Esters acylpalladium complexes

The Pd-catalyzed hydrogenoiysis of acyl chlorides with hydrogen to give aldehydes is called the Rosenmund reduction. Rosenmund reduction catalyzed by supported Pd is explained by the formation of an acylpalladium complex and its hydrogenolysis[744]. Aldehydes can be obtained using other hydrides. For example, the Pd-catalyzed reaction of acyl halides with tin hydride gives aldehydes[745]. This is the tin Form of Rosenmund reduction. Aldehydes are i ormed by the reaction of the thio esters 873 with hydrosilanes[746,747]. 

The acylpalladium complex formed from acyl halides undergoes intramolecular alkene insertion. 2,5-Hexadienoyl chloride (894) is converted into phenol in its attempted Rosenmund reduction[759]. The reaction is explained by the oxidative addition, intramolecular alkene insertion to generate 895, and / -elimination. Chloroformate will be a useful compound for the preparation of a, /3-unsaturated esters if its oxidative addition and alkene insertion are possible. An intramolecular version is known, namely homoallylic chloroformates are converted into a-methylene-7-butyrolactones in moderate yields[760]. As another example, the homoallylic chloroformamide 896 is converted into the q-methylene- -butyrolactams 897 and 898[761]. An intermolecular version of alkene insertion into acyl chlorides is known only with bridgehead acid chlorides. Adamantanecarbonyl chloride (899) reacts with acrylonitrile to give the unsaturated ketone 900[762],... 

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... 

Carboxylic acid chlorides and chloroformate esters add to tetrakis(triphenylphosphine)palladium(0) to form acylpalladium derivatives (equation 42).102 On heating, the acylpalladium complexes can lose carbon monoxide (reversibly). Attempts to employ acid halides in vinylic acylations, therefore, often result in obtaining decarbonylated products (see below). However, there are some exceptions. Acylation may occur when the alkenes are highly reactive and/or in cases where the acylpalladium complexes are resistant to decarbonylation and in situations where intramolecular reactions can form five-membered rings. 

Pd(0) complexes, Co2(CO)8 or Ni(CO)4 [1,2]. Most conveniently, Pd(0)-catalysed carbonylations of alkenes can be carried out under mild conditions in a laboratory with or without using a high pressure apparatus. Carbonylation in the presence of a small amount of HC1 is explained by the following mechanism. The first step is oxidative addition of HX to Pd(0) to generate 4. Then insertion of alkenes to H-PdX 4 gives the alkylpalladium bond 5, and the acylpalladium complex 6 is formed by subsequent CO insertion. The last step is nucleophilic attack of alcohol or water to the acylpalladium complex 6 to give the ester 7 or acid, with regeneration of H-PdX. 

For Pd-catalyzed cross-coupling reactions the organopalladium complex is generated from an organic electrophile RX and a Pd(0) complex in the presence of a carbon nucleophile. Not only organic halides but also sulfonium salts [38], iodonium salts [39], diazonium salts [40], or thiol esters (to yield acylpalladium complexes) [41] can be used as electrophiles. With allylic electrophiles (allyl halides, esters, or carbonates, or strained allylic ethers and related compounds) Pd-i73-jt-allyl complexes are formed these react as soft, electrophilic allylating reagents. 

The acylpalladium complex as the intermediate of the carbonylation can be trapped by active methylene compounds to give allenyl ketones without forming methyl esters. 

This reaction is initiated by nucleophilic attack of an oxygen atom of an amide group on an alkyne coordinated by palladium(II), forming the vinylpal-ladium intermediate 313. The insertion of CO into the C—Pd bond of 313, followed by methanolysis of the resulting acylpalladium complex, affords the esters 312 and Pd(0) catalyst. The Pd(0) is oxidized to Pd(II) by molecular oxygen, and thus the catalytic cycle operates well. 

When 2-propargyl-l,3-dicarbonyl compounds are treated with aryl iodides under a balloon of carbon monoxide 2,3,5-trisubstituted-furans containing a 5-acylmethyl group (Scheme 7a) or its enol ester (Scheme 7b) can be obtained. Formation of the acyhnethyl derivative or its enol ester depends on the aryl iodide to alkyne ratio. Excess alkyne affords the acyhnethyl derivative as the main product whereas employment of an excess of the aryl iodide favors the formation of the enol ester. The enol ester product is very likely formed from the acyhnethyl product via trapping of the corresponding enolate with an acylpalladium complex. 

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). 

It is known that the oxidative addition of aryl or vinyl halides to a low-valent palladium complex produces an aryl- or vinylpalladium complex, which reacts with carbon monoxide to afford an acylpalladium complex. If alcohol and amine are added to this reaction system, we can obtain ester or amide. " Intramolecular reactions of aryl or... 

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... 

Trapping of the [ C]acylpalladium complexes intermediately generated by addition of MeOH, BusSnH or PhSnMe3 allowed for the formation of aryl[ C]carboxylic acid methyl esters, [carbonyl- " C]aldehydes and [carbonyl- C]benzophenone derivatives as depicted in Figure 5.14. 

Keto esters are obtained by the carbonylation of alkadienes via insertion of the aikene into an acylpalladium intermediate. The five-membered ring keto ester 22 is formed from l,5-hexadiene[24]. Carbonylation of 1,5-COD in alcohols affords the mono- and diesters 23 and 24[25], On the other hand, bicy-clo[3.3.1]-2-nonen-9-one (25) is formed in 40% yield in THF[26], 1,5-Diphenyl-3-oxopentane (26) and 1,5-diphenylpent-l-en-3-one (27) are obtained by the carbonylation of styrene. A cationic Pd-diphosphine complex is used as the catalyst[27]. 

The styrene/CO polymers formed with palladium complexes of diimine ligands indeed contain ester and alkene end groups [65,66,67], Slightly more ester end groups than alkene groups are formed, showing that in addition to P-hydride elimination some termination via methanolysis of acylpalladium chain ends occurs. 

Reaction of a silyloxycyclopropane (8) with an acid chloride in the presence of a palladium catalyst also proceeds cleanly to give 1,4-keto esters in high yield (Scheme 25). A mechanism involving the interaction of (8) and an acylpalladium chloride complex in the rate-limiting step has been proposed on the basis of kinetic studies. 

The methodology is based on the findings that the acyl-0 bond in carboxylic esters and anhydrides can be readily cleaved on interaction with Pd(0) complexes to give acylpalladium carboxylate or aryloxide type complexes [33] (Scheme 1.11). 

There are two possibilities for formation of the products from the acylpalladium species. One is the direct attack of the nucleophile on the acyl ligand and the other involves coordination of the NuH to the palladium center. The palladium-bound NuH ligand, such as alcohol, is deprotonated by a base to give an acylpalladium alkoxide, which releases the ester as the reductive elimination product. Recent studies on model complexes provided some evidence supporting a route via reductive elimination [59]. 

Aryl and alkenyl halides and triflates undergo Pd-catalyzed carbonylation, offering useful synthetic methods for carboxylic acids, esters, amides, aldehydes and ketones. Facile CO insertion into aryl- or alkenylpalladium complexes generates acylpalladium intermediates 1. The intermediates are attacked by several... 

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