Enantioselective Tandem Cyclization of Alkyne‐Tethered Indoles Using Cooperative Silver(I)/Chiral Phosphoric Acid Catalysis

Abstract

We report the enantioselective synthesis of tetracyclic indolines using cooperative silver(I) and chiral phosphoric acid catalysis. A variety of alkyne-tethered indoles are suitable for this process. Mechanistic studies suggest that the in situ-generated silver(I) chiral phosphate activates both the alkyne and the indole nucleophile in the initial cyclization step through an intermolecular hydrogen bond and the phosphate anion promotes proton transfer. In addition, further modifications of the cyclization products enabled stereochemistry-function studies of a series of bioactive indolines.

Direct Carbohydroxylation of Arylalkenes with Allylic Alcohols: Cooperative Catalysis of Copper, Silver, and a Brønsted Acid

“…On the basis of our experimental results and previous reports, [17] amechanism is proposed to account for the present catalytic olefin functionalization reaction (Scheme 2). The mechanism involves two catalytic cycles starting from [L n Cu-(I)S n ] + (A)a nd ArCOOAg (C), correspondingly,w hich are generated in situ from the precatalysts Cu(MeCN) 4 PF 6 and AgSbF 6 .T he reaction of catalytic species A with 1a in the presence of aB rønsted acid could occur to generate the intermediate h 3 -allyl copper(III) carboxylate B.M eanwhile, the coordination of the catalytic C to the C = Cb ond of 2a could occur to generate the intermediate D,w hich would subsequently undergo hydration reaction to afford the intermediate organic silver E.I ntermolecular hydrogenbond interactions between B and E would produce F.A n intramolecular nucleophilic substitution-like reaction could occur to generate the intermediate G and regenerate C. Ligand exchange reaction of G with MeCN and/or solvent could finally occur to produce the desired product 3aa and regenerate A,w hich can be stabilized by phosphine ligands and coordinating solvents.…”

“…Among the copper catalysts tested [CuCl, CuBr,C uI, CuOSO 2 CF 3 , Cu(OAc) 2 ,a nd Cu(MeCN) 4 PF 6 ], the ionic-type copper complex Cu(MeCN) 4 PF 6 proved to be the best catalyst. Thel igand was then screened using PPh 3 ,1 ,4-bis(diphenylphosphino)butane (DPPB), 1,2-bis(diphenylphosphino)ethane (DPPE), 1,10-phenanthroline (1,10-phen), and racemic-2,2'-bis(diphenylphosphino)-1,1'-binaphthyl (rac-BINAP;e ntries 6a nd [14][15][16][17]. No reaction was observed when the target reaction was carried out in the absence of either ac opper catalyst or the silver Lewis acid catalyst (entries 7and 8).…”

“…The desired allylative carbohydroxylation product, (E)-1,5-diphenylpent-4-en-1-ol (3aa), was obtained in 73 %yield (entry 6). Thef inal optimization of the Brønsted acid catalyst revealed 4-NO 2 BzOH to be optimal among the Brønsted acids examined BzOH, trifluoroacetic acid (TFA), pivolic acid (PivOH), benzoic acid (BzOH), and 4-methoxybenzoic acid (4-MeOBzOH)] (entries [17][18][19][20][21]. Adecreased yield of 3aa was observed in the absence of either aBrønsted acid catalyst or ap hosphine ligand (entries 9a nd 10 versus entry 6).…”

Direct Carbohydroxylation of Arylalkenes with Allylic Alcohols: Cooperative Catalysis of Copper, Silver, and a Brønsted Acid

“…These results indicated that ac ombination of copper and silver Lewis acid catalysts is necessary for the target allylative carbohydroxylation of styrenes.T he solvent was subsequently screened using Cu(MeCN) 4 PF 6 ,P Ph 3 ,a nd 4-NO 2 BzOH as the transition metal catalyst, ligand, and Brønsted acid catalyst, respectively.P olar [1,tetrahydrofuran (THF), and acetonitrile (MeCN)] and nonpolar (toluene) solvents were investigated, and the obtained results suggested the polar solvent 1,4-dioxane to be optimal (entry 6 versus entries [11][12][13]. Thel igand was then screened using PPh 3 ,1 ,4-bis(diphenylphosphino)butane (DPPB), 1,2-bis(diphenylphosphino)ethane (DPPE), 1,10-phenanthroline (1,10-phen), and racemic-2,2'-bis(diphenylphosphino)-1,1'-binaphthyl (rac-BINAP;e ntries 6a nd [14][15][16][17]. Employing both monodentate and bidentate phosphine ligands formed the desired product 3aain satisfactory yield.…”

“…Arelatively high yield was observed when the bidentate phosphine ligand rac-BINAP was examined (entry 17). Thef inal optimization of the Brønsted acid catalyst revealed 4-NO 2 BzOH to be optimal among the Brønsted acids examined BzOH, trifluoroacetic acid (TFA), pivolic acid (PivOH), benzoic acid (BzOH), and 4-methoxybenzoic acid (4-MeOBzOH)] (entries [17][18][19][20][21]. Interestingly,t he yield of 3aa was further increased to 84 %when 2.0 equivalents of MeCN were added to the reaction mixture (entry 22), even though MeCN was demonstrated to be an unsuitable solvent (entry 12).…”