Unveiling Latent α‐Iminocarbene Reactivity for Intermolecular Cascade Reactions through Alkyne Oxidative Amination

Abstract

The chemistry of reactive (electrophilic) metallocarbenes, primarily obtained by metal-catalyzed decomposition of adiazocarbonyl compounds, has been extensively developed and offers significant opportunity for strategic CÀC, CÀO, and CÀN bond formation through a variety of well-defined reaction mechanisms. [1] Given the prevalence of nitrogen atoms in biologically and pharmaceutically relevant molecules and the intrinsic electronic similarities between carbonyl and imine functional groups, the development of analogous metallocarbene chemistry with an a imine providing the necessary electrophilic activation represents both an obvious and synthetically important extension of this chemistry. However, the requisite a-diazoimine precursors readily isomerize to 1,2,3-triazoles, and these heterocycles were long believed to represent a thermodynamic sink, thus significantly delaying the development of metallocarbene chemistry associated with such systems. [2] Nonetheless, the perception that reliable methods to access a-iminocarbene chemistry would offer the potential to build structurally complex nitrogen-containing molecules in a remarkably efficient manner has recently resulted in the development of several complementary solutions to the a-diazoimine isomerization problem (Scheme 1).Specifically, Park and co-workers showed that b oximinoesters can be diazotized with minimal rearrangement to the isomeric triazoles, thus providing an entry into a-oximino acceptor-acceptor carbenoid chemistry. [3] More generally, Fokin and co-workers demonstrated that 1,2,3-triazoles can themselves act as precursors to a-iminometallocarbene intermediates, participating predictably in a range of traditional metallocarbene reactions to generate a-functionalized imine products. [4] Implicit in this strategy is the recognition that alkynes can serve as precursors to both a-oxy-and aiminocarbenes and that oxidation of an alkyne under appropriate reaction conditions could serve to reveal the desired reactive intermediate. [5] The approach of Raushel and Fokin separates the alkyne oxidation, which is achieved through copper-catalyzed azide-alkyne cycloaddition, [6] from the rhodium-catalyzed metallocarbene generation.Studies in our laboratory have focused on the direct unveiling of an intermediate with an a-iminometallocarbene reactivity profile by a metallonitrene-initiated oxidative cascade process. In intramolecular settings we have demonstrated the feasibility of this direct approach, terminating the cascade in an array of predictable carbene reactions (oxygenylide formation/[2,3] Wittig rearrangement, [7] electrophilic aromatic substitution, and cyclopropanation). [8] Despite the inherent attractiveness of this single-step approach, reaction conditions that facilitate efficient intermolecular cascade termination are required for broad applicability and synthetic utility. Herein we describe the development of a metallonitrene-initiated alkyne oxidation cascade with intermolecular trapping of the reactive intermediate by a variety of allyl ...

Intramolecular 1,5-C(sp3)–H radical amination via Co(ii)-based metalloradical catalysis for five-membered cyclic sulfamides

“… 17 The Co( ii )-based MRC is among the few catalytic systems that are effective for amination of propargylic C–H bonds without affecting the electron-rich C C π bonds. 7d , 18 , 19 Functionalization of propargylic C–H bonds of both aryl-conjugated and alkyl-substituted alkynes led to the formation of the desired 5-membered cyclic sulfamides in high yields (entries 17–19). Moreover, the propargylic C–H bond of unprotected terminal alkynes could be also aminated selectively without interference from the acidic terminal C(sp)–H bond, as demonstrated by the formation of the cyclic sulfamide 2t in 89% yield.…”

Nucleophilic Nitrenoids Through π‐Acid Catalysis: Providing a Common Basis for Rapid Access into Diverse Nitrogen Heterocycles

“…Processes that invoke the interaction of am etal-nitrogen bond, as metallonitrenoid or metallonitrene, in the addition across the p-systema re not discussed here, but have been shownt og ive access to related intermediates. [5,6] While all the reactions discussed in this review can reasonably be viewed as proceeding through an a-imino metal carbene E synthon in ag eneral retrosynthetic sense, and may be referred to as such, it is importantt on ote that this brief analysis will not capture many important aspects of the reactivity or indeed always represent the actual intermediates involved. Twop oints in particulars hould be borne in mind in order to avoid an over-simplified pictureo ft he reactivity.F irst, both D and E have electrophilic character,s or eaction at the carbon center might occur either alongside or after elimination of the nucleofuge.…”

Synthesis of Acyclic and Cyclic Sulfamates: A Review

“…135 This method has been exploited in an intermolecular cascade reaction through alkyne oxidative amination for the synthesis of seven-membered cyclic sulfamates 122 (Scheme 38). 136 A series of substrates 120 were used to obtain the corresponding enantioenriched products, the desired seven-membered cyclic sulfamates 122.…”