Praktikable, hochaktive und enantioselektive Ferrocenyl‐Imidazolin‐Palladacyclus(FIP)‐Katalysatoren für die Aza‐Claisen‐Umlagerung von N‐para‐Methoxyphenyltrifluoracetimidaten

Asymmetric Formation of Allylic Amines with N‐Substituted Quaternary Stereocenters by PdII‐Catalyzed Aza‐Claisen Rearrangements

“…The allylic trifluoroacetamides were formed with high to excellent ee values and in good yield (Table 1, entries 1–10). As expected, the rearrangement of 3,3‐disubstituted substrates 4 is significantly slower than for substrates with R=H, for which only 0.05 mol % leads to full conversion after 1–3 days at 40 °C in most cases 2h. The lower turnover frequency is attributed to the additional substituent at the CC double bond, which hampers the attack of the imidate nitrogen atom on the olefin.…”

“…Whereas Z ‐configured 3‐monosubstituted imidates had given significantly lower reaction rates than E ‐configured substrates in previous experiments with 1 ‐X, this difference vanishes if both R and R′ are larger than H 2h. For example, both the E ‐ and Z ‐configured substrates 4 d and 4 h bearing a bulky (CH 2 ) 3 OTIPS moiety as substituent gave practically the same yield and conversion.…”

“…Recently, we have developed a planar‐chiral ferrocenyl imidazoline palladacycle catalyst 1 ‐X (FIP‐X), which displays unprecedented levels of activity for the highly enantioselective aza‐Claisen rearrangement of E ‐configured allylic PMP‐trifluoroacetimidates (PMP= p ‐methoxyphenyl) 2h. Compound 1 ‐X is in fact about 50–100 times more active for the rearrangement of this class of substrates than the best previously reported catalysts.…”

“…2.0 mol % of the catalyst precursor FIP‐Cl ( 1 ‐Cl, X=Cl), activated in situ with 3.75 equivalents AgTFA (relative to 1 ‐Cl, TFA=trifluoroacetate),2h was generally sufficient to give high conversion after 2.5 days at 50 °C using substrates in which one of the substituents R or R′ is a methyl group. The allylic trifluoroacetamides were formed with high to excellent ee values and in good yield (Table 1, entries 1–10).…”

“…Since the enantioselectivity of the rearrangement is apparently largely independent of the steric differentiation of the two residues R and R′ at the imidate 3‐position, it appeared that the enantioselectivity‐determining step is the enantioface‐selective coordination of the olefin moiety to the Pd II center (Figure 1). 11 Assuming that the olefin will coordinate (in analogy to PPh 3 )2h trans to the imidazoline N atom owing to the trans effect,12 the imidate N atom will attack the olefin at the face remote to the Pd atom. In the preferential orientation of the olefin part parallel to the ferrocene axis, the sterically undemanding C1 methylene moiety should point towards the bulky Cp Φ spectator ligand to minimize unfavorable steric interactions.…”