Synthesis of Nearly Enantiopure Allylic Amines by Aza‐Claisen Rearrangement of Z‐Configured Allylic Trifluoroacetimidates Catalyzed by Highly Active Ferrocenylbispalladacycles

Abstract

The development of the first highly active enantioselective catalyst for the aza-Claisen rearrangement of Z-configured allylic trifluoroacetimidates generating valuable almost enantiopure protected allylic amines is described. Usually Z-configured allylic imidates react significantly slower than their E-configured counterparts, but in the present study the opposite effect was observed. Z-Configured olefins have the principal practical advantage that a geometrically pure C=C double bond can be readily obtained, for example, by semihydrogenations of alkynes. Our catalyst, a C(2)-symmetric planar chiral bispalladacycle complex, is rapidly prepared from ferrocene in four simple steps. Key step of this protocol is an unprecedented highly diastereoselective biscyclopalladation providing dimeric macrocyclic complexes of fascinating structure. In the present study as little as 0.1 mol % of catalyst precursor were sufficient for most of the alkyl substituted substrates to give in general almost quantitative yields. NMR investigations revealed a monomeric structure for the active catalyst species. The bispalladacycle can also be used for the formation of almost enantiomerically pure allylic amines (ee > or =96 %) substituted with important functional groups such as ester, ketone, ether, silyl ether, acetal or protected amino moieties providing high-added-value allylic amine building blocks in excellent yield (> or =94 %). The preparative advantages should render this methodology highly appealing as a practical and valuable tool for the formation of allylic amines in target oriented synthesis.

Bispalladacycle‐Catalyzed Michael Addition of In Situ Formed Azlactones to Enones

“…The fact that both Pd centers of the FBIP system bind almost exclusively to the dehydroproline side product demonstrates that the corresponding binding constants are much higher than those of the targeted product 3‐Me‐cA , most likely due to the presence of the carboxylate moiety as anionic donor, whereas 3‐Me‐cA contains only neutral donor atoms. The anionic donor in compound 8 (i.e., the carboxylate group) binds cis , and the neutral donor (i.e., the imino group) binds trans to the imidazoline moiety as a result of a trans effect,32 in agreement with our previous findings for ferrocene bisimidazoline bispalladacycles 20b. If the formation of 3‐Me‐cA follows a bimetallic reaction pathway (see below), the coordination of one dehydroproline would already be deleterious for the catalyst performance.…”

The Asymmetric Aza‐Claisen Rearrangement: Development of Widely Applicable Pentaphenylferrocenyl Palladacycle Catalysts

“…Although ( E )‐ 25 a reacts completely within 1 d at 40 °C using only 0.05 mol % precatalyst (Table 8, entry 11), full conversion for ( Z )‐ 25 a requires 2 mol % (Table 8, entry 12). Ferrocenyl bisimidazoline bispalladacycle complexes thus remain the catalysts of choice for Z ‐configured substrates 4f,h…”

Cooperative Al(Salen)-Pyridinium Catalysts for the Asymmetric Synthesis of trans-Configured β-Lactones by [2+2]-Cyclocondensation of Acylbromides and Aldehydes: Investigation of Pyridinium Substituent Effects

“…Reactions catalyzed by salen complexes are known to proceed in several cases via bimetallic reaction pathways [ 78 ]. As part of our programme on bimetallic cooperative catalysis [ 79 , 80 , 81 , 82 ], we were therefore interested if two salen units might also cooperate to form the β-lactone products 3 . In that case the presence of a non-linear effect (NLE) would be expected [ 78 , 83 ].…”