A new chiral ammonium bromide, 1-Br, possessing diarylmethanol functionality as a substrate recognition site has been designed as a promising, dual-functioning catalyst for the highly enantioselective epoxidation of alpha,beta-unsaturated ketones under mild phase-transfer conditions. For instance, vigorous stirring of a mixture of chalcone, 1-Br (3 mol %), and 13% NaOCl in toluene at 0 degrees C for 24 h gave rise to epoxy chalcone quantitatively with 96% ee. A variety of alpha,beta-unsaturated ketones can also be epoxidized with rigorous stereochemical control, clearly demonstrating the effectiveness and utility of the present system. Further, a successful single-crystal X-ray diffraction analysis of 1-PF6 uncovered its distinctive three-dimensional molecular architecture and provided useful information for postulating the transition state.
[reaction: see text]. Highly enantioselective Michael addition of diethyl malonate to chalcone derivatives has been achieved under mild phase-transfer conditions by the successful utilization of N-spiro C(2)-symmetric chiral quaternary ammonium bromide 1 as a catalyst, which possesses diarylhydroxymethyl functionalities as a recognition site for the prochiral electrophile. This simple asymmetric Michael addition process was found to be quite effective for various chalcone derivatives, including those with heteroaromatic substituents.
Preparation of novel mono-and bidentate phosphine ligands having dendrimer moiety is reported. Monodentate (1a-c) and bidentate ligands (5a,b) were synthesized from bis(4hydroxyphenyl)phenylphosphine oxide (3) and 1,2-bis(dichlorophosphino)ethane, respectively, in high yields. The defect-free monodisperse nature of these compounds was confirmed by 31 P NMR and elemental analysis as well as by ESI mass spectra. Complexation of these ligands with PtCl 2 (COD) followed by NaBH 4 reduction in a THF/H 2 O mixture gave Pt(0) complexes 10a,b (having 1a,b as the ligands) and 13a,b (5a,b as the ligands). Monodentate ligands gave PtL 3 complexes (10a,b) and bidentate ligands gave Pt(L-L) 2 complexes (13a,b), respectively. Preliminary studies on oxidative addition of RI (R ) CH 3 or C 6 H 5 ) to 10a,b showed that the metal center is easily accessible. Further, molecular modeling of 13b showed nanoscale flattened globular structure of the complex with an approximate diameter of 4.4 nm.
Intervention of the enolate oxidation in the catalytic asymmetric phase-transfer alkylation of protected a-amino acid derivatives under aerobic conditions has been addressed, and anaerobic conditions have been introduced to obtain synthetically satisfactory chemical yields as well as a high level of enantioselectivity.Recently, we disclosed a broadly useful and practical procedure for the enantioselective synthesis of nonproteinogenic a,a-dialkyl-a-amino acids under solid-liquid phasetransfer conditions using rationally designed C 2 -symmetric chiral quaternary ammonium salts of type 1 as catalysts [(S,S)-3,4,5-trifluorophenyl-NAS-bromide [54, and (S,S)-b-naphthyl-NAS-bromide [54,839-1] from Aldrich Chemical Co. Ltd.]. 1,2 A wide variety of a,a-dialkyl-a-amino acids can be efficiently prepared with enantioselectivities as high as 99% ee either by the onepot, double alkylation of aldimine Schiff base of glycine tert-butyl ester or by the simple alkylation of aldimine Schiff base 2 derived from the corresponding a-amino acids. 3 Although excellent enantioselectivity was constantly observed, the chemical yields of the products were varied and generally modest, which prompted us to investigate the fundamental reason for this in relation to the mechanistic aspect of this asymmetric phase-transfer catalytic alkylation. Since we performed the reaction under aerobic conditions, it seemed conceivable that rapid oxidation of the in situ generated enolate with molecular oxygen could occur concurrently with the desired alkylation step, thereby resulting in a certain decrease of the chemical yield. In this letter, we address this problem and report the anaerobic conditions for the highly enantioselective alkylation of protected a-amino acid derivatives by chiral phase-transfer catalysis.Alkylation of aldimine Schiff base derived from leucine tert-butyl ester (2, R 1 = i-Bu) with benzyl bromide (1.2 equiv) in the presence of the catalyst 1 (1 mol%) and CsOH•H 2 O (5 equiv) in toluene proceeded smoothly at 0°C under aerobic conditions to give the corresponding benzylation product 3 (R 1 = i-Bu, R 2 = CH 2 Ph) in 64% isolated yield with 92% ee. 1 The observed asymmetric induction can be interpreted for by the generally proposed interfacial mechanism: the cesium enolate of 2 (R 1 = i-Bu) produced through interfacial deprotonation with CsOH•H 2 O experiences the extremely fast ion-exchange with 1 to give the corresponding chiral enolate that reacts with benzyl bromide in an asymmetric fashion as illustrated in Scheme 1. 4 This is consistent with the fact that attempted benzylation of 2 (R 1 = i-Bu) in the absence of catalyst under otherwise similar conditions afforded the racemic product 3 (R 1 = i-Bu, R 2 = CH 2 Ph) in 51% yield. Based on the plausible mechanistic profile, we assumed that the initially formed cesium enolate could be rapidly oxidized by molecular oxygen under aerobic conditions as also shown in Scheme 1, and this pathway would compete with the desired alkylation, lowering the chemical yield. Actually, upon mixing ...
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