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Structure-stability relationships in unsaturated sulfur compounds
Journal of Molecular Structure (Theochem), 285 (1993) 71-75 0166-1280/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved
71
Structure-stability relationships in unsaturated sulfur compounds. Part 2. An ab initio study of the stable conformations of divinyl sulfide, sulfoxide and sulfone Reijo Kimmelma *pa,Matti Hotokkab ‘Department of Chemistry, University*of Turku, SF-20500 Turku, Finland bDepartment of Physical Chemistry, Abo Akademi, SF-20500 Turku, Finland (Received 26 January 1993) Abstract The potential energy maps for both C-S torsions of divinyl sulfide, sulfoxide and sulfone were calculated by using the GAMES ab initio program and the 3-21G* basis set. In each case two minima were found. In the stable conformations of divinyl sulfide the vinyl groups have turned about 45” to the opposite directions or about 40” to the same direction from the s-trans,s-trans conformation. In the most stable rotamer of divinyl sulfoxide both of the vinyl groups eclipse the S=O group. In the second stable rotamer one of the vinyl groups eclipses the S=O group and the other one the lone pair electrons of the sulfur atom. In the stable conformations of divinyl sulfone the vinyl groups eclipse either different S=O groups or the same S=O group.
Introduction The structural properties of methyl vinyl sulfides, sulfoxides and sulfones have been widely studied both experimentally [l-7] and theoretically [8,9]. The properties of divinyl sulfides, sulfoxides and sulfones have also been the subject of several experimental [ 1,l O-231 and theoretical [2325] studies. According to some IR studies, divinyl sulfide and sulfoxide have two stable rotamers of which one has the planar or nearly planar s-trans,s-trans conformation (Fig. 1) [19-211. One study suggests this conformation to be also the only stable one in divinyl sulfone [20]. According to CND0/2 calculations divinyl sulfone has three stable conformations. The investigators state that perhaps it is reasonable to say that there is a tendency for the C=C bond to approach a position in which it nearly eclipses another bond * Corresponding
author.
[23]. According to ab initio calculations divinyl sulfide adopts a conformation close to s-cis,strans [24] (Fig. l), whereas results from molecular mechanics point to the s-trans,s-trans conformation [24]. Another theoretical study suggests the presence of three stable rotamers in divinyl sulfide, two of which have nearly the s-trans, s-trans and the third nearly the s-cis,s-trans conformation [25]. Thus the conformations of the stable rotamers of divinyl sulfide and especially divinyl sulfoxide and sulfone are not very well known and the results of the theoretical studies are contradictory. Therefore, it seemed interesting to study this problem more closely, especially the effect of the oxygen atoms on the conformations of the stable rotamers. In this study the structural parameters and the potential energy map for both C-S torsions of divinyl sulfide, sulfoxide and sulfone (Fig. 2) were calculated by using the GAMES ab initio program [26] and the 3-21G* basis set [27-291.
72
R. Kimmelma and M. HotokkalJ. Mol. Struct. (Theochem) 285 (1993) 71-75
7C=C /”
‘”
/
‘\
,H
F C H’ ‘H s-&s,
s-tic
s-cis, s-tratls
s-from,s-trans
180
91=92=0
Hr
F
c-sH’
Fig. 1. Some conformations
/” ii+ i.C H H
Fig, 4. The potential energy surface of &vinyl s&oxide.
of divinyl sulfide.
Results and discussion “,
c-c
H’
oivilyl slllfo~
Diviylsuyotlc
Fig. 2. Molecular structure of divinyl sulfoxide and sulfone.
0
90
180
270
Fig. 3. The potential energy surface of divinyl sulfide.
360
The potential energy surfaces of divinyl sulfide, sulfoxide and sulfone were calculated by rotating both of the vinyl groups around S-C bonds. The CSCC torsion angles were varied systematically while optimizing all the other geometrical parameters at each point. The torsion angles are
0
90
180
2io
Fig. 5. The potential energy surface of divinyl sulfone.
360
R. Kimmelrna and M. HotokkajJ. Mol. Struct. (Theochem)
285 (1993)
13
71-75
Table 1. The total energies of the minima as calculated by using the 6-31G* basis set; the conformation is specitied by giving the two CSCC torsion angles Compound
Torsion angle (deg)
Torsion angle (deg)
Total energy (a.u.)
Divinyl sul6de
180 134 -141 -120 120 110 115
180 134 141 120 120 110 -115
-552.416640 -552.419057 -552.418078 -627.219343 -627.218453 -702.070506 -702.069510
Divinyl sulfoxide Divinyl sulfone
defined in such a way that the two double bonds point in opposite directions when both angles are positive (and below 180”). The torsion angles are 0” in the planar s-cis,s-cis conformation (Fig. 1). The resulting potential energy surfaces are given in Figs. 3-5. The mapping of the potential energy surfaces is crude, but it shows clearly where the local minima are to be found. Full optimization of the geometry was performed at these points using the GAMESS program and the 6-31G* basis set [30-321. The resulting total energies are given in Table 1 and the details of the structure at the global minimum for each molecule in Table 2. The atomic and group charges are given in Tables 3 and 4, respectively. In the optimal geometry of divinyl sulfide the vinyl groups have turned about 45” to the opposite directions from the s-trans,s-trans conformation, The planar s-trans,s-trans conformation would be Table 2. The optimal structure of the molecules as obtained from full optimization by using the 6-31G* basis set Parameter
&S (pm) ?$:; so =CSC
(deg)
QCS
(deg)
QOSO (W
91 (deg) A (deg)
Divinyl sultide 177 132 _ 101 123
Divinyl sulfoxide 178 131 148 98 121
_
134 134
-120 120
Divinyl sulfone 176 132 144 103 121 120 112 112
favored by the conjugation of the lone pair electrons of the sulfur atom with the 7r orbitals of the double bonds, but probably the steric strain caused by the cx hydrogen atoms forces the molecule out of the planar conformation. Maybe that is why the SCC bond angle is 5” smaller in divinyl sulfide than in methyl vinyl sulfide [33] (the reduction of this angle takes the a hydrogen atoms further apart). Divinyl sulfide also has another stable rotamer. In this rotamer the vinyl groups have turned about 40” to the same direction from the s-trans,s-trans conformation. This result is in agreement with the theoretical study of Keiko et al. [25]. These investigators have, however, also found a third minimum for divinyl sulfide (nearly s-cis, s-trans). The experimental studies suggest that one of the stable rotamers has the s-trans, s-trans conformation [19-211, which is in reasonably good agreement with the results obtained in the present study. Table 3. The net atomic charges as obtained for the optimal structures by using the 6-31G* basis set Atom
Divinyl sulfide
Divinyl sulfoxide
Divinyl sulfone
S Cl C, C,
0.2 -0.3 -0.3 -0.4 -0.4 -
1.0 -0.4 -0.4 -0.3 -0.3 -0.8 -
1.6 -0.6 -0.6 -0.4 -0.4 -0.6 -0.6
3 0
R. Kinmwlma and M. Hotokka/J. Mol. Struct. (Theochem)
14
Table 4. The net charges of the groups Croup
Divinyl sulfide
Divinyl sulfoxide
Divinyl sulfone
-CH=CHr
-0.1 0.0 _
-0.2 +0.1 +0.2
-0.3 +0.1 +0.4
SD (SW
In the most stable rotamer of divinyl sulfoxide both of the vinyl groups eclipse the S=O group, which is reasonable because in this conformation the S=O double bond can conjugate with both of the double bonds, which stabilizes the molecule. In the second stable rotamer one of the double bonds eclipses the S=O bond and the other one the lone pair electrons of the sulfur atom. Divinyl sulfoxide is the least studied of these divinyl compounds. Two IR studies have been published, both of which state that two conformations are present [20,21], one of which is probably the s-trans,s-trans conformation [20]. In the most stable rotamer of divinyl sulfone each one of the double bonds eclipses one of the S=O bonds, which is again reasonable because of the conjugation in this conformation. The oxygen lone pair orbitals mix with the C=C 7rbonds with a weight of about 10% in divinyl sulfone. In the second stable rotamer both of the double bonds eclipse the same S=O bond, which shows that the S=O bond can conjugate with both of the double bonds at the same time. This result is in agreement with the CND0/2 study made by Hargittai et al. [23]. The second minimum energy structure with the double bonds flipped is energetically almost equally favorable in each case. The barriers are low (below 10 kJmol_‘) in divinyl sulfide and about 20 kJ mol-’ in divinyl sulfoxide and sulfone. The planar structure corresponds to a saddle point on each potential energy surface, indicating that no delocalization of the double bonds is possible. The geometrical parameters are summarized in Table 2. It can be seen that the CSC bond angle in divinyl sulfoxide is strikingly small (only 98”). In methyl vinyl sulfoxide this angle is also small (only
285 (1993) 71-75
95”; 6-31G*) [9]. This may result from different hybridization of the sulfur atom or the steric requirements of the lone pair electrons, but because there is no experimental data to verify this result, it is possible that the small value is caused by the calculation method used. Other bond angles and lengths are similar to each other in divinyl sulfide, sulfoxide and sulfone except the S=O bond length, which is 4 pm shorter in divinyl sulfone than in sulfoxide (the same tendency can be seen in methyl vinyl sulfoxide and sulfone [9]). The Mulliken population analysis shows that the sulfur atom easily donates its electrons, in particular to the strongly electronegative oxygen atoms. The net atomic charge of sulfur in divinyl sulfoxide is 1.0 and in sulfone 1.6, most of the charge being transferred to the oxygen atom(s). However, the vinyl groups also gain electron density on going from divinyl sulfide to sulfoxide and sulfone. This is probably an inductive effect. The atomic charges are added up for selected groups in Table 4. It is clearly seen that the (Y carbon atoms are indeed slightly more negatively charged in divinyl sulfoxide and sulfone than in divinyl sulfide. The net gain of 0.6 electrons observed at the cxcarbon atom in divinyl sulfide reduces to 0.3 electrons for the whole -CH= group. The p carbon atom gains 0.4 electrons but the whole =CHz group has lost 0.1 electrons, indicating that most of the electrons originate from the hydrogen atom. This observation supports the inferred inductive effect.
References R. Virtanen, Acta Chem. Stand., Ser. B, 40 (1986) 313. A.R. Katritzky, R.F. Pinzelli and R.D. Topsom, Tetrahedron, 28 (1972) 3441. S. Samdal, H.M. Seip and T. Torgrimsen, J. Mol. struct., 57 (1979) 105. V. Svdta, M. Prochazka and V. Bakos, Collect. Czech. Chem. Commun., 43 (1978) 2619. R. Kimmehna, Acta Chem. Stand., in press. E. Vajda, D. Hnyk, B. Rozsondai, J. Podlaha, J. Podlahova and J. Hasek, J. Mol. Struct., 239 (1990) 265. V.A. Naumov, R.N. Ziatdinova and E.A. Berdnikov, J. Struct. Chem., (1981) 382.
R. Kimmelma and M. Hotokka/J. Mol. Struct. (Theo&m)
285 (1993) 71-75
8 J. Kao, J. Am. Chem. Sot., 100 (1978) 4685. 9 M. Hotokka and R. Kimmelma, J. Mol. Struct. (Theothem), 276 (1992) 167. 10 G.A. Kalabin, V.M. Bzhezovskii, D.F. Kushnarev and A.G. Proidakov, J. Org. Chem. USSR, (1981) 1009. 11 R. Faure, E.J. Vincent, J.M. Ruiz and L. tina, Org. Magn. Reson., 15 (1981) 401. 12 J.C. Dyer, D.L. Harris and S.A. Evans, Jr., J. Org. Chem., 47 (1982) 3660. 13 L. Cassidei, V. Fiandanese, G. Marchese and 0. Sciacovelli, Org. Magn. Reson., 22 (1984) 486. 14 V.M. Bzhezovskii, D.F. Kushnarev, B.A. Trofimov, G.A. Kalabin, N.K. Gusarova and G.G. Efremova, Bull. Acad. Sci. USSR, Div. Chem. Sci., ll(l981) 2073. 15 G.A. Kalabin, B.A. Trofimov, V.M. Bzhezovskii, D.F. Kushnarev, S.V. Amosova, N.K. Gusarova and M.L. Al’pert, Bull. Acad. Sci. USSR, Div. Chem. Sci., 3 (1975) 501. 16 0. Kajimoto, M. Kobayashi and T. Fueno, Bull. Chem. Sot. Jpn., 46 (1973) 2316. 17 C. Miiller and A. Schweig, Tetrahedron, 29 (1973) 3973. 18 B. Fortunato and M.G. Giorgini, Gazz. Chim. Ital., 106 (1976) 1005. 19 B.A. Trofimov, Yu.L. Frolov, L.M. Sinegovskaya, V.B. Modonov, E.I. Kositsyns, S.V. Amosova, N.K. Gusarova and G.G. Efremova, Bull. Acad. Sci. USSR, Div. Chem. Sci., 2 (1977) 302. 20 A.B. Remizov, T.G. Mannafov and F.R. Tantasheva, J. Gen. Chem. USSR, (1975) 1378.
15
21 A.B. Remizov, J. Appl. Spectrosc., (1976) 1333. 22 B. Nagel’ and A.B. Remizov, J. Gen. Chem. USSR, (1978) 1089. 23 I. Hargittai, B. Rozsondai, B. Nagel, P. Bulcke, G. Robinet and J.-F. Labarre, J. Chem. Sot., Dalton Trans., (1978) 861. 24 J. Kao, C. Eyermann, E. Southwick and D. Leister, J. Am. Chem. Sot., 107 (1985) 5323. 25 V.V. Keiko, L.M. Sinegovskaya and B.A. Trofimov, Izv. Akad. Nauk SSSR, Ser. Khim., 7 (1991) 1531. 26 M.W. Schmidt, K.K. Baldridge, J.A. Boatz, J.H. Jensen, S. Koseki, M.S. Gordon, K.A. Nguyen, T.L. Windus and S.T. Elbert, Quantum Chem. Program Exch. Bull., 10 (1990) 52. 27 J.S. Binkley, J.A. Pople and W.J. Hehre, J. Am. Chem. Sot., 102 (1980) 939. 28 M.S. Gordon, J.S. Binkley, J.A. Pople, W.J. Pietro and W.J. Hehre, J. Am. Chem. Sot., 104 (1982) 2797. 29 W.J. Pietro, M.M. Francl, W.J. Hehre, D.J. DeFrees, J.A. Pople and J. S. Binkley, J. Am. Chem. Sot., 104 (1982) 5039. 30 W.J. Hehre, R. Ditchfield and J.A. Pople, J. Chem. Phys., 56 (1972) 2257. 31 M.M. Francl, W.J. Pietro, W.J. Hehre, J.S. Binkley, M.S. Gordon, D.J. DeFrees and J.A. Pople, J. Chem. Phys., 77 (1982) 3654. 32 P.C. Hariharan and J.A. Pople, Theor. Chim. Acta, 28 (1973) 213. 33 J. Kao, J. Am. Chem. Sot., 100 (1978) 4685.
71
Structure-stability relationships in unsaturated sulfur compounds. Part 2. An ab initio study of the stable conformations of divinyl sulfide, sulfoxide and sulfone Reijo Kimmelma *pa,Matti Hotokkab ‘Department of Chemistry, University*of Turku, SF-20500 Turku, Finland bDepartment of Physical Chemistry, Abo Akademi, SF-20500 Turku, Finland (Received 26 January 1993) Abstract The potential energy maps for both C-S torsions of divinyl sulfide, sulfoxide and sulfone were calculated by using the GAMES ab initio program and the 3-21G* basis set. In each case two minima were found. In the stable conformations of divinyl sulfide the vinyl groups have turned about 45” to the opposite directions or about 40” to the same direction from the s-trans,s-trans conformation. In the most stable rotamer of divinyl sulfoxide both of the vinyl groups eclipse the S=O group. In the second stable rotamer one of the vinyl groups eclipses the S=O group and the other one the lone pair electrons of the sulfur atom. In the stable conformations of divinyl sulfone the vinyl groups eclipse either different S=O groups or the same S=O group.
Introduction The structural properties of methyl vinyl sulfides, sulfoxides and sulfones have been widely studied both experimentally [l-7] and theoretically [8,9]. The properties of divinyl sulfides, sulfoxides and sulfones have also been the subject of several experimental [ 1,l O-231 and theoretical [2325] studies. According to some IR studies, divinyl sulfide and sulfoxide have two stable rotamers of which one has the planar or nearly planar s-trans,s-trans conformation (Fig. 1) [19-211. One study suggests this conformation to be also the only stable one in divinyl sulfone [20]. According to CND0/2 calculations divinyl sulfone has three stable conformations. The investigators state that perhaps it is reasonable to say that there is a tendency for the C=C bond to approach a position in which it nearly eclipses another bond * Corresponding
author.
[23]. According to ab initio calculations divinyl sulfide adopts a conformation close to s-cis,strans [24] (Fig. l), whereas results from molecular mechanics point to the s-trans,s-trans conformation [24]. Another theoretical study suggests the presence of three stable rotamers in divinyl sulfide, two of which have nearly the s-trans, s-trans and the third nearly the s-cis,s-trans conformation [25]. Thus the conformations of the stable rotamers of divinyl sulfide and especially divinyl sulfoxide and sulfone are not very well known and the results of the theoretical studies are contradictory. Therefore, it seemed interesting to study this problem more closely, especially the effect of the oxygen atoms on the conformations of the stable rotamers. In this study the structural parameters and the potential energy map for both C-S torsions of divinyl sulfide, sulfoxide and sulfone (Fig. 2) were calculated by using the GAMES ab initio program [26] and the 3-21G* basis set [27-291.
72
R. Kimmelma and M. HotokkalJ. Mol. Struct. (Theochem) 285 (1993) 71-75
7C=C /”
‘”
/
‘\
,H
F C H’ ‘H s-&s,
s-tic
s-cis, s-tratls
s-from,s-trans
180
91=92=0
Hr
F
c-sH’
Fig. 1. Some conformations
/” ii+ i.C H H
Fig, 4. The potential energy surface of &vinyl s&oxide.
of divinyl sulfide.
Results and discussion “,
c-c
H’
oivilyl slllfo~
Diviylsuyotlc
Fig. 2. Molecular structure of divinyl sulfoxide and sulfone.
0
90
180
270
Fig. 3. The potential energy surface of divinyl sulfide.
360
The potential energy surfaces of divinyl sulfide, sulfoxide and sulfone were calculated by rotating both of the vinyl groups around S-C bonds. The CSCC torsion angles were varied systematically while optimizing all the other geometrical parameters at each point. The torsion angles are
0
90
180
2io
Fig. 5. The potential energy surface of divinyl sulfone.
360
R. Kimmelrna and M. HotokkajJ. Mol. Struct. (Theochem)
285 (1993)
13
71-75
Table 1. The total energies of the minima as calculated by using the 6-31G* basis set; the conformation is specitied by giving the two CSCC torsion angles Compound
Torsion angle (deg)
Torsion angle (deg)
Total energy (a.u.)
Divinyl sul6de
180 134 -141 -120 120 110 115
180 134 141 120 120 110 -115
-552.416640 -552.419057 -552.418078 -627.219343 -627.218453 -702.070506 -702.069510
Divinyl sulfoxide Divinyl sulfone
defined in such a way that the two double bonds point in opposite directions when both angles are positive (and below 180”). The torsion angles are 0” in the planar s-cis,s-cis conformation (Fig. 1). The resulting potential energy surfaces are given in Figs. 3-5. The mapping of the potential energy surfaces is crude, but it shows clearly where the local minima are to be found. Full optimization of the geometry was performed at these points using the GAMESS program and the 6-31G* basis set [30-321. The resulting total energies are given in Table 1 and the details of the structure at the global minimum for each molecule in Table 2. The atomic and group charges are given in Tables 3 and 4, respectively. In the optimal geometry of divinyl sulfide the vinyl groups have turned about 45” to the opposite directions from the s-trans,s-trans conformation, The planar s-trans,s-trans conformation would be Table 2. The optimal structure of the molecules as obtained from full optimization by using the 6-31G* basis set Parameter
&S (pm) ?$:; so =CSC
(deg)
QCS
(deg)
QOSO (W
91 (deg) A (deg)
Divinyl sultide 177 132 _ 101 123
Divinyl sulfoxide 178 131 148 98 121
_
134 134
-120 120
Divinyl sulfone 176 132 144 103 121 120 112 112
favored by the conjugation of the lone pair electrons of the sulfur atom with the 7r orbitals of the double bonds, but probably the steric strain caused by the cx hydrogen atoms forces the molecule out of the planar conformation. Maybe that is why the SCC bond angle is 5” smaller in divinyl sulfide than in methyl vinyl sulfide [33] (the reduction of this angle takes the a hydrogen atoms further apart). Divinyl sulfide also has another stable rotamer. In this rotamer the vinyl groups have turned about 40” to the same direction from the s-trans,s-trans conformation. This result is in agreement with the theoretical study of Keiko et al. [25]. These investigators have, however, also found a third minimum for divinyl sulfide (nearly s-cis, s-trans). The experimental studies suggest that one of the stable rotamers has the s-trans, s-trans conformation [19-211, which is in reasonably good agreement with the results obtained in the present study. Table 3. The net atomic charges as obtained for the optimal structures by using the 6-31G* basis set Atom
Divinyl sulfide
Divinyl sulfoxide
Divinyl sulfone
S Cl C, C,
0.2 -0.3 -0.3 -0.4 -0.4 -
1.0 -0.4 -0.4 -0.3 -0.3 -0.8 -
1.6 -0.6 -0.6 -0.4 -0.4 -0.6 -0.6
3 0
R. Kinmwlma and M. Hotokka/J. Mol. Struct. (Theochem)
14
Table 4. The net charges of the groups Croup
Divinyl sulfide
Divinyl sulfoxide
Divinyl sulfone
-CH=CHr
-0.1 0.0 _
-0.2 +0.1 +0.2
-0.3 +0.1 +0.4
SD (SW
In the most stable rotamer of divinyl sulfoxide both of the vinyl groups eclipse the S=O group, which is reasonable because in this conformation the S=O double bond can conjugate with both of the double bonds, which stabilizes the molecule. In the second stable rotamer one of the double bonds eclipses the S=O bond and the other one the lone pair electrons of the sulfur atom. Divinyl sulfoxide is the least studied of these divinyl compounds. Two IR studies have been published, both of which state that two conformations are present [20,21], one of which is probably the s-trans,s-trans conformation [20]. In the most stable rotamer of divinyl sulfone each one of the double bonds eclipses one of the S=O bonds, which is again reasonable because of the conjugation in this conformation. The oxygen lone pair orbitals mix with the C=C 7rbonds with a weight of about 10% in divinyl sulfone. In the second stable rotamer both of the double bonds eclipse the same S=O bond, which shows that the S=O bond can conjugate with both of the double bonds at the same time. This result is in agreement with the CND0/2 study made by Hargittai et al. [23]. The second minimum energy structure with the double bonds flipped is energetically almost equally favorable in each case. The barriers are low (below 10 kJmol_‘) in divinyl sulfide and about 20 kJ mol-’ in divinyl sulfoxide and sulfone. The planar structure corresponds to a saddle point on each potential energy surface, indicating that no delocalization of the double bonds is possible. The geometrical parameters are summarized in Table 2. It can be seen that the CSC bond angle in divinyl sulfoxide is strikingly small (only 98”). In methyl vinyl sulfoxide this angle is also small (only
285 (1993) 71-75
95”; 6-31G*) [9]. This may result from different hybridization of the sulfur atom or the steric requirements of the lone pair electrons, but because there is no experimental data to verify this result, it is possible that the small value is caused by the calculation method used. Other bond angles and lengths are similar to each other in divinyl sulfide, sulfoxide and sulfone except the S=O bond length, which is 4 pm shorter in divinyl sulfone than in sulfoxide (the same tendency can be seen in methyl vinyl sulfoxide and sulfone [9]). The Mulliken population analysis shows that the sulfur atom easily donates its electrons, in particular to the strongly electronegative oxygen atoms. The net atomic charge of sulfur in divinyl sulfoxide is 1.0 and in sulfone 1.6, most of the charge being transferred to the oxygen atom(s). However, the vinyl groups also gain electron density on going from divinyl sulfide to sulfoxide and sulfone. This is probably an inductive effect. The atomic charges are added up for selected groups in Table 4. It is clearly seen that the (Y carbon atoms are indeed slightly more negatively charged in divinyl sulfoxide and sulfone than in divinyl sulfide. The net gain of 0.6 electrons observed at the cxcarbon atom in divinyl sulfide reduces to 0.3 electrons for the whole -CH= group. The p carbon atom gains 0.4 electrons but the whole =CHz group has lost 0.1 electrons, indicating that most of the electrons originate from the hydrogen atom. This observation supports the inferred inductive effect.
References R. Virtanen, Acta Chem. Stand., Ser. B, 40 (1986) 313. A.R. Katritzky, R.F. Pinzelli and R.D. Topsom, Tetrahedron, 28 (1972) 3441. S. Samdal, H.M. Seip and T. Torgrimsen, J. Mol. struct., 57 (1979) 105. V. Svdta, M. Prochazka and V. Bakos, Collect. Czech. Chem. Commun., 43 (1978) 2619. R. Kimmehna, Acta Chem. Stand., in press. E. Vajda, D. Hnyk, B. Rozsondai, J. Podlaha, J. Podlahova and J. Hasek, J. Mol. Struct., 239 (1990) 265. V.A. Naumov, R.N. Ziatdinova and E.A. Berdnikov, J. Struct. Chem., (1981) 382.
R. Kimmelma and M. Hotokka/J. Mol. Struct. (Theo&m)
285 (1993) 71-75
8 J. Kao, J. Am. Chem. Sot., 100 (1978) 4685. 9 M. Hotokka and R. Kimmelma, J. Mol. Struct. (Theothem), 276 (1992) 167. 10 G.A. Kalabin, V.M. Bzhezovskii, D.F. Kushnarev and A.G. Proidakov, J. Org. Chem. USSR, (1981) 1009. 11 R. Faure, E.J. Vincent, J.M. Ruiz and L. tina, Org. Magn. Reson., 15 (1981) 401. 12 J.C. Dyer, D.L. Harris and S.A. Evans, Jr., J. Org. Chem., 47 (1982) 3660. 13 L. Cassidei, V. Fiandanese, G. Marchese and 0. Sciacovelli, Org. Magn. Reson., 22 (1984) 486. 14 V.M. Bzhezovskii, D.F. Kushnarev, B.A. Trofimov, G.A. Kalabin, N.K. Gusarova and G.G. Efremova, Bull. Acad. Sci. USSR, Div. Chem. Sci., ll(l981) 2073. 15 G.A. Kalabin, B.A. Trofimov, V.M. Bzhezovskii, D.F. Kushnarev, S.V. Amosova, N.K. Gusarova and M.L. Al’pert, Bull. Acad. Sci. USSR, Div. Chem. Sci., 3 (1975) 501. 16 0. Kajimoto, M. Kobayashi and T. Fueno, Bull. Chem. Sot. Jpn., 46 (1973) 2316. 17 C. Miiller and A. Schweig, Tetrahedron, 29 (1973) 3973. 18 B. Fortunato and M.G. Giorgini, Gazz. Chim. Ital., 106 (1976) 1005. 19 B.A. Trofimov, Yu.L. Frolov, L.M. Sinegovskaya, V.B. Modonov, E.I. Kositsyns, S.V. Amosova, N.K. Gusarova and G.G. Efremova, Bull. Acad. Sci. USSR, Div. Chem. Sci., 2 (1977) 302. 20 A.B. Remizov, T.G. Mannafov and F.R. Tantasheva, J. Gen. Chem. USSR, (1975) 1378.
15
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