Basicity of nucleophilic carbenes in aqueous and nonaqueous solvents-theoretical predictions.
The complete basis set method CBS-QB3 was used in conjunction with the CPCM solvation model to predict both the absolute and relative pKa's of 12 nucleophilic carbenes in dimethyl sulfoxide (DMSO), acetonitrile (MeCN), and water. Average absolute pKa values in DMSO ranged from 14.4 +/- 0.16 for 3-methylthiazol-2-ylidene (12) to 27.9 +/- 0.23 in the case of bis(dimethylamino)carbene (11), while values in MeCN were determined to be between 25.7 +/- 0.16 (12) and 39.1 +/- 0.25 (11). Relative pKa calculations yielded similar results. Calculations in aqueous solution gave pKa's between 21.2 +/- 0.2 (12) and 34.0 +/- 0.3 (11). Excellent agreement between calculated and experimental pKa's was obtained for the few cases where experimental numbers are available, confirming that this theoretical approach may be used to calculate highly accurate pKa values.
Magill
Alison M
AM
School of Chemistry, University of Tasmania, Private Bag 75, Hobart, Tasmania, Australia 7001.
Cavell
Kingsley J
KJ
Yates
Brian F
BF