Acylpalladium derivatives carbonylation

Fifth, although the relative inertness of carbonyl compounds excluding acyl halides was emphasized above, most everything in chemistry is relative, and organopalladium chemistry is no exception. Thus, in the absence of faster reaction paths, Pd and its complexes may react with aldehydes via C—H activation to give acylpalladium derivatives and subsequent decarbonylation (Sect. VI.5.1), while ketones may be reduced to alcohols and even to hydrocarbons, as discussed in Sect VII.2.3.1, although the presence of proximal 7T- or n-donors may be critical in such reactions. 

Unusual cyclocarbonylation of allylic acetates proceeds in the presence of acetic anhydride and an amine to afford acetates of phenol derivatives. The cinnamyl acetate derivative 408 undergoes carbonylation and Friedel-Crafts-type cyclization to form the a-naphthyl acetate 410 under severe condi-tions[263,264]. The reaction proceeds at 140-170 under 50-70 atm of CO in the presence of acetic anhydride and Et N. Addition of acetic anhydride is essential for the cyclization. The key step seems to be the Friedel-Crafts-type cyclization of an acylpalladium complex as shown by 409. When MeOH is added instead of acetic anhydride, /3,7-unsaturated esters such as 388 are... 

In addition to alcohols, some other nucleophiles such as amines and carbon nucleophiles can be used to trap the acylpalladium intermediates. The o-viny-lidene-/j-lactam 30 is prepared by the carbonylation of the 4-benzylamino-2-alkynyl methyl carbonate derivative 29[16]. The reaction proceeds using TMPP, a cyclic phosphite, as a ligand. When the amino group is protected as the p-toluenesulfonamide, the reaction proceeds in the presence of potassium carbonate, and the f>-alkynyl-/J-lactam 31 is obtained by the isomerization of the allenyl (vinylidene) group to the less strained alkyne. 

Dibromobistriphenylphosphinepalladium(II) is an effective carbonylation catalyst in the reaction of 2-bromothiophene with aniline145 (Scheme 80) acylpalladium species are presumably intermediates in this type of reaction, which can also be used to prepare pyridine derivatives. 2-Bromothiophene... 

The insertion of carbon monoxide into azolylpalladium complexes proceeds readily and in most cases leads to the formation of carboxylic acid derivatives or ketones. In a modified version of the carbonylation 3-bromothiophene was reacted with carbon monoxide in the presence of sodium formate. This reagents converts the intermediate acylpalladium formate complex, through the release of carbon dioxide into the acylpalladium hydride (c.f 7.47.), which in turn releases thiophene carboxaldehyde as the sole product (6.62.),92 If sodium formate was replaced... 

Carbonylation of the 3-phenylallyl acetate 258 under somewhat severe conditions in the presence of tertiary amine and acetic anhydride affords the naphthyl acetate derivative 260. This interesting cyclocarbonylation is explained by the Friedel-Crafts-type cyclization of the acylpalladium 259 as an intermediate [118,119]. Even 5-phenyl-2,4-pentadienyl acetate (261) is cyclocarbonylated to afford 2-phenylphenyl acetate (262) [120],... 

The catalytic cycle is common with the single carbonylation of aryl hafides in the steps of oxidative addition of aryl halide to a Pd(0) species and the subsequent CO insertion process. Since successive CO insertion into the acylpalladium bond is not thermodynamically favorable, incorporation of another CO molecule takes the route of coordination of the CO molecule to the acylpalladium(II) intermediate. Attack by a nucleophile such as secondary amine, alcohol, and water in the presence of an appropriate base, including the amine itself, gives bis acyl type intermediate which reductively efiminates a-keto acid derivatives [131]. 

Oxidative carbonylation of alkenes is a unique reaction of Pd(II). Three types of oxidative carbonylation to give -substituted acid derivatives 130, a, -unsaturated esters 132 and succinate derivatives 134 are known, which can be understood by the following mechanism. Palladation of alkenes with PdX2, followed by CO insertion, generates the acylpalladium intermediate 129 whose reductive elimination affords -substituted carboxylic acid derivatives 130 (path a). Reaction in alcohol in the presence of a base starts by the formation of the alkoxycarbonylpalladium 128. Carbopalladation of alkene with 128 generates 131. Then y3-H elimination of the intermediate 131 yields the a-unsaturated ester 132 (path b). Further CO insertion to 131 gives the acylpalladium intermediate 133 and its alcoholysis yields the succinate derivative 134 (path c). Formation of the jS-alkoxy ester 130 (X-OR) is regarded as nucleophilic substitution of Pd-X in 131 with alcohols. 

To complete organic synthesis via Pd-catalyzed carbonylation, the acylpalladium and related derivatives, represented by RCOPdL and YCOPdL in Scheme 2, must undergo further transformations including C—Pd bond cleavage as a mandatory step. In addition, some organopalladium interconversion processes, such as acylpalladation, may take place prior to C—Pd bond cleavage. 

Before discussing the double carbonylation processes it may be helpful to understand the mechanism of the single carbonylation of aryl halides into carboxylic acid derivatives (Heck processes). The first step in the catalytic process is oxidative addition of an aryl halide to Pd(0) species formed from a catalyst precursor to yield an arylpal-ladium halide intermediate (A) in Scheme 1. Insertion of carbon monoxide into the aryl-palladium bond in A gives an acylpalladium halide complex (B). Attack of a nucleophile such as alcohol, amine, and water assisted by a base on the acylpalladium complex yields carboxylic ester, amide, and carboxylic acid, although details of the mechanism have not been unequivocally established. The palladium(O) species regenerated in the process further undergoes oxidative addition to carry out the catalytic cycle (Scheme 1). 

Besides carboxylic acids, carbonylation can give their derivatives or ketones if other nucleophiles were used to cleave the acylpalladium complex. Thus, esters and amides are formed with alcohols and amines, while ketones can be obtained in the presence of such carbanion synthons as organometallic compounds. Certainly, these processes leave a small margin for the intervention of water in any form, as in the presence of water the competition between the different nucleophiles would lower the selectivity, as, for example, in the... 

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