Thermodynamics, Kinetics, and Dynamics of the Two Alternative Aniomesolytic Fragmentations of C−O Bonds:  An Electrochemical and Theoretical Study

Abstract

Fragmentation reactions of radical anions (mesolytic cleavages) of cyanobenzyl alkyl ethers (intramolecular dissociative electron transfer, heterolytic cleavages) have been studied electrochemically. The intrinsic barriers for the processes have been established from the experimental thermodynamic and kinetic parameters. These values are more than 3 kcal/mol lower as an average than the related homolytic mesolytic fragmentations of radical anions of 4-cyanophenyl ethers. In the particular case of isomers 4-cyanobenzyl phenyl ether and 4-cyanophenyl benzyl ether, the difference in intrinsic barriers amounts to 5.5 kcal/mol, and this produces an energetic crossing where the thermodynamically more favorable process (homolytic) is the kinetically slower one. The fundamental reasons for this behavior have been established by means of theoretical calculations within the density functional theory framework, showing that, in this case, the factors that determine the kinetics are clearly different (mainly present in the transition state) from those that determine the thermodynamics and they are not related to the regioconservation of the spin density ("spin regioconservation principle"). Our theoretical results reproduce quite well the experimental energetic difference of barriers and demonstrate the main structural origin of the difference.

The effect of electron-withdrawing groups in the fragmentation of the radical anions of benzyl phenyl ethers

“…As revealed by these color changes, reduction of 2 certainly occurs, but in contrast with radical anion of 1, the resulting radical anion of 2 is not persistent enough to be detected by EPR. In accord with previous cyclic voltammetry measurements [5], this too short life time of radical anion of 2 is certainly related to mesolytic cleavages in the radical anion. …”

“…As we have discussed in previous studies [5], the p electronic state of the two radical anions is not appropriate to produce the C-O alkyl ether bond fragmentation. An intramolecular electron transfer to the r* antibonding C-O alkyl ether molecular spin orbital is required to produce the bond dissociation.…”

“…The most common reaction process of many organic radical anions is called dissociative activation [2] and describes their tendency to unimolecular bond cleavage by the addition of the extra electron. Several experimental and theoretical studies have been carried out to analyze the cleavage modes of radical anions corresponding to organic ethers with a C-O scissile bond [3][4][5][6].…”

“…The ease for cleavage to occur depends on the capacity of the system to reorganize its geometry and electronic state. In two previous works [5,6], we have studied the fragmentation of aryl benzyl ether radical anions with various substituents on the phenyl ring, concretely electron-withdrawing substituents such as -NO 2 or -CN. The nitro group has a powerful attraction for electrons; many of the chemical reactions of nitro compounds are due to this characteristic electronegativity and several different effects show up when we change one group for the other.…”

DFT Study on the SnII‐Catalyzed Diastereoselective Synthesis of Tetrahydrofuran from D–A Cyclopropane and Benzaldehyde

“…[11] Density functional theory (DFT) methods [12] have now been widely applied to various molecular systems with great success because of their efficiency and accuracy. [13][14][15] Especially the B3LYP method, [16,17] which includes Becke's three-parameter-exchange functionals and nonlocal Lee-Yang-Parr correlation functional that generally provide better results. In this paper, by taking into account the accuracy and the facility of the calculations, B3LYP/6-311G(d,p) (LANL2DZ adding one set of d-polarization function with exponent of 0.18 [18] for Sn) single-point energies based on the B3LYP/6-31G (LANL2DZ basis set [19][20][21] for Sn) optimized structures were obtained.…”