Modular assembly of cyclometalated gold(III) complexes by choosing appropriate bidentate C,N-donor ligands and ancillary ligands for chemoselective cysteine modification of peptides and proteins via C-S bond-forming reductive elimination has been achieved.

Cl 2 ] 1a-l (HC _ N = arylpyridines) and a PEG-linked complex 1m were synthesized. Complexes 1a-m are effective in catalyzing the synthesis of propargylic amines, chiral allenes and isoxazoles. Six-membered ring cyclometallated goldA C H T U N G T R E N N U N G (III) complexes 1f-l exhibited higher catalytic activity than five-membered ring cyclometallated goldA C H T U N G T R E N N U N G (III) complexes 1a-e. The diastereoselectivity of propargylic amines could be tuned by using chiral aldehyde and/ or amine substrates. Excellent enantioselectivities (90-98% ee) were achieved in chiral allene synthesis. Chiral allene racemization could be minimized by using 1f as catalyst. The PEG-linked catalyst 1m is the most catalytically active towards synthesis of propargylic amines, in which case a product turnover of 900 was achieved. Moreover, 1m could be repeatedly used for 12 reaction cycles, leading to an overall turnover number of 872.

Unprecedented stable BINOL/gold(III) complexes, adopting a novel C,O-chelation mode, were synthesized by a modular approach through combination of 1,1'-binaphthalene-2,2'-diols (BINOLs) and cyclometalated gold(III) dichloride complexes [(C^N)AuCl ]. X-ray crystallographic analysis revealed that the bidentate BINOL ligands tautomerized and bonded to the Au atom through C,O-chelation to form a five-membered ring instead of the conventional O,O'-chelation giving a seven-membered ring. These gold(III) complexes catalyzed acetalization/cycloisomerization and carboalkoxylation of ortho-alkynylbenzaldehydes with trialkyl orthoformates.

Stable bis-cyclometallated gold(III) complexes were developed as efficient catalysts for organic transformation reactions by using two strategies: (1) construction of distorted square planar gold(III) complexes and (2) dual catalysis by gold(III) complexes and silver salts.

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Unprecedented stable BINOL/gold(III) complexes, adopting an ovel C,O-chelation mode,w ere synthesized by am odular approach through combination of 1,1'-binaphthalene-2,2'-diols (BINOLs) and cyclometalated gold(III) dichloride complexes [(C^N)AuCl 2 ]. X-ray crystallographic analysis revealed that the bidentate BINOL ligands tautomerized and bonded to the Au III atom through C,O-chelation to form af ive-membered ring instead of the conventional O,O'chelation giving as even-membered ring. These gold(III) complexes catalyzed acetalization/cycloisomerization and carboalkoxylation of ortho-alkynylbenzaldehydes with trialkyl orthoformates.

A novel silver-catalyzed transformation of propargylic amine N-oxides with switchable product profiles has been developed. A diversity of enones with excellent E/Z ratios (up to > 20:1) were obtained when the reactions were conducted in aprotic solvents. In contrast, 3-chlorobenzoxymethyl ketones and a-(3-chloro)-benzoxy enones were obtained by using protic solvents. Mechanistic studies suggested that in situ generated isoxazolinium ions are the key intermediates involved in these novel silver-catalyzed reaction pathways. Applications on the chemoselective modification of cysteine-containing peptides in aqueous medium have also been achieved. COMMUNICATIONS Jian-Fang Cui et al. Scheme 2. A proposed reaction mechanism.

Silver-Catalyzed Transformation of Propargylic Amine N-Oxides to Enones and Acyloxy Ketones via Isoxazolinium Intermediates. -The propargylic amine N-oxides are formed in situ by oxygenation of the corresponding propargyl amines. Depending on the solvent, enones are obtained in aprotic and meta-chlorobenzoyloxy ketones in protic solvents. The synthetic utility of this method is demonstrated in the cysteine-selective modification of peptides. -(CUI, J.-F.; KUNG, K. K.-Y.; KO, H.-M.; HUI, T.-W.; WONG*, M.-K.; Adv. Synth. Catal. 356 (2014) 14-15, 2965-2973, http://dx.