J. Chem. SOC., Faraday Trans. 1, 1987,83, 77-83 Direct Observation of a 1,4-Hydrogen Shift in Vinyl Radicals derived from the Reaction of Alkynes with Thiyl Radicals Bruce C. Gilbert" and David J. Parry Department of Chemistry, University of York, Heslington, York YO1 5DD Loris Grossi* Istituto di Chimica Organica, Universita, Viale Risorgimento 4, Bologna, Italy E.s.r. experiments demonstrate that in vinyl radicals generated from thiyl radicals and alkynes, intermolecular abstraction of a thiol hydrogen (k z lo7 dm3 mol-l s-l) is in competition with lphydrogen shifts. A rapid 1,4-shift (k M lo5 s-l) is shown to occur in cases where the resulting radical is stabilized by the presence of a-sulphur and a-carboxy substituents, whereas in other examples a 1,hhift predominates. The occurrence of intramolecular hydrogen shifts from carbon to the heteroatom in alkoxyl and aminyl radicals is well documented, but relatively little is known about analogous carbon-to-carbon shifts (except for the reactions of phenyl radicals).' Our interest in the rearrangements of vinyl radicals2 and, in particular, in the conversion of 'CH2CH2SMe into 'CH,SCH,CH, (apparently uia a novel 1,4-shift), led us to investigate the fate of related sulphur-containing vinyl radicals using e.s.r.spectroscopy and a continuous flow system. Experimental E.s.r. spectra were recorded on a Varian E-104 spectrometer equipped with an X-band klystron and 100 kHz modulation. Splitting constants and g-values were measured by comparison with an aqueous solution of Fremy's salt [a(N) = 1.309 mT,4 g = 2.00S5].Relative radical concentrations were determined by spectrum simulation using a program supplied by Dr M. F. Chiu. A mixing chamber was employed which allows simultaneous mixing of three reagent streams ca. 40 ms before passage through the cavity of the e.s.r. spectrometer. The flow was maintained using a Watson-Marlowe 502 peristaltic pump positioned on the inlet tubing. The solutions used were as follows: stream (i) contained titanium(II1) chloride (0.008 mol dm-,) and concentrated sulphuric acid, stream (ii) contained hydrogen peroxide (0.03 mol dm-,) and stream (iii) con- tained the substrate (at a concentration up to ca. 0.5 mol drn-,) together with butynedioic acid or propynoic acid (up to ca. 0.1 mol drn-,). pH measurements were made using a Pye-Unicam PW9410 pH meter with the electrode inserted into the effluent stream.All solutions were deoxygenated both before and during use by purging with oxygen- free nitrogen. Flash-photolysis experiments were carried out on an Applied Photophysics kinetic spectrometer coupled with an excimer laser excitation source (A 308 nm), monitored by a pulsed xenon flash lamp; collecting and plotting of data was achieved via a 4500 Biomation digital oscilloscope interfaced to an Apple microcomputer. The kinetic simulation program, executed on a DEC-10 computer, was kindly provided by Prof. D. J. Waddington. The chemicals employed were all commercial samples and used as supplied. 7778 Direct Observation of a 1,4-Hydrogen Shift in Vinyl Radicals Results and Discussion E.S.R.Results Radicals were generated continuously in the cavity of the spectrometer with an aqueous flow system in which the reaction between TiIII and hydrogen peroxide was utilized to generate the hydroxyl radical:2 in the third stream was included the thiol and alkyne (normally butynedioic acid, typically l 0-2- mol dm-3), with the former in sufficient concentration (b mol dm-3) to ensure that reaction with 'OH generated the appropriate thiyl radical [k('OH + RSH) is typicallys > lo9 dm3 mol-l s-l].* When relatively high concentrations of thiol (ca. 0.03 mol dm-3) were employed, the spectra detected in most cases showed relatively strong signals with high g-values (ca. 2.0057) and low /I- and y-proton splittings characteristic of a$-di-sulphur-substituted radicals (see table 1 and fig.1):' these are evidently formed via a sequence in which the thiyl radical adds to the triple bond, the resultant vinyl radical (1) undergoes rapid intermolecular hydrogen abstraction (from more thiol) and the alkene formed traps a further thiyl radical to give (2a) or (2b) [see schemes 1 and 2 and ref. (7)l.t However, when lower concentrations of thiol were employed, different spectra were detected which reveal other fates for the intermediate vinyl radicals (see fig. 1). For example, generation of 'SCHR1R2 (R2 = C02H, R1 = H, Me) from 2-mercaptoethanoic acid and 2-mercaptopropanoic acid, respectively, in the presence of butynedioic acid led to the detection of strong signals from radicals which, on the basis of their g-values and a- and /I-splittings, are assigned to (3, R2 = R3 = R4 = C02H, R1 = H, Me), in which the radical centre has both carboxy and thioalkyl substituents; a long-range coupling to the alkenyl proton is also revealed.In similar experiments with propynoic acid and 2-mercaptoethanoic acid the expected extra long-range proton splitting was also observed. Signals from radicals (3) were first accompanied by, then replaced by, those from (2a) as [RSH] was increased. These results clearly establish that a rapid 1,4-shift can compete effectively with intermolecular abstraction (scheme I). The novel and intramolecular process shown by adducts from 2-mercaptoethanoic and 2-mercaptopropanoic acid is evidently assisted by the presence not only of the sulphur substituent but also the electron-withdrawing carboxylate group (which leads to the production of a ' merostabilized ' or 'capto-dative ' radicalg), since thiyl radicals lacking this substituent behaved in contrasting fashion.For example, with EtS' at low concentrations of thiol (< 5 x mol dm-3) two signals were detected, depending on the concentration of alkyne. For relatively low concentrations of alkyne (1 0-3 mol dm-3) the spectrum comprised a broad-lined doublet (a = 0.21 mT, g = 2.0097), characteristic of a sulphinyl radical (RSO*),10 which was replaced at higher [alkyne] by a sharper spectrum comprising a large doublet (a = 3.52 mT, g = 2.0049). Our assignment of the spectra to (5) and (6), respectively, (see scheme 2 ) is based in part on the observation of identical radicals in the corresponding reactions of a variety of thiyl radicals RS' (R = CH,CH,OH, CH2CH2C02H, Pr); further support for the correct identification of (6) derives from the parameters exhibited by some radical adducts of thiophen and its derivatives" [see e.g. (7)] and from the appearance of the expected extra (@-proton splitting and g-shift when propynoic acid was employed [cf.e.g. (S)]. We propose that for these substrates a facile 1,5-shift is followed by rapid loss of the thiyl-like radical (4), which can be readily oxidized by hydrogen peroxide to the corresponding sulphinyl radical (cf. the behaviour of other thiyl radicals under these conditions)1° or add to a second molecule of alkyne [the sequence of bond formation * Concentrations quoted are those after mixing.t The reluctance of radicals (2a, R3 = R4 = C0,H) and of analogous adducts of RS' and maleic acid to undergo fragmentation is in marked contrast to the extremely rapid scission of (J) sulphur-substituted radicals lacking the carboxylate substituents.*Table 1. E.s.r. parameters for radicals formed by reaction of thiyl radicals with alkynesa hyperfine splittings/mTd thiyl radical alkyne* [RSHIc radicals a(a-H) a@H) a (other) ge *b 'SCH,CO,H B P 'SCHMeC0,H B (X = H, OH, CO,H 'SCH,CH,X B or CH,) P 'SCH(CO,H)CH,CO,H B P (k:h i k:h low ( high low high k:h high low (3, R1 = H, R2 = R3 = R4 = CO,H) (2a, R1 = H, R2 = R3 = R4 = CO,H) (3, R' = R3 = H, R2 = R4 = CO,H) -f (3, R' = CH,, R2 = R3 = R4 = CO,H) (2a, R' = CH,, R2 = R3 = R4 = C02H) (5, R3 = R4 = C0,H) (6, R3 = R4 = CO,H) (2b, R3 = R4 = C02H) (6, R3 = H, R4 = CO,H) -f (3, R' = CH,CO,H, R2 = R3 = R4 = CO,H) (5, R3 = R4 = C0,H) l(6, R3 = R4 = C0,H) (2a, R' = CH,CO,H, R2 = R3 = R4 = C02H) (3, R1 = R4 = C02H, R3 = H, R2 = CH,CO,H) (6, R3 = H, R4 = CO,H) 1.44 - 1.46 - - 0.21 - - 1.70 - 0.21 - - 1.70 - 0.34 - 1.69 (3) 0.28 3.52 - ca.0.3 4.34 0.96 (2) 3.52 - ca. 0.3 0.92 (2) 4.34 0.22 0.28 (2) 0.24 0.10 0.28 ( 0.10 - - ca. 0.35 (2) - 0.07 - - ca. 0.3 0.25 (0.18 - 2.0050 9 2.0056 9 2.0051 ? 2 2.0050 2.0057 t 2.0097 2.0049 9 2.0057 < 2.0041 2.0048 2.0097 9 2.0049 $. 2.0057 2.0051 2.0041 Q P a Typically pH ca. 1. B = butynedioic acid, P = propynoic acid. High: typically 3 x lop2 mol drn-,; low: 3 x mol dm-,. fO.O1 mT. fO.0001. f Complex signals, including radicals of type (2).4 \o80 Direct Observation of a I,4-Hydrogen Shgt in Vinyl Radicals R3 R4 \ I c =c. / R3 \ /R4 S /c=c\H 'CHR'R2 kadd 1 'SCHR1R2 H / \ R'R'CHS Scheme 1. in (6) remains to be established]. At higher concentrations of thiol (ca. 0.03 mol dm-3) intermolecular reaction of the intermediate vinyl radicals evidently occurs, as indicated by the detection in increased concentrations of radicals of type (2b) (see table 1) togetter, in some cases, with complex signals attributed to the corresponding hydroxyl-radical adducts of the alkenes thus formed. Reaction of mercaptosuccinic acid led to the detection of radicals of type (2) (at high [thiol]) and (3), as well as (5) and (6) (at lower [thiol]), indicating that 1,4- and 1,Sshifts occur for this substrate at a comparable rate.Kinetic Analysis From simple steady-state analysis of the competition between the 1,4-shift and inter- molecular abstraction we conclude that kabs = 1O2k1,, dm3 mol-l. Flash photolysis of an aqueous solution of the disulphide [SCH,CH,OHJ, (5 x lo-, mol dm-3) and maleic acid (1.4 x mol dm-3) at room temperature led to the pseudo-first-order disappearance of the weak absorption12 from the appropriate thiyl radical (A = 330 nm), from which the rate constant for addition was determined13 as ca. 5 x lo7 dm3 mol-l s-l. In similar experiments with butynedioic acid, absorption from a product obscured the decay: the rate constant for addition is expected to be similar to that found for maleic acid [see e.g. ref. (14)]. We have employed a kinetic simulation program in an attempt to match the variation in the steady-state concentrations of (2a), (2b) and (3) (as measured by e.s.r.) with [RSH]; it has been previously pointed out that a pseudo-steady state is achieved in the cavity in the TilI1-H,O, system and that such analysis is a~pr0priate.l~ We haveB.C . Gilbert, D. J . Parry and L. Grossi 0 X 81 t Fig. 1. E.s.r. spectra obtained during the oxidation of 2-mercaptopropanoic acid (2 x mol dm-3) at pH ca. 1. 9 2.0057 mol dm-3) with 'OH in the presence of butynedioic acid (3 x (3, R' = Me, RZ = R3 = R4 = CO,H) X HozC \ foZH S '=c\H \dMeCOz H HOZC /COZH R4 = C02H) 0 'C-C-SCHMeCOZH (2a, R' = Me, R2 = R3 = \ H0, CCHMeS 'H used parameters as follows: k('OH+RSH), 5 x lo9 dm3 mol-1 s - ~ ; ~ k(Ti111+H202), 2 x lo3 dm3 mol-1 s-l [ref.(16)] ; k(RS' + alkyne or alkene), 5 x lo7 dm-3 mol-1 s-l; all radical-radical termination rates, lo9 dm3 mol-1 s-l; and a time between mixing and observation of 0.04 s (measured using a spectrophotometric method and the Fe3+-thio- cyanate reaction17). The observed behaviour of those substrates which exhibit a 1,4-shift is reproduced with absolute values of kabs and kl,4 of ca. lo7 dm3 mol-l s-l and ca. lo5 s-l, respectively. In the rather more complex behaviour exhibited by those substrates which undergo a 1,5-shift, the rate of rearrangement also appears to be ca. lo5 s-l (and within the range reported for comparable shifts in vinyl radicals with simple alkyl chair@). Conclusions The rapidity of the intramolecular 1 $shifts in the vinyl radicals described here presumably reflects a contribution to the exothermicity of reaction associated with the formation of a C(sp2)-H bond at the expense of a C(sp3)-H bond (ca.40 kJ rn0l-l).l9 The apparent facility of the corresponding 1,4-shift in certain cases appears similarly to reflect this, together with the extra stability associated with production of a radical in which the unpaired electron is delocalized onto sulphur and the carboxy group: that the effect of the latter is crucial (with calculated contribution to the stabilization energy of ca. 40 kJ mo1-1)20 is indicated by the lack of a corresponding abstraction reaction82 Direct Observation of a 1,4-Hydrogen Shgt in Vinyl Radicals 'SCH2CHzX kadd 1 R'C*CR4 IR4 L c . R3 / \CH2CH2X S R\ /R4 / ,c=c \ S H C H , - ~ H X -CHI -CHX I S' H (4) 1 \ /R4 '0s /" =c\H R3 ( 5 ) R3 \ /R4 ,c = c, S H CHzCH2X \ kadd I 'SCH,CH2X R3 S ' 'H \.IR4 C - C - SCHz CHZ X \ CHZCHZX (2b) Scheme 2. 2.60 0.225 2.90 0.20 H HO2C H 0 k 7 $ ) . 0 3 H 1.69 +-I H H g = 2.005 1 g = 2.0038 (k < lo4 s-l) in the absence of the carboxylic acid group. Consideration of the likely geometry of the intermediate vinyl radicals and inspection of models show that the relatively long C-S bonds (expected21 to be ca. 0.18 nm) and small LCSC (ca. looo) allow a distance of closest approach of the radical centre and the &hydrogen (for 1,4-shift) of ca. 0.16 nm. We suggest that similar favourable geometric considerations, as well as the radical stabilization conferred by an a-sulphur substituent play an equally important role in bringing about the rapid 1,4-shifts observed for Q-sulphur-substituted alkyl radical^.^ We thank the S.E.R.C.for a studentship (for D. J.P.) and NATO for a research grant (567/84) to L.G.B. C. Gilbert, D. J . Parry and L. Grossi 83 References 1 A. L. J. Beckwith and K. U. Ingold, in Rearrangements in Ground and Excited States, ed. P. de Mayo 2 J. Foxall, B. C. Gilbert, H. Kazarians-Moghaddam, R. 0. C. Norman, W. T. Dixon and G. H. 3 L. Lunazzi, G. Placucci and L. Grossi, J. Chem. SOC., Perkin Trans. 2, 1981, 703; Tetrahedron, 1983, 4 R. J. Faber and G. K. Fraenkel, J. Chem. Phys., 1967,47,2462. 5 J. Q. Adams, S. W. Nicksic and J. R. Thomas, J. Chem. Phys., 1966,46, 654. 6 See e.g. Farhataziz and A. B.Ross, Selected Specific Rates of Reactions of Transients from Water in Aqueous Solution. III. Hydroxyi Radicals and Perhydroxyi Radicals and their Radical Ions (National Standard Reference Data Series, National Bureau of Standards, Washington, D.C., 1977). 7 T. Kawamura, M. Ushio, T. Fujimoto and T. Yonezawa, J. Am. Chem. SOC., 1971,93, 908. 8 See e.g. Y. Ueno, T. Miyano and M. 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SOC., 1958, 80, 2951. 18 B. C. Gilbert and D. J. Parry, J. Chem. SOC., Perkin Trans. 2, 1986, 1345. 19 S. W. Benson, Thermochemical Kinetics (Wiley, New York, 1968). 20 W. Lung-min and H. Fischer, Helv. Chim. Acta, 1983, 66, 138. 21 A. Streitwieser and C. H. Heathcock, Introduction to Organic Chemistry (Macmillan, New York, 2nd 892. K-D. Asmus, J. Phys. Chem., 1978, 82, 2777 and references therein. 85, 1365; E. I. Heiba and R. M. Dessau, J. Org. Chem., 1967,32, 3837. edn, 1981). Paper 618 1 5 ; Received 28th April, 1986