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Thes olvent-andl igand-free [2+ +2+ +2] ruthenium-promoted cycloaddition of a,wdiynes and alkynes provides af acile and efficient strategy for the synthesis of substitutedb enzene-derived systems.T he search for the optimal reaction conditions revealedt he unprecedented catalytic activity of ruthenium trichloridef or benzannulation reactions and this atom-economicalp rocess allowed the synthesis of fused arenes including dihydrobenzofurans,i soindolines,i ndanes in good to high yields.T his practical protocola lso gave rise to the preparation of pentasubstituted aromatic derivatives and was applied to the one-gram scale synthesis of af unctionalized heterocycle.Benzannulationi sa na tom-economical andastraightforward approach to access complexs tructures starting from relatively simple materials. Tr ansition metalcatalyzed [2+ +2+ +2] cycloadditions have proven to be ap owerful tooli nt he construction of highly substituted arenes.Indeed, the [2+ +2+ +2] cycloaddition hasbeen widelyr eported with several metals,t he most common being cobalt, nickel, iridium, rhodium and ruthenium. [1] Ruthenium-catalyzed [2+ +2+ +2] cycloadditions [2][3][4][5] have been reported using mainly elaborated and expensive Ru complexes such as Cp*Ru(cod)Cl, [RuCp(MeCN) 3 ]PF 6 ,[ RuCl 2 (p-cymene)] 2 , [Cp*RuCl 2 ] 2 ,Grubbs(Iand II), Hoveyda-Grubbscatalysts (Grubbs III), or Ru(ethene)(CO) 4 ,R u 3 (CO) 12 (Scheme 1) combined with organic solvents, additional ligands,s upported systems or microwavea ssistance. Due to environmental concerns to makec hemical processesg reener, the development of atom-economical synthetic procedures involving, for instance,s olvent-free conditions hasbecome crucial. [6] To the best of our knowledge,s olventless metal-catalyzed [2+ +2+ +2] cycloadditions of diynes have been scarcelyd escribeda lthough the search for environmentally friendly protocols is essential. [7] To complete our recent interest in [2+ +2+ +2] cycloaddition reactions, [8] we were wondering if am ore practical strategy couldb ee nvisaged to access fused arenesi nt he absence of solvent andi nt he absenceo fa dditional ligand by using the commercially available,c heap and easy to handle RuCl 3 ·n H 2 O. Noteworthy,M itsudo and Tanakar eported the inefficiency of RuCl 3 and the RuCl 3 /amine system toward [2+ +2+ +2] cycloadditions of terminal alkynes with dimethyl acetylenedicarboxylate andc yclotrimerization of ethyl phenylpropiolate. [9] We and others previouslyd emonstrated the efficiency of RuCl 3 ·n H 2 Oi nC À H, C À C, C À Na nd C À Ob ondf orming processes. [10,11] In this context, we report here the first efficientR uCl 3 -catalyzed [2+ +2+ +2] cycloaddition reactions of a,w-diynesa nd alkynes under solvent-freec onditions. Scheme 1. Selected[ Ru] catalysts for [2+ +2+ +2] cycloaddition reactionsofa,w-diynes.
Thes olvent-andl igand-free [2+ +2+ +2] ruthenium-promoted cycloaddition of a,wdiynes and alkynes provides af acile and efficient strategy for the synthesis of substitutedb enzene-derived systems.T he search for the optimal reaction conditions revealedt he unprecedented catalytic activity of ruthenium trichloridef or benzannulation reactions and this atom-economicalp rocess allowed the synthesis of fused arenes including dihydrobenzofurans,i soindolines,i ndanes in good to high yields.T his practical protocola lso gave rise to the preparation of pentasubstituted aromatic derivatives and was applied to the one-gram scale synthesis of af unctionalized heterocycle.Benzannulationi sa na tom-economical andastraightforward approach to access complexs tructures starting from relatively simple materials. Tr ansition metalcatalyzed [2+ +2+ +2] cycloadditions have proven to be ap owerful tooli nt he construction of highly substituted arenes.Indeed, the [2+ +2+ +2] cycloaddition hasbeen widelyr eported with several metals,t he most common being cobalt, nickel, iridium, rhodium and ruthenium. [1] Ruthenium-catalyzed [2+ +2+ +2] cycloadditions [2][3][4][5] have been reported using mainly elaborated and expensive Ru complexes such as Cp*Ru(cod)Cl, [RuCp(MeCN) 3 ]PF 6 ,[ RuCl 2 (p-cymene)] 2 , [Cp*RuCl 2 ] 2 ,Grubbs(Iand II), Hoveyda-Grubbscatalysts (Grubbs III), or Ru(ethene)(CO) 4 ,R u 3 (CO) 12 (Scheme 1) combined with organic solvents, additional ligands,s upported systems or microwavea ssistance. Due to environmental concerns to makec hemical processesg reener, the development of atom-economical synthetic procedures involving, for instance,s olvent-free conditions hasbecome crucial. [6] To the best of our knowledge,s olventless metal-catalyzed [2+ +2+ +2] cycloadditions of diynes have been scarcelyd escribeda lthough the search for environmentally friendly protocols is essential. [7] To complete our recent interest in [2+ +2+ +2] cycloaddition reactions, [8] we were wondering if am ore practical strategy couldb ee nvisaged to access fused arenesi nt he absence of solvent andi nt he absenceo fa dditional ligand by using the commercially available,c heap and easy to handle RuCl 3 ·n H 2 O. Noteworthy,M itsudo and Tanakar eported the inefficiency of RuCl 3 and the RuCl 3 /amine system toward [2+ +2+ +2] cycloadditions of terminal alkynes with dimethyl acetylenedicarboxylate andc yclotrimerization of ethyl phenylpropiolate. [9] We and others previouslyd emonstrated the efficiency of RuCl 3 ·n H 2 Oi nC À H, C À C, C À Na nd C À Ob ondf orming processes. [10,11] In this context, we report here the first efficientR uCl 3 -catalyzed [2+ +2+ +2] cycloaddition reactions of a,w-diynesa nd alkynes under solvent-freec onditions. Scheme 1. Selected[ Ru] catalysts for [2+ +2+ +2] cycloaddition reactionsofa,w-diynes.
An efficient and practical route for the preparation of highly substituted fluorenones and analogues via solvent-free ruthenium trichloride mediated [2 + 2 + 2] cycloaddition of α,ω-diynes and alkynes has been developed. This green chemistry approach involves a solventless and atom-economical catalytic process to generate densely functionalized fluorenones and related derivatives of high synthetic utility.
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