A novel application of terminal alkynes as the homogeneous catalysts for acetalization and esterification

Article abstract of DOI:10.1016/j.tet.2019.04.013

The theoretical study focused on the possible use of low-molecular-weight mono-as well as multifunctional terminal alkynes as catalysts for two reactions, which are known to be typically acid catalyzed - acetalization and esterification, is presented in this study. Multifunctional terminal alkynes [(diethynylbenzenes, triethynylbenzene, and tetrakis(4-ethynylphenyl)methane]were significantly more active than the monofunctional ones (cyclopropylacetylene, phenylacetylene, 3-cyclohexylprop-1-yne, 1-ethynyl-2-fluorobenzene, 1-ethynyl-4-fluorobenzene, 4-ethynyltoluene, 4-tert-butylphenylacetylene, and 2-ethynyl-α,α,α-trifluorotoluene), this fact can be partly explained by the higher amount of ethynyl groups per alkyne molecule. We confirmed that terminal ethynyl groups in low-molecular-weight alkynes can successfully act as acid catalytic centers for acetalization as well as for esterification.

Full text of DOI:10.1016/j.tet.2019.04.013

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Accepted Manuscript
A novel application of terminal alkynes as the homogeneous catalysts for
acetalization and esterification
Lada Sekerová, Eliška Vyskočilová, Libor Červený, Jan Sedláček
PII:
S0040-4020(19)30403-X
https://doi.org/10.1016/j.tet.2019.04.013
TET 30258
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 14 January 2019
Revised Date: 2 April 2019
Accepted Date: 3 April 2019
Please cite this article as: Sekerová L, Vyskočilová Eliš, Červený L, Sedláček J, A novel application of
terminal alkynes as the homogeneous catalysts for acetalization and esterification, Tetrahedron (2019),
doi: https://doi.org/10.1016/j.tet.2019.04.013.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
our customers we are providing this early version of the manuscript. The manuscript will undergo
copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please
note that during the production process errors may be discovered which could affect the content, and all
legal disclaimers that apply to the journal pertain.

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
ACCEPTED MANUSCRIPT

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
ACCEPTED MANUSCRIPT
A novel application of terminal alkynes as the homogeneous catalysts for acetalization and
esterification
Lada Sekerová1*, Eliška Vyskočilová1, Libor Červený1, Jan Sedláček2
1Department of Organic Technology, University of Chemistry and Technology Prague, 166 28,
Prague 6, Czech Republic
1Department of Physical and Macromolecular Chemistry, Faculty of Science, Charles University, 128
40, Prague 2, Czech Republic
*corresponding author: Lada.sekerova@vscht.cz
Abstract
The theoretical study focused on the possible use of low-molecular-weight mono- as well as
multifunctional terminal alkynes as catalysts for two reactions, which are known to be typically acid
catalyzed - acetalization and esterification, is presented in this study. Multifunctional terminal
alkynes [(diethynylbenzenes, triethynylbenzene, and tetrakis(4-ethynylphenyl)methane] were
significantly more active than the monofunctional ones (cyclopropylacetylene, phenylacetylene, 3-
cyclohexylprop-1-yne, 1-ethynyl-2-fluorobenzene, 1-ethynyl-4-fluorobenzene, 4-ethynyltoluene, 4-
tert-butylphenylacetylene, and 2-ethynyl-α,α,α-trifluorotoluene), this fact can be partly explained by
the higher amount of ethynyl groups per alkyne molecule. We confirmed that terminal ethynyl
groups in low-molecular-weight alkynes can successfully act as acid catalytic centers for acetalization
as well as for esterification.
Keywords: acid catalysis; terminal alkynes; homogeneous catalysis; acetalization; esterification
1
Introduction
Terminal alkynes may find use in many syntheses. The catalyzed addition of water to the triple bond
yields methyl ketones or aldehydes (using the specific type of catalyst to direct the reaction by anti-
Markovnikov rule) [1,2]. Cyclotrimerization and cocyclotrimerization of alkynes provide trisubstituted
benzenes or more complex cyclic products. [3,4] Acetylides (prepared by deprotonation of terminal
alkynes using a variety of organometallic bases) are used as agents for the introduction of reactive
alkyne fragments into organic molecules in various multistep syntheses of fine chemical [5]. Terminal
alkynes also serve as unique building blocks for the synthesis of mostly conjugated polymers
(polyacetylenes, polyaryleneethynylenes, polyarylenebutadiynylenes, polyarylenes) of linear or cross-
linked architecture [6-9].
The references about the use of terminal alkynes as catalysts or co-catalysts are scarce. As an
example, we found the use of terminal alkynes as co-catalysts in a binary catalytic system
RuCl2(N-heterocyclic carbene)(p-cymene)/alkyne in ring-opening metathesis polymerization (ROMP)
of norbornene derivatives [10]. The addition of alkyne tuned the activity of Ru catalysts in organic as
well as aqueous media and improved the molecular weight control of polymers in aqueous media.
The catalytic activity was tuned by variations in alkyne substituent structure. Another binary catalytic
system AuCl3/phenylacetylene [11] was developed for the boost of Ferrier rearrangement of glycals
and 2-acetoxymethylglycals with different nucleophiles in good yields with anomeric selectivity at
room temperature. This system can be also used for O-glycosylation of 1-O-acetylsugars forming
various glycosides. The carbonylation of alcohols to alkyl formates was boosted by alkyne/phosphine
catalytic system [12]. Both components of this catalytic system were essential for providing the
activity in mentioned reactions. Using phenylacetylene and tri-n-butylphosphine the highest yields
and reaction rates were obtained (selectivity 100%, 93% conversion of alcohol). Nevertheless, in
1

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
ACCEPTED MANUSCRIPT
these three cases, terminal alkynes participated as the ligands of more complex catalyst molecules
and the acidity of the acetylenic hydrogen was not probably the main reason for the catalytic activity.
As already mentioned above, the terminal alkynes contain acidic (acetylenic) hydrogen, which can be
especially in the case of arylacetylenes very easily split off and replaced with e.g. a metal cation
under formation of an acetylide [13]. In our recent research, we revealed that the acidity of
acetylenic hydrogen was sufficient to catalyze efficiently acetalization and esterification reactions,
i.e. reactions which are known to be typically acid catalyzed. We particularly confirmed and reported
as a conference contribution [14] the possible use of phenylacetylene as a homogeneous catalyst in
acetalization of aliphatic aldehydes (C3 – C7) by methanol and also in esterification of syringaldehyde
or ethylvanillin by acetic anhydride. Moreover, we found microporous polymer networks decorated
with pendant ethynyl groups as efficient heterogeneous catalysts for acetalization of aliphatic
aldehydes with methanol, esterification of carboxylic acids with methanol and izomerization of
β-pinene oxide, as well [15,16].
The aim of this study is to compare catalytic properties of a series of low-molecular-weight terminal
alkynes possessing various number of ethynyl groups [cyclopropylacetylene, 3-cyclohexylprop-1-yne,
1,3-
and
1,4-diethynylbenzenes,
1-ethynyl-2-fluorobenzene,
1-ethynyl-4-fluorobenzene,
4-ethynyltoluene, phenylacetylene, 4-tert-butylphenylacetylene, 2-ethynyl-α,α,α-trifluorotoluene,
tetrakis(4-ethynylphenyl)methane and 1,3,5-triethynylbenzene] in model acetalization and
esterification reactions. The reaction rate and yields of products are the evaluative factors for making
conclusions.
2
Results and discussion
2.1 Acidity characterization
The comparison of the catalytic activities of 12 terminal alkynes was performed based on kinetic
studies of acetalization and esterification. Both monofunctional alkynes, (cyclopropylacetylene, 3-
cyclohexylprop-1-yne, 1-ethynyl-2-fluorobenzene, 1-ethynyl-4-fluorobenzene, 4-ethynyltoluene, 4-
tert-butylphenylacetylene,
2-ethynyl-α,α,α-trifluorotoluene,
phenylacetylene
[14])
and
multifunctional alkynes [1,3-diethynylbenzene, 1,4-diethynylbenzene, 1,3,5-triethynylbenzene, and
tetrakis(4-ethynylphenyl)methane], were involved in the study. Our previous study performed with
polymer networks decorated with pendant ethynyl groups [15] showed that the hydrogen atoms of
ethynyl groups of these materials acted as acid catalytic centers. We expected similar behavior in the
case of hydrogen atoms of ethynyl groups of low molecular weight alkynes used in this study. In
order to qualitatively assess the acidity of the alkynes used, the chemical shifts (δ) of the signal of
1
1
their acetylenic hydrogen(s) in H NMR spectra were determined. The H NMR spectra were
measured in CD3OD and DMSO-d6, the δ values of ≡C-H are given in Table 1 (for full spectra see
Supplementary material).
Table 1. The chemical shift (δ) of 1H NMR signal of hydrogen in ethynyl functional groups of terminal
alkynes (in DMSO-d6 and CD3OD)
(δ) of ≡C-H (in ppm)
Terminal alkyne
Formula
in CD3OD
in DMSO-d6
Cyclopropylacetylene
(Cy3ac)
2.00
2.5
2