Acylpalladium complexes

CO is a representative species for Q, -insertion its insertion into C—Pd bonds affords acylpalladium complexes such as 15. Mechanistically, the CO insertion is 1.2-alkyl migration to coordinated CO. This is an important step in carbonyiation. SO , isonitriies, and carbenes are other species which undergo a.a-insertion. 

Furthermore, treatment of the aminopalladation product with bromine affords aziridines[176]. The aziridine 160 was obtained stereoselectively from methylamine and 1-decene in 43% yield. The aminopalladation of PdCl2 complexes of ethylene, propylene, and 1-butene with diethylamine affords the unstable ir-alkylpalladium complex 161, which is converted into the stable chelated acylpalladium complex 162 by treatment with CO[177],... 

Acyl halides react with organometallic reagents without catalysts, but sometimes the Pd-catalyzed reactions give higher yields and selectivity than the Lincatalyzed reactions. Acyl halides react with Pd(0) to form the acylpalladium complexes 846, which undergo facile transmetallation. 

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... 

Thus, (i) electron transfer from Pd(0) to cyclohexenone, for example, (ii) Pd—allyl complex formation, (iii) transmetalation forming an acylpalladium complex, and (iv) reductive elimination of Pd(0), would give either a 1,2- or a 1,4-acylation product [26] (Scheme 5.21). The role of the triphenylphosphane ligand in the regioselective formation of a 1,2-acylation product may be explained by the preferred formation of a stereochemically less crowded intermediate complex A (Scheme 5.22) and subsequent reductive elimination of Pd(0). 

The high E-stereoselectivity can be explained by the face-selective coordination of an allene to an acylpalladium complex (Scheme 16.57). 

The second mechanism involves the oxidative addition of methanol to the divalent acylpalladium complex 14 (19, Figure 12.14). This reaction has the only advantage that the new hydride initiator is formed in one step, but apart from this it is an unlikely reaction. Oxidative addition of alcohols is only known for electron-rich zerovalent palladium complexes [46],... 

The resting state of the propanoate catalysts may well be an acyl complex [60,61], while the attack of alcohol at the acylpalladium complex is considered to be the rate-determining step. It is probably more precise to say that fast preequilibria exist between the acyl complex and other complexes en route to it and that the highest barrier is formed by the reaction of alcohol and acylpalladium complex. The precise course of the reaction is still not known presumably deprotonation of the coordinating alcohol and the migratory elimination are concerted processes, accelerated by the steric bulk of the bidentate ligand. Toth and Elsevier showed that the reaction of an acetylpalladium complex and sodium methoxide is very fast and occurs already at low temperature to give methyl acetate and a palladium(I) hydride dimer [46]. 

The catalytic cycle involves the oxidative addition of RX to Pd(0), coordination and migratory insertion of CO leading to cr-acylpalladium complexes, transmetallation of the latter by organometallic compounds, followed by reductive elimination (Scheme 1). 

The Pd-catalyzed coupling of an acyl chlor.de with benzyl chlor.de to form the benzyl ketone 854 proceeds in the presence of an excess of Zn In this reaction, benzyl chloride reacts with Zn to form benzylzmc, wh.ch undergoes transmetallation with acylpalladium complex[7291. The react.on has been applied to the synthesis of riccardin B )[ ] ... 

The catalytic process (Figure 2-4) usually begins with the oxidative addition of an aryl halide or sulfonate onto the active form of the catalyst. In the presence of carbon monoxide the formed palladium-carbon bond breaks up with the concomitant insertion of a CO unit to give an acylpalladium complex. Such complexes might also be formed by the oxidative addition of acyl halides onto palladium. 

Bromobenzyl alcohol and its derivatives were converted to phthalides by the palladium catalysed insertion of carbon monoxide and intramolecular quenching of the formed acylpalladium complex. 2-Hydroxymethyl-1-bromonaphthaline, for example, gave the tricyclic product in excellent yield (3.34.). An interesting feature of the process is the use of molybdenum hexacarbonyl as carbon monoxide source. The reaction was also extended to isoindolones, phthalimides and dihydro-benzopyranones 42... 

The coupling of the same boronic acid was also achieved with 4-chlorobenzoyl chloride (6.5.), Running the reaction under anhydrous conditions the desired 2-(4 chlorobenzoyl)thiophene was obtained in good yield.7 The opening step in this process is the selective oxidative addition of the palladium into the carbonyl-chlorine bond giving an acylpalladium complex, which on subsequent transmetalation and reductive elimination gives the observed product. 

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. 

Acylpalladium complexes are readily prepared through oxidative addition of Pd° complexes to acid chlorides. PdL4 compounds, where L is a tertiary phosphine, react with acid chlorides at room temperature to give trani-L2Pd(COR)Cl complexes. Since carbon monoxide does not insert into palladium acyl bonds, Pd(C0C02R) complexes are made from oxidative addition of oxalyl chloride monoesters. 

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

The following mechanism was proposed for the carbonylation of olefin-palladium chloride complex (10). The first step is coordination of carbon monoxide to the complex. Insertion of the coordinated olefin into the palladium-chlorine bond then forms a -chloroalkylpalladium complex (IV). This complex undergoes carbon monoxide insertion to form an acylpalladium complex (V), as has been assumed for many metal carbonyl-catalyzed carbonylation reactions. The final step is formation of a )8-chloroacyl chloride and zero-valent palladium by combination of the acyl group with the coordinated chlorine. 

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