Palladium complexes, catalytic asymmetric

Another type of P-0 ligands i.e. 2-(diphenylphosphino)-2-metho -1,1-binaphthyl (shown below) is reported to show good catalytic actmty as palladium complex in asymmetric hydrosilylation reaction(59) in converting terminal olelm into optically active secondary alcohols (yield > 94 %)... 

In an extension of this work, the Shibasaki group developed the novel transformation 48—>51 shown in Scheme 10.25c To rationalize this interesting structural change, it was proposed that oxidative addition of the vinyl triflate moiety in 48 to an asymmetric palladium ) catalyst generated under the indicated conditions affords the 16-electron Pd+ complex 49. Since the weakly bound triflate ligand can easily dissociate from the metal center, a silver salt is not needed. Insertion of the coordinated alkene into the vinyl C-Pd bond then affords a transitory 7t-allylpalladium complex 50 which is captured in a regio- and stereocontrolled fashion by acetate ion to give the optically active bicyclic diene 51 in 80% ee (89% yield). This catalytic asymmetric synthesis by a Heck cyclization/ anion capture process is the first of its kind. 

At the beginning of the 1970s a convenient procedure was described for converting olefins into substituted butanedioates, namely through a Pd(II)-cata-lysed bisalkoxycarbonylation reaction. So far various catalytic systems have been applied to this process, but it took twenty years before the first examples of an enantioselective bisalkoxycarbonylation of olefins were reported. Ever since, the asymmetric bisalkoxycarbonylation of alkenes catalysed by palladium complexes bearing chiral ligands has attracted much attention. The products of these reactions are important intermediates in the syntheses of pharmaceuticals such as 2-arylpropionic acids, the most important class of... 

The synthesis, structure, and catalytic properties of a Pd11 complex with a partially hydrogenated ligand, shown in Figure 31, are described.393 This study provides the first asymmetric epoxidation of alkenes catalyzed by a palladium complex.393... 

The asymmetric hydrosilylation that has been most extensively studied so far is the palladium-catalyzed hydrosilylation of styrene derivatives with trichlorosilane. This is mainly due to the easy manipulation of this reaction, which usually proceeds with perfect regioselectivity in giving benzylic silanes, 1-aryl-1-silylethanes. This regioselectivity is ascribed to the formation of stable 7t-benzylpalladium intermediates (Scheme 3).1,S Sa It is known that bisphosphine-palladium complexes are catalytically much less active than monophosphine-palladium complexes, and, hence, asymmetric synthesis has been attempted by use of chiral monodentate phosphine ligands. In the first report published in 1972, menthyldiphenylphosphine 4a and neomenthyldiphenylphosphine 4b have been used for the palladium-catalyzed reaction of styrene 1 with trichlorosilane. The reactions gave l-(trichlorosilyl)-l-phenylethane 2 with 34% and 22% ee, respectively (entries 1 and 2 in Table l).22 23... 

A catalytic asymmetric [4+2]-cydoaddition of a vinylallene with butadiene has been achieved successfully, in which a palladium complex modified by a ferrocene-derived chiral monophosphine ligand proved to be a superior catalyst transferring chirality to the product (Scheme 16.80) [90],... 

In this chapter, recent advances in asymmetric hydrosilylations promoted by chiral transition-metal catalysts will be reviewed, which attained spectacular increase in enantioselectivity in the 1990s [1], After our previous review in the original Catalytic Asymmetric Synthesis, which covered literature through the end of 1992 [2], various chiral Pn, Nn, and P-N type ligands have been developed extensively with great successes. In addition to common rhodium and palladium catalysts, other new chiral transition-metal catalysts, including Ti and Ru complexes, have emerged. This chapter also discusses catalytic hydrometallation reactions other than hydrosily-lation such as hydroboration and hydroalumination. 

Kantchev EAB, O Brien CJ, Organ MG (2007) Palladium complexes of A-heterocyclic carbenes as catalysts for cross-coupling reactions - a synthetic chemist s perspective. Angew Chem Int Ed 46 2768-2813 Kerr MS, Rovis T (2004) Enantio selective synthesis of quaternary stereocenters via a catalytic asymmetric Stetter reaction. J Am Chem Soc 126 8876-8877 Kerr MS, Read de Alaniz J, Rovis T (2002) A highly enantio selective catalytic intramolecular Stetter reaction. J Am Chem Soc 124 10298-1029 Kerr MS, Read de Alaniz J, Rovis T (2005) An efficient synthesis of achiral and chiral 1,2,4-triazolium salts bench stable precursors for A-heterocyclic carbenes. J Org Chem 70 5725-5728... 

Habermas KL, Denmark SE, Jones TK (1994) Qrg React 45 1-158 Hagiwara E, Fujii A, Sodeoka M (1998) J Am Chem Soc 120 2474 Hamashima Y, Sasamoto N, Hotta D, Somei H, Umebayashi N, Sodeoka M (2005) Catalytic asymmetric addition of beta-ketoesters to various imines by using chiral palladium complexes. Angew Chem Int Ed Engl 44 1525-1529... 

In 1999 Trost and Schroder reported on the first asymmetric allylic alkylation of nonstabilized ketone enolates of 2-substituted cyclohexanone derivatives, e.g. 2-methyl-1-tetralone (45), by using a catalytic amount of a chiral palladium complex formed from TT-allylpaUadium chloride dimer and the chiral cyclohexyldiamine derivative 47 (equation 14). The addition of tin chloride helped to soften the lithium enolate by transmetala-tion and a slight increase in enantioselectivity and yield for the alkylated product 46 was observed. Besides allyl acetate also linearly substituted or 1,3-dialkyl substituted allylic carbonates functioned well as electrophiles. A variety of cyclohexanones or cyclopen-tanones could be employed as nucleophiles with comparable results . Hon, Dai and coworkers reported comparable results for 45, using ferrocene-modified chiral ligands similar to 47. Their results were comparable to those obtained by Trost. 

Relatively limited work on profene synthesis via carbonylation of benzyl-X derivatives has been reported from university groups. One exception is the stero-selective carbonylation of racemic benzylic bromides. The asymmetric reaction toward enantiomerically pure profenes could a priori proceed either by a kinetic resolution or by true asymmetric induction via the intermediacy of a trigonal substrate. Results from Arzoumanian et al. [35] strongly suggest that the carbonylation of 1-methylbenzyl bromide with oxazaphospholene-palladium complexes is a kinetic resolution process with a discriminative slow oxidative addition step. Best enantiomeric excess is about 64 % ee at 9 % chemical yield. Another possible way to synthesize enantiomerically pure profenes is to start from optically pure benzyl derivatives. Baird et al. investigated the carboxylation of optically active benzyl carbonates with palladium catalysts. The enantiomeric excess was only modest [36]. Thus, the development of an efficient catalytic asymmetric carbonylation of C-X derivatives is still an existing challenge. 

The coordination chemistry of many chiral ligands is directed towards preparing complexes with catalytically useful metals, most notably late transition metals. Some Rh, Pd, Pt, and Ag complexes of Diop (149) (and also (150)) have been described,327,328,381,382 along with the use of palladium Diop complexes in asymmetric palladium catalysed cross-couplings383 and alkene hydrocyanation reactions. 

This volume begins with two procedures in the area of catalytic asymmetric synthesis. The first procedure describes the synthesis of (R)-2-Dl PH ENYLPHOSPHI NO-2 -METHOXY-1,1 -BINAPHTHYL (MOP), a chiral ligand that has proven very useful in palladium-catalyzed hydrosilylation of olefins and palladium-catalyzed reduction of allylic esters by formic acid. The next procedure describes the catalytic asymmetric synthesis of nitroaldols using a chiral LANTHANUM-LITHIUM-BINOL COMPLEX, illustrated by the synthesis of (2S,3S)-2-NITRO-5-PHENYL-1,3-PENTANEDIOL. 

There has also been considerable interest and progress in the development of asymmetric Heck reactions (AHR) [68], This methodology allows for the preparation of enantiomerically enriched products from achiral substrates using a catalytic amount of a chiral palladium complex, making the process practical and economical. For example, treatment of triflate 71 with catalytic Pd(OAc)2/(R)-BlNAP provided oxindole 72 in 96% yield and 88% ee [69]. 

The search for a catalyst suitable to promote addition of the less reactive silyl enol ethers of ketones has identified a novel class of cationic transition metal complexes in two independent laboratories. The use of a chiral palladium(II) di-aquo complex in the catalytic asymmetric addition of silyl enol ethers to aldehydes (first demonstrated by Shibasaki, Sodeoka et al. [52a, 52b]) provided a clear precedent for their subsequent use with a-imino esters [53] (Scheme 27). Initial experiments focused on the reaction of various a-imino esters 82a-c with silyl enol ether 83 (1.5equiv) in the presence of the Pd diaquo complex 80a (10 mol %) in DMF. Extensive experimentation led to the formation of 84c in 67% ee, and also underscored the importance of suppressing the generation of tetrafluoroboric acid during the course of the reaction. 

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