Microphotochemistry: 4,4′-dimethoxybenzophenone mediated photodecarboxylation reactions involving phthalimides

Article abstract of DOI:10.3762/bjoc.7.121

A series of 4,4'-dimethoxybenzophenone mediated intra- and intermolecular photodecarboxylation reactions involving phthalimides have been examined under microflow conditions. Conversion rates, isolated yields and chemoselectivities were compared to analogous reactions in a batch photoreactor. In all cases investigated, the microreactions gave superior results thus proving the superiority of microphotochemistry over conventional technologies.

Full text of DOI:10.3762/bjoc.7.121

Microphotochemistry: 4,4'-Dimethoxybenzophenone
mediated photodecarboxylation reactions
involving phthalimides
Oksana Shvydkiv1, Kieran Nolan1 and Michael Oelgemöller*2
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2011, 7, 1055–1063.
1School of Chemical Sciences, Dublin City University, Dublin 9,
Ireland and 2School of Pharmacy and Molecular Sciences, James
Cook University, Townsville, QLD 4811, Australia
doi:10.3762/bjoc.7.121
Received: 16 May 2011
Accepted: 12 July 2011
Published: 02 August 2011
Email:
Michael Oelgemöller* - michael.oelgemoeller@jcu.edu.au
This article is part of the Thematic Series "Chemistry in flow systems II".
Guest Editor: A. Kirschning
* Corresponding author
Keywords:
microflow; microreactor; photochemistry; photodecarboxylation;
phthalimide
© 2011 Shvydkiv et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
A series of 4,4'-dimethoxybenzophenone mediated intra- and intermolecular photodecarboxylation reactions involving phthalim-
ides have been examined under microflow conditions. Conversion rates, isolated yields and chemoselectivities were compared to
analogous reactions in a batch photoreactor. In all cases investigated, the microreactions gave superior results thus proving the
superiority of microphotochemistry over conventional technologies.
Introduction
Organic photochemistry is a highly successful synthesis method in preparative organic chemistry [13-16]. Microflow reactors
that allows the construction of complex molecules with a “flick (μ-reactors) offer a number of advantages for photochemical
of a switch” [1-4]. Light is furthermore considered a clean transformations. In particular, their narrow reaction channels
“reagent” and consequently, photochemistry has contributed enable extensive penetration of light even at high chromophore
extensively to the growing field of Green Chemistry [5-7]. It is concentrations. In addition, products are removed from the irra-
therefore surprising that synthetic organic photochemistry has diated area thus preventing light-induced follow-up reactions or
been widely neglected by the chemical industry. In fact, most decompositions [17-19]. Recently, a number of photoreactions
photochemical production processes in industry were devel- in microreactors have therefore been described [20-23] and
oped and realized decades ago [8-11]. A major drawback of specialized micro-photoreactors for laboratory- to technical-
photochemistry as a modern research and development (R&D) scale synthesis have been developed [24-26]. We have recently
tool has been the usage of specialized reactors and lamps, which reported on acetone-sensitized photodecarboxylation (PDC)
are often considered “exotic” by synthetic chemists [12]. Over reactions of phthalimides in a commercially available microre-
the last decade, microflow chemistry has emerged as a new tool actor [27]. The photochemistry of phthalimides and its
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analogues has been intensively studied over the last decades characterization purposes from the batch processes. Due to the
[28-32]. Among the various transformations developed, small amounts used under microflow conditions, purification
photodecarboxylation reactions have emerged as efficient and and isolation of products was not attempted. Previous work has,
powerful alkylation procedures with high quantum yields of up however, demonstrated that isolated yields typically match
to 60% [33,34]. Selected transformations have also been real- conversion rates [38].
ized on a semi-technical scale using an advanced falling-film
batch reactor equipped with a 308 nm excimer light source
[35,36]. However, the established PDC protocol utilizes UVB
light for the activation step (direct or acetone sensitized), thus
limiting the desired future application of LEDs [37]. We have
therefore investigated the usage of 4,4’-dimethoxybenzophe-
none (DMBP) as a photocatalyst that absorbs readily in the
UVA region. In this publication we present preliminary results
of five DMBP mediated model transformations (Scheme 1). All
reactions were previously studied under acetone-sensitized
Figure 1: Microreactor (dwell device, mikroglas chemtech) under a UV
exposure panel (Luzchem) and connected to a syringe pump.
conditions using UVB light [27].
Wavelength matching and light penetration
When the absorption spectrum of DMBP in acetonitrile was
compared to the emission spectrum of the chosen UVA lamp
(Figure 2), its important n→π* absorption matched well with
the emission maximum of the light source. At 350 nm, an
Scheme 1: General photodecarboxylation involving phthalimides (the
broken line indicates intra- as well as intermolecular reactions).
extinction coefficient (ε350 nm) for DMBP of 496 L mol−1 cm−1
was determined. In contrast, the crucial n→π* absorption
maximum of N-alkylated phthalimides in acetonitrile lies
Results and Discussion
around 290 nm and consequently, photoprocesses induced by
Experimental setups
direct excitation may be neglected [39].
The reaction setup is shown in Figure 1. A commercially avail-
able dwell device (mikroglas chemtech) was placed under a UV The light transmission for a 1.5 mM DMBP solution in acetoni-
panel (Luzchem) fitted with five 8 W UVA lamps (λ = 350 ± 25 trile (Figure 3) was subsequently calculated using the
nm). The reactor itself was fabricated from Foturan™ glass, Beer–Lambert law [8]. As indicated by vertical lines, both set-
which has a transmission of approximately 30% at 300 nm, and ups guaranteed complete penetration of light at 350 nm. As
consisted of a heat-exchanging channel on the top and a serpen- would be expected from its much smaller path length, the light
tine reaction channel on the bottom. The reaction channel had a transmission in the microchannel was superior at 92%,
total path length of 1.15 m with 20 turns, a depth of 0.5 mm, a compared to 50% in the batch system.
width of 2 mm and a total volume of 1.68 mL. The reaction
mixture was loaded into a programmable syringe pump, α-Photodecarboxylation of N-phthaloylglycine
degassed with nitrogen, pumped through the microreactor (flow The photodecarboxylation of phthaloyl amino acids results in a
rate: 0.028 mL/min) and collected in a flask outside the irradi- formal exchange of –CO2H by –H and offers a convenient
ated area. In a parallel series of experiments, a conventional pathway to primary amines [40]. The reaction of N-phthaloyl-
Rayonet chamber reactor (RPR-200) equipped with sixteen 8 W glycine (1) in acetonitrile using 0.1 equivalents of DMBP as a
UVA lamps in a circular arrangement was used for batch reac- mediator was thus investigated as an early model transforma-
tions. A Pyrex Schlenk flask, with a transmission of approxi- tion (Scheme 2). After 1 hour, complete conversions of 1 to
mately 30% at 300 nm, of 32 mm inner diameter and equipped N-methylphthalimide (2) were achieved in the batch and
with a cold finger of 24 mm diameter, thus creating an effec- microreactor, as demonstrated by 1H NMR spectroscopy. In
tive path length of 4 mm, was inserted into the chamber. After a acetone-d6, the N–CH3 group in 2 showed a singlet at 3.11
fixed irradiation time of 1 h, which was not optimized, and ppm. DMBP remained unchanged and neither photoreduction
work-up the crude reaction products were analyzed by 1H NMR nor photopinacolization products were detectable in the crude
spectroscopy and conversions and selectivities were deter- reaction mixture [41]. An attempt was made to isolate pure 2 by
mined. In represented cases the pure products were isolated for column chromatography but it eluted together with DMBP. The
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Beilstein J. Org. Chem. 2011, 7, 1055–1063.
Figure 2: UV-spectrum of DMBP (in MeCN) versus emission spectrum of the UVA lamp. The vertical dotted line represents the cut-off wavelength of
Foturan™ and Pyrex at 300 nm (approx. 30% transmission).
Figure 3: Light-penetration profile for a 1.5 mM solution of DMBP at 350 nm. The vertical lines represent the path length in the dwell device (vertical
dashed line) versus the effective path length in the Schlenk flask (vertical dotted line).
α-photodecarboxylation is, however, known to proceed with
high selectivity [40].
Photodecarboxylative cyclizations
Two photodecarboxylative cyclization reactions were investi-
gated with potassium phthaloyl-γ-aminobutyrate (3) and potas-
sium phthalimidomethylsulfanylacetate (6) as starting materials
[42]. A main advantage of these transformations is the ease of
removal of the unreacted starting material by simple extraction.
Scheme 2: DMBP mediated α-photodecarboxylation of N-phthaloyl-
glycine (1).
In contrast to acetone-sensitized reactions [43], DMBP-medi-
ated irradiations of 3 furnished mixtures of the desired cycliza-
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Beilstein J. Org. Chem. 2011, 7, 1055–1063.
Scheme 3: Photodecarboxylation of potassium phthaloyl-γ-aminobutyrate (3).
tion product 4 and the simple decarboxylation product 5 derivative 7 could be isolated (Scheme 4). After purification by
(Scheme 3; Table 1). When conducted in the batch system, 4 column chromatography, 7 was obtained in yields of 57% for
and 5 were obtained in a ratio of 87:13. Careful column the batch system and 56% for the microreactor. In acetone-d6,
chromatographic purification gave the polycyclic product 4 in the methylene protons in the thiazolidine ring gave two sets of
an isolated yield of 29%. In the 13C NMR in acetone-d6, the doublets at 2.97/3.39 ppm and 4.36/4.93 ppm. The increased
C–OH group in 4 gave a characteristic singlet at 96.9 ppm. The yield of 7 compared to its carbon-analogue 3 suggests that the
simple decarboxylation product 5 eluted together with DMBP sulfur-atom in α-position to the carboxylate group in 6 acceler-
and could not be obtained in pure form. Its identity was thus ates photodecarboxylation [44,45]. In addition to the high
confirmed by comparison with literature data. In acetone-d6, the chemoselectivity, DMBP remained photostable and could be
terminal –CH3 group in 5 furnished a triplet at 0.90 ppm with a reisolated almost quantitatively during chromatography.
coupling constant of 7.4 Hz. Using the microreactor setup, a 4/5
mixture of 81:19 was isolated after 1 h of exposure. Despite the
slightly lower selectivity, the cyclization product 4 was isolated
in an improved yield of 47%. In both cases, DMBP showed no
signs of decomposition suggesting that the reactions had not
reached completion. Possible unreacted starting material was
removed by extraction and no recovery attempts were made.
Scheme 4: Photodecarboxylative cyclization of potassium phthalimido-
Table 1: Experimental results for the photodecarboxylation of 3.
methylsulfanylacetate (6).
batch
μ-reactor
Photodecarboxylative additions
time [h]
1
1
Phthalimides can be efficiently alkylated by photodecarboxyla-
tion of carboxylates and this methodology has emerged as a
powerful alternative to Grignard additions [46-48]. In contrast
to the acetone-sensitized procedure, the DMBP mediated reac-
tion of N-methylphthalimide (2) and potassium phenylacetate
(8) furnished the corresponding phenylmethyleneisoindolinone
4/5 ratioa
yield 4 [%]b
87/13
29
81/19
47
aDetermined by 1H NMR analysis of the crude product. bIsolated yield
after column chromatography.
When potassium phthalimidomethylsulfanylacetate (6) was 10 with high E-selectivity (Scheme 5; Table 2), as determined
used as the starting material, only the polycyclic thiazolidine by comparison with literature data. In acetone-d6, the olefinic
Scheme 5: Photodecarboxylative benzylation of 2.
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Beilstein J. Org. Chem. 2011, 7, 1055–1063.
proton gave a clear singlet at 6.70 ppm. The formation of 10 can H/D exchange at the adjacent –OH group. Under batch condi-
be explained by subsequent dehydration of the initially formed tions, the reaction had reached completion after 1 h and a 9:1
benzylated hydroxyphthalimidine 9, a process favored by the mixture of 12/13 was isolated. In addition, a large proportion of
extensive conjugation in 10 [49]. Under batch conditions, an DMBP underwent photoreduction processes [41]. Column chro-
almost complete conversion of 2 to 10 of 98% was achieved matography gave pure 12 and 13 in yields of 41% and 9%, res-
after 1 h of irradiation. Partial photoreduction of DMBP (ca. pectively. When performed in the microreactor, the conversion
10%) was furthermore observed. Using the same residence was lower with 90%, but the transformation was highly selec-
time, the transformation under microflow condition furnished a tive. Neither the photoreduction product 13 nor any decomposi-
conversion to 10 of 96% but showed no decomposition of the tion products of DMBP were identified by NMR analysis of the
mediator DMBP. Compound 10 could not be isolated in pure crude reaction mixture.
form but its NMR data was identical to that of an indepen-
dently synthesized sample [49].
Table 3: Experimental results for the photodecarboxylative addition of
11 to 2.
Table 2: Experimental results for the photobenzylation of 2.
batch
μ-reactor
batch
μ-reactor
time [h]
1
1
conversion [%]a
12/13 ratioa
yield 12 [%]b
100
90/10
41
90
time [h]
1
1
100/0
n.d.c
conversion [%]a
(E/Z)-10 ratioa
98
96
>10/1
>10/1
aDetermined by 1H NMR analysis of the crude product. bIsolated yield
after column chromatography. cYield not determined.
aDetermined by 1H NMR analysis of the crude product.
Likewise, the addition of potassium 2-(methylthio)acetate (11)
to N-methylphthalimide (2) was investigated [50]. In the larger To investigate the role of DMBP, the reaction was repeated
Rayonet (batch) reactor, the photoreduction product of under batch conditions, but in the absence of N-methylphthal-
N-methylphthalimide, i.e., compound 13, was identified next to imide (2). After one hour, DMBP was completely consumed
the expected addition product 12 (Scheme 6; Table 3). In its and its corresponding addition product 14 was obtained next to
1H NMR spectrum in acetone-d6, the addition product 12 the expected benzpinacol 15 (Scheme 7). In acetone-d6, 14 gave
showed a pair of doublets for the –CH2S group at 3.20 and 3.27 singlets at 1.96 ppm for its –SCH3- and at 3.41 ppm for its
ppm with a 2J coupling constant of 14.0 Hz. In contrast, the –CH2S group, respectively. These values closely match those
reduction product 13 gave a doublet for its –CH group at 5.78 for the related benzophenone adduct [51]. The product ratio of
ppm, which changed into a singlet upon addition of D2O due to 14/15 was determined to be 55:45.
Scheme 6: Photodecarboxylative addition of 11 to 2.
Scheme 7: Photodecarboxylative addition of 11 to DMBP.
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Beilstein J. Org. Chem. 2011, 7, 1055–1063.
Reactor comparison
cies due to possible contributions from “over-irradiation”. Once
Based on the conversions or yields achieved, the two reactor all phthalimide is consumed, photoreduction of DMBP becomes
systems showed very similar performances. Judged by the the dominant reaction due to its continuing excitation [41]. The
amounts of by-products, however, the product quality was degree of these decomposition processes can thus be used as an
somewhat superior for the microsystem. This finding is indicator for “over-irradiation”.
primarily attributed to the flow design of the dwell device
which removes the product mixture from the irradiated area and Mechanistic scenario
consequently prevents follow-up reactions. The key parameters For ketone-sensitized photodecarboxylations involving phthal-
for the batch and microreactor are compiled in Table 4. imides, energy transfer and electron transfer processes have
Compared to the Schlenk tubes (50 mL and 100 mL), the irradi- both been proposed [44,52]. A similar, simplified scenario is
ated area-to-volume (surface-to-volume) ratio of the dwell depicted in Scheme 8. Due to the comparable triplet energies of
device was five to eight times larger with 1369 m2/m3. The DMBP (T1 = 69.4 kcal/mol or 290 kJ/mol [53]) and phthalim-
dwell setup also gave the largest lamp power to irradiated area ides (2: T1 = 71 kcal/mol or 297 kJ/mol [39]), energy transfer
ratio of 1.74 W/cm2. The batch reactor incorporating the 50 mL (Scheme 8, path A) is not very efficient but has been confirmed
Schlenk flask achieved a slightly lower value of 1.50 W/cm2, spectroscopically for a related N-phthalimidocarboxylate/
whereas the larger 100 mL Schlenk vessel gave the smallest benzophenone pair [52]. Subsequent electron transfer (ET) from
ratio of 0.47 W/cm2.
the carboxylate function to the triplet excited phthalimide
furnishes an unstable carboxy radical, which undergoes rapid
decarboxylation to the corresponding carbon radical. Protona-
tion and C–C bond formation yields compounds 4 and 9, and
the latter undergoes further dehydration to 10. Alternatively,
back electron transfer (BET) and protonation generates the
simple decarboxylation products 2 and 5. Path A thus mirrors
the mechanism proposed for acetone sensitization [43]. In
contrast to carboxylates (for MeCO2− calc. EOx = 1.54 V in
MeCN versus SCE), thioethers (for Me2S: EOx = 1.23 V versus
SCE) are more readily oxidized [23]. As a result, electron
transfer to the triplet excited DMBP becomes energetically
feasible (Scheme 8, path B) [52]. Similar electron transfer
scenarios have been established for photoreactions of
N-methylphthalimide or benzophenone with either thioethers
[54,55] or alkyl- and arylthioacetic acids [48,56], respectively.
Table 4: Technical details of the two reactor types.
Parameter
batcha
μ-reactor
aperture [cm2]b
85 / 274
85 / 274
50 / 100
86.1
23.0
1.7
irradiated area [cm2]
irradiated volume [cm3]
irradiated area/volume ratio [m2/m3] 171 / 274 1369
lamp power [W]
16 × 8
5 × 8
0.46
1.74
lamp power/aperture [W/cm2]
lamp power/irradiated area [W/cm2]
1.5 / 0.5
1.5 / 0.5
aValues given for 50 mL and 100 mL flask volumes. bAssuming a cylin-
drical geometry for the Schlenk flask.
The conversion/yield per watt-hour (Wh), and the conversion/ With compound 6 or in the presence of 2, successive electron
yield per Wh per irradiated area were furthermore determined transfer generates the corresponding phthalimide radical anion.
for all transformations studied (Table 5) [38]. In all cases, the Subsequent decarboxylation, protonation and C–C bond forma-
dwell device showed significantly larger energy efficiencies tion furnish products 7 and 12. In the absence of N-methylph-
than the batch reactor. The values obtained for experiments with thalimide 2, protonation and C–C bond formation to 14 or
complete conversions represent the minimum energy efficien- photopinacolization to 15 operate instead (not shown).
Table 5: Energy efficiencies of the two reactor types.
Reaction
batcha
μ-reactora
[% Wh−1]
[% Wh−1]
[% Wh−1 cm−2]
[% Wh−1 cm−2]
1 → 2
≥0.78b
0.23
≥0.0028b
0.0027
≥2.5b
1.18
1.40
2.40
2.25
≥0.11b
0.05
3 → 4c
6 → 7c
2 → 10
2 → 12
0.45
0.0052
0.06
0.77
0.0028
0.10
≥0.78b
≥0.0028b
0.09
aBatch: 128 Wh; microreactor: 40 Wh. bMinimum values due to possible “over-irradiation”. cBased on isolated yield of 4 or 7.
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Scheme 8: Mechanistic scenario (the broken line indicates intra- and intermolecular reactions).
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