Palladium complexes oxidative carbonylation

Gabriele et al. reported that the 2-oxazolidinones 462 were synthesized by the palladium-catalyzed oxidative carbonylation of the 2-amino- 1-alkanols 461 (Scheme 144).206 The aminocarbonyl palladium complex 463 is formed as an intermediate, and subsequent ring closure gives 462. 

Following this pnblication, the anthors tested a series of Pd-NHC complexes (33-36) for the oxidative carbonylation of amino compounds (Scheme 9.8) [44,45]. These complexes catalysed the oxidative carbonylation of amino compounds selectively to the nreas with good conversion and very high TOFs. Unlike the Cu-NHC catalyst 38-X, the palladium complexes catalysed the oxidative carbonylation of a variety of aromatic amines. For example, 35 converted d-Me-C H -NH, d-Cl-C H -NH, 2,4-Me3-C H3-NH3, 2,6-Me3-C H3-NH3, and 4-Ac-C H3-NH3 to the corresponding nreas with very high TOFs (>6000) in 1 h at 150°C, in 99%, 87%, 85%, 72%, and 60% isolated yields, respectively (Pco,o2 = 3.2/0.8 MPa). 

Attempts to employ allenes in palladium-catalyzed oxidations have so far given dimeric products via jr al lyI complexes of type 7i62.63. The fact that only very little 1,2-addition product is formed via nucleophilic attack on jral ly I complex 69 indicates that the kinetic chloropalladation intermediate is 70. Although formation of 70 is reversible, it is trapped by the excess of allene present in the catalytic reaction to give dimeric products. The only reported example of a selective intermolecular 1,2-addition to allenes is the carbonylation given in equation 31, which is a stoichiometric oxidation64. 

Under appropriate conditions, alcohols and amines can undergo an oxidative double carbonylation process, with formation of oxalate esters (Eq. 34), oxamate esters (Eq. 35) or oxamides (Eq. 36). These reactions are usually catalyzed by Pd(II) species and take place trough the intermediate formation of bis(alkoxycarbonyl)palladium, (alkoxycarbonyl)(carbamoyl)palladium or bis(carbamoyl)palladium complexes, as shown in Scheme 29 (NuH, Nu H = alcohol or amine) [227,231,267,293-300]. 

On the other hand, when the oxidative carbonylation of a ,a -disubstituted propynylamines was carried out in the presence of an excess of CO2, the intermediate carbamate species could undergo cyclization with incorporation of CO2 into the five-membered cycle, either by direct nucleophilic attack of the carbamate oxygen to the triple bond coordinated to Pd(II) (Scheme 33, path a) or through the intermediate formation of a palladium carbamate complex followed by triple bond insertion (Scheme 33, path b). Carbon monoxide insertion into the Pd - C bond of the resulting stereoisomeric vinylpalladium intermediates then led to the final oxazolidi-none derivatives. 

The mechanism of the Zn chloride-assisted, palladium-catalyzed reaction of allyl acetate (456) with carbonyl compounds (457) has been proposed [434]. The reaction involves electroreduction of a Pd(II) complex to a Pd(0) complex, oxidative addition of the allyl acetate to the Pd(0) complex, and Zn(II)/Pd(II) transmetallation leading to an allylzinc reagent, which would react with (457) to give homoallyl alcohols (458) and (459) (Scheme 157). Substituted -lactones are electrosynthesized by the Reformatsky reaction of ketones and ethyl a-bromobutyrate, using a sacrificial Zn anode in 35 92% yield [542]. The effect of cathode materials involving Zn, C, Pt, Ni, and so on, has been investigated for the electrochemical allylation of acetone [543]. 

Recently, the oxidative addition of C2-S bond to Pd has been described. Methyl levamisolium triflate reacts with [Pd(dba)2] to give the cationic palladium complex 35 bearing a chiral bidentate imidazolidin-2-ylidene ligand [120]. The oxidative addition of the levamisolium cation to triruthenium or triosmium carbonyl compounds proceeds also readily to yield the carbene complexes [121], The oxidative addition of imidazolium salts is not limited to or d transition metals but has also been observed in main group chemistry. The reaction of a 1,3-dimesitylimidazolium salt with an anionic gallium(I) heterocycle proceeds under formation of the gaUium(III) hydrido complex 36 (Fig. 12) [122]. 

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. 

Palladium (II)-Nucleophile Addition across Olefins. Adding palladium complexes to olefins, either in the presence of an external nucleophile or a ligand which is attached to palladium, produces a palladium-carbon sigma-bonded complex which is not usually isolated in the case of monoolefins. Instead it decomposes and in doing so oxidizes the olefin to an organic carbonyl compound or a vinyl compound, exchanges a substituent group on the olefin, isomerizes the double bond, arylates (alkylates) the olefin, or carboxylates the olefin (2, 3). 

In the case of experiments performed under the conditions of run 6, but in the presence of 1 ml of methanol, 1.6 equivalent of dimethyl carbonate was obtained according to GC analysis. No dimethyl carbonate was observed in the absence of hydrogen chloride. Therefore, in the early stage of the carbonylation of 3, Pd/C is partly oxidized to palladium chloride (eqn. 2). This compound reacts in turn with CO and MeOH to give, according to one of the routes described in Scheme 2, dimethyl carbonate and a zerovalent palladium complex (noted [Pd]). 

The oxidative carbonylation of amines has been performed using palladium complex catalysts. Rhodium and ruthenium complexes have also been sho vn to have catalytic activity in the preparation of carbamates and ureas [93, 94]. An example is sho vn in Eq. 11.47. The usual carbonylation of amines to give formamides vas discussed in Section 11.2.3. 

The acid-catalyzed hydrocarboxylation of alkenes (the Koch reaction) can be performed in a number of ways. In one method, the alkene is treated with carbon monoxide and water at 100-350°C and 500-1000-atm pressure with a mineral acid catalyst. However, the reaction can also be performed under milder conditions. If the alkene is first treated with CO and catalyst and then water added, the reaction can be accomplished at 0-50°C and 1-100 atm. If formic acid is used as the source of both the CO and the water, the reaction can be carried out at room temperature and atmospheric pressure.The formic acid procedure is called the Koch-Haaf reaction (the Koch-Haaf reaction can also be applied to alcohols, see 10-77). Nearly all alkenes can be hydrocarboxylated by one or more of these procedures. However, conjugated dienes are polymerized instead. Hydrocarboxylation can also be accomplished under mild conditions (160°C and 50 atm) by the use of nickel carbonyl as catalyst. Acid catalysts are used along with the nickel carbonyl, but basic catalysts can also be employed. Other metallic salts and complexes can be used, sometimes with variations in the reaction procedure, including palladium, platinum, and rhodium catalysts. The Ni(CO)4-catalyzed oxidative carbonylation with CO and water as a nucleophile is often called Reppe carbonylationP The toxic nature of nickel... 

Relatively limited work on profene synthesis via carbonylation of benzyl-X derivatives has been reported from university groups. One exception is the stero-selective carbonylation of racemic benzylic bromides. The asymmetric reaction toward enantiomerically pure profenes could a priori proceed either by a kinetic resolution or by true asymmetric induction via the intermediacy of a trigonal substrate. Results from Arzoumanian et al. [35] strongly suggest that the carbonylation of 1-methylbenzyl bromide with oxazaphospholene-palladium complexes is a kinetic resolution process with a discriminative slow oxidative addition step. Best enantiomeric excess is about 64 % ee at 9 % chemical yield. Another possible way to synthesize enantiomerically pure profenes is to start from optically pure benzyl derivatives. Baird et al. investigated the carboxylation of optically active benzyl carbonates with palladium catalysts. The enantiomeric excess was only modest [36]. Thus, the development of an efficient catalytic asymmetric carbonylation of C-X derivatives is still an existing challenge. 

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