Nucleophilicity and nucleofugality of phenylsulfinate (PhSO 2-): A key to understanding its ambident reactivity

Article abstract of DOI:10.1021/ja9102056

Second-order rate constants for the reactions of the phenylsulfinate ion PhSO₂^- with benzhydrylium ions Ar₂CH ⁺ have been determined in DMSO, acetonitrile, and aqueous acetonitrile solution using laser-flash and stopped-flow techniques. The rate constants follow the correlation equation log k (20 °C) = s(N + E), which allows the determination of the nucleophile-specific parameters N and s for PhSO₂^- in different solvents. With N = 19.60, PhSO ₂^- is a slightly weaker nucleophile than malonate and azide ions in DMSO. While PhSO₂^- reacts with highly stabilized benzhydrylium ions to give benzhydryl phenyl sulfones exclusively, highly reactive benzhydrylium ions give mixtures of sulfones Ar ₂CH-SO₂Ph and sulfinates Ar₂CH-OS(O)Ph; the latter rearrange to the thermodynamically more stable sulfones through an ionization recombination sequence. Sulfones generated from PhSO₂ ^- and stabilized amino-substituted benzhydrylium ions undergo heterolysis in aqueous acetonitrile and the rate of formation of the colored benzhydrylium ions was followed spectrophotometrically by stopped-flow techniques. The ranking of the electrofugalities of the benzhydrylium ions (i.e., the relative ionization rates of Ar₂CH-SO₂Ph) was not the inverse of the ranking of their electrophilicities (i.e., the relative reactivities of Ar₂CH⁺ with nucleophiles), which was explained by differences in Marcus intrinsic barriers. While sulfones are thermodynamically more stable than the isomeric sulfinates, the intrinsic barriers for the attack of benzhydrylium ions at the oxygen of PhSO ₂^- are significantly lower than the intrinsic barriers for S-attack, and the activation energies for the attack of carbocations at sulfur are only slightly smaller than those for attack at oxygen. Because reactions of PhSO₂^- with carbocations of an electrophilicity E > -2 (i.e., carbocations which are more reactive than Ph₃C⁺) are diffusion-controlled, the regioselectivities of the reactions of PhSO ₂^- with ordinary carbocations do not reflect relative activation energies.

Full text of DOI:10.1021/ja9102056

Published on Web 03/12/2010
Nucleophilicity and Nucleofugality of Phenylsulfinate
-
(PhSO2 ): A Key to Understanding its Ambident Reactivity
Mahiuddin Baidya, Shinjiro Kobayashi, and Herbert Mayr*
Department Chemie, Ludwig-Maximilians-UniVersita¨t Mu¨nchen, Butenandtstrasse 5-13
(Haus F), 81377 Mu¨nchen, Germany
Received December 3, 2009; E-mail: herbert.mayr@cup.uni-muenchen.de
-
Abstract: Second-order rate constants for the reactions of the phenylsulfinate ion PhSO2 with benzhy-
drylium ions Ar2CH+ have been determined in DMSO, acetonitrile, and aqueous acetonitrile solution using
laser-flash and stopped-flow techniques. The rate constants follow the correlation equation log k (20 °C)
) s(N + E), which allows the determination of the nucleophile-specific parameters N and s for PhSO2- in
different solvents. With N ) 19.60, PhSO2- is a slightly weaker nucleophile than malonate and azide ions
in DMSO. While PhSO2- reacts with highly stabilized benzhydrylium ions to give benzhydryl phenyl sulfones
exclusively, highly reactive benzhydrylium ions give mixtures of sulfones Ar2CH-SO2Ph and sulfinates
Ar2CH-OS(O)Ph; the latter rearrange to the thermodynamically more stable sulfones through an ionization
recombination sequence. Sulfones generated from PhSO2- and stabilized amino-substituted benzhydrylium
ions undergo heterolysis in aqueous acetonitrile and the rate of formation of the colored benzhydrylium
ions was followed spectrophotometrically by stopped-flow techniques. The ranking of the electrofugalities
of the benzhydrylium ions (i.e., the relative ionization rates of Ar2CH-SO2Ph) was not the inverse of the
ranking of their electrophilicities (i.e., the relative reactivities of Ar2CH+ with nucleophiles), which was
explained by differences in Marcus intrinsic barriers. While sulfones are thermodynamically more stable
than the isomeric sulfinates, the intrinsic barriers for the attack of benzhydrylium ions at the oxygen of
-
PhSO2 are significantly lower than the intrinsic barriers for S-attack, and the activation energies for the
attack of carbocations at sulfur are only slightly smaller than those for attack at oxygen. Because reactions
of PhSO2- with carbocations of an electrophilicity E > -2 (i.e., carbocations which are more reactive than
Ph3C+) are diffusion-controlled, the regioselectivities of the reactions of PhSO2- with “ordinary” carbocations
do not reflect relative activation energies.
Table 1. Reactions of Arylsulfinate Ions 1 with Hard and Soft
Electrophiles
Introduction
Regioselectivities of ambident nucleophiles are usually
rationalized by the principle of hard and soft acids and bases
(HSAB)1 or, more quantitatively, by the Klopman-Salem
concept of charge and orbital controlled reactions.2
M+
RX
condition
sulfone (%) ester (%) ref.
Na+
MeI
MeI
DMF, 25 °C
MeOH, reflux
MeCN, rt
THF, 20 °C
Benzene, reflux
CH2Cl2, rt
93
98
95
93
95
0
7
2
0
0
3a
3a
3b
3c
3d
7
Using these concepts, the predominant S-methylation of the
sulfinate ion 1 by the soft electrophile iodomethane3 has been
explained by the greater softness of the sulfur center.4-6 The
exclusive O-ethylation of 1 by the triethyloxonium ion, on the
other hand, has been interpreted as the attack of the hard
oxonium ion at the hard oxygen center (Table 1).7
(nBu)4N+
a
Poly-N+(Me)3 MeI
Na+
Et3O+BF4
95
-
a Supported on Amberlyst A-26.
On the other hand, examples have been reported that do not
follow these concepts. Thus, S-methylation of arylsulfinate ions
was observed when the silver salt of p-tolylsulfinic acid was
treated with iodomethane, though silver ions have been claimed
to shift methylations with MeI toward the SN1 mechanism.3a,8
When sodium phenylsulfinate was combined with diphenylcy-
clopropenylium tetrafluoroborate, also S-attack took place,9 in
spite of the fact that carbenium ions are generally considered
to be hard electrophiles (Scheme 1).1
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In previous work, we have demonstrated that the ambident
-
nucleophiles NCS-, NC-, NO2 , and NCO-, prototypes for the
application of the HSAB model or the Klopman-Salem concept,
9
4796 J. AM. CHEM. SOC. 2010, 132, 4796–4805
10.1021/ja9102056  2010 American Chemical Society

Nucleophilicity and Nucleofugality of Phenylsulfinate
A R T I C L E S
Scheme 1. Reactions of Arylsulfinate Ion 1 with Hard Electrophiles
at the Sulfur Center
Scheme 2. Reactivity of the Phenylsulfinate Ion (1) toward
Benzhydrylium Ions Determined by Laser-Flash Photolytic
Techniques (refs 12 and 13)
do not react as predicted by these models.10 Because these
concepts have also been employed to rationalize the ambident
reactivity of the phenylsulfinate ion 1,11 we have now investi-
-
gated the reactivity of PhSO2 (1) toward benzhydrylium ions
with 6.5-ns laser pulses (266 nm, 40-60 mJ) to give benzhy-
drylium ions (Table 2), which were identified by their UV-vis
spectra.
As specified in Table 2, differently substituted phosphines
PR3 have been used as photoleaving groups. Whereas the nature
of PR3 is not crucial for the use of phosphonium ions from
highly electrophilic benzhydrylium ions, the situation is different
for phosphonium ions from highly stabilized benzhydrylium
(Ar2CH+) of variable electrophilicity. We observed a change
of regioselectivities in this reaction series, which is in disagree-
ment with the HSAB model, and we will discuss consequences
of these observations for rationalizing the ambident reactivity
of PhSO2 in general. Furthermore, we will characterize the
leaving group ability of the phenylsulfonyl group, which is
fundamental for understanding the chemistry of sulfones.
-
ions. Because phosphonium salts from Ph3P and strongly donor-
Results
+
substituted benzhydrylium ions (for example (jul)2CH-PPh3
)
easily dissociate into Ar2CH+ and PPh3, it is not possible to
prepare solutions of such phosphonium ions in the presence of
PhSO2-. In such solutions, benzhydryl phenyl sulfones are
formed rapidly, which do not yield benzhydrylium ions upon
irradiation with 266 nm laser pulses. We have, therefore, used
Rates for the reactions of the phenylsulfinate ion 1 with
benzhydrylium ions were determined either by studying the
kinetics of laser-flash photolytically generated benzhydrylium
ions or by stopped-flow techniques using stable benzhydrylium
tetrafluoroborates.
Kinetic Studies with Photolytically Generated Ar2CH+. For
the study of fast reactions, the method described in Scheme 2
was employed. Solutions of the benzhydrylphosphonium tet-
rafluoroborates (Ar2CH-PR3+BF4-) in CH3CN were irradiated
+
the more stable tributylphosphonium ions Ar2CH-PBu3 as
precursors for the photolytic generation of highly stabilized
benzhydrylium ions.
When the irradiation of the benzhydrylphosphonium ions was
performed in the presence of PhSO2Na (1-Na) in CH3CN
(containing 15-crown-5), the generated benzhydrylium ions
reacted with the phenyl sulfinate ion. As 1 was used in high
excess, the absorbances of the benzhydrylium ions decrease
monoexponentially (Figure 1), and the pseudofirst-order rate
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Zuo, W.; Zhao, L.-B.; Ren, Z.-H.; Liang, Y.-M. Synthesis 2007, 1465–
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Kielbasinski, P.; Zurawinski, R.; Drabowicz, J.; Mikolajczyk, M.
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Kondratenko, N. V.; Sambur, V. P.; Yagupol’skii, L. M. Z. Org. Khim.
1971, 7, 2382–2388. (f) Oda, M.; Kajioka, T.; Uchiyama, T.; Nagara,
K.; Okujima, T.; Ito, S.; Morita, N.; Sato, T.; Miyatake, R.; Kuroda,
S. Tetrahedron 1999, 55, 6081–6096. (g) Grossert, J. S.; Dubey, P. K.;
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Gais, H.-J.; Jagusch, T.; Spalthoff, N.; Gerhards, F.; Frank, M.; Raabe,
G. Chem.sEur. J. 2003, 9, 4202–4211. (b) Jegelka, M.; Plietker, B.
Org. Lett. 2009, 11, 3462–3465. (c) Watanabe, S.; Kurosawa, H.
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Lett. 2002, 4, 4423–4425. (e) Yokogi, M.; Kuwano, R. Tetrahedron
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K. Annu. ReV. Phys. Chem. 1987, 38, 253–270.
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E J. Org. Chem. 1989, 54, 4691–4692. (b) Grushin, V. V.; Kantor,
M. M.; Tolstaya, T. P.; Shcherbina, T. M. Russ. Chem. Bull. 1984,
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Werimont, G. J. Org. Chem. 1969, 34, 2083–2090.
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9
J. AM. CHEM. SOC. VOL. 132, NO. 13, 2010 4797

A R T I C L E S
Baidya et al.
Table 2. Electrophilicity Parameters E of the Benzhydrylium Ions (Ar2CH+) 2 and the Quinone Methide (QM) 3 and Second-Order Rate
-
Constants (k) for their Combination Reactions with PhSO2 (1) in CH3CN at 20 °C
b
a Empirical electrophilicity parameter from ref 14b, c. Photoleaving group for the photolytic generation of Ar2CH+ from Ar2CH-PR3+BF4
.
-
constants kobs were obtained by fitting the decays of the
equipment described previously.14,15 Phenylsulfinate ion 1 was
absorbances to the monoexponential function At ) A0 e-k t + C.
generally used in high excess over Ar2CH+BF4 to achieve
-
obs
From the slopes of the linear plots of kobs vs [1] (Figure 1) the
second-order rate constants k (L mol-1 s-1) were derived and
are listed in Table 2. The intercepts of the kobs vs [1] plots reflect
the background reaction. They are small for highly stabilized
benzhydrylium ions and increase with increasing electrophi-
licities of the benzhydrylium ions.
Kinetic Studies with Persistent Benzhydrylium Salts
Ar2CH+BF4-. Rate constants k e 106 L mol-1 s-1 have been
determined by mixing solutions of Ar2CH+BF4- and PhSO2Na
in a stopped-flow apparatus and following the decay of the
electrophiles’ absorbances spectrophotometrically using the
(14) (a) Mayr, H.; Patz, M. Angew. Chem., Int. Ed. Engl. 1994, 33, 938–
957. (b) Mayr, H.; Bug, T.; Gotta, M. F.; Hering, N.; Irrgang, B.;
Janker, B.; Kempf, B.; Loos, R.; Ofial, A. R.; Remennikov, G.;
Schimmel, H. J. Am. Chem. Soc. 2001, 123, 9500–9512. (c) Lucius,
R.; Loos, R.; Mayr, H. Angew. Chem., Int. Ed. 2002, 41, 91–95. (d)
Mayr, H.; Ofial, A. R. In Carbocation Chemistry; Olah, G. A.,
Prakash, G. K. S., Eds.; Wiley: Hoboken, NJ, 2004; Chapter 13, pp
331-358.
(15) (a) Mayr, H.; Kempf, B.; Ofial, A. R. Acc. Chem. Res. 2003, 36, 66–
77. (b) Mayr, H.; Ofial, A. R. Pure Appl. Chem. 2005, 77, 1807–
1821. (c) Mayr, H.; Ofial, A. R. J. Phys. Org. Chem. 2008, 21, 584–
595.
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4798 J. AM. CHEM. SOC. VOL. 132, NO. 13, 2010

Nucleophilicity and Nucleofugality of Phenylsulfinate
A R T I C L E S
Table 4. Benzhydrylations of PhSO2Na (1-Na) in DMSO or in
DMSO-d6 at Room Temperature (Details of the Individual
Experiments are Given in the Supporting Information)
δH [ppm]
4b
entry Ar2CH-X
R1
R2
time
4:5
5c
a
1
2
3
4
5
6
7
8
2t-BF4 (lil)2CH+
1 min
3 h
NMe2 1 min
3 h
20 min
OMe 1 min
1 min
0:100
0:100
5.51
Figure 1. Exponential decay of the absorbance A at 455 nm and linear
correlation of the pseudofirst-order rate constants kobs vs [1] for the reaction
of (ani)(Ph)CH+ with 1 in CH3CN at 20 °C.
2o-BF4 NMe2
0:100
5.66
0:100d
0:100e
a
2m-BF4 (mor)2CH+
5.43
5.95
2g-Cl
2g-Br
2g-Cl
or -Br
2f-Cl
OMe
OMe
OMe
46:54 6.41
Table 3. Second-Order Rate Constants (k) for the Reactions of
-
33:67j
PhSO2 (1) with the Benzhydrylium Ions (Ar2CH+) and the
1 h
0:100f
Quinone Methide (QM) at 20 °C
56:44 6.46, 6.47i 6.04
solvent
N, sa
Ar2CH+
k [L mol-1 s-1
]
9
OPh 1 min
1 min
10 2f-Br
11 2f-Cl
43:57j
0:100
DMSO
19.60, 0.60
(ind)2CH+
(jul)2CH+
(lil)2CH+
2.43 × 106
1.61 × 106
6.85 × 105
10 h
or -Br
63:37 6.41, 6.42i 5.96
b
12 2e-Cl
13 2e-Br
14 2e-Cl
Me
H
1 min
1 min
1 d
7.38 × 105
2.57 × 104
51:49j
0:100
QM
CH3CN
20.11, 0.59
13.75, 0.68
Ar2CH+
see Table 2
3.48 × 104
1.46 × 104
6.16 × 103
2.27 × 103
50W50ANd
(dma)2CH+
(pyr)2CH+
(thq)2CH+
(ind)2CH+
(dma)2CH+
(pyr)2CH+
(thq)2CH+
(ind)2CH+
or -Br
15 2d-Cl OMe
16 2d-Br
17 2d-Cl
1 min
1 min
12 d
67:33g 6.46, 6.47i 6.03
56:44j
0:100
c
or -Br
18 2b-Cl Me
19 2b-Br
14.09, 0.66e
4.41 × 104
c
Me
H
1 min
1 min
67:33g 6.42
67:33j
5.99
6.06
1.86 × 104
c
8.24 × 103
c
20 2a-Cl
H
4 h/70 °C 0:100h
3.09 × 103
a For structure, see Table 2. b δH (DMSO-d6) for Ar2CH-OS(O)Ph.
c δH (DMSO-d6) for Ar2CH-SO2Ph; 5m in CD3CN. d Isolated yield of
5o: 49%. e Isolated yield of 5m: 32%; reaction in CH3CN/15-crown-5.
f Isolated yield of 5g: 86%. g This ratio is approximate because of partial
overlap of the proton of benzhydryl chloride (Ar2CH-Cl). h Isolated
yield of 5a: 82%. i A mixture of diastereomers (1:1) was formed. j In
DMSO-d6/CD3CN (≈1:1).
a Nucleophile-specific parameters N and s according to eq 3; for
determination see below. b In the presence of 1.06 equiv of 15-crown-5.
c Rate constants at 25 °C. d 50W50AN
) 50% water and 50%
acetonitrile (v/v). e N and s parameters correspond to 25 °C.
pseudofirst-order conditions. As mentioned for the laser-flash
-
workup were unsuccessful because of its rapid heterolysis leading
experiments, the plots of kobs versus the concentration of PhSO2
to the regeneration of (lil)2CH+ and PhSO2-.
were linear, with the second-order rate constants (Table 3) being
the slopes of the correlation lines. While in DMSO all
benzhydrylium ions react quantitatively with PhSO2-, in 50%
aqueous acetonitrile (50W50AN) only highly electrophilic
benzhydrylium ions were found to react with PhSO2-. The
corresponding reactions with less electrophilic benzhydrylium
ions (E < -8) are reversible, and in some cases the degree of
ion combination was so small that rate constants for the
cation-anion combinations could not be measured. To compare
the reactivities in 50W50AN with other kinetic data, some of
which refer to 20 °C and others to 25 °C, kinetic studies in this
solvent were performed at both temperatures.
Because benzhydrylium tetrafluoroborates of electrophi-
licity E > -7 (Table 2) react with DMSO, it was not possible
to study the reactions of (mor)2CH+ and of alkoxy-substituted
benzhydrylium ions by combining the corresponding tet-
rafluoroborates Ar2CH+BF4 with PhSO2Na in DMSO solu-
-
tion. However, (mor)2CH-SO2Ph was formed and isolated
-
when (mor)2CH+BF4 was reacted with PhSO2Na/15-crown-5
in CH3CN (Table 4). Product studies with methoxy- and methyl-
substituted benzhydryl derivatives were carried out by treating
the corresponding benzhydryl bromides and chlorides (Ar2CH-
Hal) with PhSO2Na in DMSO.
As shown in Table 4, amino substituted benzhydrylium ions
(entries 1-5) on the one hand, and methoxy- and methyl-
substituted substituted benzhydryl derivatives (entries 6-17) on
the other, showed a different pattern of reactivity. While the
NMR spectra measured just after mixing (∼1 min) of PhSO2Na
Product Studies. Addition of PhSO2Na to the blue solutions of
(lil)2CH+BF4- or (dma)2CH+BF4- in DMSO-d6 leads to decolori-
zation, and the NMR spectra revealed the exclusive formation of
the sulfones (lil)2CH-SO2Ph and (dma)2CH-SO2Ph, respectively
(Table 4). The crystal structure of (dma)2CH-SO2Ph unambigu-
ously demonstrates the S-attack of the carbocation on the phenyl-
sulfinate ion.16 Attempts to isolate (lil)2CH-SO2Ph by aqueous
-
-
with 1 equiv of (lil)2CH+BF4 or (dma)2CH+BF4 in DMSO-
d6 were identical to those observed after several hours (entries
1-4), changes of the NMR spectra with time were observed,
when more electrophilic benzhydrylium ions were combined
(16) Baidya, M.; Mayr, H.; Mayer, P. Acta Crystallogr., Sec. E: Struct.
Rep. Online 2009, 65, o3035.
1
with PhSO2Na (entries 6-17). Thus, the H NMR spectrum
9
J. AM. CHEM. SOC. VOL. 132, NO. 13, 2010 4799

A R T I C L E S
Baidya et al.
Figure 2. 1H NMR spectra taken 1 min (top) and 1 h (bottom) after combining PhSO2Na with (ani)2CHCl (2g-Cl) in DMSO-d6.
observed immediately after mixing equimolar amounts of 2g-
Cl or 2g-Br and PhSO2Na in DMSO-d6 showed signals of two
compounds, which were assigned to the sulfinate ester
(ani)2CH-OS(O)Ph (4g) and the sulfone (ani)2CH-SO2Ph (5g,
Figure 2, Table 4, entries 6 and 7). The spectrum taken after
1 h showed the complete conversion of 4g into sulfone 5g
(Figure 2, Table 4, entry 8). Side products were not observed
in any of the reactions described in Table 4.
Under the same conditions, the combination of the unsym-
metrical benzhydryl chloride 2f-Cl with PhSO2Na gave a 28:
28: 44 mixture of the two diastereomers 4f and 4f′ and the
sulfone 5f (Table 4, entry 9). Page S11 in the Supporting
Information shows two signals at δ ) 6.46 and δ ) 6.47 ppm
for the benzhydrylium protons of the diastereomeric sulfinates
4f and 4f′ and the signal at δ ) 6.04 ppm for the benzhydrylium
proton of the sulfone 5f. In addition, three methoxy signals were
observed, two for the diastereomeric sulfinates (δ ) 3.75 and
3.72 ppm), and one for the sulfone at δ ) 3.71 ppm. The
assignment of these signals to the products of O-attack (4f, 4f′)
and S-attack (5f) is corroborated by the 13C NMR signals for
the benzhydryl carbons of 4f and 4f′ at δ ) 79.3 and 79.6 ppm
and one signal for the benzhydryl carbon of the sulfone 5f (δ
) 71.7 ppm). After 10 h at room temperature, only the signals
for sulfone 5f remained (Table 4, and entry 11). Similar
phenomena have been observed for 2f-Br and for the unsym-
metrical benzhydryl systems 2d-Cl, Br and 2e-Cl, Br (Table 4,
entries 12-17 and Supporting Information pp S12-S27). The
sulfones 5d-g, which were formed after extended reaction times
were stable and could be purified by column chromatography.
The dimethyl substituted benzhydryl halides 2b-Cl, Br gave 2:
1 mixtures of 4b and 5b. The rearrangement 4b f 5b was very
slow at room temperature, and after 10 days the NMR spectrum
showed the presence of 4b, 5b, and nonidentified decomposition
products. The parent benzhydryl chloride 2a-Cl did not react
with 1 at room temperature in DMSO. Heating at 70 °C for 4 h
gave 82% of the sulfone 5a, the structure of which was
confirmed by X-ray analysis.17 Details are given in the Sup-
porting Information.
Figure 3. NMR spectroscopic determination of the rate of rearrangement
of (ani)2CH-OS(O)Ph (4g) into (ani)2CH-SO2Ph (5g) in DMSO at 20 °C.
Kinetics of the Sulfinate-Sulfone Rearrangement. As shown
in the preceding section, the reaction of (ani)2CHCl with
PhSO2Na in DMSO yields a 46:54 mixture of the sulfinate 4g
and the sulfone 5g, which was eventually converted into pure
sulfone 5g. From the NMR integrals of the benzhydrylium
protons one can derive the rate of the isomerization 4g f 5g
(Figure 3). The first-order rate constant derived from the decay
of the benzhydrylium proton of 4g (k ) 8.24 × 10-4 s-1) agreed
within experimental error with that derived from the growth of
the benzhydrylium proton of 5g (k ) 8.09 × 10-4 s-1).
When this rearrangement was studied in the presence of
variable concentrations of PhSO2Na in DMSO-d6, the observed
rate constants k were found to be independent of the concentra-
tion of PhSO2Na (Supporting Information p S51). This observa-
tion excludes that the rearrangement results from a bimolecular
process, that is, backside attack of the S-atom of 1 at the
benzhydryl carbon of 4g and suggests an ionization recombina-
tion mechanism, in agreement with earlier suggestions.18
(18) For rearrangement of sulfinate esters see: (a) Drawish, D.; McLaren,
R. Tetrahedron Lett. 1962, 3, 1231–1237. (b) Darwish, D.; Preston,
E. A. Tetrahedron Lett. 1964, 5, 113–118. (c) Arcus, C. L.; Balfe,
M. P.; Kenyon, J. J. Chem. Soc. 1938, 485–493. (d) Wragg, A. H.;
McFadyen, J. S.; Stevens, T. S. J. Chem. Soc. 1958, 3603–3605. (e)
Cope, A. C.; Morrison, D. E.; Field, L. J. Am. Chem. Soc. 1950, 72,
59–67. (f) Ciuffarin, E.; Isola, M.; Fava, A. J. Am. Chem. Soc. 1968,
90, 3594–3595.
(17) Baidya, M.; Mayr, H.; Mayer, P. Acta Crystallogr., Sec. E: Struct.
Rep. Online 2009, 65, o3224.
9
4800 J. AM. CHEM. SOC. VOL. 132, NO. 13, 2010

Nucleophilicity and Nucleofugality of Phenylsulfinate
A R T I C L E S
Scheme 3. Heterolysis of Ar2CH-SO2Ph
Heterolyses of Benzhydryl Phenyl Sulfones. As mentioned
above, the highly stabilized benzhydrylium ions (lil)2CH+ and
(jul)2CH+ combine with PhSO2 in anhydrous CH3CN and
-
DMSO to give sulfones almost quantitatively, while no reaction
takes place in 50% aqueous acetonitrile. This observation
suggested the possibility of determining the rates of the
heterolysis reactions of benzhydryl phenyl sulfones (Scheme
3).19
For that purpose, colorless solutions of benzhydryl phenyl
sulfones were generated by combining benzhydrylium tetrafluo-
roborates (E < -8.0) with 4 to 40 equivalents of PhSO2Na in
CH3CN in the presence of 15-crown-5. When these solutions
were mixed with pure distilled water or aqueous acetonitrile,
the blue color of benzhydrylium ions appeared due to heterolysis
of the sulfones (Scheme 3). Monitoring this process by UV-vis
spectroscopy with a stopped-flow instrument showed that the
increase of the benzhydrylium absorbances generally followed
single exponentials (eq 1, Figure 4) from which the rate
constants kobs were derived.
Figure 5. Dependence of the observed rate constants (kobs) for the ionization
of (lil)2CH-SO2Ph and (thq)2CH-SO2Ph on the concentration of PhSO2Na
(1) in 50W50AN at 25 °C.
Table 5. Ionization Rate Constants k-1 of Benzhydryl Phenyl
Sulfones in Different Solvents at 25 °C
k-1 [s-1
]
Ar2CH-SO2Ph
50W50AN
40W60ANa
20W80ANb
(lil)2CH-SO2Ph
2.51
1.54c
10.9
1.14
2.97
1.67
1.04c
6.02
0.77
0.49c
(jul)2CH-SO2Ph
(ind)2CH-SO2Ph
(thq)2CH-SO2Ph
a 40W60AN ) 40% water and 60% acetonitrile (v/v). b 20W80AN )
20% water and 80% acetonitrile (v/v). c At 20 °C.
t
[Ar2CH+] ) [Ar2CH+]eq(1 - e-k
)
(1)
obs
The end absorbances A∞ obtained from (lil)2CH-SO2Ph and
(jul)2CH-SO2Ph in 50W50AN and 40W60AN decreased
slightly when the heterolyses were performed in the presence
of increasing concentrations of phenylsulfinate ions; concomi-
tantly the first-order rate constants kobs increased slightly (Figure
5, lower line). These effects were more pronounced when
(ind)2CH-SO2Ph and (thq)2CH-SO2Ph were exposed to aque-
ous acetonitrile under the same conditions (Figure 5, upper line)
indicating reversible ionization processes. As the observed rate
constants (kobs) for reversible reactions are the sum of forward
(k-1) and backward (k) reactions (eq 2),20 the ionization rate
constants k-1 (Table 5) were obtained as the intercepts of the
plots of kobs versus the concentrations of PhSO2Na. Table 5
shows that the ionization rate constants k-1 are generally higher
in 50W50AN than in 40W60AN solvent mixtures.
rate constants for the ion combinations. For the reactions of
(ind)2CH+ and (thq)2CH+ with PhSO2 in 50W50AN, these
-
second-order rate constants have independently been determined
from the decay of the benzhydrylium absorbances at high
-
concentrations of PhSO2 (Table 3). The observation that the
rate constants for the ion combinations, which are derived from
the heterolysis reactions (e.g., upper line in Figure 5), are
1.7-1.8 times smaller than the directly measured ones (Table
3) is probably due to the fact that the term k[PhSO2-] in eq 2
is small compared to k-1 and, therefore, does not give access
to reliable rate constants for the reverse reactions. From the
linear correlation between kobs and [PhSO2-] (Figure 1 and
Supporting Information pp S43-S47), we can exclude that this
discrepancy is due to variable ion strength.
-
kobs ) k-1 + k[PhSO2
]
(2)
Discussion
According to formal kinetics20 the slopes of the plots of kobs
vs [PhSO2-] of the heterolysis reactions (Figure 5) reflect the
second-order rate constants of the reverse reactions, that is, the
Nucleophilicity of Phenyl Sulfinate. In previous work, we have
shown that the reactions of carbocations with nucleophiles can
be described by eq 3, where k is a second-order rate constant,
E is a nucleophile-independent electrophilicity parameter, and
N and s are electrophile-independent nucleophile-specific
parameters.14,15
-
(19) Reactions involving PhSO2 as a leaving group: (a) Koutek, B.;
Pavlickova, L.; Soucek, M. Synth. Commun. 1976, 6, 305–308. (b)
Ito, S.; Tanaka, Y.; Kakehi, A.; Kondo, K.-I. Bull. Chem. Soc. Jpn.
1976, 49, 1920–1923. (c) Das, B.; Damodar, K.; Bhunia, N. J. Org.
Chem. 2009, 74, 5607–5609. (d) Rossen, K.; Jakubec, P.; Kiesel, M.;
Janik, M. Tetrahedron Lett. 2005, 46, 1819–1821. (e) Chang, Y.-F.;
Jiang, Y.-R.; Cheng, W.-C. Tetrahedron Lett. 2008, 49, 543–547. (f)
Ghera, E.; Yechezkel, T.; Hassner, A. J. Org. Chem. 1990, 55, 5977–
5982. (g) Nozaki, H.; Yamamoto, Y.; Nisimura, T. Tetrahedron Lett.
1968, 9, 4625–4626. (h) Richter, D.; Hampel, N.; Singer, T.; Ofial,
A. R; Mayr, H. Eur. J. Org. Chem. 2009, 3203–3211.
(20) (a) Maskill, H. The InVestigation of Organic Reactions and Their
Mechanisms; Blackwell Publishing: Oxford, 2006. (b) Schmid, R.;
Sapunov, V. N. Non-Formal Kinetics; Verlag Chemie: Weinheim,
1982.
Figure 4. Increase of absorbance at 632 nm during the heterolysis of
(lil)2CH-SO2Ph in 50W50AN at 25 °C.
9
J. AM. CHEM. SOC. VOL. 132, NO. 13, 2010 4801

A R T I C L E S
Baidya et al.
log k20°C ) s(N + E)
(3)
To determine the parameters N and s for phenyl sulfinate 1,
the second-order rate constants of their reactions with benzhy-
drylium ions have been plotted against the corresponding
electrophilicity parameters E (Figure 6).
Figure 6 shows that the reactions of the carbocations 2 with
1 generally follow eq 3 in the activation controlled region.
However, when highly reactive carbocations are used, the
diffusion-controlled plateau is reached (k ≈ 1010 L mol-1 s-1).
From the linear increase of log k with E, one can derive the
parameters N (from the intercepts on the abscissa) and s (from
the slopes) for the phenylsulfinate ion in different solvents (Table
3). The rate constants in acetonitrile and DMSO are almost
identical. In 50% aqueous acetontrile (v/v), phenylsulfinate is
approximately 103 times less reactive.
Can the product studies reported in Table 4 be employed to
partition the gross rate constants given in Tables 2 and 3 into
partial rate constants for S- and O-attack? This is possible for
Figure 6. Plots of log k for the reactions of phenyl sulfinate 1 with
benzhydrylium ions and a quinone methide (Tables 2, 3) versus their
electrophilicity parameters E (Table 2) in different solvents at 20 °C.
-
the reactions of PhSO2 with the benzhydrylium ions 2f and
2g, because previous investigations indicate that 2f-Br and 2g-
Br react via SN1 mechanisms.21a Table 4 unequivocally shows
that the [5]/[4] ratios observed with these carbocations after short
reaction times reflect kinetic product control. However, these
reactions proceed with diffusion rates, and the observed product
ratios do not give information about the relative reactivity of
sulfur and oxygen in activation controlled reactions.
The change of the ks/ko ratio from 1.3 for 2f (Table 4, entry
10) to 2.0 for 2g (Table 4, entry 7) shows that the percentage
of kinetically controlled S-attack increases further as one moves
to better stabilized benzhydrylium ions. Even if the ratio ks/ko
would not increase beyond 2.0 (as determined for 2g, entry 7)
for less electrophilic carbocations, one can derive that the gross
nucleophilicity N for 1 (sum of S- and O-attack) which is given
in Table 3 had only to be reduced by ∆N ) 0.35 to obtain the
21b If the ks/
-
S-specific nucleophilicity parameter N for PhSO2
.
ko ratio increases further, the correction term will be even
smaller. While the O-nucleophilicity of PhSO2- cannot precisely
be assigned, the formation of comparable amounts of 4g and
5g from 2g and PhSO2- indicates that the O-nucleophilicity of
PhSO2- cannot be much lower than its S-nucleophilicity. If one
assumes similar values of s for both sites of attack, the ratio
[5g]/[4g] ) 2.0 can only be reached if the N value for O-attack
is not more than approximately 4 logarithmic units smaller than
that for S-attack (see Figure 6).
Figure 7 compares the nucleophilicity of phenylsulfinate 1
with that of other nucleophiles. One can see that its reactivity
in DMSO and CH3CN is comparable to that of stabilized
carbanions (e.g., malonate), azide, and DABCO. It is about 3
orders of magnitude more nucleophilic than acetate, benzoate,
and CN- (all in CH3CN). Hydration has a much larger effect
on the nucleophilicity of PhSO2- than on the reactivity of ester-
stabilized carbanions. While the change from DMSO to water
reduces the N value of malonate by 4 units, the nucleophilicity
Figure 7. Comparison of the nucleophilic reactivity of the phenyl sulfinate
ion (1) with other types of nucleophiles (N values from refs 10a, b, 13, 22,
and 23).
acetonitrile, a fact which will be taken up again in the discussion
of leaving group abilities.
-
of PhSO2 is reduced by ∆N ) 6.4 when 50% water is added
Nucleofugality of Phenyl Sulfinate. Since the rate of the
rearrangement of (ani)2CH-OS(O)Ph into the corresponding
sulfone does not depend on the concentration of phenyl sulfinate
to the solution in acetonitrile. Figure 7 shows that the nucleo-
philicities of PhSO2- and Br- are almost equal in 50% aqueous
(21) (a) Phan, T. B.; Nolte, C.; Kobayashi, S.; Ofial, A. R.; Mayr, H. J. Am.
Chem. Soc. 2009, 131, 11392–11401. (b) ∆N ) 1/(s log ([5g]/([5g]
+ [4g])).
(22) Schaller, H. F.; Tishkov, A. A.; Feng, X.; Mayr, H. J. Am. Chem.
Soc. 2008, 130, 3012–3022.
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4802 J. AM. CHEM. SOC. VOL. 132, NO. 13, 2010

Nucleophilicity and Nucleofugality of Phenylsulfinate
A R T I C L E S
Figure 8. Correlation of ionization rate constants (k-1) for (lil)2CH-SO2Ph
with solvent ionizing power Y.
ions 1 in the solution, one can conclude that the rate constants
derived from Figure 3 correspond to the ionization step (C-O
cleavage). The increasing times for the rearrangement 4 f 5
with decreasing stabilization of the carbocations (2g f 2d, Table
4) are in line with this interpretation.
Figure 9. Plots of heterolysis rate constants (log k-1) for Ar2CH-SO2Ph
at 25 °C versus the electrophilicity parameters E14,15 of Ar2CH+.
The heterolyses of the corresponding sulfones (C-S cleavage)
are considerably slower and can only be observed in aqueous
solvents of high ionizing power. The Winstein-Grunwald
equation (eq 4) correlates logarithms of solvolysis rate constants
(ks) with the solvent ionizing power Y in which ks and k0 refer
to solvolysis rate constants for a substrate R-X in a given
solvent and in an 80% ethanol/water mixture, respectively.24
Because m values had been found to be close to 1 for most SN1
reactions (tBuCl: m ) 1.00) and <0.5 for SN2 reactions, m values
had been suggested as a tool for discriminating between SN1
and SN2 processes.24,25
Figure 10. Correlations of the ionization rate constants of benzhydryl
sulfones (Ar2CH-SO2Ph) versus the ionization rate constants of benzhydryl
benzoates (Ar2CH-OBz in 40W60AN, from ref 22).
log(ks/k0) ) mY
(4)
and (lil)2CH+, the weakest electrophile of this series, is far from
being the best electrofuge (Table 5, Figure 9).
Figure 8 shows a linear correlation between the ionization
rate constants k-1 for (lil)2CH-SO2Ph (Table 5) with the solvent
ionizing power Y.26 The small slopes of these correlations (m
) 0.30-0.35) which undoubtedly refer to rate-limiting ioniza-
tion processes indicate noncarbocation like transition states.
m-Values of similar magnitude have previously been observed
for the ionizations of amino-substituted benzhydryl carboxylates
in aqueous acetone.22
The consistency of the electrofugalities given in Table 5 and
depicted in Figure 9 is confirmed by the fact that the ionization
rate constants k-1 for sulfones (Table 5) correlate well with the
previously reported ionization rate constants for benzoates
(Ar2CH-OBz) in 40W60AN (Figure 10).22 The electrofugality
order of the benzhydrylium ions, therefore, differs systematically
from the electrophilicity order, but appears to be independent
of the nature of the nucleofuge. Because the heterolysis rate
constants for the benzhydryl phenyl sulfones and the benzhydryl
carboxylates have been studied with the same benzhydrylium
systems in the same solvent (40W60AN), a direct comparison
of the heterolysis rate constants is possible. The leaving group
ability of the phenylsulfonyl group (PhSO2-, C-S cleavage) is
7-11 times lower than that of benzoate (PhCO2-) and 4-7
times lower than that of acetate (CH3CO2-).22
It is remarkable that the heterolysis rate constants of the
benzhydryl phenyl sulfones do not show the inverse order of
the electrophilicities of the corresponding benzhydrylium ions
(Figure 9). More than a hundred reaction series, including the
reactions of benzhydrylium ions with phenyl sulfinate reported
in this work (Tables 2 and 3) have shown the electrophilicity
order (thq)2CH+ > (ind)2CH+ > (jul)2CH+ > (lil)2CH+.14,15 In
contrast, the order of electrofugalities is completely different,
Solvolysis rates of benzhydryl bromides and chlorides have
previously been investigated in 40W60AN.27a From the first-
order rate constants of (tol)2CHCl (2b-Cl, 2.76 s-1) and Ph2CH-
Br (2a-Br, 0.14 s-1) and structurally related benzhydryl
halides27b one can extrapolate that the phenylsulfonyl group is
a 1010 times weaker leaving group than Br- and a 108 times
weaker leaving group than Cl- in 40W60AN. On other hand,
(23) Reactivities for carbanions: (a) Lucius, R.; Loos, R.; Mayr, H. Angew.
Chem., Int. Ed. 2002, 41, 91–95. (b) Bug, T.; Mayr, H. J. Am. Chem.
Soc. 2003, 125, 12980–12986. (c) Appel, R.; Loos, R.; Mayr, H. J. Am.
Chem. Soc. 2009, 131, 704–714. For N3-: (d) Phan, T. B.; Mayr, H.
J. Phys. Org. Chem. 2006, 19, 706–713. For HOO-: (e) Minegishi,
S.; Mayr, H. J. Am. Chem. Soc. 2003, 125, 286–295. For Cl- and
Br-: (f) Minegishi, S.; Loos, R.; Kobayashi, S.; Mayr, H J. Am. Chem.
Soc. 2005, 127, 2641–2649. For glycine: (g) Brotzel, F.; Mayr, H.
Org. Biomol. Chem. 2007, 5, 3814–3820. For aniline: (h) Brotzel, F.;
Chu, Y. C.; Mayr, H. J. Org. Chem. 2007, 72, 3679–3688.
(24) (a) Grunwald, E.; Winstein, S. J. Am. Chem. Soc. 1948, 70, 846–854.
(b) Fainberg, A. H.; Winstein, S. J. Am. Chem. Soc. 1956, 78, 2770–
2777.
-
Figure 7 has shown that the S-nucleophilicity of PhSO2 is
comparable to that of Br- and Cl- in 50W50AN. This
discrepancy is again due to the high intrinsic barriers of the
S-reactivity of PhSO2- which will be discussed in detail in the
next section.
(25) Bentley, T. W.; Llewellyn, G. Prog. Phys. Org. Chem. 1990, 17, 121–
158.
(26) Bentley, T. W.; Dau-Schmidt, J.-P.; Llewellyn, G.; Mayr, H. J. Org.
Chem. 1992, 57, 2387–2392.
(27) (a) Streidl, N.; Antipova, A.; Mayr, H. J. Org. Chem. 2009, 74, 7328–
7334. (b) Streidl, N.; Nolte, C.; Mayr, H. unpublished.
9
J. AM. CHEM. SOC. VOL. 132, NO. 13, 2010 4803

A R T I C L E S
Baidya et al.
q
Table 6. Equilibrium Constants K, ∆Gq, ∆G0, ∆G0 , and Intrinsic Rate Constants (k0) for the Ionization of Ar2CH-SO2Ph in 50W50AN at
25 °C
d
e
f
g
Ar2CH+
k [M-1s-1
]
k-1 [s-1
]
K
[M-1
]
∆Gq [kJ mol-1
]
∆G0 [kJ mol-1
]
∆G0q [kJ mol-1
]
k0h [s-1
]
a
b
c
c
(lil)2CH+
(jul)2CH+
(ind)2CH+
(thq)2CH+
4.71 × 102
1.15 × 103
3.09 × 103
8.24 × 103
2.51
10.9
1.14
2.97
1.88 × 102
1.06 × 102
2.71 × 103
2.77 × 103
70.7
67.1
72.7
70.3
13.0
11.6
19.6
19.6
64.1
61.2
62.5
60.1
3.6 × 101
1.2 × 102
6.9 × 101
1.8 × 102
a k calculated by eq 3 from E ) -10.04 (Table 2) and N ) 14.09, s ) 0.66 (Table 3). b k calculated by eq 3 from E ) -9.45 (Table 2) and N )
14.09, s ) 0.66 (Table 3). c From Table 3. d From Table 5. e K ) k/k-1 f ∆Gq for the ionization reaction were calculated by using Eyring equation.
g ∆G0 for the ionization process is given by +RT ln K. h Intrinsic rate constants (k0) from Eyring equation.
.
A similar trend in the intrinsic barriers for compounds with
annelated five- and six-membered rings also controls the relative
reactivities of (lil)2CH+ and (jul)2CH+. Though (jul)2CH+ is
thermodynamically more stable than (lil)2CH+, the higher
intrinsic barrier of the 5-membered ring-system (lil)2CH+
accounts for the observation that (lil)2CH+ is less electrophilic
than (jul)2CH+. Analogously, the lower rate of ionization of
(lil)2CH-SO2Ph is due to the higher intrinsic barrier for the
formation of 5-membered ring system (lil)2CH+.
Ambident Reactivity of Phenyl Sulfinate. As shown in Figure
6, the rates of the reactions of benzhydrylium ions with PhSO2
-
Figure 11. Gibbs free energy profiles for the reactions of the phenylsulfinate
ion with amino-substituted benzhydrylium ions in 50W50AN at 25 °C
(standard state 1 M).
proceed with almost equal rates in acetonitrile and DMSO. The
rate constants for the reactions of (ani)2CH+ with PhSO2 in
-
acetonitrile (1.24 × 1010 M-1 s-1, Table 2) can, therefore, be
combined with the product ratio (4: 5 ) 1: 2.0, Table 4, entry
7) to derive the partial rate constants k ) 2.05 × 109 M-1 s-1
Intrinsic Barriers. The availability of rate and equilibrium
constants for several reactions of benzhydrylium ions with
-
PhSO2 allows us to calculate Marcus’ intrinsic barriers.
-
for the reaction of one oxygen of PhSO2 with (ani)2CH+.
According to Marcus’ equation (eq 5), the activation free energy
Combination of this rate constant with the ionization rate
constant of (ani)2CH-OS(O)Ph (k-1 ) 8.16 × 10-4 s-1 from
Figure 3) yields the equilibrium constant K ) k/k-1 ) 2.51 ×
1012 M-1 (20 °C, because the high combination rate constant k
can be assumed to be temperature-independent) which corre-
sponds to ∆G0 (20 °C) ) -69.6 kJ mol-1 for the formation of
4g from 2g and 1. Substitution of these values into the Marcus
(∆Gq) of a reaction can be expressed as a combination of the
28
reaction free energy (∆G0) and the intrinsic barrier (∆G0 ).
q
The latter term has been defined as the activation free energy
(∆Gq) of a process without thermodynamic driving force (i.e.,
for ∆G0 ) 0). The work term has been neglected in eq 5.
∆Gq ) ∆Gq0 + 0.5∆G0 + (∆G0)2/(16∆Gq0)
(5)
equation yields an intrinsic barrier of ∆G0 ) 48 kJ mol-1 for
q
the O-attack at PhSO2- by (ani)2CH+. This value is significantly
lower than ∆G0q for S-attack reported in Table 6. Though part
of this difference is due to the variation of the benzhydrylium
ion (solvent variation was found to have little influence on
Equilibrium constants for the reactions of the benzhydrylium
-
ions 2q-t with PhSO2 in 50W50AN have been calculated as
the ratio of forward/backward reaction, that is, K ) k/k-1. While
k-1 has directly been measured for these four sulfones (Table
5), the rate constants for the combination with PhSO2- (k) have
only been measured for (thq)2CH+ and (ind)2CH+ (Table 3, last
two entries). The high quality of the correlation between the
rate constants for ion combination (k) and the electrophilicity
E (illustrated by the lower line in Figure 6), justifies to employ
eq 3 for calculating the rate constants k for the reaction of
∆G0 ),22 it is obvious that O-attack is intrinsically highly favored
q
over S-attack. Taking into consideration that the combination
rate constant k ) 1.24 × 1010 M-1 s-1 used for the calculation
of ∆G0q for O-attack of 1 at (ani)2CH+ is controlled by transport
and not by activation, the intrinsic barrier ∆G0q ) 48 kJ mol-1
has to be considered as upper limit; as a consequence, the
intrinsic preference for O-attack may even be higher than derived
above. Because the thermodynamic term ∆G0 in the Marcus
equation favors S-attack, we have to conclude that both terms
-
(lil)2CH+ and (jul)2CH+ with PhSO2 as specified in Table 6.
The ionization rate constants (k-1) and the equilibrium constants
(K) at 25 °C were converted into ∆Gq and ∆G0 (Table 6) and
substitution of these values into eq 5 gives the intrinsic barriers
cancel each other to account for the similar S- and O-reactivity
-
of PhSO2
.
q
∆G0 . Table 6 shows the same ranking of intrinsic barriers for
the differently substituted benzhydryl systems (lil)2CH+
>
Conclusion
(ind)2CH+ > (jul)2CH+ > (thq)2CH+ as previously reported for
-
benzhydryl carboxylates.22
Phenylsulfinate PhSO2 is a strong nucleophile with a
reactivity in DMSO comparable to N3- and stabilized carbanions
as diethyl malonate. It undergoes diffusion-controlled reactions
with carbocations of moderate and high reactivity (E > -2),
and the ratio O- vs S-attack of these reactions cannot be
explained by transition state models, because none of these
reactions has an activation barrier. Highly reactive carbocations
(E > 0) attack oxygen and sulfur with similar rates.
The free energy diagrams in Figure 11 summarize the
influence of intrinsic barriers on the different reactivities of these
four systems. The free energy of ionization (∆G0) is the same
for (ind)2CH+ and (thq)2CH+. It is the higher intrinsic barrier
for (ind)2CH+ (annulated five-membered ring) compared to
(thq)2CH+ (annulated six-membered ring), which accounts for
the observation that (thq)2CH+ reacts faster with nucleophiles
than (ind)2CH+, and at the same time is generated faster by the
reverse reaction.
In reactions with less electrophilic carbenium ions, the ∆G0
term, which favors S-attack, is gaining importance. While the
9
4804 J. AM. CHEM. SOC. VOL. 132, NO. 13, 2010

Nucleophilicity and Nucleofugality of Phenylsulfinate
A R T I C L E S
Determination of the Rates of the Combinations of Benz-
exclusive formation of sulfones from highly stabilized carboca-
tions is due to thermodynamic product control, mixtures of
sulfones and sulfinates are formed under conditions of kinetic
control. The ambident reactivity of sulfinate PhSO2- thus is the
result of a complex interplay of intrinsic and thermodynamic
effects. It cannot be explained by the HSAB model or Klopman-
Salem concept of charge and orbital control.
-
hydrylium Ions with PhSO2-. The reactions of PhSO2 with the
colored benzhydrylium ions (Ar2CH+) in DMSO and aqueous
acetonitrile were followed photometrically at the absorption maxima
of Ar2CH+ by UV-vis spectroscopy using stopped-flow instruments
(Hi-Tech SF-61DX2 or Applied Photophysics SX.18MV-R). All
experiments were performed under pseudofirst-order conditions
(excess of PhSO2-) and the pseudofirst-order rate constants kobs were
obtained by least-squares fitting of the absorbances to the monoex-
Experimental Section
ponential function At ) A0 e-k t + C. Temperature of the solutions
obs
Materials. Commercially available acetonitrile (VWR, Prolabo,
HPLC-gradient grade) and DMSO (Acros, >99.8%, extra dry) were
used as received. The benzhydrylium tetrafluoroborates Ar2CH+
during all kinetic studies was kept constant (20 or 25 °C) using a
circulating bath thermostat.
Determination of Ionization Rates. The ionizations reactions
were followed photometrically by UV-vis spectroscopy using a
stopped-flow instrument (Hi-Tech SF-61DX2). The single mixing
14b
-
BF4 were prepared as described before. The benzhydrylphos-
phonium tetrafluoroborates and benzhydryl chlorides were synthe-
sized according to literature procedures.29,30 PhSO2Na (Acros,
>98%) and 15-crown-5 (ABC R, 98%) were purchased and used
directly without further purification.
technique was employed where
a colorless solution of
Ar2CH-SO2Ph obtained by mixing Ar2CH+BF4 with excess
PhSO2Na in CH3CN containing 15-crown-5 was used in one syringe
and the another syringe contained pure distilled water or aqueous
acetonitrile. Mixing of the colorless solution of Ar2CH-SO2Ph in
CH3CN with an equal volume of pure water or aqueous acetonitrile
inside the mixer provokes the ionization of the Ar2CH-SO2Ph. The
rates of the ionizations were followed photometrically at the
absorption maxima of Ar2CH+. Each measurement was repeated
at least five times with variable excess of the PhSO2Na. The
appearance of the benzhydrylium absorbances generally followed
-
Laser-Flash Photolysis. Laser-flash photolysis was employed
for determining the rates of the reactions of Ar2CH+ with PhSO2
-
in acetonitrile at 20 °C. For that purpose, benzhydrylium ions were
generated by irradiation of benzhydrylphosphonium tetrafluorobo-
rates (Ar2CH-PR3+BF4-) in acetonitrile with an Innolas SpitLight
600 Nd:YAG laser (fourth harmonic at λ ) 266 nm; power/pulse
of 40-60 mJ, pulse length ) 6.5 ns) in a quartz cell. The rate
constants were determined by observing the time-dependent decay
of the UV-vis absorptions of the benzhydrylium ions. The
pseudofirst-order rate constants were obtained by fitting the decay
of the UV-vis absorptions to the exponential function At ) A0
the monoexponential equation [Ar2CH+] ) [Ar2CH+]eq (1 - e-k ),
obst
obst
e-k + C.
from which the rate constants kobs were derived. Temperature of
the solutions during heterolysis studies was kept constant (20 or
25 °C) using a circulating bath thermostat.
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Acc. Chem. Res. 1987, 20, 301–308. (d) Bernasconi, C. F.; Terrier, F.
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Acknowledgment. Dedicated to Professor Paul von Rague
Schleyer on the occasion of his 80th birthday. We thank the
Deutsche Forschungsgemeinschaft for financial support (SFB 749)
and Dr. Armin R. Ofial, Konstantin Troshin, and Roland Appel
for discussions.
Supporting Information Available: Details of kinetic experi-
ments and product studies. This material is available free of
charge via the Internet at http://pubs.acs.org.
(29) Kempf, B.; Mayr, H. Chem.sEur. J. 2005, 11, 917–927.
(30) Denegri, B.; Streiter, A.; Juric, S.; Ofial, A. R.; Kronja, O.; Mayr, H.
Chem.-Eur. J. 2006, 12, 1648–1656.
JA9102056
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