Features of the mechanism of oxidative dehydrodimerization of propynol

Article abstract of DOI:10.1016/j.tetlet.2011.05.065

Analysis of kinetic regularities for the propynol oxidative coupling under the action of cupric salts in pyridine and in the presence of a buffer is undertaken. The reaction mechanism, including the formation of Cu(I) acetylides, is considered.

Full text of DOI:10.1016/j.tetlet.2011.05.065

Tetrahedron Letters 52 (2011) 3776–3778
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Tetrahedron Letters
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Features of the mechanism of oxidative dehydrodimerization of propynol
⇑
Lidiya G. Fedenok , Mark S. Shvartsberg
Institute of Chemical Kinetics and Combustion, Siberian Branch of Russian Academy of Sciences, Novosibirsk 630090, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
Analysis of kinetic regularities for the propynol oxidative coupling under the action of cupric salts in pyr-
idine and in the presence of a buffer is undertaken. The reaction mechanism, including the formation of
Cu(I) acetylides, is considered.
Received 22 March 2011
Revised 28 April 2011
Accepted 13 May 2011
Available online 19 May 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Oxidative dehydrodimerization
Mechanism
Propynol
1. Introduction
does not involve Cu(I). When Cu(I) is formed, an ‘anomalous’
mechanism with its participation is implemented. When
Oxidative dehydrodimerization (oxidative coupling) of terminal
acetylenes (Scheme 1) is an important modern method for prepar-
ing highly unsaturated dimeric, polymeric, and macrocyclic struc-
tures.1–3
[P]0 > > [Cu(II)]0, the final rate of P oxidation becomes constant. It
should be noted that oxidative coupling in aqueous ammonia un-
der the action of Cu(II) follows the same patterns as those for phe-
nyl acetylene in a buffer solution.8 The rate of dehydrodimerization
of terminal acetylenes was earlier shown to depend on the Cu(I)
concentration if the reaction conditions were favorable for the for-
mation of Cu(I) acetylides and their dimeric complexes.9 The for-
mation of acetylides and their dimeric complexes in a buffer
containing AcOH seems to be unlikely. Herein, an attempt was
made to elucidate the reasons behind the ‘anomalous’ kinetics of
P oxidation in the buffer.
The importance of this method prompts a necessity to study the
mechanism of this complex multicomponent catalytic reaction.
Earlier, the dehydrodimerization of phenyl acetylene under the ac-
tion of Cu(II) in pyridine in the presence of a buffer (amine–AcOH)
was shown to be zero order with respect to Cu(I), second order
with respect to both the substrate and Cu(II) and reverse second
order with respect to AcOH.4 The application of the buffer was nec-
essary to avoid retardation of the reaction by the developing acid-
ity. On the basis of experimental results and calculations we
proposed a mechanism for the oxidative coupling.4 The key step
was synchronous oxidation of two acetylide anions by two Cu2+
ions with simultaneous formation of the C–C bond. This mecha-
nism is valid for a number of acetylenes of various structures.5
The reactivity of acetylenes increases with enhanced acidity.
Meanwhile, the oxidative coupling of propynol (P) under the same
conditions is known to be an autocatalytic process that follows
quite different kinetic parameters.6,7 At [Cu(I)]/[Cu(II)] ratios of
P5, the reaction is zero order with respect to Cu(II), less than first
order with respect to the substrate, and approximately first order
with respect to Cu(I). At initial time of P oxidation, the mechanism
was shown to be the same as for other acetylenes, and the reaction
2. Results and discussion
Our study of the rate of P oxidative coupling versus [AcOH] at
constant [AcOꢀ] revealed unexpectedly that it increased with a rise
in the acidity of the reaction medium (Fig. 1).7 This dependence is
the key for understanding the features of P oxidative coupling. In-
deed, P can be considered as a bidentate ligand for Cu+ forming
complex (A) with participation of the p-electrons of the triple bond
and the n-electrons of the O atom (Scheme 2). This complexing en-
hances the liability of the hydroxy proton, whereas formation of
the alkoxide ion decreases the acidity of the ethynyl group. How-
ever, dissociation of the hydroxy group is reversible and the acid
in the buffer shifts the equilibrium to the left. Experimentally, this
results in a linear dependence of the reaction rate on [AcOH]. On
the contrary, dissociation of the ethynyl group of A is irreversible.
Deprotonation of the ethynyl group leads to the formation of Cu(I)
acetylides, which are highly inclined to give dimeric associates
⇑
Corresponding author. Address: Institute of Chemical Kinetics and Combustion,
Instituskaya 3, Novosibirsk 630090, Russia. Tel.: +7 383 3333 349; fax: 7 383 3307
350.
E-mail address: fedenok@kinetics.nsc.ru (L.G. Fedenok).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.05.065

L. G. Fedenok, M. S. Shvartsberg / Tetrahedron Letters 52 (2011) 3776–3778
3777
Cu(II)
Cu(I) + H+ (or H2O)
R
R
R
+
(or Cu(I) + O2)
Scheme 1.
½Pꢁt
A ¼
ð2Þ
ð3Þ
½Bꢁ
1
1 þ
þ K2
K01½CuðIÞsꢁ
½BHþꢁ
2.0
1.0
Therefore,
k3½Bꢁ½Pꢁ
þ tK2
Wf ¼
1 þ
½Bꢁ
1
K01½CuðIÞsꢁ
½BHþꢁ
k3K0 ½Bꢁ½Pꢁ ½CuðIÞ ꢁ
1
t
s
¼
1 þ K0 ½CuðIÞ ꢁ þ K0 K2½CuðIÞ ꢁ½Bꢁ=½BHþꢁ
1
s
1
s
Along with Cu(I)s, other copper complexes, which do not in practice
react with the acetylene, are present in the solution. Thus,
0.1
0.2
[AcOH], M
X
½CuðIÞꢁt ¼ ½CuðIÞsꢁ þ
Ki½L0ꢁ½CuðIÞ ꢁ þ ½Aꢁ þ ½Aꢀꢁ
ð4Þ
i
s
Figure 1. Dependence of Wf on [AcOH] with constant [AcOꢀ], maintained by adding
[NEt4]+OAcꢀ; 40 °C, [P]–1.0 M, [HN(CH2)5]–1.0 M, [Cu(II)]o–4 ꢄ 10ꢀ3 M.
where [Cu(I)]t = total concentration of Cu(I), Ki = constant for the
complexing of Cu(I) with all ligands (Li) except acetylene, L0 = [Li]/
i
[Py].
X
½CuðIÞꢁt ¼ ½CuðIÞsꢁ þ
Ki½L0ꢁ½CuðIÞ ꢁ þ K0 ½Pꢁ ꢂ ½CuðIÞ ꢁð1 þ K2½Bꢁ=½BHþꢁÞð½Pꢁ ꢃ ½PtꢁÞ
ð5Þ
ð6Þ
i
s
1
t
s
½CuðIÞꢁt
½CuðIÞsꢁ ¼
P
1 þ Ki½L0ꢁ þ K0 ½Pꢁ ð1 þ K2½Bꢁ=½BHþꢁÞ
i
1
t
k3K0 ½Bꢁ ꢂ ½CuðIÞꢁ ꢂ ½Pꢁ
1
t
t
Wf ¼
P
ð7Þ
1 þ Ki½L0ꢁ þ K0 ½Pꢁ þ K0 K2½Pꢁ ꢂ ½Bꢁ=½BHþꢁ þ K0 ½CuðIÞꢁ þ K0 K2½CuðIÞꢁ ꢂ ½Bꢁ=½BHþꢁ
i
1
t
1
t
1
t
1
t
with the participation of hydroxy groups (Scheme 2). As a result,
the reaction pathway, including the formation of dimeric Cu(I)
acetylides and their oxidation by Cu(II) into dimeric Cu(II) acety-
lides, becomes prevalent.
We assume that formation of the Cu(I) acetylide determines the
reaction rate, that is, k3 < < k1, k-1, k2, k-2, k4, k5. Therefore, the reac-
tion rate is expressed as
This kinetic expression accords with the experimental findings.
From expression 7 it can be derived that the reaction should be less
than first order with respect to acetylene. The reaction is also less
than first order with respect to Cu(I), but under the described con-
ditions,6,7 [P]0 > > [Cu(II)]0 and is nearly first order. The concentra-
tion of the base is included in the numerator of 7 in the first power,
whereas in the denominator it is included as [B]/[BH+]. This means
that the reaction is first order with respect to piperidine (the base)
at constant [B]/[BH+]. It should be emphasized that expression 7
demonstrates a linear dependence of the reaction rate on the acid
concentration, as was also found experimentally. The relationship
between the final reaction rate and the acidity of the reaction med-
ium plotted as 1/Wf versus 1/[BH+] is a straight line, which cuts off
a segment b of the ordinate axis (Fig. 2).
Wf ¼ k3½Bꢁ½Aꢁ
ð1Þ
where Wf ¼ ꢀ d½CuðIIÞꢁ – the final reaction rate. The total P concentra-
dt
tion is equal to the sum of the concentrations of free P, A and Aꢀ:
[P]t = [P] + [A] + [Aꢀ]. Using the equations for equilibrium constants
½Aꢁ
K1
½Aꢀꢁ½BHþ
½Aꢁ½Bꢁ
K10
¼
, where K01
¼
½Pyꢁ, and K2
¼
ꢁ, it can be derived that
½Pꢁ ½CuðIÞsꢁ
K2; B
K1; -Py
Cu(I)s
BH+
+
CuI
O
CuI
HO
HO
P
A
A
k1
k-1
k2
B
k3
K1 =
K2 =
k-2
CuI
HO
k4
CuII
CuI
OH
OH
k6
k5; Cu(II)
- Cu(I)
HO CuII
HO CuI
- Cu(I)
OH
HO
Scheme 2. Cu(I)s = Cu(I) solvated by pyridine; B-base (amine) ki = the rate constants for the reaction steps.

3778
L. G. Fedenok, M. S. Shvartsberg / Tetrahedron Letters 52 (2011) 3776–3778
3. Experimental
Cuprous chloride partially oxidized with O2 in the buffer solu-
6
4
tion was used as the oxidizing agent. The reaction was monitored
by a change in optical density of the solution in the region of the
absorption maximum of Cu(II) at k 680 nm. A 42 mm long thermo-
stated cuvette provided with a stirrer was used. The cuvette was
filled, under N2, with the buffer solution containing the partially
oxidized cuprous chloride, placed in a spectrophotometer (SF-10)
and thermostated. The acetylene was then introduced rapidly into
the cuvette, the reaction mixture was stirred and the measure-
ments were recorded.
2
b
tg α
+
10
5
1/[H ], l/M
Figure 2. Dependence of 1/Wf on 1/[AcOH] with constant [AcOꢀ], maintained by
adding [NEt4]+OAcꢀ; 40 °C, [P]–1.0 M, [HN(CH2)5]–1.0 M, [Cu(II)]o–4 ꢄ 10ꢀ3 M.
References and notes
1. Eglinton, G.; MacRae, B. In Advances in Organic Chemistry; Raphael, R. A., Taylor,
E. C., Wynberg, H., Eds.; Interscience Publishers: New York, London, 1963. Vol.
4, pp 225–252.
2. Shvartsberg, M. S.; Fisher, L. B. In Reactions of Acetylenic Compounds;
Troschenko, A. T., Ed.; Nauka: Novosibirsk, USSR, 1967.
3. Siemsen, P.; Livingston, R. C.; Diederich, F. Angew. Chem., Int. Ed. 2000, 39,
2632–2657.
4. Fedenok, L. G.; Berdnikov, V. M.; Shvartsberg, M. S. J. Org. Chem USSR 1973, 9,
1806–1809.
5. Fedenok, L. G.; Berdnikov, V. M.; Shvartsberg, M. S. J. Org. Chem. USSR 1974, 10,
934–936.
6. Clifford, A. A.; Waters, W. A. J. Chem. Soc. 1963, 3056–3062.
7. Fedenok, L. G.; Berdnikov, V. M.; Shvartsberg, M. S. J. Org. Chem. USSR 1978, 14,
1334–1337.
8. Fedenok, L. G.; Berdnikov, V. M.; Shvartsberg, M. S. Zh. Org. Khim. USSR 1975, 11,
2492–2497.
9. Fedenok, L. G.; Berdnikov, V. M.; Shvartsberg, M. S. J. Org. Chem. USSR 1978, 14,
1328–1333.
10. Okuro, K.; Furuune, M.; Enna, M.; Miura, M.; Nomura, M. J. Org. Chem. 1993, 58,
4716–4721.
P
1 þ Ki½L0ꢁ þ K0 ½Pꢁ
K2
k3½CuðIÞꢁt
i
1
t
b ¼
;
tg
a
¼
½Pꢁt >> ½CuðIÞꢁt
ð8Þ
k3K0 ½Bꢁ ꢂ ½Pꢁ ꢂ ½CuðIÞꢁ
1
t
t
The data obtained can be used to explain the ‘unusual’ behavior
of P in Cu-catalyzed cross-coupling with aryl iodides.10 Unlike phe-
nyl acetylene and heptyne, P does not react with aryl iodides in a
CuI-PPh3–K2CO3 system in DMSO and is regenerated in 80% yield.
However, protection of P with THP, t-butyl and acetyl groups en-
abled the reaction to give the corresponding coupled products in
37–90% yields. According to the accepted scheme of the reaction
mechanism, aryl iodide interacts with Cu(I) acetylide formed in
situ. In the case of propynol, the formation of Cu(I) acetylide was
shown to be impeded. Therefore it can be assumed that the
Cu(I)-catalyzed cross-coupling of other a-acetylenic alcohols with
11. Shvartsberg, M. S.; Kotlyarevskii, I. L.; Kozhevnikova, A. N.; Andrievskii, V. N.
Izv. Akad. Nauk SSSR, Ser. Khim. 1970, 1144–1149.
12. Kozhevnikova, A. N.; Shvartsberg, M. S.; Kotlyarevskii, I. L. Izv. Akad. Nauk SSSR,
Ser. Khim. 1973, 1168–1170.
13. Moroz, A. A.; Shvartsberg, M. S.; Kotlyarevskii, I. L. Izv. Akad. Nauk SSSR, Ser.
Khim. 1979, 851–855.
aryl iodides would also be impeded, for the same reasons.11–13
The reactivity of terminal acetylenes in oxidative couplings is
determined not only by their acidity, but also by the ability of
existing functional groups to form complexes with Cu(I). This com-
plexing facilitates the formation of a multicentered transition state
in the process of synchronous electron transfer. This feature results
in kinetic regularities in the oxidative coupling of some functional-
ized acetylenes.