Palladium Complexes with Chiral Diphospholane Ligands: Comparative Catalytic Properties and Analysis of (η³‐Allyl)palladium Species

Abstract

The chiral diphospholane ligands Duphos (1) and Duxantphos (2), which can be differentiated by their bite angle, were applied to palladium-catalysed asymmetric reactions, essentially allylic alkylations with symmetrically (3a−d) or unsymmetrically (5a,b) substituted allylic acetates as substrates. The most interesting results were found with the first series of substrates in which 2 was more reactive and/or selective than 1. The model complexes of the catalytic intermediates [Pd{(S,S)-1}(η 3 -cyclohexenyl)](BF 4 ) (10) and exo-[Pd{(R,R)-2}(η 3 -cyclohexenyl)](SbF 6 ) (11) could be characterised by X-ray crystallography.

A Comparison of (R,R)‐Me‐DUPHOS and (R,R)‐DUPHOS‐iPr Ligands in the Pd0‐Catalysed Asymmetric Allylic Alkylation Reaction: Stereochemical and Kinetic Considerations

“…Support for this conjecture comes from the observation that when the reaction was performed according to the conditions of Malaisé et al11 (Table 1, entry 3) a poorer conversion of 60 % was obtained as opposed to the quantitative yield reported by these workers using Me‐DUPHOS.…”

“…In all cases the ( R )‐enantiomer of the alkylated malonate product was the major isomer. A surprising result and contrary to the results obtained using 1a 10,11 where the ( S )‐malonate product was the major enantiomer. Drago and Pregosin10 have previously shown on the basis of extensive 1 H NMR studies on the isolated Pd‐allyl complex with an exo ‐conformation that the malonate nucleophile attacks preferentially C‐1 as this is the most electrophilic of the two terminal allyl carbons.…”

“…Our rationale was based on the supposition that the presence of bulky groups around the P atoms should give high enantioselectivities. It must be noted that, Drago and Pregosin10 have already demonstrated (as a one‐off study) an ee as high as 98 % for the alkylation of rac ‐1,3‐diphenylpropenyl acetate with dinuclear [PdCl(PhCHCHCHPh)] 2 , malonate as nucleophile and ( R , R )‐Me‐DUPHOS and Malaisé et al11 showed a similar result, but only when the sodium salt of dimethyl maloante was used [as they claimed that BSA was not an appropriate base for reactions involving ( R , R )‐Me‐DUPHOS]. In the case of ( R )‐Josiphos it was shown by Togni and Spindler that ee values of up to 93 % could be achieved for this reaction 12.…”

“…), BSA (0.3 mL, 3 equiv.) and KOAc (0.4 mg, 1 mol‐%) were added (In the case of the reaction with NaH the procedure given in ref 11. was followed).…”

Synthesis of Chiral Heterocyclic Phosphines for Application in Asymmetric Catalysis

“…The efficiency of the ligands has been proven in the Pdcatalyzed allylic alkylation of allyl acetates with dimethyl malonate as a nucleophile [114,115]. Selectivities between 24-97 %ee were observed in dependence on the solvent and the substrate used.…”