Exchange Reactions between Alkenes

It is a rare honour to be asked to write a preface to a book based on chemistry which you helped begin. My research in palladium chemistry began before 1968. Initial results were obtained by preparing arylpalladium salts by exchange reactions between Pd(OAc>2 or PdCl2 and aryl-mercurials, -lead or -tin compounds and reacting these with alkenes to... 

The first example in Table 6.6 involves an exchange reaction between two different alkenes (metathesis), and is elaborated upon in Scheme 6.55. This is a particularly important process that has gained value with the introduction of very specific catalysts that even allow rings to form with expulsion of small alkenes (i.e.. Ring Closing Metathesis [RCM]) using Grubbs catalysts. 

Under the same conditions, several types of hydrocarbon are also converted to fully deuterated compounds. The results are summarized in Table 1. Cydooctene was also transformed into fully deuterated cydooctene without a skeletal rearrangement. As shown in entries 2 and 3, saturated hydrocarbons have also been transformed into fully deuterated compounds. As described above, an interaction between allylic C-H bonds and palladium hydride induces the H-D exchange reaction for alkenes. H-D exchange in alkanes, however, cannot be explained in this way. Direct C-H activation without assistance from any functional group may be a route to the formation of fully deuterated alkanes. 

Notwithstanding the fact that certain four-center additions to alkenes, e.g. equation (94) or Fig. 22, are forbidden, bromine addition to an alkene does occur. The orbital symmetry arguments which forbid cis addition to isolated bonds favor trans addition in equation (144), in which j = 4, X may be a nucleophile or radical and Y an electrophile or radical. Therefore, additions of molecular bromine or in general X—Y to alkenes, should proceed in at least two steps. Otherwise, separated X and Y, with one electron pair between them, add in concerted fashion to the alkene. Equation (144) is effectively the prototype of numerous ionic and radical a-w exchange reactions. A wealth of information has been recorded in excellent reviews covering special aspects of this general process, e.g. electrophilic additions (de la Mare and Bolton,... 

The alkene metathesis reaction see Alkene Metathesis) exchanges alkylidene groups between different alkenes, and is catalyzed by a variety of high oxidation state, early transition metal species (equation 40). The reaction is of interest because it is the strongest bond in the alkene, the C=C bond, that is broken during the reaction. It is also commercially important in the Shell higher olefins process and in the polymerization of cycloalkenes. It is relevant to this article because carbenes are the key intermediates, and the best-known catalyst, (1), is a carbene complex. 

The early use of deuterium in place of hydrogen in the study of catalytic hydrogenation led to the recognition that the process was not simply the addition of H2 to the double bond. Horiuti and Polanyi proposed that both H2 and alkene (1) are bound to the catalyst surface and transformed to products by a sequence of elementary steps, which they represented as shown in Scheme 1, where an asterisk ( ) represents a vacant site on the catalyst.The last step, (d), is virtually irreversible under the usual hydrogenation conditions, but can be observed in the exchange reactions of D2 with alkanes. The mechanism accounts for the isomerization of an alkene if the reversal of step (c), which involves the formation of the alkyl intermediate (3), involves the abstraction of a hydrogen atom other than the one first added, and is coupled with the desorption of the alkene, (2) - (1). At present, the bond between the alkene and the metal often is represented as a ir-complex (4), as in equation (7). ... 

Delmas, M., Gaset, A. Supported acid catalysis with ion-exchange resins. II. Mechanism of the condensation reaction between aqueous formaldehyde and aromatic alkenes. J. Mol. Catal. 1982,14, 269-282. 

The alkylidene groups of an alkene pair are exchanged with one another in the presence of a transition metal catalyst. In recent times, not only reactions between two C=C double bonds but also between C=C and C=0 bonds have been investigated. The reaction includes the cleavage of double C=C bonds and the formation of new ones. Homogeneous as well as heterogeneous catalysts are effective. 

The Cu2+ montmorillonite was also found to be an effective catalyst for the addition of carboxylic acids to alkenes to give esters. Al3+-exchanged montmorillonite was shown to catalyse the reaction between ethanol and hex-l-ene producing 2- and 3-ethoxyhexanes. 

Another fundamental reaction of organic synthesis in which bond breaking and formation takes place is the metathesis reaction [1-3]. The alkene metathesis reaction [41] exchanges alkildene groups between different alkenes, and it is catalyzed by a variety of high-oxidation-state early transition-metal... 

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