Friedel-Crafts acylation of ferrocene with alkynoic acids

Article abstract of DOI:10.1016/j.jorganchem.2009.01.007

Ferrocene reacts with alkynoic acids (conjugated or nonconjugated terminal), trifluoroacetic anhydride and triflic acid (or boron trifluoride etherate) to afford the corresponding ferrocenyl ynones in good yields. Only in the case of propynoic- and 3-buty

Full text of DOI:10.1016/j.jorganchem.2009.01.007

Journal of Organometallic Chemistry 694 (2009) 1802–1806
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Journal of Organometallic Chemistry
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Friedel–Crafts acylation of ferrocene with alkynoic acids
*
_
Damian Plazuk, J. Zakrzewski
´
´
Department of Organic Chemistry, University of Łódz 90-136-Łódz, Narutowicza 68, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Ferrocene reacts with alkynoic acids (conjugated or nonconjugated terminal), trifluoroacetic anhydride
and triflic acid (or boron trifluoride etherate) to afford the corresponding ferrocenyl ynones in good
yields. Only in the case of propynoic- and 3-butynoic acid complex, untreatable reaction mixtures were
obtained. However, propynoilferrocene was obtained by desilylation of (trimethylsilyl)propynoilferro-
cene with KF/18-crown-6. Reaction with nonconjugated terminal alkynoic acids, carried out in the pres-
ence of a large excess of triflic acid (4 equiv.) results in formation of complex mixtures of products (in the
reaction with 4-pentynoic acid enol triflate of ferrocenyl 1,4-diketone and 2-ferrocenyl-5-methylfuran
were isolated). Ferrocenyl ynones obtained in this work can be used for synthesis of more complex ferr-
ocenyl systems by ‘‘click” chemistry.
Received 2 December 2008
Received in revised form 19 December 2008
Accepted 8 January 2009
Available online 14 January 2009
Keywords:
Ferrocene
Friedel–Crafts acylation
Alkynoic acid
Ynone
Ó 2009 Elsevier B.V. All rights reserved.
‘‘Click” chemistry
1. Introduction
methanesulfonic acid (TfOH) [16]. A similar system (carboxylic
acid/TFAA/H3PO4) has been employed for acylation of purely or-
Friedel–Crafts acylation of arenes is one of the most important
C–C bond forming reactions of electron-rich aromatic compounds
[1,2]. It also plays a very important role in the ferrocene chemistry
enabling an easy access to monoacyl- and 1,10-diacylferrocenes,
transformable into plethora of other derivatives via classical func-
tional group transformations [3–8]. Numerous ferrocene deriva-
tives prepared in this way found applications as ligands in
homogenous catalysis, in material science (e.g. in nonlinear optics)
and in bioorganometallic chemistry as redox-active probes or can-
didates for drugs [9,10].
In most cases saturated aliphatic or aromatic acid halides or
anhydrides in the presence of a Lewis or Brønsted acid have been
used as acylating agents. There are also a few reports on the use
of alkenoic acids halides to obtain alkenoyl ferrocenes [11–14].
Generally, these reactions are more difficult to perform because
of side reactions (e.g. cyclization to ferrocenophanones). To the
best of our knowledge, there is only one example of the Friedel–
Crafts reaction of ferrocene with an alkynoic acid derivative.
Crawford and Watts reported that reaction of ferrocene with
phenylpropiolic acid chloride and AlCl3 yields the corresponding
ynone in 41% yield [15].
ganic substrates [17,18] but it failed in the case of ferrocene [17].
We thought that it would be interesting to look whether our
system can be applied for acylation of ferrocene with unsaturated
acids, especially with alkynoic acids. This reaction would open a di-
rect route to practically unexplored class of ferrocenyl ynones,
which are expected to be versatile starting materials, e.g. in syn-
theses of ferrocenyl-substituted heterocycles [19] or for the
azide–alkyne ‘‘click” chemistry [20] (numerous synthetic applica-
tions of organic ynones have been reported [21–25]).
Herein, we report that ferrocene can be efficiently acylated, un-
der mild conditions, with alkynoic acids and TFAA in the presence
of a strong Brønstedt (TfOH) or Lewis (BF3) acid.
In our first attempts, we added ferrocene (1 mmol) and TfOH
(1.1 mmol or 4 mmol) or BF3 ꢀ Et2O (4 mmol) to solutions of conju-
gated acetylenic acids 1a–e (1.1 mmol) and TFAA (1.1 mmol) in
dichloromethane stirred previously for 1 min at rt and continued
the stirring for 2 h. Pleasingly, the workup afforded the corre-
sponding ferrocenyl ynones 2a–e in good yields (Scheme 1 and
Table 1).
Compounds 2a–c,e were characterized by spectroscopic meth-
ods and gave correct elemental analyses. Compound 2d displayed
spectroscopic data identical with those reported in the literature
[15].
It is seen from Table 1 that acylating systems based on TfOH are
more efficient than that using BF3 ꢀ Et2O. The yields of 2a–e are sat-
isfactory for reactions carried out in the presence of 1.1 equiv. of
TfOH and the increase of amount of this acid to 4 equiv. only leads
to a slight increase of the yields.
Recently, we developed a mild and efficient method of acylation
of ferrocene with saturated aliphatic and aromatic carboxylic acids
and trifluoroacetic anhydride (TFAA) in the presence of trifluoro-
* Corresponding author. Tel.: +48 42 678 4731; fax: +48 42 678 6583.
E-mail addresses: janzak@uni.lodz.pl, janzak@krysia.uni.lodz.pl (J. Zakrzewski).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.01.007

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D. Plazuk, J. Zakrzewski / Journal of Organometallic Chemistry 694 (2009) 1802–1806
1803
Fe
COOH
(1a-e)
(3a-d)
COOH
R
n
(TFAA/ acid)
(TFAA/ acid)
R
n
O
Fe
O
Fe
4a-c
(a) n=2; (b) n=3; (c) n =4; (d) n = 1
2a-e
(a) R = Me; (b) R = Pr; (c) R = (CH2)4CH ;
(d) R = Ph; (e) R = TMS
3
Scheme 1. Acylation of ferrocene with alkynoic acids (acid = TfOH or BF3 ꢀ Et2O).
Attempted reaction of ferrocene with propynoic acid (1, R = H)
gave, under the same conditions, an untreatable mixture of prod-
ucts. However, we obtained propynoilferrocene 2f in nearly quan-
titative yield by desilylation of the trimethylsilyl compound 2e
using potassium fluoride in the presence of 18-crown-6 in THF
Ccompounds 5 and 6 were separated by column chromatography
and fully characterized. Control experiments showed that neither
ynone 4a nor triflate 5 yields 6 under reaction conditions tested.
We believe that 5 and 6 might be formed on workup from diketone
7, but we failed to detect this compound in the reaction mixture
Compound 2f was already prepared by a less direct way – reac-
tion of ferrocenecarbaldehyde with ethynylmagnesium bromide
and subsequent oxidation of the alcohol formed [26].
Prompted by the above results we decided to look whether non-
conjugated alkynoic acids having a terminal acetylenic bond, 3a–d
(Scheme 1), react similarly with ferrocene to afford corresponding
ferrocenyl ynones. However, in this case we found that the amount
of TfOH and workup are of critical importance. Treatment of ferro-
cene with acids 3a–c and TFAA in the presence of 1.1 equiv. of
TfOH or 4 equiv. of BF3 ꢀ Et2O afforded expected ynones 4a–c in
high yields (Scheme 1 and Table 2).
Table 2
Acylation of ferrocene with terminal alkynoic acids.
Alkynoic acid
Product
Yield,%
In contrast, reactions carried out in the presence of 4 equiv. of
TfOH had completely different outcome and the products 4a–c
were not formed. In the case of reaction with 4-pentynoic acid
the triflate 5 was the sole isolable product (37% yield, Eq. (2)).
However, a slight change of workup (extraction after 2 h of stirring
of the reaction mixture poured onto water at rt instead of immedi-
ate extraction) afforded a mixture of 5 (49%) and 2-ferrocenyl-5-
methylfuran 6 (23%)
TfOH (1.1 equiv.)
BF3 ꢀ Et2O (4 equiv.)
3a
3b
3c
3d
4a
4b
70
91
74
–
73
83
98
–
4c
a
a
Complex, inseparable mixture of products was obtained, containing, according
to 1H NMR, FcCOCH2COCH3 and its BF2-complex [27] (when BF3 ꢀ Et2O was used).
Table 1
Acylation of ferrocene with conjugated alkynoic acids.
Alkynoic acid
Product
Yield (%)
TfOH (1.1 equiv.)
TfOH (4 equiv.)
BF3 ꢀ Et2O (4 equiv.)
1a
1b
1c
1d
1e
2a
2b
2c
2d
2e
66
70
74
73
80
74
78
80
82
73
44
61
47
52
39

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D. Plazuk, J. Zakrzewski / Journal of Organometallic Chemistry 694 (2009) 1802–1806
1804
Under the same conditions reaction of ferrocene with 5-hexy-
nyl ynones. Yields are given in Tables 1 and 2. Spectroscopic and
noic and 6-heptynoic acids, afforded, regardless workup employed,
complex inseparable mixtures.
analytical data:
Ynones 4a–c are potential starting materials for syntheses of
ferrocene-based molecular materials using the copper (I) catalyzed
azide-alkyne ‘‘click” chemistry [20]. To prove this possibility we
carried out a copper-catalysed reaction of 4a with (phenyl-
thio)methyl azide. We isolated the corresponding triazole 8 in
47% yield (the conditions of this reaction were not optimized).
2.2. FcCOC„CMe (2a)
Orange crystals. 1H NMR (200 MHz, CDCl3, d): 4.92 (t, J = 1.9 Hz,
2H, Cp), 4.57 (t, J = 1.9 Hz, 2H, Cp), 4.26 (s, 5H, Cp), 2.10 (s, 3H, CH3).
13C NMR (62.90 MHz, CDCl3, d): 181.57, 92.30, 88.37, 80.35, 79.79,
72.95, 70.36, 4.17. IR (KBr, cmꢂ1): 2966, 2931, 2279, 2213, 1617,
In conclusion, we have elaborated a simple and efficient one-
step route to both conjugated of non-conjugated terminal ferroce-
nyl alkynones starting from ferrocene and corresponding alkynoic
acids. It is worthy noting that compounds prepared by our method
are isomers of ferrocenylethynyl ketones (FcCꢁCCOR), which can
be obtained from ethynylferrocene and acyl chlorides or aryl io-
dides and CO in palladium-catalyzed reactions [28–31].
1447, 1280. MS (EI, m/z): 252 (M+). Anal. Calcd for C14H12FeO: C,
66.70; H, 4.80. Found: C, 66.49; H, 4.67.
2.3. FcCOC„CCH2CH2CH3. (2b)
Red solid. 1H NMR (200 MHz, CDCl3, d): 4.92 (t, J = 2.0 Hz, 2H,
Cp), 4.58 (t, J = 2.0 Hz, 2H, Cp), 4.26 (s, 5H, Cp), 2.44 (t, J = 6.9 Hz,
2H, CH2), 1.70 (sextet, J = 7.4 Hz, 2H, CH2), 1.11 (t, J = 7.3 Hz, 3H,
CH3). 13C NMR (62.90 MHz, CDCl3, d): 181.47, 92.57, 80.60, 80.45,
72.91, 70.43, 70.34, 21.47, 20.97, 13.53. IR (neat, cmꢂ1): 3108,
3085, 2996, 2931, 2874, 2213, 1618, 1447, 1280. MS (EI, m/z):
280 (M+). Anal. Calcd for C16H16FeO: C, 68.60; H, 5.76. Found: C,
68.65; H, 5.61.
2. Experimental
All reactions were carried out under argon. All reagents used in
this work are commercially available (Aldrich) and were used
without further purifications. Solvents were dried over appropriate
drying agents and distilled before use. Chromatographic separa-
tions were carried out using Silica gel 60 (Merck, 230–400 mesh
ASTM). The NMR spectra were run on Varian Gemini 200 BB
(200 MHz for 1H) and Bruker Advance 250 (250 MHz for 1H) spec-
trometers and IR spectra on a FT-IR Nexus spectrometer. Mass
spectra were measured on a Finnigan MAT 95 spectrometer. Ele-
mental analyses were performed by Analytical Services of the Cen-
ter of Molecular and Macromolecular Studies of the Polish
2.4. FcCOC„CCH2CH2CH2CH2CH3. (2c)
Red oil. 1H NMR (200 MHz, CDCl3, d): 4.91 (t, J = 2.0 Hz, 2H,
Cp), 4.57 (t, J = 2.0 Hz, 2H, Cp), 4.26 (s, 5H, Cp), 2.45
(t, J = 6.8 Hz, 2H, CH2), 1.64 (m, 2H, CH2), 1.46 (m, 4H, CH2),
0.95 (t, J = 7.0 Hz, 3H, CH3). 13C NMR (50 MHz, CDCl3, d):
181.42, 92.64, 80.33, 80.28, 72.79, 70.23, 70.17, 30.75, 27.35,
21.82, 18.70, 13.67. IR (neat, cmꢂ1): 3099, 3081, 2953, 2925,
2872, 2857, 2211, 1614, 1448, 1280. MS (EI, m/z): 308 (M+).
Anal. Calcd for C18H20FeO: C, 70.15; H, 6.54. Found: C, 70.11;
H, 6.41.
´
Academy of the Sciences (Łódz).
2.1. Friedel–Crafts reaction of ferrocene with alkynoic acids
A solution of an alkynoic acid (1.1 mmol) and TFAA (153
1.1 mmol) in dichloromethane (5 ml) was stirred 1 min at rt and
ferrocene (186 mg, 1 mmol) and TfOH (97 l, 1.1 mmol or 337 l,
4 mmol) or BF3 ꢀ Et2O (0.5 ml, 4 mmol) were added. The resulting
solution was stirred for 2 h. Water (15 ml) was added, the organic
layer was separated and aqueous layer extracted with dichloro-
methane. The combined organic extracts were washed with water,
dried (MgSO4) and the solvent evaporated. Column chromatogra-
phy (silica gel/dichloromethane) afforded corresponding ferroce-
l
l,
2.5. FcCOC„CPh (2d)
l
l
Dark red-violet. 1H NMR (200 MHz, CDCl3, d): 7.67 (m, 2H, Ph),
7.46 (m, 3H, Ph), 5.02 (t, J = 2.2 Hz, 2H, Cp), 4.64 (t, J = 2.1 Hz, 2H,
Cp), 4.31 (s, 5H, Cp). 13C NMR (62.90 MHz, CDCl3, d): 181.19,
132.78, 130.36, 128.66, 120.64, 89.53, 87.75, 80.62, 73.28, 70.57.
IR (KBr, cmꢂ1): 3099, 3087, 3067, 2205, 1608, 1489, 1449, 1375,
1296, 1703, 1030, 757, 689, 501; identical with those reported in
the literature [15].

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D. Plazuk, J. Zakrzewski / Journal of Organometallic Chemistry 694 (2009) 1802–1806
1805
2.6. FcCOC„CTMS (2e)
2.12. Desilylation of 2e
Dark red powder. 1H NMR (200 MHz, CDCl3, d): 4.92 (t,
J = 2.0 Hz, 2H, Cp), 4.61 (t, J = 2.0 Hz, 2H, Cp), 4.26 (s, 5H, Cp),
0.32 (s, 9H, CH3). 13C NMR (50 MHz, CDCl3, d): 180.80, 102.03,
96.24, 80.10, 73.33, 70.57, 0.59. IR (KBr, cmꢂ1): 3104, 3086, 2958,
2149, 1611, 1275, 847. MS (EI, m/z): 310 (M+). Anal. Calcd for
C16H18FeOSi: C, 61.94; H, 5.85. Found: C, 62.24; H, 6.10.
To a solution of 2e (86.5 mg, 0.279 mmol), 18-crown-6 (2.5 mg)
in THF, powdered anhydrous KF (350 mg, 6.0 mmol) was added at
rt and the resulting mixture was stirred 30 min. After addition of
water (30 ml) the product was extracted with dichloromethane
(3 ꢃ 25 ml). Flash chromatography (silica gel/dichloromethane)
afforded FcCOCꢁCH (2f) as dark red crystals. Yield 59.4 mg (99%)
1H NMR (200 MHz, CDCl3,) d 4.95 (m, 2H, Cp), 4.64 (m, 2H, Cp),
4.28 (s, 5H, Cp), 3.27 (s, 1H, CH); IR (KBr, cmꢂ1)1620 (C@O),
1449, 1274; identical with those reported in the literature [26].
2.7. FcCOCH2CH2C„CH (4a)
Orange crystals. 1H NMR (200 MHz, CDCl3, d): 4.80 (t, J = 1.9 Hz,
2H, Cp), 4.51 (t, J = 1.9 Hz, 2H, Cp), 2.97 (t, J = 6.6 Hz, 2H, CH2), 2.59
(dt, J = 6.6 Hz, 2.0 Hz, 2H, CH2), 2.00 (t, J = 2.0 Hz, 1H, CH). 13C NMR
(62.90 MHz, CDCl3, d): 201.58, 83.88, 78.34, 72.29, 69.76, 69.18,
68.65, 38.34, 13.08. IR (KBr, cmꢂ1): 1655, 3250. MS (EI, m/z): 266
(M+). Anal. Calcd for C14H15FeO: C, 67.70; H, 5.30. Found: C,
67.50; H, 5.26.
2.13. ‘‘Click” reaction of 4a with (phenylthio)methyl azide
To a solution of 4a (288 mg, 1.08 mmol) in the mixture of tert-
butanol (10 ml), ethanol (10 ml) and water (5 ml) were added
(phenylthio)methyl azide (288 mg, 1.75 mmol), CuCl (2 mg) and
Cu (50 mg as a thin wire). After 72 h stirring at ambient tempera-
ture water (40 ml) was added and the product was extracted with
dichloromethane. Flash chromatography (eluent chloroform–ethyl
acetate) and crystallization (chloroform–hexane) afforded 8 as or-
ange solid. Yield 221 mg (47%). 1H NMR CDCl3, d): 7.42 (s, 1H, CH),
7.30 (m, 5H, Ph), 5.58 (s, 2H, CH2), 4.77 (t, J = 1.7 Hz, 2H, Cp), 4.49
(t, J = 1.8 Hz, 2H, Cp), 4.11 (s, 5H, Cp), 3.11 (m, 4H, CH2CH2). 13C
NMR (DMSO-d6, d): 202.07, 146.67, 132.73, 130.45, 129.24,
127.58, 122.01, 78.82, 72.02, 69.51, 68.99, 51.46, 38.04, 19.47. IR
(KBr, cmꢂ1): 3119, 3082, 2963, 2925, 2857, 1729, 1649, 1455. MS
(EI, m/z): 431 (M+). Anal. Calcd for C23H23FeN3O: C, 61.26; H,
4.91; N, 9.74. Found: C, 61.23; H, 4.95; N, 9.81.
2.8. FcCOCH2CH2CH2C„CH (4b)
Red oil. 1H NMR (200 MHz, CDCl3, d): 4.81 (t, J = 1.9 Hz, 2H, Cp),
4.51 (t, J = 1.9 Hz, 2H, Cp), 4.22 (s, 5H, Cp), 2.89 (t, J = 7.1 Hz, 2H,
CH2), 2.35 (dt, J = 6.8 Hz, 2.8 Hz, 2H, CH2), 2.02 (t, J = 2.6 Hz, 1H,
CH), 1.94 (m, 2H, CH2). 13C NMR (50 MHz, CDCl3, d): 203.62, 83.93,
78.98, 72.13, 69.76, 69.21, 69.05, 37.68, 22.70, 17.87. IR
(neat, cmꢂ1): 3297, 3256, 3096, 2938, 2116, 1667. MS (EI, m/z):
280 (M+). Anal. Calcd for C16H16FeO: C, 68.60; H, 5.76. Found: C,
68.59; H, 5.93.
Acknowledgments
2.9. FcCOCH2CH2CH2CH2C„CH (4c)
Financial support from the Polish Ministry of Education and Sci-
ence (Grant PBZ-KBN 118/T09/12) and from the Foundation for
Polish Science is gratefully acknowledged.
Red liquid. 1H NMR (200 MHz, CDCl3, d): 4.79 (t, J = 1.9 Hz, 2H,
Cp), 4.50 (t, J = 1.9 Hz, 2H, Cp), 4.20 (s, 5H, Cp), 2.74 (t, J = 7.0 Hz,
2H, CH2), 2.27 (dt, J = 2.6 Hz, 7.0 Hz, 2H, CH2), 1.98 (t, J = 2.7 Hz,
1H, CH), 1.85 (m, 2H, CH2), 1.64 (m, 2H, CH2). 13C NMR
(62.90 MHz, CDCl3, d): 204.02, 84.09, 79.01, 72.10, 69.70, 69.25,
68.54, 38.98, 28.11, 23.54, 18.27. IR (neat, cmꢂ1): 3301, 3252,
3096, 2938, 2865, 2116, 1557. MS (EI, m/z): 294 (M+). Anal. Calcd
for C17H18FeO: C, 69.41; H, 6.17. Found: C, 69.25; H, 6.03.
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2.10. FcCOCH2CH2C(OTf)@CH2 (5a)
Orange crystals. 1H NMR (200 MHz, CDCl3, d): 5.17 (d, J = 3.6 Hz,
1H, CH), 5.09 (dt, J = 3.6 Hz, 1.0 Hz, 1H, CH), 4.80 (t, J = 1.8 Hz, 2H,
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Yellow crystals. 1H NMR (200 MHz, CDCl3, d): 6.13 (d, J = 3.0 Hz,
1H, CH), 5.92 (dq, J = 1.0 Hz, 3.0 Hz, 1H, CH), 4.55 (t, J = 1.9 Hz, 2H,
Cp), 4.23 (t, J = 1.8 Hz, 2H, Cp), 4.10 (s, 5H, Cp), 2.32 (d, J = 1.0 Hz,
3H, CH3). 13C NMR (50.0 MHz, CDCl3, d): 151.60, 150.53, 124.9,
107.00, 104.39, 69.33, 68.13, 13.72. IR (KBr, cmꢂ1): 3114, 3096,
3086, 2955, 2921,1578, 1443, 1106, 821, 792. MS (EI, m/z): 266
(M+). Anal. Calcd for C15H14FeO: C, 67.70; H, 5.30. Found: C,
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_
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