Triptycene‐Based Chiral and meso‐N‐Heterocyclic Carbene Ligands and Metal Complexes

Abstract

Based on 1-amino-4-hydroxy-triptycene, new saturated and unsaturated triptycene-NHC (N-heterocyclic carbene) ligands were synthesized from glyoxal-derived diimines. The respective carbenes were converted into metal complexes [(NHC)MX] (M=Cu, Ag, Au; X=Cl, Br) and [(NHC)MCl(cod)] (M=Rh, Ir; cod=1,5-cyclooctadiene) in good yields. The new azolium salts and metal complexes suffer from limited solubility in common organic solvents. Consequently, the introduction of solubilizing groups (such as 2-ethylhexyl or 1-hexyl by O-alkylation) is essential to render the complexes soluble. The triptycene unit infers special steric properties onto the metal complexes that enable the steric shielding of selected areas close to the metal center. Next, chiral and meso-triptycene based N-heterocyclic carbene ligands were prepared. The key step in the synthesis of the chiral ligand is the Buchwald-Hartwig amination of 1-bromo-4-butoxy-triptycene with (1S,2S)-1,2-diphenyl-1,2-diaminoethane, followed by cyclization to the azolinium salt with HC(OEt)3 . The analogous reaction with meso-1,2-diphenyl-1,2-diaminoethane provides the respective meso-azolinium salt. Both the chiral and meso-azolinium salts were converted into metal complexes including [(NHC)AuCl], [(NHC)RhCl(cod)], [(NHC)IrCl(cod)], and [(NHC)PdCl(allyl)]. An in situ prepared chiral copper complex was tested in the enantioselective borylation of α,β-unsaturated esters and found to give an excellent enantiomeric ratio (er close to 90:10).

Well‐Defined Chiral Copper NHC Complex in the Asymmetric Conjugated β‐Borylation and One‐Pot Metathesis‐Asymmetric β‐Borylation Reactions

“…However, we were unable to achieve better enantioselectivity than entry 7. Similarly, Plenio also observed enantioselectivity of 76 % ee when the same reaction was carried out in presence of NHC 13 /CuCl at −20 °C . Therefore, optimization revealed that higher selectivity could be realized at −55 °C and using diethyl ether as solvent.…”

“…[3] Subsequently,F ernµndez et al first successfully appliedan N-heterocyclic carbene (NHC)copperc omplex in enantioselective boration reactions. [4] Since then, catalysis mediated by NHC-based metal complexes has emerged as ap owerful tool for the synthesis of chiral organoboron compounds, [5][6][7] because these catalysts have severals ignificant advantages over their phosphine counterparts, [8] and fine tuning of the electronic and stericp roperties has ad ramatic impact [9] on the reactivity and selectivity of NHCs. Hoveyda first showeda nN HC-catalysed enantioselective, metal-free conjugateb oration reaction.…”

Fluorescent Dyes in Organometallic Chemistry: Coumarin‐Tagged NHC–Metal Complexes

“…Azolium salt 3· HCl serves as a convenient source for various NHC–metal complexes, which were prepared following the respective general literature procedures (Scheme ): [AuCl( 3 )], [PdCl(allyl)( 3 )], [PdCl 2 (Cl‐py)( 3 )], [RuCl 2 (ind)( 3 )(SIMes)],[7b] [MCl(cod)( 3 )] and [MCl(CO) 2 ( 3 )] (M = Rh, Ir) . All complexes are air‐stable solids, which can be stored under ambient conditions without decomposition.…”

Pentiptycene-based concave NHC–metal complexes

“…12 In order to obtain a sufficient amount of pentiptycene for the synthesis of the NHC ligands, the scale of the reaction was considerably increased (almost 20-fold) with respect to the literature procedure. 20 Our attempts to obtain the respective diimines with glyoxal and aminophenol 3 appeared promising. Quinone 1 was converted into monooxime 2 employing a literature procedure.…”

“…Triptycene phosphines have been used in organometallic chemistry, [14][15][16][17][18][19] as well as the recently reported bistriptycenebased NHC ligands, 20 but to the best of our knowledge, pentiptycene-based NHC ligands have not been reported. The use of pentiptycenes for NHC ligands offers interesting prospects, since the pentiptycene unit confers a bowl type shape to the respective NHC.…”