Divergent Synthesis of Aeruginosins Based on a C(sp3)H Activation Strategy

A general and scalable access to the aeruginosin family of marine natural products, exhibiting potent inhibitory activity against serine proteases, is reported. This was enabled by the strategic use of two recently implemented Pd‐catalyzed C(sp³)?H activation reactions. The first method allowed us to obtain the common 2‐carboxy‐6‐hydroxyoctahydroindole (Choi) core of the target molecules on a large scale, whereas the second method provided a rapid and divergent access to various hydroxyphenyllactic (Hpla) subunits, including halogenated ones. This unique strategy, together with an optimization of the fragment coupling sequence allowed the synthesis of four aeruginosins, that is, 98A–C and 298A from the chiral pool. Among them, aeruginosin 298A was synthesized on an unprecedentedly large scale. In addition, halogenated aeruginosins 98A and 98C were synthesized for the first time, thanks to a fine‐tuning of the final hydrogenation step.     

A General and Scalable Synthesis of Aeruginosin Marine Natural Products Based on Two Strategic C(sp3)H Activation Reactions

An efficient and scalable access to the aeruginosin family of marine natural products, which exhibit potent inhibitory activity against serine proteases, is reported. This synthesis was enabled by the strategic use of two different, recently implemented C(sp³)? H activation reactions. The first method led to the common 2‐carboxy‐6‐hydroxyoctahydroindole (Choi) core of the target molecules on a large scale, whereas the second one provided rapid and divergent access to the various hydroxyphenyllactic (Hpla) subunits. This strategy allowed the synthesis of the aeruginosins 98B and 298A, with the latter being obtained in unprecedentedly large quantities.     

Iron‐Catalyzed C(sp2)H and C(sp3)H Methylations of Amides and Anilides

The so‐called magic methyl effect significantly boosts the bioactivities and physical properties of pharmacologically active drugs. Direct introduction of the methyl group by C?H activation was accomplished with a versatile iron catalyst, which enabled the C?H methylation of (hetero)benzamides, anilides, alkenes, and even alkanes by triazole assistance in a chemo‐, site‐ and diastereo‐selective fashion.     

Iron‐Catalyzed C(sp2)H and C(sp3)H Arylation by Triazole Assistance

Modular 1,2,3‐triazoles enabled iron‐catalyzed C? H arylations with broad scope. The novel triazole‐based bidentate auxiliary is easily accessible in a highly modular fashion and allowed for user‐friendly iron‐catalyzed C(sp²)? H functionalizations of arenes and alkenes with excellent chemo‐ and diastereoselectivities. The versatile iron catalyst also proved applicable for challenging C(sp³)? H functionalizations, and proceeds by an organometallic mode of action. The triazole‐assisted C? H activation strategy occurred under remarkably mild reaction conditions, and the auxiliary was easily removed in a traceless fashion. Intriguingly, the triazole group proved superior to previously used auxiliaries.     

Copper‐Catalyzed Intramolecular C(sp3)H and C(sp2)H Amidation by Oxidative Cyclization

The first copper‐catalyzed intramolecular C(sp³)? H and C(sp²)? H oxidative amidation has been developed. Using a Cu(OAc)₂ catalyst and an Ag₂CO₃ oxidant in dichloroethane solvent, C(sp³)? H amidation proceeded at a terminal methyl group, as well as at the internal benzylic position of an alkyl chain. This reaction has a broad substrate scope, and various β‐lactams were obtained in excellent yield, even on gram scale. Use of CuCl₂ and Ag₂CO₃ under an O₂ atmosphere in dimethyl sulfoxide, however, leads to 2‐indolinone selectively by C(sp²)? H amidation. Kinetic isotope effect (KIE) studies indicated that C? H bond activation is the rate‐determining step. The 5‐methoxyquinolyl directing group could be removed by oxidation.     

Transition Metal‐Catalyzed Carbonylative CH Bond Functionalization of Arenes and C(sp3)H Bond of Alkanes

In this article, we present the progress made in the area of carbonylative C? H functionalization, with special emphasis on arenes and alkanes. The importance of directing group assistance and C? H functionalization using CO surrogates is also included. The budding development in the area of transition metal‐catalyzed C(sp³)? H activation makes us feel it necessary to file a summary on the past, as well as current, contributions and a prospective outlook on the transition metal‐catalyzed carbonylative transformation of C? H bonds, which is the focus of this review.     

Palladium‐Catalyzed Cross‐Dehydrogenative Functionalization of C(sp2)H Bonds

The catalytic cross‐dehydrogenative coupling (CDC) reaction has received intense attention in recent years. The attractive feature of this coupling process is the formation of a C? C bond from two C? H moieties under oxidative conditions. In this Focus Review, recent advances in the palladium‐catalyzed CDC reactions of C(sp²)? H bond are summarized, with a focus on the period from 2011 to early 2013.     

Redox‐Neutral Rhodium‐Catalyzed CH Functionalization of Arylamine N‐Oxides with Diazo Compounds: Primary C(sp3)H/C(sp2)H Activation and Oxygen‐Atom Transfer

An unprecedented rhodium(III)‐catalyzed regioselective redox‐neutral annulation reaction of 1‐naphthylamine N‐oxides with diazo compounds was developed to afford various biologically important 1H‐benzo[g]indolines. This coupling reaction proceeds under mild reaction conditions and does not require external oxidants. The only by‐products are dinitrogen and water. More significantly, this reaction represents the first example of dual functiaonalization of unactivated a primary C(sp³)? H bond and C(sp²)? H bond with diazocarbonyl compounds. DFT calculations revealed that an intermediate iminium is most likely involved in the catalytic cycle. Moreover, a rhodium(III)‐catalyzed coupling of readily available tertiary aniline N‐oxides with α‐diazomalonates was also developed under external oxidant‐free conditions to access various aminomandelic acid derivatives by an O‐atom‐transfer reaction.     

Easily Accessible Auxiliary for Palladium‐Catalyzed Intramolecular Amination of C(sp2)H and C(sp3)H Bonds at δ‐ and ε‐Positions

An easily synthesized and accessible N,O‐bidentate auxiliary has been developed for selective C? H activation under palladium catalysis. The novel auxiliary showed its first powerful application in C? H functionalization of remote positions. Both C(sp²)? H and C(sp³)? H bonds at δ‐ and ε‐positions were effectively activated, thus giving tetrahydroquinolines, benzomorpholines, pyrrolidines, and indolines in moderate to excellent yields by palladium‐catalyzed intramolecular C? H amination.     

Asymmetric Organocatalytic Direct C(sp2)H/C(sp3)H Oxidative Cross‐Coupling by Chiral Iodine Reagents

An asymmetric organocatalytic direct C? H/C? H oxidative coupling reaction of N¹,N³‐diphenylmalonamides has been well established by using chiral organoiodine compounds as catalysts, wherein four C? H bonds were stereoselectively functionalized to give structurally diverse spirooxindoles with high levels of enantioselectivity. More importantly, the findings indicated that chiral hypervalent organoiodine reagents can serve as alternative catalysts for the creation of enantioselective functionalization of inactive C? H bonds.     

Rhodium(III)‐Catalyzed Alkenylation Reactions of 8‐Methylquinolines with Alkynes by C(sp3)H Activation

The alkenylation reactions of 8‐methylquinolines with alkynes, catalyzed by [{Cp*RhCl₂}₂], proceeds efficiently to give 8‐allylquinolines in good yields by C(sp³)? H bond activation. These reactions are highly regio‐ and stereoselective. A catalytically competent five‐membered rhodacycle has been structurally characterized, thus revealing a key intermediate in the catalytic cycle.     

Iridium Complexes Containing Mesoionic C Donors: Selective C(sp3)H versus C(sp2)H Bond Activation,Reactivity Towards Acids and Bases,and Catalytic Oxidation of Silanes and Water

Metalation of a C2‐methylated pyridylimidazolium salt with [IrCp*Cl₂]₂ affords either an ylidic complex, resulting from C(sp³)?H bond activation of the C2‐bound CH₃ group if the metalation is performed in the presence of a base, such as AgO₂ or Na₂CO₃, or a mesoionic complex via cyclometalation and thermally induced heterocyclic C(sp²)?H bond activation, if the reaction is performed in the absence of a base. Similar cyclometalation and complex formation via C(sp²)?H bond activation is observed when the heterocyclic ligand precursor consists of the analogous pyridyltriazolium salt, that is, when the metal bonding at the C2 position is blocked by a nitrogen rather than a methyl substituent. Despite the strongly mesoionic character of both the imidazolylidene and the triazolylidene, the former reacts rapidly with D⁺ and undergoes isotope exchange at the heterocyclic C5 position, whereas the triazolylidene ligand is stable and only undergoes H/D exchange under basic conditions, where the imidazolylidene is essentially unreactive. The high stability of the Ir?C bond in aqueous solution over a broad pH range was exploited in catalytic water oxidation and silane oxidation. The catalytic hydrosilylation of ketones proceeds with turnover frequencies as high as 6 000 h^?1 with both the imidazolylidene and the triazolylidene system, whereas water oxidation is enhanced by the stronger donor properties of the imidazol‐4‐ylidene ligands and is more than three times faster than with the triazolylidene analogue.     

Chiral Counteranion Strategy for Asymmetric Oxidative C(sp3)H/C(sp3)H Coupling: Enantioselective α‐Allylation of Aldehydes with Terminal Alkenes

The first enantioselective α‐allylation of aldehydes with terminal alkenes has been realized by combining asymmetric counteranion catalysis and palladium‐catalyzed allylic C? H activation. This method can tolerate a wide scope of α‐branched aromatic aldehydes and terminal alkenes, thus affording allylation products in high yields and with good to excellent levels of enantioselectivity. Importantly, the findings suggest a new strategy for the future creation of enantioselective C? H/C? H coupling reactions.     

A Concise and Scalable Strategy for the Total Synthesis of Dictyodendrin B Based on Sequential CH Functionalization

A sequential C? H functionalization strategy for the synthesis of the marine alkaloid dictyodendrin B is reported. Our synthesis begins from commercially available 4‐bromoindole and involves six direct functionalizations around the heteroarene core as part of a gram‐scale strategy towards the natural product.     

Ligand‐Promoted Borylation of C(sp3)H Bonds with Palladium(II) Catalysts

A quinoline‐based ligand effectively promotes the palladium‐catalyzed borylation of C(sp³)? H bonds. Primary β‐C(sp³)? H bonds in carboxylic acid derivatives as well as secondary C(sp³)? H bonds in a variety of carbocyclic rings, including cyclopropanes, cyclobutanes, cyclopentanes, cyclohexanes, and cycloheptanes, can thus be borylated. This directed borylation method complements existing iridium(I)‐ and rhodium(I)‐catalyzed C? H borylation reactions in terms of scope and operational conditions.     

Manganese‐Catalyzed Direct Nucleophilic C(sp2)H Addition to Aldehydes and Nitriles

Herein, a manganese‐catalyzed nucleophilic addition of inert C(sp²)? H bonds to aldehydes and nitriles is disclosed by virtue of a dual activation strategy. The reactions feature mild reaction conditions, excellent regio‐ and stereoselectivity, and a wide substrate scope, which includes both aromatic and olefinic C? H bonds, as well as a large variety of aldehydes and nitriles. Moreover, mechanistic studies shed light on possible catalytic cycles.     

Cobalt‐Catalyzed C(sp2)H Alkoxylation of Aromatic and Olefinic Carboxamides

The cobalt‐catalyzed alkoxylation of C(sp²)? H bonds in aromatic and olefinic carboxamides has been developed. The reaction proceeded under mild conditions in the presence of Co(OAc)₂?4H₂O as the catalyst and tolerates a wide range of both alcohols and benzamide substrates, including even olefinic carboxamides. In addition, this reaction is the first example of the direct alkoxylation of alkenes through C? H bond activation.