Metal-imido

Organometallic complexes of the /-elements have been reported that will perform both intra-and intermolecular hydroamination reactions of alkenes and alkynes, although these lie outside of the scope of this review.149-155 Early transition metal catalysts are not very common, although a number of organometallic systems exist.156-158 In these and other cases, the intermediacy of a metal imido complex LnM=NR was proposed.159,160 Such a species has recently been isolated (53) and used as a direct catalyst precursor for N-H addition to alkynes and allenes (Scheme 35).161,162... 

Cyclopentadienylamine)scandium(2,3-dimethyl-l,3-butadiene) 7 was synthesized in good yield, as shown in Scheme 2. Complex 7 reacted with benzonitrile to form a /rz-imido complex 8, the structure of which was characterized by single crystal X-ray diffraction. This product 8 was proposed to be formed by nitrile insertion followed by an attack of another diene methylene group on the carbon atom of the imido intermediate.3 An unsaturated metal imido species was formed, which easily dimerized to produce 8. However, the yield of 8 was not reported. 

In rhenium compounds that contain two metal-imido ligands, these tend not to occupy positions trans to one another for the same reason. A trigonal-bipyramidal geometry has been identified in 2 and 3, even to the extent that the potentially chelating ligand Ph2PCH2PPh2 acts as a monodentate ligand. 

All three of these reagents have been used to prepare metal imido complexes from metal oxides. In particular the phosphinimines are very useful and have in some cases allowed multiple imido functions to be introduced on to one metal center (equation 51).75... 

The metathetical reaction of C02 with metal (Ti(IV), U(V)) arylimido-complexes led to an arylisocyanate and a metal-oxo species [4g, 115a]. In these processes, which take place under very mild conditions (ambient temperature and pressure), the metal center acts as the sink for unrequired oxygen. A common feature which characterizes these transformations from a mechanistic point of view is the intermediate formation of a metal carbimato-species through the [2+2] cycloaddition of C02 with the metal-imido complex (Scheme 6.23). By cycloreversion, the four-membered aza-metalla-cycle then converts into isocyanate and a stable metal-oxo complex. These processes, at least formally, are reminiscent of the reaction of imino-phosphoranes with C02 to give isocyanates and carbodiimides [115b,c]. 

An excellent review of transition metal imido complexes appeared in 1980 (260). Transition metal imido complexes (NR R = H, alkyl, or aryl) are most common for Second and Third Row metals, but relatively rare for First Row metals. Three classes of imido complex are known. The majority of imido complexes are monomeric containing a terminal imido group which may adopt either a linear configuration (M=N—R) containing a formal metal-nitrogen triple bond as in [Os(N-fBu)03] and rans-[M(NH)X(dppe)2]+ (M = Mo or W X = CH30,... 

In recent years, the metathesis reactions of carbodiimides, catalyzed by metal imido complexes, have been investigated. These reactions are useful to convert symmetrical carbodiimides into carbodiimides with different substituents (see Section 2.4.2.1). 

The activation of C-H bonds by d° metal centers can be investigated very effectively with complexes of the type t-Bu3SiNH)3ZrR, where R is an alkyl or aryl group, as these compounds undergo reversible elimination of hydrocarbon. In these examples, concerted mechanisms involving R-H bond interaction at d° metal imido complexes, either isolable or transient, give metathesis products via R-H elimination. ... 

Most transition metal imido (nitrene) complexes are nucleophilic at nitrogen, but certain species exhibit electrophilic character. Procedures for the catalytic oxyamination and aziridination of alkenes have been developed where imidoosmium and manganese species, respectively, are generated in situ. 

The preparation of A-methoxyaziridincs by generation of methoxynitrenium ion from dimethoxyamine and boron trifluoride is described in Section 7.2.5.9. Finally, the transition metal catalyzed aziridination of alkenes with /V-(4-methylphenylsulfonylimino)phenyliodinane, involving the in situ generation of transition metal imido species, is described in Section 7.2.7.6. 

Hydrolysis of the vinyl groups from the metal center affords olefins. Weiss, Schubert, and Schrock investigated the reaction of the alkylidyne complex W(CCMe3)Cl3(dme) with cyclohexyl isocyanate (208). Two isocyanate molecules are incorporated into the complex as shown in Eq. (212). The reaction was postulated to proceed via a cycloaddition adduct of isocyanate to the metal-carbon triple bond and cleavage of the four-mem-bered ring into metal imido and ketenyl species. Subsequent insertion of a second isocyanate into the metal-ketenyl bond would then give the observed product. 

For the direct carbonylation with group VIII transition metal catalysts two main types of mechanisms have been proposed so far, involving the formation of a metal-imido (e. g.. Structure 4) or a metallacyclic intermediate (e. g.. Structure 5) [3]. 

Scheme 2. Mechanism for the direct carbonylation involving a metal-imido intermediate [3].

In an early publication [16] the carbonylation of nitroaromatics was described as a stepwise deoxygenation of the nitro group, generating an excited singlet nitrene (probably stabilized by coordination on a metal center). Based on this description, the formation of a metal-imido intermediate was usually assumed in most of the proposed mechanisms until the mid-1980s [5, 34-38]. 

Despite several experimental facts [3] rendering a transient metal-imido species a likely source for many products of the carbonylation reaction, its role as an actual intermediate in the catalytic transformation of simple nitroaromatic substrates has never been proven. Accordingly, a type-5 mechanism (Scheme 3, involving no such intermediate) could also be operative for the formation of isocyanate. In this case, an imido complex could also be generated by a parallel minor pathway... 

Nitrogen ligands are particularly versatile in high-valent, early transition metal chemistry. Deprotonation of amines, amides, and hydrazines generates multiply bonding ligands which help to stabilize the high oxidation states. Aspects of metal-amido and metal-imido chemistry have been reviewed.79,80... 

The intermolecular hydroamination of alkynes can be catalyzed by Cp2TiMe2 (Scheme 759). It is assumed that metal imido intermediates are formed.1948,1949 Cp2TiMe2 is an efficient catalyst for the hydroamination of... 

Scheme n.17 Proposed mechanism for the hydroamination/cyclization of primary aminoalkenes involving group 4 metal imido species. 

SCHEME 11.39 Four-center transition state for 1,2-CH addition across metal-imido bond. 

Figure 5 Metal-imido relatives of the zirconocene family...

The formation of alkyne oligomers that are concomitantly formed in the hydroamination reactions catalyzed by the thorium complexes indicates that two possible different complexes can be considered as active, conceivably with inter-conversion causing the occurrence of the two parallel processes. The discernment between these two most probable mechanistic pathways to find the key organometallic intermediate, responsible for the hydroamination process, was achieved by kinetic and thermodynamic studies (Scheme 5). The first pathway proposed the insertion of an alkyne into a metal-imido (M=N) bond, as observed for early transition metal complexes [101]. The second pathway suggested the insertion of an alkyne into a metal-amido bond, as found in some lanthanide compounds [39, 58, 84, 85]... 

Several approaches have been taken to the development of efficient hydroam-ination processes and they have led to catalytic reactions that follow three main types of mechanisms (a) nucleophilic attack on activated metal-coordinated alkenes (applied mainly to late transition metals) [218], (b) cycloaddition of an alkyne and a metal imido moiety [219], (c) insertion into an M-N(amido) bond [207,220]. The subject has been reviewed recently [221]. 

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