Graphene oxide (GO)-catalyzed chemoselective thioacetalization of aldehydes under solvent-free conditions

Article abstract of DOI:10.1016/j.tetlet.2014.10.043

An efficient method for the synthesis of open chain, cyclic, and unsymmetrical dithioacetals from aryl/hetero-aryl/aliphatic aldehydes is described. The reaction is performed using graphene oxide (GO) as the catalyst under solvent-free and aerobic conditions. High chemoselectivity is observed in the reaction as aryl/alkyl ketones do not give thioketals under the condition.

Full text of DOI:10.1016/j.tetlet.2014.10.043

Accepted Manuscript
Graphene Oxide (GO)–catalyzed chemoselective thioacetalization of aldehydes
under solvent-free conditions
Babli Roy, Debasish Sengupta, Basudeb Basu
PII:
S0040-4039(14)01726-2
DOI:
http://dx.doi.org/10.1016/j.tetlet.2014.10.043
TETL 45272
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
7 August 2014
6 October 2014
8 October 2014
Please cite this article as: Roy, B., Sengupta, D., Basu, B., Graphene Oxide (GO)–catalyzed chemoselective
thioacetalization of aldehydes under solvent-free conditions, Tetrahedron Letters (2014), doi: http://dx.doi.org/
10.1016/j.tetlet.2014.10.043
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1
Tetrahedron Letters
journal homepage: www.els evier.com
Graphene Oxide (GO)–catalyzed chemoselective thioacetalization of aldehydes under
solvent-free conditions
Babli Roy, Debasish Sengupta and Basudeb Basu*
Department of Chemistry, North Bengal University, Darjeeling 734013, India
E-mail: basu_nbu@hotmail.com
ARTICLE INFO
ABSTRACT
Article history:
Received
An efficient method for the synthesis of open chain, cyclic and unsymmetrical dithioacetals from
aryl/hetero-aryl/aliphatic aldehydes is described. The reaction is performed using graphene
oxide (GO) as the catalyst under solvent-free and aerobic conditions. High chemoselectivity is
observed in the reaction as aryl/alkyl ketones do not give thioketals under the conditions.
Received in revised form
Accepted
Available online
2009 Elsevier Ltd. All rights reserved.
Keywords:
Aldehydes
Chemoselectivity
Dithioacetal
Graphene oxide
Thiols
The efficiency of graphene oxide (GO) and other chemically
modified graphenes (CMGs) as the carbocatalysts has raised
enormous interest since the seminal papers from Bielawski and
co-workers.1 Catalytic applications of carbonaceous sustainable
and easily accessible GO in diverse organic reactions is one of
the recent cherished goals.2 GO possesses rich chemical
functionality, is slightly acidic (pH 4.5 at 0.1 mgmL-1),3 and has
long been recognized as having strong oxidizing properties.4
Accordingly, GO has been mostly employed in oxidation of
different functional groups.5 For example, oxidation of benzyl
alcohol to benzaldehyde,5a,b oxidation of thiol to disulfide,5c or
amine to imine etc.5d Because of high efficiency and metal-free
catalytic activity of GO or partially reduced GO (rGO), efforts
are on to develop variety of organic transformations catalyzed by
these carbonaceous materials.2 Although GO has been utilized as
an oxidation catalyst in various organic reactions, its use as an
acid catalyst has remained relatively unexplored.5
most cases the catalyst is not recoverable, heterogeneous acid
catalysts often do not find wide applicability to various
substrates. We were interested to explore the feasibility and
catalytic efficiency of GO in thioacetalization of aldehydes and
ketones. GO has been used in the dimethylacetal formation from
aldehydes using methanol.8 But, use of thiols in place of
methanol poses the possibility of intermolecular disulfide
formation.5c We recently showed that GO can be used as a
catalyst under controlled conditions so as to avoid the oxidation
of secondary benzyl alcohols as well as minimize that of thiols.9
We herein report an efficient and practical method for the
preparation of dithioacetal from aryl/alkyl aldehydes with the aid
of catalytic amount graphene oxide (GO). The reaction is highly
selective to aldehyde carbonyl groups, can be carried out under
solvent-free and mild conditions, and applicable to variety of aryl
and alkyl aldehydes including hetero-aryl aldehydes as well as
with diverse aromatic/aliphatic thiols. As a further extension of
this reaction, unsymmetrical dithioacetals have been prepared in
excellent yields by using two different aliphatic thiols (Scheme
1).
Generally, thioacetals are formed as the result of Brønsted
and Lewis acid catalyzed condensation reaction of aldehydes and
ketones with thiols or dithiols and a vast number of methods are
available in the literature.6 Over the past decade, several
heterogeneous acid catalysts such as p-TsOH/silicagel,7a
H3PW12O40/SiO2,7b Montmorillonite K-10 Clay,7c Amberlyst-
15,7d PPA/SiO2,7e Melamine trisulfonic acid (MTSA),7f silica-
sulfuric acid (SSA),7g solid-supported dithiolanylium or
dithianylium tetrafluoroborate salts,7h and also ionic liquids,7i,j
have been employed for the preparation of thioacetals. However,
moisture sensitivity and reactivity of thioacetals towards acidic
sources for reversible reactions pose a great limitation for their
preparation in high yield and most of the methods are often
unsuitable for use in large-scale applications. Moreover, while in
R3 SH
R1
R2
S
S
R3
R3
(CH2)n
SH
GO
RT - 60 o
HS
R1
R2
R1
S
C
(CH2)n
O
R2
S
n = 2, 3
Solvent-free
R1 = Aryl, alkyl
R2 = H
R1
R2
S
R3
R4
R3 = Alkyl, aryl
R4 = Alkyl
R3 SH
R4 SH
S

2
Tetrahedron Letters
column chromatography to isolate the desired dithioacetal in 51%
Scheme 1. General scheme illustrating GO-catalyzed
diverse dithioacetals formation.
yield (entry 1). Increasing the quantity of GO (50 mg mmol−1 of
the aldehyde) revealed that nearly complete conversion of the
aldehyde to dithioacetal could be achieved at room temperature
within 3h under aerobic condition (entry 3). Significantly, there
was no detectable amount of disulfide formed in the reaction
from the oxidative dimerization of pentanethiol even under the
aerobic condition. A control experiment was performed in the
absence of the GO under similar reaction condition, which did
not produce any dithioacetal even after 24h (entry 4). In order to
see any faster conversion, the reaction was also carried out at 60
oC. This however resulted in partial conversion of the mercaptan
into the corresponding disulfide along with the desired
dithioacetal in relatively lower yield (entry 5). Moreover, the
same reaction at 60 oC and under a blanket of N2 did not stop the
formation of disulfide (entry 6). Conducting the experiments in a
solvent (THF, toluene or water) showed rather a negative effect
affording the desired dithioacetal in relatively lower yields
(entries 7-9). To scale up the reaction, one set of reaction was
performed (in 5 mmol) using the same quantity of catalyst (GO =
50 mg). Excellent yield of the dithioacetal was realized in this
case as well signifying that the minimum quantity of the catalyst
can promote the reaction in a longer time (entry 10).
Although GO is commonly used as an oxidation catalyst,4 and
thiols can produce disulfides via oxidative coupling in the
presence of GO,5c we demonstrate here that the catalytic role of
GO can be diverse and tuned by changing its loading and
reaction conditions.
To begin our study, we prepared GO from graphite powder
according to the modified Hummers method,10 followed by
exfoliation under sonication and isolation after centrifugation.
The FT-IR spectrum of the resulting GO was compared with that
of the reported absorption bands,8,11 and found fairly similar
absorption bands for the functional groups (See ESI S3.1).
Optimization of the GO-catalyzed thioacetalization was
examined with p-anisaldehyde and n-pentanethiol (1: 2.2 ratios)
as the model case and the results are shown in Table 1.
Initially, a mixture of the aldehyde and mercaptan (in 1:2.2
ratios) was gently stirred in neat at room temperature in the
presence of GO (10 mg mmol−1 of the aldehyde) under aerobic
condition. Monitoring the reaction by tlc at intervals showed the
presence of the starting aldehyde along with the desired
dithioacetal even after 24 h. The reaction was stopped and after
the separation of the catalyst, the residue was subjected to
Table 1. Optimization of dithioacetal formation from p-anisaldehyde and n-pentane thiol.a
Entry
GO (in mg mmol-1)
Reation medium
Time (h) / Temp(oC)
24 / RT
24 / RT
3 / RT
Disulfide (%)b
Not observed
Not observed
Not observed
Not observed
10
Dithioacetal (%)b
1
10
25
50
Nil
50
50
50
50
50
50
Neat
51
2
Neat
65
3
Neat
95
4
Neat
24 / RT
3 / 60
Not observed
5
Neat
61
57
35
68
55
91
6c
7
Neat
3 / 60
8
THF
20 / RT
12 / RT
12 / RT
16 / RT
traced
8
PhMe
H2O
Not observed
traced
9
10e
Neat
traced
ap-Anisaldehyde (1 mmol), n-pentanethiol (2.2 mmol).
bIsolated yield.
cReaction carried out under N2.
dDetected only on tlc, not isolated.
ep-Anisaldehyde (5 mmol), pentanethiol (11 mmol).
With this mild and solvent-free optimized condition at our
hand, we became interested to explore the general applicability of
the reaction. Accordingly, a variety of aryl aldehydes were
subjected to the reaction in the presence of thiols. The results are
presented in Table 2. It can be seen that aryl aldehydes bearing
different functional groups react easily with aliphatic thiols in the
presence of catalytic amount of GO under solvent-free aerobic
condition. The reactions were completed in 3-8 h and the
products were realized in excellent yields (Table 2, entries 1-5).
Higher temperature was required for α-naphthaldehyde and p-
chlorobenzaldehyde, which caused partial formation of disulfide
(~5-8%) (entries 6, 7). However, the reaction with p-
nitrobenzaldehyde needed much longer time and higher
temperature, and gave the dithioacetal in relatively lower yield
along with disulfide (Table 2, entry 8). Trying the reaction with
aromatic thiols afforded the desired dithioacetal in 80-85% yields
but achieved only at higher temperature (60oC) and was
associated with the formation of diaryl disulfides in 8-12%
isolated yields (entries 9-11). The disulfides however could be
separated easily from dithioacetal by column chromatography
over silica gel. In the case of using 1,2- and 1,3-dithiol,
corresponding cyclic thioacetals were obtained in excellent yields
(95-98%) and the reaction was also successful with aliphatic
cyclohexyl aldehyde (entries 12-15). However, there was no such
formation of dithioketals from a ketone under the GO-catalyzed
reaction condition (entry 16). So, there is excellent
chemoselectivity observed between the aldehyde and keto-
carbonyl groups.

3
Table 2. GO-catalyzed thioacetalization of aldehydes with different thiols.a
Entry
Aldehyde
Thiol
Time (h)
Temp.
Product
S-n-C5H11
S-n-C5H11
Yieldb (%)
95
1
3
RT
n-C5H11 SH
CHO
H3CO
H3CO
2
3
5
5
RT
RT
91
95
n-C5H11 SH
S-n-C5H11
S-n-C5H11
CHO
S-n-C7H15
S-n-C7H15
H3CO
HO
CHO
n-C7H15 SH
H3CO
HO
4
5
8
5
RT
RT
89
93
n-C5H11 SH
CHO
OH
S-n-C5H11
S-n-C5H11
OH
But SH
S-tBu
H3CO
HO
CHO
H3CO
S-tBu
HO
6c
3
60oC
87
n-C7H15-S
S-n-C7H15
n-C7H15 SH
CHO
7c
5
60oC
90
68
CySH
CHO
SCy
SCy
Cl
Cl
8c
14
60oC
S-n-C5H11
S-n-C5H11
CHO
n-C5H11 SH
O2N
O2N
9c
10
60oC
80
C6H5 SH
CHO
OH
C6H5
C6H5
S
S
OH
10c
10
60oC
85
C6H5
CHO C6H5 SH
S
C6H5
S
H3CO
H3CO
11c
10
60oC
82
98
C6H4(4-CH3)
(H3C-4)C6H4 SH
H3CO
HO
CHO
CHO
S
H3CO
HO
C6H4(4-CH3)
S
12d
5
RT
HS
SH
S
S
H3CO
H3CO

4
Tetrahedron Letters
5
13d
RT
98
98
HS
SH
H3CO
HO
CHO
S
H3CO
HO
S
14d
5
RT
CHO
OH
SH SH
S
S
OH
S
15d
5
RT
95
SH SH
O
H
S
16
24
RT/ 60oC
Not observed
O
n-C5H11 SH
n-C5H11-S
CH3
S-n-C5H11
CH3
aAldehyde : thiol : GO (1 mmol : 2.2 mmol : 50 mg) and reactions were performed at room temperature.
bIsolated yield.
cCorresponding disulfides (5-12%) were formed.
dAldehyde : thiol : GO (1 mmol : 1.1 mmol : 50 mg).
Heteroaryl
compounds
often
exhibit
promising
the formation of corresponding dithioacetals with variety of
aliphatic and benzenethiol without any difficulty (Table 3, entries
1-5). Other heteroaryl aldehyde indole-3-carbaldehyde also
produces corresponding dithioacetals with long chain aliphatic
thiols without any difficulty and in excellent yields (Table 3,
entry 6). Cyclic dithioacetals of furfural and indole-3-
carbaldehyde were obtained in excellent yields under mild
pharmacological activities and open chain dithioacetals of
heteroaryl aldehydes such as thiophene-based dithioacetals are
important scaffolds for design and discovery of new medicines.12
Since the reaction conditions are mild and solvent-free, we were
interested to extend the GO-catalyzed dithioacetalization to
thiophene aldehydes and also of other heteroaryl aldehydes.
Indeed, 5-bromothiophene-2-carbaldehyde and furfural result in
conditions
(entries
7,
8).
Table 3. GO-catalyzed thioacetalization of heteroaryl aldehydes.a
Entry
Aldehyde
Thiol
Time (h)
Product
Yield b (%)
89
1
8
But SH
S-tBu
S-tBu
CHO
S
Br
S
S
Br
2c
10
81
C6H5 SH
S C6H5
CHO
CHO
S
S
Br
Br
S C6H5
Br
Br
O
3
4
8
3
89
92
S-n-C7H15
n
-C7H15 SH
S
S-n-C7H15
n-C5H11 SH
S-n-C5H11
CHO
O
O
S-n-C5H11
5
6
3
8
88
85
CySH
S-Cy
CHO
O
S-Cy
n-C7H15-S
n-C7H15 SH
CHO
S-n-C7H15
N
H
N
H
7d
3
90
S
CHO
SH SH
O
O
S

5
8d
5
93
CHO
HS
SH
S
S
N
H
HN
aAldehyde : thiol : GO (1 mmol : 2.2 mmol : 50 mg) and reactions were performed at room temperature.
bIsolated yield.
cIsolated yield of disulfide was ~ 5%.
dAldehyde : thiol : GO (1 mmol : 1.1mmol : 50 mg).
In order to further broaden the scope of the reaction, we
analysis of the crude product mixture by HPLC (Table 4, entry
3). Although the exact reason for this selectivity is not known, it
might be attributable to the difference in reactivity between the
two thiols. Using a mixture of aliphatic or aromatic thiols,
formation of the thioacetal from benzenethiol in highest quantity
amongst the three possible dithioacetals also supports this
rationale. Since in entry 3, all three products were obtained as a
non-isolable mixture, we tried to analyze the mixture by HPLC
and compared the peaks with known products, leaving the third
peak likely to be for the unsymmetrical dithioacetal.
attempted synthesis of unsymmetrical dithioacetal using two
different thiols. Gratifyingly, the reactions of p-anisaldehyde or
vanillin with two different aliphatic thiols (n-pentyl- and n-heptyl
thiols) resulted in the formation of unsymmetrical dithioacetals as
the only product in 85-88% isolated yields (Table 4, entries 1, 2).
HPLC analysis of the crude product (before column
chromatographic purification) showed the unsymmetrical
thioether as the sole product. On the other hand, use of one
aliphatic and one aromatic thiol gave rise to a mixture of all three
possible products in varying proportions, as seen from the
Table 4. Unsymmetrical thioacetals from aryl aldehydes using two different thiols.a
Entry
Aldehyde
Thiol (A)
Thiol (B)
Time (h)
Product
Yield b (%)
88b
1
5
S-n-C5H11
S-n-C7H15
n-C5H11 SH
CHO
n-C7H15 SH
H3CO
H3CO
2
5
85b
H3CO
HO
CHO
CHO
S-n-C5H11
S-n-C7H15
n-C5H11 SH n-C7H15 SH
H3CO
HO
3c
10
13d
S-n-C5H11
S-n-C5H11
n
-C5H11 SH
C6H5 SH
H3CO
H3CO
H3CO
H3CO
39d
S-n-C5H11
S C6H5
48d
S C6H5
S C6H5
aThiol (A): thiol (B): GO (1.1 mmol:1.1 mmol:50 mg) for 1 mmol of aldehyde and the reaction was done at RT for entries 1 and 2.
bIsolated yield.
cReaction performed at 60 oC.
dProduct ratios are from HPLC analysis of the crude reaction mixture (See ESI S2.3).
As regard to the plausible mechanism for the reaction, we
wanted to probe the nature of active sites present in GO. In the
recent years, the acidic nature of GO has been attributed mostly
to the organosulfate group being originated during the oxidation
of graphite using Hummers method in the presence of sulfuric
acid,2b,8 though previous measurements of acidic pH were
attributable to the presence of surface attached carboxylic
functions.3 The FT-IR spectrum of GO showed the presence of
carboxyl (1719 cm–1; COOH stretching) and sulphate functional
groups (1052 cm–1; SO3–H stretching), which are comparable
with that reported in the literature (See ESI S3.1, Figure 3).8,11
We measured the pH of the GO in aqueous suspension (pH = 3.9;

6
Tetrahedron
50 mg in 50 ml 0.5M aqueous NaCl solution), and found it in
References and notes
fair agreement with previous observations.3 The pH of GO
remains fairly similar when measured after the reaction and also
of a mixture of n-pentane thiol and GO in water. On the other
hand, thioacetal formation is greatly retarded when the reaction is
carried out in water or in other organic solvents (Table 1, entries
7, 8 and 9). GO also gave positive Carius test and Lassaigne's test
signifying qualitatively the presence of S-containing functional
groups. The above results suggest that GO is an acid catalyst and
we tend to believe that the acidity of GO might be originating
from a combination of both carboxyl and organosulfate groups,
which is more pronounced in neat. Mechanistically, surface
active acidic functional groups of GO facilitate activation of the
aldehyde carbonyl group, more effectively in neat condition, and
subsequent nucleophilic attack by the thiol results in the eventual
formation of thioacetal.
1. Dreyer, D. R.; Bielawski, C. W. Chem. Sci. 2011, 2, 1233-1240.
2. (a) Navalon, S.; Dhakshinamoorthy, A.; Alvaro, M.; Garcia, H.
Chem. Rev. 2014, 114, 6179-6212; (b) Eigler, S.; Hirsch, A.
Angew. Chem., Int. Ed. 2014, 53, 7720-7738; (c) Su, C.; Loh, K.
P. Acc. Chem. Res. 2013, 46, 2275-2285.
3. Szabo, T.; Tombacz,; Illes, E. E.; Dekany, I. Carbon 2006, 44,
537-545.
4. Boehm, H. P.; Clauss, A.; Fischer, G.; Hofmann, U. Fifth
Conference on Carbon; Pergamon: Oxford, 1962; pp 73-80.
5. (a) Dreyer, D. R.; Jia, H.-P.; Bielawski, C. W. Angew. Chem., Int.
Ed. 2010, 49, 6813-6816; b) Mirza-Aghayan, M.; Kashef-Azar,
E.; Boukherroub, R. Tetrahedron Lett. 2012 , 53, 4962-4965; (c)
Dreyer, D. R.; Jia, H.-P.; Todd, A. D.; Geng J.; Bielawski, C. W.
Org. Biomol. Chem. 2011, 9, 7292-7295; (d) Huang, H.; Huang,
J.; Liu, Y.-M; He, H.-Y.; Cao, Y.; Fan, K.-N. Green Chem. 2012,
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Synth. Catal. 2011, 353, 528-532.
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673-676; (d) De, S. K. Tetrahedron Lett. 2004, 45, 1035-1036; (e)
De, S. K. Tetrahedron Lett. 2004, 45, 2339-2341; (f) Srivastava,
N. S.; Dasgupta K. Banik, B. K. Tetrahedron Lett. 2003, 44, 1191-
1193; (g) Kamal, A.; Chouhan, G. Tetrahedron Lett. 2002, 43,
1341-1350; (h) Muthusamy, S.; Babu, S. A.; Gunanathan, C.
Tetrahedron 2002, 58, 7897-7901; (i) Mondal, E.; Sahu, P. R.;
Bose, G.; Khan, A. T. Tetrahedron Lett. 2002, 43, 2843-2846; (j)
Muthusamy, S.; Babu, S. A.; Gunanathan, C. Tetrahedron Lett.
2001, 42, 359-362; (k) Firouzabadi, H.; Iranpoor N.; Hazarkhani,
H. J. Org. Chem. 2001, 66, 7527-7529; (l) Yadav, J. S.; Reddy, B.
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S. P.; Dantale, S. W. Tetrahedron Lett. 1995, 36, 2285-2288; (p)
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The reusability of the GO as heterogeneous acid catalyst was
examined with the combination of reactants: p-anisaldehyde and
n-pentanethiol in 1:2.2 ratios at room temperature. The GO was
recovered from the first batch of reaction by centrifugation and
washed with diethyl ether, dried and reused for subsequent four
batches of reactions. In all recycling experiments carried out at
room temperature, appreciable conversions were achieved
(Fig.1). A comparison of the FT-IR spectra of the GO before and
after use does not indicate any changes of the absorption bands,
signifying that the catalyst remain same after the reaction (See
ESI S3.1, Figure 4)
7.
(a) Ali, M. H.; Gomes, M. G. Synthesis 2005, 1326-1332; (b)
Firouzabadi, H.; Iranpoor, N.; Jafari, A. A.; Jafari, M. R. J. Mol.
Catal. A: Chem. 2006, 247, 14-18; (c) Gogoi, S.; Borah J. C.;
Barua, N.C. Synlett 2004, 1592-1594; (d) Ballini, R.; Bosica, G.;
Maggi, R.; Mazzacani, A.; Righi, P.; Sartori, G. Synthesis 2001,
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Fig. 1 Recyclability of GO in the thioacetalization of p-anisaldehyde and
n-pentanethiol
In summary, GO was found to catalyze the formation of
thioacetal from a neat mixture of aldehyde and thiol under mild,
solvent-free and aerobic conditions. Notable features of the
methodology described herein are: operational simplicity, nil or
negligible quantity of disulfide formation, applicability to a large
variety of aryl/heteroaryl aldehydes, chemoselectivity,
recyclability and environmental compatibility. Thus it provides a
practical approach for the preparation of open chain, cyclic and
unsymmetrical dithioacetals. The present method also
demonstrates that controlled loading of GO in combination with
reaction conditions could lead to the diversity in reaction types as
well as the formation of the products, and is likely to spur more
catalytic applications.
8.
Dhakshinamoorthy, A.; Alvaro, M.; Puche, M.; Fornes, V.;
Garcia, H. ChemCatChem. 2012, 4, 2026-2030.
9. Basu, B.; Kundu S.; Sengupta, D. RSC Adv. 2013, 3, 22130-
22134.
10. (a) Hummers, W. S.; Offeman, R. E. J. Am. Chem. Soc. 1958, 80,
1339; (b) Layek, R. K.; Samanta, S.; Chatterjee D. P.; Nandi, A.
K. Polymer 2010, 51, 5846-5856.
11. (a) Zhuo, Q.; Ma, Y.; Gao, J.; Zhang, P.; Xia, Y.; Tian, Y.; Sun,
X.; Zhong, J.; Sun, X. Inorg. Chem. 2013, 52, 3141-3147; (b)
Dhakshinamoorthy, A.; Alvaro, M.; Concepcion, P.; Fornes, V.;
Garcia, H. Chem. Commun. 2012, 48, 5443–5445.
Acknowledgments
12. (a) Papernaya, L. K.; Levanova, E. P.; Klyba, L. V.; Albanov, A.
I. Russ. J. Org. Chem. 2009, 45, 1036-1039; (b) Papernaya, L. K.;
Levanova, E. P.; Sukhomazova, E. N.; Albanov, A. I.; Deryagina,
E. N. Russ. J. Org. Chem. 2005, 41, 952-955.
We thank the Department of Science and Technology, India for
financial support (Grant No. SR/S1/OC-86). BR and DS thank UGC,
and CSIR, New Delhi respectively, for award of their fellowships.

7
Supplementary Material
Supplementary material that may be helpful in the review
process has been prepared and provided as a separate electronic
file in PDF.