Enantioselection aldehyde hydrophosphonylation

A salalen ligand (a hybrid salicylideneimine-salicylamine) has been coordinated to aluminium to serve as an enantioselective catalyst for aldehyde hydrophosphonylation of aldehydes.278... 

Yokomatsu T, Yamgishi T, Shibuya S (1993) Enantioselectivity for hydrophosphonylation of aromatic aldehydes catalyzed by lanthanum binaphthol complex. Remarkable electronic effect of aromatic substituents. Tetrahedron Asymmetry 4 1783-1784... 

Heteroaromatic aldehydes undergo similar enantioselective hydrophosphonylation reactions (Scheme 5-34). 

Akiyama et al. disclosed an asymmetric hydrophosphonylation in 2005 (Scheme 32) [55], Addition of diisopropyl phosphite (85a) to A-arylated aldimines 86 in the presence of BINOL phosphate (R)-M (10 mol%, R = 3,5-(CF3)j-C Hj) afforded a-amino phosphonates 87 in good yields (72-97%). The enantioselectivities were satisfactory (81-90% ee) in the case of imines derived from a,(3-unsaturated aldehydes and moderate (52-77% ee) for aromatic substrates. 

The effects of the slow addition of the aldehydes on enantioselection can be best explained as follows. Heterobimetallic catalysts such as LLB are believed to activate both nucleophiles and electrophiles. For the hydrophosphonylation of comparatively unreactive aldehydes, the activated phosphite can react only with aldehydes that are precoordinated to lanthanum. However, in the case of reactive aldehydes such as 54 and 124, the Li-activated phosphite may be able to undergo a competing reaction with the unactivated aldehyde. If such aldehydes are added in one portion, the ee of the product will thus be reduced. Slow addition of the... 

Catalytic Enantioselective Hydrophosphonylation of Aldehydes. LLB catalyzes the hydrophosphonylations of aldehydes with dimethyl phosphite to afford a-hydroxy phosphonates with high optical purity (eq 7). In some cases, the aldehyde needs to be added slowly to the mixture of LLB and phosphite in THF. For some aromatic aldehydes, another catalyst, Li[Al(binol)2] (ALB), gives better results. Imines also react with dimethyl phosphite in a highly enantioselective manner when potassium-based complexes (K3[Ln(binol)3], LnPB) are used as catalysts. ... 

Heterobimetallic catalysis mediated by LnMB complexes (Structures 2 and 22) represents the first highly efficient asymmetric catalytic approach to both a-hydro and c-amino phosphonates [112], The highly enantioselective hydrophosphonylation of aldehydes [170] and acyclic and cyclic imines [171] has been achieved. The proposed catalytic cycle for the hydrophosphonylation of acyclic imines is shown representatively in Scheme 10. Potassium dimethyl phosphite is initially generated by the deprotonation of dimethyl phosphite with LnPB and immediately coordinates to the rare earth metal center via the oxygen. This adduct then produces with the incoming imine an optically active potassium salt of the a-amino phosphonate, which leads via proton-exchange reaction to an a-amino phosphonate and LnPB. 

A detailed investigation into the hydrophosphonylation of substituted ben-zaldehydes as well as heteroaromatic aldehydes has been recently reported by Shibuya et al. [68] Therein this group found an interesting effect of thep-substit-uent on enantioselectivity. In agreement with the results reported by Shibasaki... 

The hydrophosphonylation of aldehydes can also proceed enantioselectively in the presence of chiral nonracemic Lewis acids and some bifunctional catalysts and a small subchapter discusses recent advances in this area. 

Some of the metal-based catalysts used in the asymmetric hydrophosphonylation of aldehydes (see Section 6.4) can also be applied to the phosphonylation of imines. For instance, Shibasaki s heterobimetallic BINOL complexes work well for the catalytic asymmetric hydrophosphonylation of imines. In this case lanthanum-potassium-BINOL complexes (6.138) have been found to provide the highest enantioselectivities for the hydrophosphonylation of acyclic imines (6.139). The hydrophosphonylation of cyclic imines using heterobimetallic lanthanoid complexes has been reported. Ytterbium and samarium complexes in combination with cyclic phosphites have shown the best results in the cases investigated so far. For example, 3-thiazoline (6.140) is converted into the phosphonate (6.141) with 99% ee using ytterbium complex (6.142) and dimethyl phosphite (6.108). The aluminium(salalen) complex (6.110) developed by Katsuki and coworkers also functions as an effective catalyst for the hydrophosphonylation of both aromatic and aliphatic aldimines providing the resulting a-aminophosphonate with 81-91% ee. ... 

The chiral catalyst (352) has been detected by low-temperature NMR and applied in a highly efficient and enantioselective hydrophosphonylation of aldehydes (Scheme 80). ... 

For pioneering work on the highly enantioselective one-pot reaction of in situ preformed imines using a chiral salalen-ahiminiim catalyst, see B. Saito, H. Egami, T. Katsuki, J. Am. Chem. Soc. 2007, 129, 1978-1986. Synthesis of an optically active Al(salalen) complex and its application to catalytic hydrophosphonylation of aldehydes and aldimines. 

Ternary and quaternary a-hydroxy-phosphonates, an important class of biologically active compounds, are commonly obtained by addition of dialkylphosphites onto aldehydes or ketones [30]. Well-defined mono- or bimetallic complexes of rare-earth metals, titanium, or aluminum have emerged over the past two decades as effective catalysts for this so-called hydrophosphonylation of aldehydes [31] and, with more difficulty, that of ketones [31c,d, 32], which are far less reactive because of their lower electrophilicity. In some cases, good enantioselectivities could be achieved thanks to the use of chiral metal-based precatalysts [31, 32], Despite their several similarities with rare-earth elements, we were surprised to see that discrete complexes of the large Ae metals had never been utilized to catalyze hydrophosphonylation reactions. 

The asymmetric hydrophosphonylation of aldehydes has been achieved using a catalyst based upon a triaminoiminophosphorane (Scheme 4.155) [245]. This was an interesting approach to this chemistry and generated the a-hydroxyphosphonates in excellent yields with high enantioselectivity. In addition to the hydrophosphonylation chemistry, the authors also investigated how readily common bases deprotonated phosphites in order to provide... 

In 2007, Ooi and coworkers introduced chiral tetraaminophosphonium salts as a new class of Bronsted acids [166]. Similar to the guanidine/guanidinium case, these tetraaminophosphonium salts act as Bronsted bases in their neutral/ deprotonated (triaminoiminophosphorane) form, while they can also be used as mono-functional Bronsted acids in their protonated, phosphonium form. Phos-phonium salt 67, when neutralized in situ with KO Bu, was shown to be a highly effective catalyst in the enantioselective Henry reaction of nitroalkanes with various aromatic and aliphatic aldehydes (Scheme 10.65). The same strategy was further applied to the catalytic asymmetric Henry reaction of ynals [167] and hydrophosphonylation of ynones (Scheme 10.66) [168]. Brfunctional catalysis using this scaffold were also obtained using the carboxylate salts of tetraaminophosphoniums in the direct Mannich reaction of sulfonyl imines with azlactones (Scheme 10.67) [169]. 

The Kabachnik-Fields reaction is a three-component hydrophosphonylation of imines formed in the reaction mixture from carbonyl compounds and amines [75]. In 2008, List and coworkers reported on such a reaction catalyzed by chiral phosphoric acids that combines a dynamic kinetic resolution with the concomitant generation of a new stereogenic center (Scheme 42.30). The resolution is possible when chiral racemic aldehydes 135 are used. This is because the imine formed in the first step of the reaction is in equilibrium with its achiral enamine tautomer, thereby racemizing the starting material continuously. Since one of the two enantiomers is selectively activated by the chiral phosphoric acid catalyst, the addition of phosphite 136 affords the exclusive formation of one diastereomer. All phos-phonate products 137 were obtained with good yields and moderate to excellent diastereo- and enantioselectivity [76]. 

One approach is enantioselective hydrophosphonylation. Chiral 1-hydroxyalkylphosphonates M2 could be prepared by the enantioselective hydrophosphonylation of dialkyl phosphonates Ml and aldehydes (Scheme 6.4) [50-54]. 

The base-catalysed hydrophosphonylation of aldehydes or imines (Pudovik reaction) [58] as a convenient method was widely used for the synthesis of 1-hydrox-yalkylphosphonates. Since the pioneering work of Shibuya [50] and Spilling [51] was reported, much attention has been devoted to developing enantioselective catalysts for the synthesis of chiral 1 -hydroxy alkylphosphonates. Chiral aluminium complexes were shown to be more effective chiral catalysts [59-62]. Based on the success of using A1 (salen) and A1 (salcyen) as asymmetric catalysts, Al-Schiff base complexes [63, 64] have been developed to catalyze the asymmetric addition reaction of dial-kylphosphonates and aldehydes. Tridentate Schiff base metal complexes, such as vanadium, chromium, and iron [65], have been successfully applied in many asymmetric synthetic reactions. We noticed that Al(III) complexes could catalyse the asymmetric Pudovik reaction and these ligands could be easily synthesized [66-70]. 

In order to obtain 1-hydroxyaIkylphosphonates M2 and cyclic 1-hydrox-yalkylphosphonates IVB in a higher enantiomeric excess (ee), we developed an efficient method [57] to asymmetrically synthesize M2 and IVB via enantioselective hydrophosphonylation of aldehydes using tridentate Schiff base Al(III) complexes as catalysts (Scheme 6.6). 

Under optimized reaction conditions, the asymmetric hydrophosphonylation of aldehydes with corresponding cyclic phosphonates M18-2 could be achieved in good yields with excellent enantioselectivities. On the basis of optimization, a series... 

Merino P, Marques E, Herrera RP et al (2008) Catalytic enantioselective hydrophosphonylation of aldehydes and imines. Adv Synth Catal 350 1195-1208... 

Zhou X, Liu XH, Yang X et al (2008) Highly enantioselective hydrophosphonylation of aldehydes catalyzed by tridentate schiff base aluminum(III) complexes. Angew Chem Int Ed 47 392-394... 

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