Esters bidentate

Similarly, Grignard addition to cycloalkenecaibaldehyde-derived aldimines (154) affords, upon hydrolytic work-up, optically enriched rrans-2-substituted cyclohexenecarbaldehydes (156 Scheme 28). The magnesioenamine intermediate (155) generated by the Grignard additions (only phenyl and vinyl reported) can be alkylated with methyl iodide to give, in most instances, the aldehyde (158). The amino ester bidentate ligand dictates the stereochemical control in the alkylation. The formation of the other... 

Allylic acetates react with ketene silyl acetals. In this reaction, in addition to the allylated ester 468, the cyclopropane derivative 469. which is formed by the use of bidentate ligands, is obtained[303]. Formation of a cyclopropane derivative 471 has been observed by the stoichiometric reaction of the 7r-allylpal-... 

When a bidentate phosphine is used as a ligand for the reaction of J-keto esters or /i-diketones, no dimerization takes place. Only a 2-butenyl group is introduced to give 68[49,62], Substituted dienes such as isoprene, 1,3-cyclohexa-diene, and ocimene react with carbon nucleophiles to give a mixture of possible regio- and stereoisomers of 1 1 adducts when dppp is used as a ligand[63,64]. 

The enantioselective inverse electron-demand 1,3-dipolar cycloaddition reactions of nitrones with alkenes described so far were catalyzed by metal complexes that favor a monodentate coordination of the nitrone, such as boron and aluminum complexes. However, the glyoxylate-derived nitrone 36 favors a bidentate coordination to the catalyst. This nitrone is a very interesting substrate, since the products that are obtained from the reaction with alkenes are masked a-amino acids. One of the characteristics of nitrones such as 36, having an ester moiety in the a position, is the swift E/Z equilibrium at room temperature (Scheme 6.28). In the crystalline form nitrone 36 exists as the pure Z isomer, however, in solution nitrone 36 have been shown to exists as a mixture of the E and Z isomers. This equilibrium could however be shifted to the Z isomer in the presence of a Lewis acid [74]. 

Liaw et al. reported that conversions between the neutral sparteine [Fe(NO)2] complex 133 and the anionic Fe(NO)2 [Fe(NO)2(S2C3Hg)] 137 proceed via the cationic sparteine Fe(NO)2 -complex 135 through oxidation by NO" " and transfer of the [Fe(NO)2] -unit to the chelating ligand S-(CH2)3-S 136 (Scheme 35). The resulting anionic complex 137 is stable in contrast to the cationic complex 135. The cationic complex 135 also acts as a [Fe(NO)2] donor in the presence of the DNIC [(PhS)2Fe(NO)2] 138 to yield Roussin s red ester 139. The bidentate thiol ligand S-(CH2)3-S 136 promotes the stability of the anionic DNIC Fe(NO)2 ... 

Kostic et al. recently reported the use of various palladium(II) aqua complexes as catalysts for the hydration of nitriles.456 crossrefil. 34 Reactivity of coordination These complexes, some of which are shown in Figure 36, also catalyze hydrolytic cleavage of peptides, decomposition of urea to carbon dioxide and ammonia, and alcoholysis of urea to ammonia and various carbamate esters.420-424, 427,429,456,457 Qggj-jy palladium(II) aqua complexes are versatile catalysts for hydrolytic reactions. Their catalytic properties arise from the presence of labile water or other solvent ligands which can be displaced by a substrate. In many cases the coordinated substrate becomes activated toward nucleophilic additions of water/hydroxide or alcohols. New palladium(II) complexes cis-[Pd(dtod)Cl2] and c - Pd(dtod)(sol)2]2+ contain the bidentate ligand 3,6-dithiaoctane-l,8-diol (dtod) and unidentate ligands, chloride anions, or the solvent (sol) molecules. The latter complex is an efficient catalyst for the hydration and methanolysis of nitriles, reactions shown in Equation (3) 435... 

The bidentate formate ligand of OsH(K2-02CH)(CO)(P,Pr3)2 is converted into a monodentate group by carbonylation. Thus, the reaction of this compound with carbon monoxide gives 0sH K1-0C(0)H (C0)2(P Pr3)2. Similarly, the addition of a stoichiometric amount of trimethylphosphite yields 0sH k -0C(0)H (C0) P(OMe)3 (P Pr3)2, and the addition of a stoichiometric amount of ethyne di-carboxylic methyl ester leads to 0sH K1-0C(0)H (C0)(r 2-Me02CC=CC02Me) (P Pr3)2, which in solution partially dissociates the alkyne. As is shown in... 

Hydroxycarbonylation and alkoxycarbonylation of alkenes catalyzed by metal catalyst have been studied for the synthesis of acids, esters, and related derivatives. Palladium systems in particular have been popular and their use in hydroxycarbonylation and alkoxycarbonylation reactions has been reviewed.625,626 The catalysts were mainly designed for the carbonylation of alkenes in the presence of alcohols in order to prepare carboxylic esters, but they also work well for synthesizing carboxylic acids or anhydrides.137 627 They have also been used as catalysts in many other carbonyl-based processes that are of interest to industry. The hydroxycarbonylation of butadiene, the dicarboxylation of alkenes, the carbonylation of alkenes, the carbonylation of benzyl- and aryl-halide compounds, and oxidative carbonylations have been reviewed.6 8 The Pd-catalyzed hydroxycarbonylation of alkenes has attracted considerable interest in recent years as a way of obtaining carboxylic acids. In general, in acidic media, palladium salts in the presence of mono- or bidentate phosphines afford a mixture of linear and branched acids (see Scheme 9). 

The reaction was carried out with /3-keto esters, /3-diketones, malonate, a-formyl ketones, a-cyano and a-nitro esters, cyanoacetamide, and phen-ylsulfonylacetate. (PPh3)2PdCl2 was used with sodium phenoxide. Also, Pd(OAc)2 and PPh3 are good catalysts. When bidentate ligand was used, the 1 1 rather than 1 2 addition reaction took place (56). For example, bis(diphenylphosphino) 1,2-ethane (39) produced a mixture of the following 1,4- (59) and 1,2- (60) addition products ... 

Fig. 31.10 Comparison of rate (schematic) and enantioselec-tivity for mono- and bidentate phosphorus ligands on 1 mM scale, (a) a-Dehydroamino ester, 2 bar H2 (b) jS-dehydroami-no ester, 10 bar H2.

A breakthrough in this area came when Dang and Kagan3 synthesized DIOP, a C2 chiral diphosphine obtained from tartaric acid (Fig. 6-1). DIOP-Rh(I) complex catalyzed the enantioselective hydrogenation of a-(acylamino)acrylic acids and esters to produce the corresponding amino acid derivatives with up to 80% ee. These achievements stimulated research on a variety of bidentate chiral diphosphines, and numerous chiral ligands bearing C2 symmetry have been developed as a result (see Fig. 6-1 for examples). 

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