Quaternary ammonium bifluorides chiral

Chiral Quaternary Ammonium Bifluorides Preparation and Use as Organocatalysts lor Asymmetric Carbon-Carbon Bond-Forming Reactions... 

Table 9.4 Asymmetric Mukaiyama-type aldol reactions of a glycine derivative catalyzed by chiral quaternary ammonium bifluoride 20.

The nitroaldol reaction of silyl nitronates with aldehydes promoted by ammonium fluorides, which was originally introduced by Seebach and Colvin in 1978 [24], is a useful method for the preparation of 1,2-functionalized nitroalkanols. Recently, the present authors have succeeded in developing an asymmetric version of high efficiency and stereoselectivity by using a designer chiral quaternary ammonium bifluoride of type 6 as catalyst, which was readily prepared from the corresponding bromide by the modified method C in Scheme 9.5 [25]. 

Table 9.5 Asymmetric nitroaldol reactions catalyzed by chiral quaternary ammonium bifluorides 6.

Further applications of the chiral ammonium bifluoride-catalyzed enantioselective Michael addition of silyl nitronates has been shown in the reactions with nitroalkenes as a Michael acceptor (Scheme 9.17). These studies were started by examining the reaction of nitropropane-derived silyl nitronate 23b with P-nitrostyrene, using the chiral quaternary ammonium bifluoride (R,.R)-6d. When P-nitrostyrene was treated with 23b (1.2 equiv.) in the presence of (K,f )-6d (2mol%) in THF at — 78 °C, the... 

Highly enantioselective Michael addition of silyl nitronates (105) to cyclic o /3-unsaturated ketones (106 n = 0-2) has been accomplished by the utilization of N-spiro C2-symmetric chiral quaternary ammonium bifluoride (108) as an organocatalyst, offering a new route to the enol silyl ethers of scalemic y-nitro ketones (107 70-90% ee).156... 

The chiral quaternary ammonium bifluoride (162) has been shown to catalyse a (g) highly diastereo- and enantio-selective conjugate addition of silyl nitronates (160) to nitroalkenes, giving rise to 1,3-dinitro compounds (161) (76-96% ee).213 ... 

Homogeneous Catalysis with Chiral Quaternary Ammonium Bifluorides 125... 

Table 4.2 Asymmetric Michael addition of silyl nitronate 16 to a,/ -unsaturated aldehydes and cyclohexenone catalyzed by chiral quaternary ammonium bifluorides (/ ,/ )-15. Isolation of optically active enol silyl ethers 20 and 21.

Maruoka et al. have developed and used A-spiro C2-symmetric chiral quaternary ammonium bifluorides [151] 102, 103, and more recently 104, to promote the regio- and anti-selective Mukaiyama-Michael addition of silyl nitronates to a, 3-unsaturated aldehydes [152], cyclic a,P-unsaturated ketones [153], and nitroalk-enes [154] with good yields and enantioselectivities (Scheme 2.52). Final chiral silyl enol ethers are easily hydrolyzed to the corresponding carbonyl compounds or functionalized at the a-position by reaction with electrophiles. 

C2-symmetric chiral quaternary ammonium bifluorides 123,124, and 125 served as effective organocatalysts for such reactions (Scheme 5.57). 

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