Diarylprolinol ethers catalysis

However, formation of this bond through the conjugate addition of a soft sulfur nucleophile to a,P-unsaturated aldehydes is efficiently catalysed using iminium ion catalysis [116], Using diarylprolinol silyl ether 55 the addition of a series of sulfur based nucleophiles to a variety of a,P-unsaturated aldehydes was shown to be effective (73-87% yield 89-97% ee). The products were isolated as their p-hydroxy sulfide derivatives 73 after in situ reduction of the products (Scheme 33). 

Simultaneous publication of the iminium ion catalysed hydrophosphination of a,p-unsaturated aldehydes by Melchiorre and Cordova showed diarylprolinol silyl ether 55 was effective in the conjugate addition of diphenylphosphine 74 [117, 118], Direct transformation of the products allowed for one-pot methods for the preparation of P-phosphine alcohols 75 (72-85% yield 90-98% ee), P-phosphine oxide acids 76 (65% yield 92% ee) and 3-amino phosphines 77 (71% yield 87% ee) (Scheme 34). These reports represent the first examples of the addition of P-centred nucleophiles and the resulting highly functionalised products may well have further use in asymmetric catalysis. 

This overview about developments in the field of proline-catalysis unfortunately cannot take into full account the vast field of proline-derived catalysts, such as diarylprolinols, 4-silo)yprolines or proline-silyl-ether, to name only a few. These are covered in subsequent chapters of this volume. Furthermore, other great improvements have been made by using immobilised proline catalysts, such as PEG-supported proline or polyelectrolyte-bound pro-line. Going one step further, supported proline catalysts are then applicable in the striving field of continuous-flow reactions. Recent examples include aldol, a-amination reactions and Michael reactions under such conditions. ... 

Diarylprolinol silyl ether catalyst was able to promote enantioselective allqrlations of a,p-unsaturated aldehydes with stabilised carbcations as electrophiles. Dienamine intermediates were trapped by the electrophile in two positions. The y-disubstituted aldehydes were not good substrates for y-substitution. However, linear unbranched and p-substituted a,p-unsaturated aldehydes preferred y-substitution. ot-Substituted aldehydes were unreactive under secondary amine catalysis (Scheme 8.45). 

The use of enamine catalysis in the enantioselective a-functionalization of carbonyl compounds has been reviewed, including aldol, Mannich, and alkylation processes," and a short review has examined enantioselective a-alkylation of aldehydes Benzodithiolylium tetrafluoroborate (133) is a water-stable salt and can be added enantioselectively to aldehydes at the a-position in the presence of simple chiral organocatalysts, giving the corresponding alcohol. The sulfurs can be readily cleaved with H2/Raney Ni, rendering the process a formal tf-methylation of aldehydes." a ,/3-Unsaturated aldehydes undergo enantioselective a- and y-alkylation via dien-amine activation, using a diarylprolinol TMS ether as catalyst." ... 

Propargylic esters undergo enantioselective alkylation with aldehydes (R-CHjCHO) to give propargylic alkylated products as a mixture of diastereomers (136), using cooperative catalysis by a copper(I)-BINAP complex and a diarylprolinol TMS ether." ... 

In 2009, a wide range of optically active chromans and tetrahydroquinolines were synthesised by Xiao et al. on the basis of the first enantioselective organo-catalytic intramolecular hydroarylations of phenol- and aniline-derived enals. Good yields combined with good to high enantioselectivities of up to 96% ee were obtained for the Michael adducts generated by catalysis with a diarylprolinol silyl ether (Scheme 1.83). 

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