Nitrogen-containing organic compounds, such as a-amino acids and alkaloids, are important biologically active compounds, thus the development of efficient and enantioselective methods for the construction of carbon–nitrogen bonds is a fundamental goal in modern organic synthesis. Transitionmetal-catalyzed carbene insertion into N H bonds is one of the most efficient methods to construct carbon–nitrogen bonds and the development of asymmetric versions of the N H insertion reaction has attracted considerable attention. In initial studies, chiral dirhodium catalysts were tested in intramolecular and intermolecular N H insertion reactions, however, only low to modest enantioselectivities (< 50% ee) were achieved. Since these reports, other transition metals including copper and silver have been used as catalysts, and gave enantioselectivities up to 48% ee. Recently, we reported a highly enantioselective N H insertion reaction (up to 98% ee) using a copper complex with chiral spiro bisoxazoline ligands. Subsequently, two other types of chiral copper catalysts have been developed, one with a planar chiral bipyridine ligand and the other with a binolderivative ligand, and both of these catalysts give high enantioselectivities in N H insertion reactions. Although progress on copper-catalyzed asymmetric N H insertion reactions has been substantial, they still have serious limitations. For instance, all the copper-catalyzed N H insertion reactions require high catalyst loading (5– 10 mol%) for satisfactory yields and enantioselectivities, thus more-efficient chiral catalysts are highly desirable. Because the activity of dirhodium(II) catalysts is usually superior to that of copper catalysts in nonenantioselective N H insertion reactions, the possibility of using dirhodium catalysts to achieve highly enantioselective N H insertion reactions is an intriguing one. Recently, Saito et al. reported that dirhodium(II) carboxylates and cinchona alkaloids cooperatively catalyze the asymmetric N H insertion reactions of a-diazo-a-arylacetates with anilines. The combined catalysts exhibit excellent reactivity but only modest enantioselectivity (up to 71% ee). It is generally accepted that the rhodium-catalyzed N H insertion most likely proceeds via an ylide intermediate (Scheme 1A). We speculated that the subsequent protontransfer step could be facilitated by a chiral phosphoric acid species via a seven-membered-ring transition state, and that, consequently, chiral induction could be accomplished in this step (Scheme 1B). The groups of Yu and Platz have reported that either water or alcohols can assist proton transfer in O H insertion reactions, as indicated by density functional theory calculations and ultrafast time-resolved IR spectroscopy studies. These studies stimulated our interest in exploring asymmetric N H insertion in the presence of a proton-transfer catalyst. As part of our ongoing work on the development of asymmetric carbene insertion reactions, we report herein the asymmetric N H insertion reaction cooperatively catalyzed by dirhodium(II) carboxylates and chiral spiro phosphoric acids (SPAs). Excellent reactivity and high enantioselectivity (up to 95% ee) were achieved in the presence of as little as 0.1 mol% of catalyst. In our initial study, we carried out the insertion of methyl a-diazo-a-phenylacetate (3a) into the N H bond of tert-butyl carbamate (BocNH2) in CHCl3 at 25 8C using 1 mol% of [Rh2(OAc)4] and 10 mol% of chiral SPAs 1 as the catalysts (Table 1). SPAs 1 were prepared by a simple condensation of P(O)Cl3 with 6,6’-disubstituted-1,1’-spirobiindane-7,7’-diols 2, followed by hydrolysis (Scheme 2). Diols 2 were synthesized from spinol (1,1’-spirobiindane-7,7’-diol), as described previously. In the presence of (R)-1a, the N H insertion reaction proceeded within 5 minutes to afford the insertion product in excellent yield with 11% ee (Table 1, entry 2). Control experiments showed that the SPAs alone did not promote the insertion reaction. A range of SPAs with various substituents at the 6 and 6’ positions were evaluated (Table 1, entries 3–9). All the tested SPAs afforded high yields in the N H insertion reaction. SPA (R)-1h, which bears a 6,6’-di(naphth-2-yl) group, afforded the Scheme 1. Proposed mechanism for chiral phosphoric acid induced asymmetric N H insertion.
Read full abstract
Nov 22, 2011
Y Toda + 3
Open Access
