Employment of simple transition metal (TM = Co, Fe, Cu, Pd, Pt, Au)-based photocatalyst (PC) has led to the dramatic acceleration of known TM-catalyzed reactions, as well as to the discovery of ...unprecedented chemical transformations. Compared to the conventional cooperative/dual photocatalysis (type B), this new class of unconventional PCs operates
via
a single photoexcitation/catalytic cycle, where the TM complex plays a "double duty" role by harvesting light and catalyzing the chemical transformation. Also, these TM photocatalysts participate in the bond-forming/breaking event in the transformation
via
a substrate-TM interaction, an aspect that is uncommon for conventional photocatalysis (type A). This tutorial review highlights the recent advances in this emerging area.
Employment of simple transition metal (TM = Co, Fe, Cu, Pd, Pt, Au)-based photocatalyst (PC) has led to the dramatic acceleration of known TM-catalyzed reactions, as well as to the discovery of unprecedented chemical transformations.
In recent years, visible light-induced transition metal catalysis has emerged as a new paradigm in organic photocatalysis, which has led to the discovery of unprecedented transformations as well as ...the improvement of known reactions. In this subfield of photocatalysis, a transition metal complex serves a double duty by harvesting photon energy and then enabling bond forming/breaking events mostly via a single catalytic cycle, thus contrasting the established dual photocatalysis in which an exogenous photosensitizer is employed. In addition, this approach often synergistically combines catalyst–substrate interaction with photoinduced process, a feature that is uncommon in conventional photoredox chemistry. This Review describes the early development and recent advances of this emerging field.
Selective and efficient functionalization of ubiquitous C–H bonds is the Holy Grail of organic synthesis. Most advances in this area rely on employment of strongly or weakly coordinating directing ...groups (DGs) which have proven effective for transition-metal-catalyzed functionalization of C(sp2)–H and C(sp3)–H bonds. Although most directing groups are important functionalities in their own right, in certain cases, the DGs become static entities that possess very little synthetic leverage. Moreover, some of the DGs employed are cumbersome or unpractical to remove, which precludes the use of this approach in synthesis. It is believed, that development of a set of easily installable and removable/modifiable DGs for C–H functionalization would add tremendous value to the growing area of directed functionalization, and hence would promote its use in synthesis and late-stage functionalization of complex molecules. In particular, silicon tethers have long provided leverage in organic synthesis as easily installable and removable/modifiable auxiliaries for a variety of processes, including radical transformations, cycloaddition reactions, and a number of TM-catalyzed methods, including ring-closing metathesis (RCM) and cross-coupling reactions. Employment of Si-tethers is highly attractive for several reasons: (1) they are easy to handle/synthesize and are relatively stable; (2) they utilize cheap and abundant silicon precursors; and (3) Si-tethers are easily installable and removable/modifiable. Hence, development of Si-tethers for C–H functionalization reactions is appealing not only from a practical but also from a synthetic standpoint, since the Si-tether can provide an additional handle for diversification of organic molecules post-C–H functionalization. Over the past few years, we developed a set of Si-tether approaches for C–H functionalization reactions. The developed Si-tethers can be categorized into four types: (Type-1) Si-tethers possessing a reacting group, where the reacting group is delivered to the site of functionalization; (Type-2) Si-tethers possessing a DG, designed for selective C(sp2)–H functionalization of arenes; (Type-3) reactive Si-tethers for C–H silylation of organic molecules; and finally, (Type-4) reactive Si-tethers containing a DG, developed for selective C–H silylation/hydroxylation of challenging C(sp3)–H bonds. In this Account, we outline our advances on the employment of silicon auxiliaries for directed C–H functionalization reactions. The discussion of the strategies for employment of different Si-tethers, functionalization/modification of silicon tethers, and the methodological developments on C–C, C–X, C–O, and C–Si bond forming reactions via silicon tethers will also be presented. While the work described herein presents a substantial advance for the area of C–H functionalization, challenges still remain. The use of noble metals are required for the C–H functionalization methods presented herein. Also, the need for stoichiometric use of high molecular weight silicon auxiliaries is a shortcoming of the presented concept.
Three is not a crowd: A method for the construction of imidazopyridine, imidazoquinoline, and imidazoisoquinoline frameworks has been developed. The synthetic utility of this method was demonstrated ...in a highly efficient one‐pot synthesis of the drugs alpidem and zolpidem (see scheme).
A novel method for desaturation of aliphatic amines into enamines as well as allylic and homoallylic amines has been developed. This general protocol operates via putative aryl hybrid Pd-radical ...intermediates, which combine the signature features of radical chemistry, a hydrogen atom transfer (HAT) process, and transition metal chemistry, a selective β-hydride elimination step, to achieve efficient and selective desaturation of amines. These hybrid Pd-radical intermediates are efficiently generated under mild photoinduced conditions and are capable of a 1,n-HAT (n = 5–7) event at C(sp3)–H sites. The selectivity of HAT is tunable by varying different auxiliaries, which highlight the generality of this method. Remarkably, this desaturation method, which operates under mild conditions and does not require employment of exogenous photosensitizers or oxidants, can be performed in a practical scalable fashion from simple amines.
The migrations of different groups to the metal–carbene center of RhII‐stabilized iminocarbenes that were derived from N‐sulfonyl triazoles are discussed (see scheme). The reactivity of these ...Rh‐iminocabenes can be tuned easily by variation of substituents on the parent triazole.
A novel mild, visible‐light‐induced palladium‐catalyzed hydrogen atom translocation/atom‐transfer radical cyclization (HAT/ATRC) cascade has been developed. This protocol involves a 1,5‐HAT process ...of previously unknown hybrid vinyl palladium radical intermediates, thus leading to iodomethyl carbo‐ and heterocyclic structures.
A rad transfer: A novel mild, visible‐light‐induced palladium‐catalyzed hydrogen atom translocation/atom‐transfer radical cyclization (HAT/ATRC) cascade has been developed. This protocol involves a 1,5‐HAT process of previously unknown hybrid vinyl palladium radical intermediates, thus leading to iodomethyl carbo‐ and heterocyclic structures.
Propargylic esters and phosphates are easily accessible substrates, which exhibit rich and tunable reactivities in the presence of transition metal catalysts. -Acidic metals, mostly gold and platinum ...salts, activate these substrates for an initial 1,2- or 1,3-acyloxy and phosphatyloxy migration process to form reactive intermediates. These intermediates are able to undergo further cascade reactions leading to a variety of diverse structures. This
tutorial review
systematically introduces the double migratory reactions of propargylic esters and phosphates as a novel synthetic method, in which further cascade reaction of the reactive intermediate is accompanied by a second migration of a different group, thus offering a rapid route to a wide range of functionalized products. The serendipitous observations, as well as designed approaches involving the double migratory cascade reactions, will be discussed with emphasis placed on the mechanistic aspects and the synthetic utilities of the obtained products.
This
tutorial review
highlights transition metal-catalyzed double migratory cascades of propargylic esters and phosphates toward diverse functionalized molecules.
A general, efficient, and site-selective visible light-induced Pd-catalyzed remote desaturation of aliphatic alcohols into valuable allylic, homoallylic, and bis-homoallylic alcohols has been ...developed. This transformation operates via a hybrid Pd-radical mechanism, which synergistically combines the favorable features of radical approaches, such as a facile remote C–H HAT step, with that of transition-metal-catalyzed chemistry (selective β-hydrogen elimination step). This allows achieving superior degrees of regioselectivity and yields in the desaturation of alcohols compared to those obtained by the state-of-the-art desaturation methods. The HAT at unactivated C(sp3)–H sites is enabled by the easily installable/removable Si-auxiliaries. Formation of the key hybrid alkyl Pd-radical intermediates is efficiently induced by visible light from alkyl iodides and Pd(0) complexes. Notably, this method requires no exogenous photosensitizers or external oxidants.
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