- Email: [email protected]
EPR studies of anion–radicals formed by reduction of perfluorinated α-triketones with some metals of groups I–III
Journal of Fluorine Chemistry 103 (2000) 85±91
EPR studies of anion±radicals formed by reduction of per¯uorinated a-triketones with some metals of groups I±III E.N. Shaposhnikova*, S.R. Sterlin, S.P. Solodovnikov, N.N. Bubnov, B.L. Tumanskii
Institute of Organoelement Compounds, Russian Academy of Sciences, 28 Vavilov St., 117813 Moscow, Russia Received 21 July 1999; accepted 20 October 1999
Abstract Reaction of a,b-bis-¯uorosulfatoketones i-C3F7(CFOSO2F)2C(O)RF , (RFC2F5, i-C3F7) with AcONa/AcOH afforded per¯uoro-atriketones i-C3F7C(O)C(O)C(O)RF, (RFC2F5, i-C3F7). Reduction of these triketones with metals (Li, Na, K, Mg, Cd, Zn, Tl) in THF was studied by EPR spectroscopy. Anion±radicals with unsymmetrical distribution of charge and spin densities are formed in one-electron reduction of the triketones. Unlike hydrocarbon a-polyketones, per¯uorinated triketones can undergo three-electron reduction to form paramagnetic allyl tisalkoxides. # 2000 Elsevier Science S.A. All rights reserved. Keywords: EPR; Anion±radical; Reduction; Per¯uoroalkyl a-triketones; Radical pairs
1. Introduction It is well known that ketones R(CO)nR (n1,2) readily undergo one-electron reduction to form corresponding anion±radicals [1,2,3]. If we consider a-polyketones R(CO)nR (n3±5) as higher homologues of this series we would expect that such compounds would be able to undergo reactions of multihelectron reduction to give paramagnetic particles of several types in the course of the reaction as a result of transfer of various numbers of electrons. Until now formation and recording of paramagnetic particles in the process of reduction of a-polyketones R(CO)nR (n3) has been reported only for one example, namely, H. Rock and co-authors showed that dimesityl-a-tetraone (I) gave anion±radicals (AR) by the action of alkaline metals and barium, the structure ketosemitriones was assigned to these AR [4]1. The authors [4] emphasized that although one-electron reduction of I was relatively easy (in the series of ketones and polyketones R(CO)nR0 (n1±5) the ®rst
*
Corresponding author. Fax: 7-951-355-085. According to our data, in the course of reduction of diphenyl-atriketone with amalgams of metals of I±III groups, only single-changed anion±radicals gave EPR spectra; this confirmed the conclusion made in [4] that formation of paramagnetic particles, which are products of three electron reduction of polyfluoroketones of te hydrocarbon series is impossible. 1
reduction potential was shifted to the anode region as the value of n increased [5]), formation of other paramagnetic species as a result of further reduction of I was not observed. It was shown by CVA method that reduction of I is restricted by transfer of only two electrons, evidently, to form a diamagnetic bis-enolate. Obviously, reduction potentials of a-polyketones are determined not only by the number of carbonyl groups, but also by acceptor properties of substituents R and R0 . In this connection we carried out EPR studies of reduction of ¯uoroaliphatic a-triones i-C3F7C(O)C(O)C(O)RF, (RF C2F5 (II), i-C3F7 (III)). We expected that II and III would be readily reduced owing to the electron-withdrawing effect of per¯uoroalkyl groups affording to the paramagnetic products of not only one-electron transfer, but those of three-electron reduction. 2. Synthesis of bis-fluoroalkyl-a-triones Until now only one representative of ¯uoroaliphatic atriones has been reported, per¯uoroethylisopropyl-a-trione II, obtained in 36% yield by reacting 4,5-bis(¯uorosulfonyloxy)per¯uoro-6-methylheptane-3-one (IV) with MeOH [6]. Further studies showed that the relatively poor yield of a-trione II is explained by the formation of by-products, isomeric dimethylketals (VIa,b), that is well described by a mechanism involving intermediate epoxides (Va,b):
0022-1139/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 2 - 1 1 3 9 ( 9 9 ) 0 0 2 5 8 - 4
86
E.N. Shaposhnikova et al. / Journal of Fluorine Chemistry 103 (2000) 85±91
proposed scheme is con®rmed, to a certain extent, by an increase in the relative content of dimethylketals in the products of the reaction of bis-¯uorosulfatoketone IV with MeOH in the presence of CsF. It is obvious that the undesirable intramolecular substitution of the FSO3 group could be suppressed by the action of the weaker nucleophile in a more acidic medium to increase the yield of the target a-trione. Actually, trione II was obtained in 70% yield by the reaction with AcONa/AcOH.
Bis-(hepta¯uoroisopropyl)-a-trione III was obtained similarly from VII synthesized by electrochemical ¯uorosulfation of F-2,6-dimethyl-3-heptene-5-one. However, unlike its less branched analogue, the preparation of trione III was accompanied by the formation of a multicomponent mixture of by-products including per¯uoroisobutyric acid (VIII) and 2-hydroper¯uoropropane (IX); the appearance of these products in the reaction mixture could be explained by a haloform type reaction of bis-¯uorosulfate VII (but not trione III, which transformed into tris-acetoxysemiketal under the reaction conditions):
The ability of a-¯uorosulfonyloxyketones to undergo cyclization under the action of nucleophiles to give oxiranes as a result of intramolecular substitution of FSO3 group has been previously reported [7]. The
This suggestion was con®rmed to a certain extent by increasing the yield of trione III when the reaction was carried out under conditions of incomplete conversion of product VII (VII:AcONa1:0.9).
E.N. Shaposhnikova et al. / Journal of Fluorine Chemistry 103 (2000) 85±91
87
3. EPR studies of reduction of a-triones II and III In the process of reduction of trione III by phototransfer of an electron from KI, the EPR spectrum of an anion± radical (AR) was recorded characterized by the small values of hyper®ne interaction (h®) constants of an unpaired electron with two equivalent ¯uorine nuclei of the per¯uoroisopropyl group [aF (2F)0.7 G] (Fig. 1). When the trione was reduced with sodium amalgam, interaction of the unpaired electron with ¯uorine nuclei was not pronounced. We assume that in both the cases, the structure in which density of the unpaired electron is localised on the central carbonyl group made a major contribution to the structure of the anion±radicals formed. This type of distribution of charge and density of an unpaired electron was con®rmed by quantum chemical calculations conducted by the MNDO method with UHF approximation (calculations did not take into account the effect of a counterion on the distribution of charge (q) and spin densities (r)) (Table 1). Fig. 1. ESR spectrum of product of one-electron reduction of triketone III by phototransfer of an electron from KI in THF.
The fact that the h® constants in the AR are more than 50% lower than those in the spin-adduct of trione III with the silicon-centered radical i-C3F7C(O)C [OSi(CH3)3]C(O)C3F7-i (ab-F(2F)1.7G, ag-F(12F)0.35G) [8] indicates that the spin distribution is mainly determined by mesomer structure A.
a triplet whose central component coincided with signal a (aF10.25 G). The structure in which unpaired electron density is mainly localised on the carbonyl group, adjacent to the per¯uoroethyl substituent was suggested for such a particle.
As we can see in Fig. 2, three types of particles were recorded in the process of reduction of triketone II with potassium iodide. The anion±radical in which unpaired electron density is mainly localised on the central carbonyl group, corresponded to the predominant signal a. In addition to a, signal b was recorded, characterised by interaction of an unpaired electron with one ¯uorine atom (aF5.5 G); this may be explained by reduction of the carbonyl group, adjacent to the isopropyl substituent. The third signal was
4. Reduction of a-triketones with metals of groups II-III Reduction of triketones II and III with magnesium, zinc, cadmium or thallium occurs in stages. Thus, at the ®rst stage of reduction of III (which lasted for 1 min), the EPR spectrum of the product of one-electron reduction was recorded (for the case of Zn, see Fig. 3), which contained two magnetically non-equivalent per¯uoroisopropyl groups
Table 1 MNDO/UHF calculation data for the anion±radical-product of one-electron reduction of trione III
q r
C(1)
C(2)
C(3)
O(6)
O(5)
O(4)
0.26 0.002
ÿ0.055 0.2
0.23 0.011
ÿ0.34 0.2
ÿ0.4 0.44
ÿ0.32 0.15
Linkage order C(1)-O(4) 1.77
C(2)-O(5) 1.46
C(3)-O(6) 1.83
88
E.N. Shaposhnikova et al. / Journal of Fluorine Chemistry 103 (2000) 85±91 Table 2 Hfi constants (G) of anion±radical products of one-electron reduction of triketone III Reducing agent
ab-F(F)a
ab-F(F)b
ag-F(2CF3)a
ag-F(2CF3)b
Mg/Hg Zn/Hg Cd/Hg Tl/Hg
5.0 5.1 5.0 5.2
0.25
1.35 1.35 1.65 1.5
0.25
aM
9.6 40.25
Table 3 Hfi constants (G) of anion±radicals derived from (CF3)2CFCOCOCF3 Fig. 2. ESR spectrum of product of one-electron reduction of triketone II by phototransfer of an electron from KI in THF.
and interaction of the unpaired electron with magnetic isotopes of cations appeared (Table 2). It follows from these data that species were formed in which the density of the unpaired electron and its charge are distributed in the main between two adjacent carbonyl groups (their h® parameters are close to those for anion± radicals derived from F-methyl-isopropyl-a-dione (Table 3) [9]). Thus, ketosemidione structure (X) can be assigned to the particles formed at the ®rst step of reduction of III by metals of II and III groups.
Where n2, MZn, Mg, Cd and n1, MIn, Tl. When reduction of II was carried out with metals of the second and the third groups, paramagnetic particles of two types were recorded (Figs. 4 and 5). The particles of the ®rst type were characterized by a high value of the h® constant with b-¯uorine nuclei of the ethyl group (Table 4). On the contrary, interaction of the unpaired electron with the ¯uorines of the isopropyl group was recorded in the particles
Fig. 3. ESR spectrum of product of one-electron reduction of triketone III with zinc amalgam in THF.
Reducing agent
ab-F (CF3)
ab-F (1F)
Ag-F(2CF3)
aM
Mg/Hg Zn/Hg Cd/Hg TlCp
11.5 10.5 11.25 12
1.75 1.25 1.5 1.75
1.75 3 3.1 3.5
52.25
Fig. 4. ESR spectra of products of one-electron reduction of triketone II with magnesium amalgam: a signals are assigned to AR of XIa type; b signals are assigned to AR of XIb type.
Fig. 5. ESR spectrum of product of one-electron reduction of triketone II by phototransfer of an electron from CdI2 in THF: a signals are assigned to AR of XIa type; b signals are assigned to AR of XIb type.
E.N. Shaposhnikova et al. / Journal of Fluorine Chemistry 103 (2000) 85±91
89
Table 4 Hfi constants for anion±radicals of type XIa Reducing agent
ab-F(CF2)
ag-F(CF3)
Mg/Hg Zn/Hg CdI2 Tl/Hg
11.25 10.25 10.25 11.0
1.0 1.0 0.85
aM
7.2 55
of the second type (Table 5). Therefore, ketosemidione structures (XIa,b) can be assigned to these particles.
Where n2, MZn, Mg, Cd and n1, MTl. On a more prolonged reduction of II and III (>5 min), the EPR spectra of products of one-electron reduction disappeared. Instead of them, other spectra appeared (Figs. 6 and 7) characterized by larger h® constants with b-¯uorine atoms (Tables 6 and 7) compared to the products of oneelectron interaction. These data indicate that the density of an unpaired electron is higher on carbons C(1) and C(3) and, thus, the unpaired electron is delocalized in an allyl system, therefore, the structure of triply-charged anion±radicals (XIIa,b) can be proposed for the formed AR:
XIIa: RFCF(CF3)2, XIIb: RFCF2CF3 The assumption that these AR (XIIa,b) have allyl structures is also con®rmed by comparison of h® constants with the b-¯uorines in XIIa with those in alkyl radicals containing similar substituents (cf. (CF3)2CFC FCF(CF3)2 ab-F14.8 G [10], (CF3)2CF-C F-CF2CF3 ab-F29.9 G [11]). The observed values of h® constants are smaller, because in the allyl system, the density of an unpaired electron on the terminal carbon atoms is 2/3.
Fig. 6. ESR spectrum of product of three-electron reduction of triketone III with zinc amalgam in THF.
We studied the spectra of frozen solutions of anion± radicals derived from a-triketones at various stages of reduction. The EPR spectra of adducts of one- and threecharged AR derived from III (Fig. 8) contained lines typical of dipole±dipole interactions of unpaired electrons. This allowed identi®cation of the formed radical pairs, to measure parameters of dipole±dipole interaction of unpaired electrons D, and to determine an average distance between the radical centres (Table 8). Because the value of parameter E, which characterises deviation of spin density distribution from axial symmetry for all biradical complexes, is equal to zero, it can be concluded that the chelate centre of these complexes is a tetrahedron with oxygen atoms in the tetrahedron apexes, and there is a cation in the centre of the tetrahedron (cf. [12]). We failed to record the spectra of radical pairs for derivatives II at 77 K, probably, because of too low a concentration of AR. Thus, per¯uorinated a-triketones ± contrary to their non-¯uorinated analogs-form paramagnetic species under the action of metals of groups I±III as products of not only one-electron reduction but also of three-electron reductions. The formation of AR of the latter type undoubtedly re¯ects the electron-withdrawing effect of ¯uoroalkyl substituents.
Table 5 Hfi constants for anion±radicals of type XIb Reducing agent
ab-F(F)
ag-F(2CF3)
Mg/Hg Zn/Hg Cd/Hg
4.0 4.0 ±
1.0 1.0 1.0
Fig. 7. ESR spectrum of product of three-electron reduction of triketone II with cadmium amalgam in THF.
90
E.N. Shaposhnikova et al. / Journal of Fluorine Chemistry 103 (2000) 85±91
Table 6 Hfi constants for anion±radicals, products of three-electron reduction of III
5. Experimental 19
Reducing agent
ab-F(CF2)
ab-F(F)
ag-F(2CF3)
F NMR spectra were recorded on a Bruker-WP200 SV instrument (188.3 MHz, CF3COOH as the external standard). The mass spectra were obtained on a VGMS-70E spectrometer, the energy of ionization was 70 eV. EPR spectra were recorded on a `Varian E-12A' radiospectrometer in degassed quartz tubes. Irradiation was carried out using focussed light of a Dash lamp (DRSh-1000). An electron Unipan regulator was used for thermostatic control of samples. Reduction of di- and triketones was carried out either with Li, Na, K, Zn, Mg, Cd, In, and Tl mercury amalgams or by phototransfer of an electron with MIn, MNa, K, Cd (n1,2) using a procedure reported in [3]. THF was puri®ed by standard procedures [13].
Mg/Hg Zn/Hg Cd/Hg
19.8 20.0 19.75
3.75 3.75 4.5
1.25 1.0 1.0
6. Synthesis of perfluoroethylisopropyl-a-triketone (II)
a
b
Reducing agent
ab-F(2F)
ab-F(F)
ag-F(2CF3)
Mg/Hg Zn/Hg Cd/Hg
6.45 6.8 6.9
6.45 6.8 6.9
1.2 1.2 1.25
a
b
ag-F(2CF3) 1.2 1.2 1.25
Table 7 Hfi constants for anion±radicals, products of three-electron reduction of II
Fig. 8. ESR spectrum of solutions of anion-radicals derived from triketone III at 77 K.
Table 8 Radical pairs parameters (D/E), the average distance between radical centres (L) in symmetrical biradical complexes III with metals of II group, ionic radii of elements (r) Metal (reducing agent)
D/E
L, A
r, A
Mg Zn Cd
375/0 340/0 255/0
4.20 4.36 4.77
0.65 0.74 0.97
(a) Ketone IV (15.6 g, 27 mmol) was gradually added to a mixture of dry CsF (5 g, 33 mmol) and MeOH (25 ml); after the exothermic reaction was complete, the resulting mixture was stirred for 15 min. The reaction mixture was poured into cold 30% H2SO4, the organic layer was separated, distilled over H2SO4 to collect a fraction with b.p. 105±1658C. Subsequent distillation over H2SO4 gave triketone II (1.1 g, 11%, b.p. 99±1018C) (GC-identi®cation by comparison with the authentic sample [6]) and a mixture of compounds with b.p.130±1408C which were identi®ed as dimethylketals VIa and VIb (2:1) (GC-MS). MS, m/z (Irel, %): VIa: 387 [M-CH3O] (1), 359 [MCH3OCO] (6), 299 [M-C2F5] (2), 259 [C6H3F8O2] (2), 193 [C5H6F5O2] (100), 181 [C4F7] (8), 169 [C3F7] (7), 147 [C2F5CO] (8), 131 [C3F5] (30), 119 [C2F5] (30), 97 [C2F3O] (10), 81 [C2F4] (13), 69 [CF3] (35), 59 [CH3OCO] (19), 45 [CO2H] (8), 43 [C2H3O] (5), 29 [CHO], (4). VIb: 399 [M-F] (2.0), 387 [M-MeO] (18), 354 [M-CH3O-CO] (3), 271 [M-C2F5CO] (88), 252 [M-FC2F5CO] (4), 243 [M-2F-C2F5CO] (17), 221 [MC3F7CO] (100), 209 [C5F7CO] , 197 [C3F7CO] (30), 193 [C5F7] 18, 181 [C4F7] 13, 169 [C3F7] (12), 147 [C2F5CO] (12), 131 [C3F5] (20), 119 [C2F5] (67), 100 [C2F4] (11), 81 [C2F4] (15), 69 [CF3] (65), 59 [CH3OCO] (71), 43 [C2H3O] (20), 29 [CHO], (10). (b) Sodium acetate (AcONa3H2O) (7.7 g, 72 mmol) was gradually added to a solution of compound IV (15 g, 26 mmol) in AcOH (25 ml). After the exothermic reaction was complete, the resulting mixture was stirred for 15 min and poured into 30% H2SO4. The lower layer was separated and twice distilled over conc. H2SO4 to give triketone II (6.6 g, 70%), which was identi®ed by comparison with an authentic sample (GLC) [6]. Triketone II is an orange liquid, b.p. 98±1008C. UV (l, nm): 289 (e487), 447 (e60).
E.N. Shaposhnikova et al. / Journal of Fluorine Chemistry 103 (2000) 85±91
7. Synthesis of perfluoro-4,5-bisfluorosulfonyloxy-2,6dimethylheptane-3-one VII (using a procedure reproted in [14]) Fluorosulfonic acid (100 ml) containing NaSO3F (8 g) was placed into a 150 ml one-compartment cell (anodeglassy carbon, cathode Ni). Per¯uoroethyl-3-methylbutenyl ketone [15] (70 g, 163 mmol) was added gradually into the cell for 9 h electrolysis under galvanostatic conditions (I1.8 A) at 28±308C. After the reaction was complete (16.2 A±h of electricity were passed), the mixture was poured into crushed ice, washed with water and aq.Na2CO3. The organic layer was dried over MgSO4 and distilled to give product VII (67.7 g, 66%) as a mixture of isomers, b.p. 54±648C/3 mm Hg. Found C 17.4; F 55.05; S 10.11%. C9F18O7S2. Calculated: C 17.25; F 54.6; S 10.11%. 19 F NMR : 4 (6F1), 103(1F2), 52 (1F31F4), 113(1F5), 6(6F6),ÿ 127(1F7), ÿ129 (1F8).
91
fraction with b.p. 108±1118C (7.0 g), that contained III (contaminated with AcOH). Further distillation afforded starting bisketo¯uorosulfate VII (0.9 g) (b.p. 608C/3 mm Hg). Subsequent distillation of the ®rst fraction over P2O5 gave 4.5 g of triketone III (52% on the basis of the reacted VII). Triketone III is an orange-red liquid, b.p. 108±1098C. UV (l, nm): 281 (e500), 453 (e186). Found: C 25.15, F 62.97%, C9F14O3. Calculated C 25.54, F 63.03%. MS, m/z (Irel, %): 422 [M] (2.13), 394 [M-CO] (4.26), 225 [i-C3F7 (CO)2] (2.13), 197 [C3F7CO] (100.0), 169 [C3F7] (72.34), 150 [C3F6] (8.51), 119 [C2F5] (10.64), 100 [C2F4] (21.28), 69 [CF3] (100.0), 31 [CF] (10.64). 19 F NMR: 19F: 3,5 (12F), ÿ117 (2F). Acknowledgements The work has been supported by the Russian Basic Research Foundation (grant N98-03-32933a). References
MS, m/z (Irel, %): 429 [M-C3F7CO] (8.8), 330 [C3F7CFOSO2FCF] (6.7), 299 [C3F7CFOSO2F] (4.4), 219 [C4F9] (33.3), 197 [C3F7CO] (62.2), 169 [C3F7] (33.3), 83 [SO2F] (48.9), 69 [CF3] (100.0). 8. Synthesis of perfluorodiisopropyl a-triketone III (a) A ®nely crushed portion of AcONa3H2O (18.2 g, 134 mmol) was added to a mixture of bis-¯uorosulfate VII (28.1 g, 44 mmol) and AcOH (30 ml). The reaction mixture was heated to 308C. After the start of the exothermic reaction it was stirred until gas liberation stopped (ca. 1 h). The reaction mixture was poured into cold 30% sulfuric acid; the organic layer was separated and twice distilled over H2SO4 to give a fraction with b.p. 90±1128C (12.6 g). It contained triketone III (50%) and a mixture of by-products (GC), among which per¯uoroisobutyric acid VIII and 2-hydroper¯uoropropane IX were detected and identi®ed by GC by comparison with authentic samples [16,17]. Subsequent recti®cation gave a fraction (6.5 g, 35%), b.p. 107±1108C that contained compound III and (CF3)2CFCOOH (30%) (19F NMR and GC-MS). (b) The reaction of compound VII ( 16 g, 25 mmol) with AcONa3H2O (6.2 g, 45 mmol) in AcOH (25 ml) was carried out similarly. Treatment with 30% sulfuric acid resulted in a two-phase organic layer. The starting bisketo¯uorosulfate VII (2.3 g) was separated as the lower layer. The upper layer was twice distilled over conc. sulfuric acid to give a
[1] G.A. Rassell, D.F. Lawson, H.L. Malkus, R.D. Stephens, G.R. Underwood, T. Takano, V. Malatesta, J. Am. Chem. Soc. 96 (1974) 5830. [2] G.A. Rassell, J.L. Gerlock, D.F. Lawson, J. Am. Chem. Soc. 93 (1971) 4088. [3] G.A. Rassell, J.L. Gerlock, J. Am. Chem. Soc. 96 (1974) 5838. [4] H. Bock, P. Hanel, H.-F. Herrmann, Naturforschung 45 (1990) 1197. [5] R. Gleiter, G. Kennrich, M. Langer, Angew.Chem., Int. Ed. Engl. 25 (1986) 999. [6] V.M. Rogovic, N.I. Delyagina, E.A. Avetisyan, V.F. Cherstkov, S.R. Sterlin, L.S. German, Bull. Acad. Sci. USSR, Div. Chem. Sci. 40 (1991) 1728. [7] V.M. Rogovic, N.I. Delyagina, E.I. Mysov, V.F. Cherstkov, S.R. Sterlin, L.S. German, Bull. Acad. Sci. USSR, Div. Chem. Sci. 39 (1990) 1870. [8] E.N. Shaposhnikova, S.R. Sterlin, S.P. Solodovnikov, N.N. Bubnov, I.V. Stankevich, A.L. Chistyakov, B.L. Tumanskii, Russ. Chem. Bull. 48 (1999) 470. [9] E.N. Shaposhnikova, S.R. Sterlin, S.P. Solodovnikov, N.N. Bubnov, I.V. Stankevich, A.L. Chistyakov, B.L. Tumanskii, Russ. Chem. Bull. 47 (1998) 2160. [10] B.L. Tumanskii, S.P. Solodovnikov, N.N. Bubnov, S.A. Postovoi, Yu.V. Zeifman, Bull. Acad. Sci. USSR, Div. Chem. Sci. 38 (1989) 1771. [11] S.R. Sterlin, V.F. Cherstkov, B.L. Tumanskii, E.A. Avetisyan, J. Fluor. Chem. 80 (1996) 77. [12] A.I. Prokof'ev, N.A. Malysheva, N.N. Bubnov, S.P. Solodovnikov, M.I. Kabachnik, Dokl. Akad. Nauk SSSR 252 (1980) 370. [13] A.J. Gordon, R.A. Ford, The Chemist's Companion. Wiley, New York, 1972. [14] V.M. Rogovik, Ya.I. Koval'skii, N.I. Delyagina, E.I. Mysov, V.M. Gida, V.A. Grinberg, V.F. Cherstkov, S.R. Sterlin, L.S. German, Bull. Acad. Sci. USSR, Div. Chem. Sci. 39 (1990) 1862. [15] S.D. Chepik, G.G. Belen'kii, V.F. Cherstkov, S.R. Sterlin, L.S. German, Bull. Acad. Sci. USSR, Div. Chem. Sci. 40 (1991) 446. [16] R.N. Haszeldine, J. Chem. Soc. (1953) 3559. [17] G.B. Barlow, M. Stacey, J.C. Tatlow, J. Chem. Soc. (1955) 1741.
EPR studies of anion±radicals formed by reduction of per¯uorinated a-triketones with some metals of groups I±III E.N. Shaposhnikova*, S.R. Sterlin, S.P. Solodovnikov, N.N. Bubnov, B.L. Tumanskii
Institute of Organoelement Compounds, Russian Academy of Sciences, 28 Vavilov St., 117813 Moscow, Russia Received 21 July 1999; accepted 20 October 1999
Abstract Reaction of a,b-bis-¯uorosulfatoketones i-C3F7(CFOSO2F)2C(O)RF , (RFC2F5, i-C3F7) with AcONa/AcOH afforded per¯uoro-atriketones i-C3F7C(O)C(O)C(O)RF, (RFC2F5, i-C3F7). Reduction of these triketones with metals (Li, Na, K, Mg, Cd, Zn, Tl) in THF was studied by EPR spectroscopy. Anion±radicals with unsymmetrical distribution of charge and spin densities are formed in one-electron reduction of the triketones. Unlike hydrocarbon a-polyketones, per¯uorinated triketones can undergo three-electron reduction to form paramagnetic allyl tisalkoxides. # 2000 Elsevier Science S.A. All rights reserved. Keywords: EPR; Anion±radical; Reduction; Per¯uoroalkyl a-triketones; Radical pairs
1. Introduction It is well known that ketones R(CO)nR (n1,2) readily undergo one-electron reduction to form corresponding anion±radicals [1,2,3]. If we consider a-polyketones R(CO)nR (n3±5) as higher homologues of this series we would expect that such compounds would be able to undergo reactions of multihelectron reduction to give paramagnetic particles of several types in the course of the reaction as a result of transfer of various numbers of electrons. Until now formation and recording of paramagnetic particles in the process of reduction of a-polyketones R(CO)nR (n3) has been reported only for one example, namely, H. Rock and co-authors showed that dimesityl-a-tetraone (I) gave anion±radicals (AR) by the action of alkaline metals and barium, the structure ketosemitriones was assigned to these AR [4]1. The authors [4] emphasized that although one-electron reduction of I was relatively easy (in the series of ketones and polyketones R(CO)nR0 (n1±5) the ®rst
*
Corresponding author. Fax: 7-951-355-085. According to our data, in the course of reduction of diphenyl-atriketone with amalgams of metals of I±III groups, only single-changed anion±radicals gave EPR spectra; this confirmed the conclusion made in [4] that formation of paramagnetic particles, which are products of three electron reduction of polyfluoroketones of te hydrocarbon series is impossible. 1
reduction potential was shifted to the anode region as the value of n increased [5]), formation of other paramagnetic species as a result of further reduction of I was not observed. It was shown by CVA method that reduction of I is restricted by transfer of only two electrons, evidently, to form a diamagnetic bis-enolate. Obviously, reduction potentials of a-polyketones are determined not only by the number of carbonyl groups, but also by acceptor properties of substituents R and R0 . In this connection we carried out EPR studies of reduction of ¯uoroaliphatic a-triones i-C3F7C(O)C(O)C(O)RF, (RF C2F5 (II), i-C3F7 (III)). We expected that II and III would be readily reduced owing to the electron-withdrawing effect of per¯uoroalkyl groups affording to the paramagnetic products of not only one-electron transfer, but those of three-electron reduction. 2. Synthesis of bis-fluoroalkyl-a-triones Until now only one representative of ¯uoroaliphatic atriones has been reported, per¯uoroethylisopropyl-a-trione II, obtained in 36% yield by reacting 4,5-bis(¯uorosulfonyloxy)per¯uoro-6-methylheptane-3-one (IV) with MeOH [6]. Further studies showed that the relatively poor yield of a-trione II is explained by the formation of by-products, isomeric dimethylketals (VIa,b), that is well described by a mechanism involving intermediate epoxides (Va,b):
0022-1139/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 2 - 1 1 3 9 ( 9 9 ) 0 0 2 5 8 - 4
86
E.N. Shaposhnikova et al. / Journal of Fluorine Chemistry 103 (2000) 85±91
proposed scheme is con®rmed, to a certain extent, by an increase in the relative content of dimethylketals in the products of the reaction of bis-¯uorosulfatoketone IV with MeOH in the presence of CsF. It is obvious that the undesirable intramolecular substitution of the FSO3 group could be suppressed by the action of the weaker nucleophile in a more acidic medium to increase the yield of the target a-trione. Actually, trione II was obtained in 70% yield by the reaction with AcONa/AcOH.
Bis-(hepta¯uoroisopropyl)-a-trione III was obtained similarly from VII synthesized by electrochemical ¯uorosulfation of F-2,6-dimethyl-3-heptene-5-one. However, unlike its less branched analogue, the preparation of trione III was accompanied by the formation of a multicomponent mixture of by-products including per¯uoroisobutyric acid (VIII) and 2-hydroper¯uoropropane (IX); the appearance of these products in the reaction mixture could be explained by a haloform type reaction of bis-¯uorosulfate VII (but not trione III, which transformed into tris-acetoxysemiketal under the reaction conditions):
The ability of a-¯uorosulfonyloxyketones to undergo cyclization under the action of nucleophiles to give oxiranes as a result of intramolecular substitution of FSO3 group has been previously reported [7]. The
This suggestion was con®rmed to a certain extent by increasing the yield of trione III when the reaction was carried out under conditions of incomplete conversion of product VII (VII:AcONa1:0.9).
E.N. Shaposhnikova et al. / Journal of Fluorine Chemistry 103 (2000) 85±91
87
3. EPR studies of reduction of a-triones II and III In the process of reduction of trione III by phototransfer of an electron from KI, the EPR spectrum of an anion± radical (AR) was recorded characterized by the small values of hyper®ne interaction (h®) constants of an unpaired electron with two equivalent ¯uorine nuclei of the per¯uoroisopropyl group [aF (2F)0.7 G] (Fig. 1). When the trione was reduced with sodium amalgam, interaction of the unpaired electron with ¯uorine nuclei was not pronounced. We assume that in both the cases, the structure in which density of the unpaired electron is localised on the central carbonyl group made a major contribution to the structure of the anion±radicals formed. This type of distribution of charge and density of an unpaired electron was con®rmed by quantum chemical calculations conducted by the MNDO method with UHF approximation (calculations did not take into account the effect of a counterion on the distribution of charge (q) and spin densities (r)) (Table 1). Fig. 1. ESR spectrum of product of one-electron reduction of triketone III by phototransfer of an electron from KI in THF.
The fact that the h® constants in the AR are more than 50% lower than those in the spin-adduct of trione III with the silicon-centered radical i-C3F7C(O)C [OSi(CH3)3]C(O)C3F7-i (ab-F(2F)1.7G, ag-F(12F)0.35G) [8] indicates that the spin distribution is mainly determined by mesomer structure A.
a triplet whose central component coincided with signal a (aF10.25 G). The structure in which unpaired electron density is mainly localised on the carbonyl group, adjacent to the per¯uoroethyl substituent was suggested for such a particle.
As we can see in Fig. 2, three types of particles were recorded in the process of reduction of triketone II with potassium iodide. The anion±radical in which unpaired electron density is mainly localised on the central carbonyl group, corresponded to the predominant signal a. In addition to a, signal b was recorded, characterised by interaction of an unpaired electron with one ¯uorine atom (aF5.5 G); this may be explained by reduction of the carbonyl group, adjacent to the isopropyl substituent. The third signal was
4. Reduction of a-triketones with metals of groups II-III Reduction of triketones II and III with magnesium, zinc, cadmium or thallium occurs in stages. Thus, at the ®rst stage of reduction of III (which lasted for 1 min), the EPR spectrum of the product of one-electron reduction was recorded (for the case of Zn, see Fig. 3), which contained two magnetically non-equivalent per¯uoroisopropyl groups
Table 1 MNDO/UHF calculation data for the anion±radical-product of one-electron reduction of trione III
q r
C(1)
C(2)
C(3)
O(6)
O(5)
O(4)
0.26 0.002
ÿ0.055 0.2
0.23 0.011
ÿ0.34 0.2
ÿ0.4 0.44
ÿ0.32 0.15
Linkage order C(1)-O(4) 1.77
C(2)-O(5) 1.46
C(3)-O(6) 1.83
88
E.N. Shaposhnikova et al. / Journal of Fluorine Chemistry 103 (2000) 85±91 Table 2 Hfi constants (G) of anion±radical products of one-electron reduction of triketone III Reducing agent
ab-F(F)a
ab-F(F)b
ag-F(2CF3)a
ag-F(2CF3)b
Mg/Hg Zn/Hg Cd/Hg Tl/Hg
5.0 5.1 5.0 5.2
0.25
1.35 1.35 1.65 1.5
0.25
aM
9.6 40.25
Table 3 Hfi constants (G) of anion±radicals derived from (CF3)2CFCOCOCF3 Fig. 2. ESR spectrum of product of one-electron reduction of triketone II by phototransfer of an electron from KI in THF.
and interaction of the unpaired electron with magnetic isotopes of cations appeared (Table 2). It follows from these data that species were formed in which the density of the unpaired electron and its charge are distributed in the main between two adjacent carbonyl groups (their h® parameters are close to those for anion± radicals derived from F-methyl-isopropyl-a-dione (Table 3) [9]). Thus, ketosemidione structure (X) can be assigned to the particles formed at the ®rst step of reduction of III by metals of II and III groups.
Where n2, MZn, Mg, Cd and n1, MIn, Tl. When reduction of II was carried out with metals of the second and the third groups, paramagnetic particles of two types were recorded (Figs. 4 and 5). The particles of the ®rst type were characterized by a high value of the h® constant with b-¯uorine nuclei of the ethyl group (Table 4). On the contrary, interaction of the unpaired electron with the ¯uorines of the isopropyl group was recorded in the particles
Fig. 3. ESR spectrum of product of one-electron reduction of triketone III with zinc amalgam in THF.
Reducing agent
ab-F (CF3)
ab-F (1F)
Ag-F(2CF3)
aM
Mg/Hg Zn/Hg Cd/Hg TlCp
11.5 10.5 11.25 12
1.75 1.25 1.5 1.75
1.75 3 3.1 3.5
52.25
Fig. 4. ESR spectra of products of one-electron reduction of triketone II with magnesium amalgam: a signals are assigned to AR of XIa type; b signals are assigned to AR of XIb type.
Fig. 5. ESR spectrum of product of one-electron reduction of triketone II by phototransfer of an electron from CdI2 in THF: a signals are assigned to AR of XIa type; b signals are assigned to AR of XIb type.
E.N. Shaposhnikova et al. / Journal of Fluorine Chemistry 103 (2000) 85±91
89
Table 4 Hfi constants for anion±radicals of type XIa Reducing agent
ab-F(CF2)
ag-F(CF3)
Mg/Hg Zn/Hg CdI2 Tl/Hg
11.25 10.25 10.25 11.0
1.0 1.0 0.85
aM
7.2 55
of the second type (Table 5). Therefore, ketosemidione structures (XIa,b) can be assigned to these particles.
Where n2, MZn, Mg, Cd and n1, MTl. On a more prolonged reduction of II and III (>5 min), the EPR spectra of products of one-electron reduction disappeared. Instead of them, other spectra appeared (Figs. 6 and 7) characterized by larger h® constants with b-¯uorine atoms (Tables 6 and 7) compared to the products of oneelectron interaction. These data indicate that the density of an unpaired electron is higher on carbons C(1) and C(3) and, thus, the unpaired electron is delocalized in an allyl system, therefore, the structure of triply-charged anion±radicals (XIIa,b) can be proposed for the formed AR:
XIIa: RFCF(CF3)2, XIIb: RFCF2CF3 The assumption that these AR (XIIa,b) have allyl structures is also con®rmed by comparison of h® constants with the b-¯uorines in XIIa with those in alkyl radicals containing similar substituents (cf. (CF3)2CFC FCF(CF3)2 ab-F14.8 G [10], (CF3)2CF-C F-CF2CF3 ab-F29.9 G [11]). The observed values of h® constants are smaller, because in the allyl system, the density of an unpaired electron on the terminal carbon atoms is 2/3.
Fig. 6. ESR spectrum of product of three-electron reduction of triketone III with zinc amalgam in THF.
We studied the spectra of frozen solutions of anion± radicals derived from a-triketones at various stages of reduction. The EPR spectra of adducts of one- and threecharged AR derived from III (Fig. 8) contained lines typical of dipole±dipole interactions of unpaired electrons. This allowed identi®cation of the formed radical pairs, to measure parameters of dipole±dipole interaction of unpaired electrons D, and to determine an average distance between the radical centres (Table 8). Because the value of parameter E, which characterises deviation of spin density distribution from axial symmetry for all biradical complexes, is equal to zero, it can be concluded that the chelate centre of these complexes is a tetrahedron with oxygen atoms in the tetrahedron apexes, and there is a cation in the centre of the tetrahedron (cf. [12]). We failed to record the spectra of radical pairs for derivatives II at 77 K, probably, because of too low a concentration of AR. Thus, per¯uorinated a-triketones ± contrary to their non-¯uorinated analogs-form paramagnetic species under the action of metals of groups I±III as products of not only one-electron reduction but also of three-electron reductions. The formation of AR of the latter type undoubtedly re¯ects the electron-withdrawing effect of ¯uoroalkyl substituents.
Table 5 Hfi constants for anion±radicals of type XIb Reducing agent
ab-F(F)
ag-F(2CF3)
Mg/Hg Zn/Hg Cd/Hg
4.0 4.0 ±
1.0 1.0 1.0
Fig. 7. ESR spectrum of product of three-electron reduction of triketone II with cadmium amalgam in THF.
90
E.N. Shaposhnikova et al. / Journal of Fluorine Chemistry 103 (2000) 85±91
Table 6 Hfi constants for anion±radicals, products of three-electron reduction of III
5. Experimental 19
Reducing agent
ab-F(CF2)
ab-F(F)
ag-F(2CF3)
F NMR spectra were recorded on a Bruker-WP200 SV instrument (188.3 MHz, CF3COOH as the external standard). The mass spectra were obtained on a VGMS-70E spectrometer, the energy of ionization was 70 eV. EPR spectra were recorded on a `Varian E-12A' radiospectrometer in degassed quartz tubes. Irradiation was carried out using focussed light of a Dash lamp (DRSh-1000). An electron Unipan regulator was used for thermostatic control of samples. Reduction of di- and triketones was carried out either with Li, Na, K, Zn, Mg, Cd, In, and Tl mercury amalgams or by phototransfer of an electron with MIn, MNa, K, Cd (n1,2) using a procedure reported in [3]. THF was puri®ed by standard procedures [13].
Mg/Hg Zn/Hg Cd/Hg
19.8 20.0 19.75
3.75 3.75 4.5
1.25 1.0 1.0
6. Synthesis of perfluoroethylisopropyl-a-triketone (II)
a
b
Reducing agent
ab-F(2F)
ab-F(F)
ag-F(2CF3)
Mg/Hg Zn/Hg Cd/Hg
6.45 6.8 6.9
6.45 6.8 6.9
1.2 1.2 1.25
a
b
ag-F(2CF3) 1.2 1.2 1.25
Table 7 Hfi constants for anion±radicals, products of three-electron reduction of II
Fig. 8. ESR spectrum of solutions of anion-radicals derived from triketone III at 77 K.
Table 8 Radical pairs parameters (D/E), the average distance between radical centres (L) in symmetrical biradical complexes III with metals of II group, ionic radii of elements (r) Metal (reducing agent)
D/E
L, A
r, A
Mg Zn Cd
375/0 340/0 255/0
4.20 4.36 4.77
0.65 0.74 0.97
(a) Ketone IV (15.6 g, 27 mmol) was gradually added to a mixture of dry CsF (5 g, 33 mmol) and MeOH (25 ml); after the exothermic reaction was complete, the resulting mixture was stirred for 15 min. The reaction mixture was poured into cold 30% H2SO4, the organic layer was separated, distilled over H2SO4 to collect a fraction with b.p. 105±1658C. Subsequent distillation over H2SO4 gave triketone II (1.1 g, 11%, b.p. 99±1018C) (GC-identi®cation by comparison with the authentic sample [6]) and a mixture of compounds with b.p.130±1408C which were identi®ed as dimethylketals VIa and VIb (2:1) (GC-MS). MS, m/z (Irel, %): VIa: 387 [M-CH3O] (1), 359 [MCH3OCO] (6), 299 [M-C2F5] (2), 259 [C6H3F8O2] (2), 193 [C5H6F5O2] (100), 181 [C4F7] (8), 169 [C3F7] (7), 147 [C2F5CO] (8), 131 [C3F5] (30), 119 [C2F5] (30), 97 [C2F3O] (10), 81 [C2F4] (13), 69 [CF3] (35), 59 [CH3OCO] (19), 45 [CO2H] (8), 43 [C2H3O] (5), 29 [CHO], (4). VIb: 399 [M-F] (2.0), 387 [M-MeO] (18), 354 [M-CH3O-CO] (3), 271 [M-C2F5CO] (88), 252 [M-FC2F5CO] (4), 243 [M-2F-C2F5CO] (17), 221 [MC3F7CO] (100), 209 [C5F7CO] , 197 [C3F7CO] (30), 193 [C5F7] 18, 181 [C4F7] 13, 169 [C3F7] (12), 147 [C2F5CO] (12), 131 [C3F5] (20), 119 [C2F5] (67), 100 [C2F4] (11), 81 [C2F4] (15), 69 [CF3] (65), 59 [CH3OCO] (71), 43 [C2H3O] (20), 29 [CHO], (10). (b) Sodium acetate (AcONa3H2O) (7.7 g, 72 mmol) was gradually added to a solution of compound IV (15 g, 26 mmol) in AcOH (25 ml). After the exothermic reaction was complete, the resulting mixture was stirred for 15 min and poured into 30% H2SO4. The lower layer was separated and twice distilled over conc. H2SO4 to give triketone II (6.6 g, 70%), which was identi®ed by comparison with an authentic sample (GLC) [6]. Triketone II is an orange liquid, b.p. 98±1008C. UV (l, nm): 289 (e487), 447 (e60).
E.N. Shaposhnikova et al. / Journal of Fluorine Chemistry 103 (2000) 85±91
7. Synthesis of perfluoro-4,5-bisfluorosulfonyloxy-2,6dimethylheptane-3-one VII (using a procedure reproted in [14]) Fluorosulfonic acid (100 ml) containing NaSO3F (8 g) was placed into a 150 ml one-compartment cell (anodeglassy carbon, cathode Ni). Per¯uoroethyl-3-methylbutenyl ketone [15] (70 g, 163 mmol) was added gradually into the cell for 9 h electrolysis under galvanostatic conditions (I1.8 A) at 28±308C. After the reaction was complete (16.2 A±h of electricity were passed), the mixture was poured into crushed ice, washed with water and aq.Na2CO3. The organic layer was dried over MgSO4 and distilled to give product VII (67.7 g, 66%) as a mixture of isomers, b.p. 54±648C/3 mm Hg. Found C 17.4; F 55.05; S 10.11%. C9F18O7S2. Calculated: C 17.25; F 54.6; S 10.11%. 19 F NMR : 4 (6F1), 103(1F2), 52 (1F31F4), 113(1F5), 6(6F6),ÿ 127(1F7), ÿ129 (1F8).
91
fraction with b.p. 108±1118C (7.0 g), that contained III (contaminated with AcOH). Further distillation afforded starting bisketo¯uorosulfate VII (0.9 g) (b.p. 608C/3 mm Hg). Subsequent distillation of the ®rst fraction over P2O5 gave 4.5 g of triketone III (52% on the basis of the reacted VII). Triketone III is an orange-red liquid, b.p. 108±1098C. UV (l, nm): 281 (e500), 453 (e186). Found: C 25.15, F 62.97%, C9F14O3. Calculated C 25.54, F 63.03%. MS, m/z (Irel, %): 422 [M] (2.13), 394 [M-CO] (4.26), 225 [i-C3F7 (CO)2] (2.13), 197 [C3F7CO] (100.0), 169 [C3F7] (72.34), 150 [C3F6] (8.51), 119 [C2F5] (10.64), 100 [C2F4] (21.28), 69 [CF3] (100.0), 31 [CF] (10.64). 19 F NMR: 19F: 3,5 (12F), ÿ117 (2F). Acknowledgements The work has been supported by the Russian Basic Research Foundation (grant N98-03-32933a). References
MS, m/z (Irel, %): 429 [M-C3F7CO] (8.8), 330 [C3F7CFOSO2FCF] (6.7), 299 [C3F7CFOSO2F] (4.4), 219 [C4F9] (33.3), 197 [C3F7CO] (62.2), 169 [C3F7] (33.3), 83 [SO2F] (48.9), 69 [CF3] (100.0). 8. Synthesis of perfluorodiisopropyl a-triketone III (a) A ®nely crushed portion of AcONa3H2O (18.2 g, 134 mmol) was added to a mixture of bis-¯uorosulfate VII (28.1 g, 44 mmol) and AcOH (30 ml). The reaction mixture was heated to 308C. After the start of the exothermic reaction it was stirred until gas liberation stopped (ca. 1 h). The reaction mixture was poured into cold 30% sulfuric acid; the organic layer was separated and twice distilled over H2SO4 to give a fraction with b.p. 90±1128C (12.6 g). It contained triketone III (50%) and a mixture of by-products (GC), among which per¯uoroisobutyric acid VIII and 2-hydroper¯uoropropane IX were detected and identi®ed by GC by comparison with authentic samples [16,17]. Subsequent recti®cation gave a fraction (6.5 g, 35%), b.p. 107±1108C that contained compound III and (CF3)2CFCOOH (30%) (19F NMR and GC-MS). (b) The reaction of compound VII ( 16 g, 25 mmol) with AcONa3H2O (6.2 g, 45 mmol) in AcOH (25 ml) was carried out similarly. Treatment with 30% sulfuric acid resulted in a two-phase organic layer. The starting bisketo¯uorosulfate VII (2.3 g) was separated as the lower layer. The upper layer was twice distilled over conc. sulfuric acid to give a
[1] G.A. Rassell, D.F. Lawson, H.L. Malkus, R.D. Stephens, G.R. Underwood, T. Takano, V. Malatesta, J. Am. Chem. Soc. 96 (1974) 5830. [2] G.A. Rassell, J.L. Gerlock, D.F. Lawson, J. Am. Chem. Soc. 93 (1971) 4088. [3] G.A. Rassell, J.L. Gerlock, J. Am. Chem. Soc. 96 (1974) 5838. [4] H. Bock, P. Hanel, H.-F. Herrmann, Naturforschung 45 (1990) 1197. [5] R. Gleiter, G. Kennrich, M. Langer, Angew.Chem., Int. Ed. Engl. 25 (1986) 999. [6] V.M. Rogovic, N.I. Delyagina, E.A. Avetisyan, V.F. Cherstkov, S.R. Sterlin, L.S. German, Bull. Acad. Sci. USSR, Div. Chem. Sci. 40 (1991) 1728. [7] V.M. Rogovic, N.I. Delyagina, E.I. Mysov, V.F. Cherstkov, S.R. Sterlin, L.S. German, Bull. Acad. Sci. USSR, Div. Chem. Sci. 39 (1990) 1870. [8] E.N. Shaposhnikova, S.R. Sterlin, S.P. Solodovnikov, N.N. Bubnov, I.V. Stankevich, A.L. Chistyakov, B.L. Tumanskii, Russ. Chem. Bull. 48 (1999) 470. [9] E.N. Shaposhnikova, S.R. Sterlin, S.P. Solodovnikov, N.N. Bubnov, I.V. Stankevich, A.L. Chistyakov, B.L. Tumanskii, Russ. Chem. Bull. 47 (1998) 2160. [10] B.L. Tumanskii, S.P. Solodovnikov, N.N. Bubnov, S.A. Postovoi, Yu.V. Zeifman, Bull. Acad. Sci. USSR, Div. Chem. Sci. 38 (1989) 1771. [11] S.R. Sterlin, V.F. Cherstkov, B.L. Tumanskii, E.A. Avetisyan, J. Fluor. Chem. 80 (1996) 77. [12] A.I. Prokof'ev, N.A. Malysheva, N.N. Bubnov, S.P. Solodovnikov, M.I. Kabachnik, Dokl. Akad. Nauk SSSR 252 (1980) 370. [13] A.J. Gordon, R.A. Ford, The Chemist's Companion. Wiley, New York, 1972. [14] V.M. Rogovik, Ya.I. Koval'skii, N.I. Delyagina, E.I. Mysov, V.M. Gida, V.A. Grinberg, V.F. Cherstkov, S.R. Sterlin, L.S. German, Bull. Acad. Sci. USSR, Div. Chem. Sci. 39 (1990) 1862. [15] S.D. Chepik, G.G. Belen'kii, V.F. Cherstkov, S.R. Sterlin, L.S. German, Bull. Acad. Sci. USSR, Div. Chem. Sci. 40 (1991) 446. [16] R.N. Haszeldine, J. Chem. Soc. (1953) 3559. [17] G.B. Barlow, M. Stacey, J.C. Tatlow, J. Chem. Soc. (1955) 1741.