K. S. Chen, P. J. Krusic, P. Meakin. and J. K. Kochi
2014
Electron Spin esonance Studies of Fluoroalkyl Radicals in Solution. 1. Struc Conformations, and Barriers to Hindered Internal Rotation Kuang S. Chsn, Paul J. Kruslc,+la Paul Meakln, and Jay K. Kochi’lb
’
Department of Chemistry, Indiana University, Bloomington, Indiana 4740 1 and The Central Research Department.I d E. 1. du Pont de Nemours and Company, Wi/mlngton,Delaware 19898 (ReceivedJanuary 2, 1974; Revised Manuscript Received April 23, 1974) Publicationcosts assisted by E. 1. du Pont de Nemours and Company
Methods are described for the production and esr study in solution of a variety of alkyl radicals which are substituted with one or more fluorines in the a, @, y, and 6 positions. Analysis of the isotropic ‘H and I9F coupling constants and their temperature dependence together with the selective line broadening in the esr spectra provide information about the configuration at the a-carbon center, the stable conformation, and barriers to hindered internal rotation in fluoroalkyl radicals. Thus, the presence of two a-fluorines promote a pyramidal radical center in CH&F2., CF&F2., CF$F&F2-, and CF3(CF&CFz. and line broadening studies show that the rotation barrier about the C,-Co bond is raised to 2-3 kcal/mol in comparison to f 24 kir. The reaction mixture was cooled, diic abstraction of hydrogen from 2-fluoropropane, spectra of luted with cold water, and extracted with anhydrous ether. The extracts were washed with water, dried over anhydrous radicals with good signal-to-noise ratios could only be obmagnesium sdlfate, and distilled: bp 51' (93 mm). yield served at temperatures higher than -80'. 50% [nmr 6 1.30 (L), 1.50 (d X d), 4.20 (q), 4.87 (d X q)]. Acknowledgment. K. S. C. wishes to thank the National Ethyl a-fluioropropionate (15 g) and 100 ml of 10%potassiScience Foundation for financial support. P. J. K. and P. um hydroxide aqueous solution were stirred a t room temM. are grateful to Mr. B. Gordon for experimental assisperature over a perjod of 3 hr. The reaction mixture was tance. We also thank Dr. W. J. Middleton for a sample of extracted with ether, dried, and distilled: bp 84-85' (54 mm), yield 7096 [nmr S 1.62 (d X d, 5.06 (d X q)]. tert-Butyl 1-bromo-2-fluoroethane. a-fluoropropronyl porester was prepared via the acid chloride [nmr (6 133 (s) 1.63 (d X d), 5.14 (d X q))]. References and Notes 2-Fluoropropnne was prepared by the method of Edge11 (1)(a) E. I. du Pont de Nemours and Company. (b) Department of Chemistry, Indiana University. (c) Contribution No. 239.0 from the Department of and uta the mesylate [nmr 6 1.32 (d X d), 4.86 (d X Chemistry, Indiana University. (d) Contribution No. 2132 from E. I. du septet)]. Pont de Nemours and Co. (2)H. Fischer, "Free Radicals," J. K. Kochi, Ea., Vol. II, Wiley, New York, 2- ~ l ~ o r o e and t ~ 2,2-difluoroethanol ~ ~ ~ o ~ were obtained N. Y., 1973,Chapter 19. from Colurnb-a Organic Chemicals Co. and were converted (3)W. A. Sheppard and C. M. Sharts, "Organic Fluorine Compounds," W. to the corresponding ethyl bromides by the method develA. Benjamin, New York, N. Y., 1969. (4)A. P. Stefani, fluorine Chem. Rev.. 5, 115 (1971). oped by Hoffman,dq3terd-Butyl P-fluoropropionyl perester (5)(a) M. Iwasaki, Nuorhe Chem. Rev., 5, 1 (1971);(b) A. Hudson and K. 5. was prepared from the acid chloride. J. Root, Advan. Magn. Resonance, 5, 1 (1971). Esr Measu mements. The modified Varian X-band spec(6)P. D. Sullivan and J. R. Bolton, Advan. Magn. i?eso"x, 4, 39 (1970): A. Hudson and G. R. Luckhurst, Chem. Rev., 69, 191 (1969). trometer, microwave frequency measurements, light (7)J. K. Kochi and P. J. Krusic, Chem. Soc., Spec. Pub/.,24, 147 (1969). source, and sample tubes are as described p r e v i ~ u s l y . ~ ~ ~ (8) , ~ R.~ A. Sheldon and J. K. Kochi, J. Amer. Chem. SOC.,92, 4395,5175 (1970). To miniinive the error in the g value determinations, all (9)P. Meakin and P. J. Krusic, J. Amer. Cbem. Soc., 95, 0185 (1973). measurement3 were made on spectra recorded on the same (IO)J. K. Kochi and P. J. Kruslc, J. Amer. Chem. Soc., 91, 3940 (1969). day for increasing magnetic field. Perylene cation radical g (11)(a) A. Hudson and R. A. Jackson, J. Chem. Soc., Chem. Common., 1327 (1969):(b) A. G. Davies, D. Griller, and f3. P. Roberts, J. Amer. -- 2.00258344was used as standard in the configuration emChem. Soc.. 94, 1782 (1972). ployed. The accuracy of the measurments is estimated as (12)R. V. Lloyd and M. T. Rogers, J. Amer. Chem. SOC.,95, 1512 (1973). f0.00003.Hyperfine splittings were corrected for second- (13)(a) J. H. Freed and G. K. Fraenkel, J. Chem. Phy5., 39, 326 (1963);(b) A. D. McLachlan, Proc. Roy. SOC.,Ser. A,, 280, 271 (1964);(c) J. Cooorder shifts aad ~ o ~ ~by icomputer r ~ esimulation. ~ per, A. Hudson, R. A. Jackson, and M. Townson. Mol. Phys., 23, 1155 For photolytic retluetion of alkyl halides, equal volumes (1972). The Journal of Physical Chemistry. Vol. 78. No. 20, 1974
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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 K u m g 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)
