Graphitic carbon nitride catalysed photoacetalization of aldehydes/ketones under ambient conditions

Article abstract of DOI:10.1039/c5cc08344c

Graphitic-C₃N₄ is shown for the first time to catalyse photoacetalization of aldehydes/ketones with alcohols to acetals in high yields using visible light under ambient conditions; transient charge separation over the material is effective to catalyse the reaction in the absence of Lewis or Br?nsted acids, giving a new green alternative catalyst.

Full text of DOI:10.1039/c5cc08344c

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DOI: 10.1039/C5CC08344C
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Graphitic carbon nitride catalysed photoacetalization of
aldehyde/ketones under ambient conditions
Received 00th January 20xx,
Accepted 00th January 20xx
M. Abdullah Khan, Ivo F. Teixeira, Molly M. J. Li, Yusuke Koito and S. C. Edman Tsang*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Graphitic-C3N4 is shown for the first time to catalyse also be acetalized in environmental friendly manner. Since
photoacetalization of aldehydes/ketones with alcohols to acetals such a method does not exist, we therefore set out to develop
at high yields using visible light under ambient conditions: this route in this work.
transient charge separation over the material is effective to
One exciting approach is to employ energetic light to
catalyse the reaction in absence of Lewis or Brønsted acids, polarize a semiconducting material to generate excitons under
giving a new green alternative catalyst.
transient conditions (positive hole and electron). The excitons
can then activate organic molecules for their conversion to
products (most of the organic molecules do not absorb visible
light) before they are self-recombined. Recently
semiconducting carbon nitrides consisting of earth abundant
elements of carbon and nitrogen have attracted a lot of
attention due to their ability to capture light efficiently without
any use of transition metal. In reported works, thermal
polycondensation of common organic monomers to synthesize
graphitic carbon nitrides (g-C3N4) with tunable band
positions.19–22 The graphitic planes are derived from tri-s-
triazine units inter-connected by planar amino groups with a
band gap of about 2.7 eV (see Fig. S1 of ESI). Their HOMO and
LUMO band positions are placed in such a way that the band
gap energy is capable of activating molecular O2 from air by
photoelectron to generate mild and selective O2 radicals for
organic transformations using visible light but the production
of nonselective •OH radicals are thermodynamically
prohibited.23,24 Using the transient •O2- radicals, recent reports
on selective oxidations of alcohols to aldehydes, C-H25, O-H23,24
and N-H26 oxidations over these materials have been claimed
but the kinetics are yet to be improved. Such studies prompted
us to employ this system for the acetalization reactions of
carbonyl compounds.
Acetalization reactions of aldehydes and ketones with alcohols
is one key reaction step to protect or/and introduce chemical
functionalities in carbonyl compounds during multistep
organic synthesis.1 Although many reported methods have
been employed for this conversion, most of them are using
acid catalysts which also often involve the use of toxic and
corrosive reagents.2–5 Acidic environment is unsuitable when
acid sensitive moiety groups of the substrates (e.g., carrying
silyl groups or unsaturation)1 are involved. Acetals are also
highly unstable for reversible reactions to hemiacetals and
starting substrates in acidic conditions but are stable in basic
or neutral pH.6,7 Besides, during acetalization reaction, by-
product, water is produced that requires additional physical
and chemical means for its removal in order to avoid
equilibrium shifting back to reactants.1 Acetalization reactions
are traditionally catalysed by Lewis or Brønsted acids in
aqueous medium. However, only few exceptions are
acetalizations in the presence of homogeneous examples in
organic solvents such as LiBF4,4 thiourea,6 ionic liquids,7 NBS,8,9
N,N′-bis[3,5-bis(trifluoromethyl)phenyl] and TBATB10. Apart
from separation problems incurred to these systems some of
them are toxic, costly and they suffer from a lack of generality.
Solid catalytic systems are deemed to be relatively less
deleterious to environment.11–18 It would thus be highly
desirable to develop a general acid-free, non-transition metal
containing solid catalyst so that a wide range of substrates can
•
-
In this work, we report the photocatalytic acetalization of
aldehyde/ketones over g-C3N4 in molecular oxygen and visible
light under ambient conditions. It is found that the conversion
of benzaldehyde
2 with excess methanol to corresponding
acetal is > 97 % and with selectivity > 98 % in absence of any
acid additive. As far as we are aware, this is the first example
of heterogeneous photoacetalization of aldehyde/ketones in
alcohol over g-C3N4, at room temperature and oxygen pressure
of 1 bar using visible light in environmental friendly manner
(Scheme 1). It also appears that this material can act as generic
Wolfson Catalysis Centre, Inorganic Chemistry Laboratory, Department of
Chemistry, University of Oxford, South Parks Road, Oxford, OX1 3QR, United
Kingdom. Email: edman.tsang@chem.ox.ac.uk
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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Table 1. Photoacetalization of aldehyde/ketones oVvieewr gA-rtCic3lNe O4.nline
catalyst which catalyses photoacetalization over a wide range
of aldehydes and alcohols at high yields. Notice that acetals
formation in aqueous acid medium at ambient conditions are
thermodynamic limited because of their relatively low
equilibrium constants (K~0.01-0.05 mol-1) and with a loss in
entropy when two or three molecules of starting materials
(aldehyde or ketone plus alcohol) condense to
hemiacetal/acetal products. Thus, separation and purification
of products is necessary.1 The presently observed high yields
from a single pot reaction are attributed to the preferential
strong adsorption of polar organic substrates and oxygen and
water molecules on confined surface, which enable
photoacetalization reactions to take place favourably by pre-
concentrated substrates over this solid material surface under
transient charge (excitons) separation upon light excitation.
DOI: 10.1039/C5CC08344C
No
R1
R2
T / h
Conv. / %
Sel. /%
1
2
3
4
5
C3H7
C6H5
p-(CH3)C6H4
p-(CH3O)C6H4
p-(HO)C6H4
H
H
H
H
H
20
6
4
3
6
6
10
6
6
24
24
3
24
24
93.35
97.33
87.44
83.65
0
69.76
96.72
53.91
6.50
25.35
21.22
84.48
42.72
84.68
91.90
98.30
97.44
98.20
0
98.85
97.48
94.94
85.00
66.25
70.94
86.53
93.43
90.85
6
7
8
p-(Cl)C6H4
p-(HOOC)C6H4
p-(NO2)C6H4
H
H
H
9
C6H5
C6H5CH=CH
C3H7
CH3
H
CH3
10
11
12
*C5H10

Substrate (1mM); C3N4 (25mg); methanol (5 mL); 1bar (O2); visible light;
conversion and selectivity were determined by GC. *Cyclohexanone
A moderate increase in reactivity for electron donating (-CH3 and -
OCH3) groups 3-4 at para position of substituted benzaldehyde was
Scheme 1. Photoacetalization of benzaldehyde with methanol
First, the time course for acetalization reaction of benzaldehyde observed whereas electron withdrawing (-Cl and -NO2) analogues 6-
2 with methanol was monitored and results are shown in the Figure 8 underwent acetalization considerably at a slower rate, however
1a. Under visible light irradiation, reaction proceeded at a steady all still displayed high selectivity. As expected, steric hindered
rate and reached completion at about 6 h, giving the final high acetophenone 9 although gives high electron density at the
conversion and selectivity for the acetal formation. The acetal carbonyl carbon which exhibited sluggish acetalization response,
remained to be the main product with traces of formaldehyde, but the failure of 4-Hydroxybenzaldehyde 5 to yield any product,
ester and acid (<1%) were also detected. To ascertain g-C3N4 is notwithstanding the electron donating nature of the OH group, is
required to catalyse the reaction, control experiment without g- surprising and not understood. It could due to strong inter H-
C3N4 addition was conducted. No acetal was found from bonding formed between the molecules which prohibit the
benzaldehyde under irradiation of light and oxygen pressure of 1 photoacetalization on surface. In case of α,
bar. Under same conditions in the dark with g-C3N4, negligible cinnamaldehyde 10, corresponding acetal was obtained with good
β unsaturated
product was also detected over prolong period (48 h).
yield and selectivity, while the double bond remained intact. This
clearly demonstrates the synthetic usefulness to sensitive
unsaturation moiety groups of this photocatalytic system. The
reactions of butryaldehyde 1, 2-pentaone 11, and cyclohexanone 12
without conjugated stabilization to the carbonyl group were also
successful without requiring the general use of high temperatures.
Thus, the versatility of the catalytic system to efficiently drive the
photoacetalization in the absence of acid catalysts is evident. The
extension of substrate scope for the catalytic system beyond the
reaction with methanol was realized by performing reactions with
ethanol and rather difficult but more desirable ethylene glycol
(biomass derivative)5,27 (Table 2). The reaction of benzaldehyde 13
with ethylene glycol, whose nucleophilicity is reduced due to
mutual inductive withdrawal with the proximal oxygen atoms, to
produce more stable cyclic acetal still proceeded with high
conversion and selectivity. Similarly the reaction of butyraldehyde
14 with ethylene glycol also furnished five-member cyclic 1, 3-
dioxolane product with 78 % selectivity and 70 % conversion. Under
same conditions, the reaction with ethanol 15 afforded
corresponding acetals with good selectivity nonetheless low yield.
This suggests the kinetics of the reaction is slower than the reaction
with methanol. The low selectivity of 16 is probably due observed
polymerization products. A challenging reaction28 with alcohol
larger than C3 17 (butanol) was also successful showing moderate
conversion and selectivity for the corresponding acetal product.
Fig. 1a Acetalization of benzaldehyde catalysed over g-C3N4 and b.
the Hammett plot of para substituted benzaldehyde.
To gauge the role of oxygen, the reaction was carried out in argon
atmosphere instead of using molecular oxygen. Complex
condensation products but with poor selectivity to acetal (10 %) at
relatively low conversion (35 %) were observed. These tests confirm
that reaction clearly requires heterogeneous nitride surface
whereby O2, light and substrates are essential ingredients for the
cooperative catalysis. It was found that the catalytic application is
also wide ranging and the g-C3N4 system appears to work well with
a variation of aromatic, alicyclic aldehydes and ketones under
ambient conditions using visible light.
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of •O2- we did not see any typical unselective radical reactions giving
DOI: 10.1039/C5CC08344C
a wide range of different products. Instead, we observed the ultra-
high selectivity towards acetal and the negative Hammett constant
clearly reflecting an ionic mechanism. In addition, we neither
These results extend the range and flexibility of the catalytic system
to catalyse substrates for targeted acetals using light excitation.
Table 2. Photoacetalization of selected aldehydes with different
alcohols over g-C3N4.
View Article Online
No
Substrate
Alcohol
T / h
Conv. / %
Sel. / %
observed
a significant conversion of alcohol substrate to
13
14
15
16
17
C6H5CHO
C3H7CHO
C6H5CHO
*C4H9CHO
C6H5CHO
†C2H6O2
†C2H6O2
C2H5OH
C2H5OH
C4H9OH
6
6
6
6
12
90.85
70.88
38.94
32.45
43.49
87.13
78.42
91.12
11.37
55.73
corresponding aldehyde in 2 at ambient temperature in contrast to
•
-
those previously studies due to the O2 catalytic role generated by
g-C3N4 under light irradiation at elevated temperature (in our case,
trace of formaldehyde was only observed). On addition of more
reactive H2O2 (radicals promoter) to the catalytic system 2, poor
conversion and selectivity were noted. Importantly, by blending
benzoquinone, a superoxide radical scavenger, into the reaction 2,
we did not detect much of its effect to alter the intrinsic high
conversion and selectivity of the reaction 2 (Table S2 of ESI). We
13
14
15
16
17
Substrate (1mM); C3N4 (25mg); alcohol (5 mL); 1bar (O2); visible light;
conversion and selectivity were determined by GC. †Ethylene glycol,
*Butryaldehyde
-
therefore argue that •O2 is likely retained on the carbon nitride
To elucidate the origins of the observed catalysis, kinetics data
were collected at different reaction times using benzaldehyde
derivatives with some selected para-substituted electron donating
and electron withdrawing groups of substituted benzaldehyde with
methanol, the results are shown in Figure S2 of ESI. It is revealed
that the rates of electron donating substituents are significantly
faster than those of electron withdrawing groups in comparison to
benzaldehyde. Following the kinetics at progressive increasing
conversions, the derived reaction rates follow a trend of CH3->H-
>Cl->NO2- (Table S1 of ESI). This distinctive change in reactivity of
reacting substrates suggests the possible involvement of electron
deficient charged intermediate during the photochemical
transformation. In addition, the plots of log (kX/kH) obtained from
kinetic data of the para substituted benzaldehyde against the
Hammett substitution constant (σ) showed a linear correlation
(Figure 1b) and the gradient of this fitted line gave the moderate
negative Hammett reaction constant (ρ) value of -1.9.
surface as electron trap unless elevated temperature is used to
•
-
release it into solution phase. Thus, we conclude that O2 does not
play a significant role in the charge-sensitive photoacetalization
reaction. The surprisingly high yield for photoacetalization at
ambient temperature also suggests a clear ‘pre-concentration effect’
for the polar substrates on confined surface sites presumably due
to the characteristic transient charge separation of this material.
We believe that the polar organic substrates, O2 and water could
strongly bind on this material surface during the light illumination.
Despite the fact that the activation is momentarily conducted under
the light irradiation before the recombination of exciton, the local
strong field intensity and high charge density (+ve nitrogen in close
proximity to –ve carbon in carbon nitride surface) will facilitate the
remarkable catalytic performance for the forward reaction
especially in the absence of solvent, additive or diluent. Notice that
our GC-MS clearly indicated preferential binding of polar substrates
than the less polar product on catalyst when the reaction was
continuously monitored hence reversible reaction with no acidic
protons in solution phase was not facilitated. H2O18 was still the
product from the reaction of O18 labelled benzaldehyde and dried
methanol, however, it did not seem to evolve quantitatively from
the material. Thus, the mesoporous nature of the g-C3N4 reflected
by morphological and surface characterization (Figs. S4 and S5 of
ESI) could have also trapped the by-product water dragging the
equilibrium further. In addition the material showed excellent
stability over repeated catalytic cycles (Fig. S6).
We considered the mechanism for this new but efficient light-
induced heterogeneous catalysis reaction and the particular high
yield of acetal under ambient conditions. Static and time-resolved
photo-luminescence (PL) spectroscopy was thus employed on a g-
C3N4 film. In static PL, (Fig. S3a of ESI) the observed peak around
500 nm is associated to n-π* manifold of conjugated heptazine
units suggesting a charge transfer from nitrogen (+ve hole) to
conjugated carbon (-ve electron) under the photo-excitation.29 This
matches with previous theoretical HOMO and LUMO calculations.30
It is interesting to note from Figure S3b that an interesting but
rather long luminescence time-decay curve of g-C3N4 with an
average exciton lifetime of 6.43ns, which is within the typical
timescale to initiate chemical reactions (10-10-10-5 s).31 Clearly the
semiconducting carbon nitride upon visible light excitation (
>420nm) can generate long-lived excitons (charge separation)
where the excited photogenerated electron takes residence in
conduction band (CB) and positively hole on valence band (VB) to
catalyse the photoacetalization.23,29,32,33 It has been reported that
CB of this material at -1.3 V possess a large thermodynamic driving
force with a long live photoelectron to reduce molecular O2 to form
Although more detailed studies are needed to elucidate the
mechanism,
a simplified pathway for photoacetalization is
summarized in Scheme 2. From this Scheme, visible light impinging
-
on g-C3N4 can generate surface charge separation of N and •O2
which may help to deprotonate alcohol to form adsorbed protons
on the surface initially. Such transient surface charge species will
also induce fast acetalization from polar aldehyde and alcohol by
ionic mechanism on the material surface. This may involve the
formation of oxonium ion (g) as reflected by the negative value of
the Hammett study (ρ = − 1.9) to form acetal. We anticipate that it
is stabilized by carbon nitride surface presumably through cation-π
interactions.35 Such transition state specie is known to exist in the
rate determining step in solution phase in the presence of acid
-
-
•O2 (Eo O2/•O2 = -0.16 V) but the potential of the photogenerated
hole in the VB at 1.4 V is inadequate to oxidize –OH to hydroxyl
radicals •OH (Eo -OH/•OH = 2.4V).30,34 Despite the possible existence
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Scheme 2. Proposed pathway for photoacetalization
In conclusion, we have demonstrated efficient photoacetalization
of aldehydes and ketones with alcohols using g-C3N4 as
photocatalyst, which gives impressive selectivity and conversion in
high rate and yield at ambient conditions. The reaction takes place
at room temperature in oxygen atmosphere involving substrates
and acid-free solid catalyst without adding solvent and hence is
clearly a new green synthesis method. The broad reaction scope
and remarkable versatility of this catalytic system could enable us
to tackle a wide spectrum of aldehyde/ ketones /alcohols of
interests for specific acetals production with favoured equilibrium.
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