Straightforward Synthesis of a-Amino Phosphonates

Therefore, a-aminophosphonic acids and their derivatives represent a synthetic goal in modem chemistry [162]. Among other approaches, Kabachnik-Fields reaction represents a straightforward method for the synthesis of a-amino phosphonates. There are a great number of reported racemic versions of Kabachnik-Fields reaction, and some examples will be only mentioned in the following, while scarce enantioselective cases will be treated in detail. 

This one-pot three-component reaction can be promoted by acid or base catalysts, external magnetic field [163], MW [164], or ultrasonic [165] irradiation or by heating [166]. Numerous acid catalysts have been applied to this reaction, in particular Lewis acids (e.g., InClj [167], BiClj [168], FeClj [169], YbCl3 [170], SiO /ZnCl [171], (CeClj Hp) [172], ln(OTf)3 [173], Bi(OTf)3 [174], Al(OTf)3 [175], Sn(OTf)2 [176], Mg(C10,)3 [177], Yb(PFO)3 [178], ZrOCl2-5HjO [179], Sml [180], Cdl [181], TaCl -SiOj [182], and NbCl [183]) and solid acids (e.g., montmoril-lonite KSF [184], sulfamic acid [185], amberlite IR 120 

SCHEME 12.27 One-pot synthesis of poly(aminophosphonate)s throngh K-F-RAET system. 

It worth to mention that despite the importance of the Kabachnik-Fields reaction, stereoselective versions for the synthesis of enantioenriched a-aminophosphonates are scarce [212, 213], and only few enantioselective examples have been published to date (for reviews on enantioselective catalytic direct hydrophosphonylations of imines, see Refs. [162a-c]). Organocatalytic examples use well-known chiral binol-derived phosphoric acid organocatalysts (Fig. 12.6,80 and 81) [214], and regarding metal catalysis, chiral scandium(III)-A,A -dioxide and 

In 2010, another example of the use of phosphoric acid catalyst was reported [214]. In this case, 10mol% of 3,3 -bis(3,5-bis(trifluoromethyl)phenyl)-l,l -8-binaphtyl-2,2 -diyl hydrogenphosphate 81, in toluene at 40 °C, allowed the synthesis of the corresponding a-amino phosphates with modest to good enantioselectivities (31-87% ee) and good yields (61-91%). 

---