FIGURE 3. A typical waveform collected at 520 nm and a typical
concentration series for triplet benzophenone (10-4 M) being quenched
by 2-(2-butylthioethyl)-1,3-dithiane (1a).
FIGURE 2. B3LYP/6-311+G(2d,p) optimized structures of an oxygen
centered radical of type 4 (left) and a cyclic 1,2-oxathiane radical of
type 5 (right). The structures are nearly energy-degenerate.
Hz), 0.90 (t, 3H, J ) 7.4 Hz). 13C NMR (400 MHz, CDCl3) δ
103.5, 65.1, 34.3, 32.1, 31.9, 26.6, 22.2, 13.9.
not show appreciable acceleration of the initial electron transfer.
The same is true for R-sulfides. The rationale for this behavior
of conformationally flexible ꢀ-sulfides is that their radical cations
are additionally stabilized by the formation of a partial exocyclic
S-S bond. This hypothesis is supported by DFT calculations,
which show that such stabilization can be as significant as 7
kcal/mol.
A diverse set of mass differentiated 2-(2-alkylthioethyl)-1,3-
dithianes is now made readily accessible via a straightforward
modular synthetic approach, which makes them suitable pho-
toactive tags for combinatorial encoding.
2-(2-Butylthioethyl)-1,3-dithiane (1a). From 4.27 g (22.5 mmol)
of 2-(2-butylthioethyl)-1,3-dioxolane, 2.5 mL (24.9 mmol) of 1,3-
propanedithiol, and 12.0 mL (94.6 mmol) of BF3 ·Et2O in CH2Cl2
1
(150 mL); yield 89%. H NMR (400 MHz, CDCl3) δ 4.19 (t, 1H,
J ) 7.0 Hz), 2.94-2.76 (m, 4H), 2.69 (t, 2H, J ) 7.4 Hz), 2.51 (t,
2H, J ) 7.4 Hz), 2.17-2.06 (m, 1H), 2.02 (dt, 2H, J ) 7.0, 7.4
Hz), 1.94-1.80 (m, 1H), 1.56 (quint., 2H, J ) 7.4 Hz), 1.40 (sext.,
2H, J ) 7.4 Hz), 0.91 (t, 3H, J ) 7.32 Hz). 13C NMR (400 MHz,
CDCl3) δ 46.2, 35.4, 31.9, 31.9, 30.4, 29.1, 26.2, 22.2, 13.9. MS
m/z 236 (M+, 100), 179 (25), 146 (35).
Laser flash photolysis experiments were performed with use of
the Applied Photophysics nanosecond LFP system. The third (355
nm) harmonic output from a Q-switched Nd:YAG laser was used
as the excitation source; the light is absorbed by the sensitizer and
not by the dithianes/adducts. The solvent for irradiations was
acetonitrile. The decay of triplet benzophenone (initial concentration
of benzophenone was 10-4 M) as a function of the concentration
of quencher was measured at 520 nm (Figure 3). Every sample
was degassed for 20-35 min by bubbling argon gas through the
solution.
Photochemistry and quantum yield determination were performed
in the Rayonet carousel photoreactor, using RPR-3500 lamps (max
irradiance at λ 350 nm). The samples were irradiated for 1-5, 10,
and 20 min. Quantum yields were determined at low conversions
(5 min of irradiation). A solution of benzophenone (0.1 M) and
benzhydrol (1.0 M) was used as a reference actinometer. The sample
contained the adduct (0.01 M) and benzophenone as a sensitizer
(0.1 M).
Theory and Calculation. DFT computations were carried out at
the B3LYP/6-311+G(2d,p) level. The geometries were fully
optimized and were subjected to vibrational analysis; all energies
are ZPE-corrected. The computations were carried out on a
quadruple processor Dell workstation equipped with 32 GB of RAM
with use of Gaussian 03, Revision C.02.
Experimental Section
General Procedures for Preparation of 2-(2-Alkylthioethyl)-
and 2-(2-Alkoxyethyl)-1,3-dithianes. Step A: Alkanethiol or alkanol
was dissolved in 20-30 mL of THF (for thiols) or DMF (for
alcohols), then NaH (60% in mineral oil, prewashed with hexane)
was slowly added and the resulting mixture was stirred for 15-20
min at room temperature.
Step B: The formed thiolate/alcoholate (from step A) was added
to a solution of 2-(2-bromoethyl)-1,3-dioxolane in THF or DMF
and the resulting mixture was refluxed overnight. The reaction
mixture was washed several times with water and brine and
extracted with 20 mL of CH2Cl2. The organic layer was dried over
anhydrous NaSO4 and removed under vacuum.
Step C: 2-(2-alkoxyethyl)-1,3-dioxolane or 2-(2-alkylthioethyl)-
1,3-dioxolane (from step B) and 1,3-propanedithiol were dissolved
in 100 mL of CH2Cl2, BF3 ·Et2O was added dropwise over 15-20
min, and the resulting mixture was stirred overnight at 25 °C. The
reaction mixture was washed with 5% aqueous NaOH (2 × 200
mL) and 50 mL of H2O. The organic layer was dried over anhydrous
Na2SO4. The solvent was evaporated and dried to furnish crude
2-(2-alkylthioethyl)-1,3-dithiane or 2-alkyloxyethyl)-1,3-dithiane,
which were subjected to gel filtration with pentane and pentane/
ether (15:1) as eluent.
Acknowledgment. Support of this research by the NSF
(CHE-640838) is gratefully acknowledged.
2-(2-Butylthioethyl)-1,3-dioxolane. From 2 g (22.2 mmol) of
1-butanethiol, 1.17 g (28.9 mmol) of NaH, and 4.8 g (26.7 mmol)
of 2-(2-bromoethyl)-1,3-dioxolane in THF; yield 92.5%. 1H NMR
(400 MHz, CDCl3) δ 4.95 (t, 1H, J ) 4.6 Hz), 4.01-3.90 (m, 2H),
3.90-3.80 (m, 2H), 2.71-2.55 (m, 2H), 2.55-2.47 (m, 2H), 1.94
(m, 2H), 1.56 (quint, 2H, J ) 7.4 Hz), 1.38 (sextet, 2H, J ) 7.4
Supporting Information Available: Additional experimental
information, spectra, and computational details. This material
JO800938D
6396 J. Org. Chem. Vol. 73, No. 16, 2008