N-heterocyclic silylene complexes in catalysis: new frontiers in an emerging field

The present account is a review of all N-heterocyclic silylene (NHSi) transition metal complexes that have been employed in catalytic transformations, reported up to the present time (2013). NHSi transition metal complexes now enjoy indefatigable attention since their facile isolation was realised by the report of the first isolable NHSis by West and Denk in 1994. Despite considerable research activity since then, in comparison to ubiquitous N-heterocyclic carbene (NHC) complexes, NHSi complexes are still comparatively rare. Accordingly, in comparison to the plethora of reports associated with NHC complexes, implicated in catalytic processes, only scant examples exist for NHSi complexes. Some of these reports include Heck or Suzuki type coupling, alkyne cyclotrimerisation, ketone hydrosilylation, amide reduction or Sonogashira cross-coupling reactions, and are discussed in detail here. These endeavours pave the way for new families of catalysts based on NHSis and highlight the potential future applications of these emerging and rather unexplored complexes in novel catalytic processes.