m-Carborane-Based Chiral NBN Pincer-Metal Complexes: Synthesis, Structure, and Application in Asymmetric Catalysis

Abstract

We have succeeded in synthesizing m-carborane-based chiral NBN-pincer ligands, 1,7-bis(oxazolinyl)-1,7-dicarba-closo-dodecaborane (Carbox) (7-9). The combination of bis(hydroxyamides) and 3 equiv of diethylaminosulfur trifluoride (DAST) is a key step for cyclization to form oxazoline rings in excellent yields. X-ray crystal structures of these ligands confirmed three donor sites, one central B and two flanking N atoms in fixed positions. The electrophilic halogenation of the Carbox pincer ligands with iodine and a catalytic amount of Lewis acid led to ring-opening of the oxazolines and afforded bis(haloamides) (13 and 14). The air- and moisture-stable Carbox pincer complexes of rhodium(III), nickel(II), and palladium(II) were synthesized by the oxidative addition of RhCl(3)·3H(2)O, Ni(COD)(2), and Pd(CH(3)CN)(4)[BF(4)](2) to the Carbox pincer ligands (7-9), respectively. The catalytic activity of the rhodium(III) complexes (18-20) was examined for the asymmetric conjugate reduction of α,β-unsaturated esters and reductive aldol reaction. Among these catalysts, [(S,S)-Carbox-iPr]Rh(OAc)(2)·H(2)O (18) showed the highest enantioselective catalytic ability for both asymmetric conjugate reduction and reductive aldol reaction.

The Organometallic Chemistry of Boron-Containing Pincer Ligands based on Diazaboroles and Carboranes

“…79 Similar to the report of Mirkin, the m-carborane 73 was initially dilithiated, but in this case, the dilithiation was followed by a treatment with carbon dioxide and a quenching with protons to give the corresponding carborane-dicarboxylic acid 80. After the conversion of the carboxyl group into the acid chloride 81, reaction with a chiral aminoalcohol ensured the formation of carborane bis(amide)s 82a82c.…”

New functionalized mercaptoundecahydrododecaborate derivatives for potential application in boron neutron capture therapy: Synthesis, characterization and dynamic visualization in cells

“…The 1 H and 13 1073-1057 cm -1 ) [43], indicating that intracluster bonding was not perturbed by functionalization of the icosahedron.…”

N,O-Type Carborane-Based Materials

“…It was found that an exchange coupling for both Fe III atoms in 31 was strongly antiferromagnetic. [58]. Chiral rhodium complexes 32 (chloride form) and 35 (acetate form) were found to be an effective catalyst (1 mol%) for asymmetric conjugate reaction of α,β-unsaturated esters, giving both a high enantiomeric excess (93%-94% ee).…”

“…Reaction of o-carboranylmethyl ammonium salt with commercially available phenyl aldehyde provided the phenyl(carboranylmethyl)imine 12 (Chart 2 and Scheme 3) in good yield [57]. Another interesting series of compounds is that of chiral bis(oxazolilnyl)-m-carboranes 13-14 that were synthesized via a multistep synthesis [58]. Briefly, m-carborane dicarboxylic acid was transformed to the acyl chloride with SOCl 2 and further condensed with two equivalents of the corresponding resolved amino alcohols to provide the uncycled bis(hydroxyamide)-m-carborane intermediates.…”