5.5. Comparative fluorinating ability of fluorooxy-reagents in solvents of different polarity.

A qualitative estimation of the fluorinating ability in the series of fluorooxy-reagents is not a simple task, because many of such compounds are unstable, can not be isolated individually and studied under adequate conditions. So much a fundamental theoretical approach to this problem proposed recently seems more significant. Thermodynamics of the fluorination processes has been assumed as a basis, and a qualitative criteria of the fluorinating ability of fluorooxy-reagents has been chosen as the reaction heat calculated taken into account the polarity of a solvent used for the process [292].

Quantum-chemical calculations were done by MNDO, AMI, MNDO-PM-3 methods taken into account the solvent polarity, within the frames of the model of polarizable continium developed by Tomasi.

To compare the fluorinating abilities of fluorine “ carriers” of R-OF type, their interaction with pyridine, as a fluorination object, was chosen as a reference reaction. This reaction is attractive because in addition to the comparative estimation of the carriers with each other it makes possible to compare them with a classical fluorinating reagent, N-fluoropyridinium: if the reaction enthalpy is negative, then the R-OF reagent under consideration exceeds N-fluoropyridinium cation in the fluorinating ability and yields to it in this respect if the reaction enthalpy is positive.

To fluorinate the fluorinating ability of typical fluorooxy-reagents in solvents of different polarity, calculations of the heat efficiencies ( H_reacsolvent) were made for the model exchange reactions (1-5).

To calculate the heat efficiencies of these reactions in gas phase H_reac (g) and in solvents of different polarity H_reac (s), the formation enthalpies H_f (g) of the reagents and products with complete optimization of geometry of the molecules were calculated. For charged molecular systems there was calculated free Gibbs energy G (s) of formation of a system of “polarized molecule + polarization field” ,taken into account the solvent influence within the limits of PCM model at different values of the dielectric constant .

On the basis of the obtained values of H_f (g) and G (s) the target values were calculated: the heat efficiencies of fluorination reactions (1-5) in gas phase and in solvents of different polarity H_reac (s), they are given in Table 20. The calculations were made by the authors by three quantum-chemical methods, but Table 20 represents the data only of one of them, MNDO-PM3, as the most correct one.

Table 20. Heat efficiencies (- H_reac solvent) of exchange reactions (1-5) in solvents of different polarity ( >1) and in gas phase ( =1)

Reaction		H_reac solvent kkal/mole
1	1	105,2
	10	2,6
	20	-10,6
	30	-13,5
	78,5	-16,8
2	1	121,4
	10	5,0
	20	-2,2
	30	-4,7
	78,5	-7,8
3	1	89,8
	10	-14,0
	20	-20,5
	30	-22,7
	78,5	-25,5
4	1	84,9
	10	-24,0
	20	-31,1
	30	-33,7
	78,5	-86,8
5	1	66,6
	10	-33,8
	20	-40,5
	30	-42,9
	78,5	-45,8

As it is seen from the data of Table 20, reactions under investigation (1-5) in gas phase (=1), given for comparison, are very endothermic: H_reac reaches 100 kkal/mole and are positive. At transition to polar solvents, a principal change of the heat reaction efficiencies occurs: they become negative, i.e. exothermic. Even in solvents of middle polarity ( the dielectric constant =20-30) the equilibrium of reactions (1-5) is shifted to the right-hand side, in other words, the R-OF reagents under consideration exceed N-fluoropyridinium cation in the fluorinating ability.

The comparison of the R-OF reagents with each other looks especially demonstrative : it is seen from Table 20 that according to the fluorinating ability they form the following series:

CF₃SO₂OF > FSO₂OF > CF₃COOF > CF₃OF> CH₃COOF

The series corresponds to the ideas of synthesis researchers about connection of the structure of the R-OF reagents with their reactivity. That is an additional evidence of reliability of the proposed thermodynamic method.

In conclusion, one should take into account the variation of the Gibbs curves (Fig.1) directly connected with the solvation effect : already within a range of = 30-40 the dependence of G/ degenerates , i.e. the solvation reaches saturation. Obviously , the solvents with such dielectric constant can be acceptable enough for fluorination.

Figure1. Dependence of the Gibbs free energy ( taken into acoount the solvent influence) on the dielectric constant of the solvent ( calculated within the limits of MNDO method).

As a whole, the results of the calculations of the heat efficiencies have clearly demonstrated the change of the fluorinating ability of the R-OF reagents in a form convenient for comparison in dependence on the solvent polarity. This method is the most fruitful and to some extent is unique for estimation of the fluorinating ability of the R-OF reagents and their comparison with each other.

CONCLUSION

Usually the choice of a fluorinating agent is made taken into account its availability, selectivity of the fluorination, readiness of the process course and equipment, safety of handling. It is customary to compare these factors with elemental fluorine using, because its use is the most efficient process of production of organofluorine compounds. But the high reactivity of elemental fluorine requires application of special methods that increases significantly the cost of the target products.

This paper reviews examples of application of compounds containing the O-F bonds as effective fluorinating agents. Their abilities for selective fluorination of different classes of compounds have been shown and compared. The combination of potential electron properties and steric effects is typical when hydrogen atoms are replaced with fluorine in organic molecules and is widely used in organic chemistry. Besides, in addition to the direct method of fluorination with elemental fluorine, new fluorinating reagents have appeared, which sometimes have some advantages compared with elemental fluorine and conventionally used hydrogen fluoride and fluorides of transition metals. The new fluorinating reagents do not replace the existing ones produced commercially but supplement with them. More over, a number of them have been produced on an industrial scale that makes possible their widespread study. At the same time their practical application allows changing economic factors and environment of production of fluorine-containing materials. Widespread investigations in this field have become a basis for creation of materials perspective for new technique and medicine and great technological efforts of these studies have promoted complete use of elemental fluorine in organic synthesis.

On the whole, there has been formulated a task of application of fluorinating agents not only for realization of the fluorination processes but for their application for different synthetic purposes. In our review we gave some new areas of application of compounds containing the O-F bonds for processes of oxidation and generation of reactive electrophilic particles.

We assume that the analysis of the existing data and some formulated conclusions will be useful not only for chemists working in the fluorine chemistry, but also for experts using new materials in their work and solving new complex technical tasks. The presented review on the chemistry of compounds containing the O-F bonds places at disposal of experts in this important and dynamically developing field a possibility to become familiar with the new ideas, the latest achievements and results of the investigations and also with unsolved problems.