J. Phys. Chem. 1995,99, 15035-15040
15035
Theoretical Studies on the Substitution Reactions of Sulfonyl Compounds. 2. Hydrolysis and Methanolysis of Methanesulfonyl Chloride Kiyull Yang* and In Sun Koo Department of Chemical Education, Gyeongsang National University, Chinju 660-701, Korea
Ikchoon Lee Department of Chemistry, Inha University, Inchon 402-751, Korea Received: April 14, 1995; In Final Form: July 20, 1995@
Ab initio molecular orbital calculations have been performed to study the gas-phase hydrolysis and methanolysis of methanesulfonyl chloride. The overall reaction occurs via a concerted S N mechanism ~ with a trigonalbipyramidal transition state, and the transition structure is looser when the reaction is catalyzed by additional solvent molecules. The reactivity in a mixed solvent agrees well with the gas-phase proton affinity of the hydrogen-bonded solvent complex, and the methanol-catalyzed methanolysis reaction in the gas phase is the fastest among the reactions discussed. The catalytic role of such solvent molecules appears to be bifunctional as in the hydration of a carbonyl group, but general-base catalysis is more important than general-acid catalysis. The MP2/6-3 lG* activation energy is reduced considerably by two catalytic water molecules.
Introduction Although nucleophilic substitution at sulfur is an important reaction in chemistry and biochemistry,' there have been only a few theoretical studies of this reaction. These studies have tended to focus on di- or tricoordinate sulfur systems,2 using semiempiricaland ab initio molecular orbital methods, but there seems to be no high-level theoretical work reported on substitution at tetracoordinate sulfur. It is recognized that nucleophilic substitution at tetracoordinate sulfur can proceed via a trigonal-bipyramidal intermediate (addition-elimination) or by a concerted sN2 me~hanism.~ A particularly important system which contains tetracoordinate sulfur is RS02C1; sulfonyl chlorides are important reagents in organic synthesis, and substitution reactions of these compounds bridge inorganic and organic chemi~try.~ Solvent effects and linear free energy relationships in the solvolyses of sulfonyl chlorides, especially substituted benzenesulfonyl chlorides, have received much experimental attention?v6 but little work has been done on the intrinsic nucleophilic reactivities in the gas phase.7 Robertson and co-workers founds that the kinetic solvent isotope effects for benzenesulfonyl chloride and methanesulfonyl chloride are almost the same. This result is interesting since it suggests that phenyl and methyl groups have similar electronic effects in this reaction. In part 1 of this series: we reported ab initio calculations on the hydrolysis of methanesulfonyl chloride (MSC, S ) by one water molecule. In that work, a four-membered cyclic transition state (l),in which heavy atom reorganization and proton transfer processes occur in concert, was suggested. As a continuation of these studies, this report examines the hydrolysis and methanolysis of methanesulfonyl chloride, eq 1, including the gas-phase catalytic effects of additional solvent molecules. CH,SO,Cl+ nROH
- [TS]* CH,S03R
+ HC1+ (n - 1)ROH (1)
where R = H and CH3.
* Author for correspondence. @Abstractpublished in Advance ACS Absrracfs, September 1, 1995. 0022-3654/95/2099-15035$09.00/0
In view of the well-known catalytic models for the gas-phase hydration of carbonyl groups,I0," one might consider similar models, i.e., bifunctional (2), general-acid (3),and general-base catalysis (4) for the solvolysis of sulfonyl chlorides. According to the calculations for the carbonyl hydration reaction, 3 and 4 are ineffective, with an absence of bound states on the potential energy surface. We have therefore examined the role of additional solvent molecules in bifunctional catalysis. We have also examined the reaction of MSC with a mixture of water and methanol, which mimics the solvolytic reaction in a mixed solvent.6g
Cl
Cl
1
2
61
I
CI
3
4
Computations All calculations were performed using Gaussian 92.12 Structures were fully optimized f i s t at the HF/3-21G* level and then refined at the HF/6-3 lG* level, and single-point calculations were performed with the frozen-core approximation at the second-order Mprller-Plesset perturbation (MP2) theory with the 6-31G* basis set on the HF/6-31G*-optimized geometries. In addition, for the purpose of comparison, single-point calculations were carried out at MP4(SDTQ)/6-31G* for the uncatalyzed hydrolysis reaction. The MP2 thermodynamic parameters
0 1995 American Chemical Society
15036 J. Phys. Chem., Vol. 99,No. 41,1995
Yang et al.
TABLE 1: Activation Energies (kcal mol-') for the Uncatalyzed Hydrolysis and Methanolysis of Methanesulfonyl Chloride at 0 KO
TABLE 3: MP2 Activation Parameters (kcal mol-') for the Uncatalyzed Hydrolysis and Methanolysis of Methanesulfonyl Chloride at 298 Ka
reactions
HF/3-21G*
HF/6-31G*
MP2/6-31G*
S+H S+M S H (retention) S M (retention)
32.93 (47.79) 32.09 (46.42) 43.54 42.35
56.20 (63.35) 56.26 (62.98) 66.1 1 65.97
39.28 (48.77) 36.02 (45.57) 45.79 42.50
+ +
Values in parentheses refer to the energy differences between transition structures and hydrogen-bonded reactant complexes. All other values refer to the energy differences between transition structures and separated reactants.
TABLE 2: Basis Set Dependence of the Activation Energies (kcal mol-') of Uncatalyzed Hydrolysis of Methanesulfonyl Chloride at 0 K methodhasis set HF MP2 MP4(SDTQ)
S(Al?). no. of basis functions
3-21G*
6-31G*
6-31G**
6-31 1G**
32.93 24.63 25.26 7.67 84
56.20 39.28 40.12 16.08 108
55.28 38.50 39.09 16.19 123
56.07 40.19 40.27 15.80 154
AI?
reactions S+H S+M
-TAP
Ai9
AG*
38.77 (46.17) 38.18 (46.17) 11.06 (3.43) 49.24 (49.56) 35.03 (42.53) 34.44 (42.53) 11.61 (3.23) 46.05 (45.76) 45.50 44.91 11.75 56.66
S+H
(retention) S+M 41.65 (retention)
41.06
12.41
53.47
a Values in parentheses refer to the energy differences between transition structure and hydrogen-bonded reactant complexes. All other values refer to energy differences between transition structures and separated reactants.
SHts (39.3/56.2) SHts"(43.7/39.4)
40.0
0
Correlation energy, Le., &A@) = Al?(HF) - AP(MP4).
Y v
1. W
were estimated using the HF/6-31G* frequencies. Since empirical correlations between calculated and experimental frequencies are well e~tablished,'~ the HF/6-3 lG* frequencies were scaled by 0.9 and then used to estimate MP2 thermodynamic properties at 298.15 K, 1 atm, using standard proced~res.'~ Most transition structures were located using the eigenvector following (EF)I5 procedure in conjunction with the computation of force constants at every cycle (opt = calcall, ef), and were characterized by confirming the presence of only one negative eigenvalue in the Hessian matrix. Some additional stationary points such as a slightly stable intermediate and a second transition state having a very low energy barrier were located by the intrinsic reaction coordinate (IRC) methodI6 and by linear synchronous transit (LST) calculation^,'^ which can locate a maximum along the path connecting two local minima.
Results and Discussion Uncatalyzed Reaction of Water and Methanol with MSC. Energetics. Table 1 summarizes the Hartree-Fock and MlbllerPlesset perturbation energy differences at 0 K relative to the separated reactants ( S H, or S M) and relative to the reactant complexes (SH or SM). The transition structures for reactions proceeding with retention of configuration were also located since it is natural to consider a retention mechanism for substitution at a second-row element.I8 The HF/6-3 lG* activation energies are considerably higher than those obtained at the HF/3-21G* or MP2/6-31G* levels. For the reaction in Table 1 the effect of electron correlation at the MP2 level reduces the barrier height considerably when the 6-31G* basis set is used. A similar correlation effect has been found for the gas-phase hydrolysis of formyl halides,I9 and this appears to be a general property of the HF/6-31G* basis set.20 The activation energies calculated at different levels are shown in Table 2. Since the value obtained at MP4(SDTQ)/6-311G**/ /HF/6-31G*, the highest level employed, does not differ significantly from that at MP2/6-3 lG*//HF/6-3 1G*, the latter was employed in subsequent calculations. The activation energy for the retention mechanism is higher by ca. 7 kcal mol-' (MP2) in both hydrolysis and methanolysis. Although this value is smaller than that calculated for displacement at carbon (40 kcal the retention mechanism remains improbable.
+
+
0.0
a SHrc (-9.5/-7.1)
-20.0
t
SHpc (-15.6/-20.1)
I
-40.0 I
intrinsic reaction coordinate
Figure 1. Ab initio energy profile for the hydrolysis of methanesulfonyl chloride. Values in parentheses refer to relative energies calculated at (MP2MF) with 6-31G* level at 0 K.
Table 3 collects the thermodynamic parameters of the activation process at 298.15 K, using scaled HF/6-31G* frequencies and MP2 energies. The A,?? and @ are higher when calculated from the hydrogen-bonded complex, because these are stabilized relative to the separated reactants. However, there is a compensating entropic effect, because the preassociation of reactants in these complexes significantly reduces the unfavorable entropies of activation of these cyclic processes. Consequently, the free energies of activation are virtually the same, whether these are based on the separated reactants or on the complexes. The activation energy for methanolysis is lower than that of hydrolysis by 3 kcal mol-', and this preference does not depend on how the activation energy is expressed, e.g., as electronic energy (A,??,0 K), enthalpy (e, 298 K), or free energy (AG*, 298 K). Figure 1 shows the potential energy profile for the hydrolysis of MSC; the solid and dashed lines refer to the MP2/6-31G* and the HF/6-3 1G* calculations, respectively. The single-point MP2 potential suggests a two-step process in which addition of ROH to MSC produces a diol which then eliminates HC1 with a barrier of 12 kcal mol-', considerably higher than the second banier calculated at HF/6-31G* (2 kcal mol-'). This type of mechanism, which may involve nucleophilic attack by water to form sulfonyl chloride hydrate, had previously been postulated as an alternative to di~placement.~"Although the second barrier is somewhat higher, there should be sufficient chemical activation to overcome the barrier for 1,2 eliminati~n'~ once the first banier for the addition of H20 to sulfonyl chloride has been reached because the intermediate is unstable relative to the reactant or the reactant complex. Geometries. The geometries of the structures calculated for uncatalyzed hydrolysis and methanolysis are summarized in
Substitution Reactions of Sulfonyl Compounds
J. Phys. Chem., Vol. 99, No. 41, 1995 15037 3.0 2.8
Y
5m C
0 -
2.6
D 8
n S
SHrc
SMrc
0
2.4
I v)
2.2 2.0 2.4 2.2 I
CI
SHts
SMts
CH3
SHts'
2.0 D C 0
1 .a
1
1.6
n
1.4
I .2 I
CH3
SMts'
I
1 .o
I
CI
CI
SHint
SHts"
Figure 2. HF/6-31G* structures of some stationary points for the uncatalyzed solvolysis reaction. S, H, and M refer to methanesulfonyl chloride (MSC), H20, and MeOH, respectively. rc and ts refer to reactant complex and the first transition state, respectively. ts' and ts" refer to retention transition state and the second transition state, respectively. int refers to intermediate. Figure 2. The MSC-HzO reactant complex (SHrc) has C, symmetry, with C, O', S, and C1 in the mirror plane. The MSC-MeOH complex (SMrc) has CIsymmetry. The complexes exhibit OH1-02 hydrogen bonding rather than an S-0' interaction because of the steric effects of the substituents on sulfur. This differs from the interaction in the S03-H20 complex, in which an S-0 interaction is preferred.22 In the transition states (SHts and SMts), the migrating proton (HI) has almost completely transferred to a sulfonyl oxygen (02).This degree of proton transfer is much more pronounced than in carbonyl additiont0," or in the hydration of SOs, in which the migrating hydrogen is almost symmetrically placed between two oxygen^.^^-^^ Elongation of the S-Cl bond takes place between the first transition state (ts) and the second transition state (ts"). The Pauling S-C1 bond orders24 are 0.855, 0.360, and 0.078 for ts, int, and ts", respectively. The first transition structure is tight and the second transition structure is loose: the S-Cl bond is almost cleaved in this structure. Another index for the transition structure is the percent looseness,25which is the sum of the percent lengthening of the forming bond and breaking bond at the transition structure. For the first transition structure this index is 31.9. The motions in the second transition state consist of rotation of HI around the S-O2 bond and further lengthening of the S-C1 bond. To convey these motions more explicitly, we have plotted the forming S-0' bond length versus the breaking S-C1 and 0I-H' bond lengths in Figure 3, using the geometries of IRC calculations for the hydrolysis. It is clear that the frst barrier is associated with proton transfer from water to a sulfonyl oxygen, and movement of the chlorine atom does not become significant until the intermediate has formed. This corresponds to an associative-S~2process which is shifted toward an SAN process.5d In the retention mechanism the
2.5
3.0
1.5
2.0 bond length
S-0'
Figure 3. Plots of S-0' bond length versus S-Cl and 0I-H' bond lengths obtained by IRC calculations. TABLE 4: MP2 Activation Parameters (kcal mol-') for the Solvent-Catalyzed Hydrolysis and Methanolysis of Methanesulfonyl ChloridP reactions A,!? 6(A,??)b AH$ -TAP AG* 6(AG*)b S + H + H 32.26 -1.02 34.44 20.53 54.91 5.13 S+HH
S+H+M SSHM S+M+H
S+MH S+M+M
S+MM
39.40 24.63 3 1.95 31.25 38.54 23.15 31.29
-14.65 -4.11 -12.21
39.92 26.50 32.18 32.13 38.43 24.46 30.45
15.01 21.01 15.32 21.30 15.42 21.56 15.48
54.93 41.51 47.50 54.03 53.85 46.02 45.93
-1.67 1.98 -0.03
The temperature is 0 K for A,!? and 6(A.P), and 298 K for other = &(catalyzed) parameters. Catalytic effect: Le., &e) hXs(uncata1yzed). positions of CH3 and C1 are interchanged. The alternative diol intermediate from the retention mechanism was also sought, since retention is possible if two intermediates are interconverted via Berry pseudorotation. However, no such intermediate could be found on the IRC (HF/3-21G*) path when this was followed from the retention transition structure to the product complex. Although the full reaction coordinate for the methanolysis of MSC was not calculated, the apparent reaction profile is similar to the profile for hydrolysis, and the atomic movements are not very different from those for hydrolysis. Solvent-Catalyzed Hydrolysis and Methanolysis of MSC Energetics. The reaction proceeds by an one-step process with no solvated diol intermediate, as confirmed by HF/3-21G* IRC calculations for the water-catalyzed hydrolysis. The MP2 thermodynamic activation parameters with catalytic effects are summarized in Table 4 for the reactions catalyzed by one solvent molecule. The abbreviation S H f M in the table refers to water acting as the nucleophile and methanol as the catalyst, with the energy barrier measured from the separated reactants. The abbreviation S HM refers to the same process, with the
+
+
Yang et al.
15038 J. Phys. Chem., Vol. 99,No. 41. 1995
,\,\CH3
1.976
HI//,,.
9'
I
2.030
158.7
\
(P
I
TE
0.0
l b l r e =3.34
i
2.030
CI
CI
SHHrc
SHMrc
2.031
2.031
2.730
CI
SHHts
le)
R.
10 rc = 9 08
= 7 53
Figure 5. Sequence of atomic movements for the water-catalyzed hydrolysis of methanesulfonyl chloride from the transition state (a) to the product complex (f) on the IRC calculation at the HF/3-21G* level. The length rc of the IRC is in amu"* bohr. See Figure 4 for numbering.
CI
CI
SMHrc
= 5 93
B
B Id) R. i6 73
I
(CI rc
SMMrc
I
2.630
CI
SHMts
free energy catalytic effects of methanol are -2 and 0 kcal mol-] for hydrolysis and methanolysis, respectively. Although the overall catalytic effects are not as large as in carbonyl hydration,l0
