Kinetic Solvent Effects on Hydrogen Abstraction Reactions from Carbon by the Cumyloxyl Radical. The Role of Hydrogen Bonding

Abstract

A kinetic study of the H-atom abstraction reactions from 1,4-cyclohexadiene and triethylamine by the cumyloxyl radical has been carried out in different solvents. Negligible effects are observed with 1,4-cyclohexadiene, whereas with triethylamine a significant decrease in rate constant (k(H)) is observed on going from benzene to MeOH. A good correlation between log k(H) and the solvent hydrogen bond donor parameter alpha is observed, indicative of an H-bonding interaction between the amine lone pair and the solvent.

Kinetic Solvent Effects on Hydrogen Abstraction from Phenol by the Cumyloxyl Radical. Toward an Understanding of the Role of Protic Solvents

“…More importantly, our study of the reaction of CumO • with 1,4-cyclohexadiene revealed a small increase in k H on going from MeCN, benzene, or chlorobenzene (for which k H = 6.6−6.9 × 10 7 M −1 s −1 ) to MeOH and t-BuOH (k H = 8.3 × 10 7 M −1 s −1 ), 29 with the rate constant increasing by a factor of ca. 3 when the reaction was performed in TFE solution (k H = 1.9 × 10 8 M −1 s −1 ).…”

Activation and Deactivation Strategies Promoted by Medium Effects for Selective Aliphatic C−H Bond Functionalization

“…[6] Taking compounds cyclohexane and 1,4-cyclohexadiene as reference,n osignificant effect on the k H values was observed when the reaction was studied in aprotic solvents (isooctane,b enzene,c hlorobenzene,a cetonitrile: k H = 1.1-1.2 10 6 and 6.6-6.9 10 7 m À1 s À1 for cyclohexane and 1,4cyclohexadiene,r espectively). [41][42][43] An increase in reactivity was,however, observed in protic solvents,with the highest k H values being measured for both substrates in the strong HBD solvent TFE (k H (TFE)/k H (MeCN) = 4.0 and 2.9 for cyclohexane and 1,4-cyclohexadiene,r espectively). This behavior was explained on the basis of hydrogen bonding of the solvent…”

“…Representative substrates,s uch as THF, [41] cyclohexanol (CyOH), [45] 2,2-dimethylpropanal (DMPA), [43] N,N-dimethylacetamide (DMA), [44] and triethylamine (TEA), [42,45] all undergo predominant or almost exclusive HATt oC umOC from the a-C À Hb onds (the formyl C À Hb ond with DMPA and the CÀHbonds a to the nitrogen center with DMA). The highest k H values were measured in isooctane and the lowest ones in TFE (Scheme 14), which leads to the following k H (isooctane)/k H (TFE) ratios:4.5, 7.9, 11, > 540, and > 2900, for THF,CyOH, DMPA, DMA, and TEA, respectively.…”

Modulation of Biorelevant Radical Reactions by Non‐covalent Interactions

“…The reaction of CumO• with hydrocarbons occurs with very similar rate constants in a wide range of solvents, because neither the reactants nor the transition state is strongly solvated [69]. On the other hand, the reaction of CumO• with triethylamine in methanol is sixfold slower than in MeCN [69].…”

“…On the other hand, the reaction of CumO• with triethylamine in methanol is sixfold slower than in MeCN [69]. The relevant rate constants for the reactions of Et 3 N with CumO• are collected in Table 20.4 [66,69]. The relevant rate constants for the reactions of Et 3 N with CumO• are collected in Table 20.4 [66,69].…”