Bora-amidinate as a cooperative ligand in group 2 metal catalysis

Abstract

Syntheses and crystal structures of the monomeric bora-amidinate (bam) complexes NBN-Mg·(THF) and NBN-Ca·(THF) are presented; NBN = HB[N(2,6-iPr-CH)]. The simplicity of their H NMR spectra in THF-d suggest that their monomeric solid state structures are retained in solution. NBN-Mg·(THF) in CD, however, is in equilibrium with a dimeric species. Calculations (B3PW91/6-311++G**) reveal a very high localized negative charge (NPA: -1.103) on the N atoms in NBN-Mg. The strongly basic properties of the bam ligand are in agreement with catalytic activity of these complexes in the intramolecular alkene hydroamination. A mechanism is proposed in which the bam ligand is non-innocent and cooperative, playing an active role in substrate deprotonation and product protonation.

Intramolecular Alkene Hydroamination with Hybrid Catalysts Consisting of a Metal Salt and a Neutral Organic Base

“…The activities do not follow the trends in group 2 but decrease in the order: CaI 2 > SrI 2 >> MgI 2 >> BaI 2 . This observation is in accordance with previously published trends in alkaline earth metal catalyzed hydroamination . The superior performance of CaI 2 may be explained by two antiparallel trends in group 2.…”

“…Replacing the solvent from benzene to THF generally led to faster conversion (Table ). This contrasts with earlier results, showing a decelerating effect of the polar solvent THF, which was explained by competition between metal–THF and metal‐substrate coordination …”

Alkaline Earth Metal Compounds of Methylpyridinato β-Diketiminate Ligands and Their Catalytic Application in Hydroboration of Aldehydes and Ketones

“…Recently, homoleptic complexes of alkaline earth metals have been shown as potent catalysts. For example, Harder and co-workers demonstrated the catalytic activity of bora-amidinate (bam) complexes DIPP NBN-Mg·(THF) 3 and DIPP NBN-Ca·(THF) 4 in the intramolecular alkene hydroamination, in which the bora-amidinate ligand functions as a noninnocent ligand and partakes in substrate deprotonation and product protonation . The metal–ligand cooperative catalysis has been further shown for tetranuclear Sr and Ba complexes by the same group for intramolecular alkene hydroamination, alkene hydrophosphination, pyridine hydroboration, pyridine hydrosilylation, and alkene hydrosilylation .…”

Tetranuclear Strontium and Barium Siloxide/Amide Clusters in Metal‐Ligand Cooperative Catalysis

“…The major principles, however, are similar. In both fields the catalyst L‐M‐R generally consists of a spectator ligand L and a reactive group R. We recently reported on the first examples of group 2 metal catalysts in which a dianionic bidentate ligand acts cooperatively . For example, the bora‐amidinate complex (NBN)Sr · (THF) 4 {NBN = HB[N(DIPP)] 2 , DIPP = 2,6‐diisopropylphenyl} is active in intramolecular alkene hydroamination (Scheme , top).…”