Palladium catalysts catalyzed asymmetric allylation

The palladium-catalyzed asymmetric allylic substitution using seven different phosphano-oxazoline ligands at various ligand-to-metal ratios was also studied.112 An aluminum block containing 27 wells was placed in a dry box in which the reactions were carried out in parallel. Analyses were performed by conventional chiral GC equipped with an autosampler. Such a setup allowed about 33 catalyst evaluations per day. Apparently, only a few dozen were carried out in the study, resulting in the identification of a catalyst showing an ee-value of 74% in the reaction of 4-acyloxy-2-pentene with malonate.112 It is not clear whether further ligand diversification would lead to catalysts more selective than the record set in this case by the Trost-catalyst (92% ee).113... 

While alkyl halides are typically employed as an electrophile for this transformation, Takemoto developed palladium-catalyzed asymmetric allylic alkylation of 1 using allylic acetates and chiral phase-transfer catalyst 4k, as depicted in Scheme 2.5 [ 2 3 ]. The choice of triphenyl phosphite [(PhO)3P] as an achiral palladium ligand was crucial to achieve high enantioselectivity. 

Takemoto and coworkers extended their palladium-catalyzed asymmetric allylic alkylation strategy using allyl acetate and chiral phase-transfer catalyst to the quaternization of 13 [23b]. A correct choice of the achiral palladium ligand, (PhO P, was again crucial to achieve high enantioselectivity and hence, without chiral phosphine ligand on palladium, the desired allylation product 15 was obtained with 83% ee after hydrolysis of the imine moiety with aqueous citric acid and subsequent benzoylation (Scheme 2.12). 

The molybdenum(VI) complex with BIPHEN (Schrock-Hoveyda catalyst) has been widely used as an enantioselective olefin metathesis catalyst for a variety of substrates (145). Recently, the synthesis of a tetrahydrocannabiol derivative in molybdenum-catalyzed asymmetric allylic alkylation by using Trost-ligands is carried out. Enhanced regio-, enantio-, and diastereoselectivities relative to the palladium ones have been observed (146). 

Hashizume, T, Yonehara, K., Ohe, K. and Uemura, S., A novel amphiphilic chiral hgand derived from d-glucosamine. Application to palladium-catalyzed asymmetric allylic substitution in an aqueous or organic medium, allowing catalyst recycling, J. Org. Chem., 2000, 65, 5197. 

En route to the natural products ent-CP-999,994 (165) and ent-L-733,060 (166), in 2005 Nakano et al. reported a highly enantioselective palladium-catalyzed asymmetric allylic amination (>99% ee) of the allylic acetate 162 using the Ugand L6 [61]. Conversion of the resulting allyhc amine 163 with 3-butenoic acid in the presence of DCC followed by RCM (catalyst [Ru]-II, CH2CI2, reflux, 94%) yielded the piperidinone core structure 164, which was readily converted into the target molecules 165 and 166 (Scheme 2.37). 

Asymmetric synthesis of tricyclic nitro ergoline synthon (up to 70% ee) is accomplished by intramolecular cyclization of nitro compound Pd(0)-catalyzed complexes with classical C2 symmetry diphosphanes.94 Palladium complexes of 4,5-dihydrooxazoles are better chiral ligands to promote asymmetric allylic alkylation than classical catalysts. For example, allylic substitution with nitromethane gives enantioselectivity exceeding 99% ee (Eq. 5.62).95 Phosphi-noxazolines can induce very high enatioselectivity in other transition metal-catalyzed reactions.96 Diastereo- and enantioselective allylation of substituted nitroalkanes has also been reported.9513... 

The remarkable affinity of the silver ion for hahdes can be conveniently applied to accelerate the chiral palladium-catalyzed Heck reaction and other reactions. Enantioselectivity of these reactions is generally increased by addition of silver salts, and hence silver(I) compounds in combination with chiral ligands hold much promise as chiral Lewis acid catalysts for asymmetric synthesis. Employing the BINAP-silver(I) complex (8) as a chiral catalyst, the enantioselective aldol addition of tributyltin enolates (9) to aldehydes (10) has been developed." This catalyst is also effective in the promotion of enantioselective allylation, Mannich, ene, and hetero Diels-Alder reactions. 

Asymmetric allylation using optically active allylic siliconates has been reported [96]. The allylic siliconates were prepared by asymmetric hydrosilylation of 1,3-dienes and HSiCla catalyzed by a chiral palladium catalyst followed by ethanolysis. Complete asymmetric induction of allylic siliconates to homoallylalcohols was accomplished (Sch. 55). 

Finally, carbohydrate ligands of enantioselective catalysts have been described for a limited number of reactions. Bis-phosphites of carbohydrates have been reported as ligands of efficient catalysts in enantioselective hydrogenations [182] and hydrocyanations [183], and a bifunctional dihydroglucal-based catalyst was recently found to effect asymmetric cyanosilylations of ketones [184]. Carbohydrate-derived titanocenes have been used in the enantioselective catalysis of reactions of diethyl zinc with carbonyl compounds [113]. Oxazolinones of amino sugars have been shown to be efficient catalysts in enantioselective palladium(0)-catalyzed allylation reactions of C-nucleophiles [185]. 

Other Metal Catalysts Palladium catalysts are most widely used for C—N bond formation, particularly in Buchwald-Hartwig coupling reactions [48], Halides or pseudohalides are generally used. Recently, direct aminationof C— H bonds has been developed with palladium catalysts. Pd(0)-catalyzed diamination of terminal olefins at allylic and homoallylic carbons takes place via formal sp3 C—H activation under solvent-free conditions [49]. More recently, an asymmetric version of allylic and homoallylic diamination has been successfully achieved using di-tert-butyldiaziridi-none as the nitrogen source (Equation 11.21) [50]. 

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