UNSTABLE HALOGEN-CONTAINING ORGANOLITHIUM COMPOUNDS.

7. THERMODYNAMIC CALCULATIONS OF THE DECOMPOSITION

MECHANISM OF HALOGEN-CONTAINING ORGANOLITHIUM

COMPOUNDS

V. I. Faustov, A. I. D'yachenko, and O. M. Nefedov

UDC 536.7:530.145:542.92:547.253.4'121

Unstable ~-halomethyllithiums and o-halophenyllithium decompose in solutions at low temperatures to form carbenes and dehydrobenzene, respectively. Kinetic investigations of the mechanism of the decomposition of these compounds are hampered by the presence of a high- ly exothermic step involving the recombination of the intermediate particles or their inter- action with other components of the reaction mixture following the elimination step. As a result, the rate of the elimination reaction is usually evaluated from the temperature for the beginning of the exothermic reaction, i.e., the decomposition temperature. The latter is determined by the nature of the halogen and the solvent, and in the case of compounds of the methane series, it is also determined by the number of halogen atoms and varies from --20 to --1200C, depending on these parameters [i]. At the same time, most of the laws re- lating the decomposition temperatures of halogen-containing organometallic compounds to their structure indicate that the elimination reaction takes place in one step [i, 2]:

RR1XCM --, RR1C: + MX X _r247 M

R, R I = H , Hal t X----Hal, M = m e t a l

At the same time, the values of the decomposition temperatures of a number of halogen-con- taining organolithium compounds (particularly, the thoroughly studied compounds LiCH2CI, LiCHCI2, and LiCCI3) cannot be explained satisfactorily in the framework of the scheme pre- sented.

For this reason, the present work was devoted to a thermodynamic evaluation of the pro- bability of the decomposition of halomethyllithiums and o-halophenyllithiums according to the one-step mechanism.

In the literature not only are there absolutely no data on the enthalpies of formation of halogen-containing organometallic compounds, but there are not even any seriously sub- stantiated hypotheses regarding the mutual influence of the halogen (X) and metal (M) atoms on the strength of the C--X and C--M bonds belonging to the same or adjacent C atoms. At the same time, the existing experimental and theoretical data make it possible to easily deter- mine the strength of the C--Li bond and the enthalpy of the decomposition of monofluorometh- yllithium, which, unfortunately, has not heretofore been investigated experimentally. In fact, the change in enthalpy over the course of the decomposition of this compound can be found by considering the following thermodynamic cycle, in which the enthalpies of the in- dividual steps (AH I to AHIII) are equal to the values, taken with the appropriate signs, of the strengths DX_ Y of the X--Y bonds formed or broken in a particular step:

AHI ~HII AHII I FCH~Li ~ CH2F" + Li ~ : CH~ + F + Li ' : CH~ + LiF

A H "= A H I + A H n + AHII I (1) N. D. Zelinskii Institute of Organic Chemistry, Academy of Sciences of the USSR, Moscow.

Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. i0, pp. 2183-2186, October, 1979. Original article submitted July 13, 1978.

2006 0568-5230/79/2810-2006507.50 �9 1980 Plenum Publishing Corporatio~

The values of AHII and AHII I needed for the calculation are known [3], and the value of AH I = DFCH2_Li can be determined on the basis of the results of a calculation of the enthal- py of the following isodesmic process by the MO-LCAO-SCF method [4]:

FCH~Li + CHa -+ CHsF + CHaLi + AHIv

By selecting an appropriate thermodynamic cycle, for AH I = DFCH2-Li we can obtain the expression

A H I = DFCH,-Li = DCH3-Li + DFCH~H--DcH~-H-}-AHIv (2)

Since the value of AHIV calculated in [4] by the nonempirical SCF method is--O.l kcai/ mole,* and DCH3-Li, DCHa_Fe, and DCH3_ H are equal to 47, i00, and 104 kcal/mole, respective- ly [3], according to Eq. (2), we have DFCH2_Li = 43 kcal/mole. Thus, the introduction of an F atom into the molecule of methyllithium weakens the C--Li bond by a relatively small amount (4 kcal/mole). It is likely that the introduction of each successive F atom into the mole- cule will be accompanied by further weakening of the C--Li bond by the same or a similar amount, and the analogous effect from the addition of an F atom to the adjacent C atom (for example, in o-fluorophenyllithium) or from the addition of atoms of other halogens to the methyllithium molecule should be smaller. However, because of the lack of the corresponding values for AHIV it is impossible to numerically evaluate the amount of such destabilization, and the value 5H I = 47 kcal/mole was used for the calculations of the decomposition of the other fluorinated and chlorinated methyllithiums (for which experimental values of 5HII are known). The values of AH found from Eq. (i) for the decomposition of LiCH2F, LiCHF2, LiCFz, LiCH2CI, LiCHC12, and CIaLi are presented in Table i. The table also includes the values of AHII , which are completely determined by the values of AH in our approximation. For o- fluoro- and o-chlorophenyllithium we took the value AH I = 55 kcal/mole (the strength of the bond in phenyllithium [3]), and the values of AHII, for which there are no experimental values, were found from the heats of formation of dehydrobenzene and the o-fluoro- and o- chlorophenyl radicals calculated by the semiempirical MINDO/3 [5] and MINDO/2 [6] methods (the latter was used only for the fluorine derivatives). The radicals were calculated by the "half-electronmethod" [7]. In order to evaluate the reliability of the methods used for calculating the strength of the C--X bond in the radicals, we carried out similar calculations of fluorinated and chlorinated derivatives, for which the strength of the C--X bond can be

*The specific features of the calculation of isodesmic processes characterized by a relative- ly constant correlation energy makes it possible to estimate the energy parameters to within 3-5 kcal/mole, which significantly exceeds the usual accuracy of nonempirical methods based on the one-electron approximation.

TABLE i. Enthalpies of Elimination (kcal/mole) of LiF and LiCI from Halo- gen-Containing Organolithium Compounds

Compound

LiCH2F LiCHFz LiCF3 LiCH~CI LiCUCI: LiCCI~ o-FC6H~Li o-C1C~HJA

Experiment

AHII AH

102 92 91 2~

Cale. by MINDO/3; in parentheses, by MINDO/2

AHII AH

133 (134) 43 (44) 124010) 34(20) 96(90) 6(0)

~01 35 82 !6 68 2

110(II~) 2'8(22) 82 24

*A calculation which takes into account the destabilization of the C--Li bond due to the introduction of the F substi-

tuent.

2007

TABLE 2. Bond Lengths and Bond Angles of Halomethyl Radicals Calculated by the MINDO/3 and MINDO/2 (in parentheses) Meth- ods

Compound

CIt2F CHF~ CF3

CH2C1 CHC1,_, CCh

Bond length, .~

C - - X

1,32 (1,30) 1,29(1,31) 1,30(i,31) "1,33 * 1,69 i,71 L72 t ,74 *

C - - H

1,13(J,22)

1,1(I l , l l

Bond angle, deg

XCX XCH

- n 5 ( l i 3 ) 136(111) 112(tt4) i20(113)

t 12 * 11--5 ~ 6 It0 120 i20 *

HCH

i i 3 ( 1 2 0 )

t13

*Experimental values from [8].

evaluated on the basis of experimental data [3]. The calculations of a large number of ra- dicals, including halomethyl radicals, have previously been carried out by the inorganic SCF method in the MINDO/3 parametrization [8]. The values obtained for the heats of forma- tion of the radicals were systematically underestimated by ~ i0 kcal/mole. This is attrib- uted to the explicit consideration of part of the correlational energy by the inorganic SCF method [5, 8], which is superfluous for parametrizations of the MINDO type, which have been calibrated for calculations of molecules with closed shells. From this point of view, the use of the "half-electron method" is more consistent [5]. Complete optimization of the geo- metry (the data for the halomethyl radicals are presented in Table 2) was carried out for all the compounds considered; the only restriction was the usual assumption of a planar structure for the o-halophenyl radicals and dehydrobenzene. As we see from the data ob- tained (see Table i), both semiempirical methods correctly convey the sharp weakening of the C--X bond along a series of halomethyl radicals with increasing numbers of halogen atoms. At the same time, both methods slightly overestimate the strength of the C--X bond, especial- ly in the case of the fluorinated derivatives, and this results in overestimation of the fluorinated derivatives, and this results in overestimation of the endothermicity of the re- action as a whole.

It should be noted that the MINDO/2 parametrization conveys the strengths of the C--F bond in polyfluoromethyl radicals appreciably better than does the MINDO/3 parametrization. This is possibly due to the unfortunate choice of the parameters of the F...F interaction in the MINDO/3 method, since the latter, unlike the MINDO/2 method, overestimates the values of the FCF bond angles and predicts planar structures for the CHF= and CF3 radicals. The lengths of the C--H bonds calculated in the MINDO/2 parametrization are 0.08-0.09 ~ higher than the values obtained by the MINDO/3 method due to the tendency of the MINDO/2 method to overestimate the lengths of the X--H bonds by ~ 0.i~ [6]. The C--X bond in the halophenyl4 lithiums is very weak in comparison to that in the monohalogenated methyllithiums and has the same or lower strength as that in the dihalogenated derivatives. The similar calculated values of the enthalpies of elimination of the o-halophenyllithiums and the corresponding dihalomethyllithiums suggest that their real values are also close to one another, i.e., the enthalpies of decomposition of the o-halophenyllithiums are ~ i0 kcal/mole.

According to the experimental data, the times for the decomposition of the halogen-con- taining organolithium compounds do not exceed 1 min at the decomposition temperatures, which are --120, --20, and --66~ for LiCH2CI, LiCHCI2, and LiCCI3 in THF, respectively [i]. This makes it possible to give an upper estimate for the rate constant of the reaction, and using a "normal" value of I0 ~3"5 for thd preexponential factor, we can evaluate the corresponding maximum values of the activation energy. According to these estimates, E a cannot exceed ii, 18, and 14 kcal/mole for the decomposition step in the cases of mono-, di-, and trichloro- methyllithium, respectively. Since these values are greater than AH for LiCHCI= and LiCCI3, as well as o-fluorophenyllithium (the decomposition temperature is --30~ in the same medium [2]), one-step decomposition is possible for the compounds indicated in principle. At the same time, for monochloromethyllithium the value of AH for the decomposition (which is clear- ly overestimated to the least extent among the values for all the compounds we considered) is significantly higher than the maximum activation energy; therefore, the monomolecular one-step decomposition of this compound is impossible at the experimentally observed de-

2008

composition temperature. Such a conclusion is completely consistent with the data in [i, 9], according to which the decomposition of LiCH2CI is an autocatalytic reaction.

CONCLUSIONS

The influence of the halogen on the strength of the adjacent C--Li bond has been evaluated on the basis of thermodynamic and quantum-chemical calculations, and it has been shown that monomolecular decomposition is preferable for LiCHCI2, LiCCI3, LiCFs, and o-FC6H4Li under the conditions of the experiment, while such decomposition is impossible for LiCH2CI.

LITERATURE CITED

1. 2.

3.

4.

5. 6. 7. 8. 9.

G. K~brich, Angew. Chem., 79, 15 (1967). A. I. D'yachenko, A. I. Ioffe, and O. M. Nefedov, Izv. Akad. Nauk SSSR, Ser. Khim., 1979, 366. V. I. Kondrat'ev (editor), Bond Energies, Ionization Potentials, and Electron Affinities [in Russian], Nauka (1974). J. D. Dill, P. V. R. Schleyer, and J. A. Pople, J. Am. Chem. Soc., 98, 1663 (1976). P. S. Bindham, M. J. S. Dewar, and D. H. Lo, J. Am. Chem. Soc., 97, 1285 (1975). M. J. S. Dewar and D. H. Lo, J. Am. Chem. Soc., 94, 5296 (1972). M. J. S. Dewar, J. A. Hashmall, and C. G. Venier, J. Am. Chem. Soc., 90, 1953 (1968). P. Bischof, J. Am. Chem. Soc., 98, 6844 (1976). G. KDbrich and R. H. Fischer, Tetrahedron, 24, 4343 (1968).

UNSTABLE HALOGEN-CONTAINING ORGANOLITHIUM COMPOUNDS.

8. THEORETICAL EVALUATION OF THE STRENGTH

OF CARBON--HALOGEN BONDS IN HALOGEN-CONTAINING CARBANIONS

F. I. Faustov, A. I. D'yachenko, and O. M. Nefedov UDC 541.57:546.26Z546.12: 547.253.4'121

We have experimenfally [1-3] and theoretically [4, 5] shown that geminal and vicinal halogen-containing organolithium compounds, as well as the analogous compounds of other metals [4, 6] in media of low polarity, undergo a one-step decomposition to form the cor- responding carbenes and arynes. At the same time, for the reaction of the corresponding and B hydrogen-containing organic halides with bases taking place in highly polar media and producing the same unstable particles, the following two-step ionic mechanism is traditional- ly postulated [7, 8]:

B- HCXs ~ C X Z - - :CX~ + X -

- H B (I)

X X

H (n)

Since in most cases, the reactions of the latter type proceed under mild conditions, the possibility of the realization of the ionic mechanism presupposes that the C--Hal bonds in intermediate carbanions I and II are significantly weaker than those in the corresponding un-ionized compounds. Although the weakening of the bonds in the anions is fully obvious, for a correct understanding of the mechanism of the processes involved in the generation of unstable particles from halogenated compounds we should give a numerical evaluation of the heats of dissociation of the bonds in anions of types I and II, which, along with similar data for the un-ionized organolithium compounds [5], would make it possible to determine the

" N.D. Zelinskii Institute of Organic Chemistry, Academy of Sciences of the USSR, Mos- ~ cow. Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. i0, pp. 2187- 2190, October, 1979. Original article submitted July 13, 1978.

0568-5230/79/2810-2009507.50 �9 1980 Plenum Publishing Corporation 2009