The role of electron and steric factors in the fluorination of P(III) compounds by Ph₃BiF₂

Abstract:

A reactivity of P(III) compounds towards fluorination by Ph₃BiF₂ was studied from the viewpoint of the frontier molecular orbital theory. HOMO energies of phosphites and amidophosphites were calculated. A qualitative relationship between HOMO energy levels and reactivity was found. The difference in chemical behaviour of (Me₂N)₃P and (Et₂N)₃P was attributed to a steric hinderance.

Introduction

Recently we have investigated the reaction of P(III) compounds with Ph₃BiF₂ [1] and the following P(III) reagents have been studied: (EtO)₂POH, (EtO)₃P, (EtO)₂POSiMe₃, (CF₃CH₂O)₃P, (Me₂N)₃P, (Et₂N)₃P. The reaction proceeds according to the following general scheme:

It has been shown that the reactivity of these compounds depended on the nature of substituents at a phosphorus atom.

The purpose of the present paper is to explain the behaviour of the abovementioned phosphites and amidophosphites in the fluorination reaction on the basis of the frontier molecular orbital theory.

Computational details

All the calculations were carried out semiempirically by the PC Spartan ver. 1.0 program package [2] with the AM1 effective Hamiltonian [3] on IBM compatible personal computer with Pentium-100 CPU under Windows NT. All the geometry parameters of the studied molecules were totally optimized. The geometries are shown graphically in Fig.1-6 .

Unfortunately, there is semiempirical parametrization for the Bi atom neither in the Spartan nor in the Mopac 93 programs. But according to a qualitative nature of the frontier molecular orbital theory we can omit the calculation of the LUMO of Ph₃BiF₂ from our consideration without a loss of accuracy. In fact we need to know only the relative HOMO energies of the considered phosphorus compounds for the estimation of their relative activity. Therefore we have to calculate HOMO energies of P(III) compounds only.

Results and discussion

The frontier molecular orbital theory is widely used for the analysis of the organic compounds reactivity. The criteria of applicability of the frontier orbital concept can be derived from Pearson's theory of hard and soft acids and bases [4].

According to this theory hard acids and bases have small radius and tight electronic distribution. Therefore, the mechanism of interaction between hard acids and bases is determined by electrostatic forces. On the other hand, soft acids and bases have large atomic sizes and are characterized by diffuse electronic distribution. The main interaction between soft acids and bases have, therefore, a pure quantum nature. The general tool for studying of such interactions in organic chemistry is a theory of frontier molecular orbitals.

A three coordinated phosphorus atom is a well known soft base. It has a sufficiently large atomic radius and bears a lone electron pair. Ph₃BiF₂ is a soft species too because of a large atomic radius of bismuth and three directly connected phenyl substituents, making Bi atom easy polarizable.

Thus, it is reasonable to consider the reactivity of P(III) compounds in a fluorination reaction by Ph₃BiF₂ on the basis of the frontier molecular orbital theory.

As it follows from our calculations, the shapes of the HOMO orbitals of the studied molecules are visually the same, therefore we demonstrate only the HOMO of (EtO)₃P as a sample illustration in Fig.3. It's easy to see that the phosphorus lone electron pair brings the main contribution into HOMO.

To compare the calculated HOMO energies with the reactivity of phosphorus compounds towards the fluorination reaction we introduced the following qualitative scale of reactivity:

1. A reaction proceeds at room temperature. After mixing of the reagents the reaction mixture was allowed to react during 20 hours and then the completition of the reaction was checked by NMR ³¹P spectroscopy.
2. A reaction doesn't proceed at the room temperature and needs a heating during 1 hour.
3. A reaction doesn't proceed even at prolonged heating.

The calculated HOMO energies and the relative reactivity of the P(III) compounds are presented in Table 1.

Table 1.

P(III) compound	Relative reactivity	HOMO,eV
I: (Me2N)3P II: (EtO)2POSiMe3 III: (EtO)3P	Reaction proceeds at room temperature	-7.951 -8.961 -9.805
IV: (EtO)2POH	Reaction proceeds at heating	-10.015
V: (CF3CH2O)3P	Reaction doesn't proceed even at heating	-11.172
VI: (Et2N)3P	Reaction doesn't proceed even at heating	-7.490

We can make some conclusions from the analysis of the data in Table 1. Four groups of compounds are observed on the basis of their reactivity.

The first group includes relatively active P(III) compounds (I,II,III) with HOMO energies higher than -9.805 eV. The second and the third groups are presented by compounds IV and V, respectively. We could include VI to the third group, but there are reasons to consider it separetely according to its HOMO energy level.

The remarkable difference in a chemical behaviour is observed for the first three groups, where we see the lowering of the reactivities and decreasing of HOMO energy levels simultaneously. The decreasing of the HOMO energies is also observed in the first group without a marked difference in their relative chemical reactivity. There are two reasons for such phenomenon. First, a very crude determination of the reactivity scale hides the relative reactivity of I, II and III. More accurate kinetic investigations could bring light on this fact, and probably confirm our qualitative supposition. Second, the frontier molecular orbital theory is an only qualitative tool for the investigation of a reactivity, therefore we can't expect quantitative difference in HOMO energies as compared to reaction rates.

It is interesting to compare the reactivity of VI and its HOMO energy level with the behaviour of other P(III) compounds. The nearest chemical analog of VI is I. Really their HOMO energies are very close as one could expect, but we see the opposite chemical reactivity. The reason of such a difference is clear from a comparison of the geometry of VI (Fig.6) with that of other P(III) compounds (see Fig.1-5) . It is easy to see that even in the case of bulky Me₃SiO substituent the hinderance of phosphorus atom by substituent groups is much less than it is in VI. That is why we attribute the low reactivity of VI in the fluorination reaction to the steric hinderance by the bulky Et₂N- groups.

I Fig. 1	II Fig. 2
III Fig. 3	IV Fig. 4
V Fig. 5	VI Fig. 6

Conclusions

Thus, the obtained results show the applicability and general usefulness of the frontier molecular orbitals theory for the description of the reactivity of P(III) compounds in the fluorination reaction by Ph₃BiF₂.

The qualitative relationship between the reactivity of the P(III) compounds and their HOMO energies is found. The lower HOMO energy level corresponds to a reduced reactivity.

The high reactivity caused by higher HOMO energy level can be altered by steric hinderance of bulky substituents at a phosphorus atom as it takes place in the case of VI.

Acknowledgement

We thank the Russian Foundation of Basic Research (Grant No. 94-03-08654) for the support of the investigation.

References

1. S.A. Lermontov, I.M. Rakov, N.S. Zefirov, P.J. Stang, Phosphorus, Sulfur, and Silicon,92(1994)225-229.
2. Spartan PC version 1.0, Wavefunction, Inc., 18401 Von Karman Ave., #370, Irvine, CA 92715 U.S.A.
3. M.J.S. Dewar, E.G. Zoebisch, E.F.Healy, J.J.P. Stewart, J.Amer.Chem.Soc., 107(1985)3902-3909.
4. G. Klopman(Editor), Chemical Reactivity and Reaction Paths, Wiley, New York, 1974.