Chemical interconversions in the system Tp Zn/CO2/alcohol [Tp = substituted tris(pyrazolyl)borate]

Article abstract of DOI:10.1002/cber.19971300116

The zinc hydroxide complexes Tp*Zn-OH with Tp^Cum,Me = tris(3-cumenyl-5-methylpyrazolyl)borate and Tp^tBu,Me = tris(3-tert-butyl-5-methylpyrazolyl)borate can be converted to the alkyl carbonate complexes Tp*Zn-OCOOR by reaction with dialkyl dicarbonates or with alcohol and CO₂. An alternative formation reaction is the treatment of the pyrazolyl borate with zinc perchlorate and potassium carbonate in alcohol. The interconversion between Tp^Cum,MeZn-OH and Tp^Cum.MeZn_OCOoMe in methanol-containing solution can be repeatedly performed in both directions by bubbling either CO₂ or N₂ through the solution. The alkyl carbonate complexes show a variable sensitivity towards hydrolytic destruction with reformation of the hydroxide complexes. The complexes Tp^tBu,MeZn-OCOOR (R = Me, Et) release CO₂ under high vacuum to form the alkoxide complexes Tp^tBU,Me-Zn-OR, which could not be obtained pure due to their extreme water sensitivity. Indirect evidence for their existence is also obtained by the reaction between Tp^Cum,MeZnOCOOMe and methyl iodide, forming Tp^Cum,MeZn-I and dimethyl ether. The zinc hydroxide complexes catalyse the formation of diethyl carbonate from ethanol and CO₂. VCEI Verlagsgcsellschaft mbH.