Juri ꢀc et al.
JOCNote
SCHEME 1. Affinities of Some Benzhydrylium Cations toward
ED ti hm a ent oh ly l Sulfide at PCM-B3LYP/6-311þG(2d,p) Level in
Experimental Section
General Procedure. Dimethyl sulfide (20.1 mmol) and the
appropriate benzhydryl chloride (7.4 mmol) were added to 20
mL of nitromethane and stirred for 30 min in an ice bath. A
solution of silver triflate (7.4 mmol) in nitromethane was then
added dropwise over a 10 min period. Acetonitrile was added,
and the stirring was continued for 1 h. The solid silver chloride
was removed by filtration, and removal of solvent on rotary
evaporator yielded brown oil, which crystallized in the ice bath.
Filtration and washing with small portions of petroleum and
ether led to 45-73% of the desired products.
4
-Methylbenzhydryldimethylsulfonium triflate (1): mp 58.5-
1
60.5 °C; H NMR (300 MHz, CDCl ) δ = 2.34 (s, 3H, ArCH ),
2.90 [s, 6H, (CH
ArH); C NMR (75 MHz, CDCl ) δ = 20.9 (ArCH ), 24.3
3 3
3
3
It has been shown on several occasions that the slope
þ
3
)
2
S], 6.12 (s, 1H, Ar
2
CH), 7.20-7.55 (m, 9H,
parameter (s ) can be related to the Hammett-Brown F
f
1,12
13
1
parameter.
The s parameters of HFBs are essentially the
f
[
1
(CH
37.5, 141.5 (Ar).
-Fluorobenzhydryldimethylsulfonium triflate (2): mp 94.0-
3 2 2
) S], 65.5 (Ar CH), 125.3, 127.1, 128.5, 128.9, 130.9, 133.4,
same as those obtained with 1-5 in the limits of error. Even
though the reactants’ charge states are different, the values of
4
1
sf may indicate a similar quantity of positive charge trans-
95.5 °C; H NMR (300 MHz, CDCl ) δ = 2.89 [s, 3H, (CH ) S],
2.91 [s, 3H, (CH ) S], 6.23 (s, 1H, Ar CH), 7.14 (t, 2H, J = 8.4
3 2 2 HF
3
3 2
ferred from sulfur in the reactant to benzhydrylium system in
13
the TS of 1-5 as it is developed on the benzhydrylium system
in heterolysis of HFB on the route from the reactant to TS.
To estimate the affinities of benzhydrylium ions toward
Hz, ArH), 7.44-7.68 (m, 7H, ArH); C NMR (75 MHz, CDCl
)
3
δ = 24.3 [(CH S], 64.4 (Ar CH), 126.4, 128.4, 128.6, 129.6, 130.2,
3
)
2
2
130.8, 131.0, 161.7 (Ar).
Benzhydryldimethylsulfonium triflate (3): mp 118.5-121.5 °C;
Me S in ethanol, we carried out quantum chemical calcula-
2
1
1
3
H NMR (300 MHz, CDCl
H, Ar CH), 7.42-7.64 (m, 10H, ArH); C NMR (75 MHz,
CDCl ) δ = 24.0 [(CH S], 65.1 (Ar CH), 127.2, 128.4, 129.8,
32.7 (Ar).
-Chlorobenzhydryldimethylsulfonium triflate (4): mp 111.5-
) δ = 2.88 [s, 6H, (CH ) S], 6.17 (s,
3 3 2
tions using the Gaussian 03 program suite. We applied the
polarizable continuum solvent model (PCM) with dielectric
constant for ethanol and fully optimized the geometries of 3
and 5, as well as the corresponding benzhydrylium ions and
13
1
2
3
3
)
2
2
1
4
1
Me S at PCM-B3LYP/6-311þG(2d,p) level. The results are
2
113.0 °C; H NMR (300 MHz, CDCl ) δ = 2.89 [s, 3H, (CH ) S],
3
3 2
presented in Scheme 1.
2.91 [s, 3H, (CH ) S], 6.22 (s, 1H, Ar CH), 7.27-7.61 (m, 9H,
3
2
2
1
3
Even though benzhydryl heptafluorobutyrates and di-
methylsulfonium salts solvolyze via similar barriers, the
calculated affinities of benzhydrylium ion toward hepta-
ArH); C NMR (75 MHz, CDCl
(Ar
) δ = 24.6 [(CH ) S], 65.1
3
3 2
2
CH), 126.6, 127.4, 128.6, 129.7, 129.9, 130.4, 131.2, 136.5
Ar).
3-Chlorobenzhydryldimethylsulfonium triflate (5): mp 93.5-
(
HFB
-1
fluorobutyrate ions (Eaff
= -75 kJ mol ) are consider-
1
9
5.5 °C; H NMR (300 MHz, CDCl
.97 [s, 3H, (CH S], 6.28 (s, 1H, Ar
3 2
) δ = 24.50 [(CH )
3
) δ = 2.93 [s, 3H, (CH
CH), 7.26-7.63 (m, 9H,
S], 64.5
3 2
) S],
ably higher than toward dimethyl sulfide. The results
indicate that dimethylsulfonium salts solvolyze through an
earlier transition state producing less stable intermediates
than those produced in solvolysis of HFB via later TS.
Finally, it should be discussed whether eq 1 can be applied
2
3
)
2
2
13
ArH); C NMR (75 MHz, CDCl
3
(
1
Ar CH), 118.0, 122.3, 125.7, 127.2, 128.4, 129.6, 130.8, 132.2,
34.9, 135.6 (Ar).
Kinetic Methods. Solvolysis rate constants were measured
2
to estimate the S 1 reactivity of any dimethylsulfonium salt.
N
titrimetrically by means of a TIM 856 titration manager using a
Red Rod combined pH electrode. Typically, 20-50 mg of
substrate was dissolved in 0.10-0.20 mL of dichloromethane
and injected into the solvent that was thermostated at the
required temperature ((0.01 °C). The liberated acid was con-
tinuously titrated at pH = 7.00 by using a 0.016 M solution of
NaOH in the appropriate solvent mixture. Rate constants were
averaged from at least three measurements.
Experimental data of Kevill and Anderson showed that
2 4b
unlike other sulfonium ions (e.g., tert-butyl-, arylmethyl-,
4
a
and methoxybenzyldimethylsulfonium ion, etc.) the solvolytic
reactivity of adamantyldimethylsulfonium ion in the series
of aqueous ethanols and methanols increases very slightly
1
as the water content increases. The authors stated that this
behavior is due to specific solvation caused with the cage
structure. Evidently, eq 1 cannot be applied successfully to
predict the solvolytic reactivity of adamantyldimethylsulfo-
nium ion. Nevertheless, it can be concluded that the reactivity
of a vast number of aryl- or alkyldimethylsulfonium salts can
indeed be semiquantitatively determined using the special
LFER eq 1 and that only substrates with bulk cage structure
might behave differently than the above LFER approach would
predict.
Acknowledgment. We gratefully acknowledge the finan-
cial support of this research by the Ministry of Science,
Education and Sport of the Republic of Croatia (Grant
No.006-0982933-2963)
Supporting Information Available: Correlations of log k vs
E in the series of aqueous methanol, NMR spectra ( H, C and
1
13
f
COSY) of X-substituted benzhydryl dimethylsulfonium tri-
flates, quantum chemical calculation data, and a complete ref
13. This material is available free of charge via the Internet at
http://pubs.acs.org.
(
12) Bentley, T. W. Chem.;Eur. J. 2006, 12, 6514–6520.
(13) Frisch, M. J. et al. Gaussian 03, Revision D.02, Gaussian, Inc.,
Wallingford CT, 2004 (see the Supporting Information for the full citation).
3
854 J. Org. Chem. Vol. 75, No. 11, 2010
Juric, Sandra
