Stereoselective Nucleophilic Trifluoromethylation ofN-(tert-Butylsulfinyl)imines by Using Trimethyl(trifluoromethyl)silane

Abstract

Trifluoromethylated amines are important building blocks for pharmaceutical research. [1] The CF 3 group, because of its strongly electron withdrawing nature, lowers the basicity of the amide bond towards nonspecific proteolysis [2] when these amines are incorporated into peptides, as well as modify the solubility and desolvation properties. [3] In spite of its prime importance in the drugs industry, direct asymmetric synthesis of trifluoromethylated amines is a challenge. Pirkle et al. [4] and Mosher and Wang [5] prepared 2,2,2-trifluoro-1-phenylethylamine, and Soloshonok and Ono [6] recently reported an elegant method for the preparation of perfluorinated amines by a novel [1,3]-proton shift reaction. However, all of these methods require fluorinated ketones. Nucleophilic transfer of ªCF 3 º to nitrones and imines for direct preparation of trifluoromethylated amines was recently achieved by Nelson et al. [7] and Blazejewski et al., [8] respectively. These methods suffer from low yield and lack generality. We now report the first stereoselective synthesis of trifluoromethylated amines by using TMSCF 3 2 (TMS SiMe 3 ).Our systematic investigation began as an extension of our earlier work, [9] based on the fact that imines are less electrophilic than carbonyl compounds, and that O À Si bonds are stronger than NÀSi bonds. We predicted that strongly electrophilic imines might be a solution to this problem under noncatalytic conditions. When N-sulfonylaldimines [10] were used as imine sources the reaction indeed proceeded smoothly in the presence of CsF and gave only the trifluoromethylated adducts in 45 ± 95 % yield. Next we turned our attention to sulfinylimines 1 to make this reaction stereoselective. When chiral sulfinylimines [11] were subjected to similar reaction conditions little or no products were obtained. Sulfinylimines were recovered intact, but TMSCF 3 decomposed. We surmised that sulfinylimines are not reactive enough to add TMSCF 3 . Use of different aprotic solvents and excess of reagents was not helpful. When an excess of TMSCF 3 was used, a number of unidentified fluorinated products with little or none of the expected adduct were obtained. TMS-Imidazole, [8] was recently reported to facilitate addition of TMSCF 3 to imines. In our case, however, it was ineffective. In all experiments the starting material was recovered.The above results indicate that TMSCF 3 decomposes prior to reacting with the starting material. Hence, we thought that increasing the substrate concentration might be a solution to this problem. Indeed, when neat TMSCF 3 was added to a concentrated solution of the imines, the desired adduct was obtained. The mass balance corresponds to recovered starting material. Attempts to complete the reaction by using excess of reagent in different solvents was, however, unsuccessful. Imines were treated with TMSCF 3 in the presence of a stoichiometric amount of CsF to give the corresponding trifluoromethylated sulfinamides in 50 ± 65 % yields of isolated products (Table 1, entries 1 ...

Diastereoselective Trifluoromethylation of Chiral α,β‐Unsaturated N‐tert‐Butanesulfinyl Ketimines with Ruppert‐Prakash Reagent: Asymmetric Synthesis of α‐Tertiary Trifluoromethyl Allylic Amines

“…Based on the observed diastereofacial selectivity, it is proposed that a cyclic six‐membered transition state is formed, wherein the bulky tert ‐butyl group preferentially adopts an equatorial position and the trifluoromethide ion preferably attacks the less hindered Si face of the α,β‐unsaturated ketimines 2 (Figure ). The stereocontrol mode of the current diastereoselective trifluoromethylation of the α,β‐unsaturated ketimines is completely opposite to the previously reported nucleophilic fluoroalkylation of N ‐ tert ‐butanesulfinyl aldimines (nonchelation controlled mode),–,, but similar to that of the previous precedents for the indirect mono‐ and difluoromethylation of N ‐ tert ‐butanesulfinyl ketimines . However, the exactly underlying reaction pathway of the diastereoselective trifluoromethylation of α,β‐unsaturated ketimines 2 depicted in this study is still elusive.…”