IMes chloride salt

Handling of the air- and moisture-sensitive carbene can be avoided through use of the easier-to-handle, commercially available chloride salt (IMesHCl), which can be deprotonated in situ to generate the free carbene Hgand. Independent studies conducted by Herrmann confirmed the observed high activity of IMes-based palladium complexes for the coupling of aryl chlorides [61]. 

The most effective ligand for these processes is not IMes, but instead the more bulky 2,6-diisopropylphenyl-substituted carbene, IPr, which was generated in situ from commercially available IPrHCl. Use of the preformed carbene, rather than in situ generation from the chloride salt, leads to significantly worse results. Both, alkenyl- and alkynyl-9-BBN derivatives are also suitable partners in these cross-couplings. 

Palladium NHC systems for the hydrodehalogenation of aryl chlorides and bromides and polyhalogenated aromatic substrates originate from about the same time as the first reports on Ni chemistry, and show many similarities. Initial efforts showed that the combination of PdCdba) (2 mol%), one equivalent of imidazolium chloride and KOMe produced an effective system for the reduction of 4-chlorotolu-ene, especially upon use of SIMes HCl 2 (96% yield of toluene after 1 h at 100°C) [7]. Interestingly, higher ligand to metal ratios severely inhibited the catalysis with only 7% yield of toluene achieved in the same time in the presence of two equivalents of SIMes HCl 2. Variation of the metal source (Pd(OAc)2, Pd(CjHjCN)jClj), alkoxide (NaOMe, KO Bu, NaOH/ ec-BuOH) or imidazolium salt (IMes HCl 1, IPr HCl 3, lAd HCl, ICy HCl) all failed to give a more active catalyst. 

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