Control of the free radical reaction by dynamic coordination: Unique reactivity of pyridylethyl-substituted tin hydrides

Article abstract of DOI:10.1039/a903795k

Tin hydrides having one or two pyridyl groups show unique reactivity in the reduction of alkyl halides under radical conditions, indicating the potential of intramolecular coordination for the control of radical reactions.

Full text of DOI:10.1039/a903795k

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Control of the free radical reaction by dynamic coordination: unique reactivity
of pyridylethyl-substituted tin hydrides
Seiji Suga, Takao Manabe and Jun-ichi Yoshida*
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University,
Kyoto 606-8501, Japan. E-mail: yoshida@sbchem.kyoto-u.ac.jp
Received (in Cambridge, UK) 11th May 1999, Accepted 26th May 1999
Table 1 Reduction of organic halides with tin hydrides under radical
Tin hydrides having one or two pyridyl groups show unique
conditionsa
reactivity in the reduction of alkyl halides under radical
conditions, indicating the potential of intramolecular coor-
dination for the control of radical reactions.
Yield of RH (%)b
Organic halide
C12H25Cl
Bu3SnH
2
3
The importance of polar effects in radical reactions has attracted
significant interest for many years,1 and solvent effects2 and
anchimeric assistance3 have been reported for various reactions
involving free radicals. We report here unique reactivity of tin
hydrides having pyridyl groups, suggesting the potential of
intramolecular coordination for the control of radical reactions.
This opens a new aspect of free radical chemistry. We also
report the remarkable intermolecular effect of pyridyl groups in
tin hydride reduction.
In recent work we have found the stabilization of cation
radical intermediates by dynamic coordination to tin, which in
turn favors the electron transfer from tetraalkylstannanes.4 In
connection with our interest in controlling chemical reactions
by dynamic intramolecular coordination,5,6 we envisioned that
similar coordination might also be effective for the tin radical 1.
Thus, we synthesized tin hydrides having one or two pyridyl
groups for coordination such as 2 and 3 and examined their
reaction with organic halides under radical conditions (Scheme
1).7
75
97
96
0
0
C
12H25Br
97
98
0
99
99
0
90
97
0
39
97
0
56
94
0
20
96
0
C12H25
I
Ph(CH2)3Cl
Ph(CH2)3Br
Ph(CH2)3I
1-Naphthyl-Cl
1-Naphthyl-Br
1-Naphthyl-I
C8H17CClHCH3
52
100
100
12
93
97
75
a
Reactions were normally carried out with 0.10 mmol of organic halide,
0.12 mmol of tin hydride, and 0.01 mmol of AIBN in 0.4 ml of benzene at
60 °C for 2 h under an atmosphere of argon. b Determined with GC using an
internal standard.
(0.1 equiv.), indicating that the reaction proceeded by a radical
chain mechanism. The facile 5-exo cyclization10 shown in
Scheme 3 also supported a radical mechanism. The ratio of
5-exo/6-endo was similar to that observed for Bu3SnH. The high
yields of the cyclized products also demonstrates the utility of 2
for radical carbon–carbon bond formation.11
In order to get deeper insight into the unique reactivity of 2,
we examined the effect of the pyridyl group on the stability of
the tin radical with molecular orbital calculations.12 The
N
N
SnBuH
N
SnBu2H
2
Sn
2
geometry
optimization
of
a
model
compound,
3
2-PyCH2CH2SnMe2 radical, gave three structures (Fig. 1). The
coordinated structures A and B are more stable than non-
coordinated structure C, indicating that the coordination does
1
Tin hydrides 2 and 3 were synthesized by the LiAlH4
reduction of the corresponding tin halides.4,8 The fact that the
chemical shift from the 119Sn NMR spectrum (in C6D6) of 2 (d
288.270) is similar to that of Bu3SnH (d 287.210) indicated
the absence of coordination of the pyridyl group to tin. This
sharply contrasts to the 119Sn NMR spectrum of the correspond-
ing tin halide, which indicates the presence of a definite
coordination of the pyridyl group to tin.4
The reduction of organic halides with 2, however, exhibited
unique features in contrast to that with Bu3SnH (Table 1).
Although primary alkyl bromides were reduced smoothly with
2, the corresponding chlorides were unchanged under the same
conditions.9 sec-Alkyl chlorides and aromatic chlorides were
also completely inactive towards 2 under these conditions. In
the case of Bu3SnH, however, such organic chlorides were
reduced, albeit slowly. The unique reactivity of 2 seems to be
ascribed to the lower reactivity of tin radical 1 towards organic
halides (Scheme 2).
Py
Py
R
R
H
+
+
Sn
H
Sn
R
2
1
Py
Py
X
+
Sn
+
R
X
Sn
1
Scheme 2
Br
tin hydride
AIBN
O
O
O
The reduction of alkyl bromides with 2 was completely
retarded by the addition of a catalytic amount of hydroquinone
+
R'3SnH
93%
80%
9 : 1
9 : 1
2
R
H
R
X
AIBN
Bu3SnH
Scheme 1
Scheme 3
Chem. Commun., 1999, 1237–1238
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indicate that the reactivity of tin radicals can be controlled even
by intermolecular coordination.
In summary, the results described above demonstrate the
potential of dynamic coordination for the control of radical
reactions. Future studies will hopefully delineate the detailed
mechanistic role of the coordination.
We thank Professor Ilhyong Ryu, Osaka University, for
valuable suggestions and discussions. This work was supported
by a Grant-in-Aid for Scientific Research from Monbusho.
Notes and References
Fig. 1 Optimized structures of the 2-PyCH2CH2SnMe2 radicals and their
relative energies obtained by molecular orbital calculations (MP2/
LANL2DZ).
1 M. W. Wong, A. Pross and L. Radom, J. Am. Chem. Soc., 1994, 116,
11 938; M. W. Wong, A. Pross and L. Radom, J. Am. Chem. Soc., 1993,
115, 11 050 and references therein.
2 S. Hadida, M. S. Super, E. J. Beckman and D. P. Curran, J. Am. Chem.
Soc., 1997, 119, 7406; L. Valgimigli, K. U. Ingold and J. Lusztyk, J.
Org. Chem., 1996, 61, 7947; M. S. Workentin, N. P. Schepp, L. J.
Johnston and D. D. M. Wayner, J. Am. Chem. Soc., 1994, 116, 1141;
S. S. Kim, S. Y. Kim, S. S. Ryou, C. S. Lee and K. H. Yoo, J. Org.
Chem., 1993, 58, 192, and references therein.
3 C. J. Easton and M. C. Merrett, J. Am. Chem. Soc., 1996, 118, 3035;
R. A. Jackson, K. U. Ingold, D. Griller and A. S. Nazran, J. Am. Chem.
Soc., 1985, 107, 208; K. J. Shea, D. C. Lewis and P. S. Skell, J. Am.
Chem. Soc., 1973, 95, 7768; P. S. Skell, D. L. Tuleen and P. D. Readio,
J. Am. Chem. Soc., 1963, 85, 2849; W. Thaler, J. Am. Chem. Soc., 1963,
85, 2607.
4 J. Yoshida and M. Izawa, J. Am. Chem. Soc., 1997, 119, 9361. See also
J. Yoshida, S. Suga, K. Fuke and M. Watanabe, Chem. Lett., 1999,
251.
5 Our results contrast with the pioneering work of Vedejs, which indicated
that the intramolecular coordination of the amino group enhanced the
reducing ability towards carbonyl groups: E. Vedejs, S. M. Duncan and
A. R. Haight, J. Org. Chem., 1993, 58, 3046.
6 Tin hydride reductions: I. Shibata, T. Yoshida, A. Baba and H. Matsuda,
Chem. Lett., 1991, 307; I. Shibata, T. Yoshida, A. Baba and H. Matsuda,
Chem. Lett., 1989, 619. Stille Coupling: E. Vedejs, A. R. Haight and
W. O. Moss, J. Am. Chem. Soc., 1992, 114, 6556. Electrophilic
reactions: B. Jousseaume and P. Villeneuve, J. Chem. Soc., Chem.
Commun., 1987, 513; J. C. Podesta, A. B. Chopa and L. C. Koll,
J. Chem. Res. (S), 1986, 308; H. G. Kuivila, J. E. Dixon, P. L. Maxfield,
N. M. Scarpa, T. M. Topka, K.-H. Tsai and K. R. Wursthorn,
J. Organomet. Chem., 1975, 86, 89.
7 Reduction with 2-(4-pyridyl)ethyldibutyltin hydride has been reported:
D. L. J. Clive and W. Yang, J. Org. Chem., 1995, 60, 2607.
8 M. F. Mahon, K. C. Molloy and P. C. Waterfield, Organometallics,
1993, 12, 769.
9 The reactivity of alkyl bromides is 103–106 times greater than that of
alkyl chlorides, see D. P. Curran, C. P. Jasperse and M. J. Totleben,
J. Org. Chem., 1991, 56, 7169 and references therein.
take place at the radical. Such stabilization might decrease the
reactivity of tin radicals towards organic halides,13 although a
detailed discussion should be reserved until kinetic data are
available.
The reactivity of the tin hydride having two pyridyl groups,
compound 3, was also studied (Table 1). Although organic
iodides were reduced smoothly with 3, the reactions of organic
bromides were much slower. Organic chlorides were almost
inactive towards 3. These data naturally imply that the second
pyridyl group further decreases the reactivity of the tin radical.
It is notable that the introduction of the second pyridyl group
gives us a bonus, i.e. easy separation and recovery of the tin
halide after the reaction.14 The tin halide derived from 3 can be
completely separated by acid extraction and recovered by a
follow-up base extraction, although the tin halide derived from
2 cannot be separated in a similar fashion. The easy protonation
of the second pyridyl group seems to be responsible for this
phenomena because the first pyridyl group should be used for
coordination to the tin halide (vide supra).
In order to examine the effect of intermolecular coordination
on the reactivity of the tin radical, we carried out the radical
reduction of organic halides with Bu3SnH in the presence of an
additive such as pyridine and 2,2’-bipyridyl (Fig. 2). For the
reduction of the alkyl bromide, the reaction took place smoothly
even in the presence of such additives. The reduction of the
alkyl chloride, however, was retarded significantly with an
increase in the amount of additive. It is especially remarkable
that the reaction was almost completely suppressed in the
presence of 0.25 equiv. of bipyridyl. These results clearly
10 A. N. Abeywickrema, A. L. J. Beckwith and S. Gerba, J. Org. Chem.,
1987, 52, 4072; the 6-endo cyclization product might arise via a tandem
rearrangement from the 5-exo product.
11 B. Giese, Radicals in Organic Synthesis: Formation of Carbon-Carbon
Bonds, Pergamon, Oxford, 1986; D. P. Curran, Synthesis, 1988, 417.
12 The molecular orbital calculations were carried out using Gaussian
94W. Revision B.2, M. J. Frisch, G. W. Trucks, H. B. Schlegel, P. M. W.
Gill, B. G. Johnson, M. A. Robb, J. R. Cheeseman, T. Keith, G. A.
Petersson, J. A. Montgomery, K. Raghavachari, M. A. Al-Latham, V. G.
Zakrzewski, J. V. Ortiz, J. B. Foresman, C. Y. Peng, P. Y. Ayala, W.
Chen, M. W. Wong, J. L. Andres, E. S. Replogle, R. Gomperts, R. L.
Martin, D. J. Fox, J. S. Binkley, D. J. Defrees, J. Baker, J. P. Stewart, M.
Head-Gordon, C. Gonzalez and J. A. Pople, Gaussian, Inc., Pittsburgh
PA, 1995. Geometries were optimized and vibrational frequencies were
determined.
13 K. Fukui, Acc. Chem. Res., 1971, 4, 57; I. Fleming, Frontier Orbitals
and Organic Chemical Reactions, Wiley, London, 1976.
14 Several approaches to easy work-up and separation of tin halides have
been developed. For example, D. Crich and S. Sun, J. Org. Chem., 1996,
61, 7200; D. P. Curran and S. Hadida, J. Am. Chem. Soc., 1996, 118,
2531; E. Fouquet, M. Pereyre and A. L. Rodriguez, J. Org. Chem., 1997,
62, 5242. See also ref. 7.
Fig. 2 Effect of pyridine and 2,2A-bipyridyl as additives on the reduction
of alkyl halides by Bu3SnH.
Communication 9/03795K
1238
Chem. Commun., 1999, 1237–1238