Fine-Tunable 3,4-Dihydroquinazol-2-ylidene Carbenes: Synthesis, Rhodium(I) Complexes, and Reactivity

The design and synthesis of various new six-membered
cyclic formamidinium
salts with a 3,4-dihydroquinazoline core have been reported in this
paper. Our synthetic strategy allows access to a kind of tailor-made
3,4-dihydroquinazolinium salts bearing different substituent combinations.
A series of novel 3,4-dihydroquinazolin-2-ylidene-based rhodium­(I)
complexes were prepared by the reaction of [Rh­(cod)­Cl]2 with the free carbene obtained in situ from the deprotonation of
the corresponding 3,4-dihydroquinazolinium salts with KN­(SiMe3)2. The NHCs prepared in situ can also react with
S8 or CS2 to afford the corresponding thiones
or NHC–CS2 adducts, respectively. The rhodium­(I)
complexes were transformed to the corresponding dicarbonyl complexes,
and the ν­(CO) values of the corresponding dicarbonyl Rh complexes
indicate the 3,4-dihydroquinazol-2-ylidenes are stronger electron
donors than normal five-membered NHCs. The Rh complexes are highly
active in the arylation of carbonyl compounds, and the 3,4-dihydroquinazolin-2-ylidenes
prepared in situ upon deprotonation are powerful in palladium-catalyzed
Suzuki cross-coupling reactions at room temperature with a ppm scale
catalyst loading with TONs of up to 425 000.