Silver-Catalyzed Cyclopropanation of Alkenes Using N-Nosylhydrazones as Diazo Surrogates

An efficient silver-catalyzed [2 + 1] cyclopropanation of sterically hindered internal alkenes with diazo compounds in which room- temperature-decomposable N-nosylhydrazones are used as diazo surro- gates is reported. The unexpected unique catalytic activity of silver was ascribed to its dual role as a Lewis acid activating alkene substrates and as a transition metal forming silver carbenoids. A wide range of internal alkenes, including challenging diarylethenes, were suitable for this protocol, thereby affording a variety of cyclopropanes with high efficiency in a stereoselective manner under mild conditions

α-Substituted vinyl azides: An emerging functionalized alkene

Vinyl azides are highly versatile synthons that provide access to numerous N-heterocycles and other functional groups. a-Substituted vinyl azides (azido vinylidenes) are a special class that display unique reactivity, able to react not only as azides, but also as radical acceptors, enamine-type nucleophiles, and even electrophiles, thus delivering a wide range of nitrogen-containing compounds and their derivatives. An impressive variety of intermediates – such as iminodiazonium ions, nitrilium ions, iminyl radicals, and metal enaminyl radicals – can be generated from vinyl azides and exploited in cycloadditions, C–H functionalizations, hydrolysis processes, and cascade reactions under transition metal/photoredox catalysis. In addition to presenting synthetic protocols to access vinyl azides, this Review offers a comprehensive coverage of the development of their multifaceted reactivity, and highlights their potential as versatile precursors for synthetic applications

Silver-Catalyzed Tandem C=C Bond Hydroazidation/Radical Addition/Cyclization of Biphenyl Acetylene: One-Pot Synthesis of 6-Methyl Sulfonylated Phenanthridines

A silver-catalyzed tandem carbon!carbon triple bond hydroazidation, radical addition, and cyclization of biphenyl acetylene is described under mild conditions, leading to the formation of 6-methyl sulfonylated phenanthridines in good yields. In this novel cascade reaction, most of the atoms are incorporated into the product without cleavage of the CC bond. Mechanistic studies suggest the reaction should proceed through an iminyl radical reactive intermediate

Computational Insights into Different Mechanisms for Ag-, Cu-, and Pd-Catalyzed Cyclopropanation of Alkenes and Sulfonyl Hydrazones

International audienceThe [2+1] cycloaddition reaction of a metal carbene with an alkene can produce important cyclopropane products for synthetic intermediates, materials, and pharmaceutical applications. However, this reaction is often accompanied by side reactions, such as coupling and self-coupling, so that the yield of the cyclopropanation product of non-silver transition-metal carbenes and hindered alkenes is generally lower than 50 %. To solve this problem, the addition of a low concentration of diazo compound (decomposition of sulfonyl hydrazones) to alkenes catalyzed by either CuOAc or PdCl2 was studied, but side reactions could still not be avoided. Interestingly, however, the yield of cyclopropanation products for such hindered alkenes were as high as 99 % with AgOTf as a catalyst. To explain this unexpected phenomenon, reaction pathways have been computed for four different catalysts by using DFT. By combining the results of these calculations with those obtained experimentally, it can be concluded that the efficiency of the silver catalyst is due to the barrierless concerted cycloaddition step and the kinetic inhibition of side reactions by a high concentration of alkene

Defluorinative Carboimination of Trifluoromethyl Ketones

The monofunctionalized carbodefluorination of readily accessible CF3 groups is acknowledged as an attractive approach to the preparation of partially fluorinated molecules. However, their defluorinative difunctionalization remains a challenging and unmet goal. Herein, we report an unprecedented defluorinative carboimination of trifluoromethyl ketones via a strategy of silver carbene-initiated rearrangement, in which both C–F bond and carbonyl group of the trifluoromethyl ketones were functionalized simultaneously, thus providing a straightforward synthetic method for medicinally relevant α,α-difluoroimines. The current approach involves a silver carbene-initiated intramolecular cascade process by integrating successive cleavage of C–F bond and formation of C–C and C=N bonds on a single molecule entity, which differs relevantly from the stepwise mechanism of reported carbodefluorination of CF3 groups. Mechanistic studies disclose that silver catalysis plays a critical role, particularly in the stages of C–F bond cleavage and aza-Claisen rearrangement

Silver-Catalyzed [4+3] Cycloaddition of 1,3-Dienes with Vinyl-N-triftosylhydrazones: A Practical Approach to 1,4-Cycloheptadienes

The first formal [4+3] cycloaddition of 1,3-dienes with vinyl-N-sulfonylhydrazones has been achieved by virtue of easily decomposable vinyl-N-triftosylhydrazones in the presence of silver catalysis. A series of acyclic and cyclic 1,3-dienes reacted effectively with silver(I) non-acceptor vinyl carbenes, including alkyl- and alkenyl-substituted carbenes, which are notoriously challenging, producing a broad spectrum of 1,4-cycloheptadienes with high yield and predictable stereochemistry. The products resulting from the herein described transformation include bridged and fused polycyclic systems, as well as the natural product dictyopterene C\u27 and its isomer ectocarpene. The reaction mechanism and the origin of stereoselectivity were investigated via experimental work and density functional theory calculations

Silver-Catalyzed Cyclopropanation of Alkenes Using <i>N</i>‑Nosylhydrazones as Diazo Surrogates

An efficient silver-catalyzed [2 + 1] cyclopropanation of sterically hindered internal alkenes with diazo compounds in which room-temperature-decomposable <i>N</i>-nosylhydrazones are used as diazo surrogates is reported. The unexpected unique catalytic activity of silver was ascribed to its dual role as a Lewis acid activating alkene substrates and as a transition metal forming silver carbenoids. A wide range of internal alkenes, including challenging diarylethenes, were suitable for this protocol, thereby affording a variety of cyclopropanes with high efficiency in a stereoselective manner under mild conditions

Fluoroalkyl N-triftosylhydrazones as easily decomposable diazo surrogates for asymmetric [2+1] cycloaddition : synthesis of chiral fluoroalkyl cyclopropenes and cyclopropanes

The development of asymmetric carbene transfer reactions using N-sulfonylhydrazones as the diazo surrogate is a long-term issue in organic synthesis since N-sulfonylhydrazones commonly require high temperatures for their decomposition to release reactive diazo compounds. We report on the use of fluoroalkyl-aryl ketone N-triftosylhydrazones as a class of N-sulfonylhydrazone capable of decomposing below 0 degrees C (to -40 degrees C). Their application in asymmetric [2 + 1] cycloadditions with alkynes and alkenes catalyzed by a chiral rhodium catalyst is described. This protocol affords a wide variety of fluoroalkylated cyclopropenes and cyclopropanes in high yields and high enantioselectivity and demonstrates broad functional group tolerance. It is noteworthy that these small-ring products feature a fluoroalkyl chiral quaternary carbon center. The origin of enantioselectivity for the cyclopropenation reaction of N-triftosylhydrazones with alkynes was rationalized by density functional theory calculations

Modular Synthesis of Sulfonyl Benzoheteroles by Silver-Catalyzed Heteroaromatization of Propargylic Alcohols with <i>p</i>‑Toluene­sulfonyl­methyl Isocyanide (TosMIC): Dual Roles of TosMIC

A new silver-catalyzed heteroaromatization of propargylic alcohols with <i>p</i>-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