Helical Chiral 2-Aminopyridinium Ions: A New Class of Hydrogen Bond Donor Catalysts

Abstract

Helical chiral 2-aminopyridinium ions were designed as a significantly more acidic (active) dual hydrogen-bonding catalyst than commonly used (thio)urea-based systems. The helicene framework was specifically utilized to position an inherently chiral barrier on the hydrogen-bonding side of the catalyst. The catalyst reactivity and enantioselectivity were successfully demonstrated in additions of 4,7-dihydroindoles to nitroalkenes (0.5-2 mol % catalyst loadings, up to 98:2 er).

Helicene-Based Chiral Auxiliaries and Chirogenesis

“…Further, the yields were noticeably improved from 79% (for the t-Bu derivative) to 88% (for the adamantyl derivative) under similar conditions; hence, adamantyl-containing (P)-104 is the optimal organocatalyst for this reaction (see the corresponding results summarized in Scheme 18). Further, the enhanced effectiveness of 104 in comparison with 103 indicates that the benzofused helicene framework affects the ee value by covering the space beneath the two hydrogen bonds [70], and is thus similar to the case shown in Figure 11.…”

“…In this respect, aminopyridinium salts are also capable of activating nitroalkenes through the hydrogen bond donating ability to the nitro group. For this purpose, aza [6]helicene derivatives 103 and 104, having a 2-aminopyridinium ring at the peripheral position, have been prepared [70]. Hence, the asymmetric addition of 4,7-dihydroindole 105 to nitrostyrene 106 using chiral (P)-helicenes 103 and 104 as catalysts, followed by oxidative aromatization, afforded exclusively β-nitro-indol-2-yl 107 with relatively high yield and ee (Scheme 18).…”

Dianhydrohexitols: new tools for organocatalysis. Application in enantioselective Friedel-Crafts alkylation of indoles with nitroalkenes

“…[31][32][33][34][35][36][37][38][39][40][41] The indole skeleton is considered to be one of the most useful moieties in pharmaceutical chemistry. [42][43] …”

Helicene‐Based Phosphite Ligands in Asymmetric Transition‐Metal Catalysis: Exploring Rh‐Catalyzed Hydroformylation and Ir‐Catalyzed Allylic Amination

“…[21] We have demonstrated the organocatalytic activity of parent 2-aza-[6]helicene 11 in asymmetric acyl transfer reactions to describe a moderate selectivity factor (of up to 10). [22] Recently, Carbery et al developed helicene DMAP Lewis base catalyst 12, which exhibited excellent reactivity as well as selectivity in the kinetic resolution of chiral secondary alcohols (reaching a selectivity factor of up to 116).…”