The_Catalyst_Review_December_2023 - 19

Combining Ligand Deuteration with Ligand Bulkiness in Non-heme Iron Oxidation Catalysis: Enhancing
Catalyst Lifetime and Site-Selectivity
The selective oxidation of aliphatic C-H bonds and enantioselective epoxidation of alkene functionalities are critical chemical
transformations and are typically conducted using molecular iron or manganese complexes derived from linear bis-amino-bipyridyl
(N2
Py2) ligands. Increasing the rigidity of the ligand, iron catalysts based on the BPMCN and BPBP ligands result in significant
improvements in reactivity and selectivity. Herein, the authors describe the development of a series of non-heme iron complexes
obtained by combining tri-isopropyl silyl (TIPS) groups and deuterium atoms in the ligand (Figure 1). These bulky deuterated
complexes exhibited a significant increase in catalytic performance compared to their counterparts containing only TIPS groups or
deuterium atoms.
To benchmark the performances of 1-TIPS-D4
and 2-TIPS-D4
, their catalytic performance was
compared to those of the parent complexes
devoid of the ligand modifications and to
complexes containing either modification.
Specifically, the authors tested the performance
of iron complexes in the epoxidation of ciscyclooctene
and found that bulky deuterated
complexes exhibited a significant increase
in catalytic performance compared to their
counterparts containing only TIPS groups or
deuterium atoms (Table 1).
A broad range of substrates were oxidized
with excellent yields, particularly when using
1-TIPS-D4
) via a fast or slow oxidant addition
protocol, resulting in an overall improvement in
catalytic performance. In addition, kinetic and
catalytic studies showed that deuteration does
not affect the catalytic activity and the secondary
C-H site-selectivity but increases the catalyst's
lifetime, resulting in higher conversion/yield.
Accordingly, the yield of selectively oxidized
secondary C-H products also increases with the
overall yield by using the bulky deuterated iron
complexes as catalysts.
Catalysts that combine 1-TIPS-D4
and 2-TIPS-D4
were found to demonstrate a notable and
incremental enhancement in lifetime and
efficiency in these reactions. Compared to
the catalysts that only applied one or none
of the two strategies, the combined catalysts
consistently exhibit excellent performance
in several cases that outperform literature
examples. A broad screening of substrates
showed that the TIPS-modified bulky catalysts
exhibit remarkably high regioselectivity.
Furthermore, the utilization of the slow oxidant
addition protocol contributes to further
enhancing the performance of catalyst 1-TIPS-D4
compared to non-functionalized catalysts in
catalytic oxidation. Overall, a higher product
yield and regioselectivity can be obtained using
a significantly lower amount of bulky deuterated
catalysts. Fanshi L, Meijer I, Kniestedt B, et
al. (2023) ChemCatChem doi.org/10.1002/
cctc.202301128
Figure 1. Non-heme iron complexes used in this study.
Table1. Epoxidation of cis-cyclooctene using BPBP-based catalysts at 0.25 mol%
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The Catalyst Review
December 2023
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