Lewis palladium, chiral

Silyl enol ethers derived from acetone undergo ene reactions rather than aldol reactions to give a-fimctionalyzed enol silyl ethers. Lewis acidic chiral titanium, chiral chromium, and chiral palladium catalysts have been developed to allow synthesis of silyl enol ethers having a chiral center (Scheme 3-60). 

Catalytic asymmetric Diels-Alder reactions are presented by Hayashi, who takes as the starting point the synthetically useful breakthrough in 1979 by Koga et al. The various chiral Lewis acids which can catalyze the reaction of different dieno-philes are presented. Closely related to the Diels-Alder reaction is the [3-1-2] carbo-cyclic cycloaddition of palladium trimethylenemethane with alkenes, discovered by Trost and Chan. In the second chapter Chan provides some brief background information about this class of cycloaddition reaction, but concentrates primarily on recent advances. The part of the book dealing with carbo-cycloaddition reactions is... 

Lewis acid type. Chiral copper(II), palladium(II), and titanium(IV) catalysts... 

The asymmetric fluorination of enolates by means of chiral metal complexes has been reported with Selectfluor in the presence of a chiral Lewis acid derived from TADDOL (TiCl2/TADDOL), or with F-A-sulfonimide (NFSI) with palladium complexes and chiral phosphines. 

The imines 12 (X = 4-CH3-QH4-SO2 (Ts), Ar, C02R, COR, etc.) preformed or generated in situ from N,0- or N,N-acetals or hemiacetals are another important class of Mannich reagents frequently used for diastereo- and/or enantioselective aminoalkylation reactions catalyzed by chiral Lewis acids (usually copper or palladium BINAP complexes such as 13). Among other things excellent results were obtained in the aminoalkylation of silyl enol ethers or ketene acetals [24], A typical example is the synthesis of Mannich bases 14 depicted in Scheme 5 [24b], Because of their comparatively high electrophilicity imines 12 could even be used successfully for the asymmetric aminoalkylation of unactivated alkenes 15 (ene reactions, see Scheme 5) [24h, 25], and the diastereo- and/or enantioselective aminoalkyla-... 

The palladium-catalyzed silylboration of typical allenes exhibited a strong tendency to occur at the internal double bond giving allylsilanes, which undergo allylation of aldehydes in the presence of a Lewis acid (Equation (44)).254 257 The selectivity can be varied by changing the substituents, but the boron atom always added to the central carbon. Enantioselective silylboration of terminal monosubstituted allenes was demonstrated by double asymmetric induction using chiral silylboranes and chiral catalysts (Equation (45)). 

Lewis acid catalysts activate the aldehyde by coordination to the carbonyl oxygen. Shibasaki et al. [13] were able to demon,strate that the activation of the enol ether is possible too. The reaction of the aldehyde 37 with the silyl enol ether 38 in the presence of the catalyst 39 proceeds with good, but still not excellent enantioselectivity to yield the aldol adduct 40. Only 5 mol % of the chiral palladium(II) complex 39 was used (Scheme 6a). Activation of the Pd(lI)-BINAP complex 39 by AgOTf is necessary. Therefore, addition of a small amount of water is important. 

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. 

Olefin hydrocyanation using palladium catalysts has been less well studied than with nickel. Nevertheless, zerovalent complexes of palladium, particulrly triarylphosphite complexes, hydrocyanate a wide range of olefins in useful yields (see Table 1). Early work reported the merit of excess phosphorus ligand to promote the reaction, and further paralleling the observations with nickel, Lewis acids have been used to improve catalytic activity. However, addition of ZnClj fails to improve nitrile product yield . Asymmetric induction in hydrocyanation results in optical yields of 30% in the synthesis of exo-2-cyanonorbomane using the chiral ligand DIOP, and studies on the stereochemistry of HCN and DCN addition to terminal alkenes and a substituted cyclohexene with the same catalyst have been reported. ... 

Very recently, White and coworkers introduced the chiral Lewis acid Crm(salen) as cocatalyst into Ll/Pd11 catalytic system. The oxidative allylic acetoxyaltion of terminal olefins 1 afforded the corresponding branched allylic acetates 3 in high regioselectivity and moderate enantio-selectivities (up to 63% ee) (Scheme 6) [22], The asymmetric induction possibly results from the coordination between Cr salen) and BQ, and the adduct of Cr,n(salen) BQ promotes the acetoxylation of rc-allyl-palladium complex to form enantioenriched branched allylic acetates. 

Sinou and co-workers [73] studied the influence of different surfactants on the palladium-catalyzed asymmetric alkylation of l,3-diphenyl-2-propenyl acetate with dimethyl malonate in presence of potassium carbonate as base and non-water-soluble chiral ligands. Best results in activity and enatioselectivity (> 90% ee) were observed with 2,2 -bis(diphenylphosphino)-l,l -binaphthyl (BINAP) as ligand and cetyltrimethylammonium hydrogen sulfate as surfactant in aqueous medium. Water-stable Lewis acids as catalysts for aldol reactions were developed by Kobayashi and co-workers [74]. An acceleration of the reaction was indicated in presence of SDS as anionic surfactants. An additional promotion could be observed by combination of Lewis acid and surfactant (LASCs = Lewis acid-surfactant-combined catalysts) as shown in Eq. (3). Surfactant the anion of dodecanesulfonic acid. 

Bringman et al. have described a synthesis of the unusual isoquinoline alkaloid ancistrocladine (82) starting from the chiral tetrahydroisoquinoline (79). Conversion of (79) to the ester (80), followed by a palladium catalysed coupling reaction led to the helicene-type lactone (81) which was then easily converted to (82). In a new route to the morphine ring system, Ludwig and Schafer have developed the intramolecular Lewis acid catalysed coupling of the tetrahydroisoquinoline (83) to (84) as a key step.26 The tetracycle... 

In Section 12.7.B, the Stille coupling reaction reacted a vinyl tin compound with a vinyl triflate, in the presence of palladium(O). Tetravalent tin complexes add to aldehydes and ketones, in the presence of a Lewis acid. Allyltin complexes are, by far, the most widely used of these compounds.297 a typical example is taken from the work of Keck, in which a chiral aldehyde (455) was treated with allyltributyltin, in the presence of various Lewis acids. S As shown in Table 12.19, a mixture of syn (456) and anti (457) products was obtained. The ratio of 456/457 was dependent on the structure of the R group in 455, the solvent and the Lewis acid.The anti product (457) was obtained by using the tert-butyldimethylsilyloxy derivative (sec. 7.3.A.i) of 455 with 2 equivalents of boron trifluoride in dichloromethane. The syn product is obtained preferentially when the benzyloxy derivative of 455 is used with titanium tetrachloride in dichloromethane.298... 

The asymmetric fluorination of 3-ketoesters has been achieved in 62-90% ee using F-TEDA (Selectfluor) as fluorine source in the presence of 0.5 mol% of the chiral nonracemic titanium-based Lewis acid (5.108). ° A greater range of p-ketoesters are fluorinated with higher ee using catalytic quantities of the palladium-BINAP complex (5.109) and N-fluorobenzenesulfonamide (NFSI). ° In both cases the reaction proceeds through the intermediacy of a chiral enolate. 

An interesting application of the chiral cyclopalladated complexes is palladium-promoted asymmetric Diels-Alder reactions of l-phenyl-3,4-dimethylphosphole (Scheme 4). In the original report on the Diels-Alder reaction of the phosphole reported by Nelson and co-workers, the dichloropalladium species was employed as a promoter.f In 1994, Leung showed that a chiral cyclopalladated complex was capable of promoting the Diels-Alder reaction. In this reaction, the chiral palladacycle worked as a chiral auxihary and showed almost perfect diastereoselectivity. It has been revealed that simultaneous precoordination of a diene and a dienophile to the palladium center is essential for the reaction. Thus, both dienes and dienophiles should possess Lewis basic functionahty. The diene l-phenyl-3,4-dimethylphosphole has been the only substrate examined so far, while a variety of dienophiles, such as vinylphosphine, vinyl-sulfoxide, vinylsulfide, acrylamide, vinylarsine, vinylpyridine, vinyl-pyrrole, or methylenequinuchdinone, have successfully been applied to this asymmetric Diels-Alder reaction. 

Chiral imidazolium or triazolium salt reacted with Pd(OAc)2 in the presence of Nal and KO Bu, giving a mixture of carbene-Pd complexes (Scheme These nucleophilic carbenes are regarded as replacements of phosphines with stronger Lewis basicity and attracted considerable attention recentlyThe palladium complexes formed as mixture of cis and trans isomers, which were separable by column chromatography. Upon heating in DMF at 100 °C, the cis isomer is completely isomerized into the trans isomer. In these complexes, rotations around the carbene-Pd bonds are restricted, probably due to double bond characters of the C—Pd bonds, and several diastereomers were detected. 

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]. 

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