Steric Effects in Enantioselective Allylic Alkylation Catalysed by Cationic(η³‐Allyl)palladium Complexes Bearing Chiral Pyridine‐Aziridine Ligands

Abstract

Enantiopure N,N′‐bidentate ligands (N,N′)*, containing substituted aziridine and aza‐aromatic rings, were prepared from imines derived from (S)‐valinol or (R)‐phenylglycinol and heteroaromatic aldehydes (2‐pyridine‐, 6‐benzyl‐2‐pyridine‐, 2‐ and 8‐quinolinecarbaldehyde) by the addition of iPrMgCl and subsequent cyclisation of the 1,2‐amino alcohol moiety to aziridine. Crystalline [(N,N′)*(η3‐allyl)Pd][SbF6] complexes were prepared from these ligands and used as catalysts in the alkylation of sodium dimethyl malonate with 1,3‐diphenyl‐2‐propenyl acetate and carbonate. By comparing the performances of two complexes bearing similar pyridine‐aziridine ligands but with a different C2‐aziridine substituent (Ph vs. iPr), a better enantioselectivity was provided by the Ph‐substituted ligand (90 % vs. 41 % ee), whereas the presence of a C6‐pyridine benzyl substituent caused inversion of the enantioselectivity. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Preparation, coordination and catalytic use of planar-chiral monocarboxylated dppf analogues

“…Chloro-diphenylphosphine was distilled under vacuum. Acid 2, 9c 1,3-diphenylprop-2-en-1-yl acetate (13a), 47 ethyl (1,3-diphenylprop-2-en-1-yl) carbonate (13b), 48 and di-m-chlorido-bis(Z 3 -1,3-diphenylallyl)dipalladium(II) (15) 49 were prepared following the literature procedures. Other chemicals and solvents used in workup and in crystallisations were used as received (Fluka and Aldrich).…”

Chiral amine–imine ligands based on trans-2,5-disubstituted pyrrolidines and their application in the palladium-catalyzed allylic alkylation

“…A similar inversion of enantioselectivity with a longer bridge was reported for pyridine-aziridine ligands. 29 It was found that modification on the 6-position of the pyridine ring caused reversal of the stereoselectivity for the allylic alkylation, similar to an increase in the chelate size. Ligand 34 catalyzed the reaction to afford the major product as the (R)-enantiomer.…”

“…A similar inversion of enantioselectivity with a C6-pyridine substituent was reported for pyridine-aziridine ligands. 29 Changing the electronic properties of the substituents on the 4-position of the pyridine ring has no significant effect on the stereoselectivity of the reaction. Instead, the electronic properties of the substituents on this position only affected the reactivity of these ligands.…”

“…As part of an ongoing project on the design and synthesis of ligands for asymmetric catalytic reactions, we developed a modular synthetic strategy for preparing amine-imine ligands based on trans-2,5-disubstituted pyrrolidines. 16 Since our initial report of this class of ligands, 16 several examples of both trans-2,5-disubstituted pyrrolidine-containing [20][21][22][23][24][25][26] and pyridineamine [27][28][29][30] and ligands have been reported, in addition to many tertiary diamines. 31 This type of ligand is constructed by linking a pyridine ring to the pyrrolidine ring as chiral bidentate ligands A. Nitrogen atoms from both the pyridine and pyrrolidine moieties are different electronically and are assumed to provide different binding properties to transition metals.…”

Asymmetric Synthesis of 1‐(2‐Pyrrolyl)alkylamines by the Addition of Organometallic Reagents to Chiral 2‐Pyrroleimines

“…At this point we assessed the possibility of using β‐amino alcohol 7 for the construction of new optically active, pyrrole‐containing molecules, such as aziridines by intramolecular cyclization and secondary amines through oxidative cleavage of the chiral auxiliary (Scheme ). For the first goal we followed the same protocol previously applied for the synthesis of analogous 1‐(2‐pyridyl)alkylaziridines 18b,18c. As a matter of fact, pyrrole–aziridines 9a , b , f were successfully obtained in high yields by treatment of β‐hydroxyamines 7a , b , f with triphenylphosphane and diethylazodicarboxylate (DEAD) in THF at room temperature.…”