Efficient E-Selective Transfer Semihydrogenation of Alkynes by Means of Ligand-Metal Cooperating Ruthenium Catalyst

Article abstract of DOI:10.1002/adsc.201500372

The manuscript describes an efficient protocol for the E-selective transfer semihydrogenation of internal alkynes employing a previously described bifunctional ruthenium-based PC(sp³)P complex, 20'mol% of sodium formate and nearly stoichiometric formic acid. Semihydrogenation of the terminal alkynes results in the formation of the corresponding styrenes. The mechanism of the reaction includes stepwise transfer of both the hydride and the proton of the formic acid to the substrate. This fact allowed for the development of a facile protocol for the synthesis of monodeuterated alkenes by simply applying formic acid/sodium formate/D₂O mixture as a hydrogen source. High yields, selectivity and functional group compatibility have been generally achieved.

Full text of DOI:10.1002/adsc.201500372

UPDATES
DOI: 10.1002/adsc.201500372
Efficient E-Selective Transfer Semihydrogenation of Alkynes by
Means of Ligand-Metal Cooperating Ruthenium Catalyst
Sanaa Musa,+a Amrita Ghosh,+a Luigi Vaccaro,b Lutz Ackermann,c
and Dmitri Gelmana,*
a
Institute of Chemistry, The Hebrew University, Edmond Safra Campus ( 91904 Jerusalem, Israel
Fax: (+972)-2-6585279; e-mail: dmitri.gelman@mail.huji.ac.il (homepage: http://chem.ch.huji.ac.il/~dgelman/)
b
CEMIN – Dipartimento di Chimica, Biologia e Biotecnologie, Universita di Perugia, Via Elce di Sotto, 8, Perugia, Italy
c
Georg-August-Universität Gçttingen, Institut für Organische und Biomolekulare Chemie, Tammannstrasse 2, 37077
Gçttingen, Germany
+
Equal contribution by Sanaa Musa and Amrita Ghosh.
Received: April 15, 2015; Revised: May 19, 2015; Published online: July 14, 2015
The manuscript is dedicated to Prof. Stephen L. Buchwald for the occasion of his anniversary.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500372.
Abstract: The manuscript describes an efficient pro-
tocol for the E-selective transfer semihydrogenation
of internal alkynes employing a previously de-
scribed bifunctional ruthenium-based PC(sp3)P
complex, 20 mol% of sodium formate and nearly
stoichiometric formic acid. Semihydrogenation of
the terminal alkynes results in the formation of the
corresponding styrenes. The mechanism of the reac-
tion includes stepwise transfer of both the hydride
and the proton of the formic acid to the substrate.
This fact allowed for the development of a facile
protocol for the synthesis of monodeuterated al-
kenes by simply applying formic acid/sodium for-
mate/D2O mixture as a hydrogen source. High
yields, selectivity and functional group compatibility
have been generally achieved.
functional materials, raw chemicals, etc.[3] Although
a wide spectrum of synthetic approaches to the regio-
and stereoselective synthesis of alkenes is currently
available,[4] catalytic semihydrogenation of terminal
or internal alkynes represents one of the most attrac-
tive ones, especially, given a facile access to the start-
ing materials via the reliable and robust Sonogashira
cross-coupling reaction.[5]
The vast majority of the transition metal-catalyzed
hydrogenations of alkynes lead to a more or less Z-se-
lective formation of the corresponding alkenes. Thus,
very efficient and chemo-/stereoselective Ru, Ir, V,
Nb, Pd catalyzed protocols utilizing various hydrogen
sources have been reported. The Z selectivity origi-
nates from the classical organometallic hydrogenation
mechanism that relies on the hydrometallation/reduc-
tive elimination sequence leading to the addition of
the two hydrogen atoms suprafacially to the p system
of the alkynes.[6]
Keywords: isotope labelling; ligand-metal coopera-
tion; ruthenium; semihydrogenation of alkynes;
transfer hydrogenation
The E-selective catalytic systems are less common
but have also been described. Most of them result in
the formation of nearly thermodynamic mixtures of
the alkenes due to a fast post-synthetic isomerization
of the kinetic product induced by the transition metal
catalyst. However, in some cases, the reaction pro-
ceeds via a different mechanistic sequence that dic-
Introduction
Arguably, double bonds are the most versatile func- tates formation of the E-isomer.[7]
tionality in organic chemistry, as they can be directly
We recently introduced a series of new bifunctional
transformed into, essentially, any functional group[1] pincer complexes capable of non-oxidative activation/
and, often, serve as a directing group in the manipula- formation of bonds via ligand metal-cooperation be-
tion of neighboring carbons.[2] Moreover, double tween the metal center and the ligandꢀs functional
bonds are very common structural motifs in natural sidearm (Figure 1).[8]
products, synthetic biologically active compounds,
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Sanaa Musa et al.
Table 1. Representative optimization experiments.
Entry
Conditions
Products[e]
[a]
1
2
3
4
5
6
7
8
9
1, H2[a]
7a (99% conv.)
7a (99% conv.)
none
5–7a (30% conv.)
5–7a (48% conv.)
5a (78% conv., 90% sel.)
5a (35% conv., 78% sel.)
5a (63% conv., 90% sel.)
5a (99% conv., 99% sel.)
2, H2[a]
3, H2
[b]
1, HCOOH/NEt3[b]
2, HCOOH/NEt3[b]
3, HCOOH/NEt3
3, HCOOH
3, HCOOH/NaO2CH[c]
3, HCOOH/NaO2CH[d]
Figure 1. Catalysts developed by our groups.
[a]
708C, 100 psi.
HCOOH/NEt3 (1 equiv.), 908C.
1 equiv. of formic acid, 20 mol% of sodium formate,
[b]
[c]
908C.
[d]
[e]
1.5 equiv. of formic acid, 20 mol% of sodium formate,
908C.
GC.
À
Scheme 1. Ligand-metal cooperative C H bond formation.
tion of various solvent/temperature/base/hydrogen
Mainly, the catalysts have been used for the activa- source combinations, the general trend was realized:
tion and formation of polar bonds: efficient acceptor- the Rh-based 1 and the Ir-based 2 led to the over-re-
less dehydrogenation of alcohols to ketones and duced product (dibenzyl 7a), while the Ru-based 3
esters (TON=3600) and dehydrogenative coupling of was found to be essentially inactive under the molecu-
primary amines and alcohols to imines (TON=830) lar hydrogen (entries 1–3). On the other hand, when
were reported.[9] However, they are, obviously, not HCOOH/NEt3 azeotrope replaced H2, incomplete
limited to those.[10] Thus, we also demonstrated the conversion of the starting material into 7a, 5a and 6a
direct hydroformylation of double bonds using the was observed under the catalysis of 1 and 2, whereas
same ligand-metal cooperating catalysis and proved the Ru-based 3 drove the reaction predominantly to
À
that the acidic proton does participate in the C H E-stilbene at 908C in ether-like solvents (e.g., THF,
bond formation (Scheme 1).[10] Therefore, we ques- dioxane, DME) spotting the latter as a suitable cata-
tioned if this polar mechanism can bring about some lyst (entries 4–6). Noteworthy, the lower conversions
synthetically useful reactivity of alkynes.
observed in the presence of 1 and 2 are due to the
In this manuscript, we wish to report the results of fact that the HCOOH decomposition rate catalyzed
our studies that identified a practical and synthetically by these complexes is significantly higher than the
flexible protocol for the E-selective semihydrogena- rate of the subsequent hydrogenation even at lower
tion of alkynes employing a ruthenium complex bear- temperatures.[11]
ing a suitable bifunctional PC(sp3)P pincer ligand that
Further experimentation revealed that the reaction
shows high catalytic activity and excellent selectivity indeed proceeds well at 908C in DME in the presence
when using nearly stoichiometric formic acid as a re- of 1 mol% of the catalyst 3 leading to the formation
ducing agent.
of the desired E-stilbene in 98% isolated yield and ex-
cellent chemo- and stereoselectivity, however, to
reach the full conversion in the selective way and rea-
sonable reaction time, we used 1.5 equivalents of the
formic acid and 20 mol% of sodium formate (en-
tries 7–9, Table 1).
Results and Discussion
Optimization of the Reaction Conditions
Furthermore, we found that our catalytic system is
Our initial experiments comprised attempts to accom- perfectly stable in air and the reaction can be per-
plish semihydrogenation of diphenylacetylene (4a) formed without rigorous exclusion of oxygen. Only
using different hydrogen sources in the presence of slightly reduced yields were observed when air- and
1 mol% of 1–3 (entries 1–6, Table 1). Upon examina- nitrogen-filled reactions were run side by side.
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Efficient E-Selective Transfer Semihydrogenation of Alkynes
Scope and Limitations
chemistry.[13] However, cross-coupling strategy[14] and,
in particular, the Sonogashira coupling/semihydroge-
The substrate scope of this operationally simple pro- nation sequence appear as a reliable alternative to
tocol was explored, using a range of alkynes with vari- the classical approaches. Noteworthy, the catalytic
ous electronic and steric properties, and containing systems capable of chemoselective semireduction of
different functional groups that can potentially inter- terminal alkynes into styrenes are not many,[6c,15] and,
fere with the transfer semihydrogenation reaction. therefore, 3 is a valuable addition to the arsenal of
The results are summarized in Table 2.
the existing methods, although milder catalysts are
The catalytic system performs well for a wide range known.[15e]
of electron-neutral alkynes and demonstrates excel-
lent efficiency, chemo- and stereo-E-selectivity (en-
tries 1 and 5–8, Table 2). It is noteworthy that even Mechanistic Considerations
0.1 mol% of the catalyst loading afforded a satisfacto-
ry result at 908C after 12 h (entry 5), but in order to To gain insights into the reaction mechanism, a deute-
keep the reaction time more practical we decided to rium labelling experiment was carried out
stick to the optimized 1 mol%.
(Scheme 2). Adding 2 equivalents of D2O (or
Both electron-rich (entries 2 and 3) and electron- CH3OD) to the semihydrogenation of the diphenyla-
poor (entries 4, 9 and 11) alkynes afforded the E-al- cetylene under the standard reaction conditions, selec-
kenes selectively. We found that the catalyst is com- tive formation of mono-deuterated stilbenes was ob-
patible with a variety of functional groups. For exam- served. No bis-deuterated stilbene was detected ac-
ple, even in the absence of NEt3 (under acidic condi- cording to NMR or MS analyses (see the Supporting
tions), the reaction is compatible with carboxylate or Information). Similarly, incorporation of only one
nitrile functionalities (entries 4 and 11, Table 2). How- deuterium atom was observed when substituted stil-
ever, the yields and selectivities are significantly benes were employed (Scheme 2). This experiment
lower when the carboxylate group is directly attached clearly indicates the transfer of both the hydride and
to the alkyne moiety of the substrate (entry 13). Un- the proton from the formic acid to the substrate.
expectedly, a significant drop in selectivity was ob-
On the other hand, this fact does not explain the
served when heteroaromatic and TMS-protected al- observed E-selectivity of the method. Thus, a possibili-
kynes were subjected to the reaction conditions ty of the in situ isomerization of the kinetic product
(entry 10 and 12). Steric bulk and intramolecular co- was probed. Thus, heating of the commercial Z-stil-
ordination of the neighboring functional groups to the bene with 3 for 2 h in DME results in its isomeriza-
ruthenium center may slow down the isomerization tion to the E-isomer in ca. 98% purity.[16] Almost the
rate in these cases.
same E/Z ratio (along with ca. 10% of dibenzyl) was
Unfortunately also, the acetyl moiety of 4-CH3CO- formed in the presence of the hydrogenation mixture
ꢀ
C6H4C CC6H5 (entry 9) was reduced to the corre- (formic acid/sodium formate).
sponding alcohol.
At the same time, no deuterium incorporation
Selective reduction of the E- and Z-enynes to the during the isomerization was observed when D2O or
corresponding E,E-diene is a spectacular example of CH3OD was intentionally added. These results imply
chemoselectivity of the suggested catalyst (entries 5 that fast in situ isomerization is the origin of the E-se-
and 6). Indeed, the control competition experiment lectivity of the process that may proceed also without
aimed at the hydrogenation of the 1:1 mixture of the involvement of the hydride species. The mechanism
4a and 5a by 3 under the described reaction condi- of the hydride-free isomerization may operate via in-
tions led to the exclusive formation of 5a and essen- tramolecular protonation/deprotonation[7c,17] (with the
tially no over-reduced 7a was formed, stressing the aid of the pendant OH group) of the Ru-coordinated
difference in the hydrogenation rate between the al- alkene, however, more detailed investigation is still
kynes and alkenes.
Selective semihydrogenation of terminal alkynes is
known to be difficult due to the over-reduction to the
corresponding alkanes. However, our catalyst showed
excellent reactivity in the synthesis of the correspond-
ing substituted styrenes (entries 15–21) from the dif-
ferently substituted terminal alkynes within minutes.
Of course, if the reaction time is significantly extend-
ed in these runs, formation of the over-reduced by-
products is observed.
Usually, the synthesis of functionalized styrenes is
achieved via elimination reactions[12] or Heck-type Scheme 2. Deuterium labelling experiment.
Adv. Synth. Catal. 2015, 357, 2351 – 2357
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Table 2. Representative results of transfer semihydrogenation of terminal and internal alkynes.
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UPDATES
Efficient E-Selective Transfer Semihydrogenation of Alkynes
Thus, based on these results (Scheme 2 and
Scheme 3) and our previous studies, a possible cata-
lytic cycle for the present ruthenium-catalyzed semi-
hydrogenation of alkynes is shown in Scheme 4. We
suggest that Ru-hydride species 8 are the catalytically
active intermediates in the discussed reaction. Al-
though the reactive species 8 were characterized only
spectroscopically (see the Supporting Information),
hydride formation is always observed when the pre-
catalyst 3 is heated in the presence of sodium formate.
The initial displacement of the chloride ligand, appa-
rently, requires more nucleophilic species than the
formic acid itself, which explains the necessity for the
catalytic sodium formate (20 mol%). Addition of the
sterically demanding 8 to the alkyne is apparently
faster than to the competing alkene and leads to the
formation of the hydrometallated 9 in the usual ste-
reochemistry via the expected syn-addition (step 1).
The intramolecular protonation of 9 with the aid of
the “helping OH-hand” should lead to the kinetic Z-
product and the corresponding arm-closed Ru-species
10 as was demonstrated by us previously (step 2).[9e]
Of course, the intermolecular protonolysis of 9 with
formic acid leading directly to 11 cannot be ruled out;
however, in this case, the accelerating effect of the
catalytic sodium formate is questionable (step 3). Fi-
nally, the catalytically active 8 is regenerated via ex-
trusion of CO2 (step 4). The kinetic Z-product, appa-
rently, isomerizes to the thermodynamic E-isomer.
Scheme 3. Comparative isomerization experiment.
Scheme 4. Plausible mechanism of the reaction.
Conclusions
We have reported a practical protocol for the transfer
needed. Of course, the virtue of the hydride-free iso- semihydrogenation of alkynes using almost stoichio-
merization does not rule out the usual metal-hydride metric formic acid/NaO2CH employing a Ru-based
pathway under the hydrogenation conditions.
ligand-metal cooperating catalyst developed previous-
The results of the scope and limitation studies often ly in our group. The newly developed protocol allows
exhibited very high stereoselectivity. We suspect that for the E-selective synthesis of alkenes in good to ex-
a high steric demand of the ruthenium center in 3 cellent chemo- and stereoselectivity, also those pos-
might be responsible for a kinetically favorable coor- sessing vulnerable functional groups. The suggested
dination of the cis-alkenes over the trans-alkenes thus protocol is operationally simple and even can be ap-
improving the stereoselection. The same reason may plied to the synthesis of monodeuterated alkenes by
explain the significant difference between alkynes and simply applying HCOOH/HCOONa/D2O mixture as
alkenes in terms of their hydrogenation rates (i.e., a hydrogen source.
chemoselectivity of the catalyst). Indeed, attempted
isomerization
of
Z-prop-1-ene-1,2-diyldibenzene
(Scheme 3) was sluggish in comparison with the iso-
merization of cis-stilbene under the same condi-
tions.[18]
[a]
Reaction conditions: 3 (1 mol%), formic acid (2 equiv.), sodium formate (20 mol%), DME (1 mL), 908C.
Isolated yield (average of 2 runs), purity was determined according GC or/and H NMR.
Under 0.1 mol% of the catalyst.
GC conversion with DMF as an internal standard.
[b]
[c]
[d]
1
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Experimental Section
General Procedure for the Catalytic Transfer
Semihydrogenation of Alkynes
A single-use screw-capped reaction tube was charged with 3
(1% mol) and sodium formate (20% mol) in 1,2-demethoxy-
ethane (1 mL). The alkyne (50 mg) and formic acid (2 equiv-
alents) were injected and the mixture was heated at 908C in
oil bath for the specified time. Products were isolated after
a standard work-up and purified, if needed, by flash chroma-
tography.
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Acknowledgements
This research was partially supported by the Israel Science
Foundation (ISF), Ministry of Science and the German-Isra-
eli Science Foundation (GIF).
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[16] Thermal isomerization of cis-stilbene under the same
reaction conditions in the absence of 3 was unsuccessful
in our hands. For the literature data, please see also:
G. B. Kistiakowsky, W. R. Smith, J. Am. Chem. Soc.
1934, 56, 638.
[17] a) D. S. Noyce, D. R. Hartter, F. B. Miles, J. Am. Chem.
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[18] To compare, a control isomerization of Z-prop-1-ene-
1,2-diyldibenzene with RuCl2(PPh3)3 in the presence of
formic acid in DME at 908C led to the 35/65 mixture
of E- and Z-isomers over 2 h.
Adv. Synth. Catal. 2015, 357, 2351 – 2357
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