7424
M. Kamata et al. / Tetrahedron Letters 45 (2004) 7423–7428
1O2
3O2
+
R
R
CN
NC
NC
CN
CN
hνCT
CN
NC
TCNE
R
O
+
O
CH2Cl2
NC
R
R
R
TCNE
1
2
R = H, Me
3O2
NC
CN
CN
Ph
NC
CN
CN
NC
hνCT
TCNE
+
Ph
Ph
Ph
Ph
CH2Cl2
O O
3
NC
Ph
TCNE
4
Scheme 1.
9
a proton acceptor in a Pyrex tube was irradiated by a
2-kW Xe lamp (>360nm) for 10min, 1,4-di(p-methoxy-
yield (16% yield at 18% conversion) as the sole product
(Scheme 2; entry 1 in Table 1). Prolonged irradiation
(30min) of the same solution under otherwise the same
conditions increased the conversion of 5a and the yield
of 6a (55% yield at 66% conversion: entry 2). Likewise,
photosensitized reactions of 5b and 5c predominantly
afforded the corresponding 1,4-diketones 6b (95%) and
6c (94%), respectively (entries 3 and 4). It seems that
the low reactivity of 5a compared to that of 5b and c
(entries 2–4) is ascribed to a lower oxidation potential
of the p-anisyl group than those of the p-tolyl and phe-
nyl groups. Thus, the radical ion pair, generated by the
SET of 5a with TPPBF4, is likely to be deactivated by a
back electron-transfer in competition with the formation
of 6a since the energy for the hole transfer from an aro-
matic ring to a C4–C5 bond is relatively higher in 5a+Å
than that in 5b+Å and 5c+Å. Detailed mechanistic studies
further provided the following results. In the absence
of light, 5a–c were completely recovered even after
24h. In the absence of TPPBF4, only small amounts of
1,4-diketones 6a–c were produced, respectively, from
5a–c (2–4% yields at 4–9% conversions of 5a–c). Thus,
both the presence of TPPBF4 and irradiation were
essential to promote the degradation of 5. The fluores-
cence of TPPBF4 was effectively quenched by 5a–c.à
9,10-Dicyanoanthracene, however, was not an effective
sensitizer to promote the PET reactions under the same
irradiation conditions.à In addition, the triplet photo-
sensitization using benzophenone did not significantly
increase the conversions of 5a–c and the yields of 6a–c
(entries 5–7). These results would indicate that the radi-
cal cations of 5a–c, generated by the SET with the
excited singlet state of TPPBF4, undergo elimination
,
3a,10
phenyl)butan-1,4-dione 6a
was obtained in good
All products were isolated by silica gel TLC and characterized by
their spectral data. The structures of 6 and 8 were also confirmed by
their authentic spectral data.
Selected data for 6a3a,10: colorless plates (C2H5OH); mp 156–157ꢁC;
IR (KBr, cmꢀ1) 3090, 3030, 2975, 2945, 2910, 2845, 1665 (C@O),
1603, 1575, 1510; 1H NMR (200MHz, CDCl3) d 3.39 (s, 4H), 3.87 (s,
6H), 6.90–6.98 (m, 4H), 7.97–8.07 (m, 4H); 13C NMR (50MHz,
CDCl3) d 32.32 (t, 2C), 55.45 (q, 2C), 113.65 (d, 4C), 129.87 (s, 2C),
130.31 (d, 4C), 163.41 (s, 2C), 197.30 (s, 2C).
Selected data for 7a3a: colorless prisms (CH3OH); mp 70–71ꢁC; IR
(KBr, cmꢀ1) 3030, 2960, 2850, 1713 (C@O), 1607, 1570, 1505; 1H
NMR (200MHz, CDCl3) d 2.63 (t, 2H, J = 6.7Hz), 2.80–2.93 (m,
2H), 3.00–3.08 (m, 2H), 3.81 (s, 3H), 5.98–6.03 (m, 1H), 6.83–6.93 (m,
2H), 7.28–7.37 (m, 2H); 13C NMR (50MHz, CDCl3) d 27.93 (t, 1C),
38.68 (t, 1C), 39.87 (t, 1C), 55.26 (q, 1C), 113.72 (d, 2C), 119.15 (d,
1C), 126.22 (d, 2C), 133.17 (s, 1C), 136.97 (s, 1C), 158.91 (s, 1C),
210.12 (s, 1C); MS (EI) m/z 202 (M+, 100%); UV kmax (CH3CN) 268.5
(e 20,300) nm.
Selected data for 8a3a: colorless prisms (n-hexane–CH2Cl2); mp 54–
55ꢁC; IR (KBr, cmꢀ1) 3400 (OH), 3050, 3025, 2960, 2850, 1610, 1515;
1H NMR (200MHz, CDCl3) d 3.73 (s, 3H), 6.52 (br s, 1H), 6.76 (s,
4H); 13C NMR (50MHz, CDCl3) d 55.85 (q, 1C), 114.88 (d, 2C),
116.05 (d, 2C), 149.42 (s, 1C), 153.50 (s, 1C).
Selected data for 9a: colorless oil; IR (CHCl3, cmꢀ1) 3070, 2960, 2950,
1
2860, 1712 (C@O), 1617, 1590, 1502; H NMR (200MHz, CDCl3) d
2.15–2.39 (m, 4H), 2.55–2.92 (m, 4H), 3.70 (s, 3H), 3.83 (s, 3H), 6.56–
6.74 (m, 4H), 6.88–6.98 (m, 2H), 7.37–7.48 (m, 2H); 13C NMR
(50MHz, CDCl3) d 35.79 (t, 2C), 37.13 (t, 2C), 55.26 (q, 1C), 55.51 (q,
1C), 78.55 (s, 1C), 114.03 (d, 2C), 114.14 (d, 2C), 119.82 (d, 2C),
126.56 (d, 2C), 136.31 (s, 1C), 148.87 (s, 1C), 154.19 (s, 1C), 158.91 (s,
1C), 211.12 (s, 1C).
Selected data for 10: pale yellow needles (n-hexane–CH2Cl2); mp 121–
123ꢁC; IR (CHCl3, cmꢀ1) 3050, 2960, 1683 (C@O), 1659 (C@O),
1600, 1580, 1501; 1H NMR (200MHz, CDCl3) d 4.86 (s, 2H), 7.45 (s,
1H), 7.33–7.62 (m, 11H), 7.94–8.12 (m, 4H); 13C NMR (50MHz,
CDCl3) d 45.95 (t, 1C), 123.58 (d, 1C), 126.73 (d, 2C), 128.23 (d, 4C),
128.49 (d, 2C), 128.56 (d, 2C), 128.70 (d, 2C), 129.26 (d, 1C), 132.63
(d, 1C), 133.03 (d, 1C), 136.98 (s, 1C), 139.01 (s, 1C), 141.94 (s, 1C),
152.63 (s, 1C), 190.70 (s, 1C), 195.97 (s, 1C).
Selected data for 11b: colorless crystalline; IR (CHCl3, cmꢀ1) 3080,
2960, 1709 (C@O), 1510, 1443, 1412, 1320, 1255, 1177, 1135, 1107; 1H
NMR (200MHz, CDCl3) d 2.00–2.21 (m, 2H), 2.24–2.47 (m, 2H),
2.36 (s, 3H), 2.68–2.88 (m, 2H), 3.08 (s, 3H), 7.15–7.25 (m, 2H), 7.28–
7.36 (m, 2H); 13C NMR (50MHz, CDCl3) 21.02 (q, 1C), 34.95 (t, 2C),
50.27 (q, 1C), 75.91 (s, 1C), 125.81 (d, 2C), 129.14 (d, 2C), 137.18 (s,
1C), 140.06 (s, 1C), 211.46 (s, 1C).
à The oxidation potentials of 5a–c were measured by cyclic voltamme-
try at v = 100mVsecꢀ1 in CH3CN containing 0.1M Et4NClO4
(Eo1=x2=V vs SCE: 1.72, 2.08, and 2.08, respectively, for 5a–c) and
found to be low enough to quench the excited singlet state of TPPBF4
(EꢁredðS1Þ ꢂ 2:5V vs SCE).8 Indeed, 5a–c effectively quenched the
fluorescence of TPPBF4 (kqs = 76.8, 54.4, 44.7 in CH2Cl2, respec-
tively, for 5a–c). On the contrary, the fluorescence of DCA
(EꢁredðS1Þ ꢂ 1:9V vs SCE)11 was not measurably quenched by 5b
and 5c but was quenched by 5a. DCA-sensitized photoreactions of
5a–c (DCA = 1.0 · 10ꢀ2 mmol; 2kW Xe lamp; k > 360nm; irradiated
for 30min.) produced the corresponding diketones 6a–c (2–4% yields
at 3–9% conversions of 5a–c) as the sole product.