Fe(OTf)₃‐Catalyzed Aromatization of Substituted 3‐Methyleneindoline and Benzo­furan Derivatives: A Selective Route to C‐3‐Alkylated Indoles and Benzofurans

Abstract

A simple and convenient approach was developed to the selective synthesis of 3‐substituted indoles and benzofurans by the isomerization of 3‐methyleneindoline and benzofuran derivatives catalyzed by Fe(OTf)3. The salient features of this method are the easy availability of substrates, high yield, mild reaction conditions, tolerance of a variety of functional groups, and use of an environmentally friendly catalyst. A possible mechanism has also been proposed for the isomerization process.

Metal Triflates for the Production of Aromatics from Lignin

“…[12] Other strongL ewis acids such as AlCl 3 and FeCl 3 did not show anys ignificanta ctivity in the cleavage of 1 in 1,4-dioxane or toluene (Table S2). The addition of non-nucleophilic bases such as 2,6-di-tert-butyl-4-methylpyridine [13] or NaHCO 3 completely quenched all reactivity for metal triflates that were previously successful in the cleavage of 1 (Table S2). These results, combined with the similar reactionp rogress, led us to concludet hat the triflate salts (Al(OTf) 3 3 ,a nd Hf(OTf) 4 )l ikely form triflic acid in situ and are therefore capable of catalyzingt he acidolysis of 1.N evertheless, the presence of the metal seems to modulate the acidity somewhat, depending on the stability of the triflate salt applied.…”

Bismuth(III) triflate: An Efficient Catalyst for the Synthesis of Diverse Biologically Relevant Heterocycles

“…Screening of suitable catalyst plays the crucial role among the other significant parameters during chemical synthesis. In this context, last decade has seen a tremendous outburst of the applications of various metal‐triflates as Lewis acidic catalysts for the diverse organic synthesis . In recent time bismuth (III) salts as catalyst have gained significant attention for organic transformations .…”

Iron‐Catalyzed Functionalization of 3‐Benzylideneindoline Through Tandem Csp2–Csp3 Bond Formation/Isomerization with π‐Activated Alcohols

“…Based on the experimental observations and previous reports,, a plausible mechanism was proposed (Schemeà). It is believed that FeCl 3 activates the benzylic alcohol 3a and facilitates dehydration to form carbocation 3a ′ which then activates the alkene and subsequent deprotonation of the –CH 2 unit and nucleophilic attack of the alkene to the carbocation leads to our desired 3‐substituted indole 4a .…”

“…The required starting materials, 3‐benzylidene‐1‐tosylindoline derivatives 2 , were prepared by Heck coupling of substituted 2‐bromo‐ N ‐propargylanilidines 1 , using 5 mol‐% Pd(OAc) 2 , 10 mol‐% tricyclohexylphosphine (PCy 3 ), 2.5 m aqueous K 2 CO 3 in a mixed solvent of toluene and ethanol (1:1) at 70 °C within 4 hours (Schemeà) . The reaction proceeded in a stereoselective manner via 5‐ exo‐dig cyclisation to obtain the 3‐benzylidene‐1‐tosylindoline derivatives in moderate to good yields.…”

“…However, all these methods developed for the synthesis of 3‐substituted indoles rely on the availability of preformed indole derivatives, in few cases, harsh reaction conditions, and lack of generality with respect to electrophiles and low chemical yields, which are the limitations of these strategy. In this regard, our group recently reported a new protocol for the synthesis of various 3‐substituted indole derivatives via iron‐catalyzed isomerization/cycloisomerization of substituted 3‐methyleneindoline derivatives through π‐activation of double/triple bond by iron in high yields . Due to easy accessibility of 3‐methyleneindoline derivatives through tandem Heck–Suzuki or reductive Heck coupling of 2‐halo‐ N ‐propargylanilide, this has been a potentially attractive intermediate for the synthesis of 3‐substituted indole derivatives.…”