Acylation of Ferrocene and a 1,1′-Diphosphaferrocene with Acyl Trifluoroacetates in the Presence of Trifluoromethanesulfonic (Triflic) Acid or Some Metal Triflates

Article abstract of DOI:10.1081/SCC-120027243

Ferrocene and 3,3′,4,4′-tetramethyl-1,1′ -diphosphaferrocene undergo efficient acylation by acyl trifluoroacetates (prepared in situ from carboxylic acids and trifluoroacetic anhydride) in the presence of an excess of trifluoromethanesulfonic acid or cata

Full text of DOI:10.1081/SCC-120027243

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Acylation of Ferrocene and a
1,1′‐Diphosphaferrocene with Acyl Trifluoroacetates
in the Presence of Trifluoromethanesulfonic (Triflic)
Acid or Some Metal Triflates
Damian Plażuk a & Janusz Zakrzewski a
a Department of Organic Chemistry, University of Łódź, Łódź, Narutowicza 68, 90‐136,
Poland
Version of record first published: 16 Aug 2006.
To cite this article: Damian Plażuk & Janusz Zakrzewski (2004): Acylation of Ferrocene and a 1,1′‐Diphosphaferrocene
with Acyl Trifluoroacetates in the Presence of Trifluoromethanesulfonic (Triflic) Acid or Some Metal Triflates, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 34:1, 99-107
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SYNTHETIC COMMUNICATIONSw
Vol. 34, No. 1, pp. 99–107, 2004
Acylation of Ferrocene and a
1,10-Diphosphaferrocene with Acyl
Trifluoroacetates in the Presence of
Trifluoromethanesulfonic (Triflic)
Acid or Some Metal Triflates
*
Damian Plaz˙uk and Janusz Zakrzewski
´ ´
Department of Organic Chemistry, University of Łodz,
´ ´
Łodz, Poland
ABSTRACT
Ferrocene and 3,30,4,40-tetramethyl-1,10-diphosphaferrocene undergo
efficient acylation by acyl trifluoroacetates (prepared in situ from
carboxylic acids and trifluoroacetic anhydride) in the presence of an
excess of trifluoromethanesulfonic acid or catalytic amounts (10% mol.)
of its ytterbium or hafnium salts.
Key Words: Ferrocene; Friedel–Crafts acylation; Triflic acid; Metal
triflate; 1,10-Diphosphaferrocene.
*
Correspondence: Janusz Zakrzewski, Department of Organic Chemistry, University
´ ´ ´ ´
of Łodz, 90-136 Łodz, Narutowicza 68, Poland; E-mail: janzak@uni.lodz.pl.
99
DOI: 10.1081/SCC-120027243
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

100
Plaz˙uk and Zakrzewski
Continuously growing applications of substituted ferrocenes in modern
chemical, biological and material sciences,[1,2] including chiral catalysis,[3–5]
red-ox active polymers and dendrimers,[6–9] nonlinear optical
materials,[10–12] molecular electronic devices,[13,14] biosensors,[15–18] com-
[22]
ponents of complex photochemical systems,[19–21] drugs
etc. largely
stimulated development of synthetic methods enabling introduction to this
metallocene desired carbon chains and/or functional groups. The intrinsic
high nucleophilic reactivity of ferrocene[1,2,23] makes Friedel–Crafts
acylation, hydroxyalkylation and alkylation (or alkenylation) a particularly
attractive method for synthesis of substituted ferrocenes.[23–28]
In recent years a significant progress in the Friedel–Crafts chemistry of
arenes was achieved. It involves inter alia the discovery of the catalysis by
metal trifluoromethanesulfonates (triflates),[29] superacids[30–32] and the use
of mixed anhydrides such as acyl triflates[33,34] and acyl trifluoroacetates[35,36]
as electrophilic reagents. However, up to now only a few works have been
devoted to introduction of these achievements to the ferrocene chemistry in
order to simplify synthetic procedures and to make them milder, more efficient
and eco-friendly. In most cases acid chlorides or anhydrides and AlCl3 or BF3
are used in ferrocene acylation[37] (although an interesting electrochemical
generation of the acylating agent was recently reported[38]). Roberts and Wells
reported acylation of ferrocene, 3,4-dimethylphosphaferrocene and 3,30,4,40-
tetramethyl-1,10-diphosphaferrocene with acid anhydrides in the presence of
triflic acid (TfOH).[39] They found that the use of this protic acid in place of a
classical Lewis acid (AlCl3) led to better yields of acylated products,
especially for less stable heteroferrocenes. A paper by Galli[40] briefly
mentioned that ferrocene can be acetylated by acetyl trifluoroacetate in the
presence of 85% H3PO4, but more systematic study on this system are lacking.
More recently Ranu et al.[41] described efficient and selective monoacylation
of ferrocene with acyl trifluoroacetates (prepared in situ from carboxylic acids
and trifluoroacetic anhydride, TFAA) on the solid phase of alumina.
In this paper we report the acylation of ferrocene (1) by acyl trifluoro-
acetates in the presence of triflic acid, or catalytic amounts of metal triflates
such as ytterbium triflate Yb(OTf)3 and hafnium triflate Hf(OTf)4 (Sch. 1). As
there is an increasing interest in development of synthetic chemistry of the
group 15 elements heteroferrocenes[42–44] we have tested applicability of our
method in this field using 3,30,4,40,-tetramethyl-1,10-diphosphaferrocene (2), a
stable and readily available heteroferrocene, which is known to undergo
Friedel–Crafts acylation by acyl chlorides or anhydrides in the presence of
AlCl3.[45]
When a carboxylic acid RCOOH is mixed with a stoichiometric amount
of TFAA an acyl trifluoroacetate, RCOOCOCF3, is formed. Acyl trifluro-
acetates are reactive electrophiles in aromatic acylation, but usually require

Acylation of Ferrocene
101
Scheme 1.
strong acid catalysis (trifluoroacetic acid formed in the reaction between
RCOOH and TFAA may be not sufficiently strong.[35,36] We have found that
ferrocene is slowly acetylated by a 1 : 1 mixture of acetic acid and TFAA
(1.0 molar equivalent) in dichloromethane at r.t. The isolated yield of
acetylferrocene was low (8% after 1 h, 12% after 4 h and 9% after 20 h).
However, in the presence of triflic acid (4.0 molar equivalents) the acetylation
proceeded smoothly and after 1 h acetylferrocene was isolated in 92% yield. In
the absence of TFAA no reaction was observed. The results obtained for
acylation of ferrocene and 3,30,4,40-tetramethyldiphosphaferrocene with acyl
triflates (RCOOH/TFAA) in the presence of triflic acid are gathered in Table 1
(Method A). The best yields in this method (92–97% in the case of acylation
of ferrocene) were obtained for simple aliphatic acids. Aromatic acids proved
slightly less efficient. Acylation of 2 proceeds with lower yields that acylation
of ferrocene. The yield of 2-benzoyl-3,30,4,40,-tetramethyl-1,10-diphosphafer-
rocene is comparable with that reported earlier, using (PhCO)2O/TfOH,[40]
whereas lower yield was obtained for acetylation. In no case was formation of
diacylated products observed. This may be due to the extensive protonation of

102
Plaz˙uk and Zakrzewski
Table 1. Acylation of ferrocene and 3,30,4,40,-tetramethyl-1,10-
diphosphaferrocene.
Metallocene
Methoda
Product (R)
Yield, %
1
A
B
C
A
B
A
A
B
C
A
B
A
A
B
C
A
C
3a (Me)
3a (Me)
3a (Me)
3b (Et)
92
66 (90)b
54
92
74
3b (Et)
3c (t-Bu)
3d (Ph)
3d (Ph)
97
78
12
29
3d (Ph)
3e (p-MeOPh)
3e (p-MeOPh)
3f (PhCH2)
4a (Me)
4a (Me)
4a (Me)
4b (Ph)
69
31
72
58
2
48
70
73
63
4b (Ph)
aA: RCOOH/TFAA/Metallocene/TfOH 1 : 1 : 1 : 4; B: RCOOH/
TFAA/Metallocene/Yb(OTf)3 1 : 1 : 1 : 0.1; C: RCOOH/TFAA/
Metallocene/Hf(OTf)4 1 : 1 : 1 : 0.1.
bBased on reacted 1.
acyl metallocenes in strongly acidic reaction medium, lowering their
reactivity towards electrophiles.[40]
In a series of works Kobayashi et al.[29] have shown that rare earth metal
triflates and scandium- or hafnium-triflate catalyze acylation of arenes by acid
chlorides or anhydrides. We have therefore decided to look whether triflic acid
in the Method A can be replaced by a catalytic amount of Yb(OTf)3 or
Hf(OTf)4. The results obtained for reaction of the RCOOH-TFAA (1 : 1)
system with ferrocene and 3,30,4,40,-tetramethyl-1,10-diphosphaferrocene in
dichloromethane in the presence of 10% mol of the triflate are shown in
Table 1 (Methods B and C for the ytterbium and hafnium salt, respectively).
These results show efficient catalysis by both triflates. Generally, in the
case of ferrocene Methods B and C give moderate to good yields of acylated
products (although lower than those obtained using Method A) for aliphatic
acids and only poor yields for aromatic acids. The hafnium salt proved more
efficient than its ytterbium counterpart for benzoylation and less efficient for
acetylation (Table 1). In the case of the acetylation of 2 the hafnium triflate-

Acylation of Ferrocene
103
catalyzed reaction proceeds more efficiently than the triflic acid-catalyzed
reaction.
According to earlier works,[29–36] our acylating systems could offer an
opportunity of the recycling of TFAA (as trifluoroacetic acid, TFA), TfOH (as
the barium salt) and metal triflates. However, working at a one milimole-scale
we have not tested this opportunity. A considerable advantage of our method
is also the use of carboxylic acids instead of less readily available acid
chlorides and anhydrides.
In conclusion, we have described novel and efficient acylation reactions
of ferrocene and a 1,10-diphosphaferrocene using carboxylic acids and TFAA
in the presence of triflic acid or a metal triflate.
EXPERIMENTAL
General
Dichloromethane was distilled over calcium hydride. The reactions
described here do not require an inert gas atmosphere and were carried out in
air with protection against humidity. Ferrocene was purchased from Aldrich
and used without prior purification. Compound 2 was prepared according to
the literature procedure.[46] Other reagents and solvents were obtained from
Aldrich and used as received.
General procedure for acylation of 1 and 2: A solution of a carboxylic
acid (1.0 mmol) and TFAA (1.0 mmol) in dichloromethane (5 mL) was stirred
at r.t. for 1 h. 1 or 2 (1.0 mmol) and triflic acid (4.0 mmol, Method A) or a
metal triflate (0.1 mmol, Method B and C) were then added, and the mixture
was stirred 1 h. After quenching with water, extraction with dichloromethane
the product was isolated by column chromatography (silicagel, chloroform) as
an orange-red band which followed the yellow band containing a small
amount of unreacted 1 or 2. The products were identified by spectral
comparison with the authentic samples.[37,45,47]
Acetylferrocene (3a). M.p. 85–868C (Lit.[37] 86–878C). 1H NMR
(CDCl3, d): 4.77 (t, J ¼ 1.8 Hz, 2H, H2 and H5), 4.55 (t, J ¼ 1.8 Hz, 2H, H3
and H4), 4.19 (s, 5H, unsubst. Cp), 2.37 (s, 3H, Me). IR (KBr): n(CO)
1660 cm21
.
Propionylferrocene (3b). M.p. 38–398C (Lit.[37] 34–368C). H NMR
(CDCl3, d): 4.78 (t, J ¼ 1.8 Hz, 2H, H2 and H5), 4.45 (t, J ¼ 1.8 Hz, 2H, H3
and H4), 4.19 (s, 5H, unsubst. Cp), 2.69 (q, J ¼ 7.5 Hz, 2H, CH2), 1.17 (t,
1
J ¼ 7.5 Hz, 3H, CH3). IR (KBr): n(CO) 1660 cm21
.

104
Plaz˙uk and Zakrzewski
Pivaloylferrocene (3c). M.p. 92–938C (Lit.[23] 928C). 1H NMR (CDCl3,
d): 4.84 (t, J ¼ 1.8 Hz, 2H, H2 and H5), 4.46 (t, J ¼ 1.8 Hz, 2H, H3 and H4),
4.19 (s, 5H, unsubst. Cp), 1.33 (s, 9H, t-Bu). IR (KBr): n(CO) 1655 cm21
.
Benzoylferrocene (3d). M.p. 105–1078C (Lit.[37] 105–1078C). 1H
NMR (CDCl3, d): 7.5–7.9 (m, 5H, Ph), 4.90 (t, J ¼ 1.8 Hz, 2H, H2 and H5),
4.55 (t, J ¼ 1.8 Hz, 2H, H3 and H4), 4.18 (s, 5H, unsubst. Cp), IR (KBr):
n(CO) 1664 cm21
.
(4-Methoxybenzoyl)ferrocene (3e). M.p. 74–758C (Lit.[37] 74–768C).
1H NMR (CDCl3, d): 7.93 (d, J ¼ 8.6 Hz, aromatic),d): 7.02 (d, J ¼ 8.6 Hz,
aromatic), 4.84 (t, J ¼ 1.8 Hz, 2H, H2 and H5), 4.50 (t, J ¼ 1.8 Hz, 2H, H3
and H4), 4.12 (s, 5H, unsubst. Cp). IR (KBr): n(CO) 1652 cm21
.
(Phenylacetyl)ferrocene (3f). M.p. 128–1308C (Lit.[23] 1308C). 1H
NMR (CDCl3, d): 7.26–7.35 (m, 5H, Ph), 4.89 (t, J ¼ 1.8 Hz, 2H, H2 and H5),
4.51 (t, J ¼ 1.8 Hz, 2H, H3 and H4), 4.12 (s, 5H, unsubst. Cp), 3.98 (s, 2H,
CH2). IR (KBr): n(CO) 1658 cm21
.
2-Acetyl-3,30,4,40-tetramethyl-1,10-diphosphaferrocene (4a). M.p. 68–
1
708C (Lit.[45] ꢀ718C). H NMR (CDCl3, d): 4.00 (d, J ¼ 36.2 Hz, 1H, H5),
3.71 (d, J ¼ 36.0 Hz, 2H, H20 and H50), 2.28 (d, J ¼ 2.9 Hz, 3H, acetyl), 2.39
(s, 3H, Me), 2.10 (s, 3H, Me), 2,06 (s, 3H, Me); 2.00 (s, 3H, Me). IR (KBr):
.
n(CO) 1652 cm21
2-Benzoyl-3,30,4,40-tetramethyl-1,10-diphosphaferrocene (4b). M.p.
1
66–688C (Lit.[45] ꢀ708C). H NMR (CDCl3, d): 7.3–7.6 (m, 5H, Ph), 3.94
(d, J ¼ 36.2 Hz, 1H, H5), 3.63 (d, J ¼ 36.0 Hz, 2H, H20 and H50), 2.30 (s, 3H,
Me), 2.15 (s, 3H, Me), 2,08 (s, 3H, Me); 1.98 (s, 3H, Me). IR (KBr): n(CO)
1642 cm21
.
ACKNOWLEDGMENT
We thank University of Lodz for financial support of this work (Grant
505/471).
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Received in Poland April 27, 2003