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Due to the insertion of olefin into the Pd-O bond a s -organopalladium intermediate is formed. The presence of copper in this intermediate favors its decomposition not via a b -hydride shift , which would lead to acetone, but via a heterolysis of the Pd-C bond much less common for alkyl palladium compounds with b -hydrogens resulting in a highly valuable glycol acetate. Some oxygen-containing re-oxidants may also serve as oxygen sources for the product formation. If the reduced form of the re-oxidant can be re-oxidized back by dioxygen, then this re-oxidant acts as the agent of oxygen transfer from dioxygen to olefin.
Nitrite and nitrate anions are among such re-oxidants. Products in these systems are formed as a result of the interaction between re-oxidant and olefin both coordinated on palladium. Thus, studies of the mechanisms of stoichiometric reactions of olefin oxidation by palladium complexes with nitro and nitrato ligands are of a great interest for the elucidation of the mechanisms of catalytic reactions. The interaction of olefins with these complexes depends on the olefin nature, number of N-containing ligands on palladium and solvent.
The interaction with acyclic olefins can result in aldehydes, ketones, 1,1-disubstituted alkanes, glycol derivatives and vinyl ethers 24, For cyclic olefins and methylenecycloalkanes, the product nature is much more variable since their oxidation by palladium can be accompanied by skeletal rearrangements resulting in the formation of ring-expanded, ring contracted or ring-opened products 25,26,29, The reactions were performed directly in NMR tubes at low temperatures down to K in order to stabilize organopalladium intermediates and in special cells of IR spectrometers.
Simultaneously with the registration of 1 H and 13 C NMR spectra during the olefin transformation into reaction products, the changes in nitrogen-containing fragments were monitored by IR spectroscopy. Various organopalladium compounds with the dynamics characteristic for intermediates have been detected and monitored in situ , mostly for the first time, and their structure and reactivity have been studied.
Based on obtained data the detailed mechanisms of various processes were suggested. Even in the case of the most simple olefin, i. The study of the organometallic chemistry involved allowed us to generalize the mechanism of the formation of all these products The key reaction intermediates - s -organopalladium compounds formed via the insertion of coordinated ethylene into Pd-O or Pd-N bonds - have been observed in situ by both NMR and IV Scheme 4.
The nature of oxidation products is determined in the step of the decomposition of these s -akyl palladium complexes. Scheme 5 shows the reaction routes leading to 1,1-addition products and vinyl ethers. In particularly, for the first time we registered the organometallic precursor of acetaldehyde Pd-CH 2 -CHO , which is, in fact, the intermediate of the b -hydride shift later, we observed similar species for other olefins.
Various intermediates responsible for the formation of glycol monoacetate have also be detected, e. The decomposition of the latter via a heterolysis of the Pd-C bond under the action of the coordinated acetyl nitrite nitrate gives glycol monoacetate and a nitrosyl palladium complex.
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In the absence of N-ligand, the palladium-hydroxyalkyl intermediate decomposes via the b -hydride shift resulting in acetaldehyde. Thus, it was understood the beneficial effect of nitrate ions, originally used as re-oxidants for the reduced palladium species, on the nature of the product of olefin oxidation by dioxygen catalyzed by Pd II. We have developed the oxidative acetoxylation of propylene into glycol acetate over Pd, Pd-Pt, and Pt-Rh catalysts supported on carbon, SiO 2 , and TiO 2 in acetic acid solutions of nitric acid 27,28, Palladium catalysts on carbon exhibit a high activity and an ca.
Propylene oxidation proceeds both on the catalyst surface and in the solution due to the palladium leaching. After the reaction, the catalyst can be regenerated at mild conditions. The treatment of the suspension with hydrogen results in the reduction of the dissolved palladium II complexes and the deposition of the palladium metal back on the support. After filtration, the catalyst can be used again for propylene oxidation.
Catalysis in the Fine Chemicals Industry - Scientific Update - UK
We supposed that the addition of components that serve as hydrogen activators, such as platinum or rhodium, could improve the efficiency of the reductive regeneration and catalyst stability. This work has become a basis for a new technology, which we developed and patented for the synthesis of propylene oxide 32, Propylene oxide is a key intermediate in the production of polyurethanes and polyesters and currently produced by non-catalytic methods via chlorohydrin or by propylene epoxidation with hydroperoxide, both suffering from heavy waste problems.
We developed a new process 27,33 for the propylene oxide synthesis from propylene and dioxygen in two steps: 1 oxidation of propylene into propylene glycol monoacetate using Pd, Pd-Pt and Pd-Rh supported catalysts and 2 cracking of propylene glycol monoacetate into propylene oxide and acetic acid which is recycled in a flow reactor containing a melt of potassium acetate as catalyst. Selective functionalization of sufficiently abundant monoterpenes represents a promising route for the valorization of these cheap natural products.
Some of their oxygenated derivatives alcohols, aldehydes, ketones, allylic ethers, carboxylic acid esters are commercially important materials for pharmaceutical, flavor, and perfumery industries as well as useful synthetic intermediates and chiral building blocks 34, These compounds can be produced from olefins by metal catalyzed selective oxidation and carbonylation of their double bonds ,18, Because of its commercial importance the olefin oxidation by palladium II salts has received the most considerable attention, especially when such environmentally friendly and low-priced oxidants as dioxygen and hydrogen peroxide are involved 18,19, Although these reactions have been developed into important synthetic methods, there is very little information in literature concerning their application to natural product synthesis.
The other important pathways for the synthesis of oxygen-containing derivatives of monoterpenes are metal catalyzed carbonylation, i. We have, in recent years, been studying the selective oxidation and carbonylation of monoterpenes, such as limonene 1 , a -pinene 2 , b -pinene 3 and camphene 4 , via homogeneous catalysis by metal complexes. In the present paper, some recent results in this field are presented.
Palladium catalyzed oxidation of monoterpenes by dioxygen. Both addition and isomerization reactions are reversible, with no allylic oxidation of 7 , 8 , 10 and 9 , which contain no terminal double bond, being observed. A p -allyl palladium complex was suggested as a reaction intermediate, with the coordination of the exocyclic double bond on the same palladium atom being of a critical importance for its reactivity: e.
The isomerization and addition equilibria are shifted towards the formation of 1 at higher conversions, thus, the selectivities for 5 and 6 rise significantly with the reaction time. We have found that the rate of the allylic oxidation of limonene is the first order in copper concentration.
This observation indicates that CuCl 2 has behavior far exceeding that of a simple re-oxidant. We have supposed that one of the specific functions of CuCl 2 is to assist at the decomposition of the p -allyl Pd intermediate due to the formation of bridging Pd-Cu complexes. The reactions of a -pinene 2 and b -pinene 3 under similar conditions result in a mixture of 5 ca. The results obtained show that the application of the PdCl 2 -CuCl 2 system for the oxidation of bicyclic monoterpenes is limited, since CuCl 2 , acting as a Lewis acid, promotes undesirable non-oxidative skeletal isomerizations.
We concentrated our efforts on developing a CuCl 2 -free catalyst. Although nitrate ions readily re-oxidize reduced palladium species and are reoxidized back by dioxygen, no formation of oxidation products in significant amounts are observed for limonene, a -pinene and b -pinene. In the case of camphene, however, highly selective oxidative processes occur in the presence of this catalytic system 29, Palladium catalyzed oxidation of monoterpenes by hydrogen peroxide.
A competing skeletal isomerization and solvent addition yield various by-products, the major of them being 7 , along with 13 , 8 , and bornyl acetate The esters can be easily prepared from alkyl carboxylic acids, which are readily available, stable, and non-toxic.
The copper catalysis was then used to link the alkyl unit generated in photocatalysis with a nitrogen-containing coupling partner to generate alkyl amines. The work provides a new and efficient approach for the synthesis of alkyl amines, which has potential applications in the pharmaceutical and fine chemical industries.
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It offers high selectivity, compatibility with a large number of functional groups, and broad scope. In addition, the reactions are carried out at an ambient temperature, much milder than traditional methods. The researchers also provided over 50 examples where their new method is used for the synthesis of a diverse set of alkyl anilines with high chemoselectivity and functional group compatibility.
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