2030
K. S.Chen, P. J. Krusic, andd. K. Kochi
(14) R. W. Fessendenand R. ti. Schuler, J. Chem. Phys., 43, 2704 (1965). (15) (a) K. Morokuma, L. Pederson, and M. Karplus, J. Chem. Phys., 48, 4801 (1968); (b) D. L. Beverage, P. A. Dobosh, and J. A. Pople, ibid.., 48, 4802 (1968); (c) H. Konishi and K. Morokuma, J. Amer. Chem. SOC., 94, 5603 (1972); (d) See also L. Pauling, J. Chem. Phys., 51, 2767 (1969). (16) J. K. Kochi, ID. Bakuzis, ;and P. J. Krusic, J. Amer. Chem. SOC.,95, 1516 (1973), and references cited therein. (17) (a) H. Fischer and H. Hefter, 2.Naturforsch. A, 23, 1763 (1968); (b) I. A. Zlochower, W. 4. Miller, and G. K. Fraenkel, J. Chem. Phys., 42, 3339 ( 1965). (18) M. T. Rogersand L. D. Kispert, J. Chem. Phys., 46, 3193 (1967). (19) N. Bloembergen, E. M. Purcell, and R. V. Pound, Phys. Rev., 73, 879 (1948). (20) P. J. Krusic, P. Meakin, and B. Smart, to be submitted for publication. (21) K. S. Chen and ,1. K. Koohi, J. Amer. Chem. SOC.,98, 794 (1974). (22) The p-fluoririe splittirlgs are less reliably calculated by INDO (vide infra). (23) (a) R. W. Fessenden and R. H. Schuler, J. Chem. Phys., 39, 2147 (1963); (b) For a review, see J. E. Wertz and J. R. Bolton, "Electron Spin Resonance," McGraw-Mi, New York, N. Y.,1972. (24) (a) A. J. Dobbs, 8. 6.Gilbert, and R. 0.C. Norman, J. Chem. SOC.A, 124 (1971); (b) P. J. Krusic, T. A. Rettig, and P. v. R. Schleyer, J. Amer. Chem. Soc., 9 4 995 (1972). (25) R. Livingston and H. Zeldes, J. Chem. Phys.. 44, 1245 (1966). (26) A. Hudson and K. D. J. Root, Tetrahedron, 25, 5311 (1969). (27) P. J. Krusic. unpublishedresults. (28) iNDO calculations as a function of the configuration at the 01 carbon for VI1 and IX predict normal CHQproton splittings for the planar structures (-25 G). Unpublishedresults. (29) K. S.@henand J. K. Kochi, unpublishedresults.
(30) K. S.Chen and N. Hirota, "Investigation of Rates and Mechanisms," G. Hammes, Ed., Wiley, New York, N. Y., 1973, Chapter 13, Part II. (31) (a) P. J. Krusic, P. Meakln, and J. P. Jesson, J. Phys. Chem., 75, 3438 (1971); (b) P. Meakln, E. L. Muetterties, F. N. Tebbe, and J. P. Jesson, J. Amer. Chem. SOC., 93, 4701 (1971). (32) (a) D. E. Wood, L. F. Williams, R. F. Sprecher, and W. A. Lathan, J. Amer. Chem. Soc., 94, 6241 (1972); (b) D. E. Wood, private communication. (33) D. J. Edge and J. K. Kochi, J. Amer. Chem. SOC.,94, 6485 (1972). (34) E. 8. Wilson, Jr., Advan. Chem. Phys., 2, 367 (1959); J. Dale, Tefrahedron, 22, 3373 (1966); 6. J. Karabatsos and D. J. Fenoalio, Ton Sfereochem., 5, 167 (1970). (35) R. W. Fessenden, J. Chim. Phys., 61, 1570 (1964). (36) P. J. Krusic. K. S. Chen, P. Meakin, and J. K. Kochi, to be submitted for publication. (37) A dynamic model based on a dominant sixfold component for the potential function can also account for the spectral behavior of the CH2.CH2F radical. The resulting barrier height of 1-2 kcal mol-', however, IS too large for a sixfold potential function.34 (38) A recent study of the temperature dependence of apn and a p ~ in CH2CHpF and CHpCHF2 failed to recognize the importance of the fourfold term of the potential function (I.Biddles, J. Cooper, A. Hudson, R. A. Jackson, and J. T. Wiffen, Mol. Phys., 25, 225 (1973)). (39) W. F. K. Wynne-Jones and H. Eyring, J. Chem. Phys., 3,492 (1935). (40) P. D. Bartlett and R. R. Hiatt, J. Amer. Chem. Soc., 80, 1398 (1958). (41) R. K. CrosslandandK. L. Servis, J. Org. Chem., 35, 3195 (1970). (42) W. F. Edgell and L, Parts, J. Amer. Chem. SOC.,77, 4899 (1955). (43) F. W. Hoffman, J. Org. Chem., 15, 425 (1950). (44) E. G. Segal, M. Kaplan, and G. K. Fraenkel, J. Chem. Phys., 43, 4191 (1965).
Electron Spin esonance Studies of Fluoroalkyl Radicals in Solution. II. Adducts to Fluoroolefins Kumg S. Chen, Paul J. Krusic,*!a and Jay K. Kochi*lb Department of Chemistry,lC Indiana University, Bloomingfon, Indiana 4740 1 and The CentralResearch Laboratory, I d E. 1. du Pont de Nemours and Company, Wilmingfon, Delaware 19898 (Received January 2, 1974; RevisedManuscript Received April 23, 1974)
Publicatbn costs assisted by E. 1. du Pont de Nemours and Company
The addition of chlorine, oxygen, sulfur, and silicon-centered radicals to fluoroolefins such as vinyl fluoride, vinylidene fluoride, and tetrafluoroethylene is examined. The structure and conformations of fluoroalkyil adducts with &heteroatom substituents are deduced from the magnitude of the nuclear hyperfine splittings and their temperature dependences as well as the selective line broadening in the esr spectra. These adducts exist in symmetric conformations XIIIa and b, in which the @ heteroatom is located in the plane described by the principal axis of the half-filled orbital and the C O X @bond. There are ambiguities associated with the assignment of conformations in radicals with pyramidal centers especially those with &heteroatom substituents such as sulfur and chlorine which are distorted at the p carbon. Qualitative arguinerts are put forth which favor the conformation XIIIa in which the @ heteroatom eclipses the halffilled or iital and presents the possibility of bridging such as that observed in the alkyl analogs.
Introduction1 Free-radical addition to unsaturated molecules, such as alkenes and alkynes, constitutes an important route to the formation of C-C bonds in synthesis and polymerization or telomerizatio,l prOCesSeS.2a ln particular, the formation of carbon-het,eroatom bonds results by addition of heteroatom-centered free radicals X to carbon-carbon multiple bonds, e.g.
a
I I I I
I
,--+ X-C-C-F I
I
(1)
The formation of these radical adducts is tantamount to The Journai of Physical Chemistry. Val. 78. No. 20. 7974
substitution of heteroatoms in the p position of alkyl radicals and has important Stereochemical consequences, as have studies Of bridged free Additions to fluoroolefins raise additional questions regarding the orientations in free-radical additions.3
I
b
1
(2a)
(2b)
Esr Studies of Fluoroalkyl Radicals in
Solution
2031 c
(D (D
N
(D
m
N
0
m
Y)
"
0 '
m
n
9
w
SI
m
N
"
c)
Radical
T'°C
CFsQ-CH2(>W2 h
a ~ H
aaF
'@H
-72 -129 CF30-CW2(%1F -70 -114 CF,O-CH,CF - 73 cFao-cr-I2C~c1 -128 HO-@H2d€lF~
22.71 28.13 22.72 29.39 21.42 63.47 8.10 21.42 62.27 7.73 90.61 12.10 21 65 3.470 9.70 19.1 57.6 11.2 17.7 3 . 0 ~ 14.4 -113 22.37 26.99 -106 17 31 59.21 24.48 -- 78 94.01 13.99
HO-CH~CHCI~ CHICW2e CH,CHFe GH,CFze
aCF8
2.01 1.86 3.37 3.36 0.91 2.99
'
With the background developed in the foregoing study of the structure and conformation of fluoroalkyl radicals,4 we examined in this st,udy the esr spectra of various transient fluoroalkyl radicak (derived from the addition of heteroatom-centered free ~rad~icals to fluoroolefins, especially vinyl fluoride and vinylidene fluoride. Since there are numerous product studies of a variety of a d d u c l ~ we , ~ are mainly concerned with the information which esr can provide about the structural and conformational properties of the adduct radicals. Four classes of heteroatom-centered radicals involving oxygen, chlorine, sulfur, and silicon arc? examined by the photochemical technique descriibed in the previous ~ a p e r . ~ - ~
Results Oxygen-C'entered Radical Adducts to Fluoroolefins. The uv photolyrjis of bis(trifluoromethy1) peroxide generates trifluorornethoxy radicals. In the absence of substrates the trifluoromethaxy radicals recombine in solution, and no esr spectrum i s CF',OOCF,
2CF$
(3)
owever, in the presence of fluoroolefins, intense and wellresolved esr' spectra of adducts are observed. For example, the spectrum in Figure 1 obtained from vinyl fluoride a t -TOo consists of a doublet (21.42 G) of doublets (63.47 G) split further icto n triplet (8.10 G) of quartets (3.37G). The spectrum is reiidily assignable to hyperfine couplings to the single a-protoli, 0-fluorine, the pair of /3-protons, and the three equivalent &fluorine nuclei, respectively, in the adduct I.
CF,D +
CII~===GHF --.,
CF,OCH,~HF I
g
Hyperfine splitting, G
T, "C
CHaS-CH2CHz - 70 CHaS-CHSCHF - 72 CH&H,S-CH,CHF - 75 CH,S-CH,CF, - 50 CH;S-CH,CHCH,~ -81 CH,S-CH~C(cH,),~ - 84
'aH
aaF
'pH
21.60 14.89 17.41 59.06 10.8 17.49 58.95 11.04 $8.4 4.3 21.36 24.13d 12.94 22.37d 3.1 .20
NR 0.6gb 0.gc l.Ob
*
Either cyclopropane or olefin as solvent. CH, quartet. 6-CHz triplet. a-CH3splitting. e Fromref 6. NR = not resolved ( < 0 26 ) .
A well-resolved spectrum is also obtained when a solution of bis(trifluoromethy1) peroxide in vinylidene fluoride is irradiated. The esr parameters for the adducts are collected
a In olefin as sobrent. See also ref 6. W 1 splitting. I n aqueous solutions a t ambient temperatures, W. E. Criffiths, G . F. Longster, J. Myatt and P. F. Todd, J. Chem. Sol.. B, 530 (1967). ' From ref 4.
hu =s==
:
TABLE 11: Esr Parameters of Thiyl Adducts to Fluoroolefins in Solution. Radical
H y p e r h e splitting, G
s I
Figure 2. Esr spectrum of the adduct radical CH3SCH2CHF resulting from the reaction of methylthiyl radical with vinyl fluoride at -65'.
TABLE I: Esr Pnra.metersof Oxy Radical Adducts to
Fluaroolefins in Solutiona
g
c I
Figure 1. Esr spectrum of the adduct radical CF30CH2CHFresulting from the reaction of trifluoromethoxy radical with vinyl fluoride. Proton nmr field markers are in kHz.
" .
0
(4)
CF,O
+
-
CH2=CF,
CF,OCH&F,
(5)
in Table I, together with the chloro and hydrocarbon analogs also listed for comparison. The reactions of trifluoromethoxy radical with I,l,l-trifluoropropene and 2-fluoropropene were also examined, but the adducts were formed in insufficient concentrations for esr examination. Interestingly, no allylic radicals could be observed from 2-fluoropropene, although the same reaction with tert- butoxy radicals (photochemically generated from di-tert- butyl peroxide) afforded the well-resolved spectrum of 2-fluoroallyl radical. F + (CH&C6 --. + (CHXOH
A
A
The proton hyperfine splittings in ally18 and 2-fluoroallyl radicals are comparable. F (9.18 G ) Hfi::4.&31 G) H H (14.86 G )
H
H (13.90 GI H Hfi3.88G) Sulfur Radical Adducts to Fluix-oolefins. Thiyl radicals are generated from the uv irradiation of dialkyl disulfides.6 hv
RS-SR ==== 2RS. (6) The esr spectrum of the methylthiyl adduct (R = CH3) to vinyl fluoride is shown in Figure 2. As expected, the hyperfine splitting pattern of the thiyl adduct I1 is the same as CH$
+
CH,=CHF
-
GH,S-CH,~HF LI the trifluoromethoxy adduct I, and the values of the split-
ting constants are listed in Table 11, together with those of the vinylidene fluoride adduct and related species. The presence of a resolvable splitting (0.7-1.0 6) of the methylmercapto group in the adducts to vinyl fluoride and vinylidene fluoride is remarkable in view of the absence of resolvable CH3 splitting in the ethylene adduct, The Journalof Physical Chemistry, Vol. 78. No, 20, 7974
K. S. Chen, P. J. Krusic, and J. K. Kochi
2032
TABLE 111: Esr Parameters for Silyl Radical Adducts to Fluoroolefins in Solution Hyperfine splitting, G IZadical
Figure 3. Esr spectrum of the adduct radical EtSSiCH2CHF resulting from the reactioi?of triethylsilyl radical with vinyl fluoride at -45’.
CH&CH&I12.(i Furthermore, the splitting of 4.3 G in the vinylidene fluoride adduct, CH3SCH2cF2, is unusually small for a 6 .proton ~ p l i t t i n g . ~ The addition of methylthiyl radical to tetrafluoroethylene and l,l,l-trifluoropropene was also examined, but the adducts were fxmed in insufficient concentrations to examine their esr spectra. Silicon Rtrdicnl Adducts to Fluoroolefins. Trialkylsilyl radicals are generated in solution by hydrogen abstraction from trialkylsilanes with photochemically generated tertbutoxy radicals 11”
(CH,),COOC(CH,), (CH,),CO
-k
*
K3’fhH ---
2(CH3),C6
(CH,)&OH
+
(7)
R,’Si-
(8)
The esr spectra of adducts are observed when the photolysis i s carried out in the presence of fluoroolefins. The spectrum of the trrethyl~ilyladduct (R’ = CHzCH2) to vinyl fluoride is shown in Figure 3 and the esr parameters are
+-
Et,Si.
CH,=CHF
4
Et,Si-CH,kHF
(9)
I11
listed in Table 111. The line widths are slightly broader in the silyl adolucts compared to the oxygen and sulfur analogs, and we were unable to resolve the 6-proton splittings in the methylene groups of the P-triethylsilyl substituent. Triethylsilyl radicals add to the methylene carbon of vinylidene fluoride in much the same manner as oxy and khiyl radicah. Et,Si- 4- C‘N,=CF, -* Et3Si-CHz6F2 The esr spectrum obtained from the addition of Et$% to trifluoroethyleaze shown in Figure 4 consists of a single species with a doublet (20.30 G) of doublets (63.39 G) split further into tridets of 52.51 G. The intensities of the MIF = 0 lines in the spectrum are diminished by the partially resolved seclond-order splitting and possibly by selective line broadening. Two isomeric adducts IVa and IVb are possible as sliown in eq 6. If the adduct were IVb, we would Et&.
+
CHF==CF,
-c.
Et$iCF,dHF IVa
(6a)
Et3SiCHF6F2IVb (6b) expect the a-flmrine splitting to be in the range 85-90 G, characteristic of other a,a-difluoroalkyl radicals4 On the other hand, an a-fluorine splitting of 60-65 G and an aproton splitting of 16-20 G are expected for an a-fluoroalkyl radical as comparisons in Tables 1-111 indicate.4 We suggest, therefore, that the vinylidene fluoride adduct has structure IVa. Addition of Chlorliae A t o m t o Fluoroolefins. Chlorine atoms can be generated from hydrogen chloride and tertbutoxy radicalri.1° In the presence of vinylidene fluoride, HCl -t- But@ C1. ButOH the spectrum shown in Figure 5 is obtained. The hyperfine splitting in the spectiwm at -31’ (Figure 5b) consists of a triplet (83.59 G;) of triplets (2.04 G) of 1:1:1:1quartets (8.41
-
+
The Journal of Physical Chemistry, Vol. 78 No 20, 7974
T’oc
Et3Si-CH2CH2 Et3Si-CH2CHF Et3Si-CH2CF2 Et3Si-CF2CHF EtaSi-CHz
- 119 -54 - 70 -48
aCIH
21.01 15.78 59.74 98.11 20.30 6 3 . 3 9
-98
Et3Si-CH2CHCH8e - 101 21.10 EtrSi-CH2d(CHa)2e- 101
17.67 20.70
10.85 52.51
9.36 48.72
