Solvent effects methanol fluorination

The mechanism for the fluorination of methanol by diethylaminosulfur triflouride (74) has been investigated in the gas phase and in dichloromethane using the RHF and B3LYP methods with the 6-31G+ basis set and truncated structures for the reactants.109 Solvent effects were calculated using the PCM approach. Truncating the reactant did not affect the results significantly. The results suggest that fluorination occurs via an 5 2 mechanism in both the gas phase and dichloromethane, (Scheme 27). 

Variation of the fluorine-19 chemical shift on association has been used to determine solvent effects and basicities28. Muller29 has examined the chemical shifts of 6,6,6-trifluoro-l-hexanol (TFH) in mixtures of water and organic liquids such as acetone, dioxane, THF, ethylene glycol, 2-methoxyethane, 1,2-dimethoxyethane, methanol, r-butanol, dimethyl-formamide and dimethyl sulphoxide. Typical plots are shown in Figures 2 and 3. Solvents... 

Aroyl esters of anthracene-9-methanol are photolysed in methanol to give products consistent with the anthracene-9-methyl cation as an intermediate.41 Rate constants for the solvolyses of secondary alkyl tosylates in fluorinated solvents were analysed in terms of the possible involvement of very short-lived carbocation-tosylate ion pair intermediates.42 The effect of added electrolytes on the rate of solvolysis of cumyl chloride and its -methyl derivative was studied in 90% aqueous acetone and 80% aqueous DMSO, with the results revealing a combination of a special salt effect and a mass law effect.43 Kinetic parameters obtained for the solvolysis of (8) (R1 = R2 = Me and R1 = Ar, R2 = H) show that there is substantial n, n participation in the transition state [e.g. (9). 44... 

Compounds such as fluorinated /3-ketones have been used mainly for the extraction of lanthanides and actinides, as more effective ligands than fluorinated dithiocarbamates for complexation with f-block elements. In addition to complex formation, some analytes (e.g. tervalent lanthanides) require the presence of a small amount of water containing 5% methanol — which act as matrix and solvent modifier, respectively — for quantitative extraction [89]. Also, a synergistic effect on the SFE of actinides [90,91] and of lanthanides from cellulose [92] and acid solutions [93] was observed when using a mixture of tributylphosphate (TBT) and a fluorinated /3-diketone. The effect was ascribed to competition of TBT with the matrix for the unoccupied coordination sites of lanthanides and actinides. Thus, the formation of adducts with the complexes of these analytes with fluorinated /3-ketones in supercritical COj facilitates their removal from the solid matrix. 

The evidence for fluorine hyperconjugation derived from nmr measurements available in 1970 was not deflnitive (3,4). Taft and his associates had originally argued that the fluorine substituent chemical shifts (SCS) of p-fluorine atoms in molecules with substituents which are capable of resonance interactions, for example the nitro group, exhibited a special solvent dependence and shifted to lower field as the polar character of the solvent was increased (38,39). The fluorine substituent chemical shift for the aromatic fluorine atom in 4-fluo-robenzotrifluoride, indeed, exhibits a small downfleld shift from 4.95 in 3-methylpentane to 5.75 in nitromethane to 6.05 in 75% aqueous methanol. For comparison, the substituent chemical shift for the fluorine atom in 4-fluoroni-trobenzene exhibits a shift from 9.00 in 3-methylpentane to 10.55 in nitromethane to 11.20 in 75% aqueous methanol (39). Holtz concluded on these grounds that fluorine hyperconjugation, structure XVC, might contribute to the determination of the substituent chemical shift. However, Brownlee, Dayal, and Taft subsequently showed that the effects of dipolar aprotic solvents on the... 

The rates of 5 2 and 2 reactions of several fluorinated alkyl bromides and iodides have been measured in methanol and in DMSO using azide ion and methoxide ion as the nucleophile (base). The results demonstrate the effect of changing the nucleophile, the solvent, the leaving group, and a- and -fluorine substituents on the rates of these reactions. MP2/6-3 l-l-G(d,p)-LANL2DZ level calculated AG and transition states found for both the 5 2 and the 2 reactions are consistent with the experimental results. The azide ion reactions are almost exclusively 5 2 processes, whereas the methoxide ion reactions... 

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