Silver catalyzed intramolecular cyclization for synthesis of 3-alkylideneoxindoles via C-H functionalization

A novel protocol for the preparation of various 3-alkylideneoxindoles via a silver-catalyzed aromatic C-H functionalization has been developed. The process is simple, environmentally conscious, and avoids the use of abundant bases, oxidants, or other additives.     

Room temperature hydroalkylation of electron-deficient olefins: sp3 C-H functionalization via a lewis acid-catalyzed intramolecular redox event

A practical method for the intramolecular hydroalkylation of electron-deficient olefins has been developed. The direct transformation of benzylic, tertiary, and sterically hindered secondary sp3 C-H bonds into C-C bonds under the action of a catalytic amount of a variety of Lewis acids is described. The mechanism of these transformations is proposed to involve a tandem hydride transfer/cyclization sequence.     

Highly enantioselective synthesis of tetrahydroquinolines via cobalt(II)-catalyzed tandem 1,5-hydride transfer/cyclization

A chiral catalyst prepared from N,N'-dioxide and Co(BF(4))(2)·6H(2)O was applied in the asymmetric hydride transfer initiated cyclization reaction, giving optically active tetrahydroquinolines in good yields with high enantioselectivities under mild reaction conditions. Meanwhile, in light of the absolute configuration of the product, a possible working model was proposed to explain the origin of the activation and asymmetric induction.     

Lewis acid catalyzed benzylic C-H bond functionalization of azaarenes: addition to enones

A Lewis acid catalyzed benzylic C-H bond functionalization of alkyl-substituted azaarenes is described. Sc(OTf)(3) and Y(OTf)(3) promoted the direct addition of alkyl-substituted azaarenes and benzoxazole to enones and an α,β-unsaturated N-acylpyrrole. Products were obtained in 60-96% yield.     

3-O-Acyl triggered tandem Lewis acid catalyzed intramolecular cyclization of diacetone glucose derivatives to 5-O-acyl-3,6-anhydro-d-glucose

BF₃ mediated one-pot conversion of 3-O-acyl-d-glucose-1,2:5,6-diacetonide derivatives to 5-O-acyl-3,6-anhydro-d-glucose is described through a tandem selective intramolecular cyclization sequence.     

Polyene cyclization promoted by the cross-conjugated alpha-carbalkoxy enone system. Observation on a putative 1,5-hydride/1,3-alkyl shift under Lewis acid catalysis

Polyene cyclization of compounds 3 and 4 under catalysis with AlCl3 and/or SnCl4 gave rise to complex bicyclic products 8 and 9, structures of which were highly unexpected, and X-ray analyses were invoked for unambiguously structural identification. Mechanistically, a tandem sigma-bond rearrangement process, including an unusual through-space 1,5-hydride or 1,3-alkyl shift as a key operation, is proposed.     

Palladium-catalyzed synthesis of cyclopentane-fused benzocyclobutenes via tandem directed carbopalladation/C-H bond functionalization

A new Pd-catalyzed reaction for the stereoselective synthesis of cyclopentane-fused benzocyclobutenes is described. These transformations likely proceed via carbamate-directed carbopalladation followed by intramolecular C-H activation of an alkylpalladium intermediate. The mechanistic relationship between these transformations and Pd-catalyzed reactions of gamma-(n-Boc-amino)alkenes with aryl bromides that afford pyrrolidines is discussed. Differences in reactivity between Pd-amino and Pd-amido complexes appear to play a key role in the outcome of these transformations.     

Palladium-catalyzed synthesis of 2-substituted benzothiazoles via a C-H functionalization/intramolecular C-S bond formation process

Catalytic synthesis of 2-substituted benzothiazoles from thiobenzanilides was achieved in the presence of a palladium catalyst through C-H functionalization/C-S bond formation. This method features the use of a novel catalytic system consisting of 10 mol % of Pd(II), 50 mol % of Cu(I), and 2 equiv of Bu4NBr that produced variously substituted benzothiazoles in high yields with good functional group tolerance.     

Regulation of stereoselectivity using Lewis acid in the cyclization of allenic aldehydes catalyzed by palladium complex

A highly diastereoselective synthesis of 3 is achieved from the reaction of 1 with hexamethylditin catalyzed by palladium complex in the presence of 20 mol % tris(pentafluorophenyl)borane as a Lewis acid additive for a reversal of diastereoselectivity, whereas 2 is formed in the absence of Lewis acid additive. The method described herein is successful with various substrates 1 in good yields and high levels of diastereoselectivity. [reaction: see text]     

Asymmetric cyclization of unsaturated aldehydes catalyzed by a chiral lewis acid

A highly enantioselective cyclization of prochiral unsaturated aldehydes has been accomplished with a chiral zinc reagent derived from dimethylzinc and ($\text{R}$)-(+)-1,1′-bi-2-naphthol.     

I(2) promoted domino oxidative cyclization for one-pot synthesis of 2-acylbenzothiazoles via metal-free sp(3) C-H functionalization

An I(2) promoted domino protocol was developed to construct 2-acylbenzothiazoles from simple and readily available aromatic ketones/unsaturated methyl ketones and o-aminobenzenethiols. The reaction proceeded smoothly under metal-free and peroxide-free conditions.     

Rhodium-catalyzed oxidative ortho-acylation of benzamides with aldehydes: direct functionalization of the sp2 C-H bond

A rhodium-catalyzed oxidative acylation of benzamides with aryl aldehydes via direct sp(2) C-H bond cleavage is described. In the presence of [Cp*RhCl(2)](2), AgSbF(6), and silver carbonate as an oxidant, N,N-diethyl benzamides can be effectively carbonylated to yield ortho-acyl benzamides.     

A concise synthesis of tetrabenazine: an intramolecular aza-Prins-type cyclization via oxidative C-H activation

A concise synthesis of tetrabenazine and dihydrotetrabenazine is described. The key feature of this synthesis is the intramolecular aza-Prins-type cyclization of an amino allylsilane via oxidative C-H activation.     

Syntheses of indolo[3,2,1-d,e]phenanthridines and isochromeno[3,4-a] carbazoles: palladium catalyzed intramolecular arylation via C-H functionalization

A new synthetic route has been developed for the preparation of indolo[3,2,1-d,e]phenanthridines and isochromeno[3,4-a]carbazoles via palladium catalyzed intramolecular biaryl coupling reactions. The coupling reactions proceeded smoothly and in high yields under ligand-free conditions with the catalytic system Pd(OAc)₂/Cs₂CO₃/TBAB. Under optimized reaction conditions no halogen-reduced products were observed.     

Oxidative C-arylation of free (NH)-heterocycles via direct (sp3) C-H bond functionalization

The development of a new chemical transformation, namely oxidative C-arylation of saturated (NH)-heterocycles, is described. This reaction combines dehydrogenation and arylation in one process, leading to cross-coupling of (NH)-heterocycles and haloarenes. Typical reaction conditions involve heating the reaction partners in anhydrous dioxane at 120-150 degrees C in the presence of RhCl(CO)[P(Fur)3]2 as the catalyst and Cs2CO3 as the base. Addition of tert-butylethylene as the hydrogen acceptor increases the chemical yield by diminishing the dehalogenation pathway. This method demonstrated a good substrate scope, allowing for cross-coupling of a variety of (NH)-heterocycles (e.g., pyrrolidine, piperidine, piperazine, morpholine) and halo(hetero)arenes to afford valuable heterocyclic products in one step. The preliminary mechanistic studies provided some insight regarding the key events in the proposed catalytic cycle, including beta-hydride elimination of an amido rhodium complex and carbometalation of the resulting imine. A large kinetic isotope effect [KIE (kC-H/kC-D) = 4.3] suggests that one or both beta-hydride elimination steps are rate determining. The central role for the phosphine ligand was established in controlling the partitioning between the oxidative C-arylation and N-arylation pathways.