Rhodium-amido complex

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. 

Benzene—dienes, with pentarutheniums, 6, 986 Benzen/if/indenyl—amido complexes, with Ti(IV), 4, 438 j4-Benzenes with iridium, 7, 328 rhodium, 7, 181... 

The zirconocene bis(arylamido) complex 787 was obtained by the reaction of Cp2ZrCl2 with 2 equiv. of the lithium amide605 (Scheme 195). When the reaction is carried out in a 1 1 ratio, the monoamide zirconocene chloride is generated as the major product. Reaction of in situ-generated Cp 2Zr with 2-(methylmercapto)aniline yields monoamido zirconocene hydride 788, the spectroscopic data of which suggest an interaction between the S atom and the Zr center in this complex. The bis(amido) complex 787 serves as a precursor for the synthesis of amido rhodium and iridium complexes. 

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. 

Reaction of CO with the tautomeric mixture of the two aforementioned rhodium complexes (several / flra-substituted imidoaryl groups were tested) afforded a unique bridging isocyanate complex Rh2(CO)2(ii -N,T] -C, x-ArNCO)(p-DPPM)2. The CO insertion is irreversible. Since the two initial tautomers are in equilibrium in solution, insertion of CO may in principle proceed by either of the two (Scheme 20)(next page). However, evidence was given in favour of the amido-path (path b in the Scheme), based on the fact that the cationic complex [Rh2(p-NHPh)(CO)2(DPPM)2] rapidly reacted with CO. No complex could be isolated from this last reaction, but the formation of PhNCO was detected. Two features of this mechanism are worth of note. The first is the contrast between the conclusion reached for this system (amido complex more reactive than imido one in the insertion reaction of CO) and the one reached by Bhaduri et al. [161] for the trinuclear complex Ru3(p-H)(p-NHPh)(CO)io, which, upon deprotonation of the amido group by OH, affords the inserted product [Ru3(p-H)(T] -N,ii -C,p3-PhNCO)(CO)9]. The difference is likely due to the fact that, in this latter case, the complex is trinuclear, so that the inserted CO is already coordinated to the third ruthenium atom and, especially, the formation of the new C-N bond does not require the breaking of any of the pre-existing Ru-N bonds. 

An unusual reaction in which a nitrile, RCN, is reduced to an amine, RCH2NH2, has been reported. The reaction of a-[Re2Cl4(dppbe)2] [dppbe = l,2-6w(diphenylphosphino)benzene] with RCN in the presence of HCl yields the amido complex [ReCl3(NCH2R)(dppbe)]. The reaction involves the disproportionation of the dirhodium(II) complex into a rhodium(I) and rhodium(III) species, followed by the reduction of the nitrile by rhodium(I). ... 

Whatever the route to a rhodium dihydride alkene complex, the hydrogen must be transferred sequentially to the double bond. It had always been assumed that the first C-H bond is formed / to the amido-group, so that the more stable Rh-substrate chelate is formed. This is the alkylhydride isomer observed in stoichiometric NMR studies at low temperatures, and is supported by studies under catalytic turnover conditions, assuming a normal isotope effect... 

The cyclohexyl derivative of Diop, 4.5-bis(dicyclohexylphosphinomethyl)-2.2-dimethyT1.3-dioxolane (CyDiop). is a much better cocatalyst in rhodium complexes than Diop itself in the catalytic hydrogenation of oxo groups a to amido functions. 

Concerning ring size expansion, since the first anionic sbc-membered example derived from malonic acid described by Cesar et al. [8], other sophisticated five-[5a,9], sbc-[10], and seven-membered [11] NHCs have been described and coordinated to rhodium and iridium, including sbc-membered amino/amido carbenes [10b] and caffeine-based diamino carbenes [12]. Also related to this type of ligands, Conejero and coworkers described a general method for the preparation of 2-pyridylidene-based rhodium complexes by decarboxylation of pyridinium car-boxylates [13], a method previously described by Crabtree and coworkers [14]. 

An especially important case is the enantioselective hydrogenation of a-amido-acrylic acids, which leads to a-aminoacids. A particularly detailed study has been carried out on the mechanism of reduction of methyl Z-a-acetamidocinnamate by a rhodium catalyst with a chiral diphosphine. It has been concluded that the reactant can bind reversibly to the catalyst to give either of two complexes. Addition of hydrogen at rhodium then leads to a reactive rhodium hydride and eventually to product. Interestingly, the addition of hydrogen occurs most rapidly in the minor isomeric complex, and the enantioselectivity is due to this kinetic preference. 

For example, several pincer ligands with a central amido moiety, such as those of type 11, have been developed by several groups and their complexes with metal centers such as paUadium(II), platinum(II), and rhodium(I) have been characterized. The aim was on the one hand to synthesize robust, electron-rich organometaUic compounds which easily undergo oxidative addition at the metal center and, on the other hand, to develop bifiinctional catalysts in which the central amido moiety can cooperatively act with the metal center in a catalytic event, for example, by accepting/releasing protons... 

Tye JW, Hartwig IF (2009) Computational studies of the relative rates for migratory insertions of alkcmes into square-planar, methyl, -amido, and -hydroxo complexes of rhodium. J Am Chem Soc 131(41) 14703-14712... 

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