Enantioselective Fluorination Of Oxindoles With N-fluorobenzenesulfonimide (NFSI) Analogues

This thesis is focusing on the enantioselective fluorination reactions of oxindoles with structurally modified N-fluorobenzenesulfonimides(NFSIs).It is shown that the novel NFSI(5a) analogues bearing two F(5b),t-Bu(5f) or OMe(5g) groups at the para-position of the symmetric phenyl rings lead to enhanced enantioselectivity as compared with the unmodified NFSI, but no corresponding fluorinated products were observed when those fluorinating reagent bear strong electron-withdrawing groups such as NO2(5i),CF3(5d) or OCF3(5e) were used.Using 4-methyl aniline as the starting material,after the Sandmyer reaction and condensation,5-methyl-isatin 1a was formed. Then starting from 5-substitued of isatins 1a-d, we gained oxindoles 4a-d, preceded by Grignard reaction,Boc protection, Hydrogenation.In the presence of (DHQD)2PHAL and base, the reactions of NFSI analogues with oxindoles were carried out at -80℃in dried CH3CN/CH2Cl2 (3:4) solution.5-Methyl oxindole 4a reacted with NFSI analogues 5a,5b,5f and 5g afford 6a with 70%～88% ee, but there is no corresponding target product generating when those fluorinating reagent are 5i,5d and 5e, fluorinating reagents bearing F,t-Bu or OMe groups at the para-position lead to enhanced enantioselectivity as compared with the unmodified NFSI; Oxindole 4b reacted with NFSI analogues 5a,5b,5f and 5g afford 6b with 38%～60% ee, fluorinating reagents bearing F,t-Bu or OMe groups lead to enhanced enantioselectivity as compared with the unmodified NFSI; 5-Fluoro oxindole 4c reacted with NFSI analogues 5a,5b,5f or 5g only afford racemic 6c at room temperature,and no target product generating at -80℃; 5-Methoxy oxindole 4d reacted with NFSI analogues 5a,5b,5f or 5g afford 6d with 30%～76% ee, fluorinating reagents bearing t-Bu groups lead to enhanced enantioselectivity as compared with the unmodified NFSI, whereas fluorinating reagents bearing F and OMe groups lead to low enantioselectivity. In general, fluorinating reagents containing F or t-Bu substitution have proven promising for the fluorination purpose, which might offer new insight into the development of novel fluorinating reagents with desirable enantioselectivity.