Allenes, versatile unsaturated motifs in transition-metal-catalysed [2+2+2] cycloaddition reactions

Abstract

The development of efficient reactions that enable the construction of multiple bonds and/or stereogenic centres in a single synthetic operation is of great interest for greener and step-economical syntheses of complex organic targets. Transition-metal-catalysed [2+2+2] cycloadditions excell in this regard: no fewer than three new sigma bonds and a new ring system are formed from simple unsaturated components. Allenes constitute an important subclass among these. They are more reactive than simple alkenes and generate sp(3) centres, with important stereochemical implications. In addition, further manipulation of the resulting cycloadduct through the remaining double bond is possible, increasing molecular complexity. This tutorial review highlights the value that allenes have as reagents in [2+2+2] cycloaddition and their great versatility for the development of methods to generate complex architectures.

Chemoselectivity Switching in the Rhodium‐Catalyzed Reactions of 4‐Substituted‐1‐sulfonyl‐1,2,3‐triazoles with Allenols: Noticeable Differences between 4‐Acyl‐ and 4‐Aryl‐Triazoles

“…At the end of 2017, Miura and Murakami unveiled the chemistry of acceptor/ acceptor carbenoids, which were generated from 4acyl-1-mesyl-1,2,3-triazoles, for the functionalization of aromatic C(sp 2 )ÀH bonds (Scheme 1a). [4] Recently, Miura and Murakami have described the metal-catalyzed reac-tion of 4-aryl-1-tosyl-1,2,3-triazoles with simple allenes to form pyrroles or a,b,g,d-unsaturated imines (Scheme 1b), [5] while we reported the transannulation reaction of 4-aryl-1-tosyl-1,2,3-triazoles with allenols to give 2-pyrrolines (Scheme 1c). [3] On the other hand, dramatic growth in the synthetic utility of allenes has been noticed in the last decade.…”

“…[3] On the other hand, dramatic growth in the synthetic utility of allenes has been noticed in the last decade. [4] Recently, Miura and Murakami have described the metal-catalyzed reac-tion of 4-aryl-1-tosyl-1,2,3-triazoles with simple allenes to form pyrroles or a,b,g,d-unsaturated imines (Scheme 1b), [5] while we reported the transannulation reaction of 4-aryl-1-tosyl-1,2,3-triazoles with allenols to give 2-pyrrolines (Scheme 1c). [6] Based on these results, we believed that allenols might also act as an O-containing nucleophiles able to trap acceptor/ acceptor carbenoids.…”

“…Nicely, 1-(1-tosyl-1H-1,2,3-triazol-4-yl)ethan-1-one 2, which was easily prepared following the reported procedure for its methylsulfonyl counterpart, [2] was shown to be a promising reaction partner. The use of Rh 2 (Oct) 4 as catalyst (1 mol%) and chloroform as solvent allowed the coupling between 1 a and 2 to progress at 65 8C and gave a reasonable 58% yield of product 3 a (Table 1, entry 3). The efficiency of the process could not be improved by moving to different metallic salts, solvents or reaction conditions.…”

Expeditious Preparation of Open‐Cage Fullerenes by Rhodium(I)‐Catalyzed [2+2+2] Cycloaddition of Diynes and C60: An Experimental and Theoretical Study

“…On the other hand, transition‐metal‐catalyzed [2+2+2] cycloaddition reactions appear as the method of choice for the synthesis of six‐membered rings and enable the construction of cyclohexadiene scaffolds when the reaction takes place between two alkynes and one alkene . Given that the chemical reactivity of C 60 is typical of an electron‐deficient olefin, it acts as a valuable candidate as a co‐substrate in [2+2+2] cycloadditions with alkynes . Furthermore, cycloaddition reactions with empty fullerenes show a remarkable preference for the [6,6] ring junction over [5,6] bonds…”

A Computational Study of the Intermolecular [2+2+2] Cycloaddition of Acetylene and C60 Catalyzed by Wilkinson's Catalyst

“…More than stoichiometric amounts of [NiCl 2 (PPh 3 ) 2 ] and excess PPh 3 were required to obtain a high yield of cycloadduct. In our group, we have been working on [2+2+2] cycloadditions catalyzed by rhodium for many years . It is known that rhodium catalysts are very efficient in [2+2+2] cycloadditions .…”