Ferrocenium hexafluorophosphate as an inexpensive, mild catalyst for the etherification of propargylic alcohols

Article abstract of DOI:10.1016/j.molcata.2015.06.009

Commercial ferrocenium hexafluorophosphate ([FeCp₂]PF₆) was found to be an efficient catalyst for the etherification of terminal, tertiary propargylic alcohols with primary and secondary alcohols (5 h to 3 days reaction time at 40 °C in CH₂Cl₂, 3 mol% catalyst loading). The propargylic ether products were isolated in 90-20% yields. The alcohols and propargylic alcohols were employed in an equimolar amount and no further additives were required. For a purely aromatic propargylic alcohol, the isolated yields were lower than those for a mixed aromatic-aliphatic propargylic alcohol. Through monitoring reactant consumption and product formation over time, we found that the aromatic propargylic alcohol undergoes yield-diminishing Meyer-Schuster rearrangements to the aldehyde more easily than the mixed aromatic-aliphatic propargylic alcohol. The employment of [Fe(Cp)₂]PF₆ as a single electron oxidant has the potential to add a new direction in the development of catalysts for the title reaction based on single electron transfer processes.

Full text of DOI:10.1016/j.molcata.2015.06.009

Journal of Molecular Catalysis A: Chemical 407 (2015) 221–229
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Ferrocenium hexafluorophosphate as an inexpensive, mild catalyst
for the etherification of propargylic alcohols
Matthew J. Queensen, Jordan M. Rabus, Eike B. Bauer∗
University of Missouri - St. Louis, Department of Chemistry and Biochemistry, One University Boulevard, St. Louis, MO 63121, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 May 2015
Received in revised form 8 June 2015
Accepted 9 June 2015
Available online 18 June 2015
Commercial ferrocenium hexafluorophosphate ([FeCp2]PF6) was found to be an efficient catalyst for the
etherification of terminal, tertiary propargylic alcohols with primary and secondary alcohols (5 h to 3
days reaction time at 40 ◦C in CH2Cl2, 3 mol% catalyst loading). The propargylic ether products were iso-
lated in 90–20% yields. The alcohols and propargylic alcohols were employed in an equimolar amount
and no further additives were required. For a purely aromatic propargylic alcohol, the isolated yields
were lower than those for a mixed aromatic-aliphatic propargylic alcohol. Through monitoring reactant
consumption and product formation over time, we found that the aromatic propargylic alcohol under-
goes yield-diminishing Meyer–Schuster rearrangements to the aldehyde more easily than the mixed
aromatic-aliphatic propargylic alcohol. The employment of [Fe(Cp)2]PF6 as a single electron oxidant has
the potential to add a new direction in the development of catalysts for the title reaction based on single
electron transfer processes.
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
a synthetic challenge [1]. In addition, OH units are poor leav-
ing groups, further complicating substitution chemistry. Potential
Propargylic alcohols (1 in Scheme 1) are valuable starting
materials in organic synthesis [1,2]. They are easily accessible
[3] and combine two functional groups – a triple bond and an
OH unit – in adjacent positions within one molecule, giving
rise to a multifaceted reaction landscape [1,2]. Consequently,
propargylic alcohols can be engaged in a great variety of organic
transformations. Among other reactions, propargylic alcohols can
undergo addition [4] or substitution reactions [2,5,6], they can be
transformed to allenes [7] and can rearrange to carbonyl com-
pounds [8,9,10], which has been synthetically useful in natural
product syntheses [11]. Especially cyclization reactions [12] or
addition/cyclization [13] or substitution/cyclization [14] sequences
allow access to sophisticated synthetic targets from simple starting
materials. Propargylic alcohols are, thus, powerful building blocks
in organic syntheses.
propargylic carbocation intermediates 2 (Scheme 1) in substitution
reactions can rearrange to allenic cations 3, which can react with
nucleophiles to afford allenes 6. Furthermore, propargylic alcohols
can undergo Meyer–Schuster rearrangements to aldehydes (7 in
Scheme 1) [8,9,16]. Catalysts are required for propargylic substi-
tution reactions, which are, in most cases, Lewis acids. However,
Lewis acid catalysts tend to also catalyze the Meyer–Schuster rear-
rangement to form aldehydes [8,9,17]. For example, FeCl3 catalyzes
both propargylic substitution [18] and Meyer–Schuster rearrange-
ment reactions [19,20]. Thus, the challenge is to develop catalyst
systems that promote the substitution reactions in high chemose-
lectivity.
Mechanistically, the substitution can proceed either through a
carbocation 2 [21] or an allenylidene intermediate 4 (Scheme 1)
[1,2,15,22]. Brønsted [23] as well as transition metal based Lewis
acids have been employed as catalysts for the reaction [1]. It appears
to be reasonable to assume that hard Lewis acids promote the car-
bocation route, whereas more highly sophisticated, soft transition
metal complexes are more suitable to form an allenylidene inter-
mediate (which greatly increases the selectivity of the catalyst)
[24]. A variety of catalyst systems are known to substitute OH
groups by a variety of carbon- [25], nitrogen- [26] or phosphorus-
centered [27] nucleophiles.
Propargylic substitution reactions (i.e. the substitution of the
OH group by a nucleophile, Nu, to afford the substitution prod-
uct 5, Scheme 1) are frequently applied in the functionalization
of propargylic alcohols [1,2,5,15]. The complex reaction landscape
of propargylic substitution reactions shown in Scheme 1 poses
∗
Corresponding author. Fax: +1 314 516 5342.
E-mail address: bauere@umsl.edu (E.B. Bauer).
http://dx.doi.org/10.1016/j.molcata.2015.06.009
1381-1169/© 2015 Elsevier B.V. All rights reserved.

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M.J. Queensen et al. / Journal of Molecular Catalysis A: Chemical 407 (2015) 221–229
2. Results and discussion
The starting point of our investigations was the consider-
ation that Lewis acids were known to catalyze etherification
reactions, but that Lewis acids also catalyze the Meyer–Schuster
rearrangement (vide supra). It appeared that the rearrangements of
inactivated substrates are mainly performed at elevated tempera-
tures [8,9] and they might potentially compete with substitution
reaction, resulting in unwanted side product formation. For these
(and for general) reasons, we decided to look for a catalyst sys-
tem that can perform the title reaction with terminal propargylic
alcohols at lower temperatures. Test reaction in our laboratory
showed that FeCl3 and FeCl2 catalyzed the etherification of 1,1-
diphenylprop-2-yn-1-ol (9) and benzyl alcohol at 40 ◦C (Table 1).
The conversion with FeCl2 as the catalyst (Table 1, entry 2) was
not complete after 18 h, but it was with FeCl3 (Table 1, entry 1).
However, we observed vapor formation when the alcohol starting
materials and the strong Lewis acid FeCl3 were combined. We spec-
ulated that the vapor was due to HCl and we set up a control reaction
employing HCl as the catalyst (Table 1, entries 3 and 4). Indeed, HCl
appeared to activate propargylic alcohols in some way. At differ-
ent temperatures and HCl loads, the rearrangement product along
with a small amount of the etherification product was observed
by GC, albeit at relatively low overall conversions. Brønsted acids
are known to catalyze the Meyer–Schuster rearrangement, which
might be a competing reaction when FeCl3 is utilized as catalyst
[40].
While simple catalyst systems can be very powerful, we were
looking for a less reactive and more selective catalyst that could
potentially be employed for more sophisticated target molecules
where functional group selectivity might play a role. We envisaged
[Fc]PF6 as a mild yet reactive Lewis acid, because it is a 17 valence
electron complex. Initial screening experiments employing [Fc]PF6
revealed that it does indeed catalyze etherification reactions of
propargylic alcohols (Table 1, entries 5 and 7). At room temper-
ature, the catalyzed reaction is very slow, but at 40 ◦C, the reaction
is reasonably fast (5 to 24 h) to become synthetically useful. In an
attempt to increase the reaction rate, we performed the reactions
at 65 ◦C (Table 1, entries 6 and 8). For the propargylic alcohol 9,
the product of the Meyer–Schuster rearrangement was observed
in a 80% NMR yield (entry 6, vide infra). The propargylic alcohol 8
gave, at 65 ◦C, mainly substitution product (entry 8) but we found
that 40 ◦C was sufficient for the very same reaction (entry 7). Iso-
propanol (iPrOH, a secondary alcohol) was also employed as the
alcohol nucleophile, and gave reasonable yields (Table 1, entry 9).
After these optimization efforts, we found that at a catalyst load
of 3 mol% [Fc]PF6, one equivalent of the propargylic alcohol and
one equivalent of an alcohol were converted to the corresponding
propargylic ethers (CH2Cl2 solvent, 40 ◦C, 5 h to 3 days, Table 2).
The exclusion of water and / or air was not necessary. As shown in
Table 2, the isolated yields ranged from 90 to 20%. During screen-
ing as well as isolation of the products, no other species other than
starting materials, products or occasional rearrangements were
observed (vide infra). Most significantly, no dehydration of the
propargylic alcohol 8 took place, providing the procedure chemos-
electivity. Dehydration is occasionally observed when the reaction
is performed at higher temperatures [9][9k]. We ascribe the lack
of dehydration under conditions in Table 2 to the relatively low
reaction temperature.
Scheme 1. Reaction landscape of propargylic alcohols.
The propargylic etherification reaction – i.e. the substitution of
the OH group by an oxygen-centered nucleophile – is a specific ver-
sion of a nucleophilic substitution reaction, where the nucleophile
is an alcohol. A number of Brønsted [28] and Lewis acid catalyst sys-
tems have been reported for propargylic etherification reactions.
Ruthenium-based systems have been widely employed [2,29,30],
but catalyst systems based on copper [31], rhenium [32], bismuth
[33] or gold [34] are known as well. With exceptions (as further out-
lined below), the reaction temperatures for substitution reactions
of propargylic alcohols are typically above 60 ◦C. In some cases,
additives are required for catalytic activity to unfold. There are cata-
lysts known, that perform the reaction at room temperature, mainly
based on Lewis acids in high oxidation states, such as FeCl3 [18] and
BiCl3 [33]. The gold compound Na[AuCl4] also catalyzes the etheri-
fication of internal propargylic alcohols at room temperature [34].
In general, the catalytic systems that work at room temperature
mainly employ internal propargylic alcohols [18,33,34] or utilize
propargylic esters as starting material [35].
Iron catalysis has recently gained considerable interest; iron has
a number of advantages compared to other transition metals typi-
cally employed in catalysis [36]. It is relatively non-toxic, abundant,
and, therefore, inexpensive. As a consequence, iron is increasingly
investigated as an alternative to more established transition met-
als. Accordingly, iron-based catalyst systems were also employed
in propargylic etherification reactions. Zhan reported that FeCl3
catalyzes the etherification of propargylic alcohols at room tem-
perature within 5 h, but mainly internal propargylic alcohols were
employed [18]. Zhan reported that FeCl3 also catalyzes the ether-
ification of internal propargylic esters, where the OH has been
replaced by an acetate group with increased leaving group abili-
ties [35]. Zanotti described the iron complex [Fe(Cp)(CO)2]OTf to be
catalytically active in the etherification of two different propargylic
alcohols to give six different ethers with yields ranging from 9 to
72% [37]. As mentioned above, etherification (or enol etherification)
[38] reactions of terminal propargylic alcohols frequently require
elevated temperatures. To the best of our knowledge, Zhan’s and
Zanotti’s pioneering reports are the only iron-catalyzed propargylic
etherification examples; however, the alcohol nucleophiles were
applied in excess and there is room for improvement with respect
to some of the isolated yields and the substrate scope [18,37].
As part of our continuing research in the area of the catalytic acti-
vation of propargylic alcohols [39], we are currently investigating
ruthenium based systems to serve that purpose. During the course
of our studies, we discovered that commercial ferrocenium hexaflu-
orophosphate ([FeCp2]+PF6, subsequently referred to as [Fc]PF6)
catalyzes the propargylic etherification of a number of substrates
at low temperatures. Some experiments giving further insight into
the course of the reaction are also presented.
As can be seen from Table 2, tertiary propargylic alcohols can be
employed for the reaction, however, at least one substituent on the
propargylic alcohol (R or R’) must be aromatic. Secondary propar-
gylic alcohols could not successfully be converted to ethers under
the reaction conditions and purely aliphatic secondary or tertiary
ones gave GC yields below 20%. Both benzylic and aliphatic pri-
mary alcohols can be used. Secondary alcohols can also be utilized

M.J. Queensen et al. / Journal of Molecular Catalysis A: Chemical 407 (2015) 221–229
223
Table 1
Screening reactions.
Entry Catalyst
Rꢀ
Product
Yield/%a
100
Entry Catalyst
Rꢀ
Product
Yield/%a
1
FeCl3, CH2Cl2, 40 ◦C, 18 h
CH2Ph
5
[Fc]PF6 CH2Cl2 40 ◦C, 5 h
CH2Ph
46c
2
FeCl2, CH2Cl2, 40 ◦C, 18 h
CH2Ph
45
6
[Fc]PF6 Cl2CH2 65 ◦C, 5 h
CH2Ph
80d
3
HCl (25 mol%) CH2Cl2, 40 ◦C, 18 h
CH2Ph
34b
7b
7
8
[Fc]PF6 CH2Cl2 40 ◦C, 5 h
CH2Ph
89
86
[Fc]PF6 CH2Cl2 65 ◦C, 24 h
CH2Ph
4
HCl (13 mol%) CH2Cl2, 80 ◦C, 2 days CH2Ph
30b
10b
9
[Fc]PF6 CH2Cl2 40 ◦C, 3 days iPr
44e
a
b
c
Determined by GC.
Incomplete conversion to a number of products was observed, some of which remained unidentified.
Isolated yield.
80% Meyer–Schsler rearrangement was detected by NMR.
NMR yield.
d
e
in the reaction as demonstrated with l-borneol (entry 4, where an
almost 1:1 mixture of diastereomers was isolated) and isopropanol
(iPrOH, entry 8). The secondary alcohols gave, in general, lower
yields, which we tentatively ascribed to their lower nucleophilicity
compared to primary alcohols.
and methanol (Table 2, entry 9) gave an isolated yield of only 21%,
and only 80% conversion to the ether product was observed by
GC. In general, when small alcohols, such as methanol, ethanol or
iPrOH (Table 1, entry 9) were utilized, chromatographic separation
of starting materials and products was not as easy and efficient
compared to the ether products of larger alcohol nucleophiles. We
tentatively ascribed that fact to the similar physical properties of
propargylic starting material and ether product when small alco-
hols were used.
Pure FeCl2 and FeCl3 gave different product mixtures compared
to [Fc]PF6. For example, with FeCl3, complete Meyer–Schuster rear-
rangement was observed when employed as catalyst under the
conditions in Table 2, entry 11. Thus, potential iron chloride impu-
rities cannot be responsible for the catalytic activity of [Fc]PF6.
To further investigate the reaction, we decided to follow product
formation and starting material consumption for selected reac-
tions over time by GC. For the propargylic alcohols 8 (entry 5,
Table 2 and Scheme 2, top; Fig. 1) and 9 (entry 13 and Scheme 2,
bottom; Fig. 2), the reaction with benzyl alcohol was monitored.
The reaction profile for 8 revealed a fast reaction at the early
stage, and after 5 h, the concentration of the products virtually
plateaued (Fig. 1). After one day, a slight decrease of the prod-
From the isolated yields in Table 2, some trends are obvious.
It appears that the propargylic substrate 2-phenylbut-3-yn-2-ol
(8) required, overall, a somewhat longer reaction time but gave
in general higher yields compared to the aromatic propargylic sub-
strate 1,1-diphenylprop-2-yn-1-ol (9). This aspect will be further
discussed below. When methanol was employed as the alcohol
substrate, the yields tended to be lower than for other alcohol
substrates. With 2-phenylbut-3-yn-2-ol (8) as the propargyl alco-
hol, the yield of the resulting methoxy ether was so low that we
decided to not list it in Table 2. We ascribe the low yield to the
low stability of the product. A GC analysis of the crude reaction
mixture of 8 and methanol showed almost complete consumption
of the propargylic starting material and an intense peak for the
ether product. However, all attempts to isolate the product chro-
matographically resulted in low to no yields, and we concluded
that the product decomposes during workup on the column. An
etherification reaction between aromatic propargylic substrate 9

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M.J. Queensen et al. / Journal of Molecular Catalysis A: Chemical 407 (2015) 221–229
Table 2
Isolated yields.
Entrya
1
Product
Isolated yield/%
67
Entrya
9c
Product
Isolated yield/%
21
2
3
74
69
10c
36
40
11c
4b
44
84
12c
35
46
5d
13c
6
7
86
90
20
8
a
General conditions: Propargylic alcohol (0.7 mol) and alcohol Rꢀ OH (0.7 mmol) in CH2Cl2 (1 mL) catalyzed by [Fc]PF6 (0.02 mol) at 40 ◦C for 3 days. The products were
isolated chromatographically.
b
A 50:50 mixture of diastereomers was isolated.
5 h reaction time.
18 h reaction time.
c
d
uct concentration can be observed, which we ascribed to product
decomposition through hydrolysis (not shown in Fig. 1). Water
from the atmosphere might have entered the reaction mixture, as
the experiment was not performed under the exclusion of air. No
Meyer–Schuster rearrangement to the aldehyde 11 was observed
for this substrate in the presence of the benzyl alcohol nucleophile,
which we also verified by NMR of the crude mixtures (Scheme 2,
top).
propargylic ether product 12 formed over time, but also the
Meyer–Schuster rearrangement product 13. After 8 h, the ether
and rearrangement products were observed in about equimolar
amount by GC. Thus, the isolated yields for the etherification of 1,1-
diphenylprop-2-yn-1-ol 9 (right side in Table 2) are lower because
it rearranges during the reaction in a substantial amount to the
aldehyde 13. The aldehyde rearrangement side product 13 was not
only observed by GC but also readily identified in the 1H NMR of the
crude reaction mixture, where it gave a distinct doublet at 9.4 ppm,
coupling with the alkene proton.
The situation is fundamentally different for 1,1-diphenylprop-
2-yn-1-ol 9 (Fig. 2 and Scheme 2, bottom). As can be seen, the

M.J. Queensen et al. / Journal of Molecular Catalysis A: Chemical 407 (2015) 221–229
225
Scheme 2. Etherification versus rearrangement for different propargylic alcohols as a function of time.
100
90
80
70
60
50
40
30
20
10
0
uct 12 seemed to form under kinetic control within the first eight
hours and rearrangement occurs concurrently. When the reaction
time is extended to 2 days, the ether product 12 converted and
complete Meyer–Schuster rearrangement was observed, which
is presumably a thermodynamically driven reaction. The 3,3-
diphenylacrylaldehyde product 13 is probably more stable than
both the propargylic alcohol starting material 9 and the ether prod-
uct 12.
Thus, the ether product 12 converts to the aldehyde 13, which
we did not observe for the 2-phenylbut-3-yn-2-ol ether product 10
(Fig. 1 and Scheme 2, top). Under the reaction conditions in Table 2,
the propargylic alcohol starting material 8 either does not or only
marginally rearrange to the aldehyde. We ascribe this difference
in behavior of the propargylic alcohols 8 and 9 to the aldehyde
product stabilities. Due to double conjugation, the aldehyde 13 is
more stable than the aldehyde 11, which might be a driving force
for its formation.
8
%
10
0
50
100
150
200
250
300
ꢀme / min
Fig. 1. Etherification of propargylic alcohol 8 (solid line) with benzyl alcohol fol-
lowed over time by GC, average of two runs.
When the propargylic alcohols 8 and 9 were subjected to cat-
alytic conditions in Table 2 and Scheme 2 but without any alcohol
nucleophile, some Meyer–Schuster rearrangement was observed
by GC. The reactions were somewhat inconsistent when performed
repeatedly and aldehyde products could not be isolated for both
propargylic substrates. The [Fc]PF6 is, thus, not a good catalyst for
the Meyer–Schuster rearrangement at 40 ◦C; the rearrangement
observed in Scheme 2 and Fig. 2 only efficiently takes place in
the presence of alcohol substrate. At 65 ◦C, [Fc]PF6 catalyzes the
Meyer–Schuster rearrangement of the propargylic alcohol 9 to the
corresponding aldehyde somewhat more efficiently (Table 1, entry
6).
We conclude that 1,1-diphenylprop-2-yn-1-ol 9 undergoes
rearrangement more efficiently under the conditions in Table 2,
which is responsible for the lower isolated yields but also for the
faster reaction rate, because etherification and rearrangement take
place concurrently. On the other hand, the 2-phenylbut-3-yn-2-
ol 8 appears to not rearrange in presence of an alcohol substrate
(Scheme 2, top). The lack of competing and yield-diminishing rear-
rangement is responsible for the higher propargylic ether yields
observed for the propargylic alcohol 8.
Further studies are needed in order to suggest a mechanism for
the title reaction under the reaction conditions given in Table 2.
However, some mechanistic aspects of the catalytic system pre-
sented herein can be elaborated. The allenylidene route (Scheme 1)
requires that the propargylic alcohol substrate is activated through
an allenylidene intermediate, which then is attacked by the alcohol
nucleophile [41]. Iron allenylidene complexes are known [41b,42];
Fig. 2. Etherification of propargylic alcohol 9 (dashed line) with benzyl alcohol
followed over time by GC, average of two runs.
When the reaction was continued for a total of three days
(Fig. 2, Scheme 2), rearrangement continued; furthermore, the
ether product 12 started decomposing after eight hours, presum-
ably to the propargylic alcohol starting material 9. After two days,
only the rearrangement product 13 was observed by GC and NMR,
which gave a 84% NMR yield (Scheme 2, bottom). After 8 h, the
propargylic ether product 12 was isolated in 46% yield (Table 2,
entry 13 and Scheme 2). Thus, it appeared that the reaction has
a window for the formation of 12. The propargylic ether prod-

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M.J. Queensen et al. / Journal of Molecular Catalysis A: Chemical 407 (2015) 221–229
however, we do not have experimental evidence that an iron
allenylidene intermediate is generated, as UV–vis and MS exper-
iments have been, thus far, inconclusive. While the [Fc]PF6 catalyst
is a Lewis acid, it is not a very strong one and it is question-
able if it is capable of ionizing a propargylic alcohol substrate.
On the other hand, when the reaction of entry 12 in Table 2 was
performed in the presence of either 2,2,6,6-(tetramethylpiperidin-
1-yl) oxy (TEMPO) or 2,4,6-tri-tert-butylphenol (TTBP), which are
both known to be radical scavengers, the reaction was completely
inhibited. It is possible that the radical scavengers just deactivated
the catalyst.
However, [Fc]PF6 has another role as a single electron oxidant
[43]. It is, at this point, too early to suggest a mechanism for the reac-
tion, but it appears to be possible that the title reaction proceeds
through a single electron transfer mechanism under the reaction
conditions described herein [43b,44,45]. We think the employment
of a single electron oxidant has the potential to inspire a new direc-
tion in the development of catalysts for the title reaction based on
single electron transfer processes [45].
Compared to other iron based catalyst systems known in the
literature, [Fc]PF6 performs well and complements existing sys-
tems. As already mentioned, the strict exclusion of air or water is
not necessary. The reaction is restricted to tertiary alcohols and
complements, thus, the catalyst systems that primarily work for
secondary propargylic alcohols [1,2,30,32]. The reaction tempera-
ture is 40 ◦C, which is lower than for many other catalyst systems
reported in the literature [1]. Room temperature systems have been
reported [37], but they mainly employ internal propargylic alcohols
[33,34,35]. We think the low reaction temperature allows for the
employment of propargylic alcohols with hydrogens in a ␤ posi-
tion, which tend to eliminate at higher temperatures [9][9k]. Many
reports of substitution reactions utilize propargylic alcohols with-
out ␤ -hydrogen atoms [1,32]. Most significantly, as opposed to
other systems reported in the literature, the alcohol substrate has
not to be in excess over the propargylic alcohol substrate, which
makes the employment of [Fc]PF6 attractive in cases where an
expensive alcohol substrate is one of the reactants. As demon-
strated in Table 1 (entries 3 and 4), HCl potentially interferes with
the title reaction, and we encourage researchers in the area employ-
ing very strong Lewis acids such as FeCl3 to carefully look for side
product formation.
NMR spectra were obtained at room temperature on a Bruker
Avance 300 MHz or a Varian Unity Plus 300 MHz instrument
and referenced to a residual solvent signal; all assignments are
tentative. EI and exact masses were recorded on a JEOL MStation
[JMS-700] Mass Spectrometer and HP 5988A GC–MS instrument.
Gas chromatograms were recorded on a HP 5890 instrument. IR
spectra were recorded on Thermo Nicolett 670 FTIR. Elemental
analyses were performed by Atlantic Microlab Inc., Norcross, GA,
USA.
Characterization data for the known propargylic ethers in
Table 2 are listed in the Supporting information.
General procedure for the etherification of 2-phenyl-3-butyn-
2-ol: (2-Ethoxybut-3-yn-2-yl)benzene (Table 2, entry 1) [39a]:
2-phenyl-3-butyn-2-ol (0.106 g, 0.728 mmol) was added to a 5-mL
screw cap vial and dissolved in CH2Cl2 (1 mL). Ethanol (0.036 g,
0.897 mmol) was added followed by the addition of [Fc]PF6
(0.007 g, 0.022 mmol). The vial was then sealed and heated at
40 ◦C for 3 days. The solvent was removed and the residue was
chromatographed on a neutral alumina oxide (Aluminar®) column
(2.5 × 30 cm, hexane and then CH2Cl2) to give the product as a yel-
low solid (0.085 g, 0.487 mmol, 67%). NMR (ı, CDCl3) 1H: 7.67–7.29
2
(m, 7H, aromatic), 3.69 (doublet of quintets, 1 H, JHH = 7.2 Hz,
2JHH = 4.2 Hz, OCHH’), 3.22 (doublet of quintets, 1H, JHH = 7.2 Hz,
2
2JHH = 4.2 Hz, OCHH’), 2.74 (s, 1H, C CH), 1.78 (s, 3H, CH3), 1.24 (t,
1
3H, 2JHH = 7.2 Hz, OCH2CH3) ppm. 13C{ H}: 143.3 (s, aromatic ipso),
128.6 (s, aromatic), 128.5 (s, aromatic), 127.9 (s, aromatic), 127.6
(s, aromatic*), 126.1 (s, aromatic), 84.7 (s, C CH), 75.9 (s, CC CH),
75.2 (s, C CH), 60.6 (s, OCH2), 33.2 (s, CH3), 15.6 (s, OCH2CH3)
ppm. IR (ATR, Neat): 3284 (m), 3055 (w), 2973 (m), 2926(w), 1714
(s), 1670 (s), 1575 (s), 1491 (s), 1335 (s), 1264 (s), 1057 (s), 997 (s) ␯.
(2-Phenethoxybut-3-yn-2-yl)benzene (Table 2, entry 3): 2-
phenyl-3-butyn-2-ol (0.111 g, 0.757 mmol), 2-phenylethanol
(0.093 g, 0.759 mmol), and [Fc]PF6 (0.007 g, 0.021 mmol) were
combined and worked up as described above for (2-ethoxybut-3-
yn-2-yl)benzene to give (2-phenethoxybut-3-yn-2-yl)benzene as
a yellow oil (0.118 g, 0.443 mmol, 69 %). Anal. calcd for C18H18O:
C, 86.36; H, 7.25. Found: C, 86.25; H, 7.14. NMR (ı, CDCl3) 1H:
7.41–7.38 (m, 2H, aromatic), 7.21–7.04 (m, 8H, aromatic), 3.72
2
2
2
(doublet of quintets, 1H, JHH = 1.7 Hz, JHH = 7.3 Hz, JHH = 8.5
2
2
Hz, OCHH’), 3.24 (doublet of quintets, 1H, JHH = 1.7 Hz, JHH = 7.3
3. Conclusion
2
Hz, JHH = 8.5 Hz, OCHH’), 2.79 (m, 2H, OCH2CH2Ph), 2.55 (s,
1
We presented herein a new catalyst system based on [Fc]PF6
(ferrocenium hexafluorophosphate) for the conversion of propar-
gylic alcohols to the corresponding propargylic ethers (90 to 20%
isolated yields). The reaction can be conducted at 40 ◦C, which is
lower than for many other known catalyst systems for the reac-
tion. The reagents can be employed in equimolar amounts. No
additives are required and no dehydration was observed. For a
purely aromatic propargylic alcohol, the etherification reaction
competes with an rearrangement of the alcohol to the aldehyde,
which appeared to also be catalyzed by [Fc]PF6 and which lowered
the yield for that specific substrate. Further mechanistic investiga-
tions are ongoing.
1H, C CH), 1.61 (s, 3H, CH3) ppm. 13C{ H}: 142.8 (s, aromatic),
139.2 (s, aromatic), 129.4 (s, aromatic), 129.1 (s, aromatic), 128.6
(s, aromatic), 128.3 (s, aromatic), 128.1 (s, aromatic), 126.1 (s,
aromatic), 125.9 (s, aromatic), 84.3 (s, C CH), 75.9 (CR4), 75.6 (s,
C
CH), 66.1 (s, OCH2), 36.7 (s, CH2CH2Ph), 33.1 (s, CH3) ppm. IR
(ATR, neat): ␯ 3063 (w), 2864 (w), 1708 (s), 1575 (s), 1494 (s),
1356 (s), 1218 (s), 1045 (s) cm−1. ESI-MS: m/z 235 ([M−CH3], 10%),
129 ([M−OCH2CH2Ph], 100%). EI-MS: HRMS m/z calculated for
C17H15O (corresponds to [M−CH3]): 235.1123. Found: 235.1124.
(2-Borneoylbut-3-yn-2-yl)benzene (Table 2, entry 4): 2-
phenyl-3-butyn-2-ol (0.105 g, 0.716 mmol), l-borneol (0.112 g,
0.726 mmol), and [Fc]PF6 (0.008 g, 0.024 mmol) were combined
and worked up as described above for (2-ethoxybut-3-yn-2-
yl)benzene to give (2-borneoylbut-3-yn-2-yl)benzene as a yellow
solid as a 1:1 mixture of diastereomers (0.089 g, 0.355 mmol, 44
%). NMR (ı, CDCl3) 1H: 7.74–7.71 (m, 3H, aromatic + aromatic’),
Experimental
General. All reactions were carried out in open air.
CH2Cl2,
2-phenyl-3-butyn-2-ol,
1,1-diphenyl-2-propyn-
2
3-ol, MeOH, n-butanol, benzyl alcohol, 2-phenylethanol,
(2-bromophenyl)methanol, l-borneol, (E)-dec-5-en-1-ol, 2,2,6,6-
tetramethylpiperidin-1-yl)oxy (TEMPO), 2,4,6-tri-tert-butylphenol
(TTBP), [Fc]PF6 (all Sigma–Aldrich), EtOH (Decon), and iPrOH
(Fischer) were used as received.
7.43–7.30 (m, 2H, aromatic + aromatic’), 4.10 (d, 1H, JHH = 9.2
2
Hz, OCH’), 3.97 (d, 1H, JHH = 9.1 Hz, OCH), 2.68 (s, 1H, C CH’),
2.67 (s, 1H, C CH), 2.35–2.24 (m, 1H,OCHCH2’), 2.22–2.13 (m,
1H, OCHCH2), 1.81 (s, 3H, C CCH3’), 1.79 (s, 3H, C CCH3), 1.68 (t,
2
2
2H, JHH = 4.6 Hz, OCHCH2’), 1.55 (t, 2H, JHH = 4.5 Hz, OCHCH2),

M.J. Queensen et al. / Journal of Molecular Catalysis A: Chemical 407 (2015) 221–229
227
1
3H, 2JHH = 6.3 Hz, CHCH3’) ppm. 13C{ H}: 144.1 (s), 128.3 (s), 128.0
1.41–1.22 (m, 4H, CH2CH2 + CH2CH2’), 0.96 (s, 3H, CCH3’), 0.88
(s, 3H, CCH3), 0.86 (s, 3H, CCH3’), 0.85 (s, 3H, CCH3), 0.78 (s, 3H,
(s), 126.6 (s, all aromatic), 85.6 (s, C CH), 75.4 (s, CC CH), 75.3 (s,
1
CH3 + CH3’) ppm. 13C{ H}: 144.5 (s, aromatic), 128.2 (s, aromatic),
C
CH), 68.7 (s, OCH), 33.4 (s, CH3), 24.4 (s, CH3), 24.2 (s, CH3) ppm.
127.8 (s, aromatic), 126.5 (s, aromatic), 126.4 (s, aromatic), 86.6 (s,
IR (ATR, neat): ␯ 3291 (m), 2969 (m), 2930 (m), 1597 (m), 1489 (s),
1444 (s), 1364 (s), 1220 (s), 1085 (s), 992 (s), 905 (s), 760 (s) cm−1
.
C
C
CH), 86.4 (s, C CH’), 80.3 (s, CC CH), 80.0 (s, CC CH’), 75.2 (s,
CH), 75.2 (s, C CH’), 75.0 (br s, OCH + OCH’), 49.8 (s, OCHC), 49.7
EI-MS: m/z 173 ([M−CH3], 50%), 129 ([M−iPrO], 100%).
(s, OCHC’), 47.4 (s, CC CH), 47.2 (s, CC CH’), 45.7 (s, OCHCH2CH),
45.5 (s, OCHCH2CH’), 39.5 (s, OCHCH2), 38.0 (s, OCHCH2’), 32.5 (s,
General Procedure for the etherification of 1,1-diphenylprop-2-yn-
1-ol: (1-methoxy-2-propyn-1-ylidene)bis-Benzene (Table 2, entry 9)
[17] 1,1-diphenyl-2-propyn-3-ol (0.102 g, 0.491 mmol) was added
to a 5 mL screw cap vial and dissolved in CH2Cl2 (1 mL). Methanol
(0.016 g, 0.497 mmol) was then added followed by [Fc]PF6 (0.005 g,
0.014 mmol). The vial was then sealed and heated to 40 ◦C for 5 h.
The solvent was removed and the residue was chromatographed
on a silica gel column (2.5 × 30 cm, 2:1 v/v hexanes/CH2Cl2) to
give the product 8 as a yellow oil (0.022 g, 0.099 mmol, 21%). NMR
(ı, CDCl3) 1H: 7.63–7.57 (m, 4H, aromatic), 7.41–7.30 (m, 6H,
C
CCH3), 31.9 (s, C CCH3’), 28.5 (s, CH2CH2), 28.5 (s, CH2CH2’),
27.3 (s, CH2CH2), 27.2 (s, CH2CH2’), 20.0 (s, CH3’), 20.0 (s, CH3), 19.2
(br s, CH3 + CH3’), 13.9 (s, CH3’), 13.7 (s, CH3) ppm. IR (ATR, neat): ꢀ
3302 (m), 2947 (s), 1704 (m), 1489 (s), 1446 (s), 1364 (s), 1223 (s),
1085 (s), 992 (s) cm−1. EI-MS: m/z 282 ([M], 10%), 258 ([M
C C],
10%), 243 ([M
C C CH3], 10%), 129 ([M–borneol], 100 %).
1-Bromo-2-(((2-phenylbut-3-yn-2-yl)oxy)methyl)benzene
(Table 2, entry 6): 2-phenyl-3-butyn-2-ol (0.850 g, 5.778 mmol),
(2-bromophenyl)methanol (0.721 g, 5.80 mmol), and [Fc]PF6
(0.571 g, 0.174 mmol) were added to a evacuated and nitrogen
flushed 2 neck round bottom flask and suspended in 20 mL of
CH2Cl2. The reaction was stirred at 40 ◦C for 2 days and then
purified by column chromatography (2 × 30 cm alumina column)
using hexane as eluent to give 1-bromo-2-(((2-phenylbut-3-yn-2-
yl)oxy)methyl)benzene as a yellow oil (1.269 g, 4.968 mmol, 86%).
NMR (ı, CDCl3) 1H :7.81–7.17 (m, 14H, aromatic), 5.32 (s, CH2Cl2),
2
2
aromatic), 3.41 (d, 3H, JHH = 4.3 Hz, OCH3), 2.94 (d, 1H, JHH = 4.3
Hz, C CH) ppm. 13C{ H}: 143.2 (s, aromatic), 128.4 (s, aromatic),
1
127.9 (s, aromatic), 126.9 (s, aromatic), 83.4 (s, CC CH), 81.0 (s,
C
CH), 77.8 (s, CC≡CCH), 52.7 (s, OCH3) ppm. IR (ATR, neat): ␯ 3302
(w), 1723 (m), 1672 (m), 1575 (s), 1470 (s), 1336 (s), 1266 (s), 1068
(s), 901 (s), 724 (s) cm−1. EI-MS: m/z 222 ([M], 20%), 207 ([M–CH3],
60%), 145 ([M–Ph], 100%). HRMS calculated for C16H14O: 222.1047.
Found: 222.1045.
2
2
4.83 (t, 1H, JHH=12 Hz, OCHH’), 4.39 (d, 1H, JHH = 12 Hz, OCHH’),
2
2
2.87 (d, 1H, JHH = 2.5 Hz, C CH), 1.95 (d, 3H, JHH = 2.5 Hz, CH3)
(1-Butoxyprop-2-yne-1,1-diyl)dibenzene (Table 2, entry 11): 1,1-
Diphenyl-2-propyn-3-ol (0.104 g, 0.501 mmol), n-butanol (0.037 g,
0.502 mmol), and [Fc]PF6 (0.005 g, 0.014 mmol) were combined
and worked up as described above for (1-methoxy-prop-2-yne-1,1-
diyl)dibenzene to give (1-butoxyprop-2-yne-1,1-diyl)dibenzene
as a yellow oil (0.050 g, 0.189 mmol, 40%). Anal. Calc. for C19H20O:
C, 86.32; H, 7.63 Found: C, 86.04; H, 7.38. NMR (ı, CDCl3) 1H: 7.50-
7.47 (m, 4H, aromatic), 7.25-7.15 (m, 6H, aromatic), 3.40 (t, 2H,
2JHH = 6.4 Hz, OCH2), 2.77 (s, 1H, C CH), 1.57 (quintet, 2H, 2JHH = 6.5
Hz, OCH2CH2), 1.38 (sextet, 2H, 2JHH = 7.4 Hz, CH2CH3), 0.83 (t, 3H,
ppm. 13C{ H}: 142.7 (s, aromatic), 138.4 (s, aromatic), 132.6 (s,
1
aromatic), 129.6 (s, aromatic), 129.1 (s, aromatic), 128.9 (s, aro-
matic), 128.7 (s, aromatic), 128.6 (s, aromatic), 128.3 (s, aromatic),
127.9 (s, aromatic), 127.6 (s, aromatic), 126.2 (s, aromatic), 125.2
(s, aromatic), 122.9 (s, aromatic), 84.1 (s, C CH), 76.6 (s, CC CH),
76.3 (s, C CH), 66.8 (s, OCH2), 33.1 (s, CH3) ppm. IR (ATR, neat): ␯
3290 (m), 3057 (m), 2985 (m), 2929 (w), 1729 (m), 1668 (m), 1568
(s), 1469 (s), 1441 (s), 1377 (m), 1268 (m), 1223 (m), 1089 (s), 1021
(s), 908 (m) cm−1. FAB-MS: HRMS m/z calculated for C17H15BrONa:
337.0204 and 339.0184. Found: 337.0216 and 339.0196.
2JHH = 7.3 Hz, CH2CH3) ppm. 13C{ H}: 143.6 (s, aromatic), 128.2 (s,
1
aromatic), 127.7 (s, aromatic), 126.7 (s, aromatic), 83.7 (s, CC CH),
79.8 (s, C CH), 76.7 (s, C CH), 64.4 (s, OCH2), 32.1 (s, CH2), 19.6
(s, CH2), 14.1 (s, CH3) ppm. IR (ATR, neat): ␯ 3282 (m), 3104 (m),
2957 (m), 1767 (m), 1697 (s), 1650 (s), 1548 (s), 1487 (s), 1445 (s),
1358 (s), 1024 (s), 741 (s) cm−1. EI-MS: m/z 264 ([M], 30%), 191
([M–O–n-Bu], 100%). HRMS m/z calculated for C19H20O: 264.1514.
Found: 264.1519.
(E)-(2-(Dec-5-en-1-yloxy)but-3-yn-2-yl)benzene
(Table
2,
entry 7): 2-phenyl-3-butyn-2-ol (0.103 g, 0.705 mmol), (E)-
dec-5-en-1-ol (0.111 g, 0.707 mmol), and [Fc]PF6 (0.003 g,
0.011 mmol) were combined and worked up as described above
for (2-ethoxybut-3-yn-2-yl)benzene to give (E)-(2-(dec-5-en-1-
yloxy)but-3-yn-2-yl)benzene as a yellow oil (0.180 g, 0.634 mmol,
90%). NMR (ı, CDCl3) 1H : 7.72–7.70 (d, 2H, 3JHH = 8.3 Hz, aromatic),
7.47–7.37 (m, 3H, aromatic), 5.47 (br s, 2H, HC = CH), 3.68 (q, 1H,
(1-Phenethoxyprop-2-yne-1,1-diyl)dibenzene (Table 2, entry
12): 1,1-Diphenyl-2-propyn-3-ol (0.099 g, 0.473 mmol), 2-
phenylethanol (0.059 g, 0.500 mmol), and [Fc]PF6 (0.005 g,
0.015 mmol) were combined and worked up as described
above for 1-(methoxy)prop-2-yne-1,1-diyl)dibenzene to give
2
2JHH = 6.5 Hz, OCHH’), 3.21 (q, 1H, JHH = 6.5 Hz, OCHH’), 2.78 (s,
1H, C CH), 2.07 (br. s, 4H, 2CH2), 1.83 (s, CH3), 1.66 (quint, 2H,
2JHH = 6.9 Hz, CH2), 1.45–1.39 (m, 6H, 3CH2), 0.99 (t, 3H, 2JHH = 6.9
1
Hz, CH2CH3) ppm. 13C{ H}: 143.1 (s, aromatic), 130.1 (s, aromatic),
128.4 (s, aromatic), 127.8 (s, aromatic), 126.0 (s, aromatic), 84.4 (s,
(1-phenethoxyprop-2-yne-1,1-diyl)dibenzene as
a yellow oil
C
CH), 75.8 (s, CC CH), 75.4 (s, C CH), 64.9 (s, C C), 33.1 (s, CH2),
(0.055 g, 0.168 mmol, 35%). Anal. Calc. for C23H20O: C, 88.43; H,
6.45 found: C, 88.16; H, 6.39. NMR (ı, CDCl3) 1H: 7.41–7.38 (m,
3H, aromatic), 7.17–7.13 (m, 12H, aromatic), 3.63 (t, 2H, 2JHH = 7.0
Hz, OCH2), 2.92 (t, 2H, 2JHH = 7.0 Hz, OCH2CH2), 2.75 (s, 1H, C CH)
32.5 (s, CH2), 32.5 (s, CH2), 32.0 (s, CH2), 29.5 (s, CH2), 26.3 (s, CH2),
22.4 (s, CH2), 14.2 (s, CH2CH3) ppm. IR (ATR, neat): ␯ 3303 (m),
2926 (m), 2856 (w), 1723 (m), 1672 (w), 1575 (m), 1520 (m), 1464
(s), 1264 (m), 1093 (s), 905 (s) cm−1. EI-MS: HRMS m/z calculated
for C20H28ONa: 307.2038. Found: 307.2043.
1
ppm. 13C{ H}: 143.5 (s, aromatic), 136.4 (s, aromatic), 129.3 (s,
aromatic), 128.5 (s, aromatic), 128.4 (s, aromatic), 128.4 (s, aro-
matic), 127.8 (s, aromatic), 126.7 (s, aromatic), 126.4 (s, aromatic),
83.5 (s, CC CH), 80.2 (s, C CH), 77.7 (s, C CH), 65.9 (s, OCH2), 36.7
(s, OCH2CH2) ppm. IR (ATR, neat): ␯ 3283 (m), 3058 (m), 2933 (m),
2869 (m), 1728 (s), 1595 (s), 1487 (s), 1447 (s), 1211 (s), 1063 (s),
1023 (s), 741 cm−1. EI-MS: m/z 311 ([M + H], 4%), 281 ([M–CH2 = O],
15%), 191 ([M–OCH2CH2Ph], 100%).
(Isopropoxybut-3-yn-2-yl)benzene (Table
2
entry 8): 2-
phenyl-3-butyn-2-ol (0.705 g, 4.827 mmol), isopropanol (0.491 g,
8.16 mmol), and [Fc]PF6 (0.479 g, 0.145 mmol) were combined and
worked up as described above for (2-ethoxybut-3-yn-2-yl)benzene
to give the product as an orange oil (0.184 g, 0.968 mmol, 20%).
NMR (ı, CDCl3) 1H : 7.81–7.79 (m, 1H, aromatic), 7.45–7.40 (m,
2
4H, aromatic), 3.95 (septet, 1H, JHH = 6.3 Hz, OCH), 2.78 (s, 1H,
Meyer-Schuster rearrangement of 1,1-diphenyl-2-propyn-3-ol
(Scheme 2, bottom): 1,1-diphenylprop-2-yn-1-ol (0.145 g, 0.697
C
CH), 1.84 (s, 3H, CH3), 1.38 (d, 3H, 2JHH = 6.3 Hz, CHCH3), 1.06 (d,

228
M.J. Queensen et al. / Journal of Molecular Catalysis A: Chemical 407 (2015) 221–229
mmol), benzyl alcohol (0.073 g, 0.678 mmol), and [Fc]PF6 (0.007 g,
0.021 mmol) were combined in CH2Cl2 (3 mL) and heated at 40 ◦C
for 3 days. The solvent was removed and the residue filtered
through a short pad of silica gel. From the recovered mass (0.192 g),
an NMR yield of 84% for the aldehyde 13 was calculated. The NMR
showed besides the rearranged product only benzyl alcohol. NMR
(ı, CDCl3) 1H: 9.43 (d, 0.65H, 2JHH = 8.0 Hz, HC = O of 13), 7.35-7.20
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Acknowledgment
We would like to thank the National Science Foundation (NSF
CHE-1300818) for financial support. Further funding from the
National Science Foundation for the purchase of the NMR spectrom-
eter (CHE-9974801) and the purchase of the mass spectrometer
(CHE-9708640) is acknowledged.
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Appendix A. Supplementary data
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