Dearomative Insertion of Fluoroalkyl Carbenes into Azoles Leading to Fluoroalkyl Heterocycles with a Quaternary Center.

The skeletal ring expansion of heteroarenes through carbene insertion is gaining popularity in synthetic chemistry. Efficient strategies for heterocyclic ring expansion to access heterocycles containing a fluoroalkyl quaternary carbon center through fluoroalkyl carbene insertion are highly desirable because of their broad applications in medicinal chemistry. Herein, we report a general strategy for the dearomative one-carbon insertion of azoles using fluoroalkyl N-triftosylhydrazones as fluoroalkyl carbene precursors, resulting in ring-expanded heterocycles in excellent yields with good functional-group compatibility. The broad generality of this methodology in the late-stage diversification of pharmaceutically interesting bioactive molecules and versatile transformations of the products has been demonstrated.

Site-Selective C-H Allylation of Alkanes: Facile Access to Allylic Quaternary sp3 -Carbon Centers.

The construction of allylic quaternary sp3 -carbon centers has long been a formidable challenge in transition-metal-catalyzed alkyl-allyl coupling reactions due to the severe steric hindrance. Herein, we report an effective carbene strategy that employs well-defined vinyl-N-triftosylhydrazones as a versatile allylating reagent to enable direct assembly of these medicinally desirable structural elements from low-cost alkane feedstocks. The reaction exhibited excellent site selectivity for tertiary C-H bonds, broad scope (>60 examples and >20 : 1:0 r. r.) and good efficiency, even on a gram-scale, making it a convenient alternative to the well-known Trost-Tsuji allylation reaction for the formation of alkyl-allyl bonds. Combined experimental and computational studies were employed to unravel the mechanism and origin of site- and chemoselectivity of the reaction.

Transition-metal-free azide insertion of N-triftosylhydrazones using a non-metallic azide source.

Benzylic azides, an important class of active organic synthons, were synthesized in high yields from the easily accessible N-triftosylhydrazones with stable TMSN3 under mild conditions. The reaction features high efficiency and excellent functional group tolerance, as illustrated by gram-scale synthesis and the synthesis of drug-like molecules. Mechanistic studies reveal that azidation occurs at the electron-deficient diazo-carbon via the elimination of N2 by an azide ion.

Silver-Catalyzed Vinylcarbene Insertion into C-C Bonds of 1,3-Diketones with Vinyl-N-triftosylhydrazones.

We describe the silver-catalyzed formal insertion of a vinylcarbene into unstrained C(CO)-C bonds of 1,3-diketones using vinyl-N-triftosylhydrazones as vinylcarbene precursors. This method allows the rapid synthesis of otherwise inaccessible 2-vinyl-substituted 1,4-diketones from relatively simple substrates. This mild catalytic protocol exhibits a good functional group tolerance and substrate scope and allows for good chemoselectivity control. The preliminary theoretical calculations suggest that the reaction proceeds through a stepwise enol cyclopropanation/retro-aldol pathway.

Silver-Catalyzed Stereoselective Aminosulfonylation of Alkynes.

A silver-catalyzed intermolecular aminosulfonylation of terminal alkynes with sodium sulfinates and TMSN3 is reported. This three-component reaction proceeds through sequential hydroazidation of the terminal alkyne and addition of a sulfonyl radical to the resultant vinyl azide. The method enables the stereoselective synthesis of a wide range of ß-sulfonyl enamines without electron-withdrawing groups on the nitrogen atom. These enamines are found to be suitable for a variety of further transformations.

Silver-Catalyzed [2+1] Cyclopropenation of Alkynes with Unstable Diazoalkanes: N-Nosylhydrazones as Room-Temperature Decomposable Diazo Surrogates.

The [2+1] cycloaddition of alkynes with diazo compounds represents one of the most powerful and reliable methods for the construction of cyclopropenes. However, it remains a formidable challenge to accomplish the cyclopropenation of alkynes with non-stabilized diazoalkanes, owing to the fact that such compounds are unstable and prone to detonation. Herein, we report a general silver-catalyzed cyclopropenation reaction of alkynes with unstable diazoalkanes, by for the first time the discovery and application of N-nosylhydrazones as room-temperature decomposiable diazo surrogates. This method allows for the efficient assembly a wide variety of cyclopropene derivatives that are otherwise difficult to access by conventional methods.

Tandem O-H Insertion/[1,3]-Alkyl Shift of Rhodium Azavinyl Carbenoids with Benzylic Alcohols: A Route To Convert C-OH Bonds into C-C Bonds.

Alcohols are among the most abundant and commonly used organic feedstock in industrial processes and academic research. The first tandem O-H insertion/[1,3]-alkyl shift reaction reported is between benzylic alcohols and rhodium azavinyl carbenoids derived from N-sulfonyl-1,2,3-triazoles, which provides a strategically novel way of cleaving C-OH bonds and forming C-C bonds. The substrate scope is broad, capable of covering 1°-, 2°-, and 3°-benzylic alcohols. Moreover, it constitutes a new and powerful synthetic method for constructing α-aminoketones. Mechanistic studies suggest that a [1,3]-alkyl shift of oxonium ylides is responsible for cleavage of the C-OH bonds.

Silver-Catalyzed Isocyanide-Isocyanide [3+2] Cross-Cycloaddition Involving 1,2-Group Migration: Efficient Synthesis of Trisubstituted lmidazoles.

Imidazole ring is an important five-membered aromatic heterocycle that is widely present in natural products and synthetic molecules. The isocyanide-isocyanide [3+2] cross-cycloaddition reaction constitutes a straightforward method to access imidazoles starting from the easily available chemicals. So far, only three successive reports are known and all lead to the formation of 1,4-disubstituted imidazoles. Here, we report the first isocyanide-isocyanide [3+2] cross-cycloaddition reaction allowing for the formation of 1,4,5-trisubstituted imidazoles under silver catalysis. An unexpected 1,2-migration of sulfonyl, alkoxycaybonyl, and carbamoyl groups took place during the cyclization process that is responsible for the formation of trisubstituted imidazoles. This report displayed a mechanistically novel synthetic method toward a variety of imidazole derivatives, which are otherwise difficult to access by conventional methods.

Chemo- and regioselective reductive deoxygenation of 1-en-4-yn-ols into 1,4-enynes through FeF3 and TfOH co-catalysis.

We report chemo- and regioselective direct reductive deoxygenation of 1-en-4-yn-3-ols into 1,4-enynes through FeF3 and TfOH cooperative catalysis under NBSH-mediated conditions. Further, we show the efficient synthesis of a pharmaceutically significant 1,4-enyne. The present methodology can also be used for chemo- and regioselective direct deoxygenation of other alcohols.

Silver-catalyzed cascade reaction of tosylmethyl isocyanide (TosMIC) with propargylic alcohols to (E)-vinyl sulfones: dual roles of TosMIC.

An unprecedented silver-catalyzed cascade reaction of tosylmethyl isocyanide (TosMIC) with propargylic alcohols for the efficient synthesis of (E)-vinyl sulfones has been developed where TosMIC plays a dual role as both the reactant in the allenylation of propargylic alcohols and the sulfonyl source.

Silver-Catalyzed Cross-Coupling of Isocyanides and Active Methylene Compounds by a Radical Process.

Isocyanides are versatile building blocks, and have been extensively exploited in C-H functionalization reactions. However, transition-metal-catalyzed direct C-H functionalization reactions with isocyanides suffer from over-insertion of isocyanides. Reported herein is a radical coupling/isomerization strategy for the cross-coupling of isocyanides with active methylene compounds through silver-catalysis. The method solves the over-insertion issue and affords a variety of otherwise difficult to synthesize ß-aminoenones and tricarbonylmethanes under base- and ligand-free conditions. This report presents a new fundamental C-C bond-forming reaction of two basic chemicals.

Silver-Catalyzed Tandem Hydroazidation/Alkyne-Azide Cycloaddition of Diynes with TMS-N3: An Easy Access to 1,5-Fused 1,2,3-Triazole Frameworks.

A general cascade hydroazidation and alkyne-azide 1,3-dipolar cycloaddition of diynes using silver catalysis is reported. A wide variety of diynes participated in the reaction with trimethylsilyl azide (TMS-N3) in the presence of H2O, affording the corresponding 1,5-fused-1,2,3-triazoles in good-to-excellent yields. This unprecedented protocol is operationally simple with a broad substrate scope, good functional group tolerance, and high reaction efficiency, thus providing easy access to various fused 1,2,3-triazoles.

DAST-promoted Beckmann rearrangement/intramolecular cyclization of acyclic ketoximes: access to 2-oxazolines, benzimidazoles and benzoxazoles.

The first example of DAST-promoted Beckmann rearrangement/intramolecular cyclization of acyclic ketoximes is described. This unique protocol represents a direct and effective pathway to 2-oxazolines, benzimidazoles and benzoxazoles in moderate to good yields.

Formation of benzo[f]-1-indanone frameworks by regulable intramolecular annulations of gem-dialkylthio trienynes.

An atom-economic route to benzo[f]-1-indanone frameworks has been developed starting from the readily available gem-dialkylthio trienynes by intramolecular annulations. The chemoselectivity of the intramolecular cyclizations can be regulated by both the base and the type of gas atmosphere used in the reaction, thus allowing the divergent synthesis of the corresponding functionalized benzo[f]-1-indanones in good to excellent yields.

Decarbonylative C-C bond-forming reactions of saccharins by nickel catalysis: homocoupling and cycloaddition.

Decarbonylation of saccharins by nickel catalysis enables two kinds of CC bond-forming reactions; homocoupling of saccharins to form biaryls and cycloaddition with alkynes to form benzosultams. The former represents the first reported nickel-catalyzed decarbonylative CC homocoupling reaction, whereas the latter constitutes a powerful method to pharmaceutically relevant benzosultams. The reactions proceed with good functional-group tolerance and excellent regioselectivity.

Lewis and Brønsted acid cocatalyzed reductive deoxyallenylation of propargylic alcohols with 2-nitrobenzenesulfonylhydrazide.

Invited for the cover of this issue is the group of Prof. Xihe Bi at Northeast Normal University, China. The cover illustrates the reductive deoxyallenylation of challenging sterically hindered tertiary propargylic alcohols, through the reaction with 2-nitrobenzenesulfonylhydrazide (NBSH), by the combined use of FeF3 and HOTf catalysts. Read the full text of the article at 10.1002/chem.201404692.

Modular synthesis of sulfonyl benzoheteroles by silver-catalyzed heteroaromatization of propargylic alcohols with p-toluenesulfonylmethyl isocyanide (TosMIC): dual roles of TosMIC.

A new silver-catalyzed heteroaromatization of propargylic alcohols with p-toluenesulfonylmethyl isocyanide (TosMIC) has been developed that provides an efficient and modular approach to sulfonyl benzoheteroles. For the first time, TosMIC plays a dual role in one reaction: sulfonyl source and ligand. An unprecedented deoxysulfonylation/hydration/condensation reaction pathway is disclosed.

Lewis and Brønsted acid cocatalyzed reductive deoxyallenylation of propargylic alcohols with 2-nitrobenzenesulfonylhydrazide.

Reductive deoxyallenylation of sterically hindered tertiary propargylic alcohols was realized on reaction with 2-nitrobenzenesulfonylhydrazide (NBSH) by the combined use of Lewis and Brønsted acid catalysts. This method features a broad substrate scope, mild reaction conditions, and good functional-group tolerance, and affords various mono-, di-, and trisubstituted allenes in good-to-excellent yields. The synthetic utility of this method was demonstrated by the synthesis of 2H-chromenes and 1,2-dihydroquinolines.

Silver-catalyzed cyclization of 2-pyridyl alkynyl carbinols with isocyanides: divergent synthesis of indolizines and pyrroles.

Divergent syntheses of indolizines and 2,4-disubstituted pyrroles by the silver-catalyzed cyclization of 2-pyridyl alkynyl carbinols with isocyanides are reported. These methods provide an effective route to highly functionalized indolizines and 2,4-disubstituted pyrroles in good to excellent yields. The 2,4-disubstituted pyrroles are synthesized by an unprecedented regioselective [3+2] cycloaddition of terminal alkynes with isocyanides.

General silver-catalyzed hydroazidation of terminal alkynes by combining TMS-N3 and H2O: synthesis of vinyl azides.

A general hydroazidation of unactivated alkynes using silver catalysis is reported. The reactions of diverse terminal alkynes with trimethylsilyl azide (TMS-N(3)) in the presence of H(2)O afforded the corresponding vinyl azides in good to excellent yields. This reaction has a broad substrate scope, good functional group tolerance, simple operation, and high reaction efficiency, thus providing an easy access to various functionalized vinyl azides.