Lewis Acid-Catalyzed Selective [2 + 2]-Cycloaddition and Dearomatizing Cascade Reaction of Aryl Alkynes with Acrylates

Abstract

Combined Lewis acid, consisting of two or more Lewis acids, sometimes shows unique catalytic ability, and it may promote reactions which could not be catalyzed by any of the Lewis acids solely. On the other hand, the development of efficient methods for the facile synthesis of cyclobutenes and densely functionalized decalins is an attractive target for synthetic chemists due to their versatile synthetic utilities and widespread occurrence in natural products. Herein, we wish to report an efficient method for the assembly of cyclobutenes and densely functionalized decalin skeletons through In(tfacac)-TMSBr catalyzed selective [2 + 2]-cycloaddition and dearomatizing cascade reaction of aryl alkynes with acrylates with high chemo- and stereoselectivity. The obtained cyclobutene could be easily converted into cyclobutane as well as synthetically useful 1,4- and 1,5-diketones with high chemo- and stereoselectivity. On the basis of mechanistic studies, plausible reaction mechanisms were proposed for both the [2 + 2]-cycloaddition and the dearomatizing cascade reaction. Finally, the computational studies of reaction mechanisms were conducted, and the results suggest that the combined Lewis acid could efficiently promote both reactions.

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

“…[10] More recently,R ajanbabu and co-workers achieved cobaltcatalyzed 1,3-enyne/ethylene [2+ +2] cycloaddition and (asymmetric) hydrovinylation in at andem fashion. Virtually no [2+ +2] cycloadduct was observed either using InBr 3 (10 mol %) alone as the catalyst [15] or using InBr 3 (10 mol %) in place of In (entries 11 and 12), suggesting that alow-valent cobalt species is catalytically active.Note also that the present [2+ +2] cycloaddition did not proceed at all under thermal conditions,atatemperature up to 100 8 8C. With ac atalytic system comprising CoBr 2 (5 mol %), dppf (5 mol %), and In (20 mol %), a1 :1 mixture of 1a and 2a underwent clean conversion in DCE at 60 8 8C, affording the desired [2+ +2] cycloadduct 3aa as as ingle regio-and stereoisomer in 89 % yield (entry 1).…”

“…While CH 2 Cl 2 could be used as alternative solvent (entry 9), other solvents such as MeCN were rather detrimental (entry 10). Virtually no [2+ +2] cycloadduct was observed either using InBr 3 (10 mol %) alone as the catalyst [15] or using InBr 3 (10 mol %) in place of In (entries 11 and 12), suggesting that alow-valent cobalt species is catalytically active.Note also that the present [2+ +2] cycloaddition did not proceed at all under thermal conditions,atatemperature up to 100 8 8C.…”

Me3SiBr/InCl3 catalyzed allylation of alcohols: Identifying the combined Lewis structure and investigating the reaction mechanism

“…Michael addition of enol acetates, as well as Prins reactions . Recently, Loh's group has developed a methodology to produce versatile intermediates of nature products with indium/silicon‐based Lewis acid catalyst . The mechanism of [2 + 2] cycloaddition of aryl alkynes was investigated by DFT, and the calculations suggested that the [2 + 2] cycloaddition is catalyzed by the combined Lewis acid via a stepwise pathway.…”

Facile Synthesis of Polysubstituted Indolizines via One‐Pot Reaction of 1‐Acetylaryl 2‐Formylpyrroles and Enals

“…[12] As part of our ongoing interest in the synthesis of carbo-and heterocycles, [13] we envisaged an ovel strategy for the synthesis of highly functionalized indolizines might be achieved via oxidative [4+ +2] annulation of 1-acetylaryl 2-formylpyrroles and enals. Solventc oncentration investigation indicated that 2.0 mL is proper, generating 5, 6-dihydroindolizine 3a in up to 89 %y ield with 17:1 dr (entries [12][13][14][15]. Notably,Z ou group reported "one-pot" multicomponent methodt oi ndolizines and pyrido[1,2-a]indoles via pyridine-ring construction.…”

“…In fact, retaining the highly reactive formyl moiety under basic and oxidative conditions is one of main challenges for this strategy.H erein, we present the successful development of one-pot synthetic route to indolizines based on the formal [4+ +2] annulation of 1-acetylaryl 2-formylpyrroles with enals, followed by oxidative aromatization. In the absence of LiCl, product yield and dr dramatically dropped (entry 16), indicating that Lewis acid [14,15] LiCl plays ac ritical role in this annulation. [10c] We selected the reactiono fc innamaldehyde 2a and readily available1 -acetylphenyl 2-formylpyrrole 1a as the model reaction for optimizing conditions.…”