Intramolecular assistance of electron transfer. Oxidative cleavage of the carbon-tin bond of tetraalkylstannanes

Article abstract of DOI:10.1021/ja970899t

Experimental and theoretical studies of intramolecular assistance by carbonyl groups and heteroatoms in the electron transfer driven cleavage of the carbon-tin bond of tetraalkylstannanes has been carried out. Carbonyl groups and heteroatoms (oxygen and n

Full text of DOI:10.1021/ja970899t

J. Am. Chem. Soc. 1997, 119, 9361-9365
9361
Intramolecular Assistance of Electron Transfer. Oxidative
Cleavage of the Carbon-Tin Bond of Tetraalkylstannanes
Jun-ichi Yoshida* and Mitsuhiko Izawa
Contribution from the Department of Synthetic Chemistry and Biological Chemistry,
Graduate School of Engineering, Kyoto UniVersity, Kyoto 606-01, Japan
ReceiVed March 21, 1997. ReVised Manuscript ReceiVed July 26, 1997X
Abstract: Experimental and theoretical studies of intramolecular assistance by carbonyl groups and heteroatoms in
the electron transfer driven cleavage of the carbon-tin bond of tetraalkylstannanes has been carried out. Carbonyl
groups and heteroatoms (oxygen and nitrogen) in an appropriate position of tetraalkylstannane facilitate the electron
transfer and selective cleavage of the carbon-tin bond. Although the coordination of the carbonyl groups and
heteroatoms to tin was not detected in the neutral molecules, molecular orbital calculations indicate that the coordination
of such groups to tin in the cation radical intermediate stabilizes the system and weakens the carbon-tin bond.
These results offer striking examples of the intramolecular assistance of electron transfer driven reactions.
Introduction
Scheme 1
Electron transfer in solution is generally assisted by the
stabilization of the resulting cation radical or anion radical by
the coordination of the solvent molecules or ions, and such
coordination is usually very weak or not present before the
electron transfer (Scheme 1a). So, if a substrate has a specific
coordinating site to stabilize the developing ionic center, the
electron transfer should be facilitated significantly (Scheme 1b).
Such coordination is also expected to facilitate subsequent
chemical processes such as fragmentation. However, only a
limited number of papers have been published for such
intramolecular assistance of an electron transfer driven reaction.1
We report the importance of such intramolecular assistance in
oxidative cleavage of the carbon-tin bond of tetraalkylstan-
nanes.
The electron transfer driven cleavage of carbon-metal σ
bonds2 has received significant mechanistic study and is of
demonstrated synthetic utility. For example, the initial one-
electron oxidation gives the σ-cation radical intermediate and
its subsequent fragmentation3 gives products which are often
synthetically useful.4 In our study aimed at new methods for
the activation of organometallic compounds toward electron
transfer, we encountered an interesting observation indicating
both the facilitation of electron transfer and the control of the
subsequent fragmentation reaction by the intramolecular coor-
dination. Such coordination was not detected in the starting
neutral molecule. In this paper we describe experimental and
theoretical studies of intramolecular assistance by dynamic
coordination of carbonyl groups and heteroatoms to tin in the
electron transfer driven reaction of tetraalkylstannanes.
Results and Discussion
Electrochemical Study. We found that the oxidation
potential of (3-oxobutyl)tributylstannane (1)5 (Ed ) 1.41 V vs
Ag/AgCl) was less positive than that of tetrabutylstannane (Ed
) 1.67 V) (Table 1) by 0.26 V, indicating that the electron
transfer is facilitated by 6.0 kcal/mol. Although the activation
of organometallic compounds toward electron transfer by
extracoordination has been extensively studied,6 in our case there
is no indication of the coordination of the carbonyl group in
the neutral molecule of 1 by IR spectroscopy (the carbonyl
vibration band at 1717 cm-1).7 Usually the presence of electron-
withdrawing groups on the metal is essential for such coordina-
tion, but there is no electron-withdrawing group on the tin atom
in the present case. Therefore, the electron transfer seems to
X Abstract published in AdVance ACS Abstracts, September 15, 1997.
(1) (a) Sankararaman, S.; Lau, W.; Kochi, J. K. J. Chem. Soc., Chem.
Commun. 1991, 396-398. (b) Olabe, J. A.; Haim, A. Inorg. Chem. 1989,
28, 3277-3278.
(2) (a) Chanon, M.; Rajzmann, M.; Chanon, F. Tetrahedron 1990, 46,
6193-6299. (b) Kochi, J. K. In ComprehensiVe Organic Synthesis Trost,
B. M., Fleming, I., Eds.; Pergamon: Oxford, 1991; Vol. 7, Chapter 7.4, pp
849-889.
(3) (a) Shaik, S.; Reddy, A. C.; Ioffe, A.; Dinnocenzo, J. P.; Danovich,
D.; Cho, J. K. J. Am. Chem. Soc. 1995, 117, 3205-3222. (b) Dinnocenzo,
J. P.; Zuihof, H.; Lieberman, D. R.; Simpson, T. R.; MaKechney, M. W. J.
Am. Chem. Soc. 1997, 119, 994-1004 and references and cited therein.
(4) Synthetic utility of electron transfer induced bond cleavage of group
14 metal compounds. For example, photoelectron transfer reactions: (a)
Yoon, U. C.; Mariano, P. S. Acc. Chem. Res. 1992, 25, 233-240. (b)
Mizuno, K.; Ikeda, M.; Toda, S.; Otsuji, Y. J. Am. Chem. Soc. 1988, 110,
1288-1290. Electrochemical reactions: (c) Yoshida, J. Top. Current Chem.
1994, 170, 40-81. (d) Yoshida, J.; Ishichi, Y.; Isoe, S. J. Am. Chem. Soc.
1992, 114, 7594-7595. Chemical electron transfer reactions: (e) Narasaka,
K.; Kohno, Y. Bull. Chem. Soc. Jpn. 1993, 66, 3456-3463 and references
cited therein.
(5) The organotin compounds having a carbonyl group at the â-position
are called tin homoenolates. See, for example, Kuwajima, I.; Nakamura,
E. In ComprehensiVe Organic Synthesis; Trost, B. M., Fleming, I., Eds.;
Pergamon: Oxford, 1991; Vol. 2, Chapter 1.14, pp 441-454.
(6) (a) Corriu, R. J. P.; Young, J. C. In The Chemistry of Organic Silicon
Compound; Patai, S., Rappoport, Z., Eds.; Wiley: Chichester, 1989; Part
2, Chapter 20, pp 1241-1288. (b) Kumada, M.; Tamao, K.; Yoshida, J. J.
Organomet. Chem. 1982, 239, 115-132 and references cited therein.
(7) It was reported that the carbonyl stretching frequencies of (3-
oxobutyl)trimethylstannane and (3-oxobutyl)dimethylchlorostannane are
1710 and 1684 cm-1, respectively: Kuivila, H. G.; Dixon, J. E.; Maxfield,
P. L.; Scarpa, N. M.; Topka, T. M.; Tsai, K.-H.; Wursthorn, K. R. J.
Organomet. Chem. 1975, 86, 89-107.
S0002-7863(97)00899-8 CCC: $14.00 © 1997 American Chemical Society

9362 J. Am. Chem. Soc., Vol. 119, No. 40, 1997
Yoshida and Izawa
Table 2. Oxidation Potentials of Heteroatom-Substituted
Table 1. Electrochemical Oxidation of Ketoorganostannanes and
Related Compounds
Tetraorganostannanes
a The oxidation potentials were determined by rotating disk electrode
voltammetry in 0.1 M Bu4NClO4/CH2Cl2 by using a glassy carbon
working electrode and a Ag/AgCl reference electrode.
solvent,11 although it is well documented that the fragmentation
of the organometallic cation radical usually affords a cationic
metal and an organic free radical.12 Such a possibility is denied
by the fact that the workup without chloride did not give the
chlorostannanes.
To examine the fate of the alkyl group, the anodic oxidation
of the organotin compound having large decyl groups was
carried out (eq 1). Decene was formed as a mixture of
regioisomers suggesting the intermediacy of decyl cation.
a The oxidation potentials were determined by rotating disk electrode
voltammetry in 0.1 M Bu4NClO4/CH2Cl2 by using a glassy carbon
working electrode and a Ag/AgCl reference electrode. b The preparative
electrochemical oxidation reactions were usually carried out with 0.25
mmol of the substrate with 0.75 mmol of 2,6-lutidine in 4.5 mL of
0.17 M Bu4NClO4/CH2Cl2 with use of a divided cell equipped with a
carbon rod anode and a platinum plate cathode at 0 °C under constant
current conditions. c Isolated yields. Yields in parentheses were
determined by 1H NMR with use of an internal standard. d Isolated yeild.
Yields in parentheses were determined by GC analysis with use of an
internal standard. e 32% of the starting material was recovered un-
changed.
be facilitated by the coordination of the carbonyl group to tin
in the cation radical intermediate.8
Preparative electrochemical oxidation of compound 1 in Bu4-
NClO4/CH2Cl2 followed by workup with aqueous NaCl gave
rise to the selective cleavage of the butyl-tin bond and the
addition of the chlorine atom to tin producing the pentacoor-
dinate organotin compound (2) that exhibited the carbonyl
It should be emphasized that intramolecular assistance for
the electron transfer is also effected by other groups containing
heteroatoms such as oxygen and nitrogen (Table 2).13 The effect
of the pyridyl group is remarkable.14 Although 119Sn NMR
indicates that the pyridyl group does not coordinate to tin at
the neutral stage (-12.15 ppm in CDCl3)16 the introduction of
a pyridyl group resulted in a dramatic decrease of the oxidation
potential (∆Ed ) 0.53 V), suggesting the coordination to tin in
the cation radical intermediate.17 It is also noteworthy that the
oxidation potential decreases with the increase of the number
of the pyridyl groups. In the case of tris [2-(2-pyridyl)ethyl]-
butylstannane, the simultaneous coordination of three pyridyl
groups to tin seems to be difficult, if we assume that the
maximum coordination number of tin is six. Probably the
entropic factor plays some role.
vibration band at 1686 cm-1 7
.
It is interesting that the oxidation potentials of organotin
compounds having a carbonyl group at different positions
(compound 3 and 5) were found to be more positive than that
of 1. It is also noteworthy that the carbon-tin bond cleavage
of 3 and 5 by the preparative anodic oxidation was not selective
and that both the butyl-tin bond cleavage products (compounds
4 and 6) and chlorotributylstannane were obtained. Therefore,
the position of the carbonyl group seems to be crucial for both
the promotion of electron transfer and the selective bond
cleavage. This is consistent with the IR spectra of the products
(2, 4, and 6); compound 2 exhibits the largest shift for the
carbonyl vibration, indicating that the coordination to form the
five-membered ring is more effective than those for the six-
and seven-membered rings. The ester carbonyl group (com-
pound 7) was also effective.9
The formation of chlorostannanes (2, 4, 6, and 8) may suggest
that the fragmentation of the cation radical intermediate produces
the tin cation stabilized by the carbonyl group, which reacts
with chloride ion in the workup with aqueous NaCl. There is,
however, another possibility to be considered. The fragmenta-
tion of the cation radical produces the tin radical,10 which
abstracts the chlorine atom from dichloromethane used as the
(10) It has been reported that the oxidative cleavage of the carbon-tin
bond gives the carbocation and the tin radical if the electron donating group
is attached on the carbon. Eaton, D. F. J. Am. Chem. Soc. 1980, 102, 3278-
3281.
(11) The abstract of halogen atom from organic halides by tin radicals
is well-known. For example, see: Curran, D. P.; Jasperse, C. P.; Totleben,
M. J. J. Org. Chem. 1991, 56, 7169-7172. See also ref 9.
(12) For example, Eaton, D. F. Pure Appl. Chem. 1984, 56, 1191-1202.
(13) Intramolecular coordination of a nitrogen atom at tin promoting the
C-Sn bond cleavage has been reported. For example: (a) Vedejs, E.;
Haight, A. R.; Moss, W. O. J. Am. Chem. Soc. 1992, 114, 6556-6558. (b)
Jousseaume, B.; Villeneuve, P. J. Chem. Soc., Chem. Commun. 1987, 513-
514.
(14) [2-(2-Pyridyl)ethyl]tributyltin was prepared by hydrostannylation of
2-vinylpyridine.15
(8) Facilitation of electrophilic reactions of tetraorganostannane by the
carbonyl coordination has been reported. See ref 7 and: Podesta, J. C.;
Chopa, A. B.; Koll, L. C. J. Chem. Res. (S) 1986, 308-309.
(9) Nagao and Ochiai reported the effective intramolecular coordination
of acetal oxygen to tin although an electronegative chlorine atom is present
on the tin: Ochiai, M.; Iwaki, S.; Ukita, T.; Matsuura, Y.; Shiro, M.; Nagao,
Y. J. Am. Chem. Soc. 1988, 110, 4606-4610.
(15) Mahon, M. F.; Molloy, K. C.; Waterfield, P. C. Organometallics
1993, 12, 769-774.
(16) Davies, A. G. In ComprehensiVe Organometallic Chemistry; Wilkin-
son, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon: Oxford, 1982; Vol.
2, Chapter 11, pp 519-628.
(17) The coordination of the pyridyl group to tin in [2-(2-pyridyl)ethyl]-
diphenyltin halides has been reported. See ref 17.

Intramolecular Assistance of Electron Transfer
J. Am. Chem. Soc., Vol. 119, No. 40, 1997 9363
Table 3. Relative Energies of Cation Radicals Obtained by ab
Initio MO Calculationsa
relative total energies (eV)
HF/3-21G*// MP2/3-21G*// B3LYP/3-21G*// HF/LANL2DZ//
structure HF/3-21G* HF/3-21G*
HF/3-21G* HF/HFLANL2DZ
A
B
C
-1.44
-1.30
0
-1.34
-1.15
0
-1.03
-0.82
0
-1.84
-1.72
0
a Calculations were carried out with use of G94W.
Scheme 2
Figure 1. Optimized structures of the cation radical of (3-oxopropyl)-
trimethylstannane obtained by ab initio MO calculations (HF/3-21G*).
The preparative electrochemical oxidation of [2-(2-pyridyl)-
ethyl]tributylstannane in Bu4NClO4/CH2Cl2 followed by workup
with aqueous NaCl gave rise to the selective cleavage of the
butyl-tin bond and addition of the chlorine atom to tin
producing the pentacoordinate organotin compound. Therefore,
the pyridyl group is also effective for the selective cleavage of
the carbon-tin bond.
lecular coordination. Such stabilization favors the electron
transfer to decrease the oxidation potential. It is also noteworthy
that in A and B one of the carbon-tin bonds is selectively
elongated. Therefore, the intramolecular coordination is also
responsible for the selective cleavage of the C-Sn bond.
Reaction Mechanism. On the basis of the experimental
results and computational study, we propose a reaction mech-
anism involving the coordination of a carbonyl group or a
heteroatom in an appropriate position to tin stabilizing the cation
radical intermediate. Such nucleophilic assistance also facilitates
the selective cleavage of the carbon-tin bond (Scheme 2).
The picture emerging from the existence of pre-coordination
in the neutral molecule might also explain the present results.
Since such pre-coordination was not detected by the IR and
NMR studies, the population of the pre-coordinated complexes
is very small even if they exist. The pre-coordinated species
are, however, expected to have lower oxidation potentials.
Therefore, even if their population is very small, there is a
possibility that the electron transfer might selectively take place
from such species. However, it seems to be difficult to
distinguish such a mechanism from the mechanism based on
the stabilization of the cation radical intermediate. At some
point in the reaction coordinate, the geometry changes to achieve
the intramolecular coordination to facilitate the electron transfer.
If such a geometry change occurs in a very early stage, it is
regarded as the pre-coordination. If the coordination takes place
in a very late stage, it is regarded as the stabilization of the
cation radical. There is another possibility that involves the
initial partial geometry change to decrease the oxidation potential
followed by the ionization and subsequent geometry change to
stabilize the resulting cation radical. Some part of the facilita-
tion of the electron transfer might also be attributed to a rapid
follow up chemical reaction, because the coordination facilitates
the subsequent bond cleavage. Therefore, we must consider
“dynamic coordination” instead of “static coordination” to
account for the intramolecular assistance of electron transfer.
Another important point is that a benzene ring is also effective
for the facilitation of the electron transfer, although its magnitude
is not so large. Presumably, π-coordination1b to the tin atom
stabilizes the developing cation radical intermediate.
Computational Studies. To get a deeper insight into the
intramolecular assistance for the electron transfer and the
subsequent fragmentation, we carried out a computational study
using ab initio molecular orbital calculations.18 (3-Oxopropyl)-
tributylstannane was chosen as a model compound. The
geometry optimization at the HF/3-21G* level gave three
structures (A, B, and C in Figure 1). In A the carbonyl group
coordinates tin to form the trigonal bipyramidal structure. In
B the carbonyl group also coordinates tin, but the structure of
B is quite different from that of A. In C there is no coordination
of the carbonyl group to tin. One of the C-Sn bonds in A and
B is remarkably lengthened by 44-67% relative to the other
C-Sn bonds,19 implying that such a bond is selectively
weakened by the coordination. In A the C-Sn bond trans to
the carbonyl oxygen is elongated whereas in B the C-Sn bond
cis to the carbonyl oxygen is much longer than usual. In C
four C-Sn bonds have similar lengths, although one of them
is slightly longer than the others. The optimization at HF/
LANL2DZ also gave similar structures.
The total energies calculated at several levels indicated that
the carbonyl-coordinated conformers (A and B) are much more
stable than the noncoordinated conformer C (Table 3), and that
A is slightly more stable than B irrespective of the level of the
calcuation. Thus, the molecular orbital calculations indicated
that the cation radical intermediate is stabilized by the intramo-
Conclusions
The findings of the present study offer striking examples of
the intramolecular assistance of electron transfer driven reac-
tions. The dynamic intramolecular coordination facilitates the
electron transfer and controls the subsequent fragmentation
reactions. Such dynamic coordination provides a new method
for the activation of substrate molecules toward electron transfer
and the control of the subsequent reaction of the cation radical
intermediate. Various types of electron transfer driven reactions
(18) The ab initio MO calculations were carried out with the Gaussian
94W program. All the optimized structures (UHF/3-21G*) were local
minima according to the vibration analysis. The calculations from ECP
(HF/LANL2DZ) also gave similar optimized structures.
(19) MNDO calculations of tetraalkyltin cation radical indicated that the
bond to the apical alkyl group is lengthened by ca. 13%. Dewar, M. J. S.;
Grady, G. L.; Kuhn, D. R. Organometallics 1985, 4, 1041-1044.

9364 J. Am. Chem. Soc., Vol. 119, No. 40, 1997
Yoshida and Izawa
1.24-1.35 (m, 6H), 1.42-1.52 (m, 6H), 1.71-1.82 (m, 2H), 2.13 (s,
3H), 2.44 (t, 2H, J ) 7.2 Hz); 13C NMR (75 MHz, CDCl3) δ 8.35,
8.36, 13.56, 21.63, 27.26, 29.10, 29.80, 48.43, 209.45; IR (neat) 2957
(s), 2924 (s), 1719 (s), 1458 (m), 1358 (m), 1171 (m), 1071 (w), 596
(w) cm-1; MS (EI) m/e (%) 319 (M+ - Bu, 100), 291 (12), 235 (7),
205 (SnCH2CH2CH2COCH3, 5), 177 (SnBu, 23), 121 (6); HRMS calcd
for C13H27OSn (M+ - Bu) 319.1084, found 319.1078.
(5-Oxohexyl)tributylstannane (7). To a solution of NaOH (453
mg, 18.9 mmol) in H2O (4 mL)/MeOH (4 mL) was added (5-oxo-4-
methoxycarbonylhexyl)tributylstannane (417 mg, 0.933 mmol). The
reaction mixture was refluxed for 20 min, and then 1.2 mL (21 mmol)
of acetic acid was added. The mixture was extracted with ether, and
the organic phase was washed with saturated aqueous NaHCO3 and
brine and dried over MgSO4. After evaporation of the solvent, the
crude product was purified via flash chromatography (hexane:ethyl
acetate ) 30:1) to obtain 316 mg (0.812 mmol, 87%) of the title
compound: 1H NMR (300 MHz, CDCl3) δ 0.75-0.84 (m, 8H), 0.89
(t, 9H, J ) 7.2 Hz), 1.29 (t, 6H, J ) 7.3 Hz), 1.41-1.61 (m, 10H),
2.13 (s, 3H), 2.43 (t, 2H, J ) 7.2 Hz); 13C NMR (75 MHz, CDCl3) δ
8.44, 8.56, 13.56, 26.57, 27.25, 28.48, 29.12, 29.68, 43.30, 209.45; IR
(neat) 2957 (s), 2926 (s), 1721 (s), 1464 (m), 1481 (w), 1358 (m),
1169 (m), 1071 (w), 961 (w), 691 (m); MS (EI) m/e 333 (M - Bu,
100), 277 (10), 235 (6), 177 (SnBu, 19), 121 (7). Anal. Calcd for
C18H38OSn: C, 55.55; H, 9.84. Found: C, 55.85; H, 10.12.
will hopefully be developed on the basis of this concept and
widely utilized in organic synthesis.
Experimental Section
General. 1H and 13C NMR spectra were measured on a Varian
Gemini 2000 spectrometer with Me4Si as an internal standard. 119Sn
NMR spectra were measured on a JEOL JNM-A400 spectrometer with
Me4Sn as an internal standard. CDCl3 was used as the solvent unless
stated otherwise. IR spectra were measured with a Shimazu FTIR-
8100 spectrophotometer. Mass spectra were obtained with a JEOL
IMS-300 spectrometer. Thin-layer chromatography (TLC) was carried
out by using Merck precoated silica gel F254 plates (thickness 0.25 mm),
and silica gel used for flash chromatography was Wakogel C300. Gel
permeation chromatography (GPC) was carried out with a Japan
Analytical Industry LC 908 with CHCl3 as the eluent.
Diethyl ether and tetrahydrofuran was distilled from benzophenone
ketyl. Dichloromethane was washed with water and distilled from P2O5,
then removal of the trace of acid was carried out by distillation from
dried K2CO3 and distillate was stored over molecular sieves 4A. Olefins
were distilled under appropriate pressure before the hydrostannylation
reaction.
All reactions were carried out under Ar atmosphere unless otherwise
noted.
Voltammetry. The oxidation potentials were measured by rotating
disk electrode voltammetry, which was carried out with a Bioanalytical
Systems BAS 100B and a Nikko Keisoku RRDE-1 with a glassy carbon
working electrode, platinum wire counter electrode, and Ag/AgCl
(saturated aq KCl) reference electrode in 0.1 M Bu4ClO4/CH2Cl2 at
1000 rpm. The sweep rate was 10 mV/s. Cyclic voltammetry was
also carried out with a BAS 100B with a glassy carbon working
electrode. All the organotin compounds showed irreversible oxidation
waves, and the peak potentials shifted with the sweep rate.
(3-Methoxypropyl)tributylstannane. To a solution of sodium
methoxide (6.7 mmol) in THF (5 mL)/MeOH (10 mL) was added 2.27
g (5.5 mmol) of (3-bromopropyl)tributylstannane. The mixture was
refluxed overnight, and then 0.10 mL (1.8 mmol) of acetic acid was
added. The reaction mixture was extracted with ether, and the organic
phase was washed with water and dried over MgSO4. After the solvent
was evaporated, the crude product was purified with flash chromatog-
raphy (hexane:ethyl ether ) 50:1) to obtain 1.28 g (0.352 mmol, 64%)
of the title compound: 1H NMR (300 MHz, CDCl3) δ 0.73-0.85 (m,
8H), 0.89 (t, 9H, J ) 7.1 Hz), 1.24-1.36 (m, 6H), 1.42-1.53 (m, 6H),
1.71-1.81 (m, 2H), 3.32 (t, 2H, J ) 6.9 Hz), 3.34 (s, 3H); 13C NMR
(75 MHz, CDCl3) δ 4.43, 8.68, 13.57, 26.79, 27.31, 29.13, 58.45, 76.34;
IR (neat) 2957 (s), 2924 (s), 1420 (w), 1307 (m), 1340 (w), 1201 (m),
1119 (s), 693 (m) cm-1; MS (EI) m/e (%) 307 (M+ - Bu, 100), 193
(SnCH2CH2CH2OCH3, 11), 177 (SnBu, 30); HRMS calcd for C12H27-
OSn (M+ - Bu) 307.1084, found 307.1070.
(3-Oxobutyl)tributylstannane (1). To methylmagnesium iodide
prepared from 2.38 g (17.0 mmol) of MeI and 472 mg (19.4 mmol) of
Mg in 40 mL of diethyl ether was added dropwise 2.73 g (7.93 mmol)
of (2-cyanoethyl)tributylstannane. The mixture was stirred overnight
at room temperature, then 4 mL of aqueous NH4Cl was added carefully
at 0 °C. The resulting mixture was filtered, and the solvent was
removed under reduced pressure. Purification of the crude product via
flash chromatography (silica gel, hexane:ethyl acetate ) 100:1) followed
by distillation (bulb-to-bulb, 170 °C/1 mmHg) afforded 1.70 g (4.71
mmol, 59%) of the title compound: 1H NMR (300 MHz, CDCl3) δ
0.80-0.85 (m, 8H), 0.89 (s, 9H, J ) 7.2 Hz), 1.30 (sextet, 6H, J ) 7.3
Hz), 1.42-1.55 (m, 6H), 2.14 (s, 3H), 2.60 (t, 2H, J ) 8 Hz); 13C
NMR (75 MHz, CDCl3) δ 1.91, 8.89, 13.56, 27.26, 28.90, 29.06, 40.85,
210.72; IR (neat) 2957 (s), 2924 (s), 1717 (s), 1375 (w), 1360 (m),
1177 (m), 1136 (w), 1072 (w), 961 (w), 689 (m) cm-1; MS (EI) m/e
(%) 305 (M+ - Bu, 100), 249 (7), 191 (SnCH2CH2COCH3, 16), 121
(4); HRMS calcd for C12H25OSn (M+ - Bu) 305.0927, found 305.0925.
(3-Oxobutyl)tridecylstannane. To a solution of trichloro(3-ox-
obutyl)stannane20 (2.96 g, 10.0 mmol) in THF (30 mL) was added decyl
magnesium bromide (prepared from 6.70 g (30.3 mmol) of decyl
bromide and 946 mg (38.9 mmol) of Mg in 30 mL of THF) at -78 °C
for 1 h. THF (30 mL) was added and the mixture was stirred at -78
°C for 4.5 h. The precipitate was removed by a short column (silica
gel, hexane:ethyl acetate ) 3:1) and the eluent was washed with brine
and dried with MgSO4. After removal of the solvent, the crude product
was purified via flash chromatography (hexane:ethyl acetate ) 50:1)
to obtain the title compound (3.53 g, 5.75 mmol, 58%). The product
was further purified with GPC: 1H NMR (300 MHz, CDCl3) δ 0.79-
0.92 (m, 17H), 1.26 (br s, 42H), 1.48 (br, 6H), 2.14 (s, 3H), 2.59 (t,
2H, J ) 8.1 Hz); 13C NMR (75 MHz, CDCl3) δ 1.97, 9.26, 14.00,
22.59, 26.81, 28.93, 29.18, 29.27, 29.59, 31.85, 34.37, 40.91, 210.81;
IR (neat) 2924 (s), 2853 (s), 1719 (s), 1358 (m), 1175 (m), 1136 (w),
722 (w), 689 (w) cm-1; MS (FAB) m/e 473 (M+ - C10H21, 100), 331
(10), 261 (13), 191 (SnCH2CH2COCH3, 48); HRMS calcd for C24H49-
SnO (M+ - C10H21) 473.2805, found 473.2831.
[2-(2-Pyridyl)ethyl]tributylstannane. A mixture of Bu3SnH (828
mg, 2.84 mmol), AIBN (25 mg, 0.15 mmol), and 2-vinylpyridine (376
mg, 3.58 mmol) was stirred at 60 °C for 3 h. Flash chromatography
(hexane:ethyl acetate ) 20:1) of the crude product afforded 707 mg
(1.78 mmol, 63%) of the title compound. 1H NMR (300 MHz) δ 0.78-
0.84 (m, 6H), 0.88 (t, 9H, J ) 7.2 Hz), 1.16-1.21 (m, 2H), 1.29 (sextet,
6H, J ) 7.3 Hz), 1.40-1.51 (m, 6H), 2.95-3.01 (m, 2H), 7.06-7.11
(m, 1H), 7.16-7.18 (m, 1H), 7.56-7.62 (m, 1H), 8.50-8.52 (m, 1H);
13C NMR (75 MHz) δ 8.73, 8.80, 13.60, 27.31, 29.12, 35.31, 120.85,
122.00, 136.41, 149.24, 165.12; IR (neat) 3007 (w), 2955 (s), 2922
(s), 1591, (s), 1568 (m), 1464 (m), 1433 (s), 1375 (m), 749 (s) cm-1
;
MS (EI) m/e (%) 340 (M+ - Bu, 100), 226 (SnCH2CH2C5H4N, 26).
Anal. Calcd for C19H35SnN: C, 57.60; H, 8.90; N, 3.54. Found: C,
57.34; H, 9.04, N, 3.43.
Bis[2-(2-pyridyl)ethyl]dibutylstannane. To a solution of [2-(2-
pyridyl)ethyl]tributylstannane (260 mg, 0.656 mmol) in THF (1 mL)
was added BuLi (1.57 M in hexane, 0.46 mL, 0.72 mmol) at -78 °C
within 2 min. The resulting deep red solution was stirred for 33 min
at -78 °C, and a solution of Bu2SnCl2 (90 mg, 0.30 mmol) in THF (1
mL) was added within 3 min. THF (1 mL) was added and the mixture
was stirred for 10 min at -78 °C, warmed to room temperature, and
stirred at this temperature for 1 h. The reaction mixture was poured
into 10 mL of 1 N HCl, and the mixture was washed with 10 mL of
hexane. The aqueous phase was separated, and KOH was added until
the pH of the solution became 10. The mixture was extracted with 20
mL of ether, and the organic phase was separated and dried over
MgSO4. The removal of the solvent under reduced pressure afforded
96 mg of crude product. Flash chromatography (hexane:ethyl acetate
) 3:1) gave 77 mg (0.17 mmol, 57%) of the title compound: 1H NMR
(300 MHz) δ 0.77-0.87 (m, 10H), 1.17-1.33 (m, 8H), 1.38-1.46 (m,
4H), 2.95-3.01 (m, 4H), 7.05-7.10 (m, 2H), 7.14-7.17 (m, 2H), 7.56-
7.61 (m, 2H), 8.49-8.52 (m, 2H); 13C NMR (75 MHz) δ 8.88, 9.11,
13.54, 27.26, 29.00, 35.10, 120.82, 122.00, 136.35, 149.14, 164.87;
(4-Oxopentyl)tributylstannane (3). This compound was prepared
in 55% yield from (3-cyanopropyl)tributylstannane according to a
similar procedure for the preparaiton of 1: 1H NMR (300 MHz, CDCl3)
δ 0.73-0.79 (m, 2H), 0.80-0.85 (m, 6H), 0.89 (t, 9H, J ) 7.2 Hz),
(20) Ryu, I.; Sonoda, N. Organometallics, 1990, 9, 277-280.

Intramolecular Assistance of Electron Transfer
J. Am. Chem. Soc., Vol. 119, No. 40, 1997 9365
IR (neat) 3067 (w), 3007 (w), 2955 (s), 2923 (s), 1592 (s), 1563 (m),
1473 (s), 1433 (s), 1375 (w), 1150 (m), 1049 (m), 749 (s); MS (EI)
m/e (%) 389 (M+ - Bu, 100), 340 (M - CH2CH2C5H4N, 65) 226
(SnCH2CH2C5H4N, 52). Anal. Calcd for C22H34SnN2: C, 59.35; H,
7.70; N, 6.29. Found: C, 59.52; H, 7.94; N, 6.10.
MHz, CDCl3) δ 9.17, 13.94, 19.22, 27.04, 28.31, 29.24, 40.64, 217.52;
IR (neat) 2957 (s), 2924 (s), 1686 (s), 1420 (w), 1366 (m), 1335 (w),
1183 (m), 1125 (w), 1076 (w), 702 (w) cm-1; MS (EI) m/e (%) 305
(M+ - Cl, 20), 283 (M+ - Bu, 100), 191 (M+ - 2Bu, 36); HRMS
calcd for C8H16ClOSn (M+ - Bu) 282.9912, found 282.9905.
Tris[2-(2-pyridyl)ethyl]butylstannane. To a solution of [2-(2-
pyridyl)ethyl]tributylstannane (438 mg, 1.55 mmol) in THF (10 mL)
was added BuLi (1.57 M in hexane, 3.8 mL, 6.0 mmol) at -76 °C
within 2 min. The resulting deep red solution was stirred for 1 h at
-73 °C, then 438 mg (1.55 mmol) of BuSnCl3 was added at -73 °C.
The solution was stirred for 20 min at -73 °C, then stirred for 90 min
at 0 °C. To the reaction mixture was added 10 mL of 1 N HCl and 30
mL of hexane. The organic phase was separated and washed with 10
mL of 1 N HCl. The aqueous phase and the washing was combined,
and NaOH was added until the pH of the solution became 10. The
mixture was extracted twice with 50 mL of ether. The combined
organic phase was dried over MgSO4, and the solvent was removed
under reduced pressure. Flash chromatography (ethyl acetate) of the
crude product gave 458 mg (0.927 mmol, 60%) of the title compound.
This product was further purified with GPC (CHCl3) (50% yield): 1H
NMR (300 MHz) δ 0.75-0.86 (m, 5H), 1.08-1.30 (m, 8H), 1.34-
1.42 (m, 2H), 2.94-3.00 (m, 6H), 7.05-7.09 (m, 3H), 7.13-7.15 (m,
3H), 7.54-7.60 (m, 3H), 8.49-8.51 (m, 3H); 13C NMR (75 MHz) δ
9.23, 9.64, 13.51, 27.25, 28.89, 34.93, 120.75, 121.98, 136.27, 149.05,
164.75; IR (neat) 3065 (m), 3007 (m), 2951 (s), 2919 (s), 1591 (s),
1568 (m), 1473 (s), 1433 (s), 1150 (m), 994 (m), 750 (s) cm-1; MS
(FAB) m/e (%) 438 (M+ - Bu, 9), 389 (M+ - C2H4C5H4N, 100), 269
(Bu2SnCl, 18), 177 (BuSn, 17), 155 (SnCl, 19). Anal. Calcd for
C25H33SnN3: C, 60.75; H, 6.73; N, 8.50. Found: C, 61.01; H, 6.73;
N, 8.46.
Chloro(4-oxopentyl)dibutylstannane (4) was purified with PTLC
(hexane:ethyl acetate ) 10:1): 1H NMR (300 MHz, CDCl3) δ 0.92 (t,
6H, J ) 7.2 Hz), 1.21-1.44 (m, 10H), 1.61-1.72 (m, 4H), 1.89-1.97
(m, 2H), 2.20 (s, 3H), 2.60 (t, 2H, J ) 5.3 Hz); 13C NMR (75 MHz,
CDCl3) δ 13.54, 18.25, 18.79, 20.10, 26.67, 27.87, 30.33, 40.66, 215.29;
IR (neat) 2957 (s), 2923 (s), 1694 (s), 1377 (m), 1184 (m), 1076 (w),
720 (w), 673 (w), 594 (w), 523 (w) cm-1; MS (EI) m/e 319 (M+ - Cl,
9), 297 (M+ - Bu, 100), 269 (Bu2SnCl, 18), 177 (BuSn, 17), 155 (SnCl,
19). Anal. Calcd for C13H27ClOSn: C, 44.17; H, 7.70. Found: C,
44.45; H, 7.73.
Chloro(5-oxyhexyl)dibutylstannane (6) was purified with PTLC
(hexane:ethyl acetate ) 10:1): 1H NMR (300 MHz, CDCl3) δ 0.92 (t,
6H, J ) 7.2 Hz), 1.24-1.42 (m, 10H), 1.59-1.67 (m, 8H), 2.15 (s,
3H), 2.47 (t, 2H, J ) 6.8 Hz); 13C NMR (75 MHz, CDCl3) δ 13.46,
17.36, 17.55, 25.20, 26.33, 26.72, 27.32, 27.70, 42.98, 209.26; IR (neat)
2957 (s), 2924 (s), 1707 (s), 1418 (m), 1360 (m), 1173 (w), 1076 (w),
700 (w), 675 (w) cm-1; MS (EI) m/e 333 (M+ - Cl, 6) 311 (M+
-
Bu, 100), 269 (Bu2SnCl, 13), 177 (BuSn, 9), 155 (SnCl, 15). Anal.
Calcd for C14H29ClOSn: C, 45.75; H, 7.95. Found: C, 45.46; H, 7.91.
Chloro[2-(2-pyridyl)ethyl]dibutylstannane was purified with PTLC
(hexane:ethyl acetate ) 3:1): 1H NMR (300 MHz, CDCl3) δ 0.86 (t,
6H, J ) 7.2 Hz), 1.26-1.39 (m, 8H), 1.53-1.67 (m, 6H), 3.42 (t, 2H,
J ) 7.4 Hz); 13C NMR (75 MHz, CDCl3) δ 13.30, 13.51, 19.78, 26.72,
28.11, 32.27, 122.77, 124.41, 138.85, 145.59, 162.00; IR (neat) 3060
(w), 3023 (w), 2955 (s), 2923 (s), 1603 (s), 1439 (s), 1289 (m), 1186
(2-Phenylethyl)tributylstannane. To Grignard reagent prepared
from 650 mg (3.53 mmol) of phenylethyl bromide and 105 mg (4.32
mmol) of Mg in 5 mL of diethyl ether was added 952 mg (2.92 mmol)
of Bu3SnCl dropwise. The mixture was stirred overnight at ambient
temperature. To the reaction mixture was added 0.65 mL of aqueous
NH4Cl carefully with stirring. The resulting mixture was filtered, and
the filtrate was dried over MgSO4. After removal of the solvent the
crude product was purified via flash chromatography (hexane) followed
by bulb-to-bulb distillation (190 °C/2 mm Hg) to obtain the title
compound (2.40 mmol, 950 mg, 82%): 1H NMR (300 MHz, CDCl3)
δ 0.76-0.80 (m, 6H), 0.82-0.91 (m, 9H), 1.10-1.16 (m, 2H), 1.22-
1.34 (m, 6H), 1.39-1.51 (m, 6H), 2.78-2.83 (m, 2H), 7.16-7.30 (m,
5H); 13C NMR (75 MHz, CDCl3) δ 8.67, 10.66, 13.60, 27.30, 29.13,
32.94, 125.63, 127.80, 128.41, 146.06; IR (neat) 3063 (w), 3027 (w),
2957 (s), 2924 (s), 1603 (w), 1495 (m), 1454 (m), 1375 (m), 1071 (w),
747 (m), 698 (s) cm-1; MS (EI) m/e 339 (M+ - Bu, 100); HRMS
calcd for C16H27Sn (M+ - Bu) 339.1135, found 339.1134.
General Procedure for the Anodic Oxidation of Organotin
Compounds. The anodic oxidation reactions were carried out in a
divided cell equipped with a carbon rod anode (15 mm × φ 6 mm)
and a platinum plate cathode (15 mm × 10 mm). In the anodic chamber
were placed an organotin compound (0.25 mmol), 0.17 M Bu4NClO4/
CH2Cl2 (4.5 mL), 2,6-lutidine (0.75 mmol), and molecular sieves 4A
(300 mg). In the cathodic chamber were placed 0.17 M Bu4NClO4/
CH2Cl2 (4.5 mL) and molecular sieves 4A (300 mg). The constant
current electrolysis (5 mA) was carried out at 0 °C with magnetic
stirring until 2.0 F/mol of electricity was consumed. After completion
of the reaction, saturated aqueous NaCl (2 mL) was added to the anodic
chamber and the mixture was stirred for several minutes at 0 °C, and
then warmed to room temperature. The mixture in the anodic chamber
was collected. After the addition of saturated aqueous NaCl (3 mL),
the mixture was stirred at room temperature for several min. The
volatile materials were removed under reduced pressure. Ether (15
mL) was added, and the mixture was stirred vigorously. After removal
of the insoluble materials by filtration, the organic phase was separated,
and the aqueous phase was extracted with ether. The combined organic
phase was dried over MgSO4. After removal of the solvent the crude
product was purified by using preparative TLC (Merck precoated silica
gel F-254 plates (thickness 0.25 mm) or GPC (Jaigel 1H, 2H, CHCl3)).
(m), 1013 (s), 762 (s); MS (EI) m/e 340 (M+ - Cl, 23), 318 (M+
-
Bu, 100), 285 (17), 224 (37), 155 (SnCl, 15). Anal. Calcd for C15H26-
ClNSn: C, 48.11; H, 7.00; N, 3.74. Found: C, 48.39; H, 6.87; N,
3.59.
Chloro(2-methoxycarbonylethyl)dibutylstannane (3) was purified
with PTLC (hexane:ethyl acetate ) 10:1): 1H NMR (300 MHz, CDCl3)
δ 0.91 (t, 6H, J ) 7.4 Hz), 1.28-1.44 (m, 10H), 1.59-1.68 (m, 4H),
2.77 (t, 2H, J ) 7.5 Hz), 3.78 (s, 3H); 13C NMR (75 MHz, CDCl3) δ
11.66, 13.51, 18.76, 26.58, 27.84, 29.71, 53.36, 181.52; IR (neat) 2957
(s), 2924 (s), 1690 (s), 1443 (s), 1358 (s), 1184 (m), 1128 (m), 681
(m) cm-1; MS (EI) m/e (%) 321 (M+ - Cl, 23), 299 (M+ - Bu, 100),
207 (M+ - 2Bu, 19); HRMS calcd for C8H16ClO2Sn (M+ - Bu)
298.9861, found 298.9869.
Chloro(3-oxybutyl)didecylstannane was purified with GPC
(CHCl3): 1H NMR (300 MHz, CDCl3) δ 0.88 (t, 6H, J ) 6.6 Hz),
1.17 (t, 2H, J ) 7.4 Hz), 1.26-1.30 (m, 32H), 1.60-1.68 (m, 4H),
2.26 (s, 3H), 2.97 (t, 2H); 13C NMR (75 MHz, CDCl3) δ 8.79, 13.98,
19.13, 22.56, 25.72, 28.80, 29.09, 29.22, 29.51, 29.54, 31.82, 33.64,
40.21, 217.37; IR (neat) 2924 (s), 1688 (s), 1466 (m), 1366 (s), 1335
(m), 1181 (m), 1125 (w); MS (FAB) m/e 473 (M+ - Cl, 100), 367
(M+ - C10H21, 24), 331 (9), 191 (28); HRMS calcd for C14H28ClOSn
(M+ - C10H21) 367.0851, found 367.0838. Anal. Calcd for C24H49-
ClOSn: C, 56.77; H, 9.73. Found: C, 56.79; H, 9.72.
Acknowledgment. This work was partially supported by
Grant-in-Aid for Scientific Research on Priority Areas (No. 283,
Innovative Synthetic Reactions) from Monbusho. We thank Mr.
Haruo Fujita of our department for the measurement of the 119
-
Sn NMR spectra, Dr. Shigeru Yamago for valuable discussion,
and Mr. Mitsuru Watanabe and Mr. Takao Manabe for the
assistance of the electrochemical measurements. We dedicate
this paper to Professor Hans Scha¨fer in honor of this 60th
birthday.
Supporting Information Available: Listings of geometries
of cation radicals obtained by MO calculations (3 pages). See
any current masthead page for ordering and Internet access
instructions.
Chloro(3-oxobutyl)dibutylstannane (2) was purified with PTLC
(hexane:ethyl acetate ) 10:1); 1H NMR (300 MHz, CDCl3) δ 0.91 (t,
6H, J ) 7.2 Hz), 1.18 (t, 2H, J ) 7.1 Hz), 1.26-1.42 (m, 8H), 1.57-
1.67 (m, 4H), 2.27 (s, 3H), 2.98 (t, 2H, J ) 7.4 Hz); 13C NMR (75
JA970899T