Reaction with simple olefins mechanism

Cobalt hydrocarbonyl is a very reactive compound. It reacts extremely rapidly with triphenylphosphine, probably by a first-order dissociation mechanism, producing cobalt hydrotricarbonyl triphenylphosphine (44). This demonstrates the very ready replacement of one ligand by another. Cobalt hydrocarbonyl also catalyzes the isomerization of olefins. Under conditions of the hydroformylation reaction, olefin isomerization is observed. But there is controversy as to whether or not rearranged aldehydes (aldehydes which cannot be produced by simple addition to the starting olefin) are produced mainly by rearrangement of an intermediate in the reaction (28, 75, 55) or by reaction of isomerized olefins (55). 

Additions to Aromatic Hydrocarbons. A variety of photochemical additions to aromatic hydrocarbons have been reported. Benzene and its derivatives add to maleic anhydride74-76 as well as to simple olefins,77-80 isoprene,81 acetylene derivatives,79,82 and alcohols.83 The mechanism of the maleic anhydride-benzene reaction is discussed in Section IV. A.4. Naphthalene forms a photoadduct with dimethyl acetylenedicarboxylate62 and with acrylonitrile8211 while anthracene behaves similarly with maleic anhydride84 and with 1,2-benzanthracene.85 The photoaddition of several aromatic amines to anthracene has been reported to proceed via a charge transfer complex86,87 in fact, the majority of these addition reactions may proceed in this manner. 

The addition of hydrogen halides to simple olefins, in the absence of peroxides, takes place by an electrophilic mechanism, and the orientation is in accord with Markovnikov s rule.116 When peroxides are added, the addition of HBr occurs by a free-radical mechanism and the orientation is anti-Markovnikov (p. 751).137 It must be emphasized that this is true only for HBr. Free-radical addition of HF and HI has never been observed, even in the presence of peroxides, and of HCI only rarely. In the rare cases where free-radical addition of HCI was noted, the orientation was still Markovnikov, presumably because the more stable product was formed.,3B Free-radical addition of HF, HI, and HCI is energetically unfavorable (see the discussions on pp. 683, 693). It has often been found that anti-Markovnikov addition of HBr takes place even when peroxides have not been added. This happens because the substrate alkenes absorb oxygen from the air, forming small amounts of peroxides (4-9). Markovnikov addition can be ensured by rigorous purification of the substrate, but in practice this is not easy to achieve, and it is more common to add inhibitors, e.g., phenols or quinones, which suppress the free-radical pathway. The presence of free-radical precursors such as peroxides does not inhibit the ionic mechanism, but the radical reaction, being a chain process, is much more rapid than the electrophilic reaction. In most cases it is possible to control the mechanism (and hence the orientation) by adding peroxides... 

Excited cyclic a,/3-unsaturated ketones attack simple olefins too in their ground state. For example, Eaton carried out the reaction in Equation 13.51.660 The exact mechanism is not understood, since the characteristics of the reaction vary With some reactants it is regiospecific and/or stereospecific and with others it is neither.67... 

The reaction of formamide with aromatic compounds under ultraviolet irradiation is still unexplored and only preliminary results have so far been obtained. In the cases already studied it has been found that this reaction must be sensitized with a ketonic sensitizer, usually acetone, in order to take place. The mechanism of the photoamidation of aromatic compounds certainly differs from the one of simple olefins. The detailed mechanism still awaits further experimental evidence, and in some cases involves, most probably, radical combinations and not addition of radical to unsaturated systems. Interactions of the excited sensitizer with aromatic compounds, having in some cases triplet energies similar or just a bit higher than those of the sensitizers used, must be brought into consideration. Experimentally it has been shown that the photosensitized amidation of benzene leads to benzamide (11),... 

Simple olefins, such as cyclohexene, styrene, or tetramethylene, were unreactive even at 60°C. For facile reaction, the olefin must be substituted with powerful electron-attracting substituents capable of stabilizing a negative charge. A schematic mechanism showing the stepwise nucleophilic addition of (Ph3P)2M02 to the olefin may be represented as follows ... 

Though the mechanism of even the simple reaction of osmium tetroxide with an olefin remains uncertain, there has been considerable controversy over the mechanism of the process when a chiral cinchona alkaloid ligand is also involved . K.B. Sharpless et al., J. Org. Chem. 1996, <57, 7978. 

The rates of the overall reactions can be related to the rate law expressions of the individual steps by using the steady state approximation. However simple kinetic data alone may not distinguish a mechanism where, for example, a metal and an olefin form a small amount of complex at equilibrium that then goes on to react, from one in which the initial complex undergoes dissociation of a ligand and then reacts with the olefin. As a reaction scheme becomes more complex such steady state approximations become more complicated, but numerical methods are now available which can simulate these even for complex mixtures of reactants. 

From the preceding discussions, the salient features of the sulfur atom—olefin systems may be summarized as follows (a) triplet state atoms react with olefins to produce only episulfide in an almost completely stereospecific process, and (f>) the interaction of singlet state sulfur atoms with olefins produces mercaptans, and probably episulfides as well. The kinetics of the reaction is not in accord with the simple mechanism which includes one or more product-forming steps with the olefin molecule in competition with the abstraction reaction from COS, even if allowances are made for collisional relaxation of S( Z)) atoms by COS and the olefin. For this reason, the possibility should be kept in... 

Knyazev, V. D., V. S. Arutyunov and V. I. Vedeneev The mechanism of 0( P) atom reactions with ethylene and other simple olefins, Int. J. Chem. Kinetics 24 (1992) 545-562. 

This interpretation incorporates the major features of the mechanism of olefin polymerization with Ziegler-Natta catalysts and shows how such features may be deduced. There is still much to learn about these complex systems however, we may have little doubt but what further work will define this extremely complex reaction, with as much precision as has been applied to more simple reactions. 

It has been observed that cycloadditions of methyl cinnamate with simple alkenes in dilute solution are catalyzed by Lewis acids such as boron trifluoride86. The mechanism of these reactions involves electronic excitation of a ground state methyl cinnamate-Lewis acid complex followed by reaction of the excited complex with ground state olefin. The catalytic effect of the Lewis acid results from an increase either in excited state lifetime or reactivity of the complexed versus free ester. This was discovered in an investigation of the photochemical reactivity of coumarin in the presence of Lewis acids87. 

Of particular interest for this review are those reactions of 02 that closely resemble those found in the autoxidation. Since both 02 and R02 react with many olefins to form hydroperoxides, some basis for distinguishing between these reactants can be important in understanding the detailed mechanism of oxidation of a specific compound, particularly in photooxidations where both type I and II processes can occur. Many simple substituted olefins give very similar mixtures of hydroperoxides by the two pathways however, certain structural units do give markedly different hydroperoxides from R02 and 02 and are useful as criteria for mechanism. Examples are 1,2-dimethylcyclohexene [190]... 

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