Converting Caffeine to Electronically Different N‑Heterocyclic Carbenes with a Hypoxanthine Backbone

The synthesis of the new mixed amino–amido N-heterocyclic
carbene 2, featuring a hypoxanthine structural unit,
starting from caffeine via the hypoxanthinium precursor 4 is described. NHC 2 forms adducts with group 6 elements,
coordinates to L2ClRh fragments (L2 = COD, (CO)2), or dimerizes to yield olefin 8. The donor
properties of NHC 2 were evaluated by IR spectroscopic
assessment of its TEP value (2058 cm–1), which is
typical for an amino–amido NHC. Dicarbenes are also easily
obtained by converting the imidazole moieties of the hypoxanthinium
cation 4 or the olefin 8 into NHC functions.
X-ray structure determinations of a Rh complex of carbene 2 and the olefinic dicarbene precursor 13 are presented.