Chemistry Letters Vol.33, No.6 (2004)
717
dex KMF/0T ¼ sMF=s0T (s: mM/t) was introduced to show MFE.
In the present case, K9.4T/0T ¼ 1:44 and K7.05T/0T ¼ 1:41.
The same experiments were repeated several times and aver-
ages of the indices KMF/0T were calculated as shown in Figure 2,
which also includes the result obtained in the experiments under
0.4 T MF (neodymium magnet).
ꢀg mechanism is applicable, ISC predominates in the presence
of a magnetic field. Consequently, increase of the concentration
of 2 results in acceleration of the reaction rate.
There is a possibility that the present MFE is caused by
increase of oxygen partial pressure.10
It should be emphasized that the reproducibility of the MFE
in this study is good enough to perform the experiments under
various conditions.
2.0
1.46
singlet
t-BuOO
triplet
1.45
1.5
t-BuOO
I
O
I
O
1.02
*
1
2
O
O
1.0
magnetic
field
intersystem crossing
*
0.5
0.0
Figure 4. Intersystem crossing mechanism to interpret the
acceleration of the radical oxidation of isochroman (1) to 1-
isochromanone (7).
9.4 T
(n=3)
7.05 T
(n=6)
0.4 T
(n=2)
The authors are grateful to Dr. Yoshifumi Tanimoto of
Hiroshima University for his kind discussion and encourage-
ment.
Figure 2. The indices KMF/0T obtained by the experiments us-
ing 400 MHz (9.4 T), 300 MHz (7.05 T) NMR spectrometers
and a neodymium magnet (0.4 T).
It is clear that 9.4-T and 7.05-T MF increase the reaction rate
in the range of 45–46% while 0.4-T MF does not affect the reac-
tion rate.
References and Notes
1
ne.jp/grape/jacr/Pub/m 06/06 symp 4.pdf).
a) Y. Tanimoto, H. Hayashi, and S. Nagakura, Chem. Phys.
Lett., 41, 267 (1976). b) Y. Sakaguchi, H. Hayashi, and S.
Nagakura, Bull. Chem. Soc. Jpn., 53, 39 (1980). c) M.
Wakasa, K. Nishizawa, H. Abe, G. Kido, and H. Hayashi,
J. Am. Chem. Soc., 121, 9191 (1999). d) R. De, Y. Fujiwara,
B. Zhang, and Y. Tanimoto, Bull. Chem. Soc. Jpn., 73, 1573
(2000).
The same experiments were performed in benzene instead of
ethyl acetate, affording K9.4T/0T ¼ 2:14 (n ¼ 1) and K7.05T/0T
2
¼
2:12 ꢄ 0:005 (n ¼ 2).
To further confirm the MFE, a ‘cross-experiment’ was intro-
duced: A freshly prepared mixture of 3 and BPBI in ethyl acetate
was delivered into two NMR tubes, one (S7.05T!0T) of which
was set in the 300 MHz NMR probe (30 ꢃC) and the other
(S0T!7.05T) immersed in a water bath (30 ꢃC). After 4 h, the sam-
ples were exchanged with each other. The reaction course was
monitored by taking out a small portion (ca. 10 mL) of the mix-
ture every 30 min, which was subjected to HPLC analysis
(Figure 3). It is also obvious from this experiment that 7.05-T
MF accelerates the oxidation.
3
R. Z. Sagdeev, Y. N. Molin, K. M. Salikhov, T. V. Leshina,
M. A. Kamha, and S. M. Shein, Org. Magn. Reson., 5, 603
(1973).
4
5
6
M. Wakasa and H. Hayashi, Mol. Phys., 100, 1099 (2002).
S. Fukuyoshi and T. Kusumi, Chem. Lett., 2001, 230.
M. Inotani, S. Fukuyoshi, and T. Kusumi, Tetrahedron Lett.,
42, 7451 (2001).
10000
8000
6000
4000
2000
0
10000
8000
6000
4000
2000
0
7.05 T
0 T
7
8
9
M. Ochiai, T. Ito, H. Takahashi, A. Nakanishi, M. Toyonari,
T. Sueda, S. Goto, and M. Shiro, J. Am. Chem. Soc., 118,
7716 (1996).
The temperature of the probe was also confirmed by the
NMR chemical shift difference between CH3 and OH signals
of methanol.
a) U. E. Steiner and T. Ulrich, Chem. Rev., 89, 51 (1989). b)
S. Nagakura, H. Hayashi, and T. Azumi, ‘‘Dynamic Spin
Chemistry,’’ Kodansha/Wiley, Tokyo (1998). c) H. Hayashi,
Y. Sakaguchi, and M. Wakasa, Bull. Chem. Soc. Jpn., 74,
773 (2001). d) Y. Tanimoto and Y. Fujiwara, ‘‘Handbook
of Photochemistry and Photobiology,’’ ed. by H. S. Nalwa,
American Science Publishers (2003), Chap. 10, pp 413—
445.
7.05 T
0 T
(a)
(b)
µ
µ
0
200
400
600
0
200
time/min
400
600
time/min
Figure 3. (a) The reaction course of the sample, 7.05 T!0 T.
(b) The reaction course of the sample, 0 T!7.05 T.
The MFE observed in this study may be interpreted by the
ꢀg mechanism of radical pair mechanism.9 Since the present
radical oxidation is a thermal reaction, homolysis of 1 proceeds
through a singlet radical pair, which either recombines to form 1,
diffuses to radical 2 and tert-butylperoxide radical, or converts to
a triplet radical pair by intersystem crossing (ISC) (Figure 4).
The triplet radical pair can no more recombine to 1 (Pauli theory)
and easily releases 2 and the peroxide radical. In case where the
10 a) N. I. Wakayama, T. Okada, J. Okano, and T. Ozawa,
Jpn. J. Appl. Phys., 40, L269 (2001). b) T. Okada, N. I.
Wakayama, L. Wang, H. Shingu, J. Okano, and T. Ozawa,
Electrochim. Acta, 48, 531 (2003).
Published on the web (Advance View) May 17, 2004; DOI 10.1246/cl.2004.716