Olefins solvent-free system

Cerveny et al. (100 report an investigation of the hydrogenation of 12 olefinic substrates in the liquid state with 5% Pt on silica gel as catalyst under usual conditions and without solvents. The reaction rates related to 2,3-dimethyl-2-butene and the relative adsorption coefficients obtained in systems of various pairs of substrates and by recalculation using Eq. (25) to 2,3-dimethyl-2-butene are given in Table IV. Comparison between the measured reaction rates and the rates of hydrogenation in solvents (71 has revealed that relations existing between the rates of hydrogenation in a solvent-free system approximately correspond to those determined in... 

The investigation of the effect of olefins on the course of hydrogenation in solvent-free systems, similarly to the investigation of these relations in systems with solvents, demonstrated the necessity of a complex view of the whole system, because the behavior of substrates in hydrogenation may be affected by all the components present in the reaction mixture. 

Solvent-free 1,3-dipolar cycloaddition of diphenylnitrUimine 13, generated in situ from hydrazonoyl chloride, to olefins 184 was efficiently catalyzed by a combination of porous calcium hydroxyapatite, serving both as base and as solid support, and microwaves. A key feature of this catalyst/solvent-free system is the recy-... 

However, all these systems suffer from high concentrations of chloride ion, so that substantial amounts of chlorinated by-products are formed. For these reasons there is a definite need for chloride- and copper-free systems for Wacker oxidations. One such system has been recently described, viz., the aerobic oxidation of terminal olefins in an aqueous biphasic system (no additional solvent)... 

Monflier, E., Tilloy, S., Blouet, E., Barbaux, Y., Mortreux, A. Wacker oxidation of various olefins in the presence of per(2,6-di-0-methyl)-P-cyclodextrin mechanistic investigations of a multistep catalysis in a solvent-free two-phase system. J. Mol. Catal. A Chemical 1996,109, 27-35. 

This conclusion is in agreement with a remark of Lewis who stated that slower reacting systems tend to show a greater effect under microwave radiation than faster reacting ones [82]. In this way, during solvent-free Wittig olefination with phosphoranes, it was shown that the benefit of MW irradiation increases with less reactive systems. The best stabilized phosphoranes do not react at all in the solid state with aldehydes or ketones under conventional heating but necessitate MW irradiation [83]. 

Mechanochemical aminochlorination of electron-deficient olefins with chloramine-T offers an access to highly functionalized systems (Scheme 3.79) [53]. Variety of additives was tested in the reaction of 1,3-diphenylpropenone to find out that reactions promoted by Phl(OAc)2 (50mol%) are the most efficient to produce vicinal chloroamino compounds 295. Hydrates of metal chlorides (Cu, Fe, Co, Ni, Cr, Ce, Sn, and In) were less active and Cul nonreactive. Optimized reaction conditions were then employed for various electron-deficient olefins 294 (Table 3.40). Comparison with reactions carried out in organic solvents showed that the solvent-free mechanochemical reactions were the most efficient in terms of product yield and reaction time (entry 1). 

Metal alkoxides have a well-established role in catalytic reactions. In Chapter 7, a brief review on the history, characteristics and synthetic routes for preparing metal alkoxides are illustrated. The catalytic processes performed by these catalysts include polymerization of different olefin oxides and cyclic esters, asymmetric reduction of aldehydes and ketones, oxidation of sulfides and olefins, and a variety of asymmetric reactions. The remainder of the chapter discusses characteristics of these catalytic systems. Other challenges separate from the metal alkoxide catalysis involve development of catalytic protocols in solvent-free or in green solvent conditions, viz., H O or liquid CO. The second challenge is recovery of catalyst without loss of its activity. Supporting metal alkoxide onto inorganic solids, especially magnetic ones, may effectively solve the later problem. 

Sc(OTf) 3-catalyzed 1,3-dipolar cycloaddition reactions of phenyl aziridines with olefins such as cyclic enol ethers and allyltrimethylsilane also proceeded well [36]. Sc(OTf)3 could also be used as a catalyst for [3 + 2] cycloaddition of aziridines with various nitriles to give the corresponding imidizalines in good to excellent yields under solvent-free conditions (Scheme 12.20) [37]. Although a relatively large amount of Sc (OTf) 3 was used in this reaction, the fast reaction rate, mild conditions and experimental operational ease are the features of this system. 

The general reactivity of higher a-olefins is similar to that observed for the lower olefins. However, heavier a-olefins have low solubihty in polar solvents such as water consequentiy, in reaction systems requiting the addition of polar reagents, apparent reactivity and degree of conversion maybe adversely affected. Reactions of a-olefins typically involve the carbon—carbon double bond and can be grouped into two classes (/) electrophilic or free-radical additions and (2) substitution reactions. 

The previous sections show that certain ionic liquids, namely the chloroalumi-nate(III) ionic liquids, are capable of acting both as catalyst and as solvent for the polymerization of certain olefins, although in a somewhat uncontrolled manner, and that other ionic liquids, namely the non-chloroaluminate(III) ionic liquids, are capable of acting as solvents for free radical polymerization processes. In attempts to carry out polymerization reactions in a more controlled manner, several studies have used dissolved transition metal catalysts in ambient-temperature ionic liquids and have investigated the compatibility of the catalyst towards a range of polymerization systems. 

For long chain olefins, the hydroformylation generally proceeds slowly and with low selectivity in two-phase systems due to their poor solubility in water. Monflier et al. recently reported a conversion of up to 100% and a regioselectivity of up to 95% for the Rh-catalyzed hydroformylation of dec-l-ene in water, free of organic solvent, in the presence of partially methylated 6-cyclodextrins (Eq. 3.42).173... 

For cationic polymerisation of olefins in solvents of DC appreciably less than ca. 10 and for those of heterocyclic monomers in all solvents of DC up to perhaps 15-20, this is not so. For such systems the polymerisations are probably at least dieidic (free ions and ion-pairs) and a lowering of the temperature will increase the DC of the ion pairs. Thus in such systems the change of temperature affects not only k p and k"p, but also the relative abundance of the different types of chain-carriers therefore the proper interpretation of the apparent activation energies is difficult and by no means obvious. 

In asymmetric hydrogenation of olefins, the overwhelming majority of the papers and patents deal with hydrogenation of enamides or other appropriately substituted prochiral olefins. The reason is very simple hydrogenation of olefins with no coordination ability other than provided by the C=C double bond, usually gives racemic products. This is a common observation both in non-aqueous and aqueous systems. The most frequently used substrates are shown in Scheme 3.6. These are the same compounds which are used for similar studies in organic solvents salts and esters of Z-a-acetamido-cinnamic, a-acetamidoacrylic and itaconic (methylenesuccinic) acids, and related prochiral substrates. The free acids and the methyl esters usually show appreciable solubility in water only at higher temperatures, while in most cases the alkali metal salts are well soluble. 

The general features of the isomerization are compatible with a free radical cation chain mechanism, featuring electron transfer from unreacted olefin to rearranged radical cation. This chain mechanism was firmly established in several other isomerizations by the observation of quantum yields greater than unity. Thus, the dicyanoanthracene sensitized irradiation of m-stilbene results in nearly quantitative isomerization (> 98%) to the trans-isomer. In this system, the quantum yield increases with increased ds-stilbene concentration, solvent polarity, salt concentration, as well as decreasing light intensity [159]. 

---