Key words
C–H bond functionalization - amines - heterocycles - catalysis - synthesis
1
Introduction
The development of methods for the C–H bond functionalization of amines continues to be a topic of significant interest. Given the potential to lead to real-world applications, coupled with the intellectually stimulating nature of the field, this sustained high level of interest is hardly surprising. A plethora of approaches have emerged over the years, exhibiting significant mechanistic diversity. In addition, an almost overwhelming number of contributions continue to be published at an ever-accelerating pace, making it challenging to keep up with what has already been accomplished, and to put new discoveries into perspective. The rapid speed of development can also obscure what has already been done well versus which transformations need further improvement (regarding scope, ease of use, cost, scalability, etc.), and which worthwhile unsolved challenges remain to be addressed. The goal of this Graphical Review is to provide a concise overview of the manifold methods that achieve the functionalization of sp3 C–H bonds in amines and their protected derivatives (e.g., amides, carbamates, N-aryl amines, etc.). We aim to cover the most important methods while highlighting the underlying mechanisms. Throughout, we have attempted to trace the origin of each approach back to a seminal report or important literature precedent. A focus is placed on historical contributions, key innovations, and the most recent cutting-edge advances. While reactions are grouped by mechanism, clear categorization of a given process is not always possible. Clearly, certain transformations would fit well into different categories. Due to the format of this review and the vast number of contributions published to date, this overview could not possibly be comprehensive, nor does it aim to be. Coverage extends to the end of 2020, with selected contributions from early 2021. We hope that this review will offer something of value to novices and experts alike. Feedback from the community is welcomed, so that a future, updated version of this review can be improved upon.
Regarding the structure of this Graphical Review, abbreviated references including prior reviews are provided within the Figures at the appropriate places. Full references are shown in the reference section and are grouped by Figure number. A note on the use of color: Amine substrates are shown in black, while groups that are being added are colored in light or dark blue. Catalysts are shown in purple or green. Other colors are used on occasion to highlight certain aspects (e.g., green for directing groups, red for hydrogens that are being functionalized, and orange for curly arrows).
(from left to right) Subhradeep Dutta was born and raised in West Bengal, India. He earned a B.Sc. degree in chemistry from Calcutta University (India) in 2016 and an M.Sc. degree in chemistry from the Indian Institute of Technology Kanpur (IITK) in 2018 under the guidance of Prof. Basker Sundararaju. In August 2018, he moved to the University of Florida (USA) for his graduate studies, joining the group of Prof. Daniel Seidel. His research focuses on developing methods towards the C–H bond functionalization of cyclic amines.
Bowen Li was born and raised in Shandong, P. R. of China. He earned a B.Sc. degree in the School of Chemistry and Chemical Engineering at Shanghai Jiao Tong University (P. R. of China) working with Prof. Wanbin Zhang. In 2019, he moved to the University of Florida (USA) for his graduate studies, joining the group of Prof. Daniel Seidel. His research focuses on asymmetric catalysis and C–H bond functionalization.
Dillon Rickertsen was born in Denver, Colorado, USA. He earned a B.Sc. degree in the Department of Chemistry at the University of Colorado, Denver (USA), working with Prof. Scott Reed. In 2019, he moved to the University of Florida for his graduate studies, joining the group of Prof. Daniel Seidel. His research is focused on developing methodologies for the C–H bond functionalization of amines.
Daniel Valles was born in Caracas, Venezuela and raised in Weston, Florida, USA. He attended the California Institute of Technology (Caltech) (USA) working with Prof. Peter Dervan, Prof. Sarah Reisman, and Dr. Scott Virgil. In 2018, he started his Ph.D. research at the University of Florida under the direction of Prof. Daniel Seidel. His research focuses on the functionalization of C–H bonds on cyclic amines.
Daniel Seidel studied chemistry at the Friedrich-Schiller-Universität Jena (Germany) and at the University of Texas at Austin (USA) (Diplom 1998). He performed his graduate studies in the lab of Prof. Jonathan L. Sessler, obtaining his Ph.D. in 2002. From 2002–2005, he was an Ernst Schering Postdoctoral Fellow in the group of Prof. David A. Evans at Harvard University (USA). He started his independent career at Rutgers University (USA) in 2005 and was promoted to Associate Professor in 2011 and Full Professor in 2014. In the summer of 2017, his research group moved to the University of Florida (USA).
Figure 1 Deprotonation of tertiary amines.[1]
Figure 2 Deprotonation of protected amines, part I.[2]
Figure 3 Deprotonation of protected amines, part II.[3]
Figure 4 Deprotonation of protected amines, part III.[4]
Figure 5 Deprotonation of protected amines, part IV.[5]
Figure 6 Transition-metal-catalyzed reactions with substrates containing directing groups, part I.[6]
Figure 7 Transition-metal-catalyzed reactions with substrates containing directing groups, part II.[7]
Figure 8 Transition-metal-catalyzed reactions with substrates containing directing groups, functionalization of amino acid derivatives.[8]
Figure 9 Transition-metal-catalyzed reactions with substrates containing directing groups, catalytic enantioselective approaches.[9]
Figure 10 Transition-metal-catalyzed reactions involving transient directing groups (TDGs).[10]
Figure 11 Native-amine-directed transition-metal-catalyzed reactions.[11]
Figure 12 Undirected transition-metal-catalyzed reactions.[12]
Figure 13 Hydroaminoalkylation.[13]
Figure 14 Oxidative methods, stoichiometric metal-based oxidants.[14]
Figure 15 Oxidative methods, stoichiometric nonmetallic oxidants.[15]
Figure 16 Oxidative preparation of building blocks.[16]
Figure 17 Metal-catalyzed cross-dehydrogenative-coupling (CDC) reactions.[17]
Figure 18 Metal-catalyzed cross-dehydrogenative-coupling (CDC) reactions with oxygen as the terminal oxidant.[18]
Figure 19 Iodine-catalyzed cross-dehydrogenative-coupling (CDC) reactions.[19]
Figure 20 Acceptorless cross-dehydrogenative-coupling (CDC) reactions with hydrogen evolution.[20]
Figure 21 Catalytic enantioselective cross-dehydrogenative-coupling (CDC) reactions.[21]
Figure 22 Oxidative β-functionalization.[22]
Figure 23 Oxidative formation of sulfur-rich heterocycles.[23]
Figure 24 Reactions involving amine N-oxides.[24]
Figure 25 Dehydrogenation/aromatization.[25]
Figure 26 Hydrogen borrowing.[26]
Figure 27 Condensation-based methods involving azomethine ylide intermediates, aromatization.[27]
Figure 28 Condensation-based methods involving azomethine ylide intermediates, pericyclic reactions.[28]
Figure 29 Condensation-based methods involving azomethine ylide intermediates, redox-neutral 3-component coupling reactions.[29]
Figure 30 Condensation-based methods involving azomethine ylide intermediates, redox-annulations.[30]
Figure 31 Internal redox transformations involving [1,n]-H transfers, the ‘tert-amino effect’.[31]
Figure 32 Lewis and Brønsted acid catalyzed internal redox transformations involving [1,n]-H transfers.[32]
Figure 33 Catalytic enantioselective internal redox transformations involving [1,n]-H transfers.[33]
Figure 34 Internal redox transformations involving [1,n]-H transfers in non-conjugated systems.[34]
Figure 35 (Redox-neutral) methods involving intermolecular hydride transfer.[35]
Figure 36 Li-amide-based imine and 1-azaallyl anion generation from unprotected azacycles.[36]
Figure 37 Reactions involving carbenes or metal carbenoids.[37]
Figure 38 Hofmann–Löffler–Freytag (HLF) reaction.[38]
Figure 39 Miscellaneous radical-based methods.[39]
Figure 40 Electrochemical approaches, cation pool method.[40]
Figure 41 Electrochemical approaches, 9-azabicyclo[3.3.1]nonane N-oxyl (ABNO) catalysis.[41]
Figure 42 Intramolecular hydrogen atom transfer (HAT).[42]
Figure 43 Direct hydrogen atom transfer (HAT).[43]
Figure 44 Photoredox approaches, part I.[44]
Figure 45 Photoredox approaches, part II.[45]
Figure 46 Indirect hydrogen atom transfer (HAT).[46]
Figure 47 Deconstructive functionalization.[47]
