Table 2 Reaction of different a,b-unsaturated lactones with methylpyrroli-
dine 2a under different reaction conditions; 0.1 equivalent of the
semiconductor with respect to 1a was added
the ring of 2a,c and never at the methyl group. Therefore, we
propose a tautomeric equilibrium of the iminium ions 7 and 8.11
As described previously, only one diastereomer was isolated.3
Such a bimolecular reaction between two reactive intermediates
appears more realistic if the overall reaction takes place near the
surface of the semiconductor.
Oxidation of an a-aminoalkyl radical by excited TiO2 is also
possible. Therefore, we searched to diminish the two-electron
oxidation by changing the semiconductor (Table 3, entry 4).
When ZnS was used as sensitiser, only product 4g could be
isolated. Despite the more rapid conversion, the yield of the
desired product 4g remained low. Probably, this semiconductor
is less oxidative due to the higher energy level of its valence
band edge12 and its surface properties.
The most apparent difference between the reaction conditions
of the homogeneous and the heterogeneous catalysis is the
concentration of the tertiary amine. In the case of homogeneous
catalysis, acetonitrile must be used as solvent to obtain the best
results3 and the reaction is slower and less chemoselective when
the tertiary amine is used as solvent. These results might be
explained by the higher polarity of acetonitrile solutions which
stabilise the radical ion pairs formed by electron transfer and
decrease the rate of the back electron transfer. In the case of
heterogeneous catalysis, the electron transfer from the amine to
the sensitiser takes place at the surface of the semiconductor.
Molecules at interfaces are less mobile than the same molecules
in solution. Therefore, the back electron transfer can be slowed
down only by enhancement of the subsequent deprotonation
step. This acid–base reaction is facilitated by the presence of a
large excess of tertiary amine. A higher concentration of the
tertiary amine favours also the hydrogen abstraction of the
oxoallyl radical 6 in the chain propagation.
Irradiation
time/h
Conversion Yielda Ratio
1
(%)
(%)
c(3)/c(4)
1b
1c
2
90
100
100
64
98
90
45/55
43/57
44/56
2
1db
3.5
1e
13
20
76
44/56
a Based on conversion of 1. b The starting concentration was
1022 mol L21
.
Table 3 Reaction of (5R)-menthyloxy-2[5H]furanone 1a with N-methylpi-
peridine 2c under different reaction conditions; 0.1 equivalent of the
semiconductor (SC) with respect to 1a was added
We thank Professor J. P. Pete for his support and for helpful
discussions. S. M. thanks the Ministère de la Recherche for a
doctoral fellowship.
Yielda
Notes and references
† Experimental: a Rayonet photochemical chamber reactor equipped with
lamps emitting at l = 350 nm was used as the light source.
Entry Semiconductor c(1a)/mol L21 Conversion (%) 4g
5
A suspension of the substrate (7.5 mmol) and the semiconductor (0.15
mmol) in 150 ml of the tertiary amine was irradiated in Pyrex-tubes (outside
diameter: 4 cm) under vigorous stirring with a magnetic stir bar. The
mixture was filtered through Celite and the solvent was recycled by
distillation under reduced pressure. The residue was purified by flash
chromatography (silica gel, eluent: light petroleum–ethyl acetate: 2+1)
1
2
3
4
TiO2
TiO2
TiO2
ZnS
5 3 1022
5 3 1022
1022
72
15
34
94
—
74
b
Trace 90
28
23
62
Trace
1022
a Based on conversion of 1a. b The amine was distilled over CaH2 under
argon and the semiconductor was kept at 100 °C for 48 h.
1 B. Giese, Radicals in Organic Synthesis: Formation of Carbon–Carbon
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product 5 resulting from a Michael addition of piperidine to 1a
was isolated (Table 3, entry 1). Due to their lower reactivity of
a-aminoalkyl radicals derived from N-methylpiperidine, the
oxidation of these radicals became competitive and demethyla-
tion occurred.11 Even under strictly anhydrous conditions, this
side reaction took place (entry 2). However, when the
concentration of compound 1a was reduced, the radical addition
could be observed and product 4g was isolated with moderate
yield (entry 3). This result indicates that 1 or, more likely, the
oxoallyl radical 6 might participate in the demethylation process
via an electron transfer from the radical cation to 7 (Scheme 2).
It should be noted that the radical addition always takes place on
9 W. Schindler, F. Knoch and H. Kisch, Chem. Ber., 1996, 129, 925; R.
Künnerth, C. Feldmer, F. Knoch and H. Kisch, Chem. Eur. J., 1995, 1,
441.
10 J. Eriksen, Photoinduced Electron Transfer, Part A. ed. M. A. Fox and
M. Chanon, Elsevier, Amsterdam, 1988, ch 1.10; P. V. Kamat, Chem.
Rev., 1993, 93, 267.
11 C. Ferroud, P. Rool and J. Santamaria, Tetrahedron Lett., 1998, 39,
9423; G. Pandey, Synlett, 1992, 546; X. Zhang, Y. S. Jung, P. Mariano,
M. A. Fox, P. S. Martin and J. Merkert, Tetrahedron Lett., 1993, 34,
5239; M. Bietti, A. Cuppoletti, C. Dagostin, C. Florea, C. Galli, P.
Gentili, H. Peride and C. R. Caia, Eur. J. Org. Chem., 1998, 2425.
12 T. Sakata, Photocatalysis, ed. N. Serpone and E. Pelizzetti, J. Wiley &
Sons, New York, 1989, p. 311.
Scheme 2 Possible mechanism of the oxidation of an a-aminoalkyl radical
by an oxoallyl radical introducing the demethylation of 2c.
Chem. Commun., 2001, 1576–1578
1577