Hydrogenation Accompanied by Side Reactions

In studies of structure effects it is necessary that the catalyst should appear as a constant parameter. Generally, such studies may be performed using a great variety of catalysts, but the possibility of generalization of the data is very restricted. This phenomenon is especially marked in the cases of hydrogenation of olefinic substrates, which on some catalysts proceed selectively, while on others they are accompanied by side reactions, mainly by isomerization and sometimes also by hydrogenolysis. 

Alkylation by -halo ethers is of preparative interest only in special cases and is often accompanied by side reactions, e.g., loss of hydrogen halide. The subject has been reviewed several times.504... 

Removal of hydrogen halide is often accompanied by side reactions in which the halide is hydrolysed by the inorganic base ... 

When aromatic hydrocarbons are treated with aluminum chloride the autocondensation is accompanied by side reactions (ring contraction, partial hydrogenation, etc.) reactions of phenyl and naphthyl ethers are, however, smoother, and Scholl and Seer278 obtained good yields of 4,4 -diethoxy-l,l -binaphthalene from 1-ethoxynaphthalene and aluminum chloride in nitrobenzene. 

It has been shown that the isomerization of n-pentane in the presence of aluminum chloride or aluminum bromide containing catalysts was accompanied by side reactions, characterized by the formation of butanes and tarry material. These side reactions can be inhibited by the use of hydrogen or organic additives. 

The hydrosilylation of alkenes and alkynes catalyzed by transition metal complexes is often accompanied by side reactions such as isomerization, polymerization, and hydrogenation of unsaturated compounds and/or redistribution and dehydrogenation of silicon hydrides as well as reactions in which both substrates take part, such as dehydrogenative silylation (4,13). The latter reaction, which in certain conditions permits direct production of unsaturated silyl compound, has been a subject of intense study over the last two decades. 

Addition of anhydrous hydrogen chloride to vinylidene fluoride is reported to be accompanied by a hazardous (explosive) side reaction [53]. 

Eaborn and Taylor147 measured first-order rate coefficients for sulphonation of some aromatics in mixtures of trifluoroacctic acid-aqueous sulphuric acid, as sulphonation proved to be a troublesome side reaction accompanying hydrogen exchange in these media. They introduced a technique which has been found useful by later workers and makes use of the high solubility of sulphonic acids in... 

In contrast with exo (top) facial selectivity in the additions to norbomene 80 [41], Diels-Alder reaction between isodicyclopentadiene 79 takes place from the bottom [40] (see Scheme 32). To solve this problem, Honk and Brown calculated the transition state of the parent Diels-Alder reaction of butadiene with ethylene [47], They pointed ont that of particular note for isodicyclopentadiene selectivity issue is the 14.9° out-of-plane bending of the hydrogens at C2 and C3 of butadiene. The bending is derived from Cl and C4 pyramidalization and rotation inwardly to achieve overlap of p-orbitals on these carbons with the ethylene termini. To keep the tr-bonding between C1-C2 and C3-C4, the p-orbitals at C2 and C3 rotate inwardly on the side of the diene nearest to ethylene. This is necessarily accompanied by C2 and C3 hydrogen movanent toward the attacking dienophile. They proposed that when norbomene is fused at C2 and C3, the tendency of endo bending of the norbomene framework will be manifested in the preference for bottom attack in Diels-Alder reactions (Schane 38). 

In principle, carbometallation of an alkene (RCH=CH2) with a coordinatively unsaturated organotransition metal compound (R1 M I. ) can produce a monomeric carbometallation product 1 (Scheme 6). This reaction may not, however, stop at this stage. It can be accompanied by other processes of which (i) hydrogen-transfer hydrometallation to produce a potentially thermodynamically more favorable mixture of a 1,1-disubstituted alkene and a hydrometallation product 2 and (ii) polymerization to produce polyalkenes 3 are representative. The extents to which these side-reactions occur are functions of relative rates of various competing processes. For example, accumulation of the monomeric carbometallation product 1 can be favored in cases where the starting R1 MTL is more reactive toward alkenes than 1. The organometal/alkene ratio is also an important parameter, since neither of the two side-reactions can proceed after all of the starting alkene has reacted. 

Hydrogen transfer reactions from an alcohol to a ketone (typically acetone) to produce a carbonyl compound (the so-caUed Oppenauer-type oxidation ) can be performed under mild and low-toxicity conditions, and with high selectivity when compared to conventional methods for oxidation using chromium and manganese reagents. While the traditional Oppenauer oxidation using aluminum alkoxide is accompanied by various side reactions, several transition-metal-catalyzed Oppenauer-type oxidations have been reported recently [27-29]. However, most of these are limited to the oxidation of secondary alcohols to ketones. 

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