Olefins, activated dienes

Ffirai and Toshima have published several reports on the synthesis of transition-metal nanoparticles by alcoholic reduction of metal salts in the presence of a polymer such as polyvinylalcohol (PVA) or polyvinylpyrrolidone (PVP). This simple and reproducible process can be applied for the preparation of monometallic [32, 33] or bimetallic [34—39] nanoparticles. In this series of articles, the nanoparticles are characterized by different techniques such as transmission electronic microscopy (TEM), UV-visible spectroscopy, electron diffraction (EDX), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) or extended X-ray absorption fine structure (EXAFS, bimetallic systems). The great majority of the particles have a uniform size between 1 and 3 nm. These nanomaterials are efficient catalysts for olefin or diene hydrogenation under mild conditions (30°C, Ph2 = 1 bar)- In the case of bimetallic catalysts, the catalytic activity was seen to depend on their metal composition, and this may also have an influence on the selectivity of the partial hydrogenation of dienes. 

A similar system based on rhodium has been studied (123) and was found to be less active than the equivalent iridium catalysts. Selective hydrogenation of acetylenes to olefins and dienes to monoolefins can be performed using the rhodium system, and the authors note that although propan-2-ol is an effective source of hydrogen (via oxidation to acetone), mild pressures of hydrogen gas can also be employed. 

Regents which are active upon irradiation with benzene are olefins and dienes None of these react photochemically with borazine. Recently, the photolysis of benzene at X = 184.9 run with D2 has been shown to produce a small amount of CgHsD (4> = 0.01) This is contrast to the very efficient deuteration at the boron site of borazine (4 = 0.90). Hexafluoroacetone as the absorbing species reacts with borazine to produce a B-alkoxyborazine substitution product. Reaction of this reagent with benzene, on the other hand, involves the CF3 radicals and the products are addition rather than substitution products ss)... 

This review is limited to the polymerization of hydrocaibon dienes and olefins by means of organolithium initiators. It is not intended to include activated olefins or dienes that can be polymerized by bases of far lower reactivity or that do not involve direct caibon-lithium bonding. 

Additions to nonactivated olefins and dienes are important reactions in organic synthesis [1]. Although cycloadditions may be used for additions to double bonds, the most common way to achieve such reactions is to activate the olefins with an electrophilic reagent. Electrophilic activation of the olefin or diene followed by a nucleophilic attack at one of the sp carbon atoms leads to a 1,2- or 1,4-addition. More recently, transition metals have been employed for the electrophilic activation of the double bond [2]. In particular, palladium(II) salts are known to activate carbon-carbon double bonds toward nucleophilic attack [3] and this is the basis for the Wacker process for industrial oxidation of ethylene to acetaldehyde [41. In this process, the key step is the nucleophilic attack by water on a (jt-ethylene)palladium complex. 

Asymmetric monoepoxidation of conjugated dienes has been accomplished via (salen)Mn(ni)-catalyzed [salen = A,A -bis(salicylidene)ethylenediamine] oxidation. The reaction exhibits regioselectivity for attack at cis double bonds of c/i,trawi-conJugated dienes, and affords trans epoxides as the major products from cis olefins. Thus, diene 14 gave optically active fraw5-vinylepoxide 15 as the major product with 87% ee as shown in equation 16. 

Several metal macrocycles, like iron phthalocyanine and cobalt salophen were tested in these palladium-catalyzed aerobic oxidation of olefins and dienes. These tnacrocycles were able to activate the molecular oxygen, but in the homogeneous system several problems have arised, such as poor solubility, self-degradation and the difficulty to reuse theses complexes. A... 

Nonconjugated dienes bound to electrophilic transition-metal centers are activated toward nucleophilic attack analogous to olefin activation by transition metals discussed in 5.8.2.3.4. Although secondary reactions or rearrangements can occur, the pattern of the nucleophilic addition step (Nuc = nucleophile) leading to a, n complexes is shown... 

Table 11.1 Catalytic activity of Rh colloidal dispersions for hydrogenation of olefins and dienes. Adapted from Ref [15].

Olefin Activation. Mechanistic studies on the 1,4-oxidation of 1,3-dienes led to the discovery of a new palladium catalyst [Pd(DA)2], which was readily prepared from the reaction of Pd2(dba)3 and the Diels-Alder adduct derived from 1,3-cyclohexadiene and/>-benzoquinone. With p-benzoquinone as the stoichiometric oxidant, this palladium complex proves more reactive and selective than the Pd(II) carboxylate, typically used in... 

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