Physical Organic Chemistry of Transition Metal Carbene Complexes. 14.1 Thermodynamic Acidity Measurements of Fischer Carbene Complexes in Acetonitrile
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Abstract
The pKa values of five Fischer carbene complexes of the general structure (CO)5CrC(OR)CHR‘R‘‘, of two (CO)4(PR3)CrC(OCH3)CH3 type complexes and of one aminocarbene complex, were determined in acetonitrile by a spectrophotometric method. These pKa values span a range from 22.2 for (CO)5CrC(OCH3)CH3 to 27.6 for Deprotonation of the carbene complexes was achieved with strong amine bases such as TMG, DBU, and BEMP. Attempts at measuring the pKa of some other carbene complexes failed either because of instability of the respective anions (e.g., (CO)5CrC(OCH3)CH(CH3)2) or because the pKa was too high for deprotonation to occur ((CO)5CrC(N(CH3)2)CH3). Similarities and differences in the structural dependence of these acidities in acetonitrile compared to other solvents such as 50% acetonitrile−50% water, DMSO, and THF are discussed.
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