Benzimidazole complex, alkene

Another interesting example of dehydrative C-C coupling involves the alkylation of benzimidazole 36 with allyl alcohol 37, which is catalysed by complex 39 [15], The reaction is believed to proceed by alkene complex formation with the allyl alcohol 37 with loss of water from the NH proton of the NHC ligand and OH of the allyl alcohol to give an intermediate Ji-allyl complex. The initially formed 2-allylbenzimidazole isomerises to a mixture of the internal alkenes 38 (Scheme 11.9). 

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. 

Chiral complexes 27 were tested in oxidative Heck-type reactions of boronic acids with acyclic alkenes (Equation (9.2)). Product yields were modest however, enantioselectivities were excellent (9(C98% ee). Other examples of functionalised benzimidazol-2-ylidene Pd complexes include 28 and 29 (Figure 9.6), which required high temperatures and long reaction times to afford reasonable conversions. ... 

Cationic complex 49, related to Crabtree s catalyst, was one of the first NHC complexes used for TH of ketones, alkenes and nitro compounds in refluxing isopropanol (Figure 13.Related neutral complexes 50 were also very active in the reduction of acetophenone in KOH/z-PrOH." Complexes of Rh and Ir with benzimidazol-2-ylidenes 51 provided 99% yield after 90min in the TH of acetophenone and cyclohexanone in KOH/z-PrOH." Such activity was attributed to the hemilabile-OMe group. Also, pyridinylimidazol-2-ylidene Ir complex 52 was applied to the reduction of benzophenone and p-nitrobromobenzene." ... 

An interesting application of the non-innocent behavior of NHCs at transition metal centers was developed by Bergman and Ellman. They found that cyclization of substituted benzimidazoles was efficiently catalyzed by [RhCl(COE)2]2/PCy3 at 135 °C (COE = cyclooctene). Mechanistic studies isolated a square planar NHC-rhodium(i)/alkene complex 71 as the resting state of the catalyst. Calculations proposed the insertion of the alkene moiety into the NHC-Rh bond to form a zwitterionic intermediate, 72, to be the rate determining step with a computed activation barrier of +47 kcal mol (Scheme 4.22a). The N to Rh proton transfer in 72 formed 73, from which reductive elimination of the cyclized product could occur, this being N-bound to Rh in 74. Similar results were obtained with a related hydropyrimidine-based system. ... 

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