Zirconium imido complexes

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. 

Amino alcohol-N-oxides as precursors of chiral oxazolidines synthesis of (R)-(-)-cryp-tostyline I. Heterocycles 1993, 36, 1763-1769. Blum, S. A. Walsh, P. J. Bergman, R. G. Epoxide-opening and group-transfer reactions mediated by monomeric zirconium imido complexes./. Am. Chem. Soc. 2003, 325, 14276-14277. 

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). 

The reactivity of zirconium imido complexes supported by a dibenzotetraaza[14]annulene (taa) macrocycle ligand toward a range of unsaturated substrates has been examined. Reaction with t-butyl isocyanate cleanly generated the cycloaddition product Zr N(2,6-( -Pr)2C6H3)C(0)N(t-Bu) (Me4taa) (equation 21). ... 

Zirconium imido complexes, generated by thermolysis of zirconium alkyl amides and bis(amides)104 or tetrazenes105 in the presence of excess norbornene, gave the [2 + 2] cycloaddition products with exo configuration, e.g., 11 (X-ray)104. 

Bergman has also reported an example of C-H addition to a zirconium-nitrogen double bond. The complex Cp2Zr(NHR)Me loses methane to generate an imido complex that can either be trapped with THF or reacted with benzene (Eq. 19). No reactions with alkanes were reported [86]. 

S. A. Blum, V. A. Rivera, R. T. Ruck, F. E. Michael, R. G. Bergman, Synthetic and mechanistic studies of strained heterocycle opening reactions mediated by zirconium(IV) imido complexes, Organometallics 24 (2005) 1647. 

Group 4 bis(amidate)bis(amido) complexes have also been identified as precatalysts for the more challenging hydroamination of alkenes. The majority of investigations in this field focus on the intramolecular cychzation of aminoalkenes with zirconium-based catalysts. [64e] Neutral group 4 bis(amidate) zirconium amido or imido complexes are efficient precatalysts for the intramolecular cychzation of primary amines to form pyrrolidine and piperidine products (Scheme 12). The monomeric imido complex can be generated by reaction of the bis(amido) complex with 2,6-dimethylaniline and trapped with triphenylphosphine oxide. [64e] The bis(amido) and imido complexes... 

Wolczanski and Bergman concurrently showed that the addition of hydrocarbon C-H bonds occurs to zirconium(rV) imido complexes, as shown in Equations 6.55 and 6.56. - The shylamidozircononium-imido complex reacted with both alkanes and arenes, whereas the zirconocene-imido complex reacted with arenes. Kinetic studies showed that the generation of the unsaturated zirconium-imido complex is rate limiting and that the unsaturated zirconocene compoimd is generated reversibly by dissociation of THE prior to the C-H bond cleavage process. Most likely, this reaction occurs by formation of an alkane or arene complex and subsequent reaction of the alkane or arene complex to generate the amido alkyl or amido aryl complexes. - ... 

The [2+2] reactions of the zirconium-imido compounds with alkynes and alkenes occurs by a mechanism similar to that for the [2+2] reactions of carbenes with alkynes and alkenes. The alkene or alkyne first binds to an open coordination site at the metal, and this coordination is followed by conversion of the alkyne or alkene complex to the metallacyhc product (e.g. Equation 13.76). Thus, the [2+2] reaction requires a 16-electron intermediate to bind the olefin or alkyne, even though the metallacyHc product and the imido complex have the same overall electron count. In support of the coordination of alkyne or alkene, albeit weakly, to the d° metal center, the rate of the reaction of alkynes with the 18-electron zirconocene-imido compound containing bound pyridine-N-oxide was inhibited by added pyridine-N-oxide (Equation 13.76). ... 

The first hydroaminations by this mechanism were reported by Bergman with zircono-cene complexes and by Livinghouse with monocyclopentadienyl titanium and zirconium complexes. Bergman reported the intermolecular addition of a hindered aniline to an alkyne. The hindrance of the aniline was important to prevent formation of stable dimeric complexes containing bridging imido groups. Livinghouse reported intramolecular reactions that occurred at lower temperatures over shorter times. The intramolecularity of this process allows the [2+2] cycloaddition of the imido complex with the alkyne to be faster than the dimerization. 

Bergman et al. reported that imido complex of zirconium (S,S)-97 was able to resolve racemic allenes even at room temperature [70]. The reaction proved to be... 

Efficient reactivity with secondary amines points clearly toward alternative mechanistic profiles from the weU-estabhshed [2-E2] cycloaddition of a metal imido to a C-C multiple bond. Indeed, early computational work on a cyclopentadienyl system suggested that while the [2-E2] cycloaddition step is energetically feasible, the subsequent protonolysis step for catalyst turnover is turnover-limiting [40, 59]. More recently, Scott and coworkers [60] showed experimentally that proton transfer in the release of product and generation of the requisite zirconium imido complex can be turnover-limiting (Scheme 15.10). 

As noted previously, Sadow and coworkers initially proposed the turnover-limiting, proton-assisted C-N bond formation for an impressively reactive zwitterionic-zirconium catalyst While this catalyst is limited to primary amine substrates, consistent with an imido complex playing a key role in the catalytic cycle, careful deuterium-labehng experiments suggests that proton-assisted C-N bond formation is involved in the turnover-Umiting step (Scheme 15.12). 

Reduction of Cp2ZrCl2 with 1.5equiv. of sodium amalgam affords the fulvalene complex CpZr(/r-Cl) 2(/r-C5H4-C5H4) 28.35 COMC(1995) summarized a number of phosphido, sulfide, imido, and alkyl complexes derived from 28. Cuenca and co-workers have since reported further investigations into the chemistry of 28, leading to new zirconium(iv) /(-imido. //-(r/2.r/2-N.N-hydrazido), and //-(r/ -C r/2-C,N-isocyanido) dinuclear fulvalene derivatives.36... 

The reaction involves initial formation of the zirconium-imido species, followed by [2-1-2] cycloaddition with the C—C unsaturation (Scheme 13). This is consistent with the observation that bis(amidate) complexes do not mediate hydroamination with secondary amine containing substrates. The cyclic transition state of the intramolecular reaction determines the regioselectivity of the reaction followed by successive protonation of the intermediate metallacycle and release of product to regenerate the catalyticaUy active imido species. 

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