Shell higher olefins process , carbene

Since NHC are excellent phosphane mimics, the use of functionalised carbenes as ligands in olefin polymerisation catalysts would be self evident. An excellent model reaction is the commercially used Shell Higher Olefin Process (SHOP), initially developed by Keim [100-102], Here, enolisable ketophosphanes are used as ligands for nickel(II) catalysts (see Figure 2.13). A variant of this is the use of phosphino phenol ligands introduced by Heinicke [103-105]. 

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. 

It is still unclear how the initiation step in alkene metathesis occurs and how the initial carbene forms. Commercial applications of metathesis include the triolefin process, in which propylene is converted to ethylene and butene, the neohexene process, in which the dimer of isobutylene, Me3CCH=CMe2, is metathesized with ethylene to give Me3CCH=CH2, an intermediate in the manufacture of synthetic musk, and a 1,5-hexadiene synthesis from 1,5-cy-clooctadiene and ethylene. Two other applications, SHOP and ROMP (Shell higher olefins process and ring-opening metathesis polymerization), are discussed in the next section. 

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