Phosphine ligands allylic compounds

Asymmetric allylation of carbon nucleophiles has been carried out extensively using Pd catalysts coordinated by various chiral phosphine ligands and even with nitrogen ligands, and ee > 90% has been achieved in several cases. However, in most cases, a high ee has been achieved only with the l,3-diaryl-substitiitcd allylic compounds 217, and the synthetic usefulness of the reaction is limited. Therefore, only references are cited[24,133]. 

The plausible mechanism of this ruthenium-catalyzed isomerization of allylic alcohols is shown in Scheme 15. This reaction proceeds via dehydrogenation of an allylic alcohol to the corresponding unsaturated carbonyl compound followed by re-addition of the metal hydride to the double bond. This mechanism involves dissociation of one phosphine ligand. Indeed, the replacement of two triphenylphosphines by various bidentate ligands led to a significant decrease in the reactivity.37... 

The reaction starts with an oxidative addition of an allylic compound to palladium(O) and a Tt-allyl-palladium complex forms. Carboxylates, allyl halides, etc. can be used. In practice one often starts with divalent palladium sources, which require in situ reduction. This reduction can take place in several ways, it may involve the alkene, the nucleophile, or the phosphine ligand added. One can start from zerovalent palladium complexes, but very stable palladium(O) complexes may also require an incubation period. Good starting materials are the 7t-allyl-palladium intermediates ... 

Ruthenium complexes containing this phosphine ligand are able to reduce a variety of double bonds with enantiomeric excesses above 95%. In order to achieve high enantio-selectivity, the compound to be reduced must show a strong preference for a specific orientation when complexed with the catalyst. This ordinarily requires the presence of a fimctional group that can coordinate with the metal. The ruthenium binaphthyldiphosphine catalyst has been used successfully with unsaturated amides,11 allylic and homoallylic alcohols,12 and unsaturated carboxylic acids.13... 

Neutral ligands such as tertiary phosphines convert 3-allyl complexes of platinum(II) into the 1-allyl compound, and only in the presence of a large excess of added phosphine is the allyl ligand displaced. 

Combinations of eight different ligands and twelve different metal salts were screened for their efficiency to catalyze the allylation of /i-dicarbonyl compounds. The assay identified not only the well known catalyst system Pd(OAc)2 combined with a phosphine ligand but also the combination [ IrCl(cod) 2] and iPr-pybox or 1,10-phenanthroline as efficient catalysts. These are the first examples of non-phosphane iridium catalysts capable of allylic alkylations. 

These allyl cation complexes 229 are electrophilic and react with a variety of nucleophiles, most notably with the stabilised enolates of P-dicarbonyl compounds such as malonates. The immediate product is again a Jt-complex of Pd(0) 230 but there is now no leaving group so the Pd(0) drops off and is available for a second cycle of reactions. Though the reaction strictly requires Pd(0), the more convenient Pd(II) compounds are often used with phosphine ligands. Reduction to Pd(0) occurs either because the phosphine is a reducing agent or by oxypalladation and p-elimination. 

Activated methylene compounds such as dimethyl malonate have found substantial utility in palladium catalyzed allylic substitution reactions. Accordingly, the Krapcho decarboxylation is often used in conjunction with these reactions. As an example, the first total synthesis of enantiomerically pure (-)-wine lactone has utilized the sequence of reactions.27 First, the allylic substitution reaction of 2-cyclohexen-l-yl acetate (49) with alkali sodium dimethylmalonate yielded 51 with high enantioselectivity, as a result of the use of chiral phosphine ligand 50. The malonate was then subjected to Krapcho decarbomethoxylation using NaCl, H2O, and DMSO at 160 °C to yield 52. This reaction has been used similarly following the allylic substitution reaction with other malonate derivatives.28-30... 

As an example, a new palladium based method has been developed for the alkylation of tyrosine residues [34], In this reaction, allylic carbonates, esters, and carbamates are activated by palladium(O) complexes in aqueous solution, resulting in the formation of electrophilic zr -allyl complexes (such as 16), Fig. 10.3-8(a). These species react at pH 8-10 with the phenolate anions of tyrosine residues, resulting in the formation of aryl ether 17 and regeneration of the Pd(0) catalyst. The reaction requires no organic cosolvent, is catalytic in palladium, and requires P(m-QjH4S03 )3 as a water-soluble phosphine ligand. In contrast to alkyl or allylic halides, the inert character of the allyloxycarbonyl compounds used in this reaction ensures that nonspecific... 

Cyclometallation of phosphine ligands is a pathway that can deactivate catalysts. Addition of aryl C-H bonds is most favorable, and addition of the primary C-H bonds in methylphosphines is more favorable than addition of the methylene C-H bonds in higher alkylphosphines. Catalytic reactions initiated by compounds containing cyclometallated phosphorus ligands as stable and convenient precursors to active catalysts have been reported, some of which are shown in Figure 6.4. Less common are catalysts that become activated by cyclometallation one example that catalyzes allylic substitution is shown in Figure 6.5. ... 

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