Migratory deinsertion

Aside from two-center (Patterns 1 and 2) and three-center (Patterns 3, 4, 11, and 12) processes, most of the processes shown in Scheme 1.3 are four-center processes involving either addition (Patterns 5—10) or 0-bond metathesis (Pattern 13). In this context, it should be noted that addition is simply a four-center metathesis in which one molecule happens to be multiply-bonded. In addition to these metathetical processes, there is yet another fundamentally important four-center metathetical process termed migratory insertion and deinsertion (Patterns 14 and 15). It should be clear from Patterns 14 and 15 shown in Scheme 1.3 that distinction between insertion and deinsertion is only a relative and semantic issue. In the current discussion, a process involving cleavage of the C—Zr bond is termed migratory insertion, while the reverse process is termed migratory deinsertion. 

As shown in equation 7.59, OA of the aldehyde yields the cis-hydridoacylrhodium complex, 39. Heating results in ligand dissociation to give 40, which undergoes migratory deinsertion (to be discussed in Chapter 8) to produce 41. Isomerization of 41 to a d.v-hydridoalkyl complex presumably occurs before final the final RE step to yield the C-H elimination product. These reactions have applications in catalytic industrial process known as olefin hydroformylation (Section 9-2). Equation 7.60 illustrates straightforward RE to form a new C-H bond. 

IV. Migatory insertion and migratory deinsertion Pattern 8 Migratory deinsertion Pattern 18 Migratory insertion... 

Despite these interesting and promising possibilities, the current scope of the in situ generation of organopalladium derivatives via migratory deinsertion is still rather limited. 

This reaction profile, also called carbonylation, governs the reactivity of Pd-carbonyl complexes. Anionic M[Pd(CO)l3], for instance, catalyzes the reductive carbonylation of esters.f On the other hand, Pd(CO)(PPh3)3 was reported to catalyze the carboxymethy-lation of organic halides and the cyclocarbonylation of cinnamyl halides.f " However, the Pd-CO complexes are most often generated in situ from preformed alkyl -palladium complexes and CO under stoichiometric or catalytic conditions, for example, in the copolymerization of alkenes and CO. Decarbonylation reactions also involve the intermediacy of Pd-CO complexes. In this case, migratory deinsertion (Sect, n.3.1), that is, the microscopic reversal of the migratory insertion, takes place. 

Migratory insertion and its microscopic reversal, that is, migratory deinsertion, are two of the 20 or so fundamental processes discussed in Sect. 1.2 in which Pd participates. In principle, they should be observed with a wide variety of substrates. In reality, however, the current scope of these processes observable with organopalladiums is almost totally limited to those that involve CO and related compounds, such as isonitriles. Conse-qnently, the migratory insertion-deinsertion chemistry of organopalladium compounds at present is essentially synonymous with their carbonylation-decarbonylation reactions. 

Another potentially useful but not yet widely employed class of electrophiles are sul-fonyl chlorides and other halides that can also undergo oxidative addition-migratory deinsertion (Scheme 22). 

The influence of the electronics on the rate of the migratory deinsertion was measured with para-substituted benzoylrhodium(III) complexes (Scheme 1.52) [68]. An electron-deficient aryl group migrated faster than an electron-rich one. The rate of the migratory deinsertion was also affected by the spectator ligand migratory deinsertion of the chloride complex was faster than that of the bromide complex. 

The migratory deinsertion process is important in the context of the transition metal-catalyzed or -mediated extrusion of the carbonyl group from aldehydes, ketones, and acid derivatives [70]. For example, treatment of aldehydes with a rhodium complex induced decarbonylation through oxidative addition of the... 

Aldehydes undergo a decarbonylation reaction by action of a transition metal complex [17]. For example, benzaldehyde was decarbonylated with Wilkinson complex to furnish benzene along with a rhodium carbonyl complex 47 (Scheme 7.14) [17e,f]. Oxidative addition of the aldehydic C-H bond to rhodium, migratory deinsertion of CO, and reductive elimination operate in sequence for the decarbonylation reaction. Decanal was also decarbonylated to furnish a mixture of nonane and nonene, which were produced via the alkylpalladium intermediate 48 (Scheme 7.15) [17d]. 

PropargyUc alcohol 124 was fragmented into the alkene 128 and CO in the presence of a ruthenium catalyst (Scheme 7.47) [66]. Oxidative addition of the C-H bond to ruthenium affords alkynylruthenium 125, which is converted into the allenylidene complex 126 with elimination of hydroxide anion. The hydroxide then adds to the allenylidene carbon directly connected to the ruthenium center, leading to the formation of acylruthenium 127 via tautomerization. Subsequent migratory deinsertion of CO followed by reductive elimination gives 128. 

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