An Electron Spin Resonance Method for Monitoring the Progressive Replacement of Fluorine by Alkoxy Groups in Perfluorobenzophenonel

FREDERICK PETER SARGENT AND MARSHALL GRANT BAILEY Resenrch Che~nistry Brrr~zch, Alo~nic Energy of Cn11ndr1 Li~rrited,

W/~iteshell Nlrclerrr Resenrcl~ Establish~ne~~t, Pi~zmvn, Ma~zitoba ROE ILO

Received July 6 , 1973

The use of electron spin resonance (e.s.1.) to follow the course of a chemical reaction which does not involve paramagnetic intermediates is reported. The principle of the method is the conversion of the reaction product into a paramagnetic species which may be characterized by e.s.r. In the present example, photoconversion of ketones into radical anions is used to follow the successive displacement of fluorine from perfluorobenzophenone.

On rapporte I'utilisation de la rksonnance paramagnktique Clectronique (1.p.e.) pour etablir la nature du chemin suivi par une reaction chimique qui n'implique pas d'inter~nediaires paramagnetiques. Le principe de la mithode est de transformer le produit de la reaction en une espece paramagnttique caractirisable par r.p.e. A titre d'exemple, on utilise la photoconversion de cetones en radicaux anions pour examiner les deplacements successifs du fluor en perfl~~orobenzophCnone.

[Traduit par le journal] Can. J . Chem., 51,4088 (1973)

Introduction Displacement of fluorine froin highly fluo-

rinated aromatic coinpounds is well known and is an important synthesis route for many partly fluorinated conlpounds ( I ) . During our studies (2) of the photochemistry of perfluorobenzo- phenone, (CGF5),C0, henceforth referred to as PFB, a simple method for demonstrating and monitoring the successive replace~nent of fluorine by methoxide groups in basic methanol solutions was discovered. This replacement process is generalized in eqs. 1 and 2. 11 1 C,,F,,, + C H 3 0 - ->C,,F ,,,- , 0 C H 3 + F -

[21 CnFP(OCH3), + CH3O- C,,F,- I(OCH~),,+ I + F-

The method involves photoconversion of the ketone into its radical anion (3, 4). The number of fluorines replaced lnay be deduced from the changes in the e.s.r. spectra.

Experimental The e.s.r. spectra were obtained with a Varian V-4500

spectrometer. The magnetic field sweep was calibrated with aqueous solutions of Frenly's salt assuming a, to be 13.0 G. All chemicals were reagent grade and were used as received. Solutions of methoxide were prepared by dissolving freshly cut pieces of sodium in methanol. The photolyses were performed in the sample cavity of the e.s.r. spectrometer with a 100-W, compact-arc, medium- pressure, mercury lamp.

Results and Discussion A methanolic solution containing 0.1 mollkg

PFB and 1 mollkg methoxide was prepared by

'AECL No. 4637.

mixing. Aliquots of this were taken at various times after mixing and photolyzed i l l situ in the e.s.r. spectrometer. The spectrum shown in Fig. 1 was observed inlmediately after mixing. This closely resembles that reported by Brown a n d Williams ( 5 ) , the interpretation being given in Table 1. We also assign this spectrum t o the radical anion of 4,4'-diniethoxyoctofl~~orobenzo- phenone, (C,F,OMe),CO. T h e presence of this ketone shows that the two para fluorines of P F B which have a hyperfine splitting of 8.36 G (2), have been replaced by methoxy groups which d o not give a resolvable hyperfine splitting, in the e.s.r. spectrum of the anion.

FIG. 1. The e.s.r. spectrum observed during the photolysis of solutions of methoxide and perfluorobenzo- phenone irnniediately after mixing.

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SARGENT AND BAILEY: PEF

TABLE 1. The e.s.r. hyperfine splitting (G) for the observed spectra. The numbers in

parentheses are the number of equivalent fluorine nuclei

Position

Spectrum ortho meta

Fig. 1 4.57 (4) 1.14 (4) Fig. 2 4 .65(2) 0 .87(4) Fig. 3 5.90 (1) 1 .6 (2)

Photolysis of an aliquot of the mixture I h after mixing gave the spectrum shown in Fig. 2 and described in Table 1. The prominent features of the spectrum are a triplet of quintets. These originate from an anion containing only six fluorine atoms. The four fluorines of 0.87 G were assigned to the tneta positions. The remain- ing two fluorine nuclei are clearly at ortl~o posi- tions but the spectra do not indicate which positions. From steric considerations, this radical was assigned to the ketyl of 2,2',4,4'-tetra- methoxyhexafluorobenzophenone. The presence of this ketone shows that after the two para fluorines are substituted, two ortho fluorines are replaced by methoxide.

Photolysis of an aliquot of the mixture 1 week after mixing gave the spectr~lm shown in Fig. 3 and Table 1. The major features of this are a doublet of triplets which would arise from three fluorine nuclei, two of which are equivalent. The latter have a hyperfine splitting of 1.58 G and undoubtedly occupy tneta positions. The re- maining fluorine with splitting of 5.9 G must

FIG. 2. The e.s.r. spectrum observed during the photolysis of solutions of rnethoxide and perfluorobenzo- phenone 1 h after mixing.

FIG. 3. The e.s.r. spectrum observed during the photolysis of solutions of methoxide and perfluorobenzo- phenone one week after mixing.

originate from an ortl~o position. Therefore this spectrum shows the presence of a heptamethoxy- trifl~~orobenzophenone. The sharp central fea- tures of the spectrum in Fig. 3 arise from residual tetramethoxyhexafluorobenzophenone.

When solutions of PFB and methoxide were mixed and continuously photolyzed, similar changes are observed. Therefore the successive nucleophilic replacement of fluorine by methoxy groups are not affected by the photolysis. The latter merely serves as a probe to determine the progress of the reaction.

Summary The observations reported demonstrate the

use of e.s.r. to follow the successive replace- ments of fluoririe in perfli~orobenzopheno~le by methoxy groups. The principle of the method is the conversion of the products of a chemical reaction into a paramagnetic species which may be characterized by e.s.r. This conversion lnay be made by any means suitable to the system under study. In the present example, the well- known photoconversion of ketones t o radical anions (3,4) was used. Other methods c o ~ ~ l d involve radiolysis, thermolysis, electrolysis, the reaction with alkali metals, or oxidation- reduction reactions.

1. J . C. TATLOW. Endeavour, 22, 89 (1963). 2. F. P. SARGENT and M. G. BAILEY. Can. J . Chem. 49,

2350 (1971). 3. G. A. RUSSELL and E. J . CELLS. Tetrahedron Lett. 20,

1333 (1963). 4. P. B. A y s c o u c ~ and F. P. SARGENT. Chem. Com-

mun. 94 (1963). 5. J. K. BROWN and W. G. WILLIAMS. Trans. Faraday

SOC. 64, 298 (1968).

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