Titanocene fixation-reduction

Titanocene chemistry provides an appropriate basis for a discussion of mechanism. Figure 4 shows a recent interpretation of the titanocene fixation-reduction process (13) submitted for publication by the Olives three or four months after our titanocene-N2 paper appeared (II). The mechanism is a complicated one, based essentially on ESR results meas-... 

Figure 4. Proposed titanocene fixation-reduction process...

Figure 5. Evaluation of proposed titanocene fixation-reduction mechanism...

Sulphurated borohydrides can reduce oximes to amines the intermediate hydroxylamines are isolable. Sodium borohydride reduces nitriles to amines in high yield in the presence of Raney nickel as catalyst. The titanocene-promoted fixation-reduction of molecular nitrogen has been utilized in the conversion of ketones into amides and acid chlorides into nitriles in an overall reductive deoxygenation, as exemplified in Scheme 109. 

Secondly, in the Olive mechanism, the fixing species is formed by a three-electron change starting with titanocene dimer. We subjected ti-tanocene to a three-electron naphthalenide reduction and have not been able to observe any nitrogen fixation of any kind with the resulting species. 

Finally, the Olive paper demands an ammonia/titanium ratio of 1 1. In our titanocene experiments involving N2 fixation and reduction, we clearly get an ammonia/titanium ratio of 2 1. The early hydrogen abstraction work which we reported may seem loosely to fit the Olive scheme but is a very slow reaction — 3 weeks), undoubtedly occurring subsequent to the fixation operation but possibly accounting for the Olive experimental observations. I think the very rapid reaction —fiO min) that is normally observed does not conform to the Olive interpretation but is actually a titanium (II)-induced fixation without hydrogen removal from solvent. 

Recently, the unusual behavior of reaction systems that involve titanocene toward the normally quite inert molecule N2 has been the subject of considerable interest. The fixation and reduction of molecular nitrogen has been observed in mixtures of titanocene dichloride and ethyl magnesium bromide and in other systems containing titanocene derivatives (100-108). 

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