Tin Hydride-Promoted Radical Chain Reactions
J . Org. Chem., Vol. 61, No. 26, 1996 9407
Ma t er ia ls. The following solvents were purified by
distillation: tetrahydrofuran (THF) from sodium benzophe-
none; dichloromethane (CH2Cl2) from P2O5; N,N-dimethylfor-
mamide (DMF) from CaH2; N,N-dimethylacetamide (DMA)
from CaH2. Unless otherwise noted, all materials were
obtained from commercial suppliers and used without further
purification. Triphenyltin hydride,16 triphenyltin iodide,17
triphenyltin benzenethiolate,18 (tert-butyldimethylsilyl)tri-
phenyltin,19 and triphenyltin triflate20 were prepared according
to reported procedures.
Sch em e 4
Electr ooxid a tion of Tr ip h en yltin Hyd r id e. A mixture
of triphenyltin hydride (100 mg, 0.28 mmol) and Bu4NClO4
(50 mg, 0.146 mmol) in THF (7 mL) was placed in a beaker-
type undivided cell fitted with two platinum electrodes (1.5 ×
2 cm2). A constant current (5 mA/cm2) was supplied at room
temperature for 46 min (1.5 F/mol). The reaction mixture was
concentrated under diminished pressure, and the residue was
diluted with hexane/CH2Cl2 (1/1). The solution was washed
with brine, dried (Na2SO4), and concentrated in vacuum. The
residue was recrystallized from hexane to afford Ph3SnSnPh3
(83 mg, 83%): IR (KBr) 3066, 3012,1481, 1429, 1073, 1021,
triphenyltin radical. The triphenyltin compounds, there-
fore, undergo oxidation to generate the corresponding tin
cation. These results shows that only electrooxidation
of triphenyltin hydride and electroreduction of triphenyl-
tin iodide can generate tin radical (Ph3Sn•) in the
electrolysis media.
Therefore, reaction modes of the triphenyltin deriva-
tives are highly dependent on the nature of the substit-
uents (Y). The reaction modes of the triphenyltin de-
rivatives are divided into six categories as illustrated in
Scheme 4.
The electrogenerated species, e.g., tin radical, tin anion,
and tin cation, are of great interest from synthetic points
of view. The present work was mainly focused on the
production of tin radical. We believe that the two other
electrogenerated species (cation and anion) may also find
interesting applications in organic synthesis.
1
998, 727, 698 cm-1; H-NMR (200 MHz, CDCl3) δ 7.25-7.42,
7.44-7.54, 7.59-7.65 (m, 30H); 13C-NMR (50 MHz, CDCl3) δ
128.66, 128.78, 137.44, 139.07. 1H-NMR and 13C-NMR spectra
are identical with those of the authentic sample.13
Electr or ed u ction of Tr ip h en yltin Ch lor id e. Electroly-
sis was carried out in an H-type divided cell fitted with
platinum anode (1.5 × 2 cm2) and lead cathode (1.5 × 2 cm2).
Triphenyltin chloride (300 mg, 0.78 mmol) was placed in the
cathodic compartment, and a DMF solution of Et4NOTs (250
mg, 0.83 mmol/7 mL, each) was charged in both the anodic
and cathodic compartments. The mixture was electrolyzed
under a constant current density (3.3 mA/cm
2) with stirring
at room temperature. After passage of 1.5 F/mol of electricity
(188 min), the catholytes were poured into cold aqueous 5%
HCl and extracted with hexane/CH2Cl2 (1/1). The combined
extracts were washed with aqueous NaHCO3 and brine and
dried (Na2SO4). After removal of the solvents, the residue was
recrystallized from hexane to afford Ph3SnSnPh3 (142 mg,
73%) whose IR and 1H-NMR spectra are identical with those
described above.
Exp er im en ta l Section
Gen er a l. 1H NMR (200 MHz) and 13C NMR spectra (50
MHz) were recorded on a Varian VXR-200 spectrometer, and
chemical shifts are reported in part per million (δ) downfield
from TMS. IR spectra were obtained on a J EOL RFX-3002
grating infrared spectrophotometer. Column chromatography
was carried out on a Merck silica gel 60, 230-400 mesh ASTM.
All preparative electrolyses were carried out twice under a
nitrogen atmosphere. Cyclic voltammetry was performed
under an argon atmosphere.
Electr or ed u ction of Tr ip h en yltin lod id e. Electrolysis
was carried out in an H-type divided cell fitted with a platinum
anode (1.5 × 2 cm2) and a lead cathode (1.5 × 2 cm2).
Triphenyltin iodide (100 mg, 0.21 mmol) was placed in the
cathodic compartment, and a DMF solution of Et4NOTs (250
mg, 0.83 mmol/7 mL, each) was charged in both the anodic
and cathodic compartments. The mixture was electrolyzed
under a constant current density (3.3 mA/cm2) with stirring
at room temperature. After passage of 1.2 F/mol of electricity
(39 min), the catholytes were poured into water and the
mixture was extracted with a mixed solution of hexane/CH2-
Cl2 (1/1). The combined extracts were washed with brine and
dried (Na2SO4). After removal of the solvents, the residue was
chromatographed on a silica gel column with hexane to give
Electr och em ica l Setu p a n d Electr och em ica l P r oce-
d u r e for Cyclic Volta m m etr y. Cyclic voltammetry was
performed with a homemade potentiostat15 and a wave-form
generator, PAR Model 175. The cyclic voltammograms were
recorded with a Nicolet 3091 digital oscilloscope. Experiments
were carried out in a three-electrode cell connected to a
Schlenck line. The counter electrode was a platinum wire of
ca. 1 cm2 apparent surface area; the reference was a saturated
calomel electrode (Tacussel) separated from the solution by a
bridge (3 mL) filled with a 0.3 M n-Bu4NBF4 solution of THF.
Then, 12 mL of THF containing 0.3 M n-Bu4NBF4, was poured
into the cell followed by 0.024 mmol of the tin derivative (2 ×
10-3 M). The cyclic voltammetry was performed at a disk
electrode (a gold or platinum disk made from cross section of
wire (L ) 0.5 mm) sealed into glass) with a scan rate of 0.2 V
1
Ph3SnSnPh3 (31 mg, 43%) whose IR and H-NMR spectra are
fully identical with those described above.
Electr olysis of Tr ip h en yltin Ben zen eth iola te. P r oce-
d u r e A. Electrolyses were carried out in an H-type divided
cell fitted with two platinum electrodes (1.5 × 2 cm2).
A
s-1
. In the case of Ph3SnI, the cyclic voltammetry was
solution of triphenyltin benzenethiolate (400 mg, 0.87 mmol)
and Bu4NClO4 (1 g, 2.92 mmol) in DMA (7 mL) was placed in
the anodic compartment, and the same solution was placed
in the cathodic compartment. A constant current density (6.7
mA/cm2) was supplied under stirring at room temperature.
After passage of 4 F/mol of electricity (280 min), the catholyte
was filtered and washed with MeOH and ether to afford Ph3-
performed at a rotating disk electrode (a glassy carbon disk
(L ) 2 mm) inserted into a Teflon holder, Tacussel EDI 65109)
with a scan rate of 0.02 V s-1 and an angular velocity of 105
rad s-1 (Tacussel controvit). The same reference electrode
(SCE) was used for cyclic voltammetry performed in DMF. In
this case the filling solution was 3 mL of DMF containing
n-Bu4NBF4 (0.3 M).
With this series of experiments on triphenyltin derivatives,
we did not have to meet with problems of coating and
passivation of the electrodes which usually occur with tin
derivatives. It might be due to the presence of the phenyl
groups on the tin atom.
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