Gold(I) Complexes of ImPyDippDipp and ImPyMesMes: Biaryl L‑Shaped N‑Heterocyclic Carbene Analogues of IPr and IMes

Imidazol-2-ylidenes, IPr and IMes, represent by far the
most important
and widely utilized N-heterocyclic carbenes in organic synthesis and
catalysis. Herein, we report the synthesis, catalytic activity, and
structural and electronic characterization of ImPyDippDipp and ImPyMesMes,
sterically bulky and easily accessible biaryl L-shaped N-heterocyclic
carbene analogues of IPr and IMes. These ligands exploit the rigid
imidazo­[1,5-a]­pyridin-3-ylidene architecture to merge
the properties of the biaryl scaffold with the electron-rich characteristics
of the carbene center. The catalytic activity is evaluated in the
gold­(I)-catalyzed hydration of alkynes and cyclization N-propargylamides,
two model reactions for π-activation of alkynes that have found
broad application in organic synthesis. Structural and electronic
evaluation indicates that biaryl L-shaped ImPyDippDipp and ImPyMesMes
ligands are more sterically demanding and more electron σ-donating
and π-accepting than the classical imidazol-2-ylidnes, IPr and
IMes. Both of these L-shaped ligands show excellent catalytic activity
in gold­(I)-catalyzed hydration of alkynes and cyclization of N-propargylamides
compared to their imidazol-2-ylidene congeners, IPr and IMes. Considering
the tremendous impact of imidazol-2-ylidenes in homogeneous catalysis,
we anticipate that this class of biaryl L-shaped NHCs will be rapidly
and widely adopted to complement IPr and IMes N-heterocyclic carbenes.