Hydrogen-bonding catalysis of sulfonium salts

Abstract

Although quaternary ammonium and phosphonium salts are known as important catalysts in phase-transfer catalysis, the catalytic ability of tertiary sulfonium salts has not yet been well demonstrated. Herein, we demonstrate the catalytic ability of trialkylsulfonium salts as hydrogen-bonding catalysts on the basis of the characteristic properties of the acidic α hydrogen atoms on alkylsulfonium salts.The importance of onium salt compounds has been established in the field of organic chemistry. 1 Alkyl-ammonium, phosphonium, and sulfonium salts are some of the most important and reliable onium salt reagents in organic synthesis (Fig. 1). These compounds are often utilized as very useful reagents in the construction of organic building blocks, and the reactions using these reagents appear in textbooks of organic chemistry as important, named reactions. 2,3 Furthermore, quaternary ammonium and phosphonium salts are also known as reliable catalysts, which are used to promote a wide variety of organic transformations as phase-transfer and/or base catalysts. 4 Despite the wide synthetic utility of onium salt compounds as reagents and catalysts, the catalytic ability of tertiary sulfonium salts has not yet been demonstrated well in organic synthesis. 5 The limitation of sulfonium salt catalysts has been attributed mainly to the high reactivity and instability of the compounds with acidic α hydrogen atoms. To create new possibilities for sulfonium salts as catalysts, we focused on the hydrogen-bonding abilities of α hydrogen atoms on alkylsulfonium salts when we reported the use of type 1 tetraalkylammonium salts as hydrogen-bonding catalysts on the basis of the characteristic properties of the α hydrogen atoms (Fig. 2). 6,7 Herein, we demonstrate the catalytic ability of trialkylsulfonium salts in hydrogen-bonding catalysis. 8 Fig. 1 Onium salts in organic synthesis.Based on the design of type 1 tetraalkylammonium salts as effective hydrogen-bonding catalysts, 6 we focused on simple type 2 trialkylsulfonium salts (Fig. 2). The structures of the α hydrogen atoms that binded to the iodide anion compared favorably to the X-ray crystal structures of ammonium iodide 1a and sulfonium iodide 2a. 9,10 Furthermore, we expected the acidity of the α hydrogen atoms of 2a to approximate the acidity of 1a, based on the reported pKa values. 11 Fig. 2 Comparison between ammonium iodide 1a and sulfonium iodide 2a.

Hydrogen‐Bonding Catalysis of Alkyl‐Onium Salts

“…An aza-Diels-Alder reactiono fN-phenylbenzaldimine with aD anishefsky's diene was employed as a model reaction to evaluate the ability of sulfonium salts to activate imines (Scheme 14). [32] Cyclic trialkylsulfonium iodide 9a showedr eactivity that was highert han that of piperidine-derived ammonium iodides 2a,a nd the reactionw ith 9a provided at arget aza-Diels-Alder product in ag ood yield (81 % yield). Sulfonium barfate 9b further improved the catalytic ac-tivity to provide the product in very high yield (98 %y ield).…”

“…To create new possibilities for trialkylsulfonium salts, our group also became interested in the hydrogen‐bonding catalysis of cyclic trialkylsulfonium salts 9 when we determined that cyclic tetraalkylammonium salts 1 worked as hydrogen‐bonding catalysts on the basis of the characteristic properties of the α‐hydrogen atoms (Figure , see also Schemes and ) . The structure and anion binding mode of sulfonium iodide 9 a is similar to that of ammonium iodide 1 a .…”

“…The hydrogen‐bonding catalytic abilities of cyclic trialkylsulfonium salts 9 in Mannich‐type reactions involving an N ‐acylisoquinoline derivative have been discussed and compared with the catalytic activities of cyclic tetraalkylammonium salts 1 and 2 (Scheme ) . As mentioned in Scheme , ammonium iodide catalyst 1 a bearing carboxylate groups at the α‐carbons was moderately promoted a Mannich‐type reaction.…”

“…To expand the utility of cyclic trialkylsulfonium salts 9 as hydrogen‐bonding catalysts, we next became interested in the activation of imines. An aza‐Diels–Alder reaction of N ‐phenylbenzaldimine with a Danishefsky's diene was employed as a model reaction to evaluate the ability of sulfonium salts to activate imines (Scheme ) . Cyclic trialkylsulfonium iodide 9 a showed reactivity that was higher than that of piperidine‐derived ammonium iodides 2 a , and the reaction with 9 a provided a target aza‐Diels–Alder product in a good yield (81 % yield).…”

Chiral Tertiary Sulfonium Salts as Effective Catalysts for Asymmetric Base‐Free Neutral Phase‐Transfer Reactions

“…On the other hand, when the reaction was performed in the H 2 O/toluene biphasic solvent system, the generated TfOH was moved into the aqueous phase.C onsequently,c ontact between sulfonium enolate A and TfOH was suppressed, and the formation of sulfonium enolate A was promoted. Theu rea moiety of catalyst 2c interacted with maleimide 5a via hydrogen bonds,t hus returning aw ell-organized complex B.I ti sn oteworthy that hydrogen-bonding interactions between the a-hydrogen atoms of the trialkylsulfonium salt moiety [14] on catalyst 2c and the substrates may also be important in organizing the transition-state structure. [15] Ah ighly stereoselective conjugate addition then occurred to give intermediate C.P rotonation of intermediate C by TfOH, which was generated during the reaction of sulfonium triflate 2c with oxindole 4a, afforded product 6aa with regeneration of sulfonium salt catalyst 2c.…”