Imido mechanism

Scheme 4 Imido mechanism for intermolecular hydroamination of alkynes...

Scheme 5 Imido mechanism for hydroamination/cyclization of aminoalkenes...

More recently, neutral zirconium-based catalysts capable of performing reactions with both primary and secondary amines in intra- [55-57] and intermolecular [57, 58] reactions were reported. The imido mechanism is obviously impossible, and an insertion mechanism, similar to the lanthanide-like mechanism shown in Scheme 2 was proposed [55]. The isolation of an insertion intermediate in an intermolecular alkyne hydroamination reaction is compelling evidence in favor of the insertion mechanism [58]. 

Only a limited number of organoactinide catalysts have been investigated for the hydroamination/cyclization of aminoalkenes (Fig. 4, Table 2) [55, 96-98]. The constrained geometry catalysts 11-An (An = Th, U) show high activity comparable to the corresponding rare earth metal complexes and can be applied for a broad range of substrates [55, 96, 97]. The ferrocene-diamido uranium complex 12 was also catalytically active for aminoalkene cyclization, but at a somewhat reduced rate [98]. Mechanistic studies suggest that the actinide-catalyzed reaction occurs via a lanthanide-like metal-amido insertion mechanism and not via an imido mechanism (as proposed for alkyne hydroaminations), because also secondary aminoalkenes can be cyclized [55, 98]. 

Further reduction and protonation of the imido group leads to the formation of a Mo(I) ammine complex (8b) in a mechanism similar to the fluoro system in the classic Chatt cycle. The Mo(I) ammine complex 8b is first reduced to the corresponding Mo(0) complex 8c at which stage the ammine ligand is exchanged with... 

The latter transformation requires the use of a small amount of an acid or its ammonium salt. By using [Cp2TiMe2] as the catalyst, primary anilines as well as steri-cally hindered tert-alkyl- and sec-alkylamines can be reacted.596 Hydroamination with sterically less hindered amines are very slow. This was explained by a mechanism in which equlibrium between the catalytically active [L1L2Ti=NR] imido complex and ist dimer for sterically hindered amines favors a fast reaction. Lantha-nade metallocenes catalyze the regiospecific addition of primary amines to alkenes, dienes, and alkynes.598 The rates, however, are several orders of magnitude lower than those of the corresponding intramolecular additions. 

Both expressions predict a rate maximum (as observed), but the exact mechanism cannot be inferred from the evidence above. Other cases of a similar kind are the hydroxylaminolysis of formamide35 which exhibits a pH-rate maximum at pH 6.2-6.5 and the reaction of amines with imido esters to give amidines36. Both reactions show kinetics consistent with the presence of a tetrahedral... 

Zirconium imido complexes have been used to carry out S 2 reactions of allylic chloride, bromide, iodide, and alkyl, aryl, and trimethylsilyl ethers in high yields at room temperature.12 The syn stereochemistry, an inverse secondary (k /k Oy = 0.88 obtained using the ( )-l-(r-butyldimethylsilyloxy)-3-deuterioprop-2-ene and the rate expression led the authors to suggest the reactions occurred via the mechanism in Scheme 4 with transition state (9). 

Scheme 68) <86H(24)1565>. The most likely mechanism of this reaction involves radical anion formation (414), cleavage, successive electron addition, and cyclization of the intermediary imido-ester (415) to an imidazole derivative (416) (Scheme 68). 

Although the mechanism of reduction of N2 in biological systems is by no means clear, the reactions of coordinated N2 in simple systems are reasonably well understood. The principal types of reaction sequence involve hydrazido, imido, and nitrido species (see Section 9-19). 

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. ... 

Heine ° has pointed out that this reaction represents an example of the reverse of the reaction on pyrolysis of imido esters. Heine has recently shown that the pyrolytic isomerization of cis- and rra j-l-p-nitrobenzoyl-2,3-diphenylaziridines into 2-p-nitrophenyl-4,5-diphenyl-2-oxazolines is a stereospecific process. These results are consistent with a mechanism that involves either a four-membered transition state or a short-lived tight ion-pair intermediate that collapses to the oxazoline before racemisation can occur . The pyrolysis of l,3-diaroyl-2-aryl-aziridines results in a different kind of reaction, in which a-benzamidobenzal-acetophenones are produced, viz. 

In the managanese system [Mn3(i7-C5Me5)3(/i,-NO)3(/i,3-NO)] (279), very similar chemistry is observed but in addition, reduction of the nitrosyl group occurs, yielding the imido cluster [Mn3(i7-C5Me5)3(/u,3-NH)(/i2-NO)3] in low yield. Such a reaction undoubtedly proceeds by a nitrido intermediate, though the exact mechanism is, at present, unknown. 

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... 

A vanadium(II) tmeda complex with 2,6-dichlorothiophenolate has been reported with both S- and Cl-donor atoms (217).874 The thiolate donor atoms are trans to each other while the Cl atoms are trans to the N atoms in the tmeda ligand.874 The complex will react with azobenzene to give the phenylimido complex where one of the chloro-donor ligands is now trans to the imido nitrogen (218) 874 this complex is structurally similar to other imidovanadium(IV) complexes.643-646 A V11 cysteine methyl ester complex has been shown to reduce water to H2 but the mechanism of this process is not known.875... 

Addition of 2 equiv. of primary amines RNH2 to a suspension of Cp TiCl3 and phosphoalkynes P=CR in toluene at room temperature gives azaphosphatitanacyclobutane derivatives (Scheme 218). The mechanism of the formation of these complexes is thought to proceed via Ti-imido intermediates.595... 

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