Olefins into Metal-Acyl Bonds

Insertions of Olefins into Metal-Acyl Bonds 

Indeed, direct measurements of the rates of insertion of CO and ethylene into alkyl-metal olefin and acylmetal olefin complexes show that the insertion of ethylene into the metal-acyl linkage is faster than the insertion of ethylene into the metal-alkyl linkage. Comparisons of these rates for insertions into cationic palladium complexes containing phenanthroline and bis-diphenylphosphinopropane as ancillary ligand have been made by Brookhart and co-workers. These reactions are shown in Equations 9.69 and 9.70. A summary of the barriers for insertion is provided in Table 9.2. The rate of insertion of ethylene into the metal-acyl bond is orders of magnitude faster than the rate of insertion of ethylene into the metal-alkyl bond. - -  

Insertions of Alkynes into Metal-Carbon Bonds 

The insertions of alk5mes into metal-carbon o-bonds are less common than either the insertions of olefins into metal-carbon bonds or the insertions of alkynes into metal-hydride bonds. Nevertheless, several examples of this reaction have been studied, and many examples are part of catalytic processes. Most of the insertions of alkynes into metal-carbon bonds occur by concerted migratory insertion pathivays and provide products from cis addition of the metal and hydrocarbyl group across the carbon-carbon multiple bond, as predicted on theoretical groimds by Thom and Hoffmann. In some cases, the products from trans addition are observed, but these kinetic products are thought to result from isomerization of the vinyl group in reaction intermediates formed by cis addition. 

The insertions of alkynes into metal-carbon bonds are thermodynamically more favored than the insertions of olefins into metal-carbon bonds because the cleavage of one carbon-carbon TT-bond in an alk3me requires less energy than the cleavage of the C-C n-bond in an olefin and the sp -C-M bond in the product of alkyne insertion is stronger than the sp -C-M bond in the product of alkene insertion. The insertions of alkynes into the vinyl complexes that result from alkyne insertion are also favored thermodynamically. Thus, multiple insertions of alkynes to form polyacetylenes, just like the multiple insertions of alkenes to form polyolefins, are knoivn. Because of the conducting properties of polyacetylenes, the transition-metal-catalyzed polymerization of alkynes to form polyacetylenes has been studied.  

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If a group, such as (C(0)0CH2CH2CH2CH=CH2), was used, the olefin was conforma-tionally constrained to bind perpendicular to the Pd—C bond and insertion-cyclization did not occur. Alkene insertion reactions into metal-acyl bonds usually occur more rapidly than insertions into metal-alkyl bonds. 

The second insertion in the productive cycle of Fig. 14 would involve the chelated acyl complex (5). Again it might have been difficult to convert this to the olefin complex (6) in the nickel case as the strong chelate Ni-oxygen bond has to be weakened. However, for nickel it seems that (5) is replaced with the five-coordinated acyl complex 5a by uptake of one additional CO. However, 5a is not amenable for ethylene uptake as a first step in the insertion of ethylene into the metal-acyl bond since ethylene will have to replace the more strongly bound CO (>10 kcal mol ). It is thus Ukely that the CO/ethylene polymerization cycle is blocked by a species such as 5a or the four-coordinated chelate (5) of Fig. 14. 

In parallel with the directed hydroarylation of olefins, a series of papers described the formation of ketones from heteroarenes, carbon monoxide, and an alkene. Moore first reported the reaction of CO and ethylene with pyridine at the position a to nitrogen to form a ketone (Equation 18.28). Related reactions at the less-hindered C-H bond in the 4-position of an A/-benzyl imidazole were also reported (Equation 18.29). - Reaction of CO and ethylene to form a ketone at the ortho C-H bond of a 2-arylpyridine or an N-Bu aromatic aldimine has also been reported (Equations 18.30 and 18.31). Reaction at an sp C-H bond of an N-2-pyridylpiperazine results in both alkylative carbonylation and dehydrogenation of the piperazine to form an a,p-unsaturated ketone (Equation 18.32). The proposed mechanism of the alkylative carbonylation reaction is shown in Scheme 18.6. This process is believed to occur by oxidative addition of the C-H bond, insertion of CO into the metal-heteroaryl linkage, insertion of olefin into the metal-acyl bond, and reductive elimination to form the new C-H bond in the product. 

The following mechanism was proposed for the carbonylation of olefin-palladium chloride complex (10). The first step is coordination of carbon monoxide to the complex. Insertion of the coordinated olefin into the palladium-chlorine bond then forms a -chloroalkylpalladium complex (IV). This complex undergoes carbon monoxide insertion to form an acylpalladium complex (V), as has been assumed for many metal carbonyl-catalyzed carbonylation reactions. The final step is formation of a )8-chloroacyl chloride and zero-valent palladium by combination of the acyl group with the coordinated chlorine. 

The mechanism of this transformation is unclear at the present time, but two possibilities are pictured below. In the first (Fig. 5), loss of a CO ligand and binding of the acetylene initially provides the T -alkyne complex 17. Subsequent loss of a second equivalent of CO allows for coordination of the alkene to give 17a. Insertion of the olefin into the titanium-carbon bond of the alkyne complex produces metallacyclopentene 18. The insertion of CO generates acyl complex 19 which, upon reductive elimination, yields the observed cyclopentenone product. A second plausible mechanism (Fig. 6) involves initial formation of metal-... 

C-C cleavage of strained rings and ketones has been used to develop useful catalytic reactions. For example, vinylcyclopropanes and vinylcyclobutanes react with alkynes (Equation 6.66) to generate products from 5+2 and 6+2 addition processes that form seven- and eight-membered ring products by overall transformations that are homologs of the Diels-Alder reaction. " The mechanism of these catalytic reactions continues to be studied, but these reactions most likely occur by coordination of the olefin to rhodium and insertion of the metal into the cyclopropene or cyclobutane. Decarbonylation of dialkyl ketones, including relatively unstrained cyclic ketones, has been reported and most likely occurs by oxidative addition into the acyl-alkyl C-C bond, subsequent de-insertion of CO, and C-C reductive elimination. 

A simple catalytic cycle for hydroacylation is shown in part A of Scheme 18.19. Hydroacylation occurs by oxidative addition of the formyl C-H bond to generate an acyl hydride complex. Insertion of olefin into the metal hydride then generates an alkyl acyl intermediate. These complexes undergo reductive elimination, as described in Chapter 8. Although these basic steps constitute the catalytic cycle, many other processes occur outside of this cycle in the catalytic system. Some of these steps lying off the cycle lead to poisoning of the catalyst and others are unproductive reversible processes that have been revealed by H/D exchange experiments. Part B of Scheme 18.19 shows a catalytic cycle that includes these side processes. 

The hydroacylation of olefins with aldehydes is one of the most promising transformations using a transition metal-catalyzed C-H bond activation process [1-4]. It is, furthermore, a potentially environmentally-friendly reaction because the resulting ketones are made from the whole atoms of reactants (aldehydes and olefins), i.e. it is atom-economic [5]. A key intermediate in hydroacylation is a acyl metal hydride generated from the oxidative addition of a transition metal into the C-H bond of the aldehyde. This intermediate can undergo the hydrometalation ofthe olefin followed by reductive elimination to give a ketone or the undesired decarbonyla-tion, driven by the stability of a metal carbonyl complex as outlined in Scheme 1. 

Various unsaturated compounds can be inserted into the metal alkyl, aryl, and alkenyl complexes to give new organometallic complexes having various functional groups. The insertions of carbon monoxide (CO) and isocyanide (CNR) into transition metal-carbon a-bond are particularly important processes, since a carbon unit can be increased in the process and the acyl type complexes formed by the insertion processes can be subjected to further transformations to synthesize useful organic compounds. For example, the CO inserhon constitutes a fundamental step in industrially important processes such as hydroformylation of olefins, acetic acid synthesis from methanol and CO, Fischer-Tropsch process, amidocarbonylation, olefin and CO copolymerizahon processes as well as in a variety of laboratory syntheses of carbonyl containing compounds. 

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