Mechanism and regiochemistry of azametallacyclobutene formation from imidozirconocene complexes and alkynes

Lee, Sun Yeoul; Bergman, Robert G.

doi:10.1016/0040-4020(94)01117-i

Cited by 53 publications

(1 citation statement)

References 23 publications

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“…While a similar amido mechanism may be operative for the cationic zirconium species, , recent cyclohydroamination studies using neutral group 4 complexes allow the possibility of an imido mechanism (Scheme ) analogous to Bergman’s hydroamination of alkynes/allenes . Nevertheless, while the [2+2] cycloadditions of group 4 imides with alkynes and allenes have been investigated using both experimental − and computational − methods, such a reaction of unstrained alkenes (e.g., at VIII , Scheme ) is otherwise essentially unprecedented. , …”

Section: Introductionmentioning

confidence: 99%

Mechanism of Catalytic Cyclohydroamination by Zirconium Salicyloxazoline Complexes

et al. 2010

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The mechanism of hydroamination/cyclization of primary aminoalkenes by catalysts based on Cp*LZr(NMe(2))(2) (L = κ(2)-salicyloxazoline) is investigated in a range of kinetic, stoichiometric, and structural studies. The rate law is found to be d[substrate]/dt = k[catalyst](1)[substrate](0) for all catalysts and aminoalkenes studied. The overall rate is similar for formation of five- and six-membered rings, and a substantial KIE (k(H)/k(D)) is observed, indicating the involvement of N-H bond-breaking in a rate-determining step (RDS) which is not ring-closure. Remarkably, the reaction proceeds at the same rate in THF as it does in toluene, but added non-cyclizable amine slows the reaction, indicating that while the metal is not acting as a Lewis acid in the RDS, the activated substrate is involved. Also in contrast to other catalysts, increasing steric bulk improves the rate, and the origins of this are investigated by X-ray crystallography. Thermodynamic parameters extracted from eight independent kinetic studies indicate moderate ordering (ΔS(double dagger) = -13 to -23 cal/K·mol) and substantial overall bond disruption (ΔH(double dagger) = 17 to 21 kcal/mol) in the rate-determining transition state. Secondary amines are unreactive, as is a catalyst with a single aminolyzable site, thus excluding an amido mechanism. A catalytic cycle involving rate-determining formation of a reactive imido species is proposed. Stoichiometric steps in the process are shown to be feasible and have appropriate rates by synthetic and in situ NMR spectroscopic studies. The fate of the catalyst in the absence of excess amine (at the end of the catalytic reaction) is conversion to a metallacyclic species arising from CH activation of a peripheral substituent.

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Section: Introductionmentioning

confidence: 99%

Mechanism of Catalytic Cyclohydroamination by Zirconium Salicyloxazoline Complexes

et al. 2010

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Versatile Behavior of β-Heterosubstituted Zirconaindene Complexes: M−Cl Bond Activation (M = Si, P, Au Fragments)

Owsianik¹,

Zabłocka²,

Igau³

et al. 2001

Eur. J. Inorg. Chem.

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Early Transition Metalα-Diazoalkane Complexes

et al. 2000

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The reaction of diazoalkanes with transition metals results either in coordination through the nitrogen atoms or in formation of a carbon ± metal bond. Many N-bonded substituted transition metal diazoalkanes have been studied, [1] but only four series of C-bonded late transition metal compounds have been isolated. [2] a-Metalated diazoalkanes L n MÀC(N 2 )R containing an early transition metal have not yet been reported. Treatment of zirconocene Zr IV complexes with diazoalkanes results in insertion into ZrÀC, [3] ZrÀH, [3c] ZrÀP, [4] and zirconium ± metal bonds [5] to give the corresponding hydrazonato ligands I [Eq. (1)].We have shown that 1-aza-zirconacyclopentene complexes 1 a and 1 b [6] (see Scheme 1) contain a strongly electrophilic metal center (mostly because of the inductive electronwithdrawing properties of the s-bonded nitrogen atom) and a rather nucleophilic imido group that does not share its lonepair electron density with the adjacent metal center because of the strain in the metallacycle. [7] Indeed phosphanes 1 a and 1 b activate CÀH bonds in relatively acidic carbonic acids, such as acetylenes and methylene compounds. [8] Although electrophilic, metal-mediated, aliphatic C À H bond cleavage is precedented, [9] its application to the functionalization of complex substrates is very rare. The present report deals with a novel electrophilic sp 2 -CÀH bond activation and its application for the preparation of a-diazomethylzirconium complexes, which are the first C-metalated diazoalkanes L n MC(N 2 )R with an early transition metal.Treatment of 1 a and 1 b with ethyl diazoacetate (2) in THF at room temperature gave the light yellow, thermally stable solids 4 a and 4 b (Scheme 1) in excellent yields (> 88 %). The 31 P NMR spectra of 4 a and 4 b display a singlet at d 49.4 and 58.5, respectively. The total disappearance of the sp 2 -CH (202.3 MHz, CDCl 3 ): d 116.96; 1 H NMR (500 MHz, CDCl 3 ): d 8.48 (br, 1 H), 7.54 (d, 1 H, J 4,5 7.5 Hz), 7.51 (d, 1 H, J 6,7 7.5 Hz), 7.45 (s, 1 H), 7.19 ± 7.35 (m, 7 H), 6.12 (m, 1 H), 4.90 (m, 1 H), 4.36 (m, 1 H), 3.45 ± 4.00 (m, 8 H), 3.50 (m, 3 H), 3.26 (m, 1 H), 3.13 (m, 1 H), 2.96 (m, 1 H), 2.23 (m, 1 H), 2.01 ± 2.06 (m, 1 H), 1.82 ± 1.95 (m, 7 H), 0.91 (s, 9 H), 0.08 (2 s); HRMS (FAB): calcd for C 40 H 53 N 5 O 7 PSi: 774.3452; found: 774.3455.15: DBU (37 mL, 0.25 mmol) was added to a stirred solution of 12 (38.6 mg, 0.05 mmol) and 3'-O-TBDMS-thymidine (35.6 mg, 0.1 mmol) in dry THF (2 mL) at room temperature. After 20 min, the 31 P NMR spectrum of the reaction mixture showed a single signal at around d 140. The mixture was then loaded onto a short column to filter off DBU. After evaporation of the solvent, the residue was dissolved in dry CH 2 Cl 2 , and Beaucages reagent (10 mg, 0.05 mmol) was added at room temperature. After stirring for 5 min, 31 P NMR spectroscopy revealed a single peak at around d 68. The solvent was removed, the residue was dissolved in THF, and TBAF was added. After 2 h, the solvent was removed in vacuo. CH 2 Cl 2 was added to the resi...

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Mechanism and regiochemistry of azametallacyclobutene formation from imidozirconocene complexes and alkynes

Cited by 53 publications

References 23 publications

Mechanism of Catalytic Cyclohydroamination by Zirconium Salicyloxazoline Complexes

Mechanism of Catalytic Cyclohydroamination by Zirconium Salicyloxazoline Complexes

Versatile Behavior of β-Heterosubstituted Zirconaindene Complexes: M−Cl Bond Activation (M = Si, P, Au Fragments)

Early Transition Metalα-Diazoalkane Complexes

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