Metal-free oxidative C-C bond formation of active methylenic sp C-H bonds with benzylic sp C-H and allylic sp C-H bonds mediated by DDQ

An efficient and simple method for the oxidative coupling of benzylic and allylic sp³ C-H bonds with active methylenic sp³ C-H bonds under metal-free conditions was developed by employing 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) as an oxidant. The reaction was shown to proceed smoothly for various 1,3-dicarbonyl compounds with a range of benzylic and allylic substrates in good to excellent yields.     

Catalytic intermolecular allylic C-H alkylation

The first electrophilic Pd(II)-catalyzed allylic C H alkylation is reported, providing a novel method for formation of sp3-sp3 C C bonds directly from C H bonds. A wide range of aromatic and heteroaromatic linear (E)-alpha-nitro-arylpentenoates are obtained as single olefin isomers in excellent yields directly from terminal olefin substrates and methyl nitroacetate. The use of DMSO as a pi-acidic ligand was found to be crucial for promoting functionalization of the pi-allylPd intermediate. Products from this reaction are valuable synthetic intermediates and are readily transformed to amino esters via selective reduction and optically enriched alpha,alpha-disubstituted amino acid precursors via asymmetric conjugate addition.     

Highly efficient copper-catalyzed nitro-Mannich type reaction: cross-dehydrogenative-coupling between sp3 C-H bond and sp3 C-H bond

A novel C-C bond formation method was developed via the cross-dehydrogenative-coupling (CDC) reaction catalyzed by using copper bromide in the presence of an oxidizing reagent, tert-BuOOH. The CDC reaction provides a simple and efficient catalytic method to construct beta-nitroamine via the reaction between sp3 C-H and sp3 C-H bonds.     

CuBr-catalyzed direct indolation of tetrahydroisoquinolines via cross-dehydrogenative coupling between sp3 C-H and sp2 C-H bonds

A novel and efficient C-C bond formation method was developed via the cross-dehydrogenative coupling (CDC) reaction of indoles and tetrahydroisoquinolines catalyzed by copper bromide in the presence of an oxidizing reagent, tert-BuOOH. The CDC reaction provides a simple and efficient catalytic method to construct indolyl tetrahydroisoquinolines via a combination of sp3 C-H bond and sp2 C-H bond followed by C-C bond formation.     

Palladium-catalyzed alkylation of sp2 and sp3 C-H bonds with methylboroxine and alkylboronic acids: two distinct C-H activation pathways

Palladium-catalyzed alkylations of sp2 and sp3 C-H bonds with either methylboroxine or alkylboronic acids were developed. Ag2O or AgCO3 is used as a crucial oxidant and promoter for the transmetalation step. Ether, ester, alcohol, and alkene functional groups are tolerated. A new C-H activation pathway differing from the cyclometalation process is elucidated using methylboroxine as the coupling partner.     

Intra/intermolecular direct allylic alkylation via Pd(II)-catalyzed allylic C-H activation

The first catalytic direct alkylation of allylic C-H bonds via Pd(II)-catalysis is described in the absence of base. Polysubstituted cyclic compounds can also be constructed by the intramolecular direct allylic alkylation.     

Catalytic enantioselective alkynylation of prochiral sp3 C-H bonds adjacent to a nitrogen atom

[reaction: see text] The construction of chiral carbon centers via the first catalytic asymmetric alkynylation of prochiral CH2 groups was developed by using a copper-catalyzed double activation of sp3 and sp C-H bonds. Optically active 1-alkynylated tetrahydroisoquinolines were obtained by this method.     

Copper-catalyzed amidation of allylic and benzylic C-H bonds

[Structure: see text] A copper-catalyzed amidation of allylic and benzylic C-H bonds with both primary and secondary sulfonamides is described. The reaction is applicable to the coupling of a diverse set of hydrocarbon species with aryl, heteroaryl, and alkyl sulfonamides and is tolerant of a variety of functional groups. Mechanistic insight has been gained through the isolation of a benzylic acetate intermediate, which was demonstrated to undergo facile conversion to the substituted sulfonamide product under copper catalysis.     

Palladium-catalyzed oxygenation of unactivated sp3 C-H bonds

This communication describes a new palladium-catalyzed method for the oxygenation of unactivated sp3 C-H bonds. A wide variety of alkane substrates containing readily available oxime and/or pyridine directing groups are oxidized with extremely high levels of chemo-, regio-, and in some cases diastereoselectivity. The substrate scope of these reactions is discussed, and the high selectivities are rationalized on the basis of the requirements of putative palladium alkyl intermediates.     

Palladium-catalyzed alkylation of aryl C-H bonds with sp3 organotin reagents using benzoquinone as a crucial promoter

The combination of directed C-H activation, batch-wise addition of tetraalkyltin reagents, and rate enhancement by benzoquinone and microwave irradiation provides a promising strategy for the direct coupling of C-H bonds with organometallic reagents. A variety of tetraalkyltins were coupled to C-H bonds to give the alkylated products in good yields by using 5 mol % Pd(OAc)2 as the catalyst. Benzoquinone was shown to be essential for the C-H activation when substrates containing non-pi-conjugated chelating groups are used. Monitoring the formation and reductive elimination of the Pd(Ar)(Me)L2 complex also revealed that benzoquinone promotes the reductive elimination step. Microwave irradiation enhances the reaction rate drastically. The versatility of this protocol was demonstrated by using substrates containing either oxazoline or pyridine as directing groups.     

Propargylation of 1,3-dicarbonyl compounds with 1,3-diarylpropynes via oxidative cross-coupling between sp3 C-H and sp3 C-H

A highly efficient oxidative-coupling reaction between diarylpropargylic sp(3) C-H and active methylenic sp(3) C-H was achieved with DDQ as oxidant. The reaction afforded a direct method for the propargylation of 1,3-dicarbonyl compounds, thus providing a concise synthesis of the corresponding products.     

Enantioselective gold-catalyzed functionalization of unreactive sp3 C-H bonds through a redox-neutral domino reaction

Selectivity is golden: The first asymmetric redox-neutral domino reaction catalyzed by gold that results in the direct functionalization of unreactive sp(3) C-H bonds has been reported. This method consists of a heterocyclization/1,5-hydride transfer/cyclization reaction and provides synthetically valuable azepines with high enantioselectivities and in high yields (Tf = triflate; see scheme).     

Catalytic asymmetric allylic and homoallylic diamination of terminal olefins via formal C-H activation

This paper describes a catalytic asymmetric diamination process for terminal olefins at allylic and homoallylic carbons via formal C-H activation using di-tert-butyldiaziridinone as nitrogen source with a catalyst generated from Pd2(dba)3 and chiral phosphorus amidite ligand. A wide variety of readily available terminal olefins can be effectively diaminated in good yields with high regio-, diastereo-, and enantioselectivities.     

Intermolecular amidation of unactivated sp2 and sp2 C-H bonds via palladium-catalyzed cascade C-H activation/nitrene insertion

This communication describes the Pd(OAc)2-catalyzed intermolecular amidation reactions of unactivated sp2 and sp3 C-H bonds using primary amides and potassium persulfate. The substrates containing a pendent oxime or pyridine group were amidated with excellent chemo- and regioselectivities. It is noteworthy that reactive C-X bonds were well-tolerated and a variety of primary amides can be effective nucleophiles for the Pd-catalyzed C-H amidation reactions. For the reaction of unactivated sp3 C-H bonds, beta-amidation of 1 degrees sp3 C-H bonds versus 2 degrees C-H bonds is preferred. The catalytic reaction is initiated by chelation-assisted cyclopalladation involving C-H bond activation. Preliminary mechanistic study suggested that the persulfate oxidation of primary amides should generate reactive nitrene species, which then reacted with the cyclopalladated complex.     

Catalytic alkylation of benzylic C-H bonds with 1,3-dicarbonyl compounds utilizing oxygen as terminal oxidant

The oxidative alkylation of benzylic C-H bonds with 1,3-dicarbonyl compounds was developed using oxygen as the terminal oxidant in the presence of catalytic amounts of FeCl₂, CuCl and NHPI.     

Catalytic and enantioselective allylic C-H activation with donor-acceptor-substituted carbenoids

In this perspective we give an overview of enantioselective C-H activation at allylic sites by means of rhodium(II)-stabilized donor-acceptor-substituted carbenoids. This methodology has been proven to be both an equivalent to established asymmetric reaction sequences and a new synthetic approach with no established counterpart in organic synthesis.     

Palladium-catalyzed oxidative carbonylation of benzylic C-H bonds via nondirected C(sp3)-H activation

A new strategy for generating benzylpalladium reactive species from toluenes via nondirected C(sp(3))-H activation has been developed. This led to construction of an efficient Pd-catalyzed reaction protocol for the oxidative carboxylation of benzylic C-H bonds to form substituted 2-phenylacetic acid esters and derivatives from inexpensive, commercially available starting materials.     

Catalytic asymmetric synthesis of chromenes and tetrahydroquinolines via sequential allylic alkylation and intramolecular Heck coupling

Highly enantioselective synthesis of chiral chromenes and tetrahydroquinolines is achieved by combining asymmetric copper-catalyzed allylic substitution with Grignard reagents and an efficient intramolecular Heck reaction. Moreover, the exocyclic double bond formed in the cyclisation was subjected to RCM, hydroboration and hydrogenation illuminating the synthetic versatility of these heterocycles.     

Molybdenum-catalyzed asymmetric allylic alkylation of 3-alkyloxindoles: reaction development and applications

We report a full account of our work towards the development of Mo‐catalyzed asymmetric allylic alkylation reactions with 3‐alkyloxindoles as nucleophiles. The reaction is complementary to the Pd‐catalyzed reaction with regard to the scope of oxindole nucleophiles. A number of 3‐alkyloxindoles were alkylated successfully under mild conditions to give products with excellent yields and good‐to‐excellent enantioselectivities. Applications of this method to the preparation of indoline alkaloids such as (?)‐physostigmine, ent‐(?)‐debromoflustramine B, and the indolinoquinoline rings of communesin B are reported.