Flexible, Bowl-Shaped N-Heterocyclic Carbene Ligands: Substrate Specificity in Iridium-Catalyzed Ketone Hydrosilylation

A series of benzimidazolium chlorides was synthesized as precursors to N-heterocyclic carbene ligands, with N-substituents varying in size from 3,5-xylyl (1a) to first-generation dendritic 3,5-bis(3,5-di-tert-butylphenyl)phenyl (1b), to the second-generation 3,5-bis[3,5-bis(3,5-di-tert-butylphenyl)phenyl]phenyl (1c). The dendritic side groups of 1b and 1c form a flexible, bowl-like cavity. Iridium complexes of 1a−c were synthesized and were shown to be catalytically active for the hydrosilylation of aryl methyl ketones. The dendritic ligands 1b and 1c effect a moderate level of substrate specificity in the competitive hydrosilylation of ketones of varying size. In the competitive hydrosilylation of acetophenone versus 3-(3,5-di-tert-butylphenyl)acetophenone, acetophenone is consumed approximately 3.7 times more quickly using the second-generation ligand 1c. Using the control ligand 1a, this ratio is 1.8

Flexible, Bowl-Shaped N-Heterocyclic Carbene Ligands: Substrate Specificity in Iridium-Catalyzed Ketone Hydrosilylation

A series of benzimidazolium chlorides was synthesized as precursors to N-heterocyclic carbene ligands, with N-substituents varying in size from 3,5-xylyl (1a) to first-generation dendritic 3,5-bis(3,5-di-tert-butylphenyl)phenyl (1b), to the second-generation 3,5-bis[3,5-bis(3,5-di-tert-butylphenyl)phenyl]phenyl (1c). The dendritic side groups of 1b and 1c form a flexible, bowl-like cavity. Iridium complexes of 1a−c were synthesized and were shown to be catalytically active for the hydrosilylation of aryl methyl ketones. The dendritic ligands 1b and 1c effect a moderate level of substrate specificity in the competitive hydrosilylation of ketones of varying size. In the competitive hydrosilylation of acetophenone versus 3-(3,5-di-tert-butylphenyl)acetophenone, acetophenone is consumed approximately 3.7 times more quickly using the second-generation ligand 1c. Using the control ligand 1a, this ratio is 1.8

Flexible, Bowl-Shaped N-Heterocyclic Carbene Ligands: Substrate Specificity in Iridium-Catalyzed Ketone Hydrosilylation

A series of benzimidazolium chlorides was synthesized as precursors to N-heterocyclic carbene ligands, with N-substituents varying in size from 3,5-xylyl (1a) to first-generation dendritic 3,5-bis(3,5-di-tert-butylphenyl)phenyl (1b), to the second-generation 3,5-bis[3,5-bis(3,5-di-tert-butylphenyl)phenyl]phenyl (1c). The dendritic side groups of 1b and 1c form a flexible, bowl-like cavity. Iridium complexes of 1a−c were synthesized and were shown to be catalytically active for the hydrosilylation of aryl methyl ketones. The dendritic ligands 1b and 1c effect a moderate level of substrate specificity in the competitive hydrosilylation of ketones of varying size. In the competitive hydrosilylation of acetophenone versus 3-(3,5-di-tert-butylphenyl)acetophenone, acetophenone is consumed approximately 3.7 times more quickly using the second-generation ligand 1c. Using the control ligand 1a, this ratio is 1.8

Rigid, Sterically Diverse N-Heterocyclic Carbene-Pyridine Chelates: Synthesis, Mild Palladation, and Palladium-Catalyzed Allylic Substitution

A series of four 1-(pyridin-2-yl)benzimidazolium salts was synthesized as precursors to rigid, bidentate N-heterocyclic carbene-pyridine ligands. With the goal of exploring the interplay between steric and electronic asymmetry in catalysis, the ligands are constructed with very small (H, Me) or very bulky (2,6-diisopropylphenyl, 2,4,6-triisopropylphenyl) groups adjacent to the nitrogen and carbon donor atoms. Cationic palladium-allyl complexes were synthesized under mild conditions, using triethylamine as base and potassium or silver hexafluorophosphate as counterion source. The structures of three complexes were verified using X-ray crystallography. All four palladium complexes were active catalysts for the allylic substitution reaction between allylic carbonates and either sodium diethyl-2-methylmalonate or N-methylbenzylamine, although significant ligand-controlled effects on regioselectivity were not observed under the conditions studied

Intramolecular Alkyne Hydroalkoxylation and Hydroamination Catalyzed by Iridium Hydrides

Iridium(III) hydrides prove to be air-stable active catalysts for intramolecular hydroalkoxylation and hydroamination of internal alkynes with proximate nucleophiles. The cyclization follows highly selective 6-endo-dig regiochemistry when regioselectivity is an issue

Rigid, Sterically Diverse N-Heterocyclic Carbene-Pyridine Chelates: Synthesis, Mild Palladation, and Palladium-Catalyzed Allylic Substitution

A series of four 1-(pyridin-2-yl)benzimidazolium salts was synthesized as precursors to rigid, bidentate N-heterocyclic carbene-pyridine ligands. With the goal of exploring the interplay between steric and electronic asymmetry in catalysis, the ligands are constructed with very small (H, Me) or very bulky (2,6-diisopropylphenyl, 2,4,6-triisopropylphenyl) groups adjacent to the nitrogen and carbon donor atoms. Cationic palladium-allyl complexes were synthesized under mild conditions, using triethylamine as base and potassium or silver hexafluorophosphate as counterion source. The structures of three complexes were verified using X-ray crystallography. All four palladium complexes were active catalysts for the allylic substitution reaction between allylic carbonates and either sodium diethyl-2-methylmalonate or N-methylbenzylamine, although significant ligand-controlled effects on regioselectivity were not observed under the conditions studied

Disubstituted Imidazolium-2-Carboxylates as Efficient Precursors to N-Heterocyclic Carbene Complexes of Rh, Ru, Ir, and Pd

N,N‘-Disubstituted imidazolium-2-carboxylates are efficient precursors to NHC complexes of Rh, Ir, Pd, and Ru

Intramolecular Alkyne Hydroalkoxylation and Hydroamination Catalyzed by Iridium Hydrides

Iridium(III) hydrides prove to be air-stable active catalysts for intramolecular hydroalkoxylation and hydroamination of internal alkynes with proximate nucleophiles. The cyclization follows highly selective 6-endo-dig regiochemistry when regioselectivity is an issue

Rigid, Sterically Diverse N-Heterocyclic Carbene-Pyridine Chelates: Synthesis, Mild Palladation, and Palladium-Catalyzed Allylic Substitution

A series of four 1-(pyridin-2-yl)benzimidazolium salts was synthesized as precursors to rigid, bidentate N-heterocyclic carbene-pyridine ligands. With the goal of exploring the interplay between steric and electronic asymmetry in catalysis, the ligands are constructed with very small (H, Me) or very bulky (2,6-diisopropylphenyl, 2,4,6-triisopropylphenyl) groups adjacent to the nitrogen and carbon donor atoms. Cationic palladium-allyl complexes were synthesized under mild conditions, using triethylamine as base and potassium or silver hexafluorophosphate as counterion source. The structures of three complexes were verified using X-ray crystallography. All four palladium complexes were active catalysts for the allylic substitution reaction between allylic carbonates and either sodium diethyl-2-methylmalonate or N-methylbenzylamine, although significant ligand-controlled effects on regioselectivity were not observed under the conditions studied

The Mechanism of Markovnikov-Selective Epoxide Hydrogenolysis Catalyzed by Ruthenium PNN and PNP Pincer Complexes

The homogeneous catalysis of epoxide hydrogenolysis to give alcohols has recently received significant attention. Catalyst systems have been developed for the selective formation of either the Markovnikov (branched) or anti-Markovnikov (linear) alcohol product. Thus far, the reported catalysts exhibiting Markovnikov selectivity all feature the potential for Noyori/Shvo-type bifunctional catalysis, with either a RuH/NH or FeH/OH core structure. The proposed mechanisms of epoxide ring-opening have involved cooperative C–O bond hydrogenolysis involving the metal hydride and the acidic pendant group on the ligand, in analogy to the well-documented mechanism of polar double-bond hydrogenation exhibited by catalysts of this type. In this work, we present a combined computational/experimental study of the mechanism of epoxide hydrogenolysis catalyzed by Noyori-type PNP and PNN complexes of ruthenium. We find that, at least for these ruthenium systems, the previously proposed bifunctional pathway for epoxide ring-opening is energetically inaccessible; instead, the ring-opening proceeds through opposite-side nucleophilic attack of the ruthenium hydride on the epoxide carbon, without the involvement of the ligand N–H group. For both catalyst systems, the rate law and overall barrier predicted by density functional theory (DFT) are consistent with the results from kinetic studies