Hydrogenation alkadienes

The metal-catalysed hydrogenation of multiply unsaturated hydrocarbons is, of necessity, more complex than that of monoolefins. The problems encountered in alkyne and alkadiene hydrogenation are essentially similar and it is appropriate, therefore, that the two systems are considered together. 

Bivalent radicals derived from unbranched alkenes, alkadienes, and alkynes by removing a hydrogen atom from each of the terminal carbon atoms are named by replacing the endings -ene, -diene, and -yne by -enylene, -dienylene, and -ynylene, respectively. Positions of double and triple bonds are indicated by numbers when necessary. The name vinylene instead of ethenylene is retained for —CH=CH—. 

Common precursors for the enantioselective hydrogenation with the use of cationic rhodium are the tetrafluoroborate salts of bis-alkadiene rhodium. As a solvent one often uses dichloromethane or methanol, in which tetrafluoroborate is indeed weakly or non-coordinating. The first alkadiene at rhodium is rapidly replaced in situ by the phosphorus ligands added. 

Structure effects on the rate of selective or total oxidation of saturated and unsaturated hydrocarbons and their correlations have been used successfully in the exploration of the reaction mechanisms. Adams 150) has shown that the oxidation of alkenes to aldehydes or alkadienes on a BijOj-MoOj catalyst exhibits the same influence of alkene structure on rate as the attack by methyl radicals an excellent Type B correlation has been gained between the rate of these two processes for various alkenes (series 135, five reactants, positive slope). It was concluded on this basis that the rate-determining step of the oxidation is the abstraction of the allylic hydrogen. Similarly, Uchi-jima, Ishida, Uemitsu, and Yoneda 151) correlated the rate of the total oxidation of alkenes on NiO with the quantum-chemical index of delo-calizability of allylic hydrogens (series 136, five reactants). 

W-Substituted 2,4-alkadien-l-ols such as 474 add the alkyllithium/(—)-sparteine complex preferentially to the 2-position to form via the alkoxide the corresponding allyllithium intermediates 475 . Protic workup leads to a mixture of ii/Z-alkenols 476 and 477 on catalytic hydrogenation the -branched alcohols 478 are isolated (equation 130). 

A further problem arises when one considers the hydrogenation of alkynes and alkadienes containing four or more carbon atoms. In such cases, it is possible that the intermediate olefin may be formed in more than one isomeric form. Hence, the stereospecificity of the hydrogenation must be considered and attempts made to explain the observed stereospecificity in terms of the mechanism (see Sect. 4.3). 

In the ensuing discussion, it will be assumed that structure L is the relevant species in alkyne hydrogenation, and that the catalytically active adsorbed state of an alkadiene can be represented as a jr-olefin complex in which either one or both olefinic bonds interact with the surface. 

It is generally agreed that the kinetics and the distributions of deuter-ated products from the reactions of alkynes or alkadienes with deuterium are satisfactorily interpreted in terms of the consecutive addition of two hydrogen atoms, of unspecified origin, to the adsorbed hydrocarbon to yield the monoolefin. The identity of the distributions of deuteroethyl-enes from the reaction of acetylene with equilibrated and non-equil-ibrated hydrogen—deuterium mixtures also provides strong evidence for such a mechanism [91]. 

The interaction of an alkyne or alkadiene with deuterium leads to the formation of deuteroalkenes whose isotopic composition yields valuable information regarding possible reaction mechanisms. In an attempt to interpret in detail the deuteroalkene distributions, two approaches have been used. The first, due to Bond [163], is a simplified version of the general theory proposed by Kemball for the hydrogenation of ethylene (see Sect. 3.4) and has been used to interpret the results of the reaction of acetylene with deuterium [163—165]. The method comprises a steady state analysis of the reaction scheme... 

If the alkene or alkadiene has at least one hydrogen on the carbon adjacent to the double bond, reaction with singlet oxygen may give hydroperoxides. The mechanism of this reaction is related to [4 + 2] cycloadditions and is presumed to occur through a HLickel pericyclic transition state (see Section 21-10D) ... 

Catalyzed hydrogenation of alkynes, alkenynes, and alkadienes.1 This catalyst effects highly c/s-selective hydrogenation of triple bonds of alkynes and alkenynes, with easy recovery of the complex by filtration. It also effects only 1,2-addition in hydrogenation of even hindered 1,4-substituted 1,3-butadienes. 

Both of these reactions have very important industrial uses (Section 14.3.9). In order to obtain alkene streams of sufficient purity for further use, the products of steam-cracking or catalytic cracking of naphtha fractions must be treated to lower the concentration of alkynes and alkadienes to very low levels (<5ppm). For example, residual alkynes and dienes can reduce the effectiveness of alkene polymerisation catalysts, but the desired levels of impurities can be achieved by their selective hydrogenation (Scheme 9.4) with palladium catalysts, typically Pd/A Os with a low palladium content. A great deal of literature exists,13,37 particularly on the problem of hydrogenating ethyne in the presence of a large excess of... 

The base-catalyzed reaction of alkylaromatics with olefins is unique in that it allows the size of the alkyl group of an arylalkane to be increased. Arylalkanes suitable for this reaction are those which contain a benzylic hydrogen. The olefins most useful for this reaction are ethylene, propylene, conjugated alkadienes, and styrene and its derivatives. Sodium and potassium are very effective catalysts. Sodium usually requires the presence of a chain precursor to initiate the reaction. 

The polymers of rubber plastics have unsaturated hydrocarbon chain structure, since they are polymerized from alkadienes. The general formula of poly(l,3-butadiene) or butadiene rubber (BR) and polyisoprene or natural rubber (NR) is drawn in Scheme 12.5, where X is hydrogen in BR and methyl group in synthetic polyisoprene or NR. The free radical mechanism of thermal decomposition starts by homolytic scission of the alkyl C-C bonds. Two primary macroradicals (4 and 5) are formed for which the rearrangement... 

The monoenes formed on hydrogenating 1,3-alkadienes correspond to 1,2- and 1,4-adducts of hy-drogen. - - To account for the main products of the reaction of D2 with 1,3-butadiene over Pd/AhOs, 1-butene-3,4- /2 and nani-2-butene-1,4- 2, Meyer and Burwell assumed that the diene is adsorbed on the surface in the trans conformation. Addition of deuterium to a terminal carbon atom forms an allylic... 

A combination of 1-alkenylboronates and 1-halo-1,3-alkadienes is expected to lead to the same trienes, but this combination is generally not recommended because of the experimental problems related to the instability of dienyl halides and a side reaction involving elimination of hydrogen halides with bases to produce the corresponding enyne. 

The hydrogenation of unsaturated hydrocarbons continues to attract attention by reason both of the practical importance and of the theoretical interest of this system. This is particularly true for the selective hydrogenation of alkadienes and alkynes in alkenes rich cuts [74]. Fundamental studies [75] have shown that Pd is the most active and selective metal for these reactions. Some reasons for the better performances exhibited by Pd-based catalysts can be learned from the mechanism and the surface complexes involved in these transformations. 

The dependence on Pd particle size of the hydrogenations of alkenes, alkadienes, and alkynes has been studied in the gas and liquid phases and for pure reactants or simulated real feedstocks. This is illustrated by some typical examples later. 

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