Palladium complexes vinyl ethers

A three-component coupling involving three alkenes was employed using a stoichiometric amount of palladium to generate a bicyclic acetal 6.91, which could be converted to an epimer of the prostaglandin, PGF2 (Scheme 6.31). The three-component coupling involves nucleophilic attack of the alcohol 6.89 onto an ethyl vinyl ether-palladium complex 6.93, intramolecular alkene insertion, intermolecular insertion of... 

The reactions of nitrones constitute the absolute majority of metal-catalyzed asymmetric 1,3-dipolar cycloaddition reactions. Boron, aluminum, titanium, copper and palladium catalysts have been tested for the inverse electron-demand 1,3-dipolar cycloaddition reaction of nitrones with electron-rich alkenes. Fair enantioselectivities of up to 79% ee were obtained with oxazaborolidinone catalysts. However, the AlMe-3,3 -Ar-BINOL complexes proved to be superior for reactions of both acyclic and cyclic nitrones and more than >99% ee was obtained in some reactions. The Cu(OTf)2-BOX catalyst was efficient for reactions of the glyoxylate-derived nitrones with vinyl ethers and enantioselectivities of up to 93% ee were obtained. 

T[[dotb]he nature of the initial attack by the water (eq. 10) is a matter of some controversy (205,206). Stereochemical and kinetic studies of model systems have been reported that support trans addition of external water (207,208) or internal addition of cis-coordinated water (209), depending on the particular model system under study. Other palladium-catalyzed oxidations of olefins in various oxygen donor solvents produce a variety of products including aldehydes (qv), ketones (qv), vinyl acetate, acetals, and vinyl ethers (204). However the product mixtures are complex and very sensitive to conditions. 

In accessing chiral allyl vinyl ethers for Claisen rearrangement reactions, Nelson et al. employed the iridium-mediated isomerization strategy. Thus, the requisite enantioenriched diallyl ether substrate 28 was synthesized via a highly enantioselective diethylzinc-aldehyde addition protocol10 (Scheme 1.1k). The enantioselective addition of Et2Zn to cinnamaldehyde catalyzed by (—)-3-exo-morpholinoisobomeol (MIB 26)11 provided an intermediate zinc alkoxide (27). Treatment of 27 with acetic acid followed by 0-allylation in the presence of palladium acetate delivered the 28 in 73% yield and 93% ee. Isomerization of 28 with a catalytic amount of the iridium complex afforded the allyl vinyl ether... 

Olefin isomerization catalyzed by ruthenium alkylidene complexes can be applied to the deprotection of allyl ethers, allyl amines, and synthesis of cyclic enol ethers by the sequential reaction of RCM and olefin isomerization. Treatment of 70 with allyl ether affords corresponding vinyl ether, which is subsequently converted into alcohol with an aqueous HCl solution (Eq. 12.37) [44]. In contrast, the allylic chain was substituted at the Cl position, and allyl ether 94 was converted to the corresponding homoallylic 95 (Eq. 12.38). The corresponding enamines were formed by the reaction of 70 with allylamines [44, 45]. Selective deprotection of the allylamines in the presence of allyl ethers by 69 has been observed (Eq. 12.39), which is comparable with the Jt-allyl palladium deallylation methodology. This selectivity was attributed to the ability of the lone pair of the nitrogen atom to conjugate with a new double bond of the enamine intermediate. 

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 ... 

Palladium/phosphine-sulfonate neutral complexes, reported by Drent, Pugh, and coworkers, could incorporate several polar comonomers such as methyl acrylate, vinyl ethers, methyl vinyl ketones, and silyl vinyl ethers. Other common comonomers such as vinyl acetate, acrylonitrile, and vinyl chloride showed very low comonomer incorporation, between 1% and 2% [48]. 

Most recently, the synthesis of chalcones from aryl bromides in the presence of PPha as a ligand was achieved (Scheme 7.16) [35]. Later on, this group extended our methodology to vinyl ethers [36]. Based on both experimental results and DFT calculations, a proposed mechanism for this reaction is shown in Scheme 7.17. It begins with the oxidative addition of ArX to the Pd° center to form the corresponding aryl palladium complex. Followed by the coordination and insertion of CO, the respective acyl palladium complex is produced. After coordination, addition, and elimination processes, the desired chalcone is produced. Under the assistant of the base, Pd is regenerated and starts the next reaction cycle. 

The oxidative functionalization of olefins through ir-olefin complexes of palladium also has a long history, including the industrial production of acetaldehyde and vinyl acetate. Related reactions, including the conversion of olefins to vinyl ethers and enamines, have been studied in more recent times for fine chemical synthesis. These oxidative C-0 and C-N bond formations have been conducted with a variety of oxidants, including Oj, and have been studied as both intermolecular and intramolecular processes. 

This divergent oxidative reactivity of alcohols and carboxylic acids with olefins is illustrated in Equation 16.103. Bofli reactions generate products from p-hydrogen elimination of an alkoxyalkyl or acetoxyalkyl complex. However, the vinyl ether product generated by p-hy-drogen elimination undergoes reaction with a second equivalent of alcohol to form the acetal in a process catalyzed by the acidic medium or the action of palladium(II) as Levris add. 

C=C double bond of aP-unsaturated steroidal esters. Several reports of addition reactions catalysed by transition metal compounds have also appeared. Diphenyl-phosphine adds to alkyl vinyl ethers with a high regioselectivity, in the presence of nickel(ii) and palladium(ii) complexes, giving the Markownikolf adducts (99). ... 

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