Brønsted Acid Catalyzed Friedel–Crafts‐Type Coupling and Dedinitrogenation Reactions of Vinyldiazo Compounds

Abstract

The direct Friedel–Crafts‐type coupling and dedinitrogenation reactions of vinyldiazo compounds with aromatic compounds using a metal‐free strategy are described. This Brønsted acid catalyzed method is efficient for the formation of α‐diazo β‐carbocations (vinyldiazonium ions), vinyl carbocations, and allylic or homoallylic carbocation species via vinyldiazo compounds. By choosing suitable nucleophilic reagents to selectively capture these intermediates, both trisubstituted α,β‐unsaturated esters, β‐indole‐substituted diazo esters, and dienes are obtained with good to high yields and selectivity. Experimental insights implicate a reaction mechanism involving the selective protonation of vinyldiazo compounds and the subsequent release of dinitrogen to form vinyl cations that undergo intramolecular 1,3‐ and 1,4‐ hydride transfer processes as well as fragmentation.

Intermolecular [5 + 1]-Cycloaddition between Vinyl Diazo Compounds and tert-Butyl Nitrite to 1,2,3-Triazine 1-Oxides and Their Further Transformation to Isoxazoles

“…A probable mechanism for the synthesis of the triazine Noxide 2 through formal intramolecular [5 + 1] cycloaddition is proposed in Scheme 5. Consistent with previous findings on the nucleophilicity of the terminal carbon of vinyldiazo esters, 12,22 1 selectively undergoes nitrosyl exchange with TBN to generate the nitroso-vinyl diazonium intermediate A and tert-butoxide, or either its azo-tert-butoxide or tert-butyl alcohol from proton exchange with HFIP. Then intramolecular nucleophilic association of the nitrosyl nitrogen with the terminal nitrogen of the diazonium ion occurs followed by the deprotonation to furnish the 1,2,3-triazine 1-oxide 2.…”

“…We reasoned that increasingly abundant vinyldiazo compounds might enable selective electrophilic addition to form vinyl diazonium ions and, in doing so, provide a convenient entry to valuable reactive intermediates. Indeed, metallo-vinyl carbenes have made possible highly stereoselective [3+ n ]-cycloaddition reactions (Scheme d) and, recently, bis­(trifluoro­methane­sulfonyl)­imide (HNTf 2 ) has been found to be an effective Brønsted acid catalyst to exclusively protonate the vinylogous position of vinyldiazo compounds to form highly reactive vinyl carbocations after the loss of dinitrogen (Scheme e) . Cycloadditions involving dinitrogen from the diazo functional group, including widely established [3 + 2] dipolar cycloadditions and the rare [4 + 2] Diels–Alder reaction, have been reported, and the nucleophilic trapping of diazonium ions is a classic transformation .…”

“…Indeed, metallo-vinyl carbenes have made possible highly stereoselective [3+n]-cycloaddition reactions (Scheme 2d) 11 and, recently, bis(trifluoromethanesulfonyl)imide (HNTf 2 ) has been found to be an effective Brønsted acid catalyst to exclusively protonate the vinylogous position of vinyldiazo compounds to form highly reactive vinyl carbocations after the loss of dinitrogen (Scheme 2e). 12 Cycloadditions involving dinitrogen from the diazo functional group, including widely established [3 + 2] dipolar cycloadditions 13 and the rare [4 + 2] Diels−Alder reaction, 14 have been reported, and the nucleophilic trapping of diazonium ions is a classic transformation. 15 Also, alkyl nitrites are well-known to be effective nitrosyl transfer reagents.…”

Highly Regio‐, Stereo‐, and Enantioselective Copper‐Catalyzed B−H Bond Insertion of α‐Silylcarbenes: Efficient Access to Chiral Allylic gem‐Silylboranes

“…The main byproducts of the reaction were conjugated diene 3 a′ and alkenylsilane 3 a′′ . 3 a′ is probably generated by 1,4‐hydride transfer [15] of the copper carbene intermediate, while 3 a′′ is likely formed from partial reduction of 3 a′ . The use of chiral bisphosphine ligand L1 , chiral phosphoramidite L2 , phosphine‐oxazoline L3 , or pyridine bisoxazoline L4 and bisoxazoline ligand L5 gave poor results (entries 2–6).…”

Conversion of α-Diazoketones into 1-Bromo-2-alkyl- or 2-arylpent-4-en-2-ols using Tin-Mediated Allylation/Propargylation

“…A plausible mechanism is shown in Scheme 4 . 8 The first step of the reaction is tin insertion to allyl bromide followed by 1,2-addition to the electrophilic carbonyl group of 1a to form the active intermediate 5 . Intermediate 5 decomposes by extrusion of nitrogen to give active carbene, which reacts with HBr generated during work up process.…”