Olefin complexes oxidative functionalization

A common pathway in palladium-catalyzed oxidation reactions is that the 7r-olefin complex formed reacts with a nucleophile, either external or coordinated, and the new organometallic intermediate may then undergo a number of different reactions (Scheme l) (i) an intramolecular hydride shift leads to ketone formation (ii) a )6-elimination results in the formation of a vinyl functionalized olefin (iii) an oxidative cleavage of the palladium-carbon bond produces a 1,2-functionalized olefin and (iv) an insertion reaction, exemplified by insertion of an olefin, leads to formation of a new palladium-carbon bond, which may be cleaved according to one of the previous processes ()6-elimination or oxidative cleavage). In all cases palladium has removed 2 electrons from the organic molecule, which becomes oxidized. These electrons, which end up on Pd(0), are in turn transferred to the oxidant and Pd(II) is regenerated, in this way a palladium(II)-catalyzed oxidation is realized. 

The chiral anisole derivative 119 has been used in the synthesis of several asymmetric functionalized cyclohexenes (Fig. 27) [55]. In a reaction sequence similar to that employed with racemic anisole complexes (see above), 119 adds an electrophile and a nucleophile across C4 and C3, respectively, to form the cy-clohexadiene complex 120. The vinyl ether group of 120 can then be reduced by the tandem addition of a proton and hydride to C2 and Cl respectively, affording the olefin complex 121. Direct oxidation of 121 liberates the cyclohexenes of the type 122 having the initial asymmetric auxiliary still intact. Alternatively, the auxiliary may be cleaved under acidic conditions to afford q -allyl complexes, which can undergo reactions with nucleophiles regioselectively at Cl. Oxidative decomplexation liberates the cyclohexenes 123-127. 

Recent wide-spreading of the concept of atom economy in chemical synthesis, and economical and ecological requirements have fueled significant interest to selective oxidative functionalization of substrates with C-H and C=C bonds. Catalytic functionalization of CH bonds or olefin C=C bonds with a platinum(ll) or a palladium(ll) complex leading to products with new C-O bonds may represent a mechanistically more complex case as compared to the reaction sequence shown in Fig. 1. A C-H or C=C bond activation step by a transition metal complex leading... 

Abstract The oxidative functionalization of olefins is an important reaction for organic synthesis as well as for the industrial production of bulk chemicals. Various processes have been explored, among them also metal-catalyzed methods using strong oxidants like osmium tetroxide. Especially, the asymmetric dihydroxylation of olefins by osmium(Vlll) complexes has proven to be a valuable reaction for the synthetic chemist. A large number of experimental studies had been conducted, but the mechanisms of the various osmium-catalyzed reactions remained a controversial issue. This changed when density functional theory calculations became available and computational studies helped to unravel the open mechanistic questions. This mini review will focus on recent mechanistic studies on osmium-mediated oxidation reactions of alkenes. 

The oxidative functionalization of olefins mediated by transition metal oxides leads to a variety of products including epoxides, 1,2-diols, 1,2-aminoalcohols, and 1,2-diamines [1]. Also the formation of tetrahydrofurans (THF) from 1,5-dienes has been observed, and enantioselective versions of the different reactions have been developed. Although a lot of experimental data has been available, the reaction mechanisms have been a subject of controversial discussion. Especially, osmium (VIII) complexes play an important role there, as the proposal of a stepwise mechanism [2] for the dihydroxylation (DH) of olefins by osmium tetroxide (OSO4) had started an intense discussion about the mechanism [2—11],... 

The oxidative functionalization of olefins through ir-olefin complexes of palladium also has a long history, including the industrial production of acetaldehyde and vinyl acetate. Related reactions, including the conversion of olefins to vinyl ethers and enamines, have been studied in more recent times for fine chemical synthesis. These oxidative C-0 and C-N bond formations have been conducted with a variety of oxidants, including Oj, and have been studied as both intermolecular and intramolecular processes. 

Wacker-type oxidations have not only been used to prepare commodity chemicals, but have been used in complex molecule synthesis. Examples of the use of three classes of the oxidative functionalizations of olefins in natural products synthesis are shown in Schemes 16.27-16.30. 

Another interesting feature of Pd complexes is their ability to use oxygen as terminal oxidant for oxidative functionalizations. Several examples of oxidative C yi—H olefinations (oxidative Heck reactions) proceed with O2 as a reagent. Furthermore, aerobic dehydrogenative aryl couplings with high site selectivity have been developed. ... 

The reactivity of a remarkable electronically unsaturated tantalum methyli-dene complex, [p-MeCgH4C(NSiMe3)2]2Ta( = CH2)CH3, has been investigated. Electrophilic addition and olefination reactions of the Ta = CH2 functionality were reported. The alkylidene complex participates in group-transfer reactions not observed in sterically similar but electronically saturated analogs. Reactions with substrates containing unsaturated C-X (X = C, N, O) bonds yield [Ta] = X compounds and vinylated organic products. Scheme 117 shows the reaction with pyridine N-oxide, which leads to formation of a tantalum 0x0 complex. ... 

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