Kinetic Stabilization of Quinone Dianions via Hydrogen Bonding by Water in Aprotic Solvents

It is well-established that very weak acids such as water and alcohols strongly H-bond to quinone dianions, Q2–, in aprotic solvents. This results in thermodynamic stabilization of Q2– and a shift of the formal potential of the Q–/2– couple to less negative values. This study shows that the strong H-bonding of water also results in a type of kinetic stabilization of Q2–. CVs of the naturally occurring naphthoquinone Vitamin K1 in very dry 0.1 M NBu4PF6/CH2Cl2 show a reversible Q0/– wave but a chemically irreversible Q–/2– wave. Similar behavior is seen with anthraquinone and duroquinone. Evidence suggests that this is due to nucleophilic attack of Q2– on CH2Cl2 to give ether products. Addition of water results not only in the expected positive shift in potential of the second wave but also in an increase in the chemical reversibility. This indicates that the H-bonding of water to Q2– blocks the irreversible reaction with CH2Cl2 by significantly decreasing the rate of that reaction. Further experiments show that the kinetic stabilization by water is great enough that it can slow the reduction of Q2– with iodomethane, a very reactive electrophile, in CH3CN.