Stable P-heterocyclic carbenes: scope and limitations.
The conjugate acids (PHCH+s) of P-heterocyclic carbenes (PHCs) are prepared by formal [3+2] cycloaddition of a 1,3-diphosphaallyl or 1,3-phosphinophosphenium cation with various nitriles. The effect of the phosphorus substituent on the fate of the cyclization and on that of the counteranion and base in the subsequent deprotonation reaction are reported. Two PHCs that are indefinitely stable in the solid state are described. In solution, one of them, made from acetonitrile, undergoes a facile [3+2] cycloreversion, whereas the other, based on dimethyl cyanamide, is stable, presumably owing to its zwitterionic structure, which involves a tricoordinate pentavalent phosphorus atom. The reactivity of PHCs is strongly driven by the high electrophilicity of the phosphorus centers, as demonstrated by their reactivity with water and benzaldehyde. Although both PHCs reported in this paper are direct analogues of the least-basic NHCs, their basicity is comparable to those of the more strongly basic NHCs (as determined by comparison of the carbonyl stretching frequencies of their corresponding cis-[RhCl-(CO)2(L)] complexes).
Masuda
Jason D
JD
UCR-CNRS Joint Research Chemistry Laboratory (UMI 2957), Department of Chemistry, University of California, Riverside, CA 92521-0403, USA.
Martin
David
D
Lyon-Saunier
Celine
C
Baceiredo
Antoine
A
Gornitzka
Heinz
H
Donnadieu
Bruno
B
Bertrand
Guy
G
D002384:Catalysis; D018360:Crystallography, X-Ray; D006571:Heterocyclic Compounds / Q000138:chemical synthesis* / Q000737:chemistry; D006838:Hydrocarbons / Q000737:chemistry; D008697:Methane / Q000031:analogs & derivatives* / Q000737:chemistry; D008958:Models, Molecular; D015394:Molecular Structure; D010720:Phosphines / Q000737:chemistry
D006571:Heterocyclic Compounds; D006838:Hydrocarbons; D010720:Phosphines; C030011:carbene; D008697:Methane