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J. Phys. Chem. A 2010, 114, 8480–8487
Stereoelectronic Effects in r-Carbanions of Conformationally Constrained Sulfides, Sulfoxides, and Sulfones Joachim Podlech† Institut fu¨r Organische Chemie, Karlsruher Institut fu¨r Technologie (KIT), Fritz-Haber-Weg 6, 76131 Karlsruhe, Germany ReceiVed: July 9, 2010
Stabilizing effects in oxygenated thiane- and 1,3-dithiane-derived R-carbanions were investigated computationally. All isomers bearing sulfide, sulfoxide, and sulfone functional groups and combinations thereof were investigated by DFT calculations and NBO analyses. Their stabilities and the stereoelectronic effects present in these compounds were compared. Stabilizing effects of the respective functional groups were further estimated by calculation of isodesmic reactions. It turned out that nC f σ*S-O interactions, where both the nC and the SdO bond are in an antiperiplanar conformation, have the highest stabilizing effects. Similar stabilizing interactions are effective in carbanions with an equatorial lone pair at the carbon; here a productive nC f σ*S-C interaction is possible. nC f σ*S-O interactions, where the SdO bond is part of a sulfoxide are significantly more effective than in sulfones. Calculations of isodesmic reactions show similar trends but suggest the presence of additional electrostatic effects and possibly, to some extent, steric effects. Introduction R-Carbanions of sulfides, sulfoxides, and sulfones play an important role in various named and unnamed reactions in organic chemistry.1 The stability of these carbanions, i.e., acidity of the parent protonated compounds is significantly, albeit not exclusively, ruled by stereoelectronic effects.2 In the structurally related R-carbanions of carbonyl compounds (the enolates) a stabilization is due to interaction of the anionic lone pair (donor) with the π* orbital of the carbonyl group (acceptor). It has been noted already in 1976 by Epiotis, Bernardi, Wolfe, and co-workers3 that in sulfur compounds σ*S-C and σ*S-O orbitals are the acceptors dominating the stability of the respective carbanions. This has furthermore been intensively studied by Alabugin et al.4 The extent of these effects is important not only for the stability of these compounds but also for the structure of the carbanions and thus for the stereochemical course of their reactions. While stereoelectronic effects in sulfides, sulfoxides, and sulfones have been studied repeatedly,5 the corresponding effects in their respective anions have hardly been addressed.3a,6,7 The stabilization of an R-deprotonated sulfoxide with its lone pair antiperiplanar to an SdO bond has already been reported by Tsuchihashi et al. in 1973,8 albeit without explanation. Boche et al. solved the structure of sulfoxide R-carbanions by X-ray crystallography and found a pyramidalization of the R-carbon with the lone pair anti to the SdO bond.6f,9 They performed ab initio calculations to confirm this effect, although at a rather low level of theory. The structures of sulfide10 and sulfone9,11 R-carbanions were similarly elucidated by X-ray crystallography. During our studies on the nucleophilic addition to alkylidene bissulfoxides12 we similarly argued that an anionic lone pair evolving during these reactions is stabilized by an antiperiplanar SdO bond. This stabilization has been confirmed by ab initio calculations,13 by NMR spectroscopic investigations,14 and by kinetic experiments.15 In this manuscript a detailed investigation on the stability of the R-carbanions derived from sulfides, † E-mail:
[email protected].
sulfoxides, and sulfones and on the stabilizing interactions effective in these compounds based on ab initio calculations is given. Stereoelectronic Effects Six-membered rings with rigid geometries and with welldefined conformational minima are suitable systems for the investigation of stereoelectronic effects.16 Included in these studies were all isomeric S-oxygenated R-carbanions of thiane and 1,3-dithiane comprising sulfides, sulfoxides, sulfones, and derivatives bearing combinations of these functional groups (Figure 1, Table 1). Furthermore R-anions with equatorial or axial lone pair at the R-carbons were differentiated. All calculations were performed with and without the consideration of solvent effects. Though relative energies and the stereoelectronic effects differ to some extent for solvated and nonsolvated compounds, these differences proved to be of no conceptual significance. All discussions in this manuscript are based on data obtained with consideration of solvation effects. Nevertheless, the data obtained for nonsolvated substrates are given in the Supporting Information. Stereoelectronic effects were quantified by natural bond orbital (NBO) analysis (Tables 2 and 3).17,18 Deleting offdiagonal Fock matrix elements gives energies of stereoelectronic effects between the respective orbitals. The main effects causing the stability of these carbanions are first nC f σ*S-C interactions (Figure 2, top), which are especially possible in carbanions with an equatorial lone pair. Here the donor (the lone pair) is in a perfect antiperiplanar arrangement2a with the acceptor, the σ*S-C orbital. Axial lone pairs at the carbon are best stabilized when an SdO bond is in an antiperiplanar orientation (Figure 2, second row). Here a σ*S-O is the acceptor orbital. This nC f σ*S-O interaction leads to significant stabilization of the respective carbanions. Besides these dominant stereoelectronic effects, minor contributions from stereoelectronic effects have to be considered to allow a correct explanation of the carbanions
10.1021/jp1063758 2010 American Chemical Society Published on Web 07/27/2010
Stereoelectronic Effects in Sulfur-Functionalized R-Carbanions
Figure 1. Parent compounds and carbanions used for calculations.
relative stabilities. Among these are σC-H f σ*S-C and σC-H f σ*S-O interactions, which also apply vice versa (σS-C f σ*C-H and σS-O f σ*C-H), since C-H bonds are not only acceptable donors but also suitable acceptors. Since these energetic contributions are (comparatively) small, they will not be generally discussed in the following sections. Interaction of the sulfur’s axial lone pairs with an antiperiplanar C-H bond (nS f σ*C-H) contributes with 12-25 kJ/mol. This lies in the same range as for corresponding oxygen-containing substrates, e.g., in tetrahydropyran, nO f σ*C-H: 24.2 kJ/mol.19 Further stabilization is possible in carbanions, where the equatorial lone pair at the carbon is not in an antiperiplanar orientation with an axial SdO bond. Simplistic treatment of this interaction usually leads to a neglect of these effects. Nevertheless, after rehybridization of the sp3 lone pair toward an orbital with higher p character,20 an interaction becomes possible, contributing up to 26 kJ/mol to the stability of the respective compounds (Figure 2, third row). The p character of the lone pair in these compounds rises to 92% while it would be only 75% for sp3-hybridized carbon atoms. Similarly, this rehybridization allows for better interaction of an axial nC with S-C bonds contributing up to 20 kJ/mol (fourth row). A further consequence of this rehybridization toward orbitals with higher p character is that these orbitals now have significantly higher energies, thus allowing a better interaction with antibonding orbitals. When no interaction of the lone pair at the carbon is possible at all, the p character is reduced below 75% (only 60% is some cases), which allows the adjacent orbitals at that atom (two S-C bonds and the C-H bond) to raise their respective energy levels to facilitate additional stabilizing interactions.21 NBO Analyses Thiane anion 1e bearing an equatorial anionic lone pair is 17 kJ/mol more stable than anion 1a due to an nC f σ*S-C interaction, which contributes with 60 kJ/mol to the stability of the molecule and an nS f σ*C-H interaction (21 kJ/mol). No
J. Phys. Chem. A, Vol. 114, No. 32, 2010 8481 significant stereoelectronic effect with participation of the nC orbital is present in isomer 1a (Tables 1 and 2). Within the ensemble of thiane-derived sulfoxides, carbanion 2a bearing an axial sulfoxide and an axial nC orbital stabilized by an nC f σ*S-O interaction (131 kJ/mol) is significantly more stable than the other isomers. Stereoelectronic effects in the other isomers are much less stabilizing. No energy barrier prevented inversion of the configuration at C-2 of anion 2a toward isomer 2e during optimization; a minimum structure was not obtained. Anion 3e is essentially stabilized by an nC f σ*S-C interaction, while no significant stabilization is obvious in anion 3a. Nevertheless, an nC f σ*S-O interaction contributing 19 kJ/ mol to the stability of 3a is possible though these orbitals are in a nonfavorable synclinal orientation. Sulfone-derived anions 4e and 4a have almost identical energies. An nC f σ*S-O interaction, where the SdO bond is part of a sulfone group, contributes substantially less than the corresponding effect in sulfoxide groups (cf. 131 kJ/mol in sulfoxide 2a). This lessstabilizing interaction in sulfone anion 4a (97 kJ/mol) competes with three major effects working in anion 4e: an nC f σ*S-C hyperconjugation (54 kJ/mol), a σC-H f σ*S-O (19 kJ/mol), and a synclinal nC f σ*S-O interaction (17 kJ/mol) into the axial SdO bond, which can occur after rehybridization of the nC lone pair to about 83% p character. Nevertheless, the somewhat higher stability of carbanion 4e as compared with isomer 4a is consistent with the prior finding that sulfone R-anions preferentially take a bisecting conformation (Figure 3).9,22 The stability of structurally related anions derived from oxygenated dithianes 5-14 is ruled by similar effects. Deprotonated 1,3-dithiane 5e with an equatorial lone pair at position C-2 is 26 kJ/mol more stable than isomer 5a. This is essentially due to nC f σ*S-C (2 × 51 kJ/mol) and nS f σ*C-H hyperconjugation (2 × 18 kJ/mol). The significant stereoelectronic effects in the less stable isomer 5a are σC-H f σ*S-C and σS-C f σ*C-H interactions summing to about 20 kJ/mol. The most stable monooxygenated 1,3-dithiane anion 6a bears an axial SdO bond and an axial anionic lone pair, giving rise to a most favorable nC f σ*S-O stabilization (111 kJ/mol). Nevertheless, nC f σ*S-C and nC f σ*SO-C interactions, each contributing about 41-49 kJ/mol, and nS f σ*C-H interactions (25 and 18 kJ/mol, respectively) lead to similar relative stabilities of isomers 6e and 7e (as compared with isomer 6a) bearing an equatorial lone pair at C-2. Again, an nC f σ*S-O interaction with contribution of the axial SdO bond is possible in anion 6e (26 kJ/mol) after rehybridization of the nC orbital to about 88% p character. No appreciable stabilization is possible in isomer 7a. Doubly oxygenated isomers derived from 1,3-dithiane comprise a sulfone 8 and three isomeric bissulfoxides 9-11. Within this ensemble energetic differences are obvious. Anions derived from the mixed sulfone-sulfide 8 are considerably more stable than those derived from the bissulfoxides 9-11, exemplifying the most general tendency of compounds to disproportionate exothermally.23 Here the combination of a sulfide and a sulfoxide function is lower in energy than two sulfoxide functions. This observation was confirmed by calculating the absolute energy of dimethyl sulfide (-477.9554 hartree), dimethyl sulfoxide (-553.1433), and dimethylsulfone (-628.3545), showing that the disproportionation of two molecules of dimethyl sulfoxide to dimethyl sulfide and dimethylsulfone is exothermic (-61.0 kJ/mol). Carbanion 8e bearing an equatorial lone pair is more stable than isomer 8a: The former is stabilized by two nC f σ*S-C interactions (41 and 46 kJ/mol) and further stereoelec-
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TABLE 1: Absolute and Relative Energies of Dithiane- and Thiane-Derived Carbanionsa type C 5H 9 S
-
C5H9OS-
C5H9O2SC4H7S2C4H7OS2-
C4H7O2S2-
C4H7O3S2-
C4H7O4S2-
compound
Eel [hartree]
Eel + ZPE [hartree]b
Erel [kJ/mol]c
µ [debye]d
p character [%] (hybridization)e
1e 1a 2e 2a 3e 3a 4e 4a 5e 5a 6e 6a 7e 7a 8e 8a 9e 9a 10e 10a 11e 11a 12e 12a 13e 13a 14e 14a
-594.2056 -594.1995 f -669.4225 -669.4124 -669.4110 -744.6496 -744.6479 -953.0993 -953.0897 -1028.3036 -1028.3047 -1028.3041 -1028.2972 -1103.5346 -1103.5274 f -1103.5137 -1103.4996 -1103.5068 -1103.5004 -1103.4948 f -1178.7346 -1178.7286 -1178.7263 -1253.9545 -1253.9523
-594.0809 -594.0746 f -669.2934 -669.2839 -669.2822 -744.5155 -744.5140 -953.0007 -952.9909 -1028.2014 -1028.2019 -1028.2018 -1028.1947 -1103.4269 -1103.4201 f -1103.4069 -1103.3941 -1103.4002 -1103.3944 -1103.3886 f -1178.6231 -1178.6174 -1178.6150 -1253.8380 -1253.8361
≡0.0 16.5 f ≡0.0 24.9 29.5 ≡0.0 4.0 ≡0.0 25.6 1.3 ≡0.0 0.3 19.0 ≡0.0 18.0 f 52.7 86.2 70.1 85.4 100.6 f ≡0.0 14.9 21.1 ≡0.0 5.0
8.7 6.4 f 7.2 10.6 9.8 10.2 8.7 8.9 6.2 9.8 7.4 10.4 8.8 10.4 8.4 f 9.9 10.5 7.9 11.2 10.6 f 9.3 10.9 8.9 11.1 9.2
81(sp4.4) 72(sp2.5) f 87(sp6.5) 80(sp3.9) 72(sp2.6) 83(sp4.7) 83(sp4.8) 82(sp4.5) 62(sp1.6) 88(sp7.3) 80(sp4.1) 79(sp3.9) 60(sp1.5) 83(sp5.0) 77(sp3.4) f 92(sp11.0) 82(sp4.6) 77(sp3.4) 78(sp3.5) 62(sp1.6) f 89(sp8.3) 79(sp3.7) 73(sp2.7) 85(sp5.7) 88(sp7.6)
a Electronic energies: B3LYP/6-31++G (d,p)//B3LYP/6-31++G(d,p). Solvent model: cpcm. Solvent: tetrahydrofuran. b Electronic energies including zero point vibrational energies (ZPE). ZPEs were scaled by the factor 0.9806. c Relative energies based on the energy of the most stable isomer of the respective type. d Electric dipole moment. e Hybridization and p character at the anionic carbon as obtained by NBO analyses. f No minimum structure; no significant barrier while optimization toward the formation of 2a, 9a, or 12a, respectively.
TABLE 2: Stereoelectronic Effects in Thiane-Derived Carbanions (in kJ/mol) interaction
1e
nC(eq) f σ*(S-C) nC(ax) f σ*(S-C) nC(ax) f σ*(S-Oax) nC(ax) f σ*(S-Oeq) nC(eq) f σ*(S-Oax) nC(eq) f σ*(S-Oeq) σ(C-H) f σ*(S-C) σ(C-H) f σ*(S-Oax) σ(S-C) f σ*(C-H) σ(S-Oax) f σ*(C-H) nS f σ*(C-H)
59.7
1a
2a
6.3
7.7 130.6
3e
3a
50.4
4e 54.4
5.1
6.4 96.8 9.9
19.1 16.7 1.6
3.1 6.8
8.8
4a
7.3
11.4 18.6
4.0
12.6
6.3
9.2 3.3
20.6
tronic effects summarizing to about 55 kJ/mol, while the latter benefits essentially only from one nC f σ*S-O interaction (84 kJ/mol). Within the group of bissulfoxides, anion 9a with two axial SdO groups bearing the lone pair in an axial orientation has the lowest energy. Its stability is a consequence of a most effective nC f σ*S-O interaction (2 × 89 kJ/mol). The p character of the nC orbital is 92% in this compound. With this, the nC f σ*S-O hyperconjugation is obviously still possible, but the interaction should be facilitated by the higher energy of the orbital. Only one of these nC f σ*S-O interactions (94 kJ/ mol) is present in anion 10a with one SdO bond axial and one SdO bond equatorial. No nC f σ*S-O interaction is possible in anion 11a bearing two equatorial SdO groups, making this isomer 48 kJ/mol higher in energy than isomer 9a. The lack of possible interactions of the anionic lone pair results in a decrease of the p character of the lone pair to 62%, allowing for an increase of the adjacent bond’s p character (11a: S-C 83%, C-H 72%; cf. in 9a: S-C 78%, C-H 66%). Anions 10e and 11e with an equatorial lone pair both benefit from nC f σ*S-C
13.2
interactions worth about 2 × 35 kJ/mol. The significant differences in the stabilizing contributions of the nC f σ*S-O and the nC f σ*S-C interactions result a nonminimum structure of anion 9e. No energy barrier prevented inversion of configuration toward isomer 9a during optimization. The energetic differences of trioxygenated substrates are explained by similar stereoelectronic effects. An nC f σ*S-O interaction, where the SdO bond is part of a sulfoxide provides stabilizing effects on the order of 95 ( 11 kJ/mol in average (Table 4), while it only contributes 75 ( 6 kJ/mol of stabilizing effects when the SdO bond is part of a sulfone. Consequently, the differences in energy of these isomers are not as pronounced as within the bissulfoxides. Anion 13e stabilized by two nC f σ*S-C interactions (30 and 43 kJ/mol) is even more stable than anion 13a, which is essentially stabilized by only one nC f σ*S-O interaction (73 kJ/mol) where the SdO bond is part of a sulfone. No minimum energy was obtained for anion 12e. Tetraoxygenated substrates 14e and 14a have about the same energy. This cannot be explained easily, when only the
5.0
5.5
8.9a
5a
6e
24.8
2.8
14.1
26.1f
44.9
44.6
c
11.8
7.3
7.1
4.8
9.6a/ 8.8b 111
6a
7e
17.7 12.4
2.1g
48.9 41.3
4.3 5.6
6.7 5.0
2.5 20.8
16.5
14.6h 1.5i
8.6
7.4
8.4
4.1
84.3 3.2i
5.7g
46.3
8a
11.2a/ 9.5d
d
8e 41.0
8.5a/ 7.2c
7a
e
11.2
6.7
6.1b 89.4
9a
13.1
3.6
14.0
17.8f 0.4g
37.3 43.1
10e
7.5 10.4
5.0 8.5
13.8g
5.4b/ 6.2c 94.4
10a
14.6
5.4g
33.5
11e
4.9
6.0
10.7g
5.5c
11a
11.6 8.3
6.0 8.2
7.0b/ 7.8d 86.0 73.0 7.1i
12a
11.6
3.0
16.1
9.3h 1.6g/ 3.1i
43.0
30.3
13e
7.2
7.1
10.4
4.5
2.1
13.0
18.9h 0.3i
8.3
7.2
70.7 5.3i
73.0 3.4i/ 12.6g
14a
9.2d
36.7
14e
6.1c/ 5.8d
13a
Ø 46.4 ( 4.5 35.6 ( 4.8 44.0 ( 1.3 42.0 ( 4.9 e 95.2 ( 11.1 75.3 ( 6.2 e e e 5.3 ( 1.1 5.1 ( 0.6 7.1 ( 1.0 8.6 ( 1.3 14.0 ( 0.1 15.2 ( 1.9 6.0 ( 1.6 6.3 ( 1.3 11.2 ( 0.6 8.1 ( 0.6 3.2 ( 0.5 2.6 ( 0.4 20.4 ( 3.2 12.9 ( 1.3
a
nC(ax) f σ (S-C). nC(ax) f σ*(SOax-C). nC(ax) f σ*(SOeq-C). nC(ax) f σ*(SO2-C). This line summarizes different types of stereoelectronic effects. A comparison is not reasonable. f nC f σ*(S-Oax). g nC f σ*(S-Oeq). h nC f σ*(SO-Oax). i nC f σ*(SO-Oeq).
b
18.4
5e
51.1
interaction
nC(eq) f σ*(S-C) nC(eq) f σ*(SOeq-C) nC(eq) f σ*(SOax-C) nC(eq) f σ*(SO2-C) nC(ax) f σ*(S-C) nC(ax) f σ*(S-Oax) nC(ax) f σ*(SO-Oax) nC(ax) f σ*(S-Oeq) nC(eq) f σ*(S-Oax) nC(eq) f σ*(S-Oeq) σ(C-H) f σ*(S-C) σ(C-H) f σ*(SOeq-C) σ(C-H) f σ*(SOax-C) σ(C-H) f σ*(SO2-C) σ(C-H) f σ*(S-Oax) σ(C-H) f σ*(SO-Oax) σ(S-C) f σ*(C-H) σ(SOeq-C) f σ*(C-H) σ(SOax-C) f σ*(C-H) σ(SO2-C) f σ*(C-H) σ(S-Oax) f σ*(C-H) σ(SO-Oax) f σ*(C-H) nS f σ*(C-H) nS(SOeq) f σ*(C-H)
TABLE 3: Stereoelectronic Effects in 1,3-Dithiane-Derived Carbanions (in kJ/mol)
Stereoelectronic Effects in Sulfur-Functionalized R-Carbanions J. Phys. Chem. A, Vol. 114, No. 32, 2010 8483
Figure 2. Most important stereoelectronic effects in sulfur-containing carbanions.
Figure 3. Preferred conformation in sulfone-derived R-anions.
TABLE 4: Stereoelectronic Effects with an Anionic Lone Pair as Donor
a Anionic lone pair: donor. Boldface bond: acceptor. b Values for compounds 1-4//values for compounds 5-14. c Isomer 2e bearing this structural motif turned out to be no minimum structure.
prominent interactions are considered: isomer 14a is essentially stabilized by an nC f σ*S-O interaction (2 × 71 kJ/mol), while anion 14e benefits from a much smaller nC f σ*S-C stabilization
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Figure 4. β-Anions of sulfur-containing heterocycles.
(2 × 37 kJ/mol). A comprehensive listing of all pertinent stereoelectronic effects in isomer 14e additionally contains synclinal nC f σ*S-O interactions (2 × 19 kJ/mol) possible due to a 85% p character at the nC orbital, and σC-H f σ*S-O interactions (2 × 13 kJ/mol). It turned out that the stereoelectronic effects of interest in these compounds have similar energies with very small standard deviations, when one compares the contributions of identical structural patterns (Table 4). nC f σ*S-C interactions in sulfide functions, which significantly contribute in the four dithiane derivatives 5-8e, contribute with 46 ( 5 kJ/mol. Nevertheless, the corresponding stereoelectronic effect in the thiane derivative 1e is significantly higher (60 kJ/mol) than in the respective 1,3dithiane derivatives. However, this can easily be understood: interaction of one structural motif (here the anionic lone pair) with two functional groups instead of one leads to better overall stabilization, but the single stabilizing effects should be lower for each single interaction. The nC f σ*S-C stabilizations differ significantly in sulfides, sulfoxides, and sulfones. It even plays a role, whether the sulfoxide bears an equatorial or an axial SdO bond. Here the clearly distinguishable average energetic contributions are 36 ( 5 kJ/mol (equatorial SdO, entry 2), 44 ( 1 (axial SdO, entry 3), and 42 ( 5 (SO2, entry 4). This also turned out to be true for nC f σ*S-O interactions. It makes a significant difference, whether the SdO bond is part of a sulfoxide (95 ( 11 kJ/mol, entry 5) or a sulfone (75 ( 6 kJ/ mol, entry 6). Though this publication deals with sulfur-derived R-carbanions, a short paragraph should be spent to have a glimpse on the corresponding β-anions (Figure 4). As noted, R-anions of thianes and 1,3-dithianes are in part stabilized by nC f σ*S-C interactions. An equivalent nC f σ*C-S stereoelectronic effect, where sulfur and carbon changed their places, should be similarly stabilizing. Alabugin noted that a σC-H f σ*C-S interaction in 1,3-dithiane is almost 4 times more effective than a σC-H f σ*S-C hyperconjugation (27.2 versus 7.5 kJ/mol).4,24 In accord with the increased importance of this bond-weakening interactions, thiane- and dithiane-derived carbanions 16-21e bearing this structural pattern turned out to not be minima structures suffering a β-elimination.25 Isodesmic Reactions A further relevant method for the investigation of stabilizing effects within molecules is the calculation of isodesmic (homodesmotic) reactions.26 Figure 5 exemplifies possible isodesmic reactions for compounds investigated herein. The stabilizing influence of the sulfur in carbanion 1e is quantified by additional calculation of the corresponding nondeprotonated thiane, of the cyclohexyl anion, and of cyclohexane (top). Summarized energies of the starting materials should be lower than for the
Figure 5. Exemplary isodesmic reactions.
TABLE 5: Isodesmic Correlations for Thiane Derivativesa
a Energy values were not available for compound 2e. No isodesmic comparison is possible.
products by the additional stabilization energy operative in carbanion 1e. Similar calculations can be made for the 1,3dithiane derivatives. Here the carbanion is compared either with the respective dithiane (2nd row) or with two molecules of thiane derivatives (third and fourth row). These isodesmic correlations, which are summarized in Tables 5 and 6, clearly reflect the relative stabilities as given in Table 1. They furthermore give insight into all stabilizing effects working in these compounds, including not only stereoelectronic interactions but also steric effects and dipole-dipole interactions. Again, the stabilization of a deprotonated, thiane-derived sulfoxide is highest in carbanion 2a; here the stabilization of the carbanion with axial lone pair by an axial SdO bond contributes 125 kJ/mol, which corresponds approximately with the dominant nC f σ*S-O interaction (131 kJ/mol) working in
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TABLE 6: Isodesmic Correlations for Dithiane Derivativesa
a
Energy values were not available for compounds 9e and 12e. No isodesmic comparison is possible.
Figure 6. Charge effects in sulfone anions.
this compound. Isomer 4e shows the highest stabilization energy (156 kJ/mol) of all thiane-derived carbanions, which is about twice the sum of the lone pair’s stereoelectronic effects (∼73 kJ/mol). A similar comparison can be made for sulfone anion 4a. Obviously an additional effect is working in the sulfone anions, which is not sufficiently explained by stereoelectronic interactions. An electrostatic effect might be working in these compounds,2h which is possible since SdO bonds can roughly be described by single bonds with charge separation (Figure 6).27 The sulfur in the (uncharged) sulfone 4 has an NBO charge of +2.13 while the sulfurs in sulfoxides 2 and 3 have NBO charges of +1.21 and +1.20, respectively. Isodesmic correlations for 1,3-dithiane-derived carbanions 5-14e and 5-14a allow for similar conclusions. Comparison with dithianes 5-14 gives energetic values that are roughly the sum of two isolated stabilizations obtained for thiane derivatives (Table 6, entries 1-18a). The stabilization energy for the sulfoxide anion 6a (-164 kJ/mol, Table 6, entry 4a) is about the sum of the values obtained for carbanions 1a (-47 kJ/mol, Table 5, entry 2) and 2a (-125 kJ/mol, Table 5, entry 2). Comparison of the carbanion’s energies with those obtained for the two underlying thiane-derived compounds (entries 1-18b)
gives stabilization energies generally 10-25% lower than those obtained from comparison with dithianes (entries 1-18a). The lower values should be due to the fact that the stabilization of an anionic center by two functional groups is not additive but somewhat smaller. The fact that these differences steadily increase from 10% (for the first entries in the table) to 25% (the last entries) might be attributed to a steric effect. Steric hindrance should roughly increase from the first to the last entries in the table. The higher the steric hindrance, the lower is the stability of the respective compounds when compared with the less sterically hindered thiane derivatives. Conclusion R-Carbanions of sulfides, sulfoxides, and sulfones are stabilized by stereoelectronic, by electrostatic, and to some extent by steric effects. Sulfide R-anions are significantly stabilized, when the carbon-centered lone pair is antiperiplanar to S-C bonds. R-Anions of sulfoxides are stabilized by interaction of the carbon-centered lone pair with an antiperiplanar SdO bond. A similar nC f σ*S-O interaction in sulfones is less effective due to competing nC f σ*S-C interactions. Nevertheless, these stereoelectronic effects are accompanied by an electrostatic effect, which is especially effective in sulfones. Computational Methods All structures were fully optimized at the B3LYP28/6-31++G (d,p)29 level using the Gaussian 03 program package.30 Analytic
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frequencies were calculated at that level of theory to identify the structures as minima on the potential energy surface. Zeropoint vibrational energies (ZPE) were included in the calculation of relative energies and of isodesmic reactions. A scaling factor of 0.9806 was applied.31 The NBO 3.1 program was used as implemented in the Gaussian 03 program package.17 Stereoelectronic effects were quantified by deletion of the corresponding off-diagonal elements of the Fock matrix in the NBO basis. Optimizations, vibrational analyses, and NBO calculations were performed with and without consideration of solvent. Solvent was modeled with the CPCM-SCRF method32 with tetrahydrofuran as solvent. This solvent supplies a significant solvation but avoids alteration of the molecule’s electronic properties by a too high polarity. Results obtained from calculations without the consideration of a solvent field were neither included nor discussed in this manuscript, but these data are given in the Supporting Information. Acknowledgment. This work was supported by the Deutsche Forschungsgemeinschaft (DFG). Supporting Information Available: Results without consideration of solvent effects (energies, stereoelectronic effects, isodesmic correlations) and computational details on all minimum structures. Complete ref 30. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) General:(a) Mikołajczyk, M.; Drabowicz, J.; Kiełbasin˜ski, P. Chiral sulfur reagents; CRC Press: Boca Raton, FL, 1997. Chemistry of sulfides: (b) Gro¨bel, B.-T.; Seebach, D. Synthesis 1977, 357–402. (c) Metzner, P.; Thuillier, A. Sulfur reagents in organic synthesis; Academic Press: London, 1994. Chemistry of sulfoxides: (d) Ferna´ndez, I.; Khiar, N. Chem. ReV. 2003, 103, 3651–3705. Chemistry of sulfones: (e) Simpkins, N. S. Sulphones in organic synthesis; Pergamon Press: Oxford, U.K., 1993. (f) Delouvrie´, B.; Fensterbank, L.; Na´jera, F.; Malacria, M. Eur. J. Org. Chem. 2002, 3507–3525. (2) Stereoelectronic effects: (a) Epiotis, N. D.; Yates, R. L.; Larson, J. R.; Kirmaier, C. R.; Bernardi, F. J. Am. Chem. Soc. 1977, 99, 8379– 8388. (b) Kirby, A. J. The anomeric effect and related stereoelectronic effects at oxygen; Springer: Berlin, 1983. (c) Deslongchamps, P. Stereoelectronic effects in organic chemistry; Pergamon Press: Oxford, U.K., 1983. (d) The anomeric effect and associated stereoelectronic effects; Thatcher, G. R. J., Ed.; American Chemical Society: Washington, DC, 1993. (e) Wiberg, K. B.; Castejon, H. J. Am. Chem. Soc. 1994, 116, 10489–10497. (f) Juaristi, E.; Cuevas, G. The anomeric effect; CRC Press: Boca Raton, FL, 1995; (g) Weinhold, F.; Landis, C. Valency and bonding. A natural bond orbital donor-acceptor perspectiVe; Cambridge University Press: Cambridge, U.K., 2005; (h) Glendening, E. D.; Shrout, A. L. J. Phys. Chem. A 2005, 109, 4966–4972. (i) Fleming, I. Molecular orbitals and organic chemical reactions; John Wiley & Sons: Chichester, U.K., 2010. (j) See as well: Eliel, E. L.; Hutchins, R. O. J. Am. Chem. Soc. 1969, 91, 2703–2715. (3) (a) Epiotis, N. D.; Yates, R. L.; Bernardi, F.; Wolfe, S. J. Am. Chem. Soc. 1976, 98, 5435–5439. (b) Barbarella, G.; Dembech, P.; Garbesi, A.; Bernardi, F.; Bottoni, A.; Fava, A. J. Am. Chem. Soc. 1978, 100, 200– 202. See as well: (c) Aggarwal, V. K.; Worrall, J. M.; Adams, H.; Alexander, R.; Taylor, B. F. J. Chem. Soc., Perkin Trans. 1997, 1, 21–24. (4) (a) Alabugin, I. V. J. Org. Chem. 2000, 65, 3910–3919. (b) Alabugin, I. V.; Zeidan, T. A. J. Am. Chem. Soc. 2002, 124, 3175–3185. (5) (a) Bien, S.; Celebi, S. K.; Kapon, M. J. Chem. Soc., Perkin Trans. 1990, 2, 1987–1990. (b) Juaristi, E.; Cuevas, G.; Flores-Vela, A. Tetrahedron Lett. 1992, 33, 6927–6930. (c) Juaristi, E.; Cuevas, G. Tetrahedron Lett. 1992, 33, 1847–1850. (d) Salzner, U.; von R. Schleyer, P. J. Am. Chem. Soc. 1993, 115, 10231–10236. (e) Juaristi, E.; Cuevas, G.; Vela, A. J. Am. Chem. Soc. 1994, 116, 5796–5804. (f) Juaristi, E.; Ordon˜ez, M. Tetrahedron 1994, 50, 4937–4948. (g) Aggarwal, V. K.; Worrall, J. M.; Adams, H.; Alexander, R. Tetrahedron Lett. 1994, 35, 6167–6170. (h) Aggarwal, V. K.; Davies, I. W.; Franklin, R.; Maddock, J.; Mahon, M. F.; Molloy, K. C. J. Chem. Soc., Perkin Trans. 1994, 1, 2363–2368. (i) Cuevas, G.; Juaristi, E.; Vela, A. J. Phys. Chem. A 1999, 103, 932–937. (j) Alabugin, I. V.; Manoharan, M.; Zeidan, T. A. J. Am. Chem. Soc. 2003, 125, 14014–14031. (k) Roux, M. V.; Temprado, M.; Jime´nez, P.; Da´valos, J. Z.; Notario, R.; Martı´n-Valca´rcel, G.; Garrido, L.; Guzma´n-Mejı´a, R.; Juaristi, E. J. Org. Chem. 2004, 69, 5454–5459. (l) Oshida, H.; Ishii, A.; Nakayama, J. J. Org. Chem. 2004, 69, 1695–1703. (m) Juaristi, E.; Notario, R.; Roux, M. V.
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