Chiral Heterocylic Carbenes in Asymmetric Homogeneous Catalysis

Abstract

COMMUNICATIONSon tantalum or zirconium were used as molecular precursors. In these cases, however, alkyl groups are detached during the grafting process as a consequence of prot~Iysis.'~] In our case, all alkyl groups first remain at the metal since the acidic surface silanols attack the nitridomolybdenum moiety, whereas only in the second step an I-elimination occurs, yielding the corresponding alkylidene species. Further work in this area is in progress.

Synthesis of Heterocyclic Compounds via Nucleophilic Aroylation Catalyzed by Imidazolidenyl Carbene

“…Two fluorine atoms-ortho to carbonyl group of 5a-d-were continuously replaced by one oxygen atom to form xanthones 6a-d (entries [1][2][3][4]. Benzophenone 5e has 2Ј-chloro and 2-fluoro substituents instead of two o-fluoro groups.…”

“…Since 1990s, the use of NHCs as ligands has lead to significant advancements in the area of Pd-catalyzed carbon-carbon bond-forming reactions, 1) Ru-catalyzed olefin metathesis, 2) and Rh-catalyzed hydrosilylations. [3][4][5] NHCs have also attracted significant attention as organocatalysts. Several reactions have been catalyzed by NHCs, for e.g., benzoin condensation, [6][7][8][9][10][11] Stetter reaction, [12][13][14] transesterification/acylation reaction, 15,16) and cyanosilylation.…”

Copper-Catalyzed Allylic Substitution and Conjugate Addition with Alkylboranes

“…The N substituted NHC ligand (L6) 37 with a hydroxyalkyl arm containing a stereogenic center induced slight catalytic activity and enantioselectivity (entry 2), but further investigation based on hydroxylated ligands was not fruitful. The ring unsaturated C 2 symmetric NHC ligand (L7) 38 having 1 (1 naphthyl)ethyl groups at both nitrogen atoms imparted moderate yields and enantioselectivities (46%, 41% ee in THF; 62%, 43% ee in toluene) (entries 3 and 4).…”

Rh-N-Heterocyclic Carbene (NHC) Complex-Catalyzed Addition of Phenylboronic Acid to N-Sulfonyl and N-Phosphinoyl Aldimines

“…Due to their strong s-donating properties, NHCs can form metal complexes that have high stabilities toward heat, moisture, and air, and they have higher catalytic abilities than their phosphine counterparts. [2][3][4][5][6] NHCs are now used as ligands for transition metals in many important chemical transformations such as Pd-catalyzed coupling reactions, 7) Ru-catalyzed olefin metathesis, 8) Rh-catalyzed hydrosilylations, [9][10][11] and Cu-catalyzed conjugate addition reactions. [12][13][14] Moreover, NHCs have attracted considerable attention as organocatalysts in several reactions such as benzoin condensation, [15][16][17][18][19][20] Stetter reaction, [21][22][23] transesterification/acylation reactions, 24,25) and nucleophilic substitution reactions.…”