Intramolecular "hydroiminiumation and -amidiniumation" of alkenes: a convenient, flexible, and scalable route to cyclic iminium and imidazolinium salts.
Addition of a stoichiometric amount of HCl to alkenylaldimines, -formamidines, and -amidines results in the protonation of the sp2-nitrogen atom. The resulting alkenylaldiminium, -formamidinium, and -amidinium salts can be isolated and fully characterized, including single-crystal X-ray diffraction studies. Heating solutions of these salts induces ring closure cleanly and regioselectively via formal "exo" addition of the nitrogen-hydrogen bond to the pendent carbon-carbon double bond, affording the corresponding cyclic aldiminium, dihydroisoquinolinium, and imidazolinium salts. Of special interest, novel 4,4-disubstituted imidazolinium salts are accessible via this synthetic route. Similarly, addition of phosgene to alkenyl ureas and alkenyl amides, followed by gentle heating, cleanly affords C-chloro imidazolinium, and cyclic C-chloro iminium salts, respectively. Treatment of the latter with tetrakis(triphenylphosphine)palladium allows for the preparation of the first transition-metal complex bearing a cyclic arylaminocarbene as ligand. Deuterium labeling experiments suggest that the mechanism of the hydroiminiumation and -amidiniumation reactions involves an intramolecular proton transfer to the double bond in the rate-determining step. This novel synthetic methodology gives access to a variety of N-heterocyclic carbene (NHC) and cyclic alkyl- and arylaminocarbene (CAAC) precursors.
Jazzar
Rodolphe
R
UCR-CNRS Joint Research Chemistry Laboratory (UMI 2957), Department of Chemistry, University of California, Riverside, California 92521-0403, USA.
Bourg
Jean-Baptiste
JB
Dewhurst
Rian D
RD
Donnadieu
Bruno
B
Bertrand
Guy
G
D000475:Alkenes / Q000737:chemistry*; D000578:Amidines / Q000737:chemistry; D002625:Chemistry, Organic / Q000379:methods*; D018360:Crystallography, X-Ray; D006844:Hydrocarbons, Cyclic / Q000737:chemistry; D007093:Imidazoles / Q000138:chemical synthesis / Q000737:chemistry*; D007097:Imines / Q000737:chemistry; D015394:Molecular Structure; D012492:Salts / Q000737:chemistry
D000475:Alkenes; D000578:Amidines; D006844:Hydrocarbons, Cyclic; D007093:Imidazoles; D007097:Imines; D012492:Salts