Pd-BINAP complex

Aldol reactions of silyl enolates are promoted by a catalytic amount of transition metals through transmetallation generating transition metal enolates. In 1995, Shibasaki and Sodeoka reported an enantioselective aldol reaction of enol silyl ethers to aldehydes using a Pd-BINAP complex in wet DMF. Later, this finding was extended to a catalytic enantioselective Mannich-type reaction to a-imino esters by Sodeoka s group [Eq. (13.21)]. Detailed mechanistic studies revealed that the binuclear p-hydroxo complex 34 is the active catalyst, and the reaction proceeds through a palladium enolate. The transmetallation step would be facilitated by the hydroxo ligand transfer onto the silicon atom of enol silyl ethers ... 

Zinc dibenzyl malonate complex 129, prepared by the action of Et2Zn on dibenzyl malonate, was found to be superior to alkali metal dibenzyl malonates, in terms of enantiomeric control, in the allylic substitution with ( )-130 catalysed by an in situ prepared Pd-(/ )-BINAP complex (equation 73)164. 

Their most detailed investigations focused on the Heck cyclization of iodide 18.1c to form oxindole 17.3a (Scheme 8G.18) [38a,b]. A chiral-amplification study [47] established that the catalytically active species is a monomeric Pd-BINAP complex, a conclusion also corroborated by NMR studies by Amatore and co-workers [42d,43], In addition, two possibilities for the enantioselective step of the neutral pathway were easily eliminated [38a], Oxidative addition was precluded as the enantioselective step, because iodides cyclize with very different enantioselectivities in the presence of Ag(I) salts. A scenario where migratory insertion is reversible and [l-hydridc elimination is the enantioselective step was also ruled out, because this is not consistent with the dependence of enantioselectivity on the geometry of the double bond of the cyclization precursor. 

Some optically active 3-alkoxycarbonyl-2-methylisoxazolidines were obtained by asymmetric decomposition in the presence of catalytic amount of Pd-BINAP complex. For instance, the kinetic resolution of racemic 105 by 106 afforded (+)-105 in 48% yield and with... 

Guitian et al. also reported the first cocycUzation of arynes with alkynes such as dimethyl acetyl-enedicarboxylate (DMAD) leading to phenanthrenes 101 or naphthalenes 102. The selectivity of the process can be tuned by an appropriate choice of the catalyst (Schane 12.52) [90]. These authors have also described the first enantioselective version of this process, using a Pd-(BINAP) complex as catalyst, although the nonracemic 9,12-dimethoxypentahelicene 103 was isolated in a modest yield but with reasonable ee (Scheme 12.52) [91]. Diynes can also participate in [2+2+2] cycloaddition with arynes as demonstrated by Mori et al. in the total synthesis of taiwanins C and E 104a and b (Scheme 12.52) [92]. 

Concerning enantioselective processes, Fujihara and Tamura have proved that palladium NPs containing (S)-BINAP (2,2 -bis(diphenylphosphino)-l,l -binaphthyl) as chiral stabiliser, catalyse the hydrosilylation of styrene with trichlorosilane, obtaining (S)-l-phenylethanol as the major isomer (ee = 75%) [42]. In contrast, the palladium complex [Pd(BINAP)(C3H5)]Cl is inactive for the same reaction [43]. 

Several studies were performed in order to establish the mechaiusm (5-7). The currently accepted mechartism, presented in Scheme 26.1 for the Pd(BINAP) catalyzed amination, involves the formation of a complex, Pd(BINAP)2 from a catalyst precursor (usually Pd(OAc)2 or Pd2(dba)3) and ligand this complex lies outside the catalytic cycle and undertakes dissociation of one BINAP to form Pd(BINAP) the following steps are the oxidative addition of the aryl halide to the Pd(BINAP), reaction with amine and base, and the reductive elimination of the product to reform Pd(BlNAP). 

We can see from Figure 26.3 that a complex arises at the beginning, showing peaks in both the phosphoras and fluorine NMR it is the same previously observed during the benzophenone reaction when the hexylamine reaction starts, and its product begins to form, this complex disappears and a single peak in the P is visible, referring to the Pd(BINAP)2. 

With DIOP-Pd(0) or -Ni(0) complexes as catalysts, moderate optical yields of up to 35% have been observed (126). Norbomene is convertible to the exo nitrile with up to 40% ee when a BINAP-Pd(0) complex is used (Scheme 57) (127). Ni(0) complexes of sugar-derived 1,2-diol phosphinites catalyze highly selective asymmetric addition of hydrogen cyanide to vinylarenes (128). This method gives the 2-naphthalene-2-propionitrile precursors of nonsteroid anti-inflammatory agents in up to 85% ee and in high yield. 

The first example of Pd-catalyzed enantioselective allylation to be reported was the reaction of l-(l -acetoxyethyl)cyclopentene and the sodium salt of methyl benzenesulfonylacetate in the presence of 10 mol % of a DIOP-Pd complex, which led to the condensation product in 46% ee (Scheme 85) (200). This reaction used a racemic starting material, but the enantioselection was not a result of kinetic resolution of the starting material, because the chemical yield was above 80%. However, in certain cases, the selectivity is controlled at the stage of the initial oxidative addition to a Pd(0) species. In a related reaction, a BINAP-Pd(0) complex exhibits excellent enantioselectivity the chiral efficiency is affected by the nature of the leaving group of the allylic derivatives (Scheme 85) (201). It has been suggested that this asymmetric induction is the result of the chiral Pd catalyst choosing between two reactive conformations of the allylic substrate. 

Scheme 81 shows a highly enantioselective C—C bond formation in the BINAP-Pd(II) diacetate-catalyzed reaction of aryl triflate and 2,3-dihydiofuran (193). A BINAP-Pd(0) species generated by the action of a tertiary amine on the Pd(II) complex is the actual catalyst. Enhancement of the enantioselectivity through kinetic resolution of the intermediate is indicated by the double-bond isomer having opposite absolute configuration at the arylated carbon. / -Substituted 2-pyrrolines may also be used as olefinic substrates. 

As shown in Scheme 86, the CHIRAPHOS-Pd-catalyzed reaction of 1,1,3- or 1,3,3-triphenyl-substituted allyl acetate and sodiomalonate results in optical yields of up to 86%. A BINAP-Pd(0) complex is effective for asymmetric synthesis of a-allyl-a-acetamidomalonate esters of high enantiomeric purity (202). 

Kinetic resolution using the AHR was achieved for the first time by Shibasaki et al. [123], The AHR of racemic 133 catalyzed by the Pd-(R)-tol-BINAP complex provided the desired product 134p, a potential synthetic intermediate for (+)-wortmannin,in 20% yield (134P 134a=l 1 1) and 96% ee (Scheme 6). 

In an oxidative addition, Pd(0) complex 22 with BINAP as a ligand accepts alkenyl triflate It. The resulting Pd complex 23 is cationic, since the triflate anion is bound only loosely to the palladium and dissociates from the complex.1 Syn insertion of one of the two enantiotopic double bonds of the cyclopentadienc into the alkenyl-Pd bond of complex 23 leads firs to q -allyl-Pd complex 24. This is in rapid equilibrium with t 3-allyl-Pd complex 25. Neither 24 nor 25 contains a p-H atom in a yn relationship to palladium. Moreover, internal rotation is impossible in the con form a-tionaily fixed ring system. For this reason there is no possibility of a subsequent p-hydride elimination that would once again release the palladium catalyst. In a normal Heck reaction (see discussion) the catalytic cycle would be broken at this point. 

Asymmetric cyclization to tricyclic ergolines. The ergoline system of ergot alkaloids can be obtained in fairly high enantioselectivity by cyclization of 2 catalyzed by Pd(0) complexed with a chiral ligand and a base. The ligands BINAP,... 

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 quadrant model which has been described for [Ru-BINAP] can normally be applied analogously with [Pd-BINAP] [15] and [Rh-BINAP] complexes. A new example for the latter is the hydroacylation of 4-substituted 4-pentenal 27 to 3-substituted cyclopentenones 28 using the [Rh-BINAP]C104 catalyst (Scheme 6). However, the strong dependence of the selectivity upon the nature of the substituents R suggests that stereoelectronic factors have to be taken into account as well [16]. 

An efficient asymmetric synthesis of six-membered quinoline derivatives bearing a quaternary carbon center or a spiro-ring by an ene-type cyclization of 1,7-enynes catalyzed by the cationic BINAP-Pd(ll) complex was reported <03JA4704>. 

The Heck reaction has the same scope and limitations as Pd-catalyzed car-bonylation. Pd(II) complexes are often added to the reaction mixture and are reduced in situ. Primarily trans alkenes are obtained due to conformational preferences in the rotamer from which /3-hydride elimination takes place. Sometimes the /3-hydride elimination occurs away from the new C-C bond, and in this case a new stereocenter is formed. If the starting material is prochiral, the reaction can be made asymmetric by the use of chiral phosphine ligands like BINAP (2,2 -bis(diphenylphosphino)-1,1 -binaphthyl). 

Extensive mechanistic studies have been conducted on the oxidative addition of aryl halides to Pd(0) complexes ligated by PPh3 in different media and with different additives175, 176. However, palladium complexes containing these ligands are not active catalysts for amination. Instead, one must consider the addition of aryl halides to palladium complexes bound by ligands relevant to amination. Studies of the mechanism of oxidative addition to palladium(O) complexes of P(tol-o)3, DPPF, BINAP, Q-phos, P(Bu-f)3 and an /V-heterocyclic carbene ligand have been reported. 

Amatore and coworkers have studied the addition of aryl halides to Pd(0) complexes formed by the addition of chelating phosphines to Pd2(dba)3171. Addition of these ligands to Pd2(dba)3 generates mixed phosphine/dba complexes. The kinetic behavior of the complex containing BINAP as phosphine ligand indicated that two competing pathways for addition occur, one by direct reaction of aryl halide with (BINAP)Pd(dba) and one from the (BINAP)Pd intermediate formed by dissociation of dba. 

Aldol-type reactions. The highest anti selectivity is observed with BINAP complex of Pd(OTf)j in the aldol reaction of tin enolate of cyclohexanone. The condensation of silyl enol ethers with imines provides y-keto a-amino esters. ... 

Sodeoka et al. have developed novel chiral diaqua Pd(II)-BINAP and -Tol-BINAP complexes 67 as efficient asymmetric catalysts of the aldol reaction of SEE (Scheme 10.57) [157]. These complexes are readily prepared from PdCl2(BINAP) and PdCl2(Tol-BINAP) by treatment of 2 equiv. AgBE4 in wet acetone, and are quite stable to air and moisture. The results of H NMR experiments indicate that reaction of 67b with acetophenone TMS enolate forms an O-bound Pd enolate. 

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