Scandium(III) Triflate-Catalyzed Friedel−Crafts Alkylation Reactions

Abstract

The Sc(OTf)3-catalyzed Friedel−Crafts alkylation reaction with an alcohol, an arenecarbaldehyde or an arenecarbaldehyde acetal as the alkylating agent affords a diarylmethane or an allylbenzene derivative highly selectively. The salient feature of this reaction is that only a catalytic amount of Sc(OTf)3 can effect the reaction. Furthermore, Sc(OTf)3 is recoverable and reusable after the synthetic reaction. The Sc(OTf)3-catalyzed benzylation using an arenecarbaldehyde and 1,3-propanediol or their acetal affords diarylmethane as a sole product in excellent yields in sharp contrast to the original Friedel−Crafts reaction. Since no reaction occurs in the absence of 1,3-propanediol, the reaction is considered to proceed through a redox process including a hydride shift. The hydride shift mechanism is strongly supported by the experimental evidence. The reaction of benzaldehyde with benzene in the presence of 1,3-propanediol-1,1,3,3,-d 4 gives rise to the deuterium incorporation into the benzylic carbon of diphenylmethane. Worthy of note is that 1,3-propanediol acts as the hydride source. Herein, diphenylmethyl 3-hydroxypropyl ether is assumed to be the most likely intermediate. In this reaction, Sc(OTf)3 catalyst effectively promotes initial acetal formation, electrophilic aromatic substitution, and successive intramolecular hydride transfer.

A Simple Catalyst for the Efficient Benzylation of Arenes by Using Alcohols, Ethers, Styrenes, Aldehydes, or Ketones

“…We observed that all NHC containing complexes of the type [IrCp*A C H T U N G T R E N N U N G (OTf) 2 A C H T U N G T R E N N U N G (NHC)] showed a higher activity than that shown by [{IrCp*Cl 2 …”

“…In first place, we studied the different activities of several Ir-Cp* complexes (1)(2)(3) in the benzylation of toluene using benzyl alcohols (Table 1). We observed that all NHC containing complexes of the type [IrCp*A C H T U N G T R E N N U N G (OTf) 2 A C H T U N G T R E N N U N G (NHC)] showed a higher activity than that shown by [{IrCp*Cl 2 …”

“…[1] These conditions, however, afford the concomitant production of hydrogen halides that often induce side reactions and many byproducts. As an alternative, recent reports have proposed the use of alcohols, [2][3][4] ethers, [5,6] styrenes [7,8] or benzylic acetates [9] for which milder reaction conditions are needed and side reactions are avoided.…”

Indium(III)‐Catalyzed Reductive Monoalkylation of Electron‐Rich Benzenes with Aliphatic Carboxylic Acids Leading to Arylalkane Derivatives

“…[9] However, these studies did not include a reductive monoalkylation of benzenes with aliphatic aldehydes. In this context, we found that when an aromatic compound was used as a solvent in the reductive iodination of carboxylic acids, unexpected monoalkylated aromatic com-1 ane (TMDS), and molecular iodine.…”

An Effective Bismuth‐Catalyzed Benzylation of Arenes and Heteroarenes

“…[8][9][10][11][12] Considerable progress has been made in the metal-catalyzed Friedel-Crafts alkylation of arenes to generate products containing a diarylmethane moiety commonly found in biologically active compounds. In addition to reports on lanthanide triflate catalysts, [13][14][15][16] Beller and co-workers demonstrated that late transition metal salts such as RhCl 3 , IrCl 3 , H 2 PdCl 4 , H 2 PtCl 6 and HAuCl 4 effectively catalyze the addition of benzyl acetates and benzyl alcohols to arenes. [17,18] Due to the high catalyst loading (5-10 mol %) and expenses of the late transition metals employed, this group recently disclosed an excellent iron-catalyzed benzylation of arenes.…”