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33S and 17O NMR of compounds containing the SO2 moiety. The chlorine effect
70,204-2 12 ( 1986)
JOURNAL OF MAGNETIC RESONANCE
33Sand “0 NMR of Compounds Containing the SO2 Moiety. The Chlorine Effect G . BARBARELLA,*C. CHATGILIALOGLU,*S. ROSSINI,*AND V.TUGNOLI~ *Istituto dei Composti de1Carbonio Contenenti Eteroatomi e loro Applicazioni, Consiglio Nazionale delle Ricerche, Via Tolara di Sotto 89, 40064 Ozzano Emilia, Bologna, Italy, and tlstituto Chimico “G. Ciamician, ” Universitd, Via Selmi 2. Bologna, Italy Received November 15, 1985; revised June 23, 1986 The 33Sand “0 NMR spectra of substratesX-SO*-Y, where X and Y cover a large range of group electronegativities, have been obtained. Compounds containing a chlorine atom alpha to the SO, group show a large deshielding of terminal oxygens. This chlorine effect has been rationalized in terms of interactions between molecular orbitals. 0 1986 Academic Press,Inc.
INTRODUCTION
One way of investigating the electronic distribution and structure of organosulfur compounds containing S-O bonds is through the 33Sand 170 NMR spectra of such molecules (I). However, there are several experimental difficulties with 33Sand 170 NMR, due to the low magnetogyric ratio of both nuclei as well as the low natural abundance and the large quadrupole moment (2). Furthermore 33SNMR lines very often are so broad (Wi,z up to 5 kHz) that the variation of nuclear shieldings with the chemical structure are difficult to detect (3). However, the few 33SNMR studies that have appearedso far indicate that compounds containing the SOz moiety give rise to relatively sharp 33SNMR lines (IV,,2 up to 300 Hz) (3). Thus, contrary to sulfoxides (4), sulfones are suitable for 33SNMR studies. Here we report the 33Sand I70 chemical shifts of several open-chain sulfones, XS02-Y, where the functional group is bound to ligands covering a large range of group electronegativities($6). It is worth mentioning that the geometric and conformational properties of several sulfones are well known, particularly those of open-chain ones (7), that ligand electronegativities of X-SO*-Y only slightly affect the geometry of the SO2group, and that a good linear relationship is found between the S-O bond length and the sum of the electronegativities of ligands X and Y (6). RESULTS AND DISCUSSION
Table 1 shows the 33Sand 170 natural abundancechemical shifts of the compounds X-S02-Y. Although for most of the compounds I~(~~S) and S(170)decreaseas the electronegativity of both ligands increases,there is no linear relationship between the chemical shifts and (xx + xu). However, some regular trends may be brought to light when only one of the two ligands varies. Thus Figs. 1 and 2 give the plots of c?(‘~S) 0022-2364186 $3.00 Copyright 0 1986 by Academic Press,Inc. All rights of reproduction in any form reserved.
204
205
NMR OF SULFONES TABLE 1 “S and I70 Chemical Shifts” of Compounds X-SO>-Y Compound
CH3S0@CH3 CH3S02F CH3S02CI
CH30S0,F CH,OSO&I (CH3)rNS02CI CH,SOrN(CH,),
6(“S)
6(“0)
320 320’
164 164d
335 334 335 341’ 319 364f 311 311 321 324h
170 186 238 140 150s 148 219 208 156h
Compound ClSO*Cl
CF3S0@CH3 (CH3CH2)rNS020CH3 CH3CH2SO@CH3 FSOzF
s(33s)
6(“0)
287 286’ 281* 312 352 311
296 298’ 304’ 211 228 220
b
324 337 291(
147 143 159 148m
0 Chemical shifts are in ppm from external CSr for sulfur and from external Hz0 for oxygen. Positive values indicate low field resonanceswith respect to the reference. For all “0 spectra the linewidth is 100 to 300 Hz. The experimental error is + 1 ppm. For 33Sspectra the linewidth is about 300 Hz for all compounds but CH3CHtS0,0CH3 (1500 Hz) and CH3CH2S0,Cl (2000 Hz). b For this compound we were not able to detect a sulfur-33 NMR signal. c Refs. (22, 23, 24, 31). d Refs. (25) and (31). e Ref. (25). ‘Ref. (26). 8 Ref. (27). h Ref. (30). ’Ref. (3). ’Ref. (28). m Ref. (29).
and S(170)vs group electronegativities, XY. The xv values are those obtained from geometrical and vibrational data of sulfones X-SOz-Y (6) and only the series for which at least four points are available were taken into account. The trend to proportionality observed for Sc3S)and S(“0) vs XY is good, considering the large range of xy, the differences in conformational properties (7) and, above ah, the severe approximations implied in any definition of group electronegativity (5, 6). Moreover, by considering the s(33S)and S(170)variations in this way, the following points can be made: (i) for all groups of compounds taken into account, the trend of variation of 6 with x is the same for both sulfur and oxygen, although the sensitivity of S(“0) to structural changes is higher than that of s(33S),as expected for the presence of nonbonding electrons on the oxygen atom; (ii) whereas for the series CH$02Y any increase in electronegativity of Y is accompanied by a downfield NMR shift for I70 and 33Snuclei, the opposite trend is observed for the series CHsOSO2Y and ClSO2Y, i.e. S(33S)and S(“0) decreaseas xy increases;(iii) the values for the seriesClS02Y are
206
BARBARELLA
ET AL.
r
1
2.0
1
I
3.0
I
4.0
1
5.0 3
FIG. 1. 33Sand “0 chemical shifts for the series CH3SOzY versus groupelectronegativity. The arrows indicate the chemical shifts of compounds with Y = N(CH& and Y = OCH3 corrected for the y gauche effect of the methyl groups.
always several tens of ppm higher than those for the corresponding compounds of the series CH30S02Y, although the group electronegativities for Cl and CH30 are quite similar (5, 6).
207
NMR OF SULFONES
Cl-SOyY CH,O-SO,
33S $wn 1
(0,~) -Y(
0 - -)
340-
260
"0 6 (ppm
C”3
f CA 0
220
6”
N+32
CF3
0
6
/-
\ 180
0 -2.!3
-
--L
4 -\
NkH3CH&
q
i4a
L
2.0
CF3 & o-o2. -z cl3
,
3.0
L
-\
& --
I
I
4.0
I
5.0
xY RG. 2. “S and “0 chemical shifts for the series ClSOzY and CH30SOzY versus group electronegativity. The arrows indicate the chemical shifts of compounds with Y = N(CH&s and Y = OCHs corrected for the ‘y gauche effect of the methyl groups.
In all the plots reporting S(“0) vs xy the largest deviations from the regressionline are found for N(CH& and OCH3. The Newman’s projections around the S-N and S-0 bonds for the staggeredconformation of the corresponding compounds, are
208
BARBARELLA
la
lb
ET AL.
2a
2b
The preferred conformation is known for (CH&NSO&l and CH30S02C1 to be la and 2a, respectively. However, no matter which the preferred conformation is, there is always at least one methyl group which is y gauche to the oxygen atoms of SOS. It is known that for several classes of compounds y gauche methyl groups lead to a shielding of an oxygen atom of about 10 ppm per methyl (8). If y effects, which are likely to be steric in origin, are taken into account, the 6( “0) values reported in Figs. 1 and 2 should be corrected (increased) by 20 and 10 ppm for N(CH& and OCH3, respectively. As indicated in Figs. 1 and 2, this leads to a much better fitting to the regression line. Depending upon the electronegativity of the substituents different behaviors of the chemical shifts can be observed. Within the series of CH3S02Y such behavior is similar to that of CHsY (Y = CH3, NH*, OH, F), for which a theoretical study proved that a major contribution to the shielding was the change in the size of the 2p orbitals of the carbon atom caused by electron withdrawal by the substituent (9). However, for the series CH30S02Y and ClS02Y an increase in the electronegativity of Y leads to a decrease of the chemical shift. Thus for compounds X-S02-Y, a loss of electrons at sulfur due to the Y ligand may result either in a downfield or a upfield shift, depending on the nature of the second l&and, X. Recent studies by Fliszar et al. (10) on the “N and 13C chemical shifts of several classes of compounds show that any qualitative correlation between chemical shifts and atomic charges should be interpreted with caution, since both negative or positive slopes may be observed, depending on the balance between the changes in u and ?r electron densities on the atoms in question. In the present series X-SO*-Y the participation of sulfur 3d orbitals in the chemical bonds may increase the complexity of the relationship. Thus one possible interpretation of the S(33S)and S(“0) vs xy trends must await for a more detailed knowledge on the atomic charge for this class of compounds. Attention must now be paid to the most unexpected feature of Fig. 2. In fact the line 6( “0) vs xy for the series ClS02Y lies several tens of ppm above that for the series CH30S02Y, while the lines S(33S)vs xy for both series are located in the same region. Furthermore, this “chlorine effect” on the chemical shift of terminal oxygens is additive; thus S(“0) for CH3S02CH3, CH3S02C1 and ClSO&l are 164, 238 and 296 ppm, respectively.’ Such an effect cannot be due to a conformational factor since CH3SOzY (Y = CH3, F, Cl) are known to have the same preferred conformation, the barriers to free rotation around the S-C bond being 8- 10 kJ mol-’ (7). It is generally admitted that the shielding of second-row elements is dominated by ’It has been suggestedtentatively that oxygen chemical shifts of a sulfonyl group may be rationalized in terms of T bonding effects (32).
NMR OF SULFONES
209
the local paramagnetic term, which, for a nucleus A in the well known Pople M O thleory approximation, is given by (II, 12)
&w=-~~(~-~)LZQAB
B The three terms upon which 4 depends,i.e. AE, the averageexcitation energy, (Y~)*~, the averagevalue of r for a 2p orbital, and Ce QAB,the bond-order term, are interrelated, but often one of them is predominant and C$ can be related to chemically meaningful contributions. That is, significant correlations have been found between low-lying n + H* transitions and the chemical shifts of nitrogen and oxygen atoms in a variety of compounds (II, 13). The S(33S)and S(“0) reported here are likely to result from a balance of all factors; however, one factor may be predominant when one of the ligands is a Cl atom. Variations in AE or Ce QABterms should affect in a similar way the shieldings of both sulfur and oxygen. Thus the strong deshielding observed for terminal oxygens when Y = Cl probably arises from (rw3& We suggest our data be interpreted in terms of an interaction between the oxygen lone-pairs and the low-lying antibonding orbital a&. As an example, Fig. 3 shows the sequence of molecular orbitals for CHJSOZFand CH$O&Jl. In the low part of the diagram the highestoccupied molecular orbitals obtained by photoelectron spectroscopy are reported (14) (for simplicity only the four oxygen 2p orbitals have been reported) while in the high part of the diagram the S-F and S-Cl antibonding orbitals are reported in an arbitrary scale. To our knowledge there are no experimental data on the energy values of the S-F and S-Cl antibonding orbitals; however, spectroscopic data suggesta,& to be lower in energy tlhan a&r. That is, (i) electron capture by sulfonyl chlorides appears to be a relatively fiacileprocesswhich generatesthe [R2NSO$l] 7 radicals (15); EPR parameters of such intermediates imply the unpaired electron to be located on the a& orbital (15); (ii) pulse radiolysis data suggestradical anions [RSO&l]; to be discrete intermediates (16); and (iii) electron transmission spectroscopy (ETS) indicates the existence of anionic states for mono- and polychlorinated compounds but not for fluorinated ones (17), i.e. at.+ is likely to be higher in energy than u&i. Figure 3 shows that the two lowest occupied molecular orbitals containing oxygen lone-pairs possessthe proper symmetry to interact with the low-lying antibonding orbitals. However, this kind of interaction, which should lead to a decreaseof the electronic charge on oxygen, will be much stronger in CH3S02C1than in CH3SOzF, due to a smaller energeticgap. Consequently S(“0) for the former will be much higher than for the latter. On the other hand the same kind of interaction should lead to a relatively small increase of the electronic charge on sulfur since, as indicated in the figure, the electronic distribution of ucci is largely localized on the chlorine atom; thus the contribution to S(33S)should be small compared to the contribution to S(170). In fact, in the seriesCH30S0&H3, CH30S02C1and C1S02ClS(33S)are 3 19,3 11, and Z!87ppm, respectively. However, the small changesof S(33S)arising from the chlorine effect may be somehow compensated for by small changesin AE and CB QABterms, as is probably the case for the series CH3S02CH3, CH3S02C1and ClSO$l for which the chemical shifts are 320, 335, and 287, respectively.
210
BARBARELLA
ET AL.
CH,SO&I
CH,S02F
PI
l -
aS-CI
B
FIG. 3. Sequence of molecular orbitals in methane sulfonyl chloride and fluoride. In the low part of the diagram the four highest occupied n-type orbitals are reported. For simplicity only the contribution from the oxygen 2p orbitals is presented (for a detailed description see Ref. (14)). CONCLUSIONS
33Sand “0 NMR spectra of compounds containing the SO:! moiety have been reported. Initial results show that both chemical shifts are sensitive to group electronegativity, x, of the ligands. However, an increase of x of one of the ligands causes either an increase or a decrease of the chemical shift of both, oxygen and sulfur, depending on the nature of the second ligand. The terminal oxygens of compounds for which one of the ligands is a chlorine atom are deshielded by 50 to 70 ppm, whereas for the same compounds the variation of 13(~~!3) is relatively small. We attribute this “chlorine effect” on S(“0) to an interaction between oxygen lone-pairs and the antibonding orbital of the S-Cl bond. Such an interaction leads to an increase of the chemical shift by decreasing the electronic charge, essentially through the (~~3)2p~oxygen~ term. EXPERIMENTAL
METHODS
Methyl and ethyl chlorosulfates (18, 19), i’V,Aklimethylsulfamoy1 chloride (20) and methyl ester of NJVdiethylaminosulfonic acid (20) were prepared by published meth-
NMR
OF SULFONES
211
ods. Methyl sulfonate esters were readily obtained either by the reaction of methanol with the appropriate stionyl chloride in the presenceof triethylamine or by the reaction of sodium methoxide with the corresponding sulfonyl chlorides (21). All other compounds were obtained commercially and were carefully purified before use. Spectra were run with a Bruker CXP-300 spectrometer at 23.03 MHz for sulfur and 40.67 MHz for oxygen. No lock and no spinning were needed with the highpower probe employed (horizontal tubes). The solvent was CHC& and the sample concentrations were l-2 M. The probe temperature was approximately 30°C. Typical conditions were sweepwidth 20,000 Hz, pulse width 90”, data point SK and acquisition time 50 ms. For both nuclei a 60 I.LStime was allowed after the pulse before accumulation. ACKNOWLEDGMENTS We are grateful to Dr. M. Guerra and Dr. A. Modelli for helpful discussions and to Mr. G. Bragaglia for technical assistance. REFERENCES 1. (a) R. K. HARRIS AND B. E. MANN (Eds.), “NMR and The Periodic Table,” Chapter 12, Academic Press, London, 1978; (b) J. B. LAMBERT AND F. G. RIDDEL (Eds.), “The Multinuclear Approach to NMR Spectroscopy, Chapters 11 and 19, NATO Adv. Study Inst., Series C, 1983. l?. R. K. HARRIS AND B. E. MANN (Eds.), “NMR and The Periodic Table,” Chapter 1, Academic Press, London, 1978. .1. P. S. BELTON, I. J. Cox, R. K. HARRIS, J. Chem. Sot. Faraday Trans. 2, 81,63 (1985), and references therein. 41. G. BARBARELLA, A. BONGINI, C. CHATGILIALOGLLJ, S. ROSSINI, AND V. TUGNOLI, manuscript in preparation. :i. J. E. HUHEEY, J. Phys. Chem. 69,3284 (1965); J. HINZE, M. A. WHITEHEAD AND H. H. JAFFE, J. Am. Chem. Sot. 85, 148 (1963). (5. I. HARGI~AI, 2. Naturjbrsch A 34, 755 (1979); I. HARGITTAI, Z. Naturfirsch B 38, 1304 (1983). :?. (a) I. HARGI~AI, “Sulphone Molecular Structure, Lecture Notes in Chemistry, Vol. 6,” SpringerVerlag, Berlin, 1978; (b) I. HARGITTAI, “Organic Sulfur Chemistry” (F. Bemardi, I. G. Csizmadia, and A. Man&i, Eds.), p. 68, Elsevier, Amsterdam, 1985. <3. G. BARBARELLA, P. DEMBECH, AND V. TUGNOLI, Org. Magn. Reson. 22,402 (1984); G. BARBARELLA, A. BONGINI, S. ROSSINI, AND V. TUGNOLI, Tetrahedron 41,469 1 (1985), and references therein. !I. J. B. STOTHERS, “Carbon- 13 NMR Spectroscopy,” Chapter 5, Academic Press, London, 1972. 10. (a) M. T. BERALDIN, E. VAUTHIER, AND S. FLISZAR, Can. J. Chem. 60, 106 (1982); (b) M. COMEAU, M. T. BERALDIN, E. VALJTHIER, AND S. FJXZAR, Can. J. Chem. 63,3226 (1985); (c) S. FLISZAR, G. CARDINAL, AND M. T. BERALDIN, J. Am. Chem. S’oc. 104,5287 (1982). 11. (a) G. A. WEBB, R. K. HARRIS AND B. E. MANN @Is.), “NMR and The Periodic Table,” Chapter 3, Academic Press, London, 1978; (b) G. A. WEBB, J. B. LAMBERT AND F. G. RIDDEL &Is.), “The Multinuclear Approach to NMR Spectroscopy,” Chapter 2, NATO Adv. Study Inst., Series C, 1983. I.!. J. A. POPLE, J. Chem. Phys. 37, 53 (1962). 1.3. W. H. DE JEU, Mol. Phys. 18, 31 (1970). 1,4. B. SALOUKI, H. BOCK, AND R. APPEL, Angew. Chem. Int. Ed. Engl. 11,927 (1972). 1.5. C. CHATGILIALOGLU, B. C. GILBERT, R. 0. C. NORMAN, AND M. C. R. SYMONS, J. Chem. Rex, (M)2610, (S)185 (1980). 115. C. CHATGIL~ALOGLU, unpublished results. 17. P. D. BURROW, A. MODELLI, N. S. CHON, AND K. D. JORDAN, J. Chem. Phys. 77,2699 (1982); P. D. BURROW, A. MODELLI, N. S. CHIN, AND K. D. JORDAN, Chem. Phys. Lett. 82, 270 (1981); N. S. CHIU, P. D. BURROW, AND K. D. JORDAN, Chem. Phys. Let?. 68, 12 1 (1971).
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18. W. W. BINKLEY AND E. F. DEGERING, J. Am. Chem. Sot. 60,28 10 ( 1938). 19. E. BUNCEL, Chem. Rev. 70,323 (1970). 20. W. W. BINKLEY AND E. F. DEGERING, J. Am. Chem. Sot. 61,325 1 (1939). 21. R. K. CROSSLANDAND K. L. SEWIS,J. Org. Chem. 35,3195 (1970), and references therein. 22. D. L. HARRIS AND S. A. EVANS, JR., J. Org. Chem. 47, 3358 (1982). 23. R. FAURE, E. J. VINCENT, J. M. Rurz, AND L. LENA, Org. Magn. Reson. 15,401 (1981). 24. L. CASSIDEI,V. FIANDESE,G. MARCHESE,AND 0. S~IACOVELLI, Org. Magn. Reson. 22, 486 (1984). 25. J. C. DYER, D. L. HARRIS, AND S. A. EVANS, JR., J. Org. Chem. 47, 3660 (1982). 26. R. E. WASYLISHEN, C. CONNOR, AND J. 0. FRIEDRICH, Can. .I Chem. 62,981 (1984). 27. H. A. CHRIST, P. DIEHL, H. R. SCHNEIDER, AND H. DAQHN, Helv. Chim. Acfa 44, 865 (1961). 28. B. N. FIGGIS, R. G. KIDD, AND R. S. NYHOLM, Proc. R. Sot. London Ser. A 269,469 (1962). 29. I. G. SOLOMON, A. J. KALMAREK, AND J. RANEY, Znorg. Chem. 7, 1221 (1968). 30. A. M. HAKIUNEN AND P. RUOSTESUO,Magn. Reson. Chem. 23,424 (1985). 31. R. ANNTJNZIATA AND G. BARBARELLA, Org. Magn. Reson. 22,250 (1984). 32. W. G. KLEMPERER,Angew. Chem. Znt. Ed. Engl. 17,246 (1978).
JOURNAL OF MAGNETIC RESONANCE
33Sand “0 NMR of Compounds Containing the SO2 Moiety. The Chlorine Effect G . BARBARELLA,*C. CHATGILIALOGLU,*S. ROSSINI,*AND V.TUGNOLI~ *Istituto dei Composti de1Carbonio Contenenti Eteroatomi e loro Applicazioni, Consiglio Nazionale delle Ricerche, Via Tolara di Sotto 89, 40064 Ozzano Emilia, Bologna, Italy, and tlstituto Chimico “G. Ciamician, ” Universitd, Via Selmi 2. Bologna, Italy Received November 15, 1985; revised June 23, 1986 The 33Sand “0 NMR spectra of substratesX-SO*-Y, where X and Y cover a large range of group electronegativities, have been obtained. Compounds containing a chlorine atom alpha to the SO, group show a large deshielding of terminal oxygens. This chlorine effect has been rationalized in terms of interactions between molecular orbitals. 0 1986 Academic Press,Inc.
INTRODUCTION
One way of investigating the electronic distribution and structure of organosulfur compounds containing S-O bonds is through the 33Sand 170 NMR spectra of such molecules (I). However, there are several experimental difficulties with 33Sand 170 NMR, due to the low magnetogyric ratio of both nuclei as well as the low natural abundance and the large quadrupole moment (2). Furthermore 33SNMR lines very often are so broad (Wi,z up to 5 kHz) that the variation of nuclear shieldings with the chemical structure are difficult to detect (3). However, the few 33SNMR studies that have appearedso far indicate that compounds containing the SOz moiety give rise to relatively sharp 33SNMR lines (IV,,2 up to 300 Hz) (3). Thus, contrary to sulfoxides (4), sulfones are suitable for 33SNMR studies. Here we report the 33Sand I70 chemical shifts of several open-chain sulfones, XS02-Y, where the functional group is bound to ligands covering a large range of group electronegativities($6). It is worth mentioning that the geometric and conformational properties of several sulfones are well known, particularly those of open-chain ones (7), that ligand electronegativities of X-SO*-Y only slightly affect the geometry of the SO2group, and that a good linear relationship is found between the S-O bond length and the sum of the electronegativities of ligands X and Y (6). RESULTS AND DISCUSSION
Table 1 shows the 33Sand 170 natural abundancechemical shifts of the compounds X-S02-Y. Although for most of the compounds I~(~~S) and S(170)decreaseas the electronegativity of both ligands increases,there is no linear relationship between the chemical shifts and (xx + xu). However, some regular trends may be brought to light when only one of the two ligands varies. Thus Figs. 1 and 2 give the plots of c?(‘~S) 0022-2364186 $3.00 Copyright 0 1986 by Academic Press,Inc. All rights of reproduction in any form reserved.
204
205
NMR OF SULFONES TABLE 1 “S and I70 Chemical Shifts” of Compounds X-SO>-Y Compound
CH3S0@CH3 CH3S02F CH3S02CI
CH30S0,F CH,OSO&I (CH3)rNS02CI CH,SOrN(CH,),
6(“S)
6(“0)
320 320’
164 164d
335 334 335 341’ 319 364f 311 311 321 324h
170 186 238 140 150s 148 219 208 156h
Compound ClSO*Cl
CF3S0@CH3 (CH3CH2)rNS020CH3 CH3CH2SO@CH3 FSOzF
s(33s)
6(“0)
287 286’ 281* 312 352 311
296 298’ 304’ 211 228 220
b
324 337 291(
147 143 159 148m
0 Chemical shifts are in ppm from external CSr for sulfur and from external Hz0 for oxygen. Positive values indicate low field resonanceswith respect to the reference. For all “0 spectra the linewidth is 100 to 300 Hz. The experimental error is + 1 ppm. For 33Sspectra the linewidth is about 300 Hz for all compounds but CH3CHtS0,0CH3 (1500 Hz) and CH3CH2S0,Cl (2000 Hz). b For this compound we were not able to detect a sulfur-33 NMR signal. c Refs. (22, 23, 24, 31). d Refs. (25) and (31). e Ref. (25). ‘Ref. (26). 8 Ref. (27). h Ref. (30). ’Ref. (3). ’Ref. (28). m Ref. (29).
and S(170)vs group electronegativities, XY. The xv values are those obtained from geometrical and vibrational data of sulfones X-SOz-Y (6) and only the series for which at least four points are available were taken into account. The trend to proportionality observed for Sc3S)and S(“0) vs XY is good, considering the large range of xy, the differences in conformational properties (7) and, above ah, the severe approximations implied in any definition of group electronegativity (5, 6). Moreover, by considering the s(33S)and S(170)variations in this way, the following points can be made: (i) for all groups of compounds taken into account, the trend of variation of 6 with x is the same for both sulfur and oxygen, although the sensitivity of S(“0) to structural changes is higher than that of s(33S),as expected for the presence of nonbonding electrons on the oxygen atom; (ii) whereas for the series CH$02Y any increase in electronegativity of Y is accompanied by a downfield NMR shift for I70 and 33Snuclei, the opposite trend is observed for the series CHsOSO2Y and ClSO2Y, i.e. S(33S)and S(“0) decreaseas xy increases;(iii) the values for the seriesClS02Y are
206
BARBARELLA
ET AL.
r
1
2.0
1
I
3.0
I
4.0
1
5.0 3
FIG. 1. 33Sand “0 chemical shifts for the series CH3SOzY versus groupelectronegativity. The arrows indicate the chemical shifts of compounds with Y = N(CH& and Y = OCH3 corrected for the y gauche effect of the methyl groups.
always several tens of ppm higher than those for the corresponding compounds of the series CH30S02Y, although the group electronegativities for Cl and CH30 are quite similar (5, 6).
207
NMR OF SULFONES
Cl-SOyY CH,O-SO,
33S $wn 1
(0,~) -Y(
0 - -)
340-
260
"0 6 (ppm
C”3
f CA 0
220
6”
N+32
CF3
0
6
/-
\ 180
0 -2.!3
-
--L
4 -\
NkH3CH&
q
i4a
L
2.0
CF3 & o-o2. -z cl3
,
3.0
L
-\
& --
I
I
4.0
I
5.0
xY RG. 2. “S and “0 chemical shifts for the series ClSOzY and CH30SOzY versus group electronegativity. The arrows indicate the chemical shifts of compounds with Y = N(CH&s and Y = OCHs corrected for the ‘y gauche effect of the methyl groups.
In all the plots reporting S(“0) vs xy the largest deviations from the regressionline are found for N(CH& and OCH3. The Newman’s projections around the S-N and S-0 bonds for the staggeredconformation of the corresponding compounds, are
208
BARBARELLA
la
lb
ET AL.
2a
2b
The preferred conformation is known for (CH&NSO&l and CH30S02C1 to be la and 2a, respectively. However, no matter which the preferred conformation is, there is always at least one methyl group which is y gauche to the oxygen atoms of SOS. It is known that for several classes of compounds y gauche methyl groups lead to a shielding of an oxygen atom of about 10 ppm per methyl (8). If y effects, which are likely to be steric in origin, are taken into account, the 6( “0) values reported in Figs. 1 and 2 should be corrected (increased) by 20 and 10 ppm for N(CH& and OCH3, respectively. As indicated in Figs. 1 and 2, this leads to a much better fitting to the regression line. Depending upon the electronegativity of the substituents different behaviors of the chemical shifts can be observed. Within the series of CH3S02Y such behavior is similar to that of CHsY (Y = CH3, NH*, OH, F), for which a theoretical study proved that a major contribution to the shielding was the change in the size of the 2p orbitals of the carbon atom caused by electron withdrawal by the substituent (9). However, for the series CH30S02Y and ClS02Y an increase in the electronegativity of Y leads to a decrease of the chemical shift. Thus for compounds X-S02-Y, a loss of electrons at sulfur due to the Y ligand may result either in a downfield or a upfield shift, depending on the nature of the second l&and, X. Recent studies by Fliszar et al. (10) on the “N and 13C chemical shifts of several classes of compounds show that any qualitative correlation between chemical shifts and atomic charges should be interpreted with caution, since both negative or positive slopes may be observed, depending on the balance between the changes in u and ?r electron densities on the atoms in question. In the present series X-SO*-Y the participation of sulfur 3d orbitals in the chemical bonds may increase the complexity of the relationship. Thus one possible interpretation of the S(33S)and S(“0) vs xy trends must await for a more detailed knowledge on the atomic charge for this class of compounds. Attention must now be paid to the most unexpected feature of Fig. 2. In fact the line 6( “0) vs xy for the series ClS02Y lies several tens of ppm above that for the series CH30S02Y, while the lines S(33S)vs xy for both series are located in the same region. Furthermore, this “chlorine effect” on the chemical shift of terminal oxygens is additive; thus S(“0) for CH3S02CH3, CH3S02C1 and ClSO&l are 164, 238 and 296 ppm, respectively.’ Such an effect cannot be due to a conformational factor since CH3SOzY (Y = CH3, F, Cl) are known to have the same preferred conformation, the barriers to free rotation around the S-C bond being 8- 10 kJ mol-’ (7). It is generally admitted that the shielding of second-row elements is dominated by ’It has been suggestedtentatively that oxygen chemical shifts of a sulfonyl group may be rationalized in terms of T bonding effects (32).
NMR OF SULFONES
209
the local paramagnetic term, which, for a nucleus A in the well known Pople M O thleory approximation, is given by (II, 12)
&w=-~~(~-~)LZQAB
B The three terms upon which 4 depends,i.e. AE, the averageexcitation energy, (Y~)*~, the averagevalue of r for a 2p orbital, and Ce QAB,the bond-order term, are interrelated, but often one of them is predominant and C$ can be related to chemically meaningful contributions. That is, significant correlations have been found between low-lying n + H* transitions and the chemical shifts of nitrogen and oxygen atoms in a variety of compounds (II, 13). The S(33S)and S(“0) reported here are likely to result from a balance of all factors; however, one factor may be predominant when one of the ligands is a Cl atom. Variations in AE or Ce QABterms should affect in a similar way the shieldings of both sulfur and oxygen. Thus the strong deshielding observed for terminal oxygens when Y = Cl probably arises from (rw3& We suggest our data be interpreted in terms of an interaction between the oxygen lone-pairs and the low-lying antibonding orbital a&. As an example, Fig. 3 shows the sequence of molecular orbitals for CHJSOZFand CH$O&Jl. In the low part of the diagram the highestoccupied molecular orbitals obtained by photoelectron spectroscopy are reported (14) (for simplicity only the four oxygen 2p orbitals have been reported) while in the high part of the diagram the S-F and S-Cl antibonding orbitals are reported in an arbitrary scale. To our knowledge there are no experimental data on the energy values of the S-F and S-Cl antibonding orbitals; however, spectroscopic data suggesta,& to be lower in energy tlhan a&r. That is, (i) electron capture by sulfonyl chlorides appears to be a relatively fiacileprocesswhich generatesthe [R2NSO$l] 7 radicals (15); EPR parameters of such intermediates imply the unpaired electron to be located on the a& orbital (15); (ii) pulse radiolysis data suggestradical anions [RSO&l]; to be discrete intermediates (16); and (iii) electron transmission spectroscopy (ETS) indicates the existence of anionic states for mono- and polychlorinated compounds but not for fluorinated ones (17), i.e. at.+ is likely to be higher in energy than u&i. Figure 3 shows that the two lowest occupied molecular orbitals containing oxygen lone-pairs possessthe proper symmetry to interact with the low-lying antibonding orbitals. However, this kind of interaction, which should lead to a decreaseof the electronic charge on oxygen, will be much stronger in CH3S02C1than in CH3SOzF, due to a smaller energeticgap. Consequently S(“0) for the former will be much higher than for the latter. On the other hand the same kind of interaction should lead to a relatively small increase of the electronic charge on sulfur since, as indicated in the figure, the electronic distribution of ucci is largely localized on the chlorine atom; thus the contribution to S(33S)should be small compared to the contribution to S(170). In fact, in the seriesCH30S0&H3, CH30S02C1and C1S02ClS(33S)are 3 19,3 11, and Z!87ppm, respectively. However, the small changesof S(33S)arising from the chlorine effect may be somehow compensated for by small changesin AE and CB QABterms, as is probably the case for the series CH3S02CH3, CH3S02C1and ClSO$l for which the chemical shifts are 320, 335, and 287, respectively.
210
BARBARELLA
ET AL.
CH,SO&I
CH,S02F
PI
l -
aS-CI
B
FIG. 3. Sequence of molecular orbitals in methane sulfonyl chloride and fluoride. In the low part of the diagram the four highest occupied n-type orbitals are reported. For simplicity only the contribution from the oxygen 2p orbitals is presented (for a detailed description see Ref. (14)). CONCLUSIONS
33Sand “0 NMR spectra of compounds containing the SO:! moiety have been reported. Initial results show that both chemical shifts are sensitive to group electronegativity, x, of the ligands. However, an increase of x of one of the ligands causes either an increase or a decrease of the chemical shift of both, oxygen and sulfur, depending on the nature of the second ligand. The terminal oxygens of compounds for which one of the ligands is a chlorine atom are deshielded by 50 to 70 ppm, whereas for the same compounds the variation of 13(~~!3) is relatively small. We attribute this “chlorine effect” on S(“0) to an interaction between oxygen lone-pairs and the antibonding orbital of the S-Cl bond. Such an interaction leads to an increase of the chemical shift by decreasing the electronic charge, essentially through the (~~3)2p~oxygen~ term. EXPERIMENTAL
METHODS
Methyl and ethyl chlorosulfates (18, 19), i’V,Aklimethylsulfamoy1 chloride (20) and methyl ester of NJVdiethylaminosulfonic acid (20) were prepared by published meth-
NMR
OF SULFONES
211
ods. Methyl sulfonate esters were readily obtained either by the reaction of methanol with the appropriate stionyl chloride in the presenceof triethylamine or by the reaction of sodium methoxide with the corresponding sulfonyl chlorides (21). All other compounds were obtained commercially and were carefully purified before use. Spectra were run with a Bruker CXP-300 spectrometer at 23.03 MHz for sulfur and 40.67 MHz for oxygen. No lock and no spinning were needed with the highpower probe employed (horizontal tubes). The solvent was CHC& and the sample concentrations were l-2 M. The probe temperature was approximately 30°C. Typical conditions were sweepwidth 20,000 Hz, pulse width 90”, data point SK and acquisition time 50 ms. For both nuclei a 60 I.LStime was allowed after the pulse before accumulation. ACKNOWLEDGMENTS We are grateful to Dr. M. Guerra and Dr. A. Modelli for helpful discussions and to Mr. G. Bragaglia for technical assistance. REFERENCES 1. (a) R. K. HARRIS AND B. E. MANN (Eds.), “NMR and The Periodic Table,” Chapter 12, Academic Press, London, 1978; (b) J. B. LAMBERT AND F. G. RIDDEL (Eds.), “The Multinuclear Approach to NMR Spectroscopy, Chapters 11 and 19, NATO Adv. Study Inst., Series C, 1983. l?. R. K. HARRIS AND B. E. MANN (Eds.), “NMR and The Periodic Table,” Chapter 1, Academic Press, London, 1978. .1. P. S. BELTON, I. J. Cox, R. K. HARRIS, J. Chem. Sot. Faraday Trans. 2, 81,63 (1985), and references therein. 41. G. BARBARELLA, A. BONGINI, C. CHATGILIALOGLLJ, S. ROSSINI, AND V. TUGNOLI, manuscript in preparation. :i. J. E. HUHEEY, J. Phys. Chem. 69,3284 (1965); J. HINZE, M. A. WHITEHEAD AND H. H. JAFFE, J. Am. Chem. Sot. 85, 148 (1963). (5. I. HARGI~AI, 2. Naturjbrsch A 34, 755 (1979); I. HARGITTAI, Z. Naturfirsch B 38, 1304 (1983). :?. (a) I. HARGI~AI, “Sulphone Molecular Structure, Lecture Notes in Chemistry, Vol. 6,” SpringerVerlag, Berlin, 1978; (b) I. HARGITTAI, “Organic Sulfur Chemistry” (F. Bemardi, I. G. Csizmadia, and A. Man&i, Eds.), p. 68, Elsevier, Amsterdam, 1985. <3. G. BARBARELLA, P. DEMBECH, AND V. TUGNOLI, Org. Magn. Reson. 22,402 (1984); G. BARBARELLA, A. BONGINI, S. ROSSINI, AND V. TUGNOLI, Tetrahedron 41,469 1 (1985), and references therein. !I. J. B. STOTHERS, “Carbon- 13 NMR Spectroscopy,” Chapter 5, Academic Press, London, 1972. 10. (a) M. T. BERALDIN, E. VAUTHIER, AND S. FLISZAR, Can. J. Chem. 60, 106 (1982); (b) M. COMEAU, M. T. BERALDIN, E. VALJTHIER, AND S. FJXZAR, Can. J. Chem. 63,3226 (1985); (c) S. FLISZAR, G. CARDINAL, AND M. T. BERALDIN, J. Am. Chem. S’oc. 104,5287 (1982). 11. (a) G. A. WEBB, R. K. HARRIS AND B. E. MANN @Is.), “NMR and The Periodic Table,” Chapter 3, Academic Press, London, 1978; (b) G. A. WEBB, J. B. LAMBERT AND F. G. RIDDEL &Is.), “The Multinuclear Approach to NMR Spectroscopy,” Chapter 2, NATO Adv. Study Inst., Series C, 1983. I.!. J. A. POPLE, J. Chem. Phys. 37, 53 (1962). 1.3. W. H. DE JEU, Mol. Phys. 18, 31 (1970). 1,4. B. SALOUKI, H. BOCK, AND R. APPEL, Angew. Chem. Int. Ed. Engl. 11,927 (1972). 1.5. C. CHATGILIALOGLU, B. C. GILBERT, R. 0. C. NORMAN, AND M. C. R. SYMONS, J. Chem. Rex, (M)2610, (S)185 (1980). 115. C. CHATGIL~ALOGLU, unpublished results. 17. P. D. BURROW, A. MODELLI, N. S. CHON, AND K. D. JORDAN, J. Chem. Phys. 77,2699 (1982); P. D. BURROW, A. MODELLI, N. S. CHIN, AND K. D. JORDAN, Chem. Phys. Lett. 82, 270 (1981); N. S. CHIU, P. D. BURROW, AND K. D. JORDAN, Chem. Phys. Let?. 68, 12 1 (1971).
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18. W. W. BINKLEY AND E. F. DEGERING, J. Am. Chem. Sot. 60,28 10 ( 1938). 19. E. BUNCEL, Chem. Rev. 70,323 (1970). 20. W. W. BINKLEY AND E. F. DEGERING, J. Am. Chem. Sot. 61,325 1 (1939). 21. R. K. CROSSLANDAND K. L. SEWIS,J. Org. Chem. 35,3195 (1970), and references therein. 22. D. L. HARRIS AND S. A. EVANS, JR., J. Org. Chem. 47, 3358 (1982). 23. R. FAURE, E. J. VINCENT, J. M. Rurz, AND L. LENA, Org. Magn. Reson. 15,401 (1981). 24. L. CASSIDEI,V. FIANDESE,G. MARCHESE,AND 0. S~IACOVELLI, Org. Magn. Reson. 22, 486 (1984). 25. J. C. DYER, D. L. HARRIS, AND S. A. EVANS, JR., J. Org. Chem. 47, 3660 (1982). 26. R. E. WASYLISHEN, C. CONNOR, AND J. 0. FRIEDRICH, Can. .I Chem. 62,981 (1984). 27. H. A. CHRIST, P. DIEHL, H. R. SCHNEIDER, AND H. DAQHN, Helv. Chim. Acfa 44, 865 (1961). 28. B. N. FIGGIS, R. G. KIDD, AND R. S. NYHOLM, Proc. R. Sot. London Ser. A 269,469 (1962). 29. I. G. SOLOMON, A. J. KALMAREK, AND J. RANEY, Znorg. Chem. 7, 1221 (1968). 30. A. M. HAKIUNEN AND P. RUOSTESUO,Magn. Reson. Chem. 23,424 (1985). 31. R. ANNTJNZIATA AND G. BARBARELLA, Org. Magn. Reson. 22,250 (1984). 32. W. G. KLEMPERER,Angew. Chem. Znt. Ed. Engl. 17,246 (1978).