Highly Enantioselective Hydrophosphonylation of Aldehydes Catalyzed by Tridentate Schiff Base Aluminum(III) Complexes

Abstract

Chiral a-hydroxy phosphonates and phosphonic acids are widely applied in the pharmaceutical industry owing to their biological activity, [1] and intense efforts have been made on the asymmetric hydrophosphonylation of aldehydes. To our knowledge, the first highly enantioselective hydrophosphonylation was realized by the Shibasaki group using heterobimetallic complexes ([LaLi 3 (binaphthoxide) 3 ], [AlLi(binaphthoxide) 2 ]) as tailor-made catalysts.[2] Recently, Kee and co-workers studied the catalytic performance of chiral [Al(salcyen)] and [Al(salcyan)] complexes.[3] Subsequently, the Katsuki group developed an interesting and highly efficient C 1 -symmetric [Al(salalen)] complex for the reaction.[4] An aluminum-binaphthyl Schiff base complex was also found to be effective for the reaction.[5] Although a number of works have appeared on the synthesis of chiral a-hydroxy phosphonates, [6] searching for a catalyst system that could achieve high reactivity and enantioselectivity is still challenging and interesting. As excellent chiral scaffolds, tridentate Schiff base metal complexes, especially those of vanadium, chromium, and iron, have successfully been applied in many asymmetric reactions.[7] Herein, we present a highly efficient asymmetric hydrophosphonylation of various aldehydes catalyzed by chiral tridentate Schiff base Al III complexes. Initially, tridentate Schiff base 1 a (Scheme 1), derived from l-valinol and 3,5-di-tert-butylsalicylaldehyde, reacted in situ with various aluminum(III) reagents to form complexes that catalyze the asymmetric hydrophosphonylation (Table 1, entries 1-3). The counterions of 1 a-Al III complexes showed a decisive effect on the enantioselectivity. [3a, 8] Complex 1 aEt 2 AlCl catalyzed the reaction smoothly, giving the desired products with 86 % ee, while only a racemate product was obtained in the presence of 1 a-AlEt 3 (Table 1, entry 1 vs. 3). It was interesting that catalyst 1 a-Al(OiPr) 3 showed an opposite absolute sense of stereoinduction (Table 1, entry 2). The disparate results were probably caused by the different steric and electronic properties of the counterions (Cl, Et, and OiPr), which each exhibited a different action in the catalytic cycle. [9b,d] To further improve the enantioselectivity of the reaction, [9a] the steric and electronic effects based on 1 were examined (Table 1, entries 1 and 4-8). As shown in Table 1, electron-donating groups at the position para to the OH Scheme 1. Chiral ligands used in the study. Ad = adamantyl. ee [%][c]

Self‐Assembled Bifunctional Catalysis Induced by Metal Coordination Interactions: An Exceptionally Efficient Approach to Enantioselective Hydrophosphonylation

“…[5] However, among these most outstanding examples, relatively high catalyst loadings and/or extended reaction times were generally required to induce acceptable conversions. [3][4][5][6] Clearly, the development of more efficient and practical catalytic systems for a broad range of phosphites and aldehydes (both aromatic and aliphatic) is a highly challenging topic.…”

“…[4] Quite recently, Feng and co-workers presented another highly enantioselective example promoted by the tridentate Schiff base/Al III complexes. [5] However, among these most outstanding examples, relatively high catalyst loadings and/or extended reaction times were generally required to induce acceptable conversions. [3][4][5][6] Clearly, the development of more efficient and practical catalytic systems for a broad range of phosphites and aldehydes (both aromatic and aliphatic) is a highly challenging topic.The strategy of synergistic activation by two or more reactive centers constitutes a versatile approach for the development of high-performance asymmetric catalysis.…”

“…[5] However, among these most outstanding examples, relatively high catalyst loadings and/or extended reaction times were generally required to induce acceptable conversions. [3][4][5][6] Clearly, the development of more efficient and practical catalytic systems for a broad range of phosphites and aldehydes (both aromatic and aliphatic) is a highly challenging topic.The strategy of synergistic activation by two or more reactive centers constitutes a versatile approach for the development of high-performance asymmetric catalysis. [7,8] Recently, the concept of self-assembly by metal-organic coordination has been successfully employed to construct combinatorial chiral catalysts.…”

Enantioseparation of α‐Hydroxyallylphosphonates and Phosphonoallylic Carbonate Derivatives on Chiral Stationary Phases Using Sequential UV, Polarimetric, and Refractive Index Detection

“…Chiral aluminum complexes have been reported as catalysts for highly enantioselective hydrophosphonylation of aldehydes. 13,14 Self-assembled bifunctional catalysts have also been shown to afford exceptionally efficient chiral catalysts for synthesis of 2a and 3. 15 Ooi and colleagues used an alternative approach for chiral hydrophosphonylation using a phosphonium dialkyl phosphite.…”

ChemInform Abstract: Asymmetric Intramolecular Oxa‐Michael Addition of Activated α,β‐Unsaturated Ketones Catalyzed by a Chiral N,N′‐Dioxide Nickel(II) Complex: Highly Enantioselective Synthesis of Flavanones.

“…Although [Fe(acac) 2 ] as the central metal showed ineffective asymmetric induction ( Ni(OTf) 2 L1 10 99 3(R) 7 [d] Ni ( product was obtained in 99 % yield and with 97 % ee (Table 1, entry 7). [15] Next, the structures of ligands were examined (L1-L9, Figure 1). [15] Next, the structures of ligands were examined (L1-L9, Figure 1).…”