EPR from 0-Alkyl Thioesters and 0-Alkyl Selenoesters
915
(14) R. A. Ogg, Jr., J . Am. Chem. SOC.,68, 155 (1946). (15) M. S. Matheson and L. M. Dorfman in “Pulse Radiolysis”, M.I.T. Press, Cambridge, Mass., 1969, p 170. (16) See “optical line broadenlng” and “phonon” in the Glossary to Can. J . Chem., 55 (1977), Electrons in Fluids issue: J. Jortner, private communication. (17) P. Delahay, J. Chem. Phys., 55, 4188 (1971). (18) J.-P. Dodelet, K. Shinsaka, and G. R. Freeman, Can. J. Chem., 54, 744 (1976). (19) K. Schmidt and M. Anbar, J . Phys. Chem., 73, 2846 (1969). (20) R. W. Gallant, “Physical Properties of Hydrocarbons”, Vol. 1, Gulf Publishing Co., Houston, Tex., 1968.
(21) (22) (23) (24) (25) (26) (27) (28) (29)
S. Kyropoulos, Z . Phys., 40, 507 (1926). F. Buckley and A. A. Maryott, &ti. Bur. Stand. Circ., No. 589 (1958). A. Maccoll, 0 . Rev. Chem. SOC., London, 1, 16 (1947). G. R. Freeman, Natl. Stand. Ref. Data Ser., Nati. Bur. Stand., No. 48 (1974). J. H. Baxendale and P. Wardman, Nati. Stand. Ref. Data Ser., Natl. Bur. Stand., No. 54 (1975). D. W. Johnson and 0. A. Salmon, Can. J. Chem., In press. W. Kauzmann, “Quantum Chemistry”, Academic Press, New York, N.Y., 1957, p 581. N. Q. Chako, J. Chem. Phys., 2, 644 (1934). R. S. Mulliken and C. A. Rieke, Rep. Prog. Phys., 8, 231 (1941).
Electron Paramagnetic Resonance Spectra of Some Radicals from 0-Alkyl Thioesters and 0-Alkyl Selenoesters’ D. Forrest,2a.bK. U. Ingold,*2’and
D. H. R. Barton2’
Division of Chemistry, National Research Council of Canada, Ottawa, Canada KIA OR9 and chemistry Department, Imperial College, London SW7 2AY, England (Received December 6, 1976) Publication costs assisted by the National Research Council of Canada
EPR spectra are reported for the radicals formed by addition of CF3., Me3Snq,and (EtO)zPOradicals to the C=S double bond of some thioesters and to the C=Se double bond of some selenoesters. The radicals formed by addition to thioacetate, selenoacetate, and related selenoesters adopt, for steric reasons, a conformation in which the added radical is in the eclipsed position with respect to the C, 2p, orbital. Most radicals formed by addition to thioformates and selenoformates adopt a partly staggered conformation in which the added radical lies between the eclipsed position and the C, 2p, nodal plane.
A new method for the deoxygenation of secondary alcohols has recently been de~eloped.~ The alcohol, R’OH, is first converted to an 0-alkyl thiobenzoate, R’OC(= S)C6H5,or 0-alkyl-S-methyl dithiocarbonate, R’OC(= S)SCH3,or 0-alkyl selenobenzoate, R’OC(=Se)C6H5, and the ester is then refluxed with tri-n-butylstannane in toluene. With the thiobenzoate or selenobenzoate the following free-radical chain yields the deoxygenated alcohol, R’H: n-Bu,Sn.
+ X=C
/
\
C6H5
./
-+
C6H5
n-Bu,SnXC
\
OR’
OR‘
1 1
--+
R’. + n-Bu,SnXC
I
\\
C6H5
0
R e + n-Bu,SnH -+ R’H + n-Bu,Sn. X = S, Se; R’ = secondary alkyl
The ease of this reaction implies that tri-n-butyltin adds readily to thiobenzoates and to selenobenzoates and this, in turn, suggests that the intermediate adduct radical, 1, might be examined by EPR spectroscopy. Such radicals are of interest in themselves and would be of even greater interest if a family of related radicals, 2, could be produced by the addition of transients other than n-Bu3Sn.to esters other than the benzoates, i.e. R,M*
/
+ X=C\
R’‘
/
-+
OR‘
R“
hMXC
\
OR’ 2 X = S, Se; R’ = primary, secondary, tertiary alkyl; R” = H, alkyl, aryl
There have been very few EPR spectroscopic studies of
radical additions to the C=O double bond in esters4v5 or ketones4-’ or to the C=S or C=Se double bonds in thioketones” and selenoketones11s12and there have been no studies of additions to thioesters or selenoesters. There has also been only one report on the EPR spectroscopy of carbon-centered radicals which have two different atoms from group 6 of the Periodic Table attached directly to the a carbon.13 We were also intrigued by the possibility that some 1 or 2 might undergo /3 scission sufficiently rapidly that 0-alkyl thioesters would provide a convenient route for generating R’* from R’OH at ambient temperatures. Experimental Section Materials. 0-Ethyl thioformate was prepared by the method of Mayer and Berthold:14 yellow liquid with an ozone-like smell; bp 86-87.5 “C at 750 Torr (lit.1486.5-87.5 “C); proton NMR in CDC13, 6 9.50 (1H, s), 4.54 (2 H,.q, J = 7 Hz), 1.43 (3 H, t, J = 7 Hz). 0-tert-Butyl thioformate was prepared in low yield by the procedure of Barton and M~Combie:~ yellow liquid with a vile smell; NMR in CDC13, 6 9.63 (1H, s), 1.52 (9 H, s). 0-Ethyl thioacetate was prepared by the method of Ohno et al.:15 yellow liquid; bp 109-110 “C at 760 Torr (lit.15 105-109 “C); proton NMR in CDC13,6 4.41 (2 H, q, J = 7 Hz), 2.52 (3 H, s), 1.38 (3 H, t, J = 7 Hz). 0-Ethyl thiobenzoate and 0-2-methylbutyl thiobenzoate were prepared as yellow liquids by the general procedure of Barton et a1.16 The former has a bp 74-75 “C at 0.4 Torr, and proton NMR in CDC13, 6 8.03 (2 H, m), 7.26 (3 H, m), 4.62 (2 H, q, J = 8 Hz), 1.50 (3 H, t, J = 8 Hz), and the latter has proton NMR in CDC13, 6 8.00 (2 H, m), 7.30 (3 H, m), 4.40 (2 H, d, J = 7 Hz), 1.50-0.80 (9 H, m). 0-Cholesteryl selenoformate (mp 89-91 “C) and 0-ethyl selenobenzoate were prepared as previously de~cribed.~ 0-Ethyl selenoformate was obtained as a ca. 15% solution in a mixture of dimethylformamide, methylene chloride, and tetrahydrofuran: NMR in CDC13,6 11.94 (1H, s), 4.62 The Journal of Physlcal Chemistry, Vol. 8 1 No. 9 , 1977 ~
D. Forrest, K. U. Ingokl, and D. H. R. Barton
916
TABLE I: EPR Parameters for Some Radicals Derived from Thioesters, from Di-tert-Butyl Thioketone, and for Some Related Radicalsa Radical Me,COC(H)OCMe,d Me,CSC(H)SCMe,d Me,CSC( H)OCMe, Me,CSC( CMe,), CF,SC( H)OC,H, CF,SC( H)OCMe , C F , S ~CH,)OC, ( H,' CF3SC(CMe3)2 Me, SnSC( H)OC2H, Me, SnSC(H)OCMe,' Me,SnSC( CH,)OC,H,' Me, SnSC( CMe ,), (EtO),P(O)SC(H)0C,Hs1 (EtO),P( O)SC( H)OCMe,O (EtO),P( O)SC( CH,)OC, H,' (EtO),P(O )SC(CMe, 1,
g 2.0031 2.0052 2.0033 2.0026 2.0029 2.0035 2.0031 2.0024 2.0035 2.0036 2.0034 2.0028 2.0034" 2.0035" 2.0032" 2.0026"
R"
R'O
aother
9.6(1) 15.8(1) 16.3(1)
91.3 (C,) 34.5 (C,)
17.6(1) 17.1(1) 18.3(3)
44.3 (C,) 4.3 ( 3 F ) 4.6 ( 3 F ) 6.8 ( 3 F ) 3.9 ( 3 F )
e
2.5(2) 2.4( 2)
12.2( 1) 10.9(1) 17.5(3)
1.5(2)
18.5(1) 15.7(1) 18.2(3)
2.6(2) 2.1(2)
-28.5 -8 -lOf - 9g - 6h
e
103, 109 (Sn)j 1.5(2)
a (aH)Ril/a T C
Ok
e
243, 254 (Sn)j 32.5"," (P) 95.4"JJ*4 (P) 115.0" 101"
- 38
-1 74
a Hy erfine splitting constants are given in gauss. Parameters are given at 25 "C unless otherwise specified. Data for the R,MS&CMe,), radicals are from ref 10. In the thioester adducts, 2, and in analogous radicals, R" refers to the H or alkyl that is directly bonded t o C, and R'O t o the alkoxy group bonded to C,. Temperature coefficient in mG/"C. At -100 "C. Data from ref 18. e Not resolved. f 25 t o -40 "C. g 25 to -100 "C. 25 t o -140 "C. At -60 "C. The smaller hyperfine splitting is due t o ll'Sn and the larger to ' 1 9 Sn. 25 to -80 "C. At - 100 "C. " Corrected bv the Breit-Rabi-equation. " ;laP IlaT = 2 mG/"C. O At 1 2 0 "C. P a laPI/aT = -67 mG/"C. 4 -20 to 20 "C. J
(2 H, q,J = 7 Hz), 1.50 (3 H, t, J = 7 Hz). This compound is thermally unstable and could not be prepared in pure form. Attempts to prepare O-neopentyl and 0-1adamantylmethyl selenoformates were unsuccessful. O-Methyl selenoacetate (bp 107-109 "C at 760 Torr), O-methyl selenooctadecanoate (mp 25-26 "C), O-methyl selenobenzoate (bp 86-88 "C at 0.5 Torr), and O-ethyl selenopivalate (bp 64-65 "C at 14 Torr) were prepared by Drs. P. E. Hansen2"and K. Pickerzcfollowing the general procedure of Barton and M~Combie.~ Radical Generation. The following transient R,M. radicals were generated photochemically using previously described methods" directly in the cavity of a Varian E-4 EPR spectrometer in the presence of each thioester and selenoester, usually in isopentane as the solvent: Me3Sn., Me3Si-,Me3C., Me., CF3., C6H5*,C6F6.,Me3C0., Me3CS., CF3S., and (EtO)$O. In many cases adducts were obviously not produced since the R,M- radical could be observed, and in other cases neither adducts nor R,M. could be detected. In still other cases the EPR spectra that were obtained could not be analyzed unequivocally and hence could not be assigned with certainty. This was the situation with the thiobenzoates and selenobenzoates (which were examined in toluene for reasons of solubility). Presumably the unpaired electron in 2 (R" = C6H5)is delocalized into the aromatic ring and the complexity of these spectra is due to its interaction with all the ring protons. The reaction of no R,M. radical, including Me3Sn.,with the thiobenzoates and selenobenzoates (or any of the other esters) gave an observable concentration of the R'. alkyl radical in the temperature range -80 to 25 "C. This confirms3that the /3 scission of radicals analogous to 1 is not a particularly rapid reaction at ambient and lower temperatures. The photolysis of O-ethyl selenoformate and of 0-methyl selenodecanoate in isopentane gave red selenium. The other selenoesters were photochemically more stable but all formed red or black selenium with some R,M sources. In this paper, only those 2 that have, we feel, been unequivocally identified are reported. This means the CF3-,Me3Sn.,and (EtO)&O ' adducts to the thioesters and selenoesters, together with the Me3C. adduct to O-tertbutyl thioformate. The Journal of Physical Chemistry, Vol. 81, No. 9 , 1977
Figure 1. EPR spectrum of CF,SC(H)OCMe3 in isopentane at 25 "C.
Results The EPR parameters for the radicals derived from the thioesters and selenoesters are listed in Tables I and 11, respectively. For comparative purposes, these tables include data on the radicals obtained by addition of the same R,M- radicals to di-tert-butyl thioketone:' and to di-tert-butyl selenoketone." Data on (Me3C0)2CH'8and (Me3CS)zCH18are also included in Table I for comparison with the Me3CSC(H)OCMe3radical. The EPR spectrum of CF3SC(H)OCMe3is shown in Figure 1. Under steady photolysis the concentration of all of the adducts to the thioesters and selenoesters were shown to be proportional to the square root of the incident light intensity. This indicates that all 2 decay with second-order kinetics and the majority were found to decay at rates that were close to the diffusion-controlled limit, as would be expected for radicals that are not sterically hindered about the cy carbon.lg The absence of first-order decay kinetics, combined with our failure to detect the R'. radical in any case (see Experimental Section), indicates that all 2 undergo relatively slow scission of the R'-0 bond. Discussion Conjugative delocalization of the unpaired electron onto adjacent heteroatoms, Le.
EPR from U-Alkyl Thioesters and U-Alkyl Selenoesters
917
TABLE 11: EPR Parameters for Some Radicals Derived from Selenoesters and from Di-tert-butyl Selenoketonea UH
Radical CF,SeC( CH,)OCH, CF,SeC( CMe,)OC,H,d CF, SeC(CMe, ), Me,SnSeC( CH,)OCH, Me,SnSeC( C, 7H35)OCH, Me,SnSeC(CMe,)OC,H, Me,SnSeC(CMe, 1, (Et'b),P(O jSeC("H)OC,H, (EtO),P(O)SeC(H)OChoIg (EtO),P( O)SeC(CMe,)OC,H,h (EtO),P( O)SeC(CMe,),'
R"
R'O
18.9(3)
1.7(3) 1.8(2)
g
2.0009 2.0011 2.0005 2.0045 2.0047 2.0050 2.0043 2.0035f 2.0036f 2.O02gf 2.0016f
17.3(3) 15.7(2)
7.9(3 F) 6.0(3 F1 4.6(3 F); 46.5(Ca)
C
57.9 36.6
C
C
c
56.4 54.2 11.0
C
18.1(1) 18.5(1)
uother
u77~e
2.5(2) 1.6(1) 1.2(2)
C
195, 204(Sn)e 185, 194(Sn)e 189, 198(Sn);e 52.O(Ca) 30.6f (P) 30.7f (P) 91.8f (P) 89.3f (P)
C C
48.6 C
Hyperfine splitting constants are givenjn gauss. Parameters for the selenoester adducts are reported at 25 "C unless otherwise specified. Data for the R,MSeC(CMe,), radicals are from ref 11 at - 50 "C. In the selenoester adducts, 2, R" Not resolved. d At refers to the H o r alkyl that is directly bonded t o C, and R'O t o the alkoxy group bonded t o C,. -80 "C. e The smaller hyperfine splitting is due to I1'Sn and the larger t o 1 1 9 Sn. f Corrected by the Breit-Rabi equation. g Chol = cholesteryl, in toluene as solvent. A second radical which does not contain selenium was also produced: g = U ( * ~ C$)A=t 4 O 0 C . 2 . 0 0 2 9 ; ~ ( 1H o r l P ) = 2 4 . 0 G ; a ( 2 H ) = ~ . O G ; U ( ~ ( ? ) H ) = O . ~ ~ G ; 35.0G.
plays a very important role in determining the configuration" and conformation" of alkoxyalkyl, alkylthiylalkyl, and alkylselenylalkyl radicals. Thus, conjugative delocalization increases the carbanionic character of C, and so promotes bending at the radical It also causes unhindered radicals to prefer a conformation in which the X-MR, bond lies in the C, 2p, nodal plane. Since barriers to rotation about the C,-X bond in R,MS-CH2 radicals are somewhat greater than those in analogous R,MO-CH, r a d i ~ a l s , it~ ~is-clear ~ ~ that conjugative delocalization occurs more readily to sulfur than to oxygen.30 An additional effect of the heteroatom may be to promote bending at C, by virtue of its electronega t i ~ i t y . ~ The l - ~ ~extent of bending at C, can best be estimated from the magnitude of Such measurements have shown that oxygen containing radicals are more bent than analogous sulfur containing radicals and that the amount of bending increases with multiple substitution of oxygen around C,.18,21~26,34~35 Even (R,MW3C radicals appear to be only slightly bent.36 The EPR parameters for the first three radicals listed in Table I suggest that the spectroscopic behavior of radicals containing both a-OMR, and a-SMR, is dominated by the oxygen. Thus, the g value foreMe3CSC(H)OCMe3is similar to that found for Me3COCH2,viz.,28 2.0033, *and is a preciably less than that found for Me3CSCH2, viz.,' 2.0052. The fact that the sulfur in Me3CSC(H)OCMe3does not behave as though it were just a second oxygen is indicated by the value of aHewhich is similar in magnitude (and probably in sign to judge from its temperature coefficient)l' to that for Me3COCH2(viz. but is dissimilar to the value for -17.2 G)18,26,37 (Me3CO),CH (+9.6 Comparison with data for less hindered radicals38 indicates that the spectroscopic behavior of these radicals is not a consequence of the rather large size of the R,M groups. We suggest, therefore, that the spectroscopy of a-alkoxy-a-alkylthiylalkyl radicals is dominated by the alkoxy group because this group controls the configuration of the radical. That is, alkoxy induced bending at C, so reduces conjugative delocalization to sulfur that the alkylthiyl group cannot produce its 6cnormal,?10,39,40 enhancement of the g factor. The magnitude of aHm(which depends on the spin density and degree of bending at C,) is also, therefore, about the same as in simple a-alkoxyalkyl radicals. All R,M adducts to di-tert-butyl thioketone," 3a, and to di-tert-butyl selenoketone," 3b, adopt a conformation 3 in which the R,M group is eclipsed by the C, 2p, orbital.
B
3 MRn
MRn
4
5
1
I
In this conformation the hyperconjugative interaction of M with the unpaired electron is maximized, and so aMis maximized. Those 2 which have aMvalues of about the same magnitude as is found for the corresponding 3 must, therefore, also adopt an eclipsed conformation, Le., 4, or more probably 5 since the a carbon is probably bent (see above). Usin the foregoing criterion, we deduce that the (EtO) 0 adducts to those thioesters and selenoesters which have R" = alkyl and the Me3Sn-adducts to those selenoesters which have R" = alkyl favor an eclipsed conformation (4 or 5). Steric factors are presumably responsible for this conformational preference.10i11i1g In contrast, the Me3Sn. and (EtO),l?O adducts to the thioformates and selenoformates (R" = H) favor a noneclipsed (staggered) conformation, 6 or 7, in all but one case. The
f
RnM@: H
OR
@:nR
OR'
6
7
exception is (EtO),P(O)SC(H)OCMe, which presumably favors an eclipsed,41 or nearly eclipsed, conformation because of the large size of the alkoxy group, i.e., also for steric reasons. Although the conformational preferences of some Me3Sn. and of all the Me3C. and CF3. adducts cannot be determined via uMvalues it seems most likely that they too will adopt an eclipsed conformation when R" = alkyl and a staggered conformation when R" = H. The g factors for the adducts to the thioesters provide no help in determining conformation4' because of the dominant role of the alkoxy group referred to above. However, presumably The Journal of Physical Chemistry, Vol. 81, NO. 9 , 1977
918
because the spin-orbit coupling constant for selenium is much greater than that for sulfur (1688 vs. 382 cm-'), the g values of the adducts to the selenoesters are fairly sensitive to the nature of R,M and to the conformation of the adduct." Since the g values for the CF3. adducts to the selenoesters and to di-tert-butyl selenoketone are rather similar, and since the same is true for the Me3Sn. adducts, we conclude that the conformations of the selenoester and selenoketone adducts are similar, that is, eclipsed. For the (Et0)2P0adducts the situation is not so clear-cut. However, the fact that the g values for the adducts to the selenoformates are appreciably higher than for the adduct to the selenopivalate (which is probably eclipsed, see above) or selenoketone (which is certainly eclipsed) implies that the selenoformate adducts adopt a staggered conformation. That is, in a staggered conformation the g value of such a radical will increase because there will be more conjugative delocalization of the unpaired electron onto the selenium. By making certain assumptions it is possible to estimate the extent to which the Me3Sn and (Et0)2P0 groups in their adducts to the thio- and selenoformates deviate from the eclipsed position, i.e., it is possible to get a rough value for the dihedral angle 6' (see structures 6 and 7). The magnitude of uMshould depend on 6 according to the usual relation, viz.'l
aM = A + B cos2 6' If we assume that A is negligible in comparison with E43 then B can be determined from the value of uMin the fully eclipsed (6' = 0') adducts to di-tert-butyl thioketone and selenoketone. This procedure yields values of 0 of ca. 49' for the Me3Sn.adduct to tert-butyl thioformate and of ca. 55' for the (EtO)2P0adducts to ethyl thioformate and the two selenoformates. I t is not exactly obvious why these radicals do not adopt the fully staggered conformation with 6' = 90'. However, it is worth noting that if the adducts to the selenoformates had adopted such a conformation it might not have been possible to detect them by EPR spectroscopy.12 Acknowledgment. We are grateful to Dr. G. Brunton for many helpful discussions. References and Notes (1) Issued as N.R.C.C. No. 15930. (2) (a) N.R.C.C. (b) N.R.C.C. Research Associate 1975-1977, (c) Imperial College. (3) D. H. R. Barton and S. W. McCombie, J. Chem. SOC.,Perkin Trans. 7, 1574 (1975). (4) A. J. Bowles, A. Hudson, and R. A. Jackson, J. Chem. Soc.B, 1947 11971). (5) J. Cooper, A. Hudson, and R. A. Jackson, J . Chem. SOC.,Perkin Trans. 2 , 1933 (1973). (6) A. Hudson and R. A. Jackson, Chem. Commun., 1323 (1969). (7) A. R. McIntosh and J. K. S. Wan, Can. J. Chem., 49, 812 (1971); Mol. Phys., 22, 183 (1971). ( 8 ) I. G. Neil and B. P. Roberts, J. Organometal. Chem., 102, C17 (1975). (9) W. P. Neumann, B. Schroeder, and M. Zibarth, Annalsn, 2279 (1975). (10) J. C. Scaiano and K. U. Ingokl, J. Am. Chem. Soc.,98,4727 (1976), and references cited. (1 1) J. C. Scaiano and K. U. Ingold, Chem. Commun., 205 (1976): J. Phys. Chem., 80, 1901 (1978).
The Journal of Physical Chemistry, Val. 81, No. 9 , 1977
D. Forrest, K. U. Ingold, and D. H. R. Barton (12) J. C. Scaiano and K. U. Ingold, J. Am. Chem. SOC., In press. (13) E. A. C. Lucken and B. Poncioni, J . Chem. SOC.,Perkin Trans. 2, 777 (1976). (14) R. Mayer and H. Berthold, Z . Chem., 3 , 310 (1963). (15) A. Ohno, T. Kolzuml, and G. Tsuchihashi, Tetrahedron Lett., 2083 (1968). (16) D. H. R. Barton, C. Chavis, M. K. Kaloustian, P. D. Magnus, G. A. Poulton, and P. J. West, J. Chem. Soc., WinTrans. 1, 1571 (1973). (17) G. D. Mendenhall, D. Grlller, D. Lindsay, T. T. Tidwell, and K. U. Ingold, J. Am. Chem. SOC.,96, 2441 (1974); R. A. Kaba, D. Griller, and K. U. Ingold, ibM., 96,6202 (1974); D. Grlller and K. U. Ingold, ibM., 98, 8715 (1974). (18) G. Brunton, 8. P. Roberts, A. L. J. Beckwith, P. J. Krusic, and K. U. Ingold, J. Am. Chem. SOC., submitted, and unpubllshed work from thls laboratory. (19) 0. D. Mendenhall, D. Griller, and K. U. Ingold, Chem. Brlt., 10, 248 (1974); D. Griller and K. U. Ingold, Acc. Chem. Res., 9, 13 (1976). (20) Followlng Kochi's suggestion? conflguration is here used to describe the geometry at the radical center, C, and conformation Is used to describe the geometry of the whole radical, particularly as it relates to the dihedral angle, 8, between the X-MR, bond and the C, 2p, direction. (21) J. K. Kochi, Adv. Free Radical Chem., 5 , 189 (1975). (22) A. J. Dobbs, B. C. Gilbert, and R. 0. C. Norman, J. Chem. SOC.A , 124 (1971). (23) I?. C. Bingham and M. J. S. Dewar, J. Am. Chem. Soc., 95, 7182 (1973). (24) P. J. Krusic and R. C. Bingham, J. Am. Chem. Soc., 98, 230 (1976). (25) F. Bernardi, N. D. Epiotis, W. Cherry, H. B. Schlegel, M. H. Wangbo, and S. Wolfe, J. Am. Chem. Soc., 98, 469 (1976). (26) I.Blddles, A. Hudson, and J. T. Wiffen, Tetrahedron,28, 867 (1972). (27) The rotational banier Me,CSCH2 is 25.4 kJ/md,2eand in b & & Z it is 20 f 2 kJlmol. (28) G. Brunton,' unpubllshed results. (29) E. A. C. Lucken and B. Poncioni, Helv. Chim. Acta, 55, 2673 (1972). (30) For chemical confirmation of this point, see, e.g., A. Ohno and Y. Ohnishi, Tetrahedron Leff., 4405 (1969); J. W. Timberlake, A. W. Gamer, and M. L. Hodges, IbM.,309 (1973). I t shouid also be noted that the H bond In thiones is much weaker than the H bond In ketones." (31) L. Pauling, J. Chem. Phys., 51, 2767 (1969). (32) H. M. Walborsky and P. C. Collins, J. Org. Chem., 41, 940 (1976). (33) See also M. Iwalzumi, T. Kishi, T. Isobe, and F. Watari, J. Chem. Soc., Faraday Trans. 2 , 72, 113 (1976). (34) A. Hudson and K. D. J. Root, J. Chem. SOC. B , 656 (1970). (35) A. J. Dobbs, B. C. Gilbert, and R. 0. C. Norman, J . Chem. SOC., Perkin Trans. 2, 786 (1972). (36) H. B. Stegmann, K. Scheffler, and D. Seebach, Chem. Ber., 168, 64 (1975). (37) Average value for the two aH's. For comparison, a&(av) = -16.6 G for Me3CSCH2.2s*z8 (38) From ref, 13 the gfactors and annvalues for (MeO)&H, (MeS)&H, and EtSC(H)OMe are 2.0030. 2.0051, and 2.0030. and 11.7. 14.8 and 15.4 G, respectively. (39) B. C. Gilbert, J. P. Larkin, and R. 0. C. Norman, J. Chem. SOC., Perkin Trans. 2 , 272 (1973). (40) A. J. Dobbs, "Electron Spln Resonance", Vol. 2, Chemical Society Specialist Report, London, 1974, Chapter 10. (41) The conformational preference of this radical is confirmed by the sign and magnitude of alaPllaT(seefootnote p , Table I). (42) g factors can be used to determine the conformatlon of slmple alkylthioalkyl radical^.'^*^^ For R,MSCRzff (R" = H or CMe,) they decrease from a hlgh of ca. 2.005 when R,M lles in the C, 2p, nodal plane to a low of ca. 2.0027 when the R,M group is eclipsed by the C, 2p, orbital. (43) This may not be entirely true. However, we believe we can rule out the extreme possibility that 8 = 90' In the formate adducts because, in this confirmation, spin reaches M only by spin polarization from the adjacent sulfur or selenium. The Me3SnCHzradical has approximately uqjppln density on the atom adjacent to the tin and this radical has a = 137 G.44 An Me3SnSCR," radical having 8 = 90' would be unlikely to have more than 5-20% of,#tnspln on the sulfur, and so spin polarization would produce an a of only ca. 10-25 G, that is, very much less than the 109 G actually found for Me,SnSC(H)OCMe3. (44) A. Hudson and H. A. Hussain, J . Chem. SOC. B , 793 (1969).
