Olefins into Metal-Nitrogen Bonds

Insertions of Olefins into Metal-Nitrogen Bonds 

Examples of the insertions of alkenes or alk5mes into metal-amido bonds are also rare. Examples of the insertions of alkenes into tihe M-N bonds of isolated amido complexes include the reaction of a rhodium anilide complex with alkenes to form imines witii kinetic behavior that is consistent with migratory insertion,and the formal insertion of the strongly electrophilic acrylonitrile into a platinum anilide. Additional examples include reactions of a lanthanide-amido complex generated in situ, a catalytic carboamination process in which the stereochemistry implies insertions of olefins into amides, and a catalytic hydroamination that appears to occur through an aminoalkyl complex generated by S3m addition of the iridium and amido groups across the C=C bond of norbomene. 

Equation 9.84 depicts the reaction of aniline, norbomene, and [Ir(PEt3)2(CjH )jCl] to form an iridium-aminoalkyl complex. This process is thought to occur by oxidative addition of the aniline to form an amido hydride complex, followed by insertion of the strained alkene into the iridium-nitrogen bond. Products from oxidative addition of aniline to the same iridium species were shown to form in the absence of the olefin. The syn stereochemistry of the aminoalkyl product indicated that a migratory insertion pathway was followed. 

Equation 9.85 shows the reaction of a similar rhodium-amido complex with unstrained olefins. In this case, the product is a free imine. The inverse order of the reaction in added phosphine and the zero order of the reaction in any free amine imply that the imine is formed by the sequence shown in Equation 9.86, involving olefin insertion, 3-hydrogen elimination (see Chapter 10), and tautomerization of the resulting enamine to the imine. 

Equation 9.87 involves insertion of acrylonitrile, which is electrophilic enough that the reaction is likely to occur by direct attack of the nitrogen electron pair at the olefin, not by a migratory insertion. 

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An alternative method of amine activation is opened via the oxidative addition of the N-H bond to an appropriate transition metal in a lower oxidation state. After formation of the /ff-aminoalkyl compound by insertion of the olefin into the transition-metal-nitrogen bond, the alkylamine can be generated by reductive elimination (Scheme 2), and with the reformed reduced transition metal complex the catalytic reaction can run again. 

A few final comments should be made on the insertions of substrates containing C-C multiple bonds into the bonds between a transition metal and an electronegative heteroatom. First, insertions of olefins into related thiolate and phosphide complexes are as rare as insertions into alkoxo and amido complexes. Reactions of acrylonitrile into the metal-phosphorus bonds of palladium- and platinum-phosphido complexes to give products from formal insertions have been observed, and one example is showm in Equation 9.90. However, these reactions are more likely to occur by direct attack of the phosphorus on the electrophilic carbon of acrylonitrile than by migratory insertion. Second, the insertions of alkynes into metal-oxygen or metal-nitrogen covalent bonds are rare, even though the C-C ir-bond in an alkyne is weaker than the ir-bond in an alkene. 

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 aziridination of olefins, which forms a three-membered nitrogen heterocycle, is one important nitrene transfer reaction. Aziridination shows an advantage over the more classic olefin hydroamination reaction in some syntheses because the three-membered ring that is formed can be further modified. More recently, intramolecular amidation and intermolecular amination of C-H bonds into new C-N bonds has been developed with various metal catalysts. When compared with conventional substitution or nucleophilic addition routes, the direct formation of C-N bonds from C-H bonds reduces the number of synthetic steps and improves overall efficiency.2 After early work on iron, manganese, and copper,6 Muller, Dauban, Dodd, Du Bois, and others developed different dirhodium carboxylate catalyst systems that catalyze C-N bond formation starting from nitrene precursors,7 while Che studied a ruthenium porphyrin catalyst system extensively.8 The rhodium and ruthenium systems are... 

Initial coordination to the nitrogen of the heterocycle guides insertion into the alkenyl C-H bond to form the transition metal hydride. Insertion into the pendant olefin gives the metallobicycle, which reductively ehminates to regenerate the catalytic species and form the cycloalkane. 

The formation of C—N bonds is an important transformation in organic synthesis, as the amine functionality is found in numerous natural products and plays a key role in many biologically active compounds [1]. Standard catalytic methods to produce C—N bonds involve functional group manipulations, such as reductive amination of carbonyl compounds [2], addition of nucleophiles to imines [3], hydrogenation of enamides [4—8], hydroamination of olefins [9] or a C—N coupling reaction [10, 11]. Recently, the direct and selective introduction of a nitrogen atom into a C—H bond via a metal nitrene intermediate has appeared as an attractive alternative approach for the formation of C—N bonds [12-24]. 

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