Clean and easy photochemistry

Article abstract of DOI:10.2533/chimia.2007.650

In this paper we try to convince you that the usual prejudices against photochemistry are not always well founded, supported by examples from the literature and from our own work. In fact, photochemistry can be a very powerful tool, for example in some elegant total syntheses which use key photochemical steps. Furthermore, the exploitation of chromatic orthogonality which allows wavelength-selective reactions, and recent strategies opening the possibility of enantioselective catalysis in photochemical reactions, expand the scope of this versatile synthetic tool. Finally, state-of-art LED-technology has allowed the development of small and easy-to-use photoreactors. Schweizerische Chemische Gesellschaft.

Full text of DOI:10.2533/chimia.2007.650

PHOTOCHEMISTRY
650
doi:10.2533/chimia.2007.650
CHIMIA 2007, 61, No. 10
Chimia 61 (2007) 650–654
© Schweizerische Chemische Gesellschaft
ISSN 0009–4293
Clean and Easy Photochemistry
Claire-Lise Ciana and Christian G. Bochet*
Abstract: In this paper we try to convince you that the usual prejudices against photochemistry are not always well
founded, supported by examples from the literature and from our own work. In fact, photochemistry can be a very
powerful tool, for example in some elegant total syntheses which use key photochemical steps. Furthermore, the
exploitation of chromatic orthogonality which allows wavelength-selective reactions, and recent strategies opening
the possibility of enantioselective catalysis in photochemical reactions, expand the scope of this versatile synthetic
tool. Finally, state-of-art LED-technology has allowed the development of small and easy-to-use photoreactors.
Keywords: Asymmetric reactions · Chromatic orthogonality · Organic photochemistry · Protecting groups ·
Synthesis
Introduction
O
O
A)
O
O
B)
D)
R
h"
O
R
O
Photochemistry is a powerful tool for the
synthesis of organic molecules. As the elec-
trondistributionintheelectronicallyexcited
stateisusuallyverydifferentfromthatofthe
electronic ground state, so is the reactivity
of photochemically excited molecules. It is
for this reason that photochemistry is a nat-
ural complement to standard thermal chem-
istry; unprecedented atomic connectivities
can be accessed, or reactions of otherwise
inert functional groups can occur. Extreme-
ly elegant and efficient syntheses of natural
products have been achieved this way, such
as Oppolzer’s longifolene,[1] Pirrung’s iso-
comene,[2] Wender’s cedrene,[3] Winkler’s
ingenol[4] or, more recently, Corey’s penta-
cycloanammoxic acid[5] (Scheme 1).
h"
O
O
isocomene
H
longifolene
OCH 3
C)
Cl
R
O
O
H
H
H
H
O
h"
O
h"
O
HO
HO
HO
ingenol
+
OCH3
O
OH
O
OCH3
H
N
cedrene
Cbz
E)
N
O
h"
CbzN
N
[2+2]
+
h"
-N2
O
O
O
h"
MeOH
N 2
COOMe
(CH2 )7 COOMe
pentacycloanammoxic
acid methyl ester
Scheme 1. Natural product syntheses containing a photochemical key step. A) Longifolene by
Oppolzer and Godel,[1] B) isocomene by Pirrung,[2] C) cedrene by Wender and Hobwert,[3] D) ingenol
by Winkler and coworkers,[4] E) pentacycloannamoxic acid by Mascitti and Corey.[5]
Additional attractive features of photo- i) Unpredictability (which should not be
chemical reactions are:
confused with poor reproducibility): It
is indeed true that small changes in the
structure of a photochemically well-
behaved substrate can sometimes dra-
matically modify the outcome or even
inhibit the reaction completely.
i) a relatively low cost, because no other
reagents than photons are needed,
ii) simple workup procedures (essentially
evaporation of the solvent), and
iii) atom-economy.
In view of these obvious advantages ii) Poor selectivity: It is not uncommon
*Correspondence: Prof. Dr. C.G. Bochet
University of Fribourg
Department of Chemistry
Chemin du Musée 9
CH-1700 Fribourg
Tel.: +41 26 300 87 58
of photochemical reactions, the legitimate
question should arise: why are these steps
not more often included in complex synthe-
that the expected product is formed to-
gether with significant amounts of iso-
mers and/or degradation products.
tic planning? Playing the devil’s advocate, iii) Cumbersome experimental procedures:
here are a few commonly claimed draw-
backs:
Although simple on paper, photochemi-
cal reactions can be somewhat impracti-
Fax: +41 26 300 97 38
E-Mail: christian.bochet@unifr.ch

PHOTOCHEMISTRY
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CHIMIA 2007, 61, No. 10
cal. The venerable Hanovia immersion
actions such as the acetylation of an alcohol
mercury lamp requires special glass- or the reduction of a carbonyl group with a
ware, a high-voltage power supply and metal hydride are quite insensitive to per-
numerous occasions to be photocleavable
without significant interference from most
functional groups, substrates and solvents
(Fig. 1).
This reliability has been further con-
adequate protection for the user. The in-
turbations by the environment. In a similar
troduction of the Rayonet® series made perspective, we have optimized very robust
the operation much safer, but at high photochemical reactions, such as the pho-
cost, due to the limited lifetime of the toacylation of nucleophiles to form amides,
firmed by the automated lithographic syn-
thesis of oligonucleotides in DNA chips
lamps and the need for a full set for each carbamates and esters.[7−9] Thus we found
[14]
(immobilized oligonucleotides probes)
(Scheme 4).
Photoremovable protecting groups have
of the four available wavelengths.
that, on irradiation N-acylnitroindolines
iv) No possibility for enantioselective ca- transfer acyl groups to sufficiently strong
talysis: Diastereoselective photochemi- nucleophiles, even at the relatively harm-
cal reactions are relatively common, but less wavelength of 405 nm (Scheme 2).
also been used in total synthesis of natu-
ral products. As an example, o-nitrobenzyl
real asymmetric catalysis is indeed still
a rarity.
The nature of the side chain R1 has very
little influence on the reactivity; even es-
protecting group was used in the total syn-
I
thesis of Calicheamicin γ1 . The stability of
It is the purpose of this article to show, ter-containing chains or highly conjugated
by a few selected examples from literature and potentially photoactive carotenoids are
or our own work, that, although the unre- compatible.[10] Heteroatoms are also toler-
alistic promises which were made in the ated; this allowed us to introduce three re-
60s about the panacean character of pho- agents capable of transferring Cbz, Fmoc
tochemical processes were obviously not and, to a more modest extent, Boc groups to
fulfilled, and although Ciamician’s dream[6] amines[9] (Scheme 3). We emphasize once
has remained a vision, many of the alleged again the very mild conditions required,
o-nitrobenzyl ether toward most chemical
reagents allowed an early introduction and
a clean photodeprotection in 82% yield[15]
(Scheme 5).
Poor Selectivity
A photochemical reaction is invari-
ably initiated by the absorption of a pho-
ton of the wavelength matching the energy
gap between the ground and an excited
state of the reacting functional group (the
chromophore) or the sensitizer, according
to Planck’s law ΔE = hν. Irradiation of a
polyfunctional substrate with polychro-
matic light, such as the one generated by
a medium- or low-pressure mercury lamp,
will excite all chromophores simultane-
ously, with a probability proportional to
their individual absorption coefficients, ε.
The situation is actually not as disastrous
as it may seem, because rapid internal en-
drawbacks have since found solutions.
and the absence of additional aggressive
additives such as acids, bases or even nu-
cleophilic catalysts such as DMAP.
Unpredictability
Mostofthereactions−thermalorphoto-
Protecting groups constitute a painful
chemical−generatingsimultaneouslymany necessity in organic synthesis, but the pain
bonds and stereocenters are subject to nu- is alleviated if the protection/deprotection
merous constraints in terms of substituents, steps are simple and efficient. Photolabile
steric and electronic environment and with protecting groups are advantageous be-
compatibility with other functional groups cause their cleavage requires no reagents
(a notable exception being the Diels-Alder other than light and most are stable toward
reaction). In fact, this wealth of parameters common chemical reagents. Compounds
constitutes an inherent source of unpredict- such as NVOC,[11] NPOC[12] and 5-bromo-
ability. On the other hand, simple basic re- 7-nitro-indoline[13] have been shown on
hν
O
375 or 405 nm
O
O
B r
NuH
O 2 N
Nu
47-99% yield
R 1
H 3C O
H 3C O
O
O
O
X
O
X
N
R
O 2 N
O
NO2
NO2
NO2
N
O
N
H
NO
2 R 1
NVO C
NPO C
X = NR 2, OR, SR, OPO3R, O C OR
5-bromo-7-nitroindoline
NO2
R 1 = C H 3 , (C H 2 ) 3 C l, C H 2 C H 2 C OOC H 3
O H
H 3 C O
Fig. 1. Photolabile protecting groups
O H
C 12H 15N H 2 ,
,
NuH =
,
O
4
O H
O C H 3
Scheme 2. Photoacylation of nucleophiles
hν
P G P G
P G
H N H N H N H N
P G P G P G
P G
H N H N H N H N
P G
A 1 A 1 P G
O
O
P G
C hemical
C oupling
Photo-
H 2N H 2N H N H N
deprotection
O
NR2
O
ROC O = C bz
0-99%
P G A1
O 2N
+
hν
h ν (350 nm),
NR2
R 2N H
ROC O = F moc
19-76%
N
A 5 A 5 A 6
A 6
A 4
A 2
O
C l C H 2C H 2C l
NO2
A 3 A 3 A 4
A 1 A 1 A 2
P G P G P G
P G
P G P G
A 1 A 1 P G
O
P G A2
A 1 A 1 A 2
A 2
P G
H N H N H N H N
R
H N H N H N H N
H N H N H N H N
O
ROC O = B oc
0-29%
O
NR2
Schem 4.
Scheme 3. Photochemical protection of amines with Cbz, Fmoc and Boc
groups
Scheme 4. Concept of light-directed oligonucleotide synthesis

PHOTOCHEMISTRY
652
CHIMIA 2007, 61, No. 10
ergy conversion will normally funnel all
excitations to the lowest lying excited state
C H 3
O
I
H 3C
S
of the entire molecule (Kasha rule). This
O
O
O
O C H 3
H 3C
rule is, however, by no means a law, and
we have uncovered many examples where
it is not followed.[16] In these cases, the use
of monochromatic light carefully tuned to
the excitation wavelength of the desired
chromophore or molecule is essential in or-
der to increase the chances for a selective
OSiE t 3
N
H 3C
E t 3SiO
C H 3O
O C H 3
O
O
O
E t 3SiO
O
OSi E t 3
F moc
O
O 2N
N
H 3C
H 3C O
hν
82% yield
process.[17] Then, the group to be cleaved
can be selected by changing the irradiation
C H 3
O
wavelength; a property that we have named
chromatic orthogonality. For example, the
diesters 1 and 2 can be photolyzed selec-
tively and with good yield at the ‘western’
or ‘eastern’ termini depending on which
I
H 3C
S
O
O
O
O C H 3
H 3C
OSiE t 3
N
H 3C
E t 3SiO
C H 3O
O C H 3
O
O
O
E t 3SiO
O H
OSi E t 3
F moc
O
wavelength is used[18] (Scheme 6A). Thus,
N
H 3C
H 3C O
the sequential photocleavage of the NVOC
and t-butyl ketone moieties present in 2
were exploited in a sequential solid phase
Scheme 5. The photodeprotection step in the total synthesis of
Calicheamicin γ1
I
peptide synthesis[19] (Scheme 6B).
This concept of chromatic orthogonal-
ity can also be put to good use in other types
A )
of processes, for example by combining the
O
1. hν (254 nm),
MeC N, 5 min
2. TMS C H N 2
O
O
O
O
acyl transfer reaction shown earlier with
the cleavage of a protecting group (Scheme
7A). In Scheme 7B, both the introduction
and the removal of the protecting groups are
performed photochemically, but with light
of different wavelengths.[10]
Supramolecular catalysis has also been
used as an efficient tool for regioselective
photoreactions. For example, the regiose-
lectivity of olefin [2+2] photodimerization
has been improved by Bassani and cowork-
ers[20] by using supramolecular catalysis.
H 3 C
H 3 C O
O C H 3
O C H 3
O C H 3
O C H 3
O
O
O
O
4
4
78%
O 2 N
O 2 N
1
O C H 3
1. hν (420 nm),
MeC N, 24h
1. hν (420 nm),
87%
MeC N, 24h
85%
O
2. TMSC H N 2
2. TMS C H N 2
1. hν (254 nm),
MeC N, 5 min
2. TMS C H N 2
O
O
O
O
O
H 3 C O
C H 3
H 3 C
C H 3
O
O
O
4
95%
4
O C H 3
B )
O
O
O
The substrate bears a molecular recognition
O H
1. hν (300 nm)
O
O
O
1. hν (350 nm)
O
O
O C H 3
O H
2. TMSC H N 2
O
O
2. TMS C H N 2
C H 3
3
unit which binds non-covalently (H-bonds)
to a template (barbituric acid derivative) in
order to be oriented during the cycloaddi-
tion (Scheme 8A). Indeed, irradiation (350
nm) of cinnamate ester derivative in the
presence of barbituric acid template en-
hanced the rate of formation of three prod-
ucts (out of eleven possible isomers), the
major one being the syn head-to-head dimer
(Scheme 8B).
H 3 C
C H 3
O
O
O
O
3
3
99%
O 2 N
O C H 3
87%
2
Scheme 6. Orthogonality of photolabile protecting groups
Photoacylating agent
Photolabile protecting group
hν 1
hν 2
A )
B )
+
O C H 3
O C H 3
Cumbersome Experimental
Procedures
B r
hν , (C H 2C l)2
hν , E tOH ,
C 12H 25N H 2
50-100%
+
H
C 12H 25N H 2
N
O
N
O
300 nm, 1h
350 nm, 3h
O C H 3
C 12H 25
O C H 3
It is clear that if the condition for a se-
lective reaction is to have access to relative-
ly monochromatic light, the old-fashioned
high-pressure mercury lamps are not very
useful sources (a narrow band filter has the
double inconvenience of high cost and low
photonic output and cutoff filters do not
produce monochromatic light), in contrast
to low-pressure mercury lamps which pro-
duce almost monochromatic 254 nm light.
Rayonet reactors are efficient but not very
convenient for everyday synthesis. Fortu-
nately, recent developments in UV-LED
technology are offering a solution to these
problems, as nowadays LEDs emitting
down to 254 nm are commercially available.
NO2
O
O
44-59%
Photo-protection
Photo-deprotection
Scheme 7. Photochemical protection followed by photochemical deprotection of an amine
UV-LEDs (the LUMOS reactor shown in
To date, the modest intensity of this devic-
Fig. 2), which allows the irradiation of sam-
ples in simple test or NMR tubes. As an ex-
ample, the acylations of the alcohols shown
in Scheme 2 were carried out with such a re-
actor. Another example, taken from the lit-
erature, is the Paterno-Büchi condensation
of benzaldehyde and furane, which gives a
es is still an issue for the lower end of the
spectrum (between 255 and 350 nm), but it
is already sufficient for investigative-scale
reactions (typically to convert less than 100
mg of a substrate).[7] In collaboration with a
startup company, Atlas photonics, we have
developed a compact photoreactor based on

PHOTOCHEMISTRY
653
CHIMIA 2007, 61, No. 10
h ν
A )
C O 2Me
C O 2Me
MeO 2C CO 2Me
A r A r
C O 2Me C O 2Me
C O 2Me
B )
O
H
O
H
O
MeO2C
MeO
N
N
N
N
N
OMe
n -hex n -hex
H
H
MeO
N
N
MeO
N
N
N
N
N
N
OMe
h ν
C O 2Me
H
O
H
H
N
H
N
H
N
H
N
H
A r A r
MeO2C
O
H
H
N
N
N
H
N
H
N
N
H
H
H
N
C O 2Me
N H 2
O
O
H
n -hex n -hex
O
O
Scheme 8.
A r
n -hex n -hex
major regioisomer
A r
3 formed regioisomers
Supramolecular
control of [2+2]
photodimerization
in the synthesis of pharmaceutical com- and coworkers[25] described the use of a
pounds, because it avoids transition metal catalytic quantity of a chiral agent contain-
residues that may be difficult to eliminate. ing sensitizer and complexation units for
Several strategies have been (and are be- the enantioselective catalysis of [2+2] cy-
54% yield after 16 h irradiation at 375 nm,
without any precautions other than using a
commercially available 18 mm quartz test-
tube[21] (Scheme 9).
ing) developed to obtain chiral selectivity in cloadditions. The concept is to bind the sub-
photoreactions. The main approaches are
strate to the chiral sensitizer via H-bonds.
i) reactions in confined chiral media (zeo- After excitation of the sensitizer, energy
lite, calixarene, crystals),
transfer occurs preferentially to the closer
substrate (the complexed one) and cycload-
dition afforded enantiomerically enriched 4
with modest ee (19% ee at −70 °C using
25 mol% of 6) (Scheme 11A). Performing
the reaction at room temperature or using
a catalyst without H-bonding sites did not
ii) sensitization by chiral sensitizer and
iii) templated induced photoreactions.[24]
In terms of chiral economy, it is prefer-
able to use a substoichiometric quantity of
the chiral agent. In order for this to work the
photoreaction should fulfil two conditions:
i) the energy transfer has to occur in a chi-
ral micro-environment,
ii) the substrate must not react in the ab-
sence of the chiral source.
The following examples represent re-
cent breakthroughs in this domain. Krische
lead to any enantiomeric excess. In a simi-
lar vein, Bach and coworkers[26] developed
a photoinduced electron transfer (PET)
catalyzed conjugated addition of α-amino
alkyl radicals to enones (Scheme 11B). The
Fig. 2. LUMOS reactor
catalyst is made up of an electron accepting
O
Ph
hν
H
+
O
O H
O
O H
O H
OOH
hν (468 nm)
microreactor
O
H
+
O
OOH
~60%
Scheme 9. Paterno-Büchi reaction using UV-
LEDs
O 2, Ru(t bpy)3C l 2
(-)-citronellol
Rose oxide
~40%
UV-LED based microreactors open new
horizons for photochemical syntheses. Of
the few applications that were reported until
now,[22] we choose as an illustration the pho-
tosensitized oxidation of citronellol by sin-
glet oxygen (Scheme 10), which is used to
produce rose oxide for the perfume industry.
Meyer and coworkers[23] compared a normal
batch-reactor with a microreactor in order to
evaluate both reactors’ efficiency. A slight
advantage of microreactor over batch reactor
was found concerning the space-time-turn-
overs and the photonic efficiencies.
Scheme 10. Oxidation of citronellol by singlet oxygen
O C 6H 13
O
N
O
O
O
A )
B )
h ν
5 (25% mol)
N
N
C H 3
H N
H N
H
H
-70°C , 15 min, CDCl 3,
medium pressure
H g vapor lamp
O
O
3
4
5
19% ee
O
100% conv.
N
N
h ν ( λ >300nm)
N H
O
6
O
Enantioselective Catalysis
Toluene, -60°C
N
H
O
N
H
O
N
O
6
8
7
The importance of asymmetric synthe-
5 mol% 8 , 5h : 20%ee; 61% yield
30 mol% 8 , 1h : 70%ee; 64% yield
sis must no longer be demonstrated. The
development of asymmetric photoreactions
would be advantageous compared to their Scheme 11. A) Krische’s asymmetric catalyzed [2+2] cycloaddition. B)
Bach’s PET-catalyzed conjugated addition of amine to enone.
organometallic counterparts, for example

PHOTOCHEMISTRY
654
CHIMIA 2007, 61, No. 10
hν
O
O
A )
(250W high-pressure Na lamp)
TPP (1 mol%),
H O
N
O H
r - C P L
N
O H
O H
+ O2
R
N
L-alanine (20 mol%),
D MSO, 1h
R
N
H
11a
93 % yield
56% ee
96% yield
11a
N
O
N
O H
65% ee
12
+ i-Pr2Zn
R
N
R
N
t-B u
R =
N
C OOH
N
C OO
N
N
O H
N
B )
1O 2
l - C P L
+
11a,b
R
N
H OO
R
R 2
R 2
10
11b
92% yield
76% ee
11b
R 1
R 1
9
Scheme 12. A) Amino acid catalyzed asymmetric α-oxidation of
cylohexanone with molecular oxygen. B) Catalytic enamine mechanism.
Scheme 13. CPL irradiation followed by asymmetric autocatalysis
Stryer, A. T. Lu, D. Solas, Science 1991,
251, 767.
[15] K. C. Nicolaou, C. W. Hummel, M. Na-
kada, K. Shibayama, E. N. Pitsinos, H.
Saimoto, Y. Mizuno, K. U. Baldenius, A.
that only demands to be explored and un-
derstood.
group (benzophenone) and a lactam moiety
which interacts with the substrate through
hydrogen bonding. Significant ees are ob-
tained even with 5 mol% catalyst.
Received: July 17, 2007
Córdova and coworkers[27] published the
amino acid catalyzed asymmetric α-oxida-
tion of ketones and aldehydes with molecu-
lar oxygen or air and a porphyrine sensitizer
L. Smith, J. Am. Chem. Soc. 1993, 115,
[1] W. Oppolzer, T. Godel, J. Am. Chem. Soc.
7625.
1978, 100, 2583.
[2] M. C. Pirrung, J. Am. Chem. Soc. 1981,
103, 82.
[3] P. A. Wender, J. J. Hobwert, J. Am. Chem.
Soc. 1981, 103, 688.
[4] a) J. D. Winkler, M. B. Rouse, M. F. Grea-
[16] A. Blanc, C. G. Bochet, J. Am. Chem. Soc.
2004, 126, 7174.
(TPP) (Scheme 12A). Several amino acids
[17] T. Bally, Chimia 2007, 61, 645.
are able to catalyze this reaction with high
yield, good ee and regioselectivity, for ex-
ample l-alanine catalyzed oxidation of cy-
clohexanone with 93% yield and 56% ee.
[18] a) C. G. Bochet, Angew. Chem. 2001, 40,
2071; b) A. Blanc, C. G. Bochet, J. Org.
Chem. 2002, 67, 5567.
ney, S. J. Harrison,Y. T. Jeon, J. Am. Chem.
[19] M. Kessler, R. Glatthar, B. Giese, C. G.
Soc. 2002, 124, 9726; b) J. D. Winkler, E.
C. Y. Lee, L. I. Nevels, Org. Lett. 2005, 7,
1489.
The reaction occurs via a catalytic enamine
Bochet, Org. Lett. 2003, 5, 1179.
[20] a) D. M. Bassani, Chimia 2006, 60, 175;
b) D. M. Bassani, V. Darcos, S. Mahony,
J.-P. Desvergne, J. Am. Chem. Soc. 2000,
122, 8795.
mechanism to afford peroxide intermediate
10 (Scheme 12B).
Finally, right (r) or left (l) circularly po-
larized light (CPL) was used in direct asym-
[5] V. Mascitti, E. J. Corey, J. Am. Chem. Soc.
2004, 126, 15664.
[6] G. Ciamician, Science 1912, 36, 385, “On
metric photochemistry.[28] The principle is
[21] a) Atlas Photonics, Application sheet 7:
the arid lands there will spring up indus-
trial colonies without smoke and without
smokestacks; forests of glass tubes will
extend over the plains and glass buildings
will rise everywhere; inside of these will
take place the photochemical processes
that hitherto have been the guarded secret
of the plants, but that will have been mas-
tered by human industry which will know
how to make them bear even more abun-
dant fruit than nature, for nature is not in a
hurry and mankind is”.
that two enantiomeric ground state reactants
do not absorb the same amount of CPL. As
the difference (Δε) is generally small, only
slight ee are possible without amplification.
Soai and coworkers[29] published several ar-
ticles on the asymmetric autocatalytic ad-
dition of dialkylzinc reagents to aromatic
aldehydes. Photodecomposition of racemic
pyrimidyl alkanol 11a,b by r- or l-CPL
induced a slight ee which is amplified by
autocatalysis in the addition of diisopro-
pylzinc to pyrimidyl aldehyde 12 affording
11a in 65% ee (r-CPL) and 11b 76% ee (l-
CPL) (Scheme 13). After three rounds of
consecutive autocatalysis near enantiopure
(>99.5% ee) 11a and 11b were obtained.
Paterno-Büchi; b) A. G. Griesbeck, S.
Stadtmueller, Chem. Ber. 1990, 123, 357.
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