Allylic ethers, palladium® chloride

A mixture of phenyl allyl ether palladium (Il)and bis(benzonitrile) palladium (II) chloride in dry benzene refluxed (16-20h) to give phenol... 

The oxygen atom has also been used to generate other functionalities, such as the aldehyde moiety in Kibayashi s syntheses of (—)-coniine (197) and its enantiomer (Scheme 1.43) (253). Here, reaction of tetrahydropyridine N-oxide 93 with a silylated chiral allyl ether dipolarophile 198 delivered the adduct 199 with the desired bridgehead stereochemistry via the inside alkoxy effect . Desilylation and hydrogenolytic N—O bond rupture with palladium(II) chloride provided the diol 200... 

A series of pyrido[2,3-rf pyrimidine-2,4-diones bearing substituents at C-5 and/or C-6 were synthesized using palladium-catalyzed coupling of uracil derivative 417 with vinyl substrates or allyl ethers to give the regioisomeric mixtures of 418/419 and 420/421, respectively. The ratio of the isomeric structures was dependent on the substituent R. In the case of the reaction with -butyl vinyl ether, only the product 419 was obtained. However, the reactions with acrylonitrile, ethyl acrylate, 2-trifluoromethylstyrene, and 3-nitrostyrene afforded only 418. Also, reaction with allyl phenyl ether gave only 420. The key intermediate 417 was prepared by the reaction of 6-amino-l-methyluracil with DMF-DMA (DMA = dimethylacetamide), followed by N-benzylation with benzyl chloride and vinyl iodination with iV-iodosuccinimide (NIS) (Scheme 15) <2001BML611>. 

In contrast to palladium chloride, platinous chloride does not give a ir-allylic complex on heating with allyl alcohol instead, the diallyl ether complex [PtChi CsHsOCsHs)] is formed in good yield (105). 

To conclude, palladium on charcoal is only a precursor for soluble palladium species which are active in the carbonylation of allyl ethers. The formation of these species requires the presence of chloride and proceeds jointly with the generation of dimethyl carbonate. This catalyst could be reused but we have been unable so far to recover the ester selectivities achieved with fresh catalysts. The observation of a higher content of oligomeric materials in the reaction mixture resulting from the use of the recovered catalyst led us to suggest that an increase of the surface acidity of the carrier is presumably responsible for this increase in the oligomer content. 

Interception of the ji-allyl palladium intermediate with soft nucleophiles such as morpholine and dimedone is attractive on the grounds of efficacy, economy, safety and scope. Deprotection of a phenol allyl ether in a synthesis of the antibiotic Vancomycin demonstrates that aryl iodides and chlorides remain intact [Scheme 4.227],429... 

Bis(allyl)Pd and -Pt compounds are also made by treatment of PdClj and PtClj with RMgX see Table 4. Mixed bis(allyl)Pd complexes can be obtained when an allylic RMgX reagent or allyltin reagent is slowly added at — 30°C to an ether dioxane solution of ( t-allyl)palladium chloride dimer . Removal of pentane under vacuum affords the products as light golden-yellow crystalline solids in 80% yield. 

Another example of tr-allyl ligand transfer has been described recently by Heck 159). Treatment of tr-allyl derivatives of palladium chloride with NaCo(CO)4 in ether gives tr-allyl cobalt complexes. 

The reactions and product distributions thus far reported have been exclusively concerned with hexene. It was of interest to see whether the high specificity of positional substitution could be maintained with the other hexene isomers. By positional substitution specificity is meant ester attachment on ether of the carbons involved in the original carbon-carbon double bond. Table VII shows the results of these studies. The internal olefins reacted more slowly than the a-olefin, and with both palladium chloride-cupric chloride and 7r-hexenylpalladium chloride-cupric chloride systems high substitutional specificity (> 95% ) was also maintained with 2-hexene (Table VII). However, with 3-hexene the specificity is considerably lower (80%). Whether this is caused by 3-hexene isomerization prior to vinylation or by allylic ester isomerization is not known. A surprisingly high ratio of 2-substitution to 3-substitution is found ( 7 1) in the products from 2-hexene. An effect this large... 

Kumada and his co-workers have reported an interesting series of crosscoupling reactions between phenylmagnesium bromide and allylic ethers where the regiochemistry of the product is dependent entirely on the catalyst. For example, reaction with either of the isomeric allylic ethers (42) or (43) under the influence of catalytic quantities of [l,l -bis(diphenylphosphino)fer-rocene]palladium(ii) chloride gives predominantly the internal olefln (44). By contrast, use of the corresponding nickel catalyst furnishes the terminal olefin preferentially (>81%) in each case. ... 

The second type of palladium intermediate is a rr-allyl complex of Pd(II). The TT-allyl complexes can be obtained from Pd(II) salts and allylic acetates, allyl ethers, and other allylic compounds having potential leaving groups.The same rr-allyl complexes can be prepared from alkenes by reaction with palladium chloride or palladium trifluoroacetate. " For unsymmetrical alkenes with more than one reactive allylic position, the latter method gives rise to mixtures of rr-allyl complexes. 

The synthesis of thiepins 14 was unsuccessful in the case of R1 = i-Pr,79 but if the substituents in the ortho positions to sulfur arc /erf-butyl, then thiepin 14 (R1 = t-Bu R2 = Me) can be isolated in 99% yield.80 Rearrangement of diazo compound 13 (R1 = t-Bu R2 = H), which does not contain the methyl group in position 4, catalyzed by dimeric ( y3-allyl)chloropalladium gives, however, the corresponding e.w-methylene compound. The thiepin 14 (R1 = t-Bu, R2 = H) can be obtained in low yield (13 %) by treatment of the diazo compound with anhydrous hydrogen chloride in diethyl ether at — 20 C.13 In contrast, the ethyl thiepin-3,5-or -4,5-dicarboxylates can be prepared by the palladium catalysis method in satisfying yields.81... 

A reactor was charged with allyl palladium(II) chloride dimer (4.49 mmol), triethylphosphite (8.97 mmol), and hexamethyldigermanium (98.7 mmol) and then stirred for 5 minutes at ambient temperature and treated with the dropwisc addition of fleshly distilled benzoyl chloride (89.7 mmol). After stirring for 4 hours at 110°C the mixture was filtered and concentrated. The residue was purified by column chromatography using petroleum ether/ethyl acetate, 40 1, respectively, and 7.8 g of product isolated as a yellow liquid having a max of 411.5 nm with s =1374 dm2/mol. 

Abnormal olefin arylation reactions which are of interest mechanistically and preparatively occur with some allylically substituted compounds. The ailylic esters and ethers appear normal and produce cinnamyl derivatives exclusively while ailylic alcohols and chlorides are abnormal. Ailylic alcohols and "arylpalladium acetates form 3-arylaldehydes from primary ailylic alcohols and 3-arylketones from secondary alcohols 3°). The mechanism of reaction apparently involves anti-Markovnikov addition of the palladium compound to the double bond followed by elimination of the hydrogen atom on the hydroxyl-bearing carbon rather than the benzylic hydrogen. This again would be elimination of the more electronegative hydrogen atom. 

Palladium(II) salts are known to isomerize double bonds along a hydrocarbon chain as well as to cause cis-trans isomerizations (50, 82, 179). These isomerizations are believed to occur either by Pd(II)-hydride or via ir-allyl complexes. Recently, Pd(II)-catalyzed cis-trans isomerizations that apparently do not proceed by either mechanism have been reported. For instance, cis- and fram-l,2-didi,uteroethylenes are isomerized without deuterium scrambling—a result that is inconsistent with hydride mechanisms [136). Likewise vinyl chlorides and esters are isomerized, and again it could be shown that Pd(II)-hydrides were not involved 117, 123, 124). The isomerization of vinyl ethers has also been reported recently 257). Plausible mechanisms for this isomerization include the following ... 

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