Rhodium(I)‐Catalyzed Cycloisomerization of Homopropargylallene‐Alkynes through C(sp³)−C(sp) Bond Activation

Abstract

Upon exposure to ac atalytic amount of [RhCl-(CO) 2 ] 2 in 1,4-dioxane,h omopropargylallene-alkynes underwent anovel cycloisomerization accompanied by the migration of the alkyne moiety of the homopropargyl functional group to produce six/five/five tricyclic compounds in good yields.A plausible mechanism was proposed on the basis of an experiment with 13 C-labeled substrate.T he resulting tricyclic derivatives were further converted into the corresponding bicyclo-[3.3.0] skeletons with vicinal cis dihydroxyg roups.Transition-metal-catalyzed cyclization reactions involving CÀH [1] and/or CÀC [2] bond activation steps have emerged as one of the most powerful and straightforward methods in terms of step economy for the construction of complex polycyclic frameworks.O ur recent endeavors in this field focused on the utilization of the inherent properties of the allene functional group [3] in the presence of additional p-components under rhodium catalysis as summarized in Scheme 1. [4,5] These novel rhodium-catalyzed cycloisomerization reactions of allenynes 1 [6] were tentatively surmised to proceed via the initial formation of the plausible rhodabicyclo[4.3.0] intermediates A,w hich should subse-quently collapse through several steps to various types of final products 2-6. [4,7] As af urther utilization of intermediate A,w ee nvisaged that allene-alkyne derivatives 7,with ahomopropargyl group, would produce benzocyclobutene derivatives 8 if it reacted in ap rocess similar to the conversion of 1c into 6 via intermediates A' ' and B (Scheme 2).Our initial study was carried out using 9,with adimethyl group at the allenic position to avoid unfavorable b-hydride elimination. [8] Thus as olution of 9a (R 1 = R 2 = Me) in 1,4dioxane was heated at 80 8 8Ci nt he presence of 10 mol %o f [RhCl(CO) 2 ] 2 .T he reaction reached completion within 10 min to provide the unexpected six/five/five tricyclic compound 10 a (R 1 = R 2 = Me) in 90 %y ield (Table 1, entry 1). Theexpected product 8 could not be detected in the reaction mixture.T hese conditions ([RhCl(CO) 2 ] 2 in 1,4-dioxane at 80 8 8C) were indeed found to be optimal for conversion of 9a into 10 a. [9] Thet wo derivatives 9b and 9c afforded 10 b and 10 c in 89 and 87 %yield, respectively (entries 2and 3). Upon exposure to the optimized reaction conditions,s ubstrates 9d and 9e,with aphenyl group at one of the two alkyne termini, reacted to give 10 d and 10 e in 78 and 85 %yield, respectively Scheme 1. Our previous work:R h I -catalyzed cyclization of allenynes 1.Scheme 2. This study:R h I -catalyzed cyclization of homopropargylallene-alkynes 7. Table 1: [RhCl(CO) 2 ] 2 -catalyzed cycloisomerizationo fhomopropargylallene-alkynes 9. [a] Entry 9 R 1 R 2 Product Yield [%] [b]

Zinc-Catalyzed Cyclization of Alkynyl Derivatives: Substrate Scope and Mechanistic Insights

“…This reaction yields various types of unsaturated lactones, which are prevalent structural motifs in natural and bioactive products, as well as useful compounds for the pharmaceutical and fragrance industries. 27 Numerous transition metal catalysts have been reported for this process, especially iridium, 28 rhodium, 29 palladium, 30 silver, 31 and gold. 32 The cycloisomerization reaction entails the π-activation of the C=C bond within the alkynoic acid.…”