Oxidative addition acylpalladium complexes

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],... 

Scheme 30 shows the proposed reaction mechanism, which involves the formation of an acylpalladium species as the key intermediate, in tautomeric equilibrium with a cyclic 7r-allyl complex (in this and in the following Schemes, unreactive ligands are omitted for clarity). The main reason for the high activity of the Pdl42 -based catalyst in this process lies in the very efficient mechanism of reoxidation of Pd(0), which involves oxidation of HI by 02 to I2, followed by oxidative addition of the latter to Pd(0). It is worth nothing that under these conditions Pd(0) reoxidation occurs readily without need for Cu(II) or organic oxidants. 

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 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 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. 

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. 

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. 

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. 

Palladium(O) complexes are well known to suffer oxidative addition with acid chlorides. The resulting acylpalladium(Il) complex (106) is, in contrast to the acyliron(II) complex discussed above, an electrophilic species which is subject to nucleophilic attack by various organometallics. Stille has studied the ad tion of organotins because they undergo rapid nucleophilic addition to the acylpalladium(II) complex, but do not add to either the acid chloride or react with the ketone (Scheme 42). There are also several other organometallics useful in this sense vide irfra). 

Aromatic or a,p-unsaturated acid chlorides were found to undergo nqnd oxidative addition with these palladium complexes, and the subsequent acylation of an organotin proceeded smoothly in HMPA (1-60 min) or in acetone. Formation of simple aliphatic acylpalladium(II) complexes proceeds at markedly reduced rates and the acylation of organotins is slower but still use l (10-24 h). 

In the case of a palladium-catalyzed carbonylative activation of C-X bonds, a general reaction mechanism is shown in Scheme 1.8. Normally this type of reaction starts from Pd(0) and is followed by the oxidative additional step and the coordination and insertion of CO to form the acylpalladium complex as the key... 

The Carbonylative Heck Reaction is not the same as those that were traditionally called Heck carbonylations . Heck carbonylations normally include alkoxycarbonylation, aminocarbonylation and hydroxycarbonylation, while a carbonylative Heck reaction is more related to a Heck reaction. In the late 1960s, Richard Heck developed several coupling reactions of arylmercury compounds in the presence of either stoichiometric or catalytic amounts of palladium salts [1-7]. Based on this work in 1972, he described a protocol for the coupling of iodo-benzene with styrene, which today is known as the Heck reaction [8]. In contrast to this, the catalytic insertion of olefins into acylpalladium complexes is called a Carbonylative Heck reaction . Here the acylpalladium complexes can either by CO insertion or by the oxidative addition of benzoyl precursors [9, 10]. 

The insertion of CO into Pd-carbon bonds has also been employed in several tandem/cascade reactions that afford five-membered nitrogen heterocycles [97]. A representative example of this approach to the construction of heterocydes involves synthesis of isoindolinones via the Pd-catalyzed coupling of 2-bromobenzaldehyde with two equivalents of a primary amine under an atmosphere of CO [97bj. As shown below (Eq. (1.57)), this method was used for the preparation of 144 in 64% yield. The mechanism of this reaction is likely via initial, reversible condensation of 2-bromobenzaldehyde with 2 equiv of the amine to form an aminal 145. Oxidative addition of the aryl bromide to Pd° followed by CO insertion provides the acylpalladium spedes 146, which is then captured by the pendant aminal to afford the observed product. An alternative mechanism involving intramolecular imine insertion into the Pd—C bond of a related acylpalladium species, followed by formation of a paUadium-amido complex and C—N bond-forming reductive elimination has also been proposed [97b],... 

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). 

On the left hand of Scheme 2 is shown the catalytic cycle to produce a-keto amide (Cycle 1), whereas the right-hand catalytic cycle shows the route to amide (Cycle II). The process common to both processes is oxidative addition of aryl halide to give arylpalladium halide. Further CO coordination to the arylpalladium intermediate gives a CO-coordinated complex. If CO insertion into the aryl-palladium bond takes place, an acylpalladium complex is produced to drive the double carbonylation cycle. Further coordination of CO followed by attack of amine on the carbonyl ligand produces the aroyl(carbamoyl)palladium species as the bis-acyl-type intermediate. Reductive elimination of the a-keto amide by combination of the benzoyl ligand with the carbamoyl ligand regenerates the Pd(0) species that carries the catalytic cycle. 

The mechanism (Scheme 48) ° is expected to proceed through the acylpalladium species much as in the Rosenmund reduction. Indeed, the acyl complex 56 from oxidative addition of vinyl chloride with Pd(CO)(Ph3P)3 was isolated (Scheme 49). " The reaction of acid chlorides with the same catalyst provides aldehydes. However, aliphatic acid chlorides do not reduce effectively. The phosphine ligands present in the Heck acylpalladium intermediate are thought to be the cause, allowing decarbonylation and elimination to occur. Interestingly, the formylation will not occur with the Rosenmund catalyst. 

On the other hand, there are only a few reports of catalytic nonpolymeric reactions that involve the intermolecular reaction of an alkene with an acylpalladium complex. The first example was reported in 1968 while studying the decarbonylation of acyl chlorides in the presence of various palladium salts. For example, phenylpropionyl chloride gave styrene (53%) along with l,5-diphenyl-l-penten-3-one (10%) in the presence of catalytic amounts of PdCl2. The latter compound was probably formed via reaction of the acylpalladium complex, generated via oxidative addition in the acyl-chloride bond, with styrene itself formed via decarbonylation of the acylpalladium complex followed by /S-elimina-tion (Scheme 2). 

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... 

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