Tian-Sheng Mei

Activation of remote meta -C–H bonds assisted by an end-on template

Functionalization of unactivated carbon–hydrogen (C–H) single bonds is an efficient strategy for rapid generation of complex molecules from simpler ones. However, it is difficult to achieve selectivity when multiple inequivalent C–H bonds are present in the target molecule. The usual approach is to use σ-chelating directing groups, which lead to ortho-selectivity through the formation of a conformationally rigid six- or seven-membered cyclic pre-transition state. Despite the broad utility of this approach, proximity-driven reactivity prevents the activation of remote C–H bonds. Here we report a class of easily removable nitrile-containing templates that direct the activation of distal meta-C–H bonds (more than ten bonds away) of a tethered arene. We attribute this new mode of C–H activation to a weak ‘end-on’ interaction between the linear nitrile group and the metal centre. The ‘end-on’ coordination geometry relieves the strain of the cyclophane-like pre-transition state of the meta-C–H activation event. In addition, this template overrides the intrinsic electronic and steric biases as well as ortho-directing effects with two broadly useful classes of arene substrates (toluene derivatives and hydrocinnamic acids).

Pd(II)-catalyzed amination of C-H bonds using single-electron or two-electron oxidants.

Pd(II)-catalyzed intramolecular amination of arenes is developed using either a one- or two-electron oxidant. The reaction protocol tolerates a wide range of deactivating groups including acetyl, cyano, and nitro groups. This catalytic reaction allows expedient syntheses of broadly useful substituted indolines or indoles.

Versatile Pd(OTf)2·2H2O-Catalyzed ortho-Fluorination Using NMP as a Promoter

Pd(OTf)(2) x 2 H(2)O-catalyzed ortho-fluorination of triflamide-protected benzylamines is reported. The use of N-fluoro-2,4,6-trimethylpyridinium triflate as the F(+) source and NMP as a promoter is crucial for this reaction. The conversion of triflamide into a wide range of synthetically useful functional groups makes this fluorination protocol broadly applicable in medicinal chemistry and synthesis.

Pd-Catalyzed Intermolecular C―H Amination with Alkylamines

C-H amination of N-aryl benzamides with O-benzoyl hydroxylamines has been achieved with either Pd(II) or Pd(0) catalysts. Furthermore, we demonstrate that secondary amines can be directly used with benzoyl peroxide in a one-pot procedure that proceeds via the in situ generation of the appropriate O-benzoyl hydroxylamines. This catalytic reaction provides a new disconnection for the convergent synthesis of tertiary and secondary arylalkyl amines starting from benzoic acids.

Bystanding F+ Oxidants Enable Selective Reductive Elimination from High‐Valent Metal Centers in Catalysis

Reductive elimination from partially or completely oxidized metal centers is a vital step in a myriad of carbon-carbon and carbon-heteroatom bond-forming reactions. One strategy for promoting otherwise challenging reductive elimination reactions is to oxidize the metal center using a two-electron oxidant (that is, from M((n)) to M((n+2))). However, many of the commonly used oxidants for this type of transformation contain oxygen, nitrogen, or halogen moieties that are subsequently capable of participating in reductive elimination, thus leading to a mixture of products. In this Minireview, we examine the use of bystanding F(+) oxidants for addressing this widespread problem in organometallic chemistry and describe recent applications in Pd(II) /Pd(IV) and Au(I) /Au(III) catalysis. We then briefly discuss a rare example in which one-electron oxidants have been shown to promote selective reductive elimination in palladium(II)-catalyzed C-H functionalization, which we view as a promising future direction in the field.

Versatile Pd(II)-Catalyzed C−H Activation/Aryl−Aryl Coupling of Benzoic and Phenyl Acetic Acids

The use of aryltrifluoroborates as coupling partners and O(2) as the oxidant substantially improves the scope and practicality of the Pd-catalyzed C-H activation/C-C coupling reaction. The newly discovered protocol made possible, for the first time, the ortho-coupling of electron-deficient arenes and phenyl acetic acids with organometallic reagents.

Pd(II)-Catalyzed o-C−H Acetoxylation of Phenylalanine and Ephedrine Derivatives with MeCOOOtBu/Ac2O

Pd(II)-catalyzed ortho C-H acetoxylation of triflate protected phenethyl- and phenpropylamines has been achieved with tert-butyl peroxyacetate as the stoichiometric oxidant and either DMF or CH(3)CN as the promoter. The reaction was found to tolerate a large variety of functional groups and could be combined with subsequent intramolecular amination to afford functionalized indoline derivatives.

Expedient Drug Synthesis and Diversification via ortho-C−H Iodination using Recyclable PdI2 as the Precatalyst

Pd(II)-catalyzed ortho-C-H iodination reactions of phenylacetic acid substrates have been achieved using recyclable PdI(2) as the precatalyst. This class of substrates is incompatible with the classic amide formation/ortho-lithiation/iodination sequence. The power of this new technology is demonstrated by facile drug functionalization and drastically shortened syntheses of the drugs diclofenac and lumiracoxib.

Synthesis of Indolines via Pd(II)-Catalyzed Amination of C–H Bonds Using PhI(OAc)2 as the Bystanding Oxidant

The Pd(II)-catalyzed intramolecular C-H amination of 2-pyridinesulfonyl-protected phenethylamine derivatives has been achieved using PhI(OAc)2 as a bystanding oxidant, providing access to a variety of substituted indoline derivatives in good yields. The use of the 2-pyridinesulfonyl protecting group allows for facile deprotection following C-H functionalization.

γ,δ,ε-C(sp3)–H Functionalization through Directed Radical H-Abstraction

Aliphatic amides are selectively functionalized at the γ- and δ-positions through directed radical 1,5 and 1,6 H-abstractions, respectively. The initially formed γ- or δ-lactams are intercepted by N-iodosuccinimide and trimethylsilyl azide, leading to double and triple C-H functionalizations at the γ-, δ-, and ε-positions. This new reactivity is exploited to convert alkyls into amino alcohols and allylic amines.