanathan C, Milstein D (2011) Acc Chem Res 44:588–602
13. Li H, Hall MB (2015) ACS Catal 5:1895–1913
14. Mukherjee A, Milstein D (2018) ACS Catal 8:11435–11469
Recent Advances in the Applications of Metal-Ligand Cooperation via. . .
21
complex 3’ to form ketimido complex C which is in equilibrium with an enamido
complex D formed by the reaction of an unconjugated unsaturated nitrile II’ with
complex 3’. Nucleophilic addition of a nucleophile (alcohol or amine) to the
ketimido complex C results in the formation of a new enamido complex E that
can tautomerize to form an imine complex F. Complex F undergoes C–C cleavage
reaction to release the product and regenerate the dearomatized complex 3’.
5 Summary and Outlook
In conclusion, metal-ligand cooperation operated by the dearomatization/aromatization mode of pincer complexes has allowed facile reversible activation of strong
bonds and ultimately led to the development of new catalytic transformations that are
atom-economic and environmentally benign. As the pincer ligands in these transformations are non-innocent and the bond activation or catalysis is mediated by a
synergy between metal and ligand and not only by the metal, without overall change
in the metal oxidation state, it has been possible to utilize several transition metals,
especially earth-abundant metals, in place of noble metals making the catalytic
processes more sustainable. Using the same concept new ligand designs may achieve
appropriate synergy with the main group elements and perform reversible bond
activation offering new platform to discover new catalytic reactions free from
transition metals. Development of bimetallic pincer complexes with the ability to
perform metal-ligand cooperation can also bring new modes of reactivity due to
additional possibility of metal-metal cooperation.
References
1. van Koten G (1989) Pure Appl Chem 61:1681–1694
2. van der Vlugt JI, Reek JNH (2009) Angew Chem Int Ed 48:8832–8846
3. Selander N, Szabó KJ (2011) Chem Rev 111:2048–2076
4. Peris E, Crabtree RH (2018) Chem Soc Rev 47:1959–1968
5. Choi J, MacArthur AHR, Brookhart M, Goldman AS (2011) Chem Rev 111:1761–1779
6. Albrecht M, van Koten G (2001) Angew Chem Int Ed 40:3750–3781
7. van der Boom ME, Milstein D (2003) Chem Rev 103:1759–1792
8. Gunanathan C, Milstein D (2014) Chem Rev 114:12024–12087
9. Gunanathan C, Milstein D (2011) Bond activation by metal-ligand cooperation: design of
“Green” catalytic reactions based on aromatization-dearomatization of pincer complexes. In:
Ikariya T, Shibasaki M (eds) Bifunctional molecular catalysis. Springer, Berlin, pp 55–84
10. Khusnutdinova JR, Milstein D (2015) Angew Chem Int Ed 54:12236–12273
11. Milstein D (2015) PhilosTrans Royal Soc A 373:20140189
12. Gunanathan C, Milstein D (2011) Acc Chem Res 44:588–602
13. Li H, Hall MB (2015) ACS Catal 5:1895–1913
14. Mukherjee A, Milstein D (2018) ACS Catal 8:11435–11469
Recent Advances in the Applications of Metal-Ligand Cooperation via. . .
21