Ru-catalysed Alkene Metathesis

These series of investigations highlight several issues that need to be considered when undertaking computational work, especially when this involves modelling quantitative experimental data. As well as a consideration of a variety of different functional approaches, the large systems employed here presented significant conformational flexibility and steps must be taken to ensure the lowest energy 

Lord et al. studied A-substituent effects (R = H, Me, F) on the barriers for migratory insertion with both saturated and unsaturated NHCs. For the saturated species low barriers were computed to follow the trend R = F Me H. In contrast, the unsaturated species were both more sensitive to the A-substituent and showed the reverse trend (R = Me AA = +2.4kcal moC R = F ISE = +7.6kcalmol ). 

An interesting application of the non-innocent behaviour of NHCs at transition metal centres was developed by Tan et alF They found that cyclisation of substituted benzimidazoles was efficiently catalysed by [RhCl(COE)2]2/PCy3 at 135 °C (COE = cyclooctene). Mechanistic studies isolated a square planar NHC-rhodium(I)/alkene complex 48, as the resting state of the catalyst. 

One of the first mechanistic computational studies on NHC-TM chemistry was published in 1998 by Rosch on the Heck reaction.This study employed a simple diaminocarbene (DAC) model ligand and characterised the alkene insertion and p-H elimination steps of the cationic pathway, derived from a cis-[(DAC)2PdBr(Ph)] intermediate via Br /C2H4 substitution. The neutral pathway via initial DAC/C2H4 substitution) was discounted on the basis of the non-observation of free NHC experimentally and the fact that cleavage of the NHC Pd bond would be prohibitively high in energy. A mixed 

54 Favoured by Modei 1 (R = R = H) Favoured by Modei 2 and observed experimentally 

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