Hoffmann elimination mechanism

The molecular mechanism of the Hoffmann elimination involving (iV-Cl)-N-methyl-ethanolamine has been theoretically characterized by using DFT at the B3LYP/ 6-31++G computing level.49 The role of water as a solvent has been analysed by using both discrete and hybrid discrete-continuum models. The rearrangement proceeds by a water-assisted asynchronous concerted mechanism. 

Fig. 1.14. Mechanisms of C-N bond breaking, (a) Hoffmann elimination and (b) Nucleophilic substitution 29...

A carbanionic mechanism similar to that proposed by Wittig and Krauss (33) involving a Steven s rearrangement can reasonably follow (Equation 5), i.e., an intramolecular attack on ylide (III) methyl forming dimethylethylamine. Protonation of dimethylethylamine followed by Hoffmann elimination (Equation 5a) would yield ethylene. 

Although the experimental evidence suggests that the observed photogeneration of hydrogen may not involve homogeneous metal dithiolene photocatalysts, a theoretical study by Alvarez and Hoffmann addressed possible mechanisms for hydrogen elimination from ds square-planar bis(dithiolene) complexes (77). Concerted elimination of H2 from protonated sulfur atoms in the complex was proposed to be a thermally forbidden but photochemically allowed pathway, and protonation of a metal hydrido complex was also considered, as shown in Scheme 3. 

In the case of HDN, an additional interesting possibility also consistent with the heterolytic mechanism arises, since substrates like the pyridines -or intermediate alkyl or aryl amines- are sufficiently basic to promote the activation of hydrogen so as to form a metal hydride plus a protonated base (e.g. a pyridinium or an alkylammonium cation). Furthermore, some of the most widely accepted amine HDN mechanisms include the initial protonation of the amine nitrogen, followed by elimination of ammonia from the ammonium cation. Therefore, it is very easy to combine the idea of a heterolytic hydrogen activation promoted by, say n-pentylamine, with a subsequent degradation by a Hoffmann mechanism, to conform a reasonable HDN catalytic cycle. A simplified representation of this idea is given in Fig. E4. 

RX - [Ni(I), RX ], is superseded by more examples of the four types of processes concerted, radical, radical-chain, and two-step ionic. Under the mantle of reductive elimination, perhaps not quite correctly, /3 elimination of hydrogen is included since this process seems able to take place under very similar conditions to those under which a concerted reaction occurs. It is interesting to see the use of Hoffmann-type calculations in rationalizing elimination reaction pathways. Mention should be made also of the mechanism of processes involving two metal centers. 

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