Mechanisms, dinitrogen alkylation

Mechanism of alkylation of dinitrogen co-ordinated to molybdenum(O) and tungsten(0)... 

Chatt, J., Head, R.A., Leigh, G.J. and Pickett, C.J. Mechanism of alkylation and acylation of dinitrogen co-ordinated to molybdenum and tungsten J.Chem.Soc. Dalton Trans., (1979), 1638-1647... 

The alkyldiazenido complexes [MoX(N2R)(dppe)2] are generally synthesized by reaction of alkyl halides with [Mo(N2)2(dppe)2]. The reaction mechanism has been elucidated in some detail,140 and is believed to involve a step in common with substitution reactions of the bis(dinitrogen) species, namely rate-controlling reversible thermal loss of N2 (equation 14). The full reaction sequence is set out in equation (15). 

In contrast, the kinetics of the alkylation of trans-[Mo(N2)(L)(dppe)2r (L = CN or SCN) with nBuI show a first-order dependence upon both the complex and the alkyl halide (88). These kinetics and the retention of the ligand, L, in the diazenido product are consistent with the mechanism shown in Scheme 4. The electron-rich complexes (A) undergo rate-limiting, outer-sphere electron transfer to yield the alkyl radical and fra/is-[M(N2)L(dppe)2] (B). Subsequent, rapid attack of the radical on the coordinated dinitrogen of (B) yields the diazenido product (C). 

SCHEME 4. Mechanism of the alkylation of electron-rich dinitrogen complexes (phosphine ligands omitted for clarity). 

If the radical is too unstable e.g. BrCH2CH2 ) it will decompose before N-C bond formation. Alternatively, a radical which is too stable will not attack N2. It appears that alkylation and acylation of coordinated dinitrogen have similar mechanisms. ... 

A-Nitrosohydroxylamine (3.9) gives dinitrogen oxide in the usual way (3-13) via the hydroxydiazonium ion. At a very early time, this mechanism was investigated using N-labeled nitrous acid and O-alkylated hydroxylamines (Leffler and Bottner-By, 1951). 

The reactions of [W(N2)2(diphos)2] with RBr are clearly catalyzed by visible light. The homolytic fission of the R— Br bond that takes place at the metal center is preceded by the loss of one N2 molecule. The resulting C—bond is formed by an alkyl radical attack on the remaining coordinated dinitrogen. Product distribution in these photocatalyzed reactions depends on the solvent and the stability of the free radical. This mechanism is strongly supported by flash photolysis experiments. When gem-dibromides are used in the photo-catalyzed reaction, diazoalkanes are produced. With CH2Br2 for example, the diazomethane complex [WBr(diphos)2(N2CH2)]Br is obtained. More recently it has been shown that some of the diazoalkanes do not react with protonic acids, but that the unique carbon atom is attacked by nucleophiles such as MeLi to yield diazenido complexes. ... 

The mechanism of the secondary alkylation of dinitrogen and the relationship between structure and reactivity in the alkylation of co-ordinated dinitrogen J.C.S. Chem.Comm., (1980), 1024-1025... 

Dinitrogen)hydrodotris(triphenylphosphine)Co(I) has also been used to dimerize ethylene [143]. Here the dimerization of ethylene takes place at room temperature without the use of a Lewis acid. Ethylene conversion decreases with time, presumably due to partial decomposition of the catalyst. However, the decomposition is slow at 0°C. A mechanism has been proposed in which the olefin is first inserted in the cobalt-hydride bond and then a second molecule of the olefin is inserted in the cobalt-alkyl bond. Displacement of the dimer by the olefin regenerates the cyclic process. 

Reactions.— An observation that dinitrogen tetroxide oxidizes amines to the corresponding alkyl nitrates in low yield has been developed into a useful synthetic method. Conversion of an amine into its trimethylsilyl derivative alters the course of the oxidation permitting isolation of the desired nitrate in high yield (Scheme 118) in a later communication, the use of THF as solvent is reported to obviate the need for prior trimethylsilylation. This reaction seems to be general for primary and secondary amines and, although the mechanism of the reaction is unknown, retention of configuration at carbon is observed the intermediacy of the solvated diazonium salt (192) is invoked when THF is solvent. 

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