Absolute Rate Constants for Reactions of Tributylstannyl Radicals with


Department of Chemistry, North Carolina A&T, 1601 East Market Street,. Greensboro, North Carolina 27411 [email protected] Received October 6, 2003...
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Absolute Rate Constants for Reactions of Tributylstannyl Radicals with Bromoalkanes, Episulfides, and r-Halomethyl-Episulfides, -Cyclopropanes, and -Oxiranes: New Rate Expressions for Sulfur and Bromine Atom Abstraction James A. Franz,*,† Wendy J. Shaw,† Claude N. Lamb,‡ Tom Autrey,† Douglas S. Kolwaite,† Donald M. Camaioni,† and Mikhail S. Alnajjar† Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, Washington 99352, and Department of Chemistry, North Carolina A&T, 1601 East Market Street, Greensboro, North Carolina 27411 [email protected] Received October 6, 2003

Arrhenius rate expressions were determined for the abstraction of bromine atom from 2-phenethyl bromide by tri-n-butylstannyl radical (Bu3Sn•) in benzene using transient absorption spectroscopy, (log(kabs,Br/M-1 s-1) ) (9.21 ( 0.20) - (2.23 ( 0.28)/θ, θ ) 2.3RT kcal/mol, errors are 2σ) and for the abstraction of sulfur atom from propylene sulfide to form propylene, (log(ks/M-1 s-1) ) (8.75 ( 0.91) - (2.35 (1.33)/θ). Rate constants for reactions of organic bromides, RBr, with Bu3Sn• were found to vary as R ) benzyl (15.6) > thiiranylmethyl (6.2) > oxiranylmethyl (3.1) > cyclopropylmethyl (1.3) > 2-phenethyl (1.0), with kabs,Br ) 6.8 × 107 M-1 s-1 at 353 K for 2-phenethyl bromide. Bromine abstraction from R-bromomethylthiirane is about 7-fold faster than sulfur atom abstraction and is comparable to the reactivity of a secondary alkyl bromide. The potential surface for the vinylthiomethyl f allylthiyl radical rearrangement at UB3LYP/6-31G(d) and UB3LYP/6-311+G(2d,2p) levels of theory suggests that the thiiranylmethyl radical is produced about 9 kcal/mol above the allylthiyl radical on the rearrangement surface, consistent with the observed enhancement of the Br atom abstraction from the thiirane and with synchronous C-S bond scission of the thiirane ring. The selectivities reported in this work for S vs Cl and Br abstraction provide applications for radical-based synthesis and new competition basis rate expressions for trialkylstannyl radicals. Introduction The ring-opening reactions of the cyclopropylcarbinyl radical and the nitrogen and oxygen analogues are the subject of ongoing interest (eq 1, X ) NR, CH2, O, S).

are lacking. The cis-2-aziridinyl-, oxiranyl-, and cyclopropylcarbinyl radicals are predicted to exhibit activation barriers (Ea, 298 K) of 4.0, 4.4, and 7.5 kcal/mol for C-N, C-O, or C-C bond cleavage, respectively, and barriers of 10.2, 12.2, and 7.5 kcal/mol for C-C cleavage of the systems.1 Our interest in the thiiranylmethyl radical system was inspired by semiempirical calculations showing that the related phenyl-thiaspiro[2.5]octadienyl radical, 2, undergoes barrierless C-S scission (eq 2).3

The focus of this work was to determine, particularly for X ) S in eq 1, whether atom abstraction processes that produce 1 may be significantly enhanced by the synchronous, exothermic ring opening of 1 to form allylthiyl radical. High accuracy theoretical studies of the cyclopropylcarbinyl radical (1, X ) CH2) and N- and Oanalogues have been carried out using the CBS-RAD theoretical model,1 yielding predicted rates approaching experimental kinetic accuracy. The cyclopropylcarbinyl ring-opening reaction is very accurately known and serves to calibrate the theoretical models.2 Comparably accurate experimental rate expressions for the 2-aziridinyl- and oxiranylmethyl radical ring-opening reactions †

Pacific Northwest National Laboratory. North Carolina A&T. (1) Smith, D. M.; Nicolaides, A.; Golding, B. T.; Radom, L. J. Am. Chem. Soc. 1998, 120, 10223-10233. (2) Newcomb, M. J. Org. Chem. 1999, 64, 1225-1231. ‡

Work in this laboratory to characterize the vinylthiomethyl (5) f allylthiyl (6) potential surface using ab initio electronic structure calculations was underway when the work of Pasto4 appeared. Pasto demonstrated 10.1021/jo035467h CCC: $27.50 © 2004 American Chemical Society

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J. Org. Chem. 2004, 69, 1020-1027

Published on Web 01/16/2004

Rate Expressions for S and Br Atom Abstraction

that the thiiranylmethyl radical, 3, is not an intermediate on the vinylthiomethyl f allylthiyl potential surface4 and thus should undergo barrierless C-S scission to form the allylthiyl radical (eq 3). DFT calculations of the vinylthiomethyl f allylthiyl radical rearrangement, presented here at the B3LYP/6-31G(d) and B3LYP/6-311+G(2d,2p) levels of theory, confirm the findings of Pasto.4 The computational results suggest that homolytic pathways producing the thiiranylmethyl radical, such as halogen atom abstraction from R-halomethylthiiranes, should be enhanced by synchronous C-S scission producing allylthiyl radical (eq 4). Similar considerations suggest that oxiranylmethyl radicals should not exhibit enhanced rates of formation in atom transfer reactions. Oxiranylalkyl radicals (1, X ) O) have been experimentally demonstrated to undergo rapid and, thermochemistry allowing, reversible C-O and C-C cleavage, leading to products reflecting partial equilibration of allyloxyl and vinyloxyalkyl radicals depending on trapping conditions.5,6-11 Electronic structure calculations of Pasto1,4 suggest C-O cleavage of oxiranylmethyl radical will provide the kinetically favored alkoxy radical product. C-C scission products are expected only if the substituent R (eq 4) is a group that provides stabilization to the resulting radical,5,6,12,13 leading to the thermodynamically favored product.1,4,7-11 The potential well for oxiranyl radicals has been demonstrated to be shallow. Results of Krosley and Gleicher suggest that the oxiranylmethyl radical will undergo ring opening with a rate constant of >1010 s-1. Recently, Krishnamurthy et al. and Grossi et al.14,15 demonstrated the intermediacy of the oxiranyl radical by, respectively, trapping the radical with thiophenol and recording its ESR spectrum at low temperature. The magnitude of the potential well for the oxiranylmethyl radical has been addressed in calculations performed by Smith et al.1 and Pasto3,6 and was found to agree with experiments that suggest a minimal well. Finally, it has been appreciated for some years that the cyclopropylmethyl C-H bond exhibits enhanced reactivity in radical-forming reactions, as a result of the stabilization of the radical π-orbital with the HOMO of the adjacent cyclopropyl ring.16,17 However, no experimental study has directly compared the reactivity of cyclopropylcarbinyl, oxiranylmethyl, and thiiranylmethyl systems. (3) Alnajjar, M. S.; Franz, J. A. J. Am. Chem. Soc. 1992, 114, 10521058. (4) Pasto, D. J. J. Org. Chem. 1996, 61, 252-256. (5) Nonhebel, D. C. Chem. Soc. Rev. 1993, 347. (6) Ziegler, F. E.; Petersen, A. K. J. Org. Chem. 1995, 60, 26662667. (7) Breen, A. P.; Murphy, J. A. J. Chem. Soc., Chem. Commun. 1993, 191-192. (8) Davies, A. G.; Tse, M. J. Organomet. Chem. 1978, 155, 25-30. (9) Dobbs, A. J.; Gilbert, B. C.; Laue, H. A.; Norman, R. O. J. Chem. Soc., Perkin Trans. 2 1976, 1044-1047. (10) Marples, B. A.; Rudderham, J. A.; Slawin, A. M.; Edwards, A. J.; Hird, N. W. Tetrahedron Lett. 1997, 38, 3599-3602. (11) Barton, D. H.; Motherwell, R. S.; Motherwell, W. B. J. Chem. Soc., Perkin Trans. 1 1981, 2363-2367. (12) Kuivila, H. G. Acc. Chem. Res. 1968, 1, 299. (13) Krosley, K. W.; Gleicher, G. J.; Clapp, G. E. J. Org. Chem. 1992, 57, 840-844. (14) Krishnamurthy, V.; Rawal, V. J. Org. Chem. 1997, 62, 15721573. (15) Grossi, L.; Strazzari, S.; Gilbert, B. C.; Whitwood, A. C. J. Org. Chem. 1998, 63, 8366-8372. (16) Roberts, C.; Walton, J. C. J. Chem. Soc. Chem. Commun. 1984, 1109-1111. (17) Roberts, C.; Walton, J. C. J. Chem. Soc., Perkin Trans. 2 1985, 841-846.

TABLE 1. Competitive Reactions of Bu3SnH with Episulfides and Bromides in Benzene at 80 °C

Thus, this study sought to determine the rate constants for bromine atom abstraction from R-bromomethylthiirane, -oxiranes, and -cyclopropanes, with the reactivity of the thiirane structure expected to be enhanced relative to either the oxirane or cyclopropane analogues for considerations cited above. In the course of the study, it was discovered that sulfur atom was selectively and rapidly abstracted from R-chloromethylthiiranes and other thiirane systems by tri-n-butylstannyl radical (Bu3Sn•) (eq 5). Thus, we report relative and absolute rates for

abstraction of halogen atoms from a variety of R-halomethylcyclopropanes, oxiranes, and thiiranes, and Arrhenius data for abstraction of bromine atom from a primary alkyl bromide and sulfur atom from propylene sulfide. Supporting computational results characterizing the thiiranylmethyl ring-opening surface are presented. Results and Discussion Relative Rates of Bromine and Sulfur Atom Abstraction by tri-n-Butylstannyl Radical. Relative rate constants were determined by reaction of competing pairs of halide or episulfide and 2-phenethylbromide with limiting concentrations of Bu3SnH at 80 °C in benzened6. Table 1 presents relative and absolute rate constants for reaction of the tributylstannyl radical with halides and episulfides. The reaction products generated from the loss of a sulfur atom from propylene sulfide, styrene sulfide epithiobromohydrin, and epithiochlorohydrin (entries B, G, F and A, respectively) are propylene, styrene, allyl bromide, and allyl chloride. 2-Phenethyl bromide, cyclopropylcarbinyl bromide, epibromohydrin, epithiobromohydrin, and benzyl bromide (entries C, D, E, F, and H) all lose bromine to give, respectively, ethyl benzene, butene, allyl alcohol, allyl thiol, and toluene. The rate constants range from 3.5 × 107 M-1 s-1 for abstraction of sulfur atom from propylene sulfide to 1.1 × 109 M-1 J. Org. Chem, Vol. 69, No. 4, 2004 1021

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s-1 for abstraction of bromine atom from benzyl bromide at 353 K in benzene. The latter rate is approximately 16 times lower than the bimolecular diffusive encounter rate for Bu3Sn• with 2-phenethyl bromide. The reactivity of the R-bromomethylthiirane (F), bromomethyloxirane (E), and other cyclopropylcarbinyl systems (entries B and D) with Bu3Sn• relative to primary alkyl bromide invites comparison with relative rates of reactivity of primary, secondary, and tertiary alkyl bromides. Kuivila18 and Carlsson and Ingold19 reported the relative rates of reaction of Bu3Sn• with primary alkyl, secondary alkyl, and tertiary alkyl bromides to be 1:3:7 at 45 °C in nonpolar solvent. Applying the experimental value of log(A/M-1 s-1) ) 9.21 found in this work for phenethyl bromide along with the 318 K rate constant of 4.8 × 107 M-1 s-1 to the secondary and tertiary relative rates allows the published rates21 to be scaled to 353 K for comparison to the results of Table 1. The scaled relative rates at 353 K are thus predicted to be 1.0 (primary):2.7 (secondary):5.8 (tertiary). Examination of Table 1 reveals the cyclopropylcarbinyl analogues to span the range of primary, secondary, and tertiary alkyl rates, cyclopropylcarbinyl being consistent with a primary bromide, R-bromomethyloxirane similar to secondary bromide, and R-bromomethylthiirane comparable to a tertiary alkyl bromide. A narrow range of relative rates of the bromides is expected as a result of high reactivity and the correspondingly narrow range of relative enthalpy changes, ∆(∆H°) for bromine atom transfer to tin radical. Relative enthalpy changes reflect both ground-state effects on alkyl bromides as well as product radical heats of formation:

the presence of allylic chlorides, in contrast to allylic bromides where removal of the halide atom is dominant, about 7-fold faster than sulfur atom abstraction. Absolute Rates and Arrhenius Parameters of Bromine Atom Abstraction from Phenethyl Bromide by Tri-n-butylstannyl Radical. Carlsson and Ingold19 reported some of the first room-temperature absolute rate constants for reaction of stannyl radicals with halides. A decade later, Scaiano20 reported an ambient temperature rate constant for abstraction of bromine atom from a primary alkyl bromide. However, temperature-dependent rate constants for the halides have not been reported. To place the relative rates of Table 1 on an absolute basis and to determine temperature-dependent data for abstraction of sulfur atom from thiiranes, transient absorption spectroscopy was used to observe the reaction of the tin radical with primary bromide using the photolysis of di-tert-butyl peroxide in the presence of tri-n-butylstannane and 2-phenethyl bromide as depicted in eqs 6-12: t

t



BuOOtBu 9 8 2tBuO• 308 nm k7

BuO• + Bu3SNH 98 tBuOH + Bu3Sn•

∆H1

kBr

Bu3SnBr + PhCH2CH2• (8) k9

PhCH2CH2• + Bu3SnH 98 PhCH2CH2 + Bu3Sn• (9) kt

2PhCH2CH2 98 non-radical products kct

∆H2o

o

o

o

(10)

PhCH2CH2• + Bu3Sn• 98 non-radical products (11) kt′

R2Br + Bu3Sn• 98 R2• + Bu3SnBr o

(7)

Bu3SNH + PhCH2CH2Br 98

o

R1Br + Bu3Sn• 98 R1• + Bu3SnBr

(6)

2Bu3Sn• 98 non-radical products

(12)

2

∆∆H ) DH2 - ∆H1 ) -∆Hf (R Br) + ∆Hfo(R1Br) + ∆Hfo(R2•) - ∆Hfo(R1•) Thus, the relative enthalpy change for abstraction of a secondary alkyl bromine atom versus abstraction of a primary alkyl bromine atom is ∆∆H° ) -0.6 kcal/mol, and for abstraction of a bromine atom from a primary bromide versus abstraction of bromine atom from a tertiary alkyl bromide, ∆∆H° ) -1.3 kcal/mol, resulting in the narrow range of observed rates. The pattern of selectivity in abstraction of sulfur atom from thiiranes is consistent with the cleavage of the C-S bond in the rate-determining step of S atom abstraction:

The time dependence of tin radical concentration following the laser pulse is given by eq 13:

d[Bu3Sn•] ) k7[BuO•][Bu3SnH] dt kBr[Bu3Sn•][PhCH2CH2Br] + k9[Bu3SnH][PhCH2CH2•] kct[PhCH2CH2•][Bu3Sn•] - 2kt′[Bu3Sn•]2 (13) -d[Bu3Sn•] ≈ kBr[Bu3Sn•][PhCH2CH2Br] (14) dt ln([BuSn•]t0 [BuSn•]t1 ∆t

The trend in relative rates, chloromethylthiirane (A) < methylthiirane (B) < phenylthiirane (C), corresponds to the stability of the forming radical center. An important feature of Table 1 is that sulfur atoms can be removed from the thiiranyl ring with near-complete selectively in 1022 J. Org. Chem., Vol. 69, No. 4, 2004

) kobs ) kBr[PhCH2CH2Br] + C

(15)

(18) Kuivila, H. G. M., L. W.; Warner, C. R. J. Am. Chem. Soc. 1962, 84, 4, 3584-3586. (19) Carlsson, D. J.; Ingold, K. U. J. Am. Chem. Soc. 1968, 90, 70477055. (20) Scaiano, J. C. J. Am. Chem. Soc. 1980, 102, 7747-7753. (21) Macey, R. I. a. O., G. F.; Kagi Shareware, 1442-A Walnut Street #392-GO, Berkeley, CA 94709-1405; 2002.

Rate Expressions for S and Br Atom Abstraction

FIGURE 2. Plots of kobs vs [PhCH2CH2Br], M. Pseudo-firstorder analysis of transient absorption data between 1.5 and 2.5 µs for abstraction of Br atom from PhCH2CH2Br at temperatures indicated.

TABLE 2. Values of kobs (s-1) (eq 15) of Abstraction of Br Atom from Phenethyl Bromide by Tri-n-butylstannyl Radical (Concentrations and Temperatures Indicated) T ((1) (K) 284 289 299 310 318 325 333

0.0050 M

0.0102 M

0.0146 M

1.03 × 105c 2.30 × 105 4.17 × 104

2.32 × 3.39 × 105 2.02 × 105 4.36 × 105 2.90 × 105 4.09 × 105 3.06 × 105

4.13 × 5.45 × 105 4.26 × 105 5.68 × 105 5.02 × 105 6.71 × 105 5.56 × 105

105

105

0.0198 M

0.0249 Ma

5.35 × 7.22 × 105b 6.54 × 105 8.43 × 105 5.51 × 105 7.69 × 105 8.41 × 105 8.16 × 105c 9.27 × 105 8.64 × 105c 105

a Concentrations of PhCH CH Br. b Values of k c 2 2 obs. Average of two data points. All other values based on average of five data points.

FIGURE 1. (a) Transient absorbance profiles of Bu3Sn• (0.02 M Bu3SnH) at indicated PhCH2CH2Br concentrations. Each trace represents 300 transients at a laser repetition rate of 1 Hz, 2 mL/min flow, resolution of 1 ns/pt. Dividing by 400 ) 1620 M-1 cm-1 yields data in concentration units.22 (b) Numerical fit of decay of Bu3Sn•. ([PhCH2CH2Br] ) 0.0198 M, [Bu3SnH] ) 0.02 M, [tBuO•]0 ) 1.15 × 10-5 M. Rate constants k7 and k8 were optimized to fit the observed data, with values of kt′ ) 1.8 × 109 M-1 s-1, kct ) 2.0 × 109 M-1 s-1, kt ) 2.2 × 109 M-1 s-1 and k9 ) 2.3 × 106 M-1 s-1, yielding the optimized values of constants k7 ) 2.0 × 108 M-1 s-1 and k8 ) kBr) 3.0 × 107 M-1 s-1 for abstraction of Br atom by Bu3Sn•. (c) Treatment of data between 1.5 and 2.5 µs from Figure 1b. The slope of this curve provides kobs ) kBr[PhCH2CH2Br] ) 5.6 × 105 s-1, or kBr ) 5.6 × 105/0.0198 ) 2.8 × 107 M-1 s-1.

Under conditions of the low laser pulse intensity and low halide and hydride concentrations used in this study, numerical integration of the differential equations describing the time dependence of tBuO•, PhCH2CH2•, and Bu3Sn• radical concentrations from eqs 6-12 reveals that tert-butoxyl radical is fully consumed by 1 µs (see, e.g., Figure 1b), simplifying the kinetic analysis by allowing the first term of eq 13 to be neglected. Formation of the stannyl radical by alkyl radical abstraction (k9) is slow on the time scale of abstraction of Br atom by the tin radical, and the time dependence of the evolution of the Bu3Sn• is due nearly exclusively to bromine atom abstraction, with negligible contributions from radical crossand self-termination reactions (eqs 11 and 12). Kinetic fit of the observed decay of Bu3Sn• is carried out by numerical integration of eq 3, based on eqs 7-12, with optimization of rate constant kBr (eq 8) while holding values of k7, k9, kt, kct, and kt′ of eq 13 constant.21 Under

the conditions utilized, termination reactions kt, kct, and kt′ occur to negligible extent during the first 8 µs, and the choices of kt, kct, and kt′ have little effect on the optimized values of k7 and k9. Kinetic fitting is conveniently carried out using the Berkeley Madonna program.21 The kinetic fit reveals that the last three terms of eq 13 are small compared to the halogen abstraction term, prior to 2.5 µs, providing that suitably low concentrations of alkyl bromide and tin hydride are employed. High concentrations of bromide and tin hydride lead to the persistence of the tin radical due to the radical chain pathway of eqs 8 and 9. The numerical integration of eq 13 (Figure 1b) confirms that a simple logarithmic treatment of the data (Figure 1c,d) between 1.0 and 2.5 µs (eq 15) yields suitably accurate values of kBr. Thus, simple logarithmic treatment of the concentration (or equivalently, optical density) of the Bu3Sn• decay versus time was carried out to yield straight lines with the slope kobs ) kBr[PhCH2CH2Br]. Plots of kobs against [PhCH2CH2Br] provide values of kBr at each temperature (Figure 2). The resulting data is tabulated in Table 2, and temperaturedependent rate data are plotted in Figure 3. As shown below, the fit of data using data between 1.5 and 2.5 µs agrees with detailed numerical integration to within 3-4% and requires no preassignment of rate constants. Arrhenius Data for Abstraction of Sulfur Atom by Tri-n-butylstannyl Radical from Propylene Sulfide. Relative rates of abstraction of sulfur atom from propylene sulfide vs abstraction for bromine atom from 2-phenylethyl bromide in benzene-d6 by the stannyl radical were carried out over a 60 °C temperature range using GC and NMR product quantitation. Samples were sealed in Pyrex tubes following three freeze-pump-thaw J. Org. Chem, Vol. 69, No. 4, 2004 1023

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FIGURE 3. Temperature-dependent rate data for the abstraction of bromine atom from phenethyl bromide by the tributylstannyl radical. The experimental data result in an activation energy of 2.23 ( 0.28 kcal/mol and log(A/M-1 s-1) of 9.21 ( 0.20 (errors are 2σ, R2 ) 0.98), 11-60 °C, kBr (298 K) ) 3.8 ( 0.5 × 107 M-1 s-1. The temperature range is defined by the melting point of benzene and the thermal stability of the peroxide/tin hydride/bromide mixture.

FIGURE 4. Arrhenius data for abstraction of sulfur atom from propylene sulfide by tri-n-butylstannyl radical.

cycles. Solutions of 2-phenethyl bromide, propylene sulfide, azo-2-isobutyronitrile (AIBN), and internal GC/NMR standard (p-xylene) in benzene-d6 were heated in a thermostated oil bath. Ethylbenzene was measured by GC relative to internal p-xylene, and propylene sulfide was measured by comparing the vinyl hydrogen atoms of propylene to internal p-xylene standard by 1H NMR integration to yield the relative rate expression: log(kBr/kS) ) (0.46 ( 0.90) - (0.115 ( 1.3)/θ, θ ) 2.3RT kcal/mol. Combining the relative rate expression kBr/kS with the absolute rate expression for bromide abstraction from 2-phenethyl bromide, kBr, yields an Arrhenius rate expression for abstraction of sulfur atom from propylene sulfide: log(kS/M-1 s-1) ) (8.75 ( 0.92) - (2.35 ( 1.32)/ θ, θ ) 2.3RT kcal/mol, kS (298 K) ) 1.1 × 107 M-1 s-1. The results are depicted in Figure 4. Reaction of Epithiobromohydrin with 1 M Trin-butylstannane. At 80 °C in benzene-d6, R-bromomethylthiirane was converted to allylthiol, and allyl bromide with concentrations of Bu3SnH and bromide of ca. 0.05 M, with no observed autoinitiation of the reduction reaction as demonstrated by monitoring the stock solution by NMR. When 1 M Bu3SnH and 1 M R-bromomethylthiirane were combined in benzene-d6, the bromide was consumed (93% conversion, 7% residual starting bromide) at room temperature in less than 30 min forming allylthiol (78%) and allyl bromide and propylene (13%) and allylthiotributylstannane (9%), with no propylene sulfide detected. At lower concentrations of Bu3SnH and bromides (e.g., 0.05 M) employed in 1024 J. Org. Chem., Vol. 69, No. 4, 2004

kinetic experiments, allylthiotributylstannane was not formed. The absence of this product at lower reagent concentrations suggests that it does not result from attack of Bu3Sn• at the sulfur atom of R-bromomethylthiirane followed by Br loss, but rather that allylthiotributylstannane results from the cross-termination of Bu3Sn• and CH2dCHCH2S• radicals. Control experiments revealed that concentrations of propylene sulfide as low as 3 × 10-4 M would have been detectable, giving a lower limit of [allylthiol]/[propylene sulfide] > 3000. At an average [Bu3SnH] ) 0.5 M and a rate constant of 2.4 × 106 M-1 s-1 for abstraction of hydrogen atom by a primary radical,23 the lower limit for the rate constant for opening of the thiiranylmethyl radical is kre g 3000(2.4 × 106 M-1 s-1)(0.5) g 3 × 109 s-1. Rates of Bromine Atom Abstraction from Bromomethyloxiranes and -Thiiranes by Tri-n-butylstannyl Radical: Enhancement of Bromine Atom Abstraction by the Cyclopropylcarbinyl, Oxiranyl, and Thiirane Ring. The measured rate constant of bromine atom abstraction from PhCH2CH2Br (C) at room temperature, kBr ) 3.8 ( 0.5 × 107 M-1 s-1, is comparable to the rate of abstraction from n-propyl bromide previously reported by Ingold and co-workers.24 Bromine atom abstraction from R-bromomethylcyclopropane (D) is only slightly faster than from simple primary alkyl bromide. The modest effect of the cyclopropyl group on the reactivity of primary alkyl bromide is consistent with the small stabilization of a primary alkyl radical center by the cyclopropyl group, estimated in early theoretical work to be 1-1.8 kcal/mol compared to a primary alkane.25 The relative rate constant for abstraction of bromine atom from R-bromomethylthiirane (entry F of Table 1) of 6.2 reflects a significant rate enhancement of bromine atom abstraction compared to R-bromomethylcyclopropane (entry D of Table 1) of 1.3 and 2-phenylethyl bromide (entry C of Table 1) of 1.0. We interpret this rate enhancement to reflect synchronous opening of the thiirane ring with bromine atom abstraction. While rate constants for atom and group displacement reactions by Bu3Sn• reflect primarily enthalpic effects, polar effects and electron-transfer pathways, and perhaps formation of transient adducts involving the stannyl radical and the bromine atom in the displacement reactions may contribute to the description of the transition state properties of Br abstraction.13 The enthalpy changes related to the observed rate enhancements can be estimated from the thermochemistry of the vinylthiomethyl f thiiranylmethyl f allylthiyl system (eq 16 and Figure

5). The heat of formation of allylthiol, 15 kcal/mol, leads to an estimate of the heat of formation of vinylthiomethyl radical of 51 kcal/mol by assigning an S-H bond strength of 88 kcal/mol. The heat of formation of thiiranylmethyl (22) Autrey, T.; Shaw, W. J.; Franz, J. A. Unpublished work. (23) Chatgilialoglu, C.; Ingold, K. U.; Scaiano, J. C. J. Am. Chem. Soc. 1981, 103, 7739-7742. (24) Ingold, K. U.; Lusztyk, J.; Scaiano, J. C. J. Am. Chem. Soc. 1984, 106, 343-348. (25) Radom, L.; Paviot, J.; Pople, J. A.; Schleyer, P. V. R. J. Chem. Soc. Chem. Comm 1974, 58-60.

Rate Expressions for S and Br Atom Abstraction TABLE 3. Electronic Energies and Total Thermal Corrections to Enthalpy, 298 K, for Species on the Vinylthiomethyl f Allylthiyl Surfacea electronic energy, hartree/particle species

UB3LYP/ 6-311+G(2d,2p)

UB3LYP/ 6-31G(d)

enthalpy correction

allylthiyl TS1 TS2 vinylthiomethyl

-515.52197 -515.47235 -515.49605 -515.50488

-515.45165 -515.40032 -515.42747 -515.42957

0.074684 0.070495 0.072178 0.070723

a Structures optimized at the UB3LYP/6-311+G(2d,2p) and UB3LYP/6-31G(d) levels of theory. Frequencies calculated at UB3LYP/6-311+G(2d,2p).

FIGURE 5. Values of ∆Hf° (in parentheses) of allylthiyl, vinylthiomethyl, and thiiranylmethyl radicals are thermochemical estimates in kcal/mol based on values of the parent hydrocarbons allylthiol (15 ( 2), propylene sulfide (11 ( 0.5) and methylvinylthioether (18 ( 0.2) (Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. Gas-Pase Ion and Neutral Thermochemistry. J. Phys. Chem. Ref. Data 1988, 17, Suppl. 1 using BDE values for S-H (88 kcal/mol), C-H (100 kcal/mol) and SCH2-H (93.4 kcal/mol), respectively. The solid line is the potential surface for the rearrangement at B3LYP/6-31G(d), 0 K. TS1 corresponds to C2-C4 bond elongation. TS2 corresponds to torsional motion of the CH2 as the plane of the CH2 group is aligned perpendicularly to the C2-S3 bond for scission. Note that TS2 is not a saddle point on the ∠C2-S3-C4 surface since TS2 occurs in the CH2 torsional space.

radical is estimated to be 60 kcal/mol, arrived at by assigning the C-H bond dissociation energy of 101 kcal/ mol to the methyl C-H bond of propylene sulfide. This model predicts that synchronous ring opening during bromine atom abstraction would be 8-10 kcal/mol more exothermic than Br abstraction from 2-phenethylbromide, consistent with the experimental observations. Electronic Structure Calculations. Geometries and frequencies for the allylthiyl f vinylthiomethyl surface were optimized at UB3LYP/6-31G(d) and UB3LYP/ 6-311+G(2d,2p) levels of theory using the Gaussian 98 set of programs.26 The connected line of Figure 5 depicts UB3LYP/6-31G(d) energies at 0 K. Frequency calculations revealed a single negative eigenvalue for the transition structures shown in Figure 5, and all positive eigenvalues for the two ground state molecules. Enthalpy corrections to ambient temperature to UB3LYP/6-311+G(2d,2p) energies were made using scaled UB3LYP/ 6-311+G(2d,2p) frequencies following eq 17.27 Each vibration, λk, was scaled by 0.9806 for the ZPE term (x ) (26) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98; Gaussian, Inc.: Pittsburgh, PA, 1998. (27) McQuarrie, D. A. Statistical Mechanics; Harper and Row: New York, 1976.

a) and by 0.9989 for the vibrational term (x ) b). Vibrations identified as internal torsions of less than 250 cm-1 were replaced by 1/2RT.28 Geometries of the species are included in Supporting Information. Electronic struc-

ture calculations at the B3LYP/6-31G(d) and B3LYP/6311+G(2d,2p)/B3LYP/6-31G(d) levels of theory illustrate the barrierless C-S bond scission to form the allylthiyl radical. Figure 5 (connected line) shows the B3LYP/ 6-31G(d) potential surface for the vinylthiomethyl to allylthiyl radical rearrangement. The potential surface confirms the report of Pasto for the thiiranylmethyl radical in which full geometry optimization calculations resulted in the spontaneous ring opening to the allylthiyl radical.4 The inflection point observed for the thiiranylmethyl radical (indicated by the arrow, Figure 5) is consistent with the CdC bond forming concurrently with C-S bond cleavage and is strong evidence that thiiranylalkyl radical (unlike cyclopropylcarbinyl and oxiranylmethyl analogue) does not exist as a stable intermediate.1,4 These results are consistent with our previous observation, using semiempirical calculations, that thiaspirooctadienyl radicals such as 2 are not intermediate structures. In addition, Figure 5 shows the enthalpies (inside parentheses) of allylthiyl and vinylthiomethyl radicals and the transition states. The UB3LYP/6-31G(d) calculations predict that the thiiranylmethyl (TS2) radical lies 13.6 kcal/mol above the allylthiyl radical. The UB3LYP/6-311+G(2d,2p) calculations predict TS2 to lie even higher, 14.7 kcal/mol above allylthiyl radical. These values are rather substantially higher than the value estimated from thermochemical kinetics, 8 ( 2 kcal/mol. Thus, although the qualitative shape of the potential surface predicted by the DFT approach appears correct, the rather large deviation of the DFT energies for the transitional structures is a matter for concern. Finally, we note that experiments were carried out to establish a lower limit of the rate ring opening of thiiranylmethyl radical. In experiments utilizing 1 M Bu3SnH and R-bromomethylthiirane, 1H NMR measurements detected no propylene sulfide product. This observation provided a lower limit of the rate constant for (28) Scott, A. P.; Radom, L. J. Phys. Chem. A 1996, 100, 16502.

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opening of the thiiranylmethyl radical of kre > 3 × 109 s-1, consistent with barrierless ring opening. Abstraction of Bromine Atom from r-Bromomethyloxirane. The ring opening of the oxiranylmethyl radical (10) was found to have a small rate increase as compared to cyclopropylcarbinyl radical. Rearrangement

of oxiranylmethyl radical yielded only allyl alcohol from allyloxy radical (11), consistent with other literature reports indicating that the C-O bond cleavage is the kinetically favored product.1,4-11 In addition, oxiranylmethyl radical was found by reliable calculations1 to have very little enthalpic gain in forming the allyloxy radical. The increase in rate for the ring opening is thought to result from the slight enthalpic gain or possibly from polar effects in the transition state, as discussed above. This interpretation, along with our results, is consistent with the experimental evidence14,15 that suggests that the oxiranylmethyl radical is a very short-lived intermediate in the interconversion of vinyloxymethyl radical to allyloxy radical rather than a synchronous ring opening. Desulfurization of Episulfides with Tri-n-butylstannane: Synthetic Utility. In contrast to the highly reactive bromine atom in R-bromomethylthiirane, the reactivity of the chlorine atom in R-chloromethylthiirane toward tri-n-butylstannyl radical is much slower than that of the sulfur atom in the thiirane ring. The selective abstraction of sulfur atom from the thiirane ring allows the formation of allyl chloride as the only observed product. Sulfur atom was also abstracted very efficiently from both styrene sulfide and propylene sulfide. Episulfides are ordinarily converted to the corresponding olefins by reaction with triphenyl phosphine or triethyl phosphite.29-31 This work demonstrates that episulfides can also be readily desulfurized by reaction with Bu3SnH and a suitable radical initiator. The high rates of reaction of Bu3Sn• with episulfides (107-108 M-1 s-1), 5 orders of magnitude more reactive than simple dialkyl sulfides,32 guarantees that the episulfide group can be converted to the olefin in the presence of a variety of reactive functional groups. Thus, reaction of R-chloromethylthiirane (0.05 M) with a slight excess of Bu3SnH and initiator (AIBN) in benzene at 80 °C leads to the clean, quantitative formation of allyl chloride. Conclusions New Arrhenius rate expressions have been determined for the abstraction of bromine atom from phenethyl bromide and sulfur atom from propylene sulfide by tributylstannyl radical, along with relative and absolute rates of sulfur and bromine atom abstraction from episulfide, oxirane, and cyclopropylalkane systems. Rate enhancement of bromine atom abstraction from cyclopropylcarbinyl, oxiranylmethyl, and by tri-n-butylstannyl (29) Davis, R. E. J. Org. Chem. 1958, 23, 1767. (30) Schuetz, R. D.; Jacobs, R. L. J. Org. Chem. 1958, 23, 1799. (31) Nereiter, N.; Bordwell, F. G. J. Am. Chem. Soc. 1959, 81, 578. (32) Beckwith, A. L. J.; Pigou, P. E. Aust. J. Chem. 1986, 39, 7787.

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radical is as follows: R-bromomethylcyclopropane (0.85 × 108), R-bromomethyloxirane (2.1 × 108), and R-bromomethylthiirane (4.2 × 108). The increase in rate of bromine atom abstraction by tin radical from the thiiranylmethyl bromide provides evidence for synchronous thiirane ring opening providing an enthalpic enhancement of the bromine atom abstraction rate. Electronic structure calculations and thermochemical kinetic estimates show that the thiiranylmethyl radical is produced about 9 kcal/mol above the allylthiyl radical, leading to reactivity of the thiiranyl bromide comparable to secondary alkyl bromides. When halogen atom abstraction is slow, as with R-chloromethylthiirane, sulfur atom abstraction by tin radical becomes exclusive. The oxiranylalkyl counterparts show only a small rate increase in halogen atom abstraction compared to that of the cyclopropyl analogue.

Experimental Section Preparation of Reagents. Except as noted, all reagents were commercially available. R-Bromomethylthiirane, R-chloromethylthiirane, and epithiostyrene were prepared using literature33 procedures as follows. The corresponding epoxides and a slight molar equivalent excess of triphenylphosphine sulfide were combined in methylene chloride and chilled in a dry ice bath. Dry trifluoroacetic acid (2 equiv) was added dropwise. The suspension was allowed to warm to room temperature, stirred for several hours, and neutralized by addition of solid sodium bicarbonate. The mixture was filtered and concentrated by rotary evaporator. R-Bromomethylthiirane and R-chloromethylthiirane were vacuum distilled at 35-36 °C/∼0.2 Torr. Epithiostyrene was isolated by column chromatography on a silica column and further purified using rotating-plate chromatography on silica. R-Bromomethylthiirane: 1H NMR (300 MHz, benzene-d6): 4.26 (1H, ddd, J ) 9.9, 4.2, and 1.4 Hz), 3.74 (1H, m, J ) 9.9, 5.1, 4.2 Hz), 3.55 (1H, t, J ) 9.85 Hz), 3.02 (1H, dt, J ) 5.9 and 1.4 Hz), 2.69 (1H, dd, J ) 5.1 and 1.5 Hz). 13C NMR (75 MHz, benzene-d6): 36.3, 33.5, 26.4 R-Chloromethylthiirane: 1H NMR (300 MHz, benzene-d6): 3.2 (1H, ddd, J ) 6.3, 4.4, and 1.3 Hz), 2.65 (1H,t, J ) 9.6 Hz), 2.57 (1H,m), 1.87 (1H, dt, J ) 5.8 and 1.3 Hz), 1.58 (1H, dd, J ) 4.9 and 1.4 Hz). 13C NMR (75 MHz, benzened6): 48.6, 33.5, 25.2 Epithiostyrene: 1H NMR (300 MHz, benzene-d6): 7.09-6.92 (5H, m), 3.40 (1H,t, J ) 6.0 Hz), 2.28 (1H, dd, J ) 6.6 and 1.4 Hz), 2.15 (1H, dd, J ) 5.5 and 1.2 Hz). Allylthiyl-tri-(n-butyl)stannane. To a solution of 2.35 g (102.2 mmol) of potassium metal in 100 mL of methanol under nitrogen was added dropwise a solution of 7.36 g (99.2 mmol) of allylmercaptan in 50 mL of methanol. After 30 min of stirring a solution of 32.55 g (100 mmol) of Bu3SnCl in 50 mL of dry THF was added. After 1 h of stirring, 200 mL of water was added and the mixture was extracted with CH2Cl2, dried over MgSO4, concentrated to a yellow oil, and distilled (0.5 mmHg, 108-110 °C) to give pure allylthiyl-tri-(n-butyl)stannane (20 g, 56%). 1H NMR (benzene-d6): 0.9 (t, 9H), 1.051.12 (m, 6H), 1.26-1.38 (m, 6H), 1.53-1.65 (m, 6H), 3.203.23 (m,2H), 4.85-4.90 (m, 1H), 5.05-5.12 (m, 1H), 5.9-6.02 (m, 2H). Sn-H couplings were: 2J(119Sn and 117Sn-CH) ∼50 Hz, 3 119 J( Sn and 117SnCCH) ∼40 Hz, and 3J(119Sn and 117SnCCCH) ∼31 Hz. 13C NMR(benzene-d6): 14.08, 14.38, 27.92, 29.54, 30.53, 114.86, 140.24. Sn-C couplings observed were as follows: 1J(119Sn and 117Sn-C) 315, 330 Hz, 2J(119Sn and 117 Sn-C-C) 59, 61.8 Hz, 2J(119Sn-S-C and 117Sn-S-C) 5 Hz, 3 119 J( Sn-C-C-C and 117SnC-C-C) 21 Hz, and 4J(SnCCCC) ∼5 Hz. Anal. Calcd for C15H32SnS: C,49.61; H, 8.88; S, 8.83; Sn, 32.68. Found: C, 49.85; H, 9.12; S, 9.12; Sn, 32.54. (33) Chan, T. H.; Finkenbine, J. R. J. Am. Chem. Soc. 1972, 94, 2880-2882.

Rate Expressions for S and Br Atom Abstraction Transient Absorbance Spectroscopy (TAS). TAS methods were used to measure the absolute rate of halide abstraction by tri-n-butylstannyl radical. The laser flash photolysis system has been described previously.20 The attenuated output (5 mJ) of an excimer laser (Lambda Physik Compex-100, XeCl, 308 nm) was directed onto a fluorescent sample cell perpendicular to the direction of probe beam (150-W Xe lamp, model XMN-150, Optical Radiation Corp., enclosed within a Spectral Energy lamp housing, LH 150, Universal arc lamp power supply, LPS 251). The filtered output of the probe lamp was focused (L1, 250-mm cylindrical lens) at the center of the fluorescence cell. Luminescence from the solvent was minimized but not completely eliminated by placing an iris (2 mm) in the path of the probe beam placed between the flow cell and the monochromator (MC, Oriel model 77250). A second lens (L2, 50 mm) was placed before the entrance slit to the MC. In the kinetic experiments the MC was set to 400 nm to detect the absorbance maximum of the visible tributylstannyl radical absorption. A PMT tube (RCA 6199 wired in a 4-dynode chain, powered by a negative high voltage power supply, Pacific Photometric Instruments, model 226) was fixed in PMT housing attached to the exit slit of the MC. The output of the PMT tube was digitized on a Lecroy 534 transient digitizer at a resolution of 1 ns/pt and the data transferred to a PC for analysis. Stock solutions of the tri-n-butyltin hydride (0.02 M) and di-tert-butyl peroxide (0.2 M) were prepared in benzene containing phenethyl bromide (0, 5, 10.2, 19.8, and 24.9 mM) and had an absorbance at 308 of 0.30 across 1 cm. The solutions were deaerated by purging with argon and charged into a gastight syringe prior to kinetic analysis. The sample was flowed through a quartz fluorescent cell (NGS T59FL) clamped within a heated (or cooled) C-shaped anodized aluminum cell block. Measurement of the temperature of the sample, monitored at the exit port of the flow cell, was facilitated with the use of a Digisense temperature controller and thermocouple. Measurements were made at 284, 289, 299, 310, 318, 325, and 333 ( 1 K. The samples were flowed (Sage syringe pump, model 355) through a low-volume quartz fluorescent flow cell (NGS T59FL) to avoid the accumulation of photoproducts and to ensure constant reagent concentrations. Three control experiments demonstrated that the solutions were thermally and photochemically stable under the reaction conditions with flow rates of 2 mL/min and a laser repetition rate of 1 Hz: (1) Photolysis of the flowing solution was shown to be negligible after the UV output from the probe beam (150-W Xe lamp) was suitably attenuated with a long pass glass filter (LP-365). (2) A measure of the reaction products by 1H NMR before and after photolysis (flow rate of 1.0 mL and 0.5 Hz repetition rate) assured that the bromide concentration can be assumed constant under the reaction conditions. (3) 1H NMR experiments demonstrated the stability of bromide/tin hydride solutions at 60 °C. 2kt/ was determined by linearizing the data (1/∆OD) and fitting the data from 1.5 to 2.5 µs from the laser

discharge, a range where the tert-butoxy radicals are entirely consumed and stannyl radical self-termination is negligible. At temperatures above 300 K, the growth of alkyl radical also contributed to the pure first-order decay, and thus in this temperature range, 2kt/ represents data fit from 1.5 to 1.9 µs. Determination of Relative Rates. Stock solutions were prepared by combining Bu3SnH (0.02-0.07 M), azobisisobutyronitrile (AIBN, (3-5) × 10-3M), xylene (internal GC and NMR standard), 2-phenylethyl bromide (0.05-0.5 M), and competing reagent (0.05-0.1 M) in argon-purged benzene-d6 under argon. Samples (500-600 µL) of the solutions were transferred to argon-purged Pyrex tubes and sealed under vacuum after three freeze-thaw degassing cycles. The samples were heated in a Braun Thermomix 1480 oil bath for 10-12 min at 80 °C. Product and reactant concentrations were measured before and after reaction by careful 1H NMR integration and/or capillary GC measurements of products and starting materials using calibrated internal standards. The relative rates for competition of initial concentrations of reagents A0 and B0 resulting in final concentrations At and Bt is given by34

kA ln At - ln A0 ) kB ln Bt - ln B0 In practice, the products from A and B are measured. The relative rate constant kA/kB for reaction of the Bu3Sn• with reagent A forming product PA vs reaction with B forming product PB is given by the familiar relation

kA ) kB

( (

PA A0 PB ln 1 B0 ln 1 -

) )

Relative rates are presented in Table 2.

Acknowledgment. This work was supported by the Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, U. S. Department of Energy at the Pacific Northwest National Laboratory (PNNL). PNNL is operated by Battelle Memorial Institute under contract DE-ACO6-76RLO 1830. Supporting Information Available: Geometries and analytical frequencies from density functional calculations. This material is available free of charge via the Internet at http://pubs.acs.org. JO035467H (34) Walling, C.; Jacknow, B. B. J. Am. Chem. Soc. 1960, 82, 6108.

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