Bis(oxazolinyl)phenylrhodium(III) Aqua Complexes:  Synthesis, Structure, Enantioselective Allylation of Aldehydes, and Mechanistic Studies

Abstract

The reaction of (Phebox)SnMe3 (4; Phebox = 2,6-bis(oxazolinyl)phenyl) and [(cyclooctene)2RhCl]2 in the presence of CCl4 provided the air-stable and water-tolerant (Phebox)RhCl2(H2O) complexes 5. These neutral (noncationic) aqua complexes 5 acted as asymmetric catalysts for enantioselective allylation of aldehydes with allyltin reagents in the presence of 4 Å molecular sieves (MS 4A). Furthermore, these aqua complexes could be recovered quantitatively from the reaction media. Detailed mechanistic studies of this catalytic system using X-ray and NMR spectroscopy revealed that the (Phebox)RhCl2 fragment, generated by releasing H2O from aqua complex 5, is an active catalyst and the reaction proceeds by a Lewis acid catalyzed mechanism. The relative stereochemistry of the major adduct of the reaction of benzaldehyde with crotyltin reagents was anti (threo). The observed anti diastereoselectivity and si-face attack of allyltins on the carbonyl carbon of aldehydes were explained by the inverse antiperiplanar transition-state model.

Carbon–Hydrogen Bond Activation of Arenes by a [Bis(oxazolinyl)phenyl]rhodium(III) Acetate Complex

“…We have demonstrated previously that the coordination site of H 2 O in [(Phebox)RhCl 2 (H 2 O)] plays the role as the Lewis acidic position. [16] We therefore presume that a coordinatively unsaturated intermediate A with a vacant site can be formed by dissociation of H 2 O. According to the final geometry of the aryl complexes, isomerization to B might be important to accept the arene molecule.…”

Efficient Preparation of New Rhodium‐ and Iridium‐[Bis(oxazolinyl)‐3,5‐dimethylphenyl] Complexes by CH Bond Activation: Applications in Asymmetric Synthesis

“…Heating of a mixture of chiral ligand 1a or non-chiral 1b, 3 , and NaHCO 3 in methanol gave the corresponding chloride complexes…”

“…Although we also attempted several transmetallation routes with (R-Phebox)SnMe 3 , the yields have not been improved. [3] Hypothetically, we thought that C À H bond activation at the undesired 4-or 6-position of (R-Phebox)H might decrease the yields. In this context, Richards et al observed a similar phenomenon of C À H bond activation at position 4 in the reaction of (Phebox)H and Pd-A C H T U N G T R E N N U N G (OAc) 2 .…”

Chiral Bis(imidazolidine)‐Derived NCN Pincer Rh Complex for Catalytic Asymmetric Mannich Reaction of Malononitrile with N‐Boc Imines

“…[3][4][5][6][7][8][9][10] In our studies on the asymmetric catalysis,s p 2 imine coordinationo fo xazoline was replaced with sp 3 amine coordination of imidazolidine for introducing ac ooperativeN ÀHf unctionality adjacent to the metal center, and we have succeeded in the first development of ap incer bis(imidazolidine)-derived NCNÀ Pd complex for ac atalytic asymmetric Michael reaction of malononitrile with nitroalkenes.…”

“…After exploration of various synthetic routes, an NCN pincer bis(imidazolidine)C 6 H 3 RhCl 2 complex (abbreviated [PhBidine-RhCl 2 ]) was prepared by ap rocedure described in Scheme 1. [3a,4] Starting from 1,3-diformylb enzene, condensation with monobenzyl (R,R)-diphenylethylenediamine providedt he bis(imidazolidine)r ings stereoselectively.C ÀHa ctivation of 1,3-bis(imidazolidine) + . The [PhBidine-RhCl 2 ]c omplex was recrystallized from chloroform/acetonitrile/ethylene glycol, and X-ray crystallographic analysisr evealed the structure shown in Figure 2a.T he predicteda ll-trans stereochemistry of the two imidazolidine rings, due to steric repulsion between the substituents of the chiral diamine precursor,w as confirmed.…”