Palladium catalyzed oxidations with added oxidant

In 2003, Stoltz at CalTech described a palladium-catalyzed oxidative Wacker cyclization of o-allylphenols such as 55 in nonpolar organic solvents with molecular oxygen to afford dihydrobenzofurans such as 56.44 Interestingly, when (-)-sparteine was used in place of pyridine, dihydrobenzofuran 56 was produced asymmetrically. The ee reached 90% when Ca(OH)2 was added as an additive. Stoltz considered it a stepping stone to asymmetric aerobic cyclizations. In 2004, Mufiiz carried out aerobic, intramolecular Wacker-type cyclization reactions similar to 55—>56 using palladium-carbene catalysts.45 Hiyashi et al. investigated the stereochemistry at the oxypalladation step in the Wacker-type oxidative cyclization of an o-allylphenol. Like o-allylphenol, o-allylbenzoic acid 57 underwent the Wacker-type oxidative cyclization to afford lactone 58.47... 

In 1981, a stereoselective palladium-catalyzed 1,4-diacetoxylation of 1,3-dienes with p-benzoquinone (BQ) as the oxidant was reported33. It was found that chloride ions can be used as a stereochemical switch. Thus, in the absence of chloride ions trans diacetoxylation takes place, whereas in the presence of a catalytic amount of chloride ion (as added LiCl) a cis diacetoxylation takes place (Scheme 4). In both cases the reaction is highly 1,4-regioselective. The explanation for... 

In the palladium-catalyzed coupling reactions of arenes with alkenes, the cr-arylpalladium complexes react with CO to give aromatic acids in AcOH, as shown in Scheme u 97>97a 97c This carboxylation reaction of arenes with CO proceeds catalytically with respect to Pd at room temperature under atmospheric pressure of CO, when K2S2O8 is added as an oxidant and TFA is employed as a solvent. 

The palladium-catalyzed [3 + 2] cycloaddition of vinylic oxirane 20a [42] and aziridine 20b [39] with the activated olefin 4a for the formation of five membered cyclic ether 21a and pyrrolidine derivative 21b has also been reported in our laboratories. The mechanistic issue is very much similar to that discussed in Scheme 9. Pd(0) catalyst added oxidatively to 20 to produce the 7r-allylpalladium complex 22. The Michael addition of a hetero nucleophile in 22 to the activated olefin 4a gives 23 which undergoes intramolecular nucleophilic attack on the inner 7r-allylic carbon atom to give the cy-clized products 21 and Pd(0) species is generated (Scheme 10). Similarly, the palladium-catalyzed [3 + 2] cycloaddition of vinylic oxirane 20a with the N-losylimincs 24 is also known (Scheme 11) [43]. Intermolecular cycloaddition of vinyl epoxides and aziridines with the heterocumulenes such as isocyanates, carbodiimides and isothiocyanates is also known [44,45]. Alper et al. reported the regio- and enatioselective formation of the thiaolidine, oxathiolane, and dithiolane derivatives by the palladium-catalyzed cyclization reaction of 2-vinylthiirane with heterocumulenes [46]. 

To ensure complete oxidation of aldehydes to carboxylic acids, Aulin-Erdtman and Tomita (1963) treated the ozonation products of lignin model compounds with hydrogen peroxide. Soteland (1971) applied hydrogen peroxide to a groundwood ozonation product and found that it degraded the components to unidentified volatile substances. To simplify the product mixture, reduction is sometimes used to stop further oxidation. Tanahashi et al. (1975) applied palladium-catalyzed hydrogenation, while Kondo et al. (1987) added dimethyl sulfide. 

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... 

Palladium-catalyzed intermolecular oxidations of dienes with carboxylic acids and alcohols as donors give 1,4-addition products. This chemistry has been studied extensively by Backvall. Early studies involved 1,4-additions of two acetoxy or alkoxy groups across a diene,More recently, intermolecular additions of two different nucleophiles have been developed. The ability to control the stereochemistry of the additions across cyclic dienes makes this procedure particularly valuable. As shown in Scheme 16.25, conditions for either cis or trans additions have been developed. Reactions conducted in the absence of added chloride form products from trans 1,4-addition, while reactions conducted in the presence of added chloride form products from cis 1,4-addition. 

Intramolecular oxidative aminations of olefins have alsobeen studied, and many of these intramolecular processes were observed prior to the analogous intermolecular variants. The oxidative aminations of alkenes with arylamines and arylamine derivatives catalyzed by palladium complexes were shown by Hegedus to form indoles (Equation 16.120). These reactions were conducted with orf/io-allylaniline and ort/zo-allylaniline derivatives as substrate, Pd(NCMe)jCl2 as catalyst, and benzoquinone as oxidant. Intramolecular reactions of N-tosylated aliphatic amines were reported by Larock. ° For example, the tosylamide in Equation 16.121 imdergoes cyclization in high yield in the presence of dioxygen with Pd(OAc)j as catalyst in DMSO. A related reaction (Equation 16.122) was reported recently in toluene solvent with added pyridine. ... 

In 2009, Ackermann and coworkers reported a palladium catalyst derived from an air-stable secondary phosphane oxide (l-Ad)2P(0)H that enables efficient intramolecular a-arylations of amides with aryl chlorides. Good yields were obtained for several oxindole derivatives. The interesting fact was that they discovered an unprecedented intramolecular palladium-catalyzed a-arylation route to azaoxindoles (Scheme 8.50) [90]. 

The palladium-catalyzed cross-coupling of secondary phosphine oxides with aryl halides in water has also been reported using discrete paUadacycles as catalysts (Scheme 4.204) [340], No added ligands were needed for the metal center in this reaction although several additives were added to the reaction mixture including potassium fluoride and added TBAB. Aryl bromides, chlorides, and iodides were successfully converted into phosphine oxides using this approach, but aryl chlorides were sluggish. 

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