Lewis platinum, chiral

Williams group observed low enantioselectivities for the Michael addition of a prochiral nucleophile, ethyl 2-cyanopropionate 623, to methyl vinyl ketone 624 catalyzed by chiral platinum complexes (Scheme 8.196)." The NMR analysis indicated that these cationic Pt complexes act as Lewis acids toward nitriles. The X-ray crystal structure as well NMR analysis showed that the solvent ligand that is readily displaced by an organic substrate is situated cis to the nitrogen donor in the Pt complex and, therefore, is in a chiral pocket created by the oxazoline ring. 

Strukul has recently shown that platinum catalyst [(triphosPO)Pt]2+ used to promote the Baeyer-Villiger oxidation with H2O2 operated by Lewis acid catalysis and promoted the leaving of the OH group.33 This dual catalysis property was considered unique to Pt catalysts. Chiral platinum systems have been applied to cyclic ketones to give moderate enantioselectivity of lactones.32... 

In extension of this work, a platinum complex with a chiral phosphine ligand and a BINOL template was employed as a metallomonomer [28]. After removal of the template and activation of the complex with silver salts (generation of Lewis acidic platinum centers) the MIP was used as a catalyst for an asymmetric ene reaction. Enantioselectivities of 72% ee were observed (Fig. 15). This compares well to... 

The asymmetric arylation or alkylation of racemic secondary phosphines catalyzed by chiral Lewis acids in many cases led to the formation of enantiomerically enriched tertiary phosphines [120-129]. Chiral complexes of ruthenium, platinum, and palladium were used. For example, chiral complex Pt(Me-Duphos)(Ph)Br catalyzed asymmetric alkylation of secondary phosphines by various RCH2X (X=C1, Br, I) compounds with formation of tertiary phosphines (or their boranes) 200 in good yields and with 50-93% ee [121]. The enantioselective alkylation of secondary phosphines 201 with benzyl halogenides catalyzed by complexes [RuH (/-Pr-PHOX 203)2] led to the formation of tertiary phosphines 202 with 57-95% ee [123, 125]. Catalyst [(R)-Difluorophos 204)(dmpe]Ru(H)][BPh4] was effective at asymmetric alkylation of secmidaiy phosphines with benzyl bromides, whereas (R)-MeOBiPHEP 205/dmpe was more effective in the case of benzyl chlorides (Schemes 65, 66, and 67) [125—127]. 

The use of chiral platinum complex as Lewis acid catalysts for the aldol reaction has been documented and studied by several groups. The platinum catalysts are generated upon the treatment of platinum salts and phosphines in generally. The PCP-type chiral platinum complexes 31 have been shown to function effectively in the aldol addition of methyl isocyanoacetate and aldehydes in excellent yields and promising enantioselectivities (Scheme 9) (78-80). The first... 

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