Dinitrogen elimination

The first carbene ever isolated was la, which was prepared using the most classical route to transient carbenes, namely, the decomposition of diazo compounds. The [bis(diisopropylamino)phosphino](trimethylsilyl)diazomethane precursor (la) was obtained by treatment of the lithium salt of trimethylsilyldiazomethane with 1 equiv of bis(diisopropylamino)chlorophosphine. Dinitrogen elimination occurs by photolysis (300 nm) or thermolysis (250 °C under vacuum) affording carbene la as a red oily material in 80% yield (Scheme 8.1). Carbene la is stable for weeks at room temperature and can even be purified by flash distillation under vacuum (10-2 Torr) at 75-80 °C. 

In addition, it has been demonstrated that the reaction may take a different course depending on the nature of the dienophile [167b]. Thus, cyclopropene reacts as expected in the first step however, upon dinitrogen elimination, pyran 447 forms. Reaction with a deficiency of cyclooctyne produces furan 448 in 41% yield. However, when excess of cyclooctyne is used, the product of the tandem cycloaddition 449 forms in 74% yield (Scheme 16.90). 

This procedure for the synthesis of ethyl 3-nitroacrylate is essentially that of Stevens and Emmons.2 Four major changes have been introduced by the submitters rapid introduction of dinitrogen tetroxide no purification of the intermediate nitro iodo ester use of dry, finely powdered sodium acetate for elimination of hydroiodic acid and percolation of the final product through a mat of alumina. With these modifications, the preparation is reproducible and highly efficient (80-90% overall). 

In conclusion, it is very likely that the influence of solvents on the change from the heterolytic mechanism of dissociation of the C —N bond in aromatic diazonium ions to homolytic dissociation can be accounted for by a mechanism in which a solvent molecule acts as a nucleophile or an electron donor to the P-nitrogen atom. This process is followed by a one- or a two-step homolytic dissociation to an aryl radical, a solvent radical, and a nitrogen molecule. In this way the unfavorable formation of a dinitrogen radical cation 8.3 as mentioned in Section 8.2, is eliminated. 

The [MoOI(prP4)] precursor 3 was finally converted to the corresponding dinitrogen complex by electrochemical reduction with a Hg pool electrode in the presence of dinitrogen and phenol. The latter reagent was added as a weak acid to induce protonation of the oxo group and subsequent elimination as water. The blue solution of the Mo oxo complex thereby turned... 

A [4 + 2]-cycloaddition reaction of 1,3,4-oxadiazole 195 was followed by isomerization and elimination of dinitrogen to provide a pyrrole [160]. 

Apparently, the alkylated sulfonylhydrazines fragmented with the spontaneous elimination of o-nitrobenzenesulfinic add to form the diazenes followed by the loss of dinitrogen leading to 15. A benzannulated analog was likewise synthesized [30]. 

Ta1 adducts with ethylene have been obtained as highly air sensitive solids by reduction of the corresponding Ta111 compounds under argon (equation 85),292 or by reductive elimination of H2 from [TaH2ClL4] (Scheme 9). A similar procedure, but under dinitrogen, gave Tav nitrenes (Section 34.2.3.6). The same Tam precursor (60) provided o alkyl derivatives (equation 86). Complex (63) catalyzes the selective dimerization of ethylene to 1-butene. 

These are not accessible from coordinated dinitrogen and the preparative routes involve alkylation (equation 172) or protonation (equation 173) of hydrazido(2—) precursors or direct use of the hydrazine with elimination of hydrogen halide (equation 174). 

The suggestion for the catalytic cycle is presented in Scheme 4.18. Initially, an unsaturated iron complex is formed by expulsion of both dinitrogen molecules. Next, coordination of the alkene takes place, which is preferred since activation of hydrogen is also feasible. After alkene coordination, oxidative addition of hydrogen yields a formally 18-electron complex. Insertion of the alkene gave an alkyl complex, which recreated the starting complex via reductive elimination. Notably, the alkene complex also supports an isomerization of the double bond, hence an extension of possible intermediates is conceivable. 

M-1s-1) to form RhCl(PPh3)2 and RhCl(H2C=CH2)(PPh3)2, respectively however, reaction with H2 to form the dihydride is much slower, (1 x 10 M-- -s-- -). Also described are flash photolysis studies of the dinitrogen species IrCl(N2) (PPI13) 2 and the dihydride H2lrCl(C0)(PPh3)2- In both cases, the transient IrCl(PPh3)2 is formed. These results indicate that CO labilization from the Ir(lII) dihydride is a facile photochemical pathway and the photo-reductive elimination of H2 is a more complicated mechanism than previously inferred. 

The reaction of cycloalkeno-l,2,3-selenadiazole 183 with a mixture of [Pd2(dba)3] and trialkylphosphine in toluene under reflux for 1 h gave the novel complexes 51 in 36-55% yields (dba = dibenz[ ] anthracene) <2004POL2967, 1998CC1305>. The molecular structure of complex 51 (n = 1, R = Bu) was determined by X-ray crystallography. The proposed mechanism is shown in Scheme 18. Insertion of palladium(O) into the selenium-nitrogen bond of 1,2,3-selenadiazole occurs, followed by 1,3-dipolar addition of a selenaketocarbene 185 formed in situ by thermal elimination of dinitrogen with the elimination of a trialkylphosphine. 

On the other hand, the reaction of 1,2,3-selenadiazole 166 with 1 equiv of Pt(PPh3)4 in toluene at 140 °C (3 h) led to the formation of new selenoplatinum complex 52 in 35% yield (Equation 13) <2005TL1001>. This reaction may involve the insertion of di(triphenylphosphino)platinum into the selenadiazole ring, followed by 1,3-dipolar addition of an intermediate formed in situ by thermal elimination of dinitrogen with the elimination of triphenylphosphine, similar to the reaction with [Pd2(dba)3] and trialkylphosphine described above. The structure of complex 52 was established by X-ray analysis. Complex 52 is a selective catalyst for the hydrosilylation of terminal alkynes. 

For the concerted decomposition of N-nitroso compounds in (58), we require only that the orbitals labeled and three-membered ring and forbidden for the five-membered ring. The elimination of dinitrogen oxide from N-nitrosoaziridines is syn stereospecific (Clark and... 

Because of the dependence of the PL intensity of TiC>2 on the nature of the gas-phase molecules introduced (alcohols) and its reversibility upon elimination of the molecules by flowing dinitrogen, there is hope that such an effect can be applied to gas sensors. With the combined use of several techniques (PL, time-resolved femtosecond diffuse reflectance spectroscopy, multiple internal reflection IR absorption), the dynamics and role of photogenerated electrons and holes in the absence or presence of metals (notably platinum) are now better understood, at both the gas-solid and liquid-solid interfaces. It is also likely that not only TiOz, but other types of semiconductors will be more thoroughly investigated in the future. 

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