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Nitro and Nitroso Metathesis Reactions with Monomeric Zirconium Imido Complexes.

ABSTRACT: The bis(cyclopentadienyl)(tert-butylimido)zir-conium complex 1 undergoes ambient-temperature metathesis reactions with nitro- and nitrosoarenes, providing a rare nonphotochemical synthesis of cis-azoxy and cis-azo compounds, respectively. 2-Nitro-2-methylpropane also undergoes metathesis to give trans-(tert-butylazoxy)-2-methylpropane. This reaction was studied kinetically, and the rate was found to be first order in both 1 and substrate and inversely proportional to the concentration of tetrahydrofuran, with activation parameters DeltaH(double dagger) = 15 +/- 2 kcal mol(-1) and DeltaS(double dagger) = -26 +/- 3 cal mol(-1) K(-1).

SUBMITTER: Blum SA

PROVIDER: S-EPMC1383656 | biostudies-literature | 2004 Aug

REPOSITORIES: biostudies-literature

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Nitro and Nitroso Metathesis Reactions with Monomeric Zirconium Imido Complexes.

Blum Suzanne A SA Bergman Robert G RG

Organometallics 20040801 17

The bis(cyclopentadienyl)(tert-butylimido)zir-conium complex 1 undergoes ambient-temperature metathesis reactions with nitro- and nitrosoarenes, providing a rare nonphotochemical synthesis of cis-azoxy and cis-azo compounds, respectively. 2-Nitro-2-methylpropane also undergoes metathesis to give trans-(tert-butylazoxy)-2-methylpropane. This reaction was studied kinetically, and the rate was found to be first order in both 1 and substrate and inversely proportional to the concentration of tetrahy ...[more]

PMID: 16508695

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Project description:We have investigated both the chemical reduction of (BDI)Nb(V) imido complexes (BDI = HC[C(Me)NAr](2); Ar = 2,6-(i)Pr(2)-C(6)H(3)) to the formal Nb(III) oxidation state and the ability of these Nb(III) complexes to behave as two-electron reductants. The reduction of the Nb(V) species was found to depend heavily on the nature of available supporting ligands, but the chemistry of the reduced compounds proceeded cleanly with a number of unsaturated organic reagents. Accordingly, the novel Nb(V) bis(imido) complexes supported by the monoazabutadiene (mad) ligand (mad)Nb(N(t)Bu)(NAr)(L') (L' = py, thf) were formed by either KC(8) reduction of (BDI)Nb(N(t)Bu)Cl(2)(py) in the absence of strong π-acids or by H(2) reduction of the Nb(V) dimethyl complex (BDI)Nb(N(t)Bu)Me(2) in THF. These products are likely formed though an intramolecular, 2 e(-) reductive C-N bond cleavage, as has been observed previously for related Group 4 systems, suggesting that transient Nb(III) intermediates were present in both cases. In the presence of 1,2-bis(dimethylphosphino)ethane (dmpe), KC(8) reduction of (BDI)Nb(N(t)Bu)Cl(2)(py) was arrested at the Nb(IV) oxidation state to give (BDI)Nb(N(t)Bu)Cl(dmpe), which was characterized by solution-state EPR spectroscopy as a Nb-centered paramagnet with strong coupling to the two equivalent phosphorus nuclei (A(iso){(93)Nb} = 120.5×10(-4) cm(-1), A(iso){(31)P} = 31.0×10(-4) cm(-1), g(iso) = 1.9815). When strong π-acids were used to intercept the thermally unstable Nb(III) complex (BDI)Nb(N(t)Bu)(py) prior to reductive cleavage of the ligand C-N bond, the thermally stable Nb(III) species (BDI)Nb(N(t)Bu)(CX)(2)(L″) (X = O, L″ = py; X = NXyl, L″ = CNXyl; Xyl = 2,6-Me(2)-C(6)H(3)) were obtained in good yields. The Nb(III) complexes (BDI)Nb(N(t)Bu)py, (BDI)Nb(N(t)Bu)(CO)(2)(py) and (BDI)Nb(N(t)Bu)(CO)(2) were subsequently investigated for their ability to serve as two-electron reducing reagents for both metal-ligand multiple bond formation and for the reduction of organic π-systems. The reduction of mesityl azide by (BDI)Nb(N(t)Bu)(py) and diphenylsulfoxide by (BDI)Nb(N(t)Bu)(CO)(2) led to the monomeric bis(imido) and dimeric oxo complexes (BDI)Nb(N(t)Bu)(NMes)(py) and [(BDI)Nb(N(t)Bu)](2)(μ2-O)(2), respectively. MeLi addition to (BDI)Nb(N(t)Bu)(CO)(2)(py) resulted in the formation of a Nb-acylate via methide addition to one of the carbonyl carbons. The acylate product was revealed to have a short Nb-C(acylate) bond distance (2.059(4) Å), consistent with multiple Nb-C bond character resulting from Nb(III) back-bonding into the acylate carbon. The interaction of (BDI)Nb(N(t)Bu)(CO)(2) with two equivalents of 4,4'-dichlorobenzophenone resulted in the clean, quantitative formation of the corresponding pinacol coupling product, but introduction of the ketone in 1: 1 molar ratios resulted in mixtures of the pinacol product and the starting material, suggesting that ketone coordination to the Nb(III) complex may be reversible. Relatedly, addition of 1-phenyl-1-propyne to (BDI)Nb(N(t)Bu)(CO)(2) formed a thermally unstable 1: 1 Nb/alkyne complex, as characterized by NMR and IR spectroscopies; reaction of this species with HCl/MeOH yielded a 2: 1 mixture of 1-phenyl-1-propene and the free alkyne, suggesting a high degree of covalency in the Nb-C bonds.

Dataset Information

Nitro and Nitroso Metathesis Reactions with Monomeric Zirconium Imido Complexes.

Publications

Nitro and Nitroso Metathesis Reactions with Monomeric Zirconium Imido Complexes.

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