Abstract
While Lewis acids and metal amides are among the most frequently used metal species, they are believed to be incompatible when combined. Here we describe a Lewis acid/metal amide hybrid, which contains electron-withdrawing groups and basic and bulky nitrogen functional groups in the same metal complex, as a novel catalyst. We have synthesized In(N(SiMe3)2)2Cl (In(HMDS)2Cl) and In(HMDS)2OTf as Lewis acid/metal amide hybrids, which showed excellent catalytic activity for the reaction of nitrones with terminal alkynes to give synthetically useful propargyl hydroxylamines. It is noted that neither the Lewis acids (InCl3, In(OTf)3) nor the metal amides (In(HMDS)3) have activity; only the hybrids worked well, and the catalytic activity of the hybrids was shown to be much higher than that of previously reported catalysts for this reaction. The concept of a Lewis acid/metal amide hybrid as a catalyst may be expanded to broad acid/base catalysis.
Highlights
Lewis acids have o en been used in organic chemistry as promoters and catalysts.[1,2] Friedel–Cra s acylation and alkylation with AlCl3 are typical examples;[3] more recently, several stereoselective reactions catalyzed by Lewis acids have been developed.[4,5] On the other hand, lithium diisopropylamide (LDA) and potassium hexamethyldisilazide (KHMDS) are typical examples of metal amides;[6,7] these amides abstract acidic protons to generate nucleophilic species such as carbanions for carbon–carbon bond-forming reactions (Fig. 1A)
The concept of a Lewis acid/metal amide hybrid as a catalyst may be expanded to broad acid/base catalysis
Acetylide I reacts with nitrone 1 to give adduct III via addition step II, in which the Lewis acidity of the indium bearing electron-withdrawing group X is important to activate nitrone 1. Both the Lewis acidity and Brønsted basicity of In(HMDS)2X are crucial for this reaction, and an exquisite balance between the Lewis acidity and Brønsted basicity of the Lewis acid–metal amide hybrids is key for this reaction
Summary
A Lewis acid/metal amide hybrid as an efficient catalyst for carbon–carbon bond formation†. While Lewis acids and metal amides are among the most frequently used metal species, they are believed to be incompatible when combined. We describe a Lewis acid/metal amide hybrid, which contains electron-withdrawing groups and basic and bulky nitrogen functional groups in the same metal complex, as a novel catalyst. We have synthesized In(N(SiMe3)2)2Cl (In(HMDS)2Cl) and In(HMDS)2OTf as Lewis acid/metal amide hybrids, which showed excellent catalytic activity for the reaction of nitrones with terminal alkynes to give synthetically useful propargyl hydroxylamines. It is noted that neither the Lewis acids (InCl3, In(OTf)3) nor the metal amides (In(HMDS)3) have activity; only the hybrids worked well, and the catalytic activity of the hybrids was shown to be much higher than that of previously reported catalysts for this reaction. The concept of a Lewis acid/metal amide hybrid as a catalyst may be expanded to broad acid/base catalysis
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