Z-Selective phosphine promoted 1,4-reduction of ynoates and propynoic amides in the presence of water

Article abstract of DOI:10.1039/d1ob00909e

Phosphine-mediated reductions of substituted propynoic esters and amides in the presence of water yield the partially reduced α,β-unsaturated esters and amides with highZ-selectivity. The competitivein situ ZtoE-isomerization of the product in some cases lowers theZtoEratios of the isolated α,β-unsaturated carbonyl products. Reaction time and the amounts of phosphine and water in the reaction mixture are the key experimental factors which control the selectivity by preventing or reducing the rates ofZ- toE-product isomerization. Close reaction monitoring enables isolation of theZ-alkenes with high selectivities. The computational results suggest that the reactions could be highlyZ-selective owing to the stereoselective formation of theE-P-hydroxyphosphorane intermediate.

Full text of DOI:10.1039/d1ob00909e

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Z-Selective phosphine promoted 1,4-reduction of
ynoates and propynoic amides in the presence of
water†
Cite this: Org. Biomol. Chem., 2021,
19, 6092
Fabian Seifert,a Denis Drikermann,a Johannes Steinmetzer, b You Zi,a
a
Stephan Kupfer b and Ivan Vilotijevic
*
Phosphine-mediated reductions of substituted propynoic esters and amides in the presence of water
yield the partially reduced α,β-unsaturated esters and amides with high Z-selectivity. The competitive
in situ Z to E-isomerization of the product in some cases lowers the Z to E ratios of the isolated
α,β-unsaturated carbonyl products. Reaction time and the amounts of phosphine and water in the reac-
tion mixture are the key experimental factors which control the selectivity by preventing or reducing the
rates of Z- to E-product isomerization. Close reaction monitoring enables isolation of the Z-alkenes with
high selectivities. The computational results suggest that the reactions could be highly Z-selective owing
to the stereoselective formation of the E-P-hydroxyphosphorane intermediate.
Received 10th May 2021,
Accepted 11th June 2021
DOI: 10.1039/d1ob00909e
rsc.li/obc
us to conduct a more detailed study of the selectivity in these
1,4-reduction reactions (Scheme 1).5
Introduction
Reduction of electron deficient alkynes is a straightforward
Phosphine mediated reduction of alkynes in the presence of
path for synthesis of α,β-unsaturated carbonyl compounds. water has been reported as a metal-free alternative to reductions
Numerous methods have been developed for selective partial of electron deficient alkynes using transition metal catalysts.1a–d
reduction of such alkynes to the corresponding alkenes. Most Almost uniformly, these reactions have been reported to prefer-
of these methods preferentially produce the more stable, entially produce E-alkenes. A recent study by Whittaker disclosed
E-alkenes.1 Methods to selectively produce Z-alkenes from the for the first time that these reactions, depending on the reaction
corresponding alkyne are still in high demand. The existing conditions, can produce the Z-alkene preferentially when
methods that typically rely on transition metal catalyzed hydro- ynoates are used as starting materials.1e Here, we report that
genations2 and catalysis by frustrated Lewis pairs3 leave space phosphine-promoted reductions of ynoates and propynoic
for further development of more general and mild reduction amides in the presence of water can be highly Z-selective and we
methods that avoid the use of hydrogen gas or expensive present a mechanistic analysis of these reactions with a focus on
catalysts.
factors that determine stereochemical outcome of the reaction
In our work on phosphine catalyzed trans-hydroboration of supported by quantum chemical simulations “pysisyphus”,6
a
alkynes, we observed a competitive 1,4-reduction of electron recently introduced efficient external optimizer written in
deficient alkynes under forcing conditions with substrates that Python which can also be used to describe excited states.
do not undergo the desired hydroboration reaction.4 The
origin of the 1,4-reduction products was attributed to the
ability of the phosphine to promote alkyne reduction in the
presence of water. The recent reports of stereoselective difunc-
tionalizations of alkynes promoted by phosphines compelled
Results and discussion
Reduction of methyl 3-phenylpropiolate P-3 was taken as a
model for optimization of the reaction conditions (Table 1). A
brief survey of solvents identified that the reactions in ethereal
aInstitute of Organic Chemistry and Macromolecular Chemistry,
Friedrich Schiller University Jena, Humboldtstr. 10, 07743 Jena, Germany.
E-mail: ivan.vilotijevic@uni-jena.de
solvents mixable with water gave highest yields when used in
1 : 5 v/v mixtures with water in the presence of 1.0 equiv. of
PBu3. Further optimization was focused on reactions in 1,4-
dioxane. The use of higher amount of phosphine resulted in
reduced selectivity. The amount of water in the reaction
mixture proved to be an important determinant of the E : Z
product ratio.
bInstitute for Physical Chemistry and Abbe Center of Photonics,
Friedrich Schiller University Jena, Helmholtzweg 4, 07743 Jena, Germany
†Electronic supplementary information (ESI) available: Detailed experimental
procedures and characterization data for new compounds and the details of
computational study on reaction mechanism. See DOI: 10.1039/d1ob00909e
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of the Z-products is the goal (Table 1, entries 2–6). When water
was completely left out, the reaction proceeded with only 14%
yield and resulted in equal amounts of the reduced species
(Table 1, entry 1). Final optimization effort focused on temp-
erature and overall concentration of the reaction mixture.
While increased temperature and increased concentration
strongly influenced the reaction rates, the effects on the yield
and selectivity remained modest (Table 1, entries 7–12).
Reactions using tertiary triarylphosphines, alkylarylpho-
sphines and other trialkyl phosphines followed similar selecti-
vity trends. Tri-n-butylphosphine,
a commonly used in-
expensive nucleophilic phosphine, was chosen for evaluation
of the reaction scope due to the favorable reaction kinetic
profile over a range of substrates.
The scope of the reactions for ynoates has been tested under
the optimized conditions with a range of diversely substituted
aryl ynoates. The influence of substituents on reaction rates
required close monitoring of the reaction course for each sub-
strate. These studies led to the conclusion that the selectivity for
the Z-product significantly decreases with extended reaction
times. This was attributed to a competitive isomerization of the
initially produced Z-enone to the thermodynamically more
stable E-enone in the presence of phosphine. Such isomeriza-
tion in the presence of a nucleophilic catalyst is well documen-
ted and our experiments corroborated previous reports.7 The
previous reports of related reduction reactions being E-selective
were likely a consequence of rapid isomerization that can occur
under the conditions employed in previous studies.1d,e In con-
trast to previous reports, the conditions employed in our study
served a purpose of suppressing the Z to E isomerization.
Namely, minimal amount of the nucleophilic phosphine (1
equiv.) was used in the reaction together with the minimal
amount of water (1 equiv.). With increased amount of phos-
phine and increased amount of water, the competitive isomeri-
zation occurs at higher rates leading to diminished selectivities.
It became apparent that the reaction time is of crucial inter-
est if the Z-product was to be isolated with high selectivity.
Each reaction should be quenched at an opportune time point
to allow isolation of the desired Z-alkene in high yields (as a
consequence of high conversion of starting material) while
preventing significant isomerization to the E-product.
Monitoring the reactions by NMR spectroscopy allowed us to
decide on when to quench the reaction for each substrate. The
reactions are highly Z-selective, but the observed ratios of Z-
and E-products depends on the rate of isomerization which
was higher for the more electrophilic enones that undergo the
reversible conjugate addition of the nucleophilic catalysts at
higher rates resulting in faster isomerization. Consequently,
the reductions of p-methyl substituted ynone P-5 provide a
large window of reactions times where the product mixtures
feature high ratios of Z-and E-products Z-5 and E-5. In contrast
to this, p-chloro substituted ynone P-8 featured lower selecti-
vity for the Z-product because of the fast isomerization
Scheme 1 Previously reported phosphine-mediated and phosphine-cata-
lyzed reductions and difunctionalizations of electron deficient alkynes that
produce α,β-unsaturated alkenes and comparison to this work.
Table 1 Reduction of methyl 3-phenylpropiolate P-3 in 1,4-dioxane
and tri-n-butylphosphine, [P-3] = 125 mmol L−1
PBu3 H2O
Entry [eq.] [eq.] [°C] [min] of 3 [%]
T
t
NMR-yield E/Z
P-3
[%]
[mol%]
1
2
3
4
5
6
7
8
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0
70
70
70
70
70
70
40
50
60
80
90
300
30
30
30
30
30
30
30
30
30
30
14
1 : 1
11
5
2
2
2
2
3
4
4
4
3
4
1.0
2.0
3.0
4.0
6.0
1.0
1.0
1.0
1.0
1.0
1.0
95 (93)a
93
1 : 25 (1 : 25)a
1 : 17
95
95
93
83
1 : 14
1 : 13
1 : 11
1 : 25
86
1 : 25
9
87 (91)a
89
1 : 25 (1 : 25)a
1 : 25
10
11
12
90
90
1 : 17
1 : 14
100 30
a Yield and E/Z-ratio after column chromatography. NMR-yields were
determined by 1H NMR-spectroscopy with dimethylsulfone as an
internal standard.
When the water content in the reaction mixture was varied (Scheme 2).
between 1 and 6 equivalents, a clear trend emerged showing
Further evaluation of the reaction scope established that
that the minimum amount of water should be used if isolation this trend is general. Electron rich ynones such as P-12 and
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Scheme 2 1H NMR monitoring of the partial reductions of 2-ynoates to
corresponding 2-enoates. Conditions: 2-ynoate (1.00 equiv.), PBu3 (1.00
equiv.), H2O (1.00 equiv.), 60 °C, 0.60 mL 1,4-dioxane-d8. NMR-yield
■
Scheme 3 Scope of the reaction for various ynoates under the opti-
mized conditions. Isolated yields for Z-product are listed together with
the E/Z selectivities.
of 2-enoate in (Z)-configuration.
NMR yield of 2-enoate in (E)-
configuration.
Unconsumed 2-ynoate. E/Z-Ratios were determined
based on the intensity of integrals in relationship to internal standard
(dimethylsulfone).
consequence of the lower rates of isomerization, but it could
also be attributed to the higher steric demand of the dialkyla-
mides used in the study. Although electron poor propynoic
P-13 provided the Z-product with high selectivity and in good amides provided the reduction products in higher yield due to
yields. Electron poor substrates P-11, P-15, P-16 and P-17 were the higher reaction rates, the selectivity of the reduction of
converted to the corresponding alkenes in good yields but nitro-substituted P-32 remained low with 1 : 1.8, yet higher
with diminished selectivities. The reactions of alkyl substi- than for the corresponding ynoate (Scheme 4). Lower selectiv-
tuted ynoates led to competitive isomerization to the corres- ities observed with electron poor propynoic amides are consist-
ponding all-E-diene. Ynone P-19 also proved to be a competent ent with the hypothesis that higher rate of isomerization leads
substrate for the Z-selective reductions giving an 88% com- to diminished selectivity.
bined yield of Z- and E-products and ratio of 3.8 : 1 (Scheme 3).
Three possible reaction pathways are presented in
To further demonstrate the utility of this method, we inves- Scheme 5. It has been established that electron poor alkynes
tigated the reductions of propynoic amides. While expecting undergo conjugate addition of nucleophilic phosphines result-
lower reaction rates due to the generally lower electrophilicity ing in a zwitterionic enolate intermediate ii which can depro-
of propynoic amides, the focus was directed to how this will tonate sufficiently acidic protic additives, in this case water, to
influence the reaction selectivity and the rates of Z to E isomer- produce intermediate iii,1e,4,8 (see Fig. S1–S3 in the ESI† docu-
ization. Free propynoic amides did not produce any of the ment for calculated reaction profile). The vinylphosphonium
acrylamide products under the optimized reaction conditions. ion features possible electrophilic sites at C1, C2, C3 and the
In contrast to this, dialkyl propynoic amides were smoothly phosphorus atom. Since the ester hydrolysis products are not
reduced to the corresponding acrylamides in moderate yields. observed in appreciable amount, nucleophilic attack of the
The generally lower yields compared to the reaction of ynoates hydroxide at C1 is considered unlikely. This is also the case
were attributed to the lower reaction rates. Selectivities in the with attack at C3 as it is not clear how the resulting intermedi-
reactions of propynoic amides, on the other hand, proved to ate would lead to the reaction products. Hydroxide addition at
be more robust and at the same level or higher than those in C2 (path A) would result in formation of a phosphonium ylide
the reductions of the corresponding esters. This could be the vii. Similar transformations have been proposed in the related
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Scheme 4 Scope of the reaction for various propynoic amides under
the optimized conditions. Isolated yields for Z-product are listed
together with the E/Z selectivities.
Scheme 5 Mechanistic considerations of possible reaction pathways
for the partial reduction of ynoates to ynones. Stereochemical assign-
ments for intermediates are omitted for simplicity. (a) Representation of
possible pathways A, B and C. (b) Possible explanation for Z-selectivity of
the partial reduction.
phosphine catalyzed alkyne difunctionalization reactions.4,5
Upon proton transfer, intermediate vii would lead to the
betaine viii and subsequently to the oxaphosphetane ix which,
via elimination of phosphine oxide, produces the enoate finding is further supported by the reduction of the Mayer
product Z-3. Such intermediates have been proposed to be bond-order from 1.05 in the zwitterion – typical for a regular
involved in Wittig type olefinations.8a and are also invoked in single bond – to merely 0.51. The intermediate iv and its
phosphine promoted reductions of epoxides that require deprotonated form v could lead to the oxaphosphetane inter-
heating to high temperatures and presumably feature high mediate x (path C). We considered this pathway unlikely as it
kinetic barriers.9 In agreement with the literature,9 the per- involves 4-endo-trig type cyclization. Instead, P-oxophosphorane
formed quantum chemical calculations predict high energy v could fragment to produce phosphine oxide and the vinyl
barriers to be involved in the formation of Z-3 along path A anion vi which is rapidly protonated to afford the enone Z-3.
(see Fig. S11†). Overall, water addition to the zwitterion Such fragmentation is supported by the simulations which
appears to be the rate-limiting step in this proposed pathway. clearly reveal the weakening of the P-C2 (single) bond in v, thus
The computational results are in line with the experimental initiating the complete cleavage of the P-C2 bond to form vi
finding and show that the reduction of P-3 using tributyl- (Fig. S15 in the ESI†). This mechanism has been invoked in
phosphine is highly Z-selective, as three of four investigated recently reported substitutions of phosphonium salts10 and
channels yield Z-3. Detailed information regarding the compu- studied in detail in the pioneering work of Byrne and Gilheany
tational results assessing the thermodynamics of path A are on hydrolysis and alcoholysis of phosphonium ylides.11 Our pre-
presented in the ESI.†
liminary computational results indicate that this fragmentation
The vinylphosphonium intermediate could also undergo occurs with a low kinetic barrier which favors path B as a
nucleophilic attack of the hydroxide ion on the phosphorus mechanism of the reduction over paths A and C. Direct SN2-type
atom resulting in the pentavalent P-hydroxyphosphorane iv substitution at the phosphorus atom of intermediate v to gene-
(paths B and C). Interestingly, the quantum chemical simu- rate the vinyl anion was excluded in the light of previous
lations reveal a considerable destabilization of the P-C2 single studies.10h,12
bond upon deprotonation (iv to v), which is evident from the
The control of stereoselectivity in these reactions is then
elongation of this bond from 1.895 (iv) to 2.234 Å (v). This reduced to the question of stereoselectivity in the formation of
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vinylphosphonium intermediate iii or the P-hydroxyphosphorane EFRE program. We thank Dr Peter Bellstedt for his support
iv. There is some evidence that addition of phosphine to ynoates with NMR, Dr Nico Ueberschaar for MS analysis and Martin
followed by protonation can be highly selective.13 Alternatively, Schröder for preliminary quantum chemical simulations.
solvation of allenic intermediate ii may be controlled by electro-
static interactions which would bring water molecules in proxi-
mity of the phosphonium salt (Scheme 5b). This would set the
stage for a fast, consecutive deprotonation of water and the
Notes and references
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reaction mixture.
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