Imido complexes insertion reactions

Inter- and intramolecular (cyclometallation) reactions of this type have been ob-.served, for instance, with titanium [408,505,683-685], hafnium [411], tantalum [426,686,687], tungsten [418,542], and ruthenium complexes [688], Not only carbene complexes but also imido complexes L M=NR of, e.g., zirconium [689,690], vanadium [691], tantalum [692], or tungsten [693] undergo C-H insertion with unactivated alkanes and arenes. Some illustrative examples are sketched in Figure 3.37. No applications in organic synthesis have yet been found for these mechanistically interesting processes. 

Reactions of bis-Cp titanacyclobutene with nitriles in the presence of PMe3 as a trapping reagent have provided access to crystalline titanocene imido complexes, for which structural details and metrical parameters are reported. When these reactions are performed in the absence of PMe3, azatitanacyclohexadienes or diazatitanacyclooctadienes are obtained as a consequence of alkyl or vinyl insertions (Scheme 548).1439... 

Reactivity of the organometallic Zr(rv) and Hf(iv) tropocoronand complexes is demonstrated by their versatile isocyanide insertion reaction pathways, leading to the formation of 7Z-iminoacyl, enediamido, rf-imme, and /i-imido... 

The lack of alkyne effect on the kinetic hydroamination rate implies that the second pathway (Scheme 5) cannot be considered a major operative route. The zero kinetic order with respect to an alkyne can be predicted for pathway 1 (Scheme 5) and is consistent with the high coordinative unsaturation of the imido complexes, allowing a fast insertion of the different alkynes with indistinct rates. This first pathway can also support the lack of reactivity when performing the reaction with bulky amines as the formation of the corresponding imido complexes is thwarted because of the hindered transition state (13), reaching the highest steric hindrance with BuNH2... 

Insertion and a-elimination reactions of early-metal-amido complexes are presented in Chapters 9 and 10 of this book. In addition to these processes, amido complexes undergo several important reactions that cause elimination of amine or formation of imido complexes. 

Cramer, Panchanatheswaran, and Gilje [37] have also reported that the CP3UCHPR3 compounds undergo insertion reactions with acetonitrile to yield uranium imido complexes (eq.(23)). Diffraction studies... 

Following the isolation of these complexes, all of the mechanistic studies on the carbonylation and reduction reactions of nitroarenes catalysed by Ru3(CO)i2, even in the presence of several promoters, have focused on the reactivity of these or related clusters [157-164]. Moreover, many studies have been also conducted on analogous osmium [165-172] and iron (see paragraph 6.6.) clusters, including insertion reactions of isocyanates, which yield potential intermediates in the carbonylation reaction (Insertion reaction of other cumulenes into the Ru-N bond will not be discussed here. However, see the paragraph of the synthesis of heterocycles later in this chapter). Although not all of the previously mentioned studies were intended to be a basis for a mechanistic understanding of the reactions here discussed, they still contain a lot of information on the possible transformations of amido or imido moieties on a trinuclear cluster. 

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. 

Some homologous monomeric iridium imido complexes have been described by Bergman et al. and their insertion reactions with CO to afford aryl or alkyl isocyanates reported [105]. However, even these eomplexes are not part of any catalytic cycle and will not be discussed. 

The insertion products of diphenylcarbodiimide into Ti(0-i-Pr)4 carry out metathesis reactions at elevated temperatures by insertion of an equivalent of carbodiimide in a head to head fashion followed by an extrusion reaction Also, insertion into a Ti-C bond is observed in the reaction of CpTiMcs with carbodiimides to form CpTiMe2[NRC(Me)NR] Insertion into Ti-C bonds is also observed in the reaction of diisopropylcarbodiimide with imido titanium cations Carbodiimides also insert into Ta-N bonds in a mixed tantalium amido/imido complex... 

The CP3UCHPR3 complexes also undergo insertion into the CsN bonds of nitriles and the C s O bonds of metal carbonyls. Thus, Cramer et al. [39] reported that insertion reactions with acetonitrile yield uranium imido complexes ... 

The mechanistic nature of these nitrene insertion reactions remains unknown. A number of possible routes can be considered (Fig. 302), including (1) direct insertion, (2) initial formation of a metal-nitrene (imido) complex followed by insertion, (3) initial addition of nitrene fragment to sulfur (i.e., oxidation of sulfur) followed by an isomeric rearrangement. On the basis that copper(II) nitrene complexes are unknown (although they have been implicated in the copper-catalyzed aziridination process) this route is ruled out and literature precedent suggests that route (3) is most likely. 

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