Homogeneous and heterogeneous photoredoxcatalyzed hydroxymethylation of ketones and keto esters: Catalyst screening, chemoselectivity and dilution effects

Article abstract of DOI:10.3762/bjoc.10.114

The homogeneous titanium- and dye-catalyzed as well as the heterogeneous semiconductor particle-catalyzed photohydroxymethylation of ketones by methanol were investigated in order to evaluate the most active photocatalyst system. Dialkoxytitanium dichlorides are the most efficient species for chemoselective hydroxymethylation of acetophenone as well as other aromatic and aliphatic ketones. Pinacol coupling is the dominant process for semiconductor catalysis and ketone reduction dominates the Ti(OiPr)4/methanol or isopropanol systems. Application of dilution effects on the TiO₂ catalysis leads to an increase in hydroxymethylation at the expense of the pinacol coupling.

Full text of DOI:10.3762/bjoc.10.114

Homogeneous and heterogeneous photoredox-
catalyzed hydroxymethylation of ketones and
keto esters: catalyst screening, chemoselectivity
and dilution effects
Axel G. Griesbeck* and Melissa Reckenthäler
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2014, 10, 1143–1150.
University of Cologne, Department of Chemistry, Organic Chemistry,
Greinstr. 4, D-50939 Köln, Germany; Fax: +49(221)470 5057
doi:10.3762/bjoc.10.114
Received: 03 February 2014
Accepted: 22 April 2014
Published: 19 May 2014
Email:
Axel G. Griesbeck* - griesbeck@uni-koeln.de
* Corresponding author
This article is part of the Thematic Series "Organic synthesis using
photoredox catalysis".
Keywords:
alkylation; carbonyl; photocatalysis; photoredox catalysis; redox;
semiconductor
Associate Editor: T. P. Yoon
© 2014 Griesbeck and Reckenthäler; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
The homogeneous titanium- and dye-catalyzed as well as the heterogeneous semiconductor particle-catalyzed photohydroxymethyl-
ation of ketones by methanol were investigated in order to evaluate the most active photocatalyst system. Dialkoxytitanium
dichlorides are the most efficient species for chemoselective hydroxymethylation of acetophenone as well as other aromatic and ali-
phatic ketones. Pinacol coupling is the dominant process for semiconductor catalysis and ketone reduction dominates the Ti(OiPr)4/
methanol or isopropanol systems. Application of dilution effects on the TiO2 catalysis leads to an increase in hydroxymethylation at
the expense of the pinacol coupling.
Introduction
Stimulated by the principles of sustainable chemical synthesis of appropriate sacrificial compounds such as electron and hole
and the progress in our understanding of catalytic and photoin- donors, elements that also appear in the natural photosynthesis,
duced electron-transfer processes, in recent years photoredox the role model for all applications. Catalysts that can function as
catalysis emerged as a new and powerful area for advanced syn- light-absorbing and as redox-activating species must combine
thesis [1-10]. There are numerous features that characterize an several features: redox-inactivity in the electronic ground states,
effective photoredox catalytic cycle: light absorption, charge optimal absorption properties in the near UV or visible region
separation, charge transport and annihilation as well as the use and appropriate redox activity in the excited states. Many potent
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Beilstein J. Org. Chem. 2014, 10, 1143–1150.
photoredox catalysts with sufficient long-term stability are tran-
sition metal complexes with excited MLCT states that can be
generated in the visible. Another important group of photocat-
alytic active compounds are semiconductor particles that absorb
in the UV-A and near visible reagion. The widely used TiO2 has
found numerous applications in photochemical water detoxifi-
cation or surface purification because of its favourable excited-
state redox properties [11]. In synthetic applications of semi-
Scheme 1: Photohydroxymethylation of carbonyl compounds and
imines.
conductor photocatalysis two clearly distinguishable reaction In order to evaluate the nature of the active catalytic species in
protocols were designated as type A and B by Kisch and the photochemical homogenous titanium-catalyzed hydroxy-
co-workers [12-15]. In the type A process, two different prod- methylation and to develop a truly catalytic process, we used a
ucts are formed from the initially formed electron–hole pair, model reaction for catalyst screening (acetophenone/methanol)
one from the substrate radical cation that is formed from elec- and applied the optimal homogenous reaction conditions
tron transfer to the semiconductor valence band hole, the other involving titanium catalysis to other ketones and keto esters.
from the substrate radical anion that is formed from reduction
Results
by the semiconductor conduction band electron. Mostly, one of
Nature of the homogeneous catalytic
titanium species
these steps consumes a sacrificial electron/hole donor. In type B
photocatalysis, combination of the radical ions leads to a new
product without the need of sacrificial components. The latter
process proceeds with a high degree of atom economy [16]. We
have recently demonstrated this for the azido-hydroperoxida-
tion of alkenes, a convenient method for the synthesis of 1,2-
amino alcohols [17,18]. In the field of C–C coupling reactions,
the direct hydroxyalkylation of carbonyl compounds and car-
bonyl analogs is a demanding task because the α-CH activation
of alcohols must occur in the presence of the acidic and nucleo-
philic hydroxy group. Thus, protection and deactivation of this
group is necessary for thermal processes. In contrast to that,
photochemical redox activation is possible in the presence of
titanium(IV) catalysts [19-22]. As shown in a series of papers
by Sato and coworkers, carbonyl compounds 1 as well as
imines couple with methanol to give the 1,2-diols or 1,2-amino
alcohols, respectively, when irradiated in the presence of stoi-
chiometric or sub-stoichiometric amounts of titanium tetrachlo-
ride (Scheme 1). In order to run these reactions to completion,
not less than 0.5 equivalents of TiCl4 were necessary which
accounts for severe catalyst consumption. Furthermore, the ad-
dition of TiCl4 to methanol solutions is cumbersome and it is
unclear what species is catalytically active. These processes
have thermal counterparts in reduced titanium-mediated chem-
The original protocol for photocatalytic hydroxymethylation
involves titanium tetrachloride in methanol as the reactive cata-
lyst/donor mixture and carbonyl compounds as the acceptor
components. During the exothermic dissolution process of
TiCl4 in methanol with formation of gaseous HCl, a slightly
yellowish solution is formed that, after irradiation with UV-A
light, turns into a bluish solution indicating the formation of
reduced titanium species. Obviously, ligand exchange reactions
lead to a series of chloro- and methoxy-titanium complexes that
have different catalytic activities. In order to simulate the
different complex stages, we applied different monomeric tita-
nium complexes of the type TiCln(OiPr)4−n (n = 0, 1, 2, 3) [25-
27] in the model process, the irradiation of a solution of aceto-
phenone (3) in methanol (Scheme 2). In the absence of any tita-
nium species, photolysis at 254 and 300 nm, respectively, led
only to trace amounts of the hydroxymethylation product 4 via a
(triplet carbonyl) excited-state hydrogen-transfer process, obvi-
ously a sluggish process under these conditions (Table 1). In the
presence of titanium complexes TiCln(OiPr)4−n, coupling and
reduction products 4 and 6 were formed without pinacol 5 for-
mation (Scheme 2).
istry, e.g., the Ti(III)/t-BuOOH-mediated hydroxymethylation In the presence of the tetraalkoxide Ti(OiPr)4, only the reduc-
of imines [23,24].
tion product 6 was detected which demonstrates that chlorotita-
Scheme 2: Model process: photocatalyzed acetophenone/methanol reaction.
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Beilstein J. Org. Chem. 2014, 10, 1143–1150.
Table 1: Homogeneous sensitizer variation for the acetophenone model reaction.
catalysta
none
irrad. wave-length (nm) yield 4 (%)b
yield 5 (%)b
yield 6 (%)b
300
254
<5c
<5c
–
–
–
–
TiCl4
300
254
33
34
–
–
–
–
TiCl3OiPr
TiCl2(OiPr)2
TiCl(OiPr)3
Ti(OiPr)4
300
254
31
47
–
–
–
–
300
254
0
60
–
–
–
–
300
254
<5d
<5d
–
–
–
–
300
254
–
–
–
–
–
46
Ti(OiPr)4/iPrOHe
Ti(OiPr)4/BF3d
Ti(OiPr)4/AlCl3d
254
254
254
–
45
49
–
–
–
43
–
–
a0.375 mmol catalyst in methanol (6 mL), 0.75 mmol acetophenone, irradiation time 72 h, rt; bisolated yields ; ctrace amounts detected in 1H NMR;
d0.75 mmol added in methanol; e0.375 mmol cat. in isopropanol.
nium complexes are crucial for the desired reaction path. The the heterogeneous TiO2 photolyses demonstrates that under
results from TiCl4 and TiCl3OiPr were nearly identical at both semiconductor conditions pinacolization becomes the major
wavelengths whereas for TiCl2(OiPr)2 catalytic activity was path, but only for aromatic ketones. Aliphatic ketones did not
preserved only for the 254 nm irradiation. These results show show conversion under TiO2 photolyses.
that different catalytically active species must exist that can be
These conditions were applied to keto ester substrates that
feature an additional trapping site for the primary hydroxy
group. We envisaged the formation of lactones from the corres-
ponding hydroxymethylation products (Scheme 4). Methyl
benzoylformate (21) gave a mixture of pinacol and the 1,2-diol
without lactone formation. The higher homologs (Table 3,
entries 2–4) resulted in the corresponding lactones with ring
sizes of 5 and 6, respectively, with the δ-keto ester (Table 3,
entry 4) leading to the δ-pentyrolactone and not the corres-
ponding seven-membered lactone. The primary product from
hydroxymethylation was also isolated from the reaction of the
ε-keto ester 25 [40].
excited in different wavelength regions. TiCl(OiPr)3 showed no
activity at all, meaning that no further ligand exchange to a
tetraalkoxide did occur from this complex. The optimal results
concerning reaction time and yields were observed for the
TiCl2(OiPr)2 species. The apparently different catalytic activity
of titanium tetraalkoxide complexes could be switched back to
hydroxymethylation in the presence of additional strong Lewis
acids such as AlCl3 or BF3. These compounds alone did not
show catalytic activity in methanol, only in combination with
Ti(OiPr)4. With the optimal homogenous reaction conditions in
hand, we applied several other aromatic and aliphatic open
chain and cyclic ketones as substrates (Scheme 3). Even
benzophenone, a notorious pinacol forming substrate, gave
moderate yields of the hydroxymethylation product 7. Among
the aromatic ketones, para-fluoroacetophenone was the most
reactive ketone. Excellent yields were obtained for 2-pentanone
where the product 18 was isolated without purification after
extraction. For comparison, the results from the Ti(OiPr)4 catal-
ysis are included in Table 2. Additonally, the comparision with
Heterogeneous and dye-sensitized photo-
catalysis
The results with the semiconductor particle TiO2 P25 under low
catalyst loading conditions appear in Table 2 for a series of
ketone substrates. In order to explore the catalyst profile we
tested other reaction conditions for the TiO2 catalysis, other
Scheme 3: Photocatalyzed acetophenone/methanol reaction: types I–III.
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Beilstein J. Org. Chem. 2014, 10, 1143–1150.
Table 2: Substrate variation under optimized conditions.
entry
Substrate
TiCl2(OiPr)2a
TiO2 P25b
yield (%)c
type II
Ti(OiPr)4a
yield (%)c
type I
yield (%)c
type I
yield (%)c
type III
1
2
3
4
5
6
7
8
Ph(CO)CH3
60 (4 [28])
28 (7 [31])
56 (10)
1 (4)
82 (5 [29])
71 (8 [24])
54 (11)
46 (6 [30])
Ph(CO)Ph
20 (7)
50 (9 [32])
2’-F-Ph((CO)CH3
4’-MeO-Ph(CO)CH3
4’-NO2-Ph(CO)CH3
4’-Me-Ph(CO)CH3
C3H7(CO)CH3
–
40 (12 [33])
–
–
23 (13 [34])
–
19 (14 [35])
27 (15 [36])
92 (18 [38])
38 (19 [39])d
–
–
6 (15)
54 (16 [34])
16 (17 [37])
–
–
–
–
cyclohexanone
a0.75 mmol ketone in methanol (6 mL), 0.5 equiv cat., irradiation time 72 h, λ = 254 nm (TiCl2(OiPr)2) and 300 nm (Ti(OiPr)4), rt; b0.75 mmol ketone in
methanol (6 mL), 1.4 wt % TiO2 P25, irradiation time 48 h irradiation, λ = 350 nm, rt; cisolated yields; dmixture consisting of 9% 1,2-diol and 29%
acetalization product with cyclohexanone (20).
Scheme 4: Photohydroxymethylation and subsequent lactonization of keto esters.
Table 3: Photohydroxymethylation of keto esters: 1,2-diol and lactone formation.
entry
compounda
R1
R2
n
1,2-diol
(%)b
lactone A
(%)b
lactone B
(%)b
1
2
3
4
5
21
22
23
24
25
Ph
Ph
Me
Me
Me
Me
Et
0
1
2
3
4
15 (26 [41])c
–
–
–
56 (28 [42])
–
Et
–
54 (29)
–
Et
–
–
–
28 (30 [43])
Me
44 (31)
–
a0.75 mmol keto ester in methanol (6 mL), 0.5 equiv TiCl2(OiPr)2, irradiation time 72 h, λ = 254 nm, rt; bisolated yields; cadditionally 11% of the corres-
ponding pinacol 27 was formed.
metal-containing heterogeneous and homogeneous catalysts as cases, the pinacol diastereoisomers were formed in nearly equal
well as the classical organic PET catalyst 9,10-dicyano- amounts. Also the change in irradiation wavelength did not lead
anthracene (DCA). The results are summarized in Table 4 for to substantial changes in conversion and chemoselectivity. The
the model reaction of acetophenone in methanol (Scheme 5). formation of formaldehyde as the final oxidation product was
Except for the Ru(bpy)3Cl2 system, all catalysts enabled a high proven qualitatively (colorless precipitation of polyformalde-
degree of conversion and high yields of the pinacol 5 were hyde was observed in most experiments) and by a GC–MS
detected. The hydroxymethylation product 4 was detected only online detection of monomeric formaldehyde.
in few experiments with a maximum yield of 6% from one TiO2
experiment. The best results were obtained for TiO2 P25 catal- From these results, we reasoned that methanol oxidation is the
ysis in the presence of molecular sieves (Table 4, entry 3). In all primary event (e.g., from the DCA results) and acetophenone
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Beilstein J. Org. Chem. 2014, 10, 1143–1150.
Table 4: Heterogeneous sensitizer variation for model reaction in comparison with optimized homogeneous conditions for model process.
entrya
catalyst
loading
conversion (%)b
yield 4 (%)b
yield 5 (%)b
1
TiO2 P25
2.8 wt %
2.8 wt %c
2.8 wt %d
2.8 wt %e
2.2 wt %
1.4 wt %
3.3 wt %
2.8 wt %
2.8 wt %
2.8 wt %
1.4 wt %
2.8 wt %
1.4 wt %
2.8 wt %
2.8 wt %
2.5 mol %
2.5 mol %f
0.5 mol %
2.5 mol %
2.5 mol %
0.5 mol %
–
77
76
96
71
89
95
81
89
79
98
95
92
95
63
65
91
65
89
27
95
96
<5%
–
–
3
–
6
1
2
–
–
–
1
–
–
–
–
–
–
–
–
1
1
–
66
58
83
48
79
82
74
69
69
70
75
75
80
46
45
50
55
82
1
2
3
4
5
6
7
8
TiO2-pigment
zinc white
9
10
11
12
13
14
15
16
17
18
19
20
21
22
WO3, <100 nm
Fe2O3, <50 nm
ZnO, 6% Al doped, <50 nm
InSnO, <50 nm
Ir(ppy)3
Ru(bpy)3Cl2
DCA
94
92
–
none
a0.75 mmol acetophenone in methanol (6 mL), irradiation time 24 h, λ = 350 nm, rt; bdetermined by GC; cλ = 300 nm; d600 mg molecular sieves were
added; e100 µL H2O was added; fIr(ppy)3 regained from entry 16.
Scheme 5: Model reaction for heterogeneous and dye-sensitized catalysis.
reduction follows resulting in the corresponding hydroxybenzyl processes is methanol oxidation [44]. By using appropriate
radicals that couple to give the pinacol 5. Under this assump- reaction conditions, every route can be switched on exclusively.
tion, a decrease in ketone concentration should favour radical Hydroxymethylation is favoured if both hydroxyalkyl radicals
combination of the hydroxybenzyl and the (more reactive and are generated in close proximity by a coupled electron transfer/
easier oxidizable) hydroxymethyl radicals. As shown in back transfer process. According to this expectation, the optimal
Table 5, this is actually the case for the semiconductor particle conditions for route I are fulfilled for TiCl2(OR)2, a species that
TiO2 P25 catalysis. If acetophenone is added to the photolysis is capable of oxidizing methanol in the excited state and simul-
solution constantly over a period of 24 h, the absolute amount taneously acting as a ground-state Lewis acid that complexes
of hydroxymethylation product 4 can be increased to 40%.
the carbonyl compound. A much weaker Lewis acid such as
Ti(OR)4 is capable of methanol oxidation but prefers
hydrogen transfer at the first or second oxidation event. The
Discussion
Three product-forming routes can be assumed for the three pinacolization route II is favoured for heterogeneous and dye-
classes of products observed in the study (Scheme 6): hydroxy- catalyzed conditions. Interestingly, the combination of TiO2
methylation (route I), pinacolization (route II) and reduction/ P25 with an organic dye prefers largely the hydrogenation route
hydrogenation (route III). The crucial primary step for all III [45].
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Beilstein J. Org. Chem. 2014, 10, 1143–1150.
Table 5: Chemoselectivity modification by application of a dilution effect.
entry
drop rate
TiO2 in methanol (30 mL)
reaction time
conversion (%)
yield 4 (%)
yield 5 (%)
1a
0.28 mmol/h
0.28 mmol/h
0.21 mmol/h
0.21 mmol/h
0.16 mmol/h
15 mg
15 mg
15 mg
10 mg
10 mg
18.7 h
18.2 h
24 h
92
82
95
89
96
36
25
33
13
40
52
38
55
60
50
2a,b
3a
4a
24 h
5c
24 h
a5 mmol acetophenone dissolved in methanol (10 mL) was slowly added to a TiO2 P25 (15 mg/ 10 mg) suspension in methanol (30 mL) irradiated
(300 nm) at 15 °C; bTiO2 P25 suspension was cooled to −5 °C; c3.72 mmol acetophenone dissolved in methanol (10 mL) was slowly added to a TiO2
P25 (10 mg) suspension in methanol (30 mL) irradiated (300 nm) at 15 °C.
dictated by diffusion kinetics (Scheme 7). The dilution experi-
ments described in Table 5 indicate that the probability for
pinacol formation is reduced by reducing the stationary concen-
tration of the aromatic ketone. It was shown that hydroxy-
methyl radicals are formed from methanol during the photo-
lysis of TiO2 in the absence of additional acceptor compounds
with formation of hydrogen and eventually formation of
formaldehyde [46]. Both hydrogen and formaldehyde were also
detected in our experiments by gas-phase analysis. Thus, higher
amounts of hydroxymethyl radicals can be produced under
lower concentration of the acceptor ketone and the probability
of hydroxybenzyl radical dimerization (i.e., route II) is
Scheme 6: Product forming routes I to III for photoredox catalysis of
methanol/carbonyl compounds.
disfavoured under these conditions.
Supporting Information
On the surface of the relatively large semiconductor particles,
combination events are rare between the hydroxymethyl radical
Supporting Information File 1
from methanol oxidation and the hydroxyalkyl radical from
ketone reduction. Thus, further oxidation of the hydroxymethyl
species to give methanol and another hydroxyalkyl radical is
feasible. The combination of two hydroxyalkyl radicals is then
Experimental part.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-10-114-S1.pdf]
Scheme 7: Photoredox initiated steps on semiconductor particle surfaces, CB, VB = conduction and valence band.
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Beilstein J. Org. Chem. 2014, 10, 1143–1150.
28.Wang, A.; Jiang, H. J. Org. Chem. 2010, 75, 2321–2326.
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