Reactions of Monoolefins

No doubt the most studied reaction in Pd(ll) chemistry is the basic Wacker process—the oxidation of ethylene to acetaldehyde in H2O. 

This reaction was known for a number of years 219), but it was the discovery by Smidt and co-workers that addition of CuClg prevented the precipitation of Pd(0) which made the reaction commercially interesting 244). Since CuCl is readily regenerated by O2, the net reaction is the oxidation of ethylene by air to acetaldehyde  

There is general agreement that at low [Pd(II)] the rate expression is 107, 152, 199) 

Although there is now agreement about the general scheme there is still discussion as to some details of the mechanism. First there is the mode of hydroxypalladation. The original suggestion was that the insertion occurred by attack of the coordinated OH on the coordinated ethylene in the slow step of the reaction. 

This step was designated the slow step on the basis of isotope effects. Deuterium-labeling experiments indicate that the decomposition step 

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The electrochemical oxidation or reduction of dienes and polyenes is generally more useful than the corresponding reaction of monoolefins which is not substituted with activating groups, since the electrode potentials required in the reaction of dienes and polyenes are generally much lower than the potentials necessary in the reaction of monoolefins. 

The mechanism of reaction of monoolefins resembles that for base-catalyzed alkylation. An example is the reaction of isobutylene (32a, b, c). 

Most Pd(II)-catalyzed reactions of monoolefins proceed via addition of Pd(II) across the olefin bond to give Pd(II)-<7-bonded carbon intermediates. Evidence for this scheme has been provided by isolation of stable adducts of this type. Usually these adducts are prepared from nonconjugated diolefins because of the special stability of this type of complex, but in some cases they have been prepared from monoolefins. 

Diazo ester/rhodium(II) carboxylate combinations other than EDA/Rh2(OAc)4 have been tested It turned out that the solubility of the rhodium(II) carboxylate greatly influenced the efficiency of cyclopropanation. For the reaction of monoolefins with ethyl diazoacetate, markedly higher yields than with Rh(II) acetate were obtained with the better soluble rhodium(II) butanoate and rhodium(II) pivalate, the latter one being soluble even in pentane. However, only poor yields resulted from the use of rhodium(Il) trifluoroacetate, even though this compound is readily soluble, Rh CCFjCOO), in contrast to the other rhodium(II) carboxylates, is able to form 1 1 complexes with olefins particularly with electron-rich ones thus, competition of olefin and diazo compound for the only available coordination site at the metal atom could be responsible for the reduced catalytic action of Rh2(CF3COO)4 (as will be seen in Section 4.1, this complex is an excellent catalyst for cyclopropanation of aromatic substrates). The diazoester substituent also has some influence on the yields. Increasing yields were obtained in the series methyl ester, ethyl ester, n-butyl... 

The Ni-catalyzed reactions of monoolefins generally yield dimer/oligomer mixtures the primary products frequently are further isomerized. However, the selectivity relative to dimerization versus oligomerization and/or formation of a particular dimer may be significantly influenced by reaction conditions, primarily by the choice of the PR3 donor and the molar ratio Ni/PR3. 

Ene reactions of monoolefins with silenes have been known for a long time308. The addition of isobutene to 54 (equation 136) has been observed to proceed at —10 °C243. A similar reaction occurs between 54 and other olefins (Table 4)246 see also Reference 266. The rates of these reactions appear to be somewhat sensitive to steric hindrance but even the stable hindered silene 41 still adds propene and isobutene115. They are very much slower than the analogous ene additions of carbonyl compounds containing a hydrogens, such as acetone. 

These types of reactions of monoolefin transition metal complexes have very significant practical application, because they allow for preparation of many compounds from inexpensive olefin hydrocarbons. Examples are provided by olefin Pd(II) complexes, which show electrophilic properties and react with water and other nucleophiles. 

L. S. Hegedus, Tetrahedron, 1984, 40, 2415-2434. Pal-ladium(II)-Assisted Reactions of Monoolefins. 

Reaction of monoolefins, PdClg and carbon monoxide in nonpolar solvents, with subsequent treatment with alcohols, yields chlorocarboxylic acid esters [513, 514]. In this case PdClg has to be present in stoichiometric quantities as a Cl source. 

Hegedus LS. Palladium(II)-assisted reactions of monoolefins. Tetrahedron 1984 40 2415-2434. 

Ohta, T. (1984) Rate constants for the reactions of diolefins with OH radicals in the gas phase. Estimate of the rate constants from those for monoolefins. J. Phys. Chem. 87, 1209-1213. 

This reaction has an accompanying endothermicity of ca 4 kJmol-1. Said differently, methylation of ethylene is some 4 kJmol-1 more exothermic than of allene. How general is this greater exothermicity of alkylation of monoolefins over that of related allenes Proceeding to the three cumulated 5-carbon dienes, we may consider the reactions... 

Several fundamental types of metathesis reactions for monoolefins or diolefins are shown in Eqs. 2-5. 

The selectivity for hydrogenation of dienes in the presence of monoolefins arises from the exceptional stability of jr-ally 1 complexes. In the case of Pt catalysts the reactions shown can compete with one another (equation 6)14. The second pathway is favored, especially when the olefin or diene must compete with excess ligands (phosphine, CO, SnCp ) for a coordination site. This is why the diene is almost completely hydrogenated before the concentration of olefin increases to the point that the olefin gains access to the catalyst. A similar phenomenon can be responsible for selectivity in hydrogenation of dienes with heterogeneous catalysts. 

The acid-catalyzed reaction of aromatics with monoolefins results exclusively in addition of alkyl groups to the aromatic ring. In contrast, the base-catalyzed reaction of aromatics with monoolefins results in alkylation... 

The side-chain alkylation reaction of aromatic hydrocarbons has also been studied using unsaturated aromatic olefins, especially styrene. Pines and Wunderlich 43) found that phenylethylated aromatics resulted from the reaction of styrenes with arylalkanes at 80-125° in the presence of sodium with a promoter. The mechanism of reaction is similar to that suggested for monoolefins, but addition does not take place to yield a primary carbanion a resonance stabilized benzylic carbanion is formed [Reaction (23a, b)j. 

Acid-catalyzed reactions of aromatics with monoolefins result in nuclear alkylation. But the base-catalyzed reactions of aromatics with olefins do not result in nuclear alkylation as long as benzylic hydrogens are available. This is true even with aromatics, such as cumene, which have deactivated benzylic hydrogens resulting in facile metalation of the ring. Apparently phenyl carbanions do not readily add to olefins. Pines and Mark (20) found that in the presence of sodium and promoters only small yields of alkylate were produced at 300° in reactions of benzene with ethylene and isobutylene and of t-butylbenzene with ethylene. With potassium, larger yields may be obtained at 190° (24)-... 

The principal types of catalysed hydrogenation reactions, together with the likely reaction products, are shown in Table 1. Of the reactions listed, the hydrogenation of monoolefins and diunsaturated hydrocarbons — acetylenes and diolefins — have attracted most attention and, for this reason, much of the ensuing discussion will be concerned with these particular reactions. 

As noted in Sect. 4.1, in the hydrogenation of diunsaturated hydrocarbons it is generally observed that both the corresponding monoolefin and alkane are formed in the initial stages of the reaction, the former product generally predominating. Further, in the reactions of alkynes and diolefins containing more than three carbon atoms, a distribution of isomeric olefins is usually observed. 

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 unpromoted hydrocyanations of monoolefins discussed so far generally involved only a few catalytic cycles on nickel. The development of a practical commercial process depended on getting many cycles. Certain Lewis acids are quite remarkable in increasing (1) catalyst cycles, (2) the linearity of products obtained, and (3) the rates of reaction. The effects depend on the Lewis acid, the phosphorus ligand used, and the olefin substrate (72). 

With monoolefins apparently simple complexes of the type [AuCI-(olefin)] were obtained (74-76), whereas di- and triolefins gave dimeric species. With norbornadiene both monomeric and dimeric compounds were isolated (74). A gold(I) complex was also obtained from the reaction of auric chloride with hexamethyl Dewarbenzene (77). 

One of the possible reactions of an electronically excited monoolefin is dimerization with a ground-state molecule. If the reaction is concerted, the transition... 

Table 6. Examples of selective disproportionation of monoolefins Type I reactions 2 A < -2 P + Q...

The most thoroughly investigated group of organolanthanide-catalyzed reactions are monoolefin transformations. These include the following processes a) hydrogenation, b) oligomerization, c) polymerization, d) hydroamination, e) hydrosilylation, and f) hydroboration, which will be discussed in the above order. 

In order to fit the initial rate data, it is not required that the bound olefin in Cp2ZrHO+, Cp2R 0+ or Cp2RO+ also inserts directly, in an intramolecular fashion. A reaction mechanism based solely on intramolecular insert ion of this bound olefin, without assuming the existence of either a bis(olefin) intermediate or the conerted, bimolecular insertion reaction of a monoolefin species with a second equivalent of monomer-does not fit the data. 

Citranaxanthene 543 is found in citrus fruits and is used as a food dye like P-carotene. The same phosphonium salt synthon 505 as used for the vitamin A synthesis is monoolefinated with the polyene dialdehyde 539. The Wittig reaction of the resulting 540 with phosphorane 541 followed by aldol condensation of the obtained 542 with acetone gives citranaxanthene 54 3 255,263) (Scheme 92). In the preparation of the polyenedial 539 l,4-dibromo-2-butene 544 is reacted with trimethyl... 

Interestingly, no reaction of bis(dithiolene) complexes with simple, monoolefins such as aliphatic olefins (ethylene, propylene, 1-hexene, etc.) had been reported until recently (77-79). Simple olefins such as ethylene, propylene, and 1-hexene have been found to react with Ni(tfd)2 under ambient conditions. The reaction is clean, selective, and reversible. The dithiolene complex does not react with H20, CO, C2H2, H2, or low concentrations of H2S under the same conditions. The reaction could therefore be useful in cleaning up petrochemical olefin feeds in which these molecules are present as contaminants. 

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