Cobalt‐Catalyzed Intermolecular [2+2] Cycloaddition between Alkynes and Allenes

Abstract

An intermolecular [2+ +2] cycloaddition reaction between an alkyne and an allene is reported. In the presence of ac obalt(I)/diphosphine catalyst, an ear equimolar mixture of the alkyne and allene is converted into a3 -alkylidenecyclobutene derivative in good yield with high regioselectivity.T he reaction tolerates av ariety of internal alkynes and mono-or disubstituted allenes bearing various functional groups.T he reaction is proposed to involve regioselective oxidative cyclization of the alkyne and allene to form a4-alkylidenecobaltacyclopentene intermediate,w ith subsequent CÀCr eductive elimination.Cyclobutane and cyclobutene derivatives are important structural elements in natural products and biologically active compounds, [1] and have also served as versatile synthetic intermediates owing to their strain-driven chemical reactivities. [2] The[ 2 + +2] cycloadditions represent the most straightforward and atom-economical approaches toward such fourmembered carbocycles, [3] where the use of an allene as one of unsaturated reactants has received considerable attention. [4] With alkyne as the other unsaturated reactant, the [2+ +2] cycloaddition leading to a3 -alkylidenecyclobutene skeleton has been extensively explored in intramolecular settings under either thermal [5] or transition metal catalyzed [6] conditions (Scheme 1a). By contrast, examples of intermolecular alkyne-allene [2+ +2] cycloadditions are sparse, [7] and limited to those involving activated alkynes,s uch as dimethyl acetylenedicarboxylate,o ra ctivated allenes,s uch as 4-vinylideneoxazoline-2-one,u nder thermal conditions.F or example,t he latter case reported by Tamaru and co-workers required the use of large excess of at erminal alkyne as the reaction partner (Scheme 1b). [7c,d] To our knowledge,s elective intermolecular [2+ +2] cycloaddition between electronically unbiased alkynes and allenes under transition-metal catalysis remains elusive,w hereas different types of allene/ alkyne cycloadditions and dimerizations have been reported using rhodium and nickel catalysts. [8] Herein, we report that ac obalt/diphosphine catalyst promotes intermolecular [2+ +2] cycloaddition between an alkyne and an allene in an ear equimolar ratio (Scheme 1c). Ther eaction tolerates ab road range of internal alkynes and mono-or disubstituted allenes, affording 3-alkylidenecyclobutenes with high regioselectivity.Previous studies by the groups of Cheng and Hilt demonstrated the ability of low-valent cobalt catalysts to promote intermolecular [2+ +2] cycloadditions between internal alkynes and strained alkenes. [9] Ogoshi and co-workers reported cobalt-catalyzed [2+ +2] cycloadditions between 1,3enynes and vinylarenes to give 1-alkenylcyclobutenes. [10] More recently,R ajanbabu and co-workers achieved cobaltcatalyzed 1,3-enyne/ethylene [2+ +2] cycloaddition and (asymmetric) hydrovinylation in at andem fashion. [11] Inspired by these precedents, [12][13][14] the present study began with exploration of the reaction between 4-octyne (1a)a nd cyclohexylallene (2...

Unveiling the Chemistry and Synthetic Potential of Catalytic Cycloaddition Reaction of Allenes: A Review

“…Ding and Yoshikai reported cobalt-catalyzed intermolecular [2+2] cycloaddition of allenes and alkynes to synthesize several 3-alkylidenecyclobutenes [ 28 ]. 4-Alkylidenecobaltcyclopentene formed as intermediate via oxidative cyclization preceded by C-C reductive elimination to form desired 3-alkylidenecyclobutenes in good yields (up to 94%) and high regioselectivities.…”