Asymmetric hydrophosphonylation of imin

Shibasaki reported the first catalytic asymmetric hydrophosphonylation of imines in 1995 (Scheme 5-45) using heterobimetallic LLB-type catalysts. 

The asymmetric catalytic hydrophosphonylation is an attractive approach for the synthesis of optically active a-amino phosphonates [84]. The first example of this type of reaction was reported by the Shibasaki group in 1995 using heterobimetal-lie lanthanoid catalysts for the hydrophosphonylation of acyclic imines [85a]. This concept has been extended to the asymmetric synthesis of cyclic a-amino phosphonates [85b—d]. Very recently, the Jacobsen group developed the first organocatalytic asymmetric hydrophosphonylation of imines [86], In the presence of 10 mol% of thiourea-type organocatalyst 71, the reaction proceeds under formation of a-amino phosphonates 72 in high yield (up to 93%) and with enantioselectivity of up to 99% ee [86], A selected example is shown in Scheme 5.42. Di-o-nitrobenzyl phosphite 70 turned out to be the preferred nucleophile. 

I. Catalytic, asymmetric hydrophosphonylation of imines promoted by the lanthanoid-potassium-BINOL catalyst (LnPB)... 

The second part of the chapter deals with several kinds of asymmetric reactions catalyzed by unique heterobimetallic complexes. These reagents are lanthanoid-alkali metal hybrids which form BINOL derivative complexes (LnMB, where Ln = lanthanoid, M = alkali metal, and B = BINOL derivative). These complexes efficiently promote asymmetric aldol-type reactions as well as asymmetric hydrophosphonylations of aldehydes (catalyzed by LnLB, where L = lithium), asymmetric Michael reactions (catalyzed by LnSB, where S = sodium), and asymmetric hydrophosphonylations of imines (catalyzed by LnPB, where P = potassium) to give the corresponding desired products in up to 98% ee. Spectroscopic analysis and computer simulations of these asymmetric reactions have revealed the synergistic cooperation of the two different metals in the complexes. These complexes are believed to function as both Brpnsted bases and as Lewis acids may prove to be applicable to a variety of new asymmetric catalytic reactions.1,2... 

Scheme 10. LnPB-catalyzed asymmetric hydrophosphonylation of imines.

Figure 7. Influence of the catalytic amount on asymmetric hydrophosphonylation of imine 59.

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. ... 

Pettersen D, Marcolini M, Bemardi L, Fini F, Herrera RP, Sgarzani V, Ricci A. Direct access to enantiomerically enriched a-aminophosphonic acid derivatives by organoca-talytic asymmetric hydrophosphonylation of imines. J. Org. Chem. 2006 71 6269-6272. 

Scheme 5-47 Asymmetric hydrophosphonylation of a cyclic imine catalyzed by heterobimetallic rare earth/alkali metal/BI-NOL complexes or by chiral titanium alkoxide complexes...

In comparison to related P(III) chemistry, metal-catalyzed additions of P-H bonds in P(V) compounds to unsaturated substrates have been studied in more detail, and several synthetically useful processes have been developed. In particular, the use of heterobimetallic BINOL-based catalysts allows asymmetric hydrophosphonylation of aldehydes and imines in high yield and enantiomeric excess. 

Groeger, H., Saida, Y., Arai, S., Martens, J., Sasai, H., and Shibasaki, M., First catalytic asymmetric hydrophosphonylation of cyclic imines highly efficient enantioselective approach to a 4-thiazolidinylphosphonate via chiral titanium and lanthanoid catalysts,Tetrahedron Lett., 37, 9291, 1996. 

Akiyama T, Morita H, Itoh J, Fuchibe K (2005a) Chiral Brpnsted acid catalyzed enantioselective hydrophosphonylation of imines asymmetric synthesis of alpha-amino phosphonates. Qrg Lett 7 2583-2585 Akiyama T, Saitoh Y, Morita H, Fuchibe K (2005b) Enantioselective Mannich-type reaction catalyzed by a chiral Bronsted acid derived from TADDOL. Adv Synth Catal 347 1523-1526... 

Scheme 21. Asymmetric hydrophosphonylation of acyclic imines catalyzed by (k)-LnPB.

In conclusion, chiral heterobimetallic lanthanoid compexes LnMB, which were recently developed by Shibasaki et al., are highly efficient catalysts in stereoselective synthesis. This new and innovative type of chiral catalyst contains a Lewis acid as well as a Bronsted base moiety and shows a similar mechanistic effect as observed in enzyme chemistry. A broad variety of asymmetric transformations were carried out using this catalysts, including asymmetric C-C bond formations like the nitroaldol reaction, direct aldol reaction, Michael addition and Diels-Alder reaction, as well as C-0 bond formations (epoxidation of enones). Thereupon, asymmetric C-P bond formation can also be realized as has been successfully shown in case of the asymmetric hydrophosphonylation of aldehydes and imines. It is noteworthy that all above-mentioned reactions proceed with high stereoselectivity, resulting in the formation of the desired optically active products in high to excellent optical purity. 

A logical extension of these themes is for one catalyst to activate both the P-nucleophile and the unsaturated electrophile. This approach has been especially popular in asymmetric hydrophosphonylation of aldehydes and imines, which has been reviewed recently [59]. 

For selected pioneering examples, see (a) D. Uraguchi, K. Sorimachi, M. Terada, J. Am. Chem. Soc. 2004, 126, 11804-11805. Organocatalytic asymmetric aza-Friedel-Crafts alkylation of furan. (b) T. Akiyama, H. Morita, J. Itoh, K. Fuchibe, Org. Lett. 2005, 7, 2583-2585. Chiral Br0nsted acid catalyzed enantioselective hydrophosphonylation of imines asymmetric synthesis of a-amino phosphonates. 

This kind of heterobimetallic complexes are excellent catalysts for a wide range of reactions, including epoxidation of enones, hydrophosphonylation of imines and aldehydes, and a range of asymmetric C-C bond formation reactions, involving Michael addition reaction, Diels-Alder reaction, aldol and nitroaldol reaction, etc. The alkaU metal has a profoimd effect on the catalytic property of these compounds. 

For activation of more unconventional substrates in the asymmetric addition to imines, Ricci and coworkers [13d] investigated the utility of asymmetric phospho-nylation. The hydrophosphonylation of imines was achieved to provide a-amino... 

Akiyama T, Morita H, Itoh J, Fuchibe K. Chiral Brpnsted acid catalyzed enantioselective hydrophosphonylation of imines asymmetric synthesis of a-amino phosphonates. Org. Lett. 2005 7 2583-2585. 

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