Iridium electrophilic addition with

Addition to linear 1,1-disubstituted allylic acetates is slower than addition to monosubstituted allylic esters. Additions to allylic trifluoroacetates or phosphates are faster than additions to allylic carbonates or acetates, and reactions of branched allylic esters are faster than additions to linear allylic esters. Aryl-, vinyl, alkynyl, and alkyl-substituted allylic esters readily undergo allylic substitution. Amines and stabilized enolates both react with these electrophiles in the presence of the catalyst generated from an iridium precursor and triphenylphosphite. 

Reactions of allylic electrophiles with stabilized carbon nucleophiles were shown by Helmchen and coworkers to occur in the presence of iridium-phosphoramidite catalysts containing LI (Scheme 10) [66,69], but alkylations of linear allylic acetates with salts of dimethylmalonate occurred with variable yield, branched-to-linear selectivity, and enantioselectivity. Although selectivities were improved by the addition of lithium chloride, enantioselectivities still ranged from 82-94%, and branched selectivities from 55-91%. Reactions catalyzed by complexes of phosphoramidite ligands derived from primary amines resulted in the formation of alkylation products with higher branched-to-linear ratios but lower enantioselectivities. These selectivities were improved by the development of metalacyclic iridium catalysts discussed in the next section and salt-free reaction conditions described later in this chapter. 

The reaction of divalent metals, such as copper, nickel, and so on, with dioxetanes in methanol leads to clean catalytic decomposition into carbonyl fragments/ The reaction rates increase with increasing Lewis acidity of the divalent metal and indicate, therefore, typical electrophilic cleavage of the dioxetane. On the other hand, univalent rhodium and iridium complexes catalyze the decomposition of dioxetanes into carbonyl fragments via oxidative addition. 

Nazarov cyclization. 2-Siloxy-4-aIkenoylfurans fail to undergo Nazarov cyclization in the presence of conventional Lewis acids, but the reaction can be brought forth with addition of an iridium complexHowever, whether the true catalyst is a highly electrophilic silicon species cannot be excluded. 

In 1993 Bergman discovered that an iridium (iii) methyl cation was capable of undergoing an exchange of the methyl group with other alkanes in a process that looked similar to the electrophilic activation of alkanes by Shilov s Pt(n) complex (Equation (17)). Theoretical treatment of this system provided evidence that the actual pathway involved oxidative addition of the alkane to give an Ir(v) dialkylhydride that then underwent reductive elimination of methane. ... 

The mechanism of the reactions of aryl halides cannot occur by the common S 2 patii for the oxidative addition of methyl halides, and most aryl halides lack substituents that would make them sufficiently electrophilic to react by nucleophilic aromatic substitution pathways. As presented in the section on radical pathways for oxidative addition, aryl halides react with metal complexes that readUy imdergo one-electron oxidation by radical mechanisms. However, metal complexes that do not readily undergo one-electron processes tend to react by two-electron mechanisms. Thus, aryl halides typically react with tP" palladium(O) complexes by concerted pathways through three-centered transition states. No strong data for a radical pathway has been gained during the many studies on the oxidative addition of aryl halides to Pd(0). In contrast, evidence that oxidative addition of aryl halides to P, iridium, Vaska-t)q)e complexes occurs by a radical pathway has been published. ... 

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. 

The reaction of organic azides with the iridium(I) carbonyl (XI) to form the nitrogen complex (XXIII) can be interpreted in terms of oxidative addition to form the intermediate (LV) which collapses to the product (XXIII) and an organic isocyanate (34). It is curious that the isocyanate returns to react with the nitrogen complex (XXIII) unless the isocyanate is intercepted by an alcohol. The rate of formation of the nitrogen complex (XXIII) follows the equation rate = ife2[IrCl(CO)(PPh3)2][RN3] 35). The azide must function as an electrophile since acid azides react about 20 times faster than phenyl azide. The postulated intermediate (LV) was not detected kinetically. 

In addition to arylboronic acids, arylboronates have also been successfully used in fluorination reactions (Scheme 7.56) [93]. These boron compounds are attractive substrates since they are typically more robust than other boron species and can often be stored for long periods of time. The catalyst system for this reaction was a copper(I) triflate species along with 2 equiv of silver fluoride. While several sources of electrophilic fluorine generated the aryl fluorides, N-fluoro-2,4,6-trimethylpyridinium hexafluorophosphate was the most effective. The chemistry displayed broad functional group tolerance with the lowest yields obtained with heteroarylboronates. The authors were also able to devise a one-pot borylation-fluorination reaction starting from the parent arene. The arene was converted into an arylboronate through an iridium-catalyzed borylation reaction in the first step of the reaction, while fluorination was achieved during the second step. This is particularly attractive since it facilitates the conversion of unfunctionalized substrates into aryl fluorides. 

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