Chiral Induction by Cinchona Alkaloids in the Rhodium(II) Catalyzed O-H Insertion Reaction

Abstract

The catalytic insertion reaction into O-H bond with a-diazocarbonyl compounds via a metal-carbenoid intermediate is a very useful organic transformation for the synthesis of oxygen-containing compounds.1-3) Remarkable advances, including enantioselective insertion, have been made in catalytic C-H insertions with a-diazocarbonyl compounds using Rh(II) complexes. [4][5][6][7][8][9] However, only limited success has been achieved for asymmetric insertions into O-H bond. 2,[10][11][12] Recently, Fu and Zhou independently reported asymmetric insertion reactions into alcoholic, 13) phenolic 14) and aquatic 15) O-H bonds with a-diazoacetate using chiral Cu(I) complexes. In contrast, chiral Rh(II) complexes gave poor enantioselectivity (8% ee) in the O-H insertion reaction, 11) while they have given remarkable results in the C-H insertion reaction. Since Rh(II) complexes are unique and powerful catalysts in the insertion reaction, we attempted to apply them to the enantioselective O-H insertion reaction.Recently, Liang et al. conducted a physicochemical approach to O-H insertion reactions using density functional theory (DFT). 10) They concluded that Rh(II) complex does not have enough affinity to the oxonium ylide intermediate to form firmly fixed stereocenter around the metal. Hence, we sought to design a novel approach to the enantioselective O-H insertion reaction catalyzed by Rh(II) complexes, independent of its own chirality. It is of interest that chiral amine additives expressed its own chirality on metal mediated epoxidation reaction using achiral Mn-salen complex. 16)Accordingly, we envisaged that an intermolecular O-H insertion of a-aryldiazoacetate and water should produce a mandelate in an enantioselective manner using cinchona alkaloids and Rh(II) complexes. Using this method, we now report that a combination of quinine (1) or quinidine (2) and dirhodium(II) tetra(triphenylacetate), Rh 2 (TPA) 4 , in the O-H insertion provides mandelate as the sole product in up to 50% ee.In our initial studies, we explored the intermolecular O-H insertion of methyl phenydiazoacetate in dichloromethane using 1 mol% of Rh 2 (TPA) 4 17,18) and 2 mol% of quinine (1) (Table 1, entry 1). [19][20][21][22] As expected, the reaction proceeded smoothly at 23°C to completion within 4 h, giving methyl mandelate in 82% yield. The enantioselectivity in this reaction was 38% ee, as determined by HPLC (Daicel Chiralcel OD-H). The preferred absolute stereochemistry was (S), which was established by comparing the sign of the optical rotation with the literature value.23) It is worth noting that enantioselectivity for a particular product was provided by the cinchona alkaloids, not by the rhodium catalyst. The enantiomeric induction in this system may be due to the formation of a chiral complex between rhodium-carbenenoid and cinchona alkaoid. To rule out the possibility that the reaction was catalyzed by quinine (1), we performed the reaction under identical conditions in the absence of Rh 2 (TPA) 4 and observed no background reaction. To ...

Catalytic X–H insertion reactions based on carbenoids

“…42 Indeed Saito and coworkers have found that cinchona alkaloid additives can deliver enantiomeric excesses of up to 50% in the O-H insertion of a-diazoesters with water catalysed by achiral rhodium(II) catalysts. 41 The Zhou group have also recently discovered that BINOL-based chiral phosphoric acids deliver high levels of enantioinduction in N-H insertion reactions. 42 These promising initial findings suggest that XHI coupled with enantioselective protonation is an area poised for further development.…”

Planar Chiral [2.2]Paracyclophane-Based Bisoxazoline Ligands: Design, Synthesis, and Use in Cu-Catalyzed Inter- and Intramolecular Asymmetric O–H Insertion Reactions

“…[15][16][17] The Cu-catalyzed O-H insertion of α-diazo esters is useful for the construction of α-alkoxycarbonyl structures, which are found in natural products and biologically active compounds. [18][19][20] A highly enantioselective version of this reaction has recently been accomplished with bisazaferrocene I, 21) Spirobox II, 17,[22][23][24][25][26] or the imidazoindolephosphine ligand III [27][28][29][30][31][32][33][34] (Fig. 2).…”

“…The Box ligands 4 with both planar and central chiralities proved to be unsuitable for Cu-catalyzed intermolecular ethanolic O-H insertion, therefore we next investigated the use of the Box ligands (S p )-1, with only planar chirality, in inter-and intramolecular aromatic O-H insertion reactions. For the intermolecular phenolic O-H insertion reaction of ethyl α-diazopropionate (7), 22,27) the enantioselectivity of the isopropyl-substituted ligand (S p )-1d was better than that of phenyl-substituted (S p )-1b, and 80% ee was achieved, with the S configuration being preferred 35) (Table 2, entries 1, 3). The insertion reaction proceeded smoothly at room temperature, and the use of CuCl as a Cu source gave the highest enantiocontrol, but the product yield was lower than that obtained with Cu(OTf) 2 (entry 6).…”

“…These trends are similar to those reported for a chiral imidazoindolephosphine-Cu complex. 27) Finally, intramolecular aromatic O-H insertion reactions 17) with Box (S p )-1d-Cu as the catalyst were examined. In the absence of a ligand, the CuOTf(C 6 H 6 ) 1/2 -catalyzed reaction of 2-diazo-o-hydroxyphenylpropanoate 10a proceeded smoothly at room temperature to afford the desired dihydrobenzofuran product 11a in only 12% yield, along with the α,β-unsaturated ester 12a (60%) ( Table 4, entry 1).…”

Rhodium(II)/Chiral Phosphoric Acid‐Cocatalyzed Enantioselective O–H Bond Insertion of α‐Diazo Esters

“…The corresponding product, a chiral α‐hydroxy ester is a key substructure of many naturally occurring compounds and active pharmaceutical ingredients (Figure ). It was demonstrated that through the combination of metal catalysts and chiral ligands ( Cinchona alkaloids, bisoxazoline, bisazaferrocene and imidazoindolephosphine etc. ), highly enantioselective insertions of α‐diazo compounds into the O–H bonds of alcohols, phenols, and water have been realized.…”