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When cobalt-mediated [2 + 2 + 2] cycloaddition reaction dares go astray: synthesis of unprecedented cobalt( iii )-complexes

Abstract : In sharp contrast with the standard [2 + 2 + 2] cycloaddition reaction of diyne/ene, cobalt-mediated cycloadditions with γ-alkylidenebutenolide led to unprecedented cobalt(III) polycyclic complexes. A plausible mechanism supported by a computational study based on an unusual fragmentation of the butenolide moiety was postulated to account for this original reaction. The transition metal-catalyzed [2 + 2 + 2] cycloaddition reaction is well established as one of the most elegant methods for the rapid construction of functionalised aromatic and hetero-aromatic polycyclic compounds, as well as 1,3-cyclohexadiene derivatives. Since the original report of a metal-mediated [2 + 2 + 2] cycloaddition reaction of acetylene derivatives, 1 many groups have demonstrated the significance of this reaction through the use of various transition metal complexes, mainly from group 8-10 elements (Ir, Ru, Ni, Fe, Co, Rh, Nb the most employed). 2 Among all the available catalysts, CpCoL 2 complexes (L = CO, PR 3 , alkenes) have been frequently employed as an active source of "CpCo". Such systems have proved their efficiency (i) for the co-cyclisation of alkynes with alkenes to give cyclohexadiene products 1 in their complexed form, 2a (ii) for the linear 2 : 1 co-oligomerisation 3 of alkynes with acyclic enol ethers (R 3 = alkoxyl) to give 1-alkoxy-1,3,5-triene derivatives 2 (Scheme 1a). It is worth noting that the use of cyclic enol ethers such as 2,3-dihydrofuran produced unconju-gated 1,3,6-trienes 3 whereas benzofuran derivatives led to the formation of the corresponding CpCo-complexed cyclohexa-dienes. 4 Herein, we report the development of a cobalt-mediated polycyclisation reaction with the original enediynes 3 bearing an alkylidenebutenolide moiety. Compared to "classic" alkenes as well as "classic" enol ethers, alkylidene-butenolides display a unique exo-cyclic double bond. This double bond, which was never been used as 2π partner in intra-or intermolecular [2 + 2 + 2] cycloaddition reactions, could have two distinct electronic properties due to the cyclic enol acetate and γ,δ-unsaturated lactone moieties. This speci-ficity would account for the unusual reactivity (Scheme 1b and c). The main features of the present work include (i) formal (2 + 2 + 3) cycloaddition reaction, (ii) unprecedented formation of cobalt(III) complexes 4 featuring both π-allyl and carbanion ligands (highlighted in green in Scheme 1c) and (iii) high levels of regio-and diastereoselectivities. These cobalt(III) complexes rearranged in acidic conditions to give bicyclo[3.3.1] non-3-en-2-one scaffolds found in polyprenylated acylphloro-glucinol natural products. Furthermore, a mechanism pathway, supported by a computational approach, based on Scheme 1 Outcomes of cobalt(I)-mediated process. † Electronic supplementary information (ESI) available: Synthesis procedures, 1 H and 13 C NMR spectra of each compounds and computational studies. CCDC 1853760, 1853762, 1853766 and 1853778. For ESI and crystallographic data in CIF or other electronic format see
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Marion Delorme, Alexander Punter, Raquel Oliveira, Corinne Aubert, Yannick Carissan, et al.. When cobalt-mediated [2 + 2 + 2] cycloaddition reaction dares go astray: synthesis of unprecedented cobalt( iii )-complexes. Dalton Transactions, Royal Society of Chemistry, 2019, 48 (42), pp.15767-15771. ⟨10.1039/C9DT03311D⟩. ⟨hal-02354762⟩



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