Platinum enolates

Chiral bis-phosphine acylplatinum complex 210 with a strong acid such as TfOH serves as an effective enantio-selective catalyst for aldol-type reactions of aldehydes with ketene silyl acetals (Equation (127)).486 The presence of water and oxygen in the catalyst preparation step is required to obtain the highly enantioselective catalyst. The intermediacy of a C-bound platinum enolate was suggested by IR and 31P NMR spectroscopies. 

Pt(II). Fujimura has developed a Pt(II)-catalyzed process for the addition of iso-butyrate-derived silyl ketene acetal 97 to aldehydes (Eq. (8.27)) [43]. The process utilizes a readily available Pt(Il) complex (98) that is generated in situ and can be easily handled in the laboratory [44]. In the presence of 5 mol% each of 98, triflic acid, and lutidine, 97 undergoes addition to aldehydes to afford a mixture of tri-methylsilyl-protected 99 and free alcohol 100 products in up to 95% ee. A thorough examination of the reaction conditions and their effect on the product selec-tivities has revealed that the addition of water and oxygen to the catalyst mixture leads to significant improvement in the optical purity of the products. A number of spectroscopic studies by P NMR and IR has led Fujimura to postulate that the reaction involves a carbon-bound platinum enolate intermediate in the catalytic cycle. 

The first example of a stable geminal enediol derivative of a carboxylic acid, isolated as its platinum(II) complex, has been reported.324 Treatment of [Pt(OH2)2(en)]2+ with /V,/V-bis(phos-phonomethyl)aminoacetic acid yields zwitterionic [Pt(bpmaa)(en)] (bpmaaH = 7V,7V-bis(phosphono-methyl)aminoacetic add), which upon crystallization from water (pH 1) affords the corresponding enol tautomer (130), as determined by X-ray crystallography and IR spectroscopy. 

Diboration of a,/ -unsaturated ketones is promoted by platinum(O) complexes. Reaction of 4-phenyl-3-buten-2-one with bis(pinacolato)diboron in the presence of a platinum catalyst affords a boryl-substituted (Z)-boron enolate, that is, a 1,4-diboration product, in high yield with high stereoselectivity (Scheme 8). The isolated boron enolate is easily hydrolyzed by exposure to water, giving / -boryl ketones in high yields.66 Similar diboration of a,/ -unsaturated ketones has also been achieved with Pt(bian)(dmfu) (bian = bis(phenylimino)acenaphthene, dmfu = dimethyl fumarate).67 Although the... 

Diboration of a,/3-unsaturated esters is catalyzed by the platinum(0)/diimine catalyst, giving a,/3-diboryl esters, that is, 3,4-addition products (Equation (13)). Although the a,/3-diboryl ester products are hydrolytically more stable than the corresponding 1,4-addition products bearing a boron ester enolate moiety, they gradually undergo hydrolysis... 

Benzyl-6-methylcyclohexanone has been prepared by the hydrogenation of 2-benzylidene-6-methylcyclohexanone over a platinum or nickel catalyst, and by the alkylation of the sodium enolate of 2-formyl-6-methylcyclohexanone with benzyl iodide followed by cleavage of the formyl group with aqueous base. The 2,6-isomer was also obtained as a minor product (about 10% of the monoalkylated product) along with the major product, 2-benzyl-2-methylcyclohexanone by successive treatment of 2-methylcyclohexanone with sodium amide and then with benzyl chloride or benzyl bromide. Reaction of the sodium enolate of 2-formyl-6-methylcyclohexanone with potassium amide in liquid ammonia formed the corresponding dianion which was first treated with 1 equiv. of benzyl chloride and then deformylated with aqueous base to form 2-benzyl-2-methylcyclohexanone.i ... 

Treatment of K[Pt(0,O-MeCOCHCOMe)(C-MeCOCHCOMe)X] with mineral acids affords a material, formulated as Pt(0, -MeCOCHCOMe)Cl(MeCOCH2COMe) in which neutral, enolic acetylacetone is rc-bonded to platinum via the C=C bond. On standing, solutions of this material in chloroform yield red [Pt( -Cl)(MeCOCHCOMe)] . Similar acidification of K[Pt(MeCOCHC-OMe)3] with HX (X = Cl, Br) yields Pt(l,3,5,7-tetramethyl-2,6,9-oxabicyclo[3.3.1]nona-3,7-diene)2X2,248 whereas sulfuric, phosphoric or nitric acid leads to polymeric [Pt(MeCOCHCOMe)2]. This compound, which is isomeric with the familiar Pt(0,O-MeC0CHCOMe)2, may aggregate by means of tridentate C,0,0-MeCOCHCOMe bridging ligands.244... 

Chloro(thienyl) complexes, with platinum(II), 8, 478 m-Chlorotributylstannane, cycloadditions, 9, 179 Chlorovinylchlorogermane, reactivity, 3, 746 Chlorovinylcobalamin, applications, 7, 45 Chlorozirconocene, preparation, 4, 930 Chlorozirconocene ketimides, preparation, 4, 911 Chlorozirconocene mono(keto enolate)s, preparation,... 

Tanaka and co-workers <19960M1524> observed the insertion of acid chlorides into SCBs in the presence of palladium or platinum catalysts. When an excess of amine was employed, SCBs undergo ring-expansion reactions to afford cyclic silyl enol ethers in good to excellent yields (Scheme 49). 

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