Functionalizations alkenes, palladium® acetate

The importance of palladium acetate lies in its ability to catalyse a wide range of organic syntheses functionalizing C-H bonds in alkanes and in aromatics, and in oxidizing alkenes. It has been used industrially in the... 

Interesting compounds, such as cyclopropylboronic acids and esters which are otherwise not easily accessible due to the apparent lack of convenient and versatile syntheses, can now be prepared by simple cyclopropanation with diazomethane. The carbene transfer to substituted and functionalized alkenes also takes place smoothly in the presence of palladium(II) acetate. Several 1-alkenylboronic acid esters 9 were converted to cyclopropylboronic acid esters 10 using this method. ... 

Allylic acetoxylation with palladium(II) salts is well known however, no selective and catalytic conditions have been described for the transformation of an unsubstituted olefin. In the present system use 1s made of the ability of palladium acetate to give allylic functionalization (most probably via a palladium-ir-allyl complex) and to be easily regenerated by a co-oxidant (the combination of benzoquinone-manganese dioxide). In contrast to copper(II) chloride (CuClj) as a reoxidant,8 our catalyst combination is completely regioselective for allcyclic alkenes with aliphatic substrates, evidently, both allylic positions become substituted. As yet, no allylic oxidation reagent is able to distinguish between the two allylic positions in linear olefins this disadvantage is overcome when the allylic acetates are to... 

Functionalized benzenes preferentially induced ortho-para substitution with electron-donating groups and meta substitution with electron-withdrawing groups (see above). Additionally, the order of reactivity found with aromatics was similar to that of electrophilic aromatic substitution. These observations implicated an electrophihc metalation of the arene as the key step. Hence, Fujiwara et al. [4b] believed that a solvated arylpalladium species is formed from a homogeneous solution of an arene and a palladium(ll) salt in a polar solvent via an electrophilic aromatic substitution reaction (Figure 9.2). The alkene then coordinates to the unstable arylpalladium species, followed by an insertion into the aryl-palladium bond. The arylethyl-palladium intermediate then rapidly undergoes )8-hydride elimination to form the alkenylated arene and a palladium hydride species, which then presumably decomposes into an acid and free palladium metal. Later on, the formation of the arylpalladium species proposed in this mechanism was confirmed by the isolation of diphenyltripalladium(ll) complexes obtained by the C-H activation reaction of benzene with palladium acetate dialkylsulfide systems [19]. 

This cyclization has been used in an efficient synthesis of chokol A 11.219, an anti-fungal compound (Scheme 11.74). The acetal 11.216, in which the tartrate moiety functions as an economical chiral auxiliary, cyclized using palladium acetate to give an 8.5 1 mixture of stereoisomers. Aeidic hydrolysis removed the acetal to give a ketone 11.217, which could be taken through to the natural product 11.219 by selective reduction of the electron-poor alkene, stereoselective addition of a methyl group, allylic oxidation and desilylation. 

The reaction mechanism is postulated to go through a Heck-like four-membered ring transition state as determined through experimental and theoretical analysis. The addition of water to the reaction did not lower the yield, and the use of palladium acetate in place of the copper catalyst gave no product Importantly, the presence of the alkene group provides a useful handle for further functionalized trifluoromethyl-ated compounds. 

Terminal alkynes react to form l-en-3-ynes in a process catalyzed by palladium acetate and tris(2,6-dimethoxyphenyl)phosphine. A number of functional groups such as internal alkenes, esters, and alcohols are tolerated, and good yields of homo- (eq 25) as well as hetero-coupled enynes (eq 26) are obtained. ... 

Sulfenamides can be synthesized by a metal-assisted reaction with disulfides and amines (eq 13). Conjugated dienols can be synthesized in a highly chemo-, regio-, and stereoselective manner from vinylic halides and allylic alcohols (eq 14). Silver acetate (or silver carbonate) and a catalytic amount of palladium(II) acetate in DMF are the preferred conditions. The reaction is very attractive because it requires only one functionalized alkene reactant and the addition of the vinylic group only occurs at the terminal carbon of the aUyUc alcohol. 4... 

Palladium-catalyzed Direct Arylation of Indoles and Thiophenes. Five-membered ring heterocycles possessing only one heteroatom and no Af-oxide function can also be arylated using palladium(II) complexes, a phosphine ligand, and an inorganic base. In one example, a tandem palladium-catalyzed Heck coupling reaction and direct intramolecular C2 arylation reaction on a )V-(2-chlorobenzyl)-5-bromoindole was reported (eq 30). The procedure, which is catalyzed by palladium acetate, uses tn-tert-butylphosphonium tetrafluoroborate as a ligand and ferf-butyl acrylate as the alkene for the Heck reaction (eq 30). ... 

In 2006, Gaunt and co-workers described a regioselective alkenylation of pyrroles under mild aerobic oxidative conditions. A catalytic amount of palladium acetate was able to selectively functionalize tert-butojycarbamate (BOC)-protected pyrrole at the C2-position while a C3-selectivity was achieved with triisopropylsilyl (TIPS)-protected pyrroles (Scheme 9.4). The aerobic conditions worked well for reactive alkenes such as acrylate derivatives, nevertheless the use of peroxide ( BuOOBz) appeared necessary in order to achieve good yields with more challenging substrates [i.e., methyl vinyl sul-fone). Interestingly, intermolecular alkenylation of pyrrole can also be effected with complete selectivity and good yields. 

Allylic halogenides containing an additional internal functional group in a suitable position, e. g., an alkene moiety or a hydroxy group, produce the corresponding cyclopentenone derivatives or lactones, respectively, via palladium-catalyzed carbonylation [43]. Related cyclocarbonylations of cinnamyl halides or acetates to form polycyclic aromatics such as naphthol derivatives have been reported (eq. (15)). Moreover, the synthetic utility of the method was demonstrated by the synthesis of acetoxybenzofurans, acetoxyindoles, and acetoxycarbazoles [44]. 

C-H o-bond activation of hydrocarbons by transition metal complexes is of considerable importance in modern organometallic chemistry and catalytic chemistry by transition-metal complexes [1], because a functional group can be introduced into alkanes and aromatic compounds through C-H o-bond activation. For instance, Fujiwara and Moritani previously reported synthesis of styrene derivatives from benzene and alkene via C-H o-bond activation of benzene by palladium(ll) acetate [2]. Recently, Periana and his collaborators succeeded to activate the C-H o-bond of methane by the platinum(ll) complex in sulfuric acid to synthesize methanol [3], Both are good examples of the reaction including the C-H o-bond activation. 

Direct functionalization of the C2 position of thiophene can be accomplished through an oxidative-Heck reaction employing palladium(II) acetate in the presence of silver carbonate. The resulting products are obtained in good to high yields with the trans-alkene isomer being favored. Typically less than 10% branched coupling products were observed (not shown). ... 

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