Imido complexes reactions

Recent investigations into the reactivity of 32 with organoazides have led to the formation of amido and imido complexes. Reaction of 32 with... 

Reactions with Transition-Metal Compounds. The numerous pubhshed products of reactions of transition-metal compounds with a2iridines can be divided into complexes in which the a2iridine ring is intact, compounds formed by reaction of a2iridine with the ligands of a complex, and complexes in which the a2iridine molecule is fragmented (imido complexes). 

Imido Complexes. The reaction of azitidines with tungsten(II) complexes leads to the formation of tungsten(IV) imido complexes (332) ... 

Titanium imido complexes supported by amidinate ligands form an interesting and well-investigated class of early transition metal amidinato complexes. Metathetical reactions between the readily accessible titanium imide precursors Ti( = NR)Cl2(py)3 with lithium amidinates according to Scheme 84 afforded either terminal or bridging imido complexes depending on the steiic bulk of the amidinate anion. In solution, the mononuclear bis(pyridine) adducts exist in temperature-dependent, dynamic equilibrium with their mono(pyiidine) homologs and free pyridine. 

A range of chloride metathesis reactions of the monomeric titanium N,N -bis(trimethylsilyl)benzamidinato-imido complexes have been described. These... 

The Cp Ti(amidinate) fragment provides a particularly useful platform for the synthesis of novel titanium imido complexes and the study of their unusual reactivity. Numerous single-, double-, and cross-coupling and imido-transfer reactions were investigated using these compounds. The synthetic routes leading to CpTi and Cp Ti amidinate derivatives are outlined in Scheme... 

The formation of a bis(guanidinate)-supported titanium imido complex has been achieved in different ways, two of which are illustrated in Scheme 90. The product is an effective catalyst for the hydroamination of alkynes (cf. Section V.B). It also undergoes clean exchange reactions with other aromatic amines to afford new imide complexes such as [Me2NC(NPr )2]2Ti = NC6F5. ... 

The rates of reaction vary with the isomer in the order 3 > 4 > 2. The imido complex may then be hydrolyzed under more vigorous acid conditions to give ammonium ion and the carboxylic acid, probably via the intermediate formation of the acyl complex. In this case the 2-isomer reacts more rapidly than the others. Treatment of the imido complexes with alkali, however, liberates the nitrile (see also Seetion B,l,e) ... 

High-valent iron-imido complexes have also been proposed as reaction intermediates in several reactions of the iron catalysis. Que and coworkers have provided evidence for Fe(IV) imide as a reaction intermediate in the reaction of [(6-PhTPA)-Fe°(CH3CN)2] " with PhI=NTs. Borovik and coworkers have also reported the formation of an amide product involving the generation of a putative iron(IV) imide [36] (Scheme 7). 

Iron-nitrene/imido complexes are proposed to be the reaction intermediates in nitrogen group transfer reactions. The nitrene group can be transferred to organic substrates. Aziridination and amination are the well-known nitrogen atom/group... 

The monomeric terminal imido complexes cp IrNR (R = tBu, SiMe2 Bu, 2,6-Me2C6H3, 2,6-1Pr2C6H3) were prepared from [cp IrCl2]2 and four equivalents of LiNHR in THF.104 All four product species were structurally characterized by X-ray diffraction and exhibit short Ir—N distances (1.712(7)—1.759(3) A) and nearly linear Ir—N C (Si) angles. Some reactions of cp IrN Bu are highlighted in Reaction Scheme 9. Complexes (77) and (78) have been studied by single-crystal X-ray diffraction. 

A first terminal imido complex of nickel (121) was prepared according to Equation (3).468 The synthesis goes via the Ni11 amido compound (122) and uses the steric bulk of the arylimido group for stabilization. The Ni11 center in (121) is planar and three-coordinate. Reaction of (121) with CO or benzyl isocyanide leads to formal nitrene transfer with formation of (124) and (125), respectively. Further reaction with CO liberates the isocyanate and carbodiimide (Equation (4)). 69... 

A very interesting study of the reaction of methyl Grignard reagents with homoleptic imido complexes of Tc(VI) has been performed [104]. Reduction of 91 with Na in THF gave the first homoleptic Tc(VI)-imido complex [Tc2(N-Ar)6] (Ar = 2,6-dimethylphenyl) [105], Under more vigorous conditions dimerization to the second homoleptic Tc(VI)-imido complex [Tc2(N-Ar)4(/i-N-Ar)2] (94)... 

Scheme 22. Reaction of homoleptic dinuclear Tc-imido complex 94 with methyl Grignard reagent...

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... 

Reversible pyridine dissociation yields the non-Lewis base stabilized imido complex [Cp(NHAr) Ti=NAr] (54). This coordinatively unsaturated species undergoes a [2 + 2] cycloaddition reaction with allene to give an azatitanocyclobutane (55), which is then protonated to the /mamido complex. Elimination of enamine occurs followed by isomerization to the energetically more favorable imine. 

The tabulation shows that the reaction favors the mixed oxo-imido complex in each case. The rate constants span a large range. The values of k for Eqs. (55)- (57) are greater than those of kr, often by a large margin. Where determined, AS has large negative values, —134 and —175 JK 1 mol-1 (62). 

With R = R = Ph and using complexes 1 or 2a, the central N -N single bond of the azine is cleaved by both metals. In this case, the bis(imido) complexes 81 were formed, treatment of which with complexes such as CpCo(C2H2)2 can give heterobimetallic bis(alkylideneamido)-bridged complexes such as 82. Mach has used this concept for the reaction of methyl-substituted titanocenes with acetoneazine. With 3, monomeric Ti(III) complexes 83 and, after activation of the methyl groups, coupled products such as 84 could be obtained [44],... 

The niobium imines can also be prepared by reaction of a niobium halo hydride with a nitrile. Thus a niobium reagent prepared from 1 and Bu3SnH reacts with nitriles to form a niobium-imido complex, which is hydrolyzed to a v/c-diamine. 

The kinetic resolution using a chiral zirconocene-imido complex 286 took place with high enantioselectivity to result in chiral allenes 287 (up to 98% ee) (Scheme 4.74) [116]. However, a potential drawback of these methods is irreversible consumption of half of the allene even if complete recovery of the desired enantiomer is possible. Dynamic kinetic resolutions avoid this disadvantage in the enantiomer-differentiating reactions. Node et al. transformed a di-(-)-L-menthyl ester of racemic allene-l,3-dicarboxylate [(S)- and (RJ-288] to the corresponding chiral allene dicarbox-ylate (R)-288 by an epimerization-crystallization method with the assistance of a catalytic amount of Et3N (Scheme 4.75) [117]. 

The nudeophile is activated by the formation of a titanium(IV)-imido complex 19. The next step is a [2 + 2] cydoaddition with one of the jt-bonds of the allene, depending on the regioselectivity leading to either 20 or 22. Compound 20 then delivers 21 by twofold stepwise proto-demetallation and the latter enamine tau-tomerizes to the imine 24 (Scheme 15.3). Compound 22, on the other hand, should provide allylamines 23, but as we shall see, there are no examples of that mode of reaction known so far. 

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. 

---