Coordination complexes as catalysts for Chan-Evans-Lam couplings

The Chan-Evans-Lam coupling of boronic acids with nucleophiles provides a heavy metal-free alternative to Buchwald-Hartwig couplings and operates under milder reaction conditions. Most commonly employed catalysts are simple copper salts, such as copper acetate. Chan-Evans-Lam couplings are notorious for being highly substrate dependent, making laborious optimization of reaction conditions necessary even for closely related substrates such as aliphatic amines and anilines.

Among the factors optimized are the solvent, the base/ligand added, the copper source (counter anion), elimination of water etc. We speculated that incorporating the necessary functionalities typically attributed to solvent, base and/or anion into the ligand framework of a coordination complex would result in a more general coupling protocol. In fact, complex 1 is one of the most active catalyst for Chan-Evans-Lam couplings and couples a large variety of N-nucleophiles using an identical, simple reaction protocol, tolerating the presence of water. None of the common site reactions in Chan-Evans-Lam couplings are observed and excess of boronic acid is not required. This catalyst is particularly active towards sterically hindered amines.

Selected publications:

73. [ACS Editors' Choice] Chan–Evans–Lam Couplings with Copper Iminoarylsulfonate Complexes: Scope and Mechanism

V. Hardouin Duparc; G. L. Bano; F. Schaper ACS Catalysis 2018, 8, 7308-7325. [PDF]

69. Sulfonato-imino copper(II) complexes : fast Chan-Evans-Lam coupling of amines and anilines

V. Hardouin Duparc; F. Schaper Dalton Transactions 2017, 46, 12766 - 12770. [PDF]

66. [Website Banner Choice] Sulfonato-diketimine copper(II) complexes : synthesis and application as catalysts in base-free Chan-Evans-Lam couplings

V. Hardouin Duparc; F. Schaper Organometallics 2017, 36, 3053-3060. [PDF]

Lactide Polymerisation

Polylactic acid is a polymer of mounting interest due to its biodegradability, medical compatibility, and the possibility to obtain the monomer from renewable resources. A catalytic challenge, however, is the development of highly active, selective and stable catalysts to obtain isotactic polylactic acid from rac-lactide, the racemic mixture of R,R- and S,S-lactide. We are exploring several underrepresented areas in the context of lactide polymerization

1. dn-metal based catalysts

Most lactide polymerization catalysts are based on d0- or d10-metal centers. We have explored the performance of other transition metal catalysts, including Mn, Fe and Cu for lactide polymerization. Copper catalyst 2 proved to be the most active copper catalyst reported, while at the same time providing excellent control over the polymer molecular weight and did not undergo any side reaction in polymerization, even under monomer-starved conditions.

Selected publications:

58. Square-Planar Cu(II) Diketiminate Complexes in Lactide Polymerization

T. J. J. Whitehorne; F. Schaper Inorg. Chem. 2013, 52, 13612-13622. [PDF]

55. NacnacBnCuOiPr: A strained geometry resulting in very high lactide polymerization activity

T. J. J. Whitehorne; F. Schaper Chem Commun. 2012, 48, 10334-10336. [PDF]

2. Catalytic-site assisted chain-end control

For isotactic polymerization, the chiral information can be either obtained from a chiral catalytic site or from the chiral chain end. In the former case, a rigid chiral catalyst is required and the unavoidable presence of the chiral chain end can be detrimental. In the latter case, isotactic stereocontrol is often difficult achieve and the role of the ligand frame work is limited to providing unselective steric bulk. In a “catalytic-site assisted chain-end” mechanism, a flexible, chiral catalytic site adapts to the chirality of the chain-end, but is responsible for the selection of the monomer. While this mechanism theoretically combines the advantages of both control mechanisms, there have been few reported instances for lactide polymerization. We have recently shown that the only reported isotactic copper-based catalysts 3 (and its derivatives) follow such a mechanism and that introduction of easy racemization in a zinc-based catalysts changed its selectivity from atactic to slightly isotactic.

Selected publications:

70. Catalytic-Site-Mediated Chain-End Control in the Polymerization of rac-Lactide with Copper Iminopyrrolide Complexes

P. Daneshmand; J. L. Jiménez-Santiago; M. Aragon--Alberti; F. Schaper Organometallics 2018, 37, 1751-1759. [PDF]

67. Mechanism and stereocontrol in isotactic rac-lactide polymerization with copper(II) complexes

P. Daneshmand; A. van der Est; F. Schaper ACS Catalysis 2017, 7, 6289-6301. [PDF]

63. Isotactic rac-Lactide Polymerization with Copper Complexes: The Influence of Complex Nuclearity

S. Fortun; P. Daneshmand; F. Schaper Angew. Chem. Int. Ed. 2015, 54, 13669-13672. [PDF]

3. Ligand-assisted activated monomer mechanism

Contrary to coordination-insertion based mechanisms, catalyzed by metal alkoxides, lactide polymerizations by an activated monomer mechanism are significantly less sensitive towards the presence of protic substrates and thus more suitable for industrial conditions. However, since the transition states of this mechanism do not involve the metal center, stereocontrol is often absent and cannot be readily influenced by the ligand framework. A recently emerged variation of this mechanism involves a participation of a basic site on the ligand in the deprotonation of the alcohol co-initiator and led to isotactic stereocontrol in group 1 and 2-based catalysts. We are currently exploring if the same mechanism can be expanded towards other metal centers.

Older projects, currently not continued

N-alkyl diketiminate ligands

Diketiminates, a class of ligands known since the 1960s, underwent a substantial revival after Brookhart’s work on late TM olefin polymerisation and they were recognized as spectator ligands of great potential, stabilizing reactive metal centers and complexes in unusual oxidation states and low coordination numbers. Most ligands employed carry ortho-disubstituted aryl substituents on nitrogen, which impart a C2v-symmetric ligand environment. N-alkyl substituted diketiminate ligands did not undergo the same revival and their applications are still mostly limited to low-molecular-weight ligands for ALD/CVD applications.

N-alkyl substituents allow the introduction of C2-symmetry into the diketiminate ligand framework. In this context, we prepared (in parallel with Harder and coworkers) the first chiral diketiminate ligand and explored its coordination chemistry with metal centers such as Zr, Cr, Cu, Zn, Al and Mg and potential applications in catalysis. In lactide polymerization, Mg-, Zr- and Cu-based diketiminate complexes proved to be among the most active complexes reported for these metals. A Mg-based catalysts with a N-alkyl diketiminate ligands proved to be the first diketiminate catalyst showing a slight preference for isotactic monomer insertion.

Selected publications:

61. Lactide polymerization catalyzed by Mg and Zn diketiminate complexes with flexible ligand frameworks

T. J. J. Whitehorne; B. Vabre; F. Schaper Dalton Transactions 2014, 43, 6339-6352. [PDF]

56. Exceptionally high lactide polymerization activity of zirconium complexes with bridged diketiminate ligands

I. El-Zoghbi; T. J. J. Whitehorne; F. Schaper Dalton Transactions 2013, 42, 9376-9387. [PDF]

50. NacnacBnMgOtBu: a diketiminate-based catalyst for the polymerisation of rac-lactide with isotactic preference

F. Drouin; T. J. J. Whitehorne; F. Schaper Dalton Trans. 2011, 40, 1396. [PDF]

46. Lactide Polymerisation with Chiral b-Diketiminate Zinc Complexes

F. Drouin; P. O. Oguadinma; T. J. J. Whitehorne; R. E. Prud'homme; F. Schaper Organometallics 2010, 29, 2139–2147. [PDF]


Bridged metallocene complexes (ansa-metallocenes) offer two advantages when compared to their unbridged counterparts: (i) the interannular bridge prevents ring rotation and enables a rigid stereochemistry and (ii) the angle of the cyclopentadienyl rings is influenced by the bridge, with consequences for the steric and electronic properties of the complexes. Our research in this area concentrates on bridged chromocenes, which have been only rarely reported.