Metal-Alkylcarbene and Quinone Carbene Complexes for C-H Bond Functionalization Reactions. Structures and Catalysis

Professor Che, Chi Ming   (Principal Investigator (PI))

36

2020-09-01

2023-08-31

999769

Metal-Alkylcarbene and Quinone Carbene Complexes for C-H Bond Functionalization Reactions. Structures and Catalysis

Alkyl carbene, C-H bond functionalization, homogeneous catalysis, metal-carbene intermediate, quinoid carbene

Chemical Sciences

Physical Sciences (P)

17302020

General Research Fund (GRF)

2020

Completed

1 The general objective is to uncover the bonding, electronic structures and reactivities of novel and/or underexplored metal-carbene complexes/intermediates and study their impacts on selective metal-catalyzed C-H functionalization and/or C-C bond formation reactions. Since most of these reaction intermediates are too short-lived or too unstable for direct detection/isolation, we plan to stabilize such elusive species through appropriate design of carbene ligands and/or macrocyclic supporting ligands, which is beneficial to their spectroscopic and structural characterizations and structure-reactivity relationship establishment. These investigations provide a straightforward way to elucidate, or shed light on, the reaction mechanisms and to gain useful insight for improving the current synthetic methodologies for related environmentally benign organic transformation reactions. Particular efforts will be focused on the structural-reactivity relationships of metal-alkylcarbene, -quinoid carbene and -perfluoroalkylcarbene complexes. The objectives stated in this proposal are as follow: 2 First, we plan to synthesize and characterize isolable group 8-10 metal complexes of the dialkylcarbene 2-adamantylidene (Ad), and study their competences in transferring Ad group under electro- and photochemical conditions. In the literature, many different metal catalysts were used for various carbene transfer reactions; however, direct and systematic comparisons between different metal catalysts are sparse, and the carbene transfer reactivity of an isolated metal Ad complex remains unexplored. In view of this existing research gap, we plan to study the electronic structures and reactivities of group 8-10 metal Ad complexes in a systematic way. Our preliminary studies showed that these species are stable and isolable. 3 Second, we aim to employ homo- and/or heterogeneous Fe, Ru, Co, Rh, Ir salen/porphyrin complexes with axial Ad ligand as robust or selective catalysts for alkylcarbene and/or arylcarbene transfer reactions such as alkene cyclopropanation and C-H insertion reactions, including asymmetric catalysis. In view of our initial success in metal-catalyzed alkylcarbene transfer reactions, we will continue to expand the substrate scope, improve stereoselectivities, increase catalytic turnover number, and even replace noble metal catalysts with those based on inexpensive metal, with special emphasis on iron complexes (which are Earth-abundant with high biocompatibility). We plan to develop efficient protocol in generating metal alkylcarbene species under ambient conditions, which is essential for highly enantio- and diastereoselective alkylcarbene transfer reactions using the novel chiral porphyrin recently designed and prepared by our group. We will also use chiral metallosalen complexes for enantioselective carbene C-H insertion. Moreover, heterogeneous catalysts, such as metal-organic frameworks (MOFs) containing M-Ad (M = Fe, Ru) porphyrins or Ir porphyrins, will be developed as recyclable catalysts for carbene C-H insertion reactions. 4 Third, we will develop dirhodium phthalocyanine (Pc) catalysts for site-selective acceptor-type carbene or diaryl carbene C-H insertion reactions with high regioselectivity. We found that, using typical dirhodium paddlewheel complexes as the catalysts, the corresponding dirhodium carbene species underwent side reactions and did not lead to the desired C-H insertion products. In view of this, we propose to use Rh-Pc complexes designed by our group for such reactions, and our preliminary studies revealed promising results using a dirhodium phthalocyanine complex as catalyst. Another advantage of our Rh-Pc system lies in the differences from the existing systems in trapping and characterizing highly reactive rhodium carbene intermediates using spectroscopies with high sensitivities, such as ESI-MS, EPR and UV-vis. Through these studies, we envision to uncover the essential and key features of effective catalysts for metal-catalyzed carbene C-H insertion reactions. 5 Finally, we will develop metal-catalyzed C-H functionalization or C-C bond formation reactions involving transfer of relatively small carbene groups such as the perfluoroalkylcarbene CHCF3. In view of the high tendency for such light species to dimerize and/or to undergo oxidation, we plan to use metal complexes supported by sterically-encumbered ligands for stabilizing the carbene intermediates. The stabilized carbene intermediates are also important for uncovering the bonding information and structural-reactivity relationship for further improvement of these carbene C-H insertion reactions.