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Allylic substitution palladium catalysis

Allyl carbonates can be cleaved by nucleophiles under palladium(O) catalysis. Allyl carbonates have been proposed for side-chain protection of serine and threonine, and their stability under conditions of /VT moc or /V-Boc deprotection has been demonstrated [107]. Prolonged treatment with nucleophiles (e.g., 20% piperidine in DMF, 24 h) can, however, lead to deprotection of Alloc-protected phenols [108,109]. Carbohydrates [110], tyrosine derivatives [107], and other phenols have been protected as allyl ethers, and deprotection could be achieved by palladium-mediated allylic substitution (Entry 9, Table 7.8). 9-Fluorenyl carbonates have been used as protected intermediates for the solid-phase synthesis of oligosaccharides [111]. Deprotection was achieved by treatment with NEt3/DCM (8 2) at room temperature. [Pg.224]

The desymmetrization of dicarbonate 206 was initiated by the addition of one equivalent of N-(3-butenyl) nosylamide 207 under palladium catalysis in the presence of Trost s chiral diphosphine ligand 205. When the first allylic substitution was completed, the reaction was warmed and the resulting intermediate 208 was treated in situ with one equivalent of a second nosylamide 209. Product 210 resulting from this double substitution reaction was submitted to a tandem intramolecular ROM/RCM to furnish key precursor 211, which was engaged in the final cyc-lization step by the reduction of the double bonds, followed by the HCl-promoted domino deprotection of the acetal and aminal formation. [Pg.378]

Keywords Allylic substitution CH activation Cross-coupling Cycloisomerization Domino reactions Metallation Multicomponent reactions Palladium catalysis... [Pg.149]

Three research groups discovered almost at the same time that non-C2-symmetrical oxazolines of the type 32 can be even more effective ligands for asymmetric catalysis than type 4 ligands (Fig. 11). For the palladium-catalyzed allylic substitutions on 62, record selectivities could be reached using 32 (X = PPhj) [30]. It seems that not only steric but also electronic factors, which cause different donor/acceptor qualities at the coordination centers of the ligand, seem to play a role here [31]. The reaction products can subsequently be converted to interesting molecules, for example 63 (Nu = N-phthalyl) can be oxidized to the amino acid ester 64 [32]. [Pg.24]

Planar chiral phosphaferrocene-oxazolines (379) constitute another family of complexes that are usefiil as ligands in asymmetric catalysis. Preparation of these takes advantage of a modified Friedel-Crafts acylation of (373) and an unusual conversion of the resulting trifluoromethyl ketone into an amide that is then cyclized to an oxazoline. The diastereomeric complexes thus formed are chromatographically separable and are used in a palladium-catalyzed asymmetric allylic substitution. Modification of this complex by using the anion derived from 3,4-dimethyl-2-phenylphosphole gives more... [Pg.2078]

The reactions of this section use stabilized carbanions formed from C—H-addic compounds by deprotonation. As phosphines are the only successful ligands known up to now, these reactions have been discussed in detail in Chapter 2, and we will again restrict this section to a few highlights. Carbanions derived from 1,3-diketones react with allylic esters enantioselectively under palladium catalysis with more than 80% ee [175]. Benzylamine is allylated by the same catalytic system, leading to substituted allyl-benzylamines with up to 97% ee (Fig. 4-32c) [176]. [Pg.214]

A remarkable product selectivity is also observed in the case of methylenecyclopropanes with geminal diphenyl substitution. Whereas under nickel catalysis [Ni(cod)2 at SO-TO C] 18 is selectively dimerized to tra7w-l,l,6,6-tetraphenyldispiro[2.1.2.1]octane, which can be obtained in 28% yield at a conversion of 40%, substrate 18 reacts in a completely different manner with palladium(O) catalysts derived from (t/ -allyl)( -cyclopentadienyl)palladium(II) and triisopropylphosphane. Besides isomerization to 19, proceeding at temperatures above 85 °C, the monospiro derivative 20 is formed as the major product. Additionally, minor amounts of a formal [3 + 3] dimer 21 can be isolated. The latter probably arises from a palladium-mediated, stoichiometric reaction as the yield of 21 could not be improved under any conditions in catalytic runs. On prolonged heating, thermal isomerization of the methylenecyclopropanes to form... [Pg.2232]

Allylic substitutions are among the most important carbon-carbon bond-forming reactions in organic synthesis. Palladium-catalyzed allylic substitutions and their asymmetric version have been extensively studied and widely used in a variety of total syntheses [78]. The palladium catalysis mostly requires soft nucleophiles such as malonate carbanions to achieve high stereo- and regioselectivity. [Pg.152]

Catalysis by palladium complexes was actively developed during this decade. Allylic substitution gave excellent results in some cases, thanks to a good fit between the structures of catalyst and substrate. There were significant improvements in the enantioselectivities of the reactions and understanding to some extent of various mechanistic details (for example see [64,65,66]. Most of the time the product was formed with one or several asymmetric centers. In rare cases axial chirality may be created, too [67]. [Pg.34]

You ye seen that palladium catalysis helps form carbon-carbon bonds that are difficult to make using conventional reactions. It can also help form carbon-heteroatom bonds that are difficult to make, and you have already seen some examples in the reactions of re-allyl complexes. Work starting in the 1990s by Buchwald and Hartwig has shown that Pd can be used to promote nucleophilic substitution at a vinylic or aromatic centre—a reaction which would not normally be possible. For example, aromatic amines can be prepared directly from the corresponding bromides, iodides, or triflates and the required amine in the presence of pal-ladium(0) and a strong alkoxide base. [Pg.1092]

Starting from dimedone, 3-allyl-substituted cyclohexa-l,4-diene-2,4-diyl bisnonafluo-robutanesulfonates have been prepared in four efficient steps. These 1,5-hexadienes under palladium catalysis first undergo intramolecular carbopalladation followed by dehydropalladation to yield a 8-methylenebicyclo[4.2.0]octa-l,4-dien derivative, which subsequently couples intermolecularly with added tert-butyl acrylate. In the presence of... [Pg.1376]

Ally lie substitution (the Tsuji-Trost reaction) is among the most synthetically useful processes in palladium catalysis. As the catalytic efficiency of allylic substitution is often moderate (5-10 mol % of Pd catalyst are usually used), and phosphine-free systems are generally inefficient, the recycling of catalyst is the only feasible way to make the process more economical. Various phase-separation techniques have been tried for this reaction. In what concerns the rate of reaction and catalytic efficiency, such ligands as TPPTS are likely to be less effective compared to PhsP.f Thus, the main reason for the use of hydrophilic ligands in allylic substitution is the design of recyclable systems. [Pg.1314]


See other pages where Allylic substitution palladium catalysis is mentioned: [Pg.200]    [Pg.14]    [Pg.1336]    [Pg.326]    [Pg.496]    [Pg.276]    [Pg.161]    [Pg.267]    [Pg.34]    [Pg.469]    [Pg.113]    [Pg.217]    [Pg.129]    [Pg.423]    [Pg.38]    [Pg.205]    [Pg.8]    [Pg.676]    [Pg.310]    [Pg.1]    [Pg.142]    [Pg.565]    [Pg.246]    [Pg.240]    [Pg.349]    [Pg.154]   
See also in sourсe #XX -- [ Pg.237 ]




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Allylation catalysis

Allylic substitution

Allyls palladium

Catalysis substitution

Palladium allylation

Palladium allylic substitution

Palladium catalysis

Palladium catalysis allylation

Palladium catalysis allylic

Palladium catalysis substitution

Palladium substitution

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