Kinetic applications of electron paramagnetic resonance spectroscopy


Matthew Burns , Guy C. Lloyd-Jones , Jonathan D. Moseley , and Joseph S. Renny. The Journal of Organic Chemistry 2010 75 (19), 6347-6353. Abstract | F...
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Kinetic Applications of Electron Paramagnetic Resonance Spectroscopy. 25. Radicals Formed by Spin Trapping with Di-tert- butyl Thioketone' J. C. Scaiano* and K. U. Ingold* Contribution from the Division of Chemistry, National Research Council of Canada, Ottawa, Ontario, Canada. Received September 26, 1975

Abstract: A variety of transient R,M- radicals (,M = C, Si, Sn, and P, but not 0)have been trapped with di-tert-butyl thioketone and the persistent adduct radicals, R,MSC(CMe3)2 ( l ) ,have been examined by EPR spectroscopy. From a comparison of the hyperfine splittings by M in 1 and in RnMCH2C(CMe3)2 and analogous radicals, and from a consideration of the g values for 1 (2,0024-2.0033), it is concluded that the R,M group eclipses the C, 2p, orbital. Competitive experiments involving the reaction of tert-butyl with thione or oxygen at -80 OC and the reaction of methyl with thione or Me3CNO at -40 " C indicate that alkyl addition to the thione has a rate constant of ca. IO6 M-I s-l at these temperatures. The CH3SC(CMe3)2 radial exists in equilibrium with a dimer at temperatures in the range -70 to -1 10 OC: AH = 9.6 f 1.5 kcal/mol; A S = 32 f 3 gibbs/mol. Above -50 :C the CH3SC(CMe3)2 radical decays with the first-order kinetics. It is concluded that CH$SC(CMe& and SiH3SC(CMe3)2 decay by an intramolecular transfer of H from M to C,. The CF$C(CMe& radical is extremely persistent. The (n-Bu)3SnSC(CMe-J2 radical decays with second-order kinetics. Arrhenius parameters for some of the decay reactions are reported.

The carbon-sulfur double bond in thioketones is known to be a fairly good trap for ~ a r b o n - c e n t e r e dand ~ - ~ sulfurcentered6 free radicals.

Experimental Section

Di-tert- butyl thioketone was prepared from di-tert- butylketimine by reaction first with methyllithium and then with carbon disulfide according to the method of Barton et aI.l4 It was purified by vacuum R,M. S=C< R,MSC< distillation and was stored under argon at -20 'C. Fortunately, the 531 nm, t 6.45 cm-l extinction coefficients of this thione (viz.,, , ,A This reaction appears to be important during the photolysis M-I; no other maxima until X >io4

0.017

E , kcal/mol

10f3 13.4 f 1.0 9.7 f 0.7 9.2 f 1.2

DimerC Yes No

No No Yes

a 1 = first-order decay of the radical over the indicated temperature range; 2 = second-order decay. A in units of s-' for first-order decays and M-l s-' for second-order decays. See text. d Decay rates!efer to CH3. generated from Me3COOCMe3 (CH3)3B. When the CH3- were generated from CH31 (n-Bu)3SnSn(n-Bu)3 the CH3SC(CMe3)2 radicals decayed about ten times more slowly, perhaps because of a chain reaction. Because of this problem the kinetics for the decay of the CD3. adduct (from CD3I and ditin) were not studied. e Azoisobutane and di-tert-butyl ketone as (CH3)3C' sources gave data in excellent agreement at temperatures >-20 'C. At lower temperatures the azo compound gives anomalous results because of the slow thermal decomposition of photochemically generated cis-azoisobutane (cf. T. Mill and R. S. Stringham, Tetrahedron Lett. 1853 (1969)). f From CF31 and (n-Bu)3SnSn(n-Bu)3. The radicals are not produced thermally in this system.

+

+

change in the equilibrium concentration of radicals with temperature could be correlated by the van't Hoff relation, K = eAS/Re-AH/RT, with AS = 32 f 3 gibbsfmol and AH = 9.6 f 1.5 kcal/mol. The change in entropy is of the magnitude expected for a simple d i m e r i ~ a t i o n . ~Since ~ , ~ ' 1,I-di-tertbutylalkyl radicals appear never to dimerize11~13~17~48 it seems very improbable that the CH3SC(CMe3)2 dimer could be head-to-head coupled. We therefore propose that it has either a head-to-tail structure, 5, or, perhaps, a tail-to-tail structure, 6. Me$,,

/CH3

Me3G.,

,CH, Me$

4c=s\ c-s' Me3C

/CH3 $Me3

Me$

j

Me$'