Palladium® complexes oxazoline ligands

Helmchen and coworkers employed a,co-amino-1,3-dienes as substrates [51]. By using palladium complexes with chiral phosphino-oxazolines L as catalysts, an enantiomeric excess of up to 80 % was achieved. In a typical experiment, a suspension of Pd(OAc)2, the chiral ligand L, the aminodiene 6/1-90 and an aryltriflate in dimethylformamide (DMF) was heated at 100 °C for 10 days. Via the chiral palladium complex 6/1-91, the resulting cyclic amine derivative 6/1-92 was obtained in 47% yield and 80% ee (Scheme 6/1.23). Using aryliodides the reaction time is shorter, and the yield higher (61 %), but the enantiomeric excess is lower (67% ee). With BINAP as a chiral ligand for the Pd°-catalyzed transformation of 6/1-90 and aryliodide, an ee-value of only 12% was obtained. 

Burgess and co-workers (38) recently reported a similar approach to the evaluation of a phosphine-oxazoline ligand system. In this paper, a series of phosphine-oxazoline ligands (132) were synthesized individually. Palladium complexes of... 

Bidentate ferrocene ligands containing a chiral oxazoline substituent possess both planar chiral and center chiral elements and have attracted much interest as asymmetric catalysts.However, until recently, preparation of such compounds had been limited to resolution. In 1995, four groups simultaneously communicated their results on the asymmetric synthesis of these structures using an oxazoline-directed diastereoselective lithiation (Scheme 8.141). " When a chiral oxazolinylferrocene 439 was metalated with butyllithium and the resulting aryllithium species trapped with an electrophile, diastereomer 442 was favored over 443. The structure of the major diastereomer 442 was confirmed, either by conversion to a compound of known stereochemistry or by X-ray crystallography of the product itself or of the corresponding palladium complex. ... 

The results from various palladium complexes derived from bis(oxazoline) ligands ent-45, 139, and 140 were examined using the above reaction and are... 

Bis(oxazoline) ligands have also been used in Wacker-type cycUzations. " For example, the phenolic derivative 238 was cyclized in the presence of ligand 13b complexed with palladium(ll) to yield the 2,3-dihydrobenzofuran 239 in 86% yield with an ee of 94% (Fig. 9.70). 

The dendritic oxazoline ligand 197 <02JOC8197> was used for the formation of palladium complexes able to catalyse enantioselective allylic alkylation reactions. 

The extraction concept is also applicable to sophisticated syntheses of fine-chemicals as recently shown by Ohe, Uemura and co-workers [35], They prepared a novel amphiphilic phosphinite-oxazoline chiral ligand based on D-glucosamine. The corresponding palladium complex was an efficient catalyst for asymmetric allylic substitution reactions and could be recycled by simple acid/base extraction and reused in the second reaction without loss of enantioselectivity. 

The arylation of secondary phosphines 201 with ortho-aiy iodides, catalyzed by generated in situ complex Pda (dba>3 x CHQ3, containing chiral ligand Et,Et-FerroTANE 207 and LiBr, led to the formatiOTi of corresponding tertiary phosphines with enantioselectivity of 90% cc [ 132,137]. The palladium complex 209 also showed high enantioselectivity in arylation of secondary phosphines [131,132]. Some examples of arylation reaction of secondary phosphines with low ee were described. The asymmetric arylation of phosphine boranes with anisyl iodide, catalyzed by chiral complex of oxazoline phosphine 208, led to the formation of enantiomerically enriched tertiary phosphines 206 with 45% ee [134]. The Pd complex 210 of (R )-t-Bu-JOSlPHOS ligand catalyzed arylation of PH(Me)(Ph)(BH3) by o-anisyl iodide with the formation of PAMP-BH3 with 10% ee (Table 3) [112]. 

Monohgated oxazolines derived from commercially available 2-ethyl-2-oxazoUne can form trans-square-planar palladium complexes by simple treatment of this ligand with inorganic palladium salts in methanolic solutions (Scheme 6.15) [72]. The catalysis, when performed in pyrrolidine, gave a yield of 32%, which corresponded to a TON of 190. The Sonogashira reaction was carried out under aerobic conditions with no precautions to exclude moisture from the solvent or substrates. 

Ricci and coworkers [64] studied oxazoline moiety fused with a cyclopenta[P]thio-phene as ligands on the copper-catalyzed enantioselective addition of Et2Zn to chalcone. The structure of the active Cu species was determined by ESI-MS. Evans and coworkers [65] studied C2-symmetric copper(II) complexes as chiral Lewis acids. The catalyst-substrate species were probed using electrospray ionization mass spectrometry. Comelles and coworkers studied Cu(II)-catalyzed Michael additions of P-dicarbonyl compounds to 2-butenone in neutral media [66]. ESI-MS studies suggested that copper enolates of the a-dicarbonyl formed in situ are the active nucleophilic species. Schwarz and coworkers investigated by ESI-MS iron enolates formed in solutions of iron(III) salts and [3-ketoesters [67]. Studying the mechanism of palladium complex-catalyzed enantioselective Mannich-type reactions, Fujii and coworkers characterized a novel binuclear palladium enolate complex as intermediate by ESI-MS [68]. 

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