Key words ligands - catalysts - transition-metal - organocatalyst - chemoselectivity - regioselectivity
- stereoselectivity - enantioselectivity
A catalyst is a substance that increases the rate of a reaction without modifying
the overall standard Gibbs energy change of the reaction. It can be an enzyme, a metal
complex, or an organic molecule. As compared to enzymes, chemical catalysts, including
transition-metal catalysts (usually derived from ligands and transition metals) and
organocatalysts, are much smaller, more diverse, and easier to modify, and consequently
they have been utilized in a tremendous number of applications in organic chemistry.
Ligands/catalysts, especially chiral examples, have played a crucial role in modern
synthetic chemistry, enabling chemists to develop numerous new transformations with
excellent control of the chemo-, regio-, stereo-, and/or enantioselectivities. During
recent decades, a large variety of ligands/catalysts have been developed, greatly
enlarging the toolbox of chemists for organic synthesis, and a huge number of challenging
transformations have been realized with the assistance of such catalysts. In this
SYNLETT cluster, we are delighted to present and discuss several novel catalysts and
interesting transformations that have been developed in recent years (Scheme [1 ]).
The highly enantioselective Michael reaction of malononitrile and α,β-unsaturated
aldehydes catalyzed by a chiral diarylprolinol silyl ether has been developed (Y.
Hayashi).[1 ] The design and synthesis of novel phenylcyclopropane-based secondary amine catalysts
and their applications in asymmetric reactions have been summarized in this cluster
(T. Kano).[2 ] A phosphazene base catalyst has been utilized to catalyze the addition reaction
of allyl sulfones possessing a diethoxyphosphoryloxy group with α,β-unsaturated ketones
(A. Kondoh & M. Terada).[3 ] Novel P ,N -ferrocenylpyrrolidine-containing-ligands have been synthesized and applied in Pd-catalyzed
allylic alkylations (P. J. Guiry).[4 ] The mechanochemical asymmetric transfer hydrogenation of diketones to provide chiral
1,3-diol derivatives with exceptionally high enantioselectivities has been developed
in the presence of a chiral ruthenium complex under solvent-free conditions (T. Cheng
& R. Liu).[5 ] A rhodium-catalyzed regio- and enantioselective direct allylation of methyl ketones
was realized by using a bisoxazolinephosphine ligand (C. Li).[6 ] 8-Quinolinyl oxazoline ligands have been utilized in the Ni-catalyzed asymmetric
reductive carbamoyl-alkylation of carbamoyl chloride tethered styrenes with unactivated
alkyl iodides, providing access to valuable chiral oxindoles with excellent enantioselectivities
(X. Wu & Y. Chen).[7 ] In addition, a novel class of chiral C
2 -symmetric multidentate ligands have been designed and developed by merging the 1,10-phenanthroline
unit with various chiral amino acids (X. Yang & Z. Zhang).[8 ]
Pd-catalyzed coupling of biphenyl-2-yl triflates with CH2 Br2 for the construction of fluorenes has been developed, which involved forming two
C(sp2 )–C(sp3 ) bonds in one step (Y. Zhang).[9 ] Recent advances in the catalytic enantioselective dihalogenation of alkenes have
been summarized (J.-W. Dong & Z.-M. Chen).[10 ] The development of kinetic resolution and desymmetrization of amines through nonenzymatic
asymmetric catalytic approaches in the last two decades has also been systematically
summarized in this cluster (X. Yang).[11 ] Recent progress on the Brønsted acid catalyzed intramolecular and intermolecular
alkyne alkoxylation/Claisen rearrangement has been discussed (L. Li & L.-W. Ye).[12 ] Additionally, a Pd-catalyzed stereospecific coupling of BINOL-bistriflates and zinc
cyanide and its application in the synthesis of 1,1′-binaphthyl-2,2′-bisoxazolines
(BOXAX) has been reported (D. Zhong & W.-B. Liu).[13 ]
Scheme 1 The catalysts and transformations covered in this cluster
We are extremely grateful to all the distinguished scientists who have contributed
to this SYNLETT cluster. The catalysts and transformations reported in this cluster
are limited examples but are highly inspiring for further development. A successful
catalyst may result in many reactions with excellent selectivities, and it is thus
important to continue to pursue new powerful catalysts in the area of synthetic chemistry.
We strongly believe that with the development of synthetic chemistry, more and more
novel catalysts and new transformations will be developed in the future.
Zhipeng Zhang
Baoguo Zhao
October 2022
