Twofold Unsymmetrical CH Functionalization of PyrDipSi-Substituted Arenes: A General Method for the Synthesis of Substitutedmeta-Halophenols

Propargyl Alcohols as Bifunctional Reagents for Divergent Annulations of Biphenylamines via Dual C-H Functionalization/Dual Oxidative Cyclization

The C-H functionalization strategy provides access to valuable molecules that previously required convoluted synthetic attempts. Dual C-H unsymmetrical functionalization, with a single bifunctional reagent, is an effective tactic. Propargyl alcohols (PAs), despite containing a reactive C≡C bond, have not been explored as building blocks via oxidative cleavage. Annulations via C-H activation are a versatile and synthetically attractive strategy. We disclose PA as a new bifunctional reagent for unsymmetrical dual C-H functionalization of biphenylamine for regioselectively annulated outcomes. On tuning the conditions, the annulation bifurcated towards an unusual dual oxidative cyclization. This method accommodates a wide range of PAs and showcases late-stage diversification of some natural products.

Synthesis of Polysubstituted Meta-Halophenols by Anion-Accelerated 2π-Electrocyclic Ring Opening

Disrotatory - thermally allowed - 2π-electrocyclic ring-opening reactions require high temperatures to proceed. Herein, we report the first anion-accelerated 2π-electrocyclic ring opening of 6,6-dihalobicyclo[3.1.0]hexan-2-ones at low temperature to give the corresponding meta-halophenols in good to high yields (18 examples, 29-92 % yield, average: 65 %). Many of the phenols have unconventional substitution patterns and are reported here for the first time. Furthermore, the strength of the methodology was shown by the total synthesis of the densely functionalized phenolic natural product caramboxin (isolated as the lactam dehydrate). The reaction mechanism underlying the anion-acceleration was investigated in an ab initio study, which concluded that base-mediated proton abstraction anti to the concurrently departing endo-bromine was the initiating step in an overall concerted reaction mechanism leading directly to the meta-halophenol.

IrIII -Catalyzed Selective ortho-Monoiodination of Benzoic Acids with Unbiased C-H Bonds

An iridium-catalyzed selective ortho-monoiodination of benzoic acids with two equivalent C-H bonds is presented. A wide range of electron-rich and electron-poor substrates undergo the reaction under mild conditions, with >20:1 mono/di selectivity. Importantly, the C-H iodination occurs selectively ortho to the carboxylic acid moiety in substrates bearing competing coordinating directing groups. The reaction is performed at room temperature and no inert atmosphere or exclusion of moisture is required. Mechanistic investigations revealed a substrate-dependent reversible C-H activation/protodemetalation step, a substrate-dependent turnover-limiting step, and the crucial role of the AgI additive in the deactivation of the iodination product towards further reaction.

Directing Group Assisted Unsymmetrical Multiple Functionalization of Arene C-H Bonds

Multiple C-H bond functionalizations promptly install diverse groups on the molecular framework and consequently fabricate complex molecular entities. This review briefly surveys the conceptual development of directing group assisted unsymmetrical multiple functionalization of arene C(sp² )-H bonds, which is exceedingly appealing and highly important.

Iridium-Catalyzed Sequential Silylation and Borylation of Heteroarenes Based on Regioselective C-H Bond Activation

An iridium-catalyzed regioselective sequential silylation and borylation of heteroarenes was developed, which represents a rare example of unsymmetrical intermolecular C-H bond difunctionalization through the introduction of two different functionalities during a one-pot transformation. Although the substrate scope for the dehydrogenative silylation of heteroarenes has been limited mainly to electron-rich five-membered rings, the current reaction proceeds with both electron-rich and electron-deficient heteroarenes with the aid of heteroatom-directing C-H bond activation. The regioselectivity of the second borylation was controlled by both steric factors and the electronic effect of the silyl group installed in the first step. In combination with the classic cross-coupling reaction, this method provides rapid access to multisubstituted heteroarenes.

Ortho-Functionalized Aryltetrazines by Direct Palladium-Catalyzed C-H Halogenation: Application to Fast Electrophilic Fluorination Reactions

A general catalyzed direct C-H functionalization of s-tetrazines is reported. Under mild reaction conditions, N-directed ortho-C-H activation of tetrazines allows the introduction of various functional groups, thus forming carbon-heteroatom bonds: C-X (X=I, Br, Cl) and C-O. Based on this methodology, we developed electrophilic mono- and poly-ortho-fluorination of tetrazines. Microwave irradiation was optimized to afford fluorinated s-aryltetrazines, with satisfactory selectivity, within only ten minutes. This work provides an efficient and practical entry for further accessing highly substituted tetrazine derivatives (iodo, bromo, chloro, fluoro, and acetate precursors). It gives access to ortho-functionalized aryltetrazines which are difficult to obtain by classical Pinner-like syntheses.

General method for the synthesis of salicylic acids from phenols through palladium-catalyzed silanol-directed C-H carboxylation

A silanol-directed, palladium-catalyzed C-H carboxylation reaction of phenols to give salicylic acids has been developed. This method features high efficiency and selectivity, and excellent functional-group tolerance. The generality of this method was demonstrated by the carboxylation of estrone and by the synthesis of an unsymmetrically o,o'-disubstituted phenolic compound through two sequential C-H functionalization processes.

Room-temperature ortho-alkoxylation and -halogenation of N-tosylbenzamides by using palladium(II)-catalyzed C-H activation

The N-tosylcarboxamide group can direct the room-temperature palladium-catalyzed C-H alkoxylation and halogenation of substituted arenes in a simple and mild procedure. The room-temperature stoichiometric cyclopalladation of N-tosylbenzamide was first studied, and the ability of the palladacycle to react with oxidants to form C-X and C-O bonds under mild conditions was demonstrated. The reaction conditions were then adapted to promote room-temperature ortho-alkoxylations and ortho-halogenations of N-tosylbenzamides using palladium as catalyst. The scope and limitation of both alkoxylations and halogenations was studied and the subsequent functional transformation of the N-tosylcarboxamide group through nucleophilic additions was evaluated. This methodology offers a simple and mild route to diversely functionalized arenes.