Angewandte
Chemie
irradiation we were unable to detect 2. 3) In the laser
experiments, with a much higher photon flux, the longer
lifetime of T1 may allow it to absorb a second photon, thus
resulting in a higher-energy triplet state Tn. The second
photon most likely comes from the same laser pulse as the
first because of fast ISC. There are no stringent wavelength
restrictions, because at the two-photon energy, the density of
Tn states will be high. This constant depopulation means that
the effective lifetime of T1 is too short to allow formation of
type II products. 4) Tn then likely undergoes ISC’ back to Sm
and then to S1 by quenching (kQ’). This repopulated S1 state
can then react to type I products (kI), although it mainly
undergoes quenching (kQ) to S0 and ISC (kISC) back to T1 as
before. This provides another channel for type I products in
the laser experiments. The low quantum yield associated with
kI (F = 0.5–3.3%) is consistent with the inefficiency of this
type I process.
In summary, we have shown that Norrish type I and
type II reactions of a dione substrate can be controlled by
appropriate choice of UV light source. It has been found that
the different mechanistic pathways are controlled by photon
flux. Specifically, in the laser experiments it is suggested that
the triplet state leading to type II products is depopulated by
further excitation.[23] This effectively shuts down the type II
pathway and favors reaction by the type I route. These
findings offer intriguing opportunities for reaction control in
synthetic organic photochemistry using high-power mono-
chromatic laser sources. For example, this study clearly
demonstrates that the formation of the lactone natural
products 7 by photochemical means would not be possible
using conventional lamp techniques; yet such products are
directly accessible by laser irradiation of readily available
diones.
The likelihood that kQ > kISC @ kI provides an explanation
for why only trace amounts (less than 1%)[20] of the type I Experimental Section
7 (E/Z): A solution of 6 in acetonitrile (0.04m, 25 mL) was placed in a
products can form upon lamp irradiation. Furthermore, as the
photon flux in the lamp is low, T1!Tn excitation is unlikely.
The reduced photon flux in the scattered laser beam experi-
ments accounts for the observed reaction of 1 by mixed type I
and II pathways.
quartz cell and irradiated at 294 nm for 20 h using the output from a
Nd:YAG pumped dye laser system (400 mW). Concentration in
vacuo and purification by flash chromatography (8% Et2O in hexane)
gave 7 as a fragrant volatile liquid (68 mg, 54% based on recovered
starting material).
It is also possible that type I products are formed from
direct reaction (kIII) of Sm, but this would be a non-Kasha
process. With a pulsed nanosecond Nd:YAG laser, however, it
is improbable that Sm could be populated directly from S1, as
the lifetime of the latter is short. A second photon absorption
would be more favorable for the photon fluxes provided by an
ultrafast laser source.[6a] As discussed above, Sm could be
accessed by ISC’ from Tn, but this pathway should be
quenched by isoprene. Triplet–triplet annihilation (T1 + T1!
S1 + S0) can also be considered as a possible explanation.[1,3,6c]
S1 could then follow the same pathway as discussed above. If it
were to proceed by this mode, however, some type II products
would be expected on laser irradiation of 1 as the T1
concentration falls to a point where bimolecular annihilation
would be much less likely.
Finally, we explored the application of these finding to the
synthesis of alkylidenephthalide natural products.[21] Both E
and Z isomers of 3-butylidinephthalide 7 can be isolated as
volatile components from the fragrant oils of a number of
plant sources.[22] We were intrigued to see if these lactones
could be synthesized from 2-propyl-1,3-indandione 6. Irradi-
ation of 6 under lamp conditions gave the indandione 3 only,
by way of a diverted type II pathway. Irradiation of 6 at
294 nm with a Nd:YAG laser gave the lactones 7 as an E/Z
mixture (Scheme 4).
(Z)-7: IR (film): n˜ = 2959 (w), 2929 (w), 1770 (s), 1686 (w),
1473 cmꢀ1; 1H NMR (CDCl3): d = 0.99 (t, 3J(H,H) = 7.3 Hz, 3H), 1.56
3
3
(dq, J(H,H) = 7.3, 7.6 Hz, 2H), 2.46 (q, J(H,H) = 7.6 Hz, 2H), 5.65
(t, 3J(H,H) = 7.9 Hz, 1H), 7.49–7.53 (m, 1H), 7.63–7.70 (m, 2H), 7.89–
7.92 ppm (m, 1H); 13C NMR (CDCl3): d = 13.9 (CH3), 22.6 (CH2),
=
27.9 (CH2), 109.6 (CH), 119.7 (CH), 124.5 (C C), 125.3 (CH), 129.4
=
=
=
(CH), 134.3 (CH), 139.6 (C C), 145.8 (C C), 167.3 ppm (C O); MS
(70 eV): m/z (%) = 189 (100), 159 (15).
(E)-7: 1H NMR (CDCl3): d = 1.03 (t, 3J(H,H) = 7.3 Hz, 3H), 1.63
(dq, J(H,H) = 7.3, 7.6 Hz, 2H), 2.54 (q, J(H,H) = 7.6 Hz, 2H), 5.86
(t, 3J(H,H) = 8.3 Hz, 1H), 7.54–7.58 (m, 1H), 7.70–7.76 (m, 1H), 7.82–
7.87 (m, 1H), 7.93–7.97 ppm (m, 1H). d = 13.9 (CH3), 22.9 (CH2), 28.1
3
3
=
(CH2), 114.1 (CH), 123.4 (CH), 125.6 (CH), 126.2 (C C), 129.6 (CH),
=
=
=
134.3 (CH), 138.4 (C C), 145.8 (C C), 166.8 ppm (C O); MS
(70 eV): m/z (%) = 189 (100), 159 (15).
3: A solution of 6 in acetonitrile (11 mm, 100 ml) was irradiated
(4.5 h) using a 125-W medium-pressure mercury lamp in a pyrex
immersion well. Purification by flash chromatography (Et2O/Hex =
3:7 to 2:3) gave 3 (28%) as a purple solid. IR (neat): n˜ = 2918 (w),
1744 (m), 1702 (s), 1585 (m), 1350 (m), 1254 (s), 766 (s) cmꢀ1
.
1H NMR (CDCl3): d = 3.26 (s, 2H), 7.83–7.87 (m, 2H), 7.97–8.00 ppm
(m, 2H). 13C NMR (CDCl3): d = 45.1(CH 2), 123.1 (CH), 135.7 (CH),
=
=
143.3 (C C), 197.4 ppm (C O); MS (70 eV): m/z (%) = 147 (100),
146 (20).
Received: October 17, 2007
Revised: November 16, 2007
Published online: February 18, 2008
Keywords: lactones · laser chemistry · photochemistry ·
.
photon flux · reaction control
[1] a) N. J. Turro, Modern Molecular Photochemistry, Benjamin/
Cummings, California, 1978; b) A. Gilbert, J. Baggot, Essentials
of Molecular Photochemistry, Blackewll Scientific, Oxford, 1991.
[2] a) M. Gonzµlez-BØjar, S. E. Stiriba, L. R. Domingo, J. PØrez-
Prieto, M. A. Miranda, J. Org. Chem. 2006, 71, 6932; b) T.
Scheme 4. Laser-specific synthesis of the naturally occurring E/Z
3-butylidinephthalides 7.
Angew. Chem. Int. Ed. 2008, 47, 2283 –2286
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2285