Highly chemoselective calcium-catalyzed propargylic deoxygenation

Article abstract of DOI:10.1002/chem.201103691

A calcium-catalyzed direct reduction of propargylic alcohols and ethers has been accomplished by using triethylsilane as a nucleophilic hydride source. At room temperature a variety of secondary propargylic alcohols was deoxygenated to the corresponding h

Full text of DOI:10.1002/chem.201103691

FULL PAPER
DOI: 10.1002/chem.201103691
Highly Chemoselective Calcium-Catalyzed Propargylic Deoxygenation
Vera J. Meyer and Meike Niggemann*[a]
Abstract: A calcium-catalyzed direct
reduction of propargylic alcohols and
ethers has been accomplished by using
triethylsilane as a nucleophilic hydride
source. At room temperature a variety
of secondary propargylic alcohols was
deoxygenated to the corresponding hy-
drocarbons in excellent yields. Further-
more, for the first time, a catalytic de-
oxygenation of tertiary propargylic al-
cohols was generally applicable. The
same protocol was suitable for an effi-
cient reduction of secondary as well as
tertiary propargylic methyl, benzyl and
allyl ethers. Substrates containing an
additional keto-, ester or secondary hy-
droxyl function were reduced with ex-
ceptional chemoselectivity at the pro-
pargylic position.
Keywords: alcohols · calcium · che-
moselectivity · direct deoxygena-
tion · reduction
Introduction
protocol is applicable in only few cases for secondary pro-
pargylic alcohols[6] and essentially one example is known for
a tertiary propargylic alcohol.[7] Substitution reactions with a
nucleophilic reductant are hampered by the poor leaving-
group character of the hydroxyl moiety. Therefore, over-stoi-
chiometric amounts of Brønsted acids or Lewis acids such as
trifluoroacetic acid (TFA) or BF3·Et2O are generally neces-
sary for a direct deoxygenation of the alcohol.[4,8] For cases
in which a selective reaction is compulsory, protection of the
alkyne with cobalt octacarbonyl is often inevitable.[9] To our
knowledge only three protocols have been described for a
direct nucleophilic reduction of propargylic alcohols.[10,11]
All three reactions are severely limited in substrate scope,
and only secondary propargylic alcohols are deoxygenated.
This deficiency in development might be attributed to two
main obstacles that are related to the reduction of propar-
gylic alcohols. First, a competing reduction of the alkyne
moiety itself, before or after the deoxygenation, is likely to
diminish the yield of the desired product.[12] In addition, it is
generally difficult to achieve selective substitution reactions
of propargylic alcohols and their derivatives with transition-
metal catalysts.[13] The reactive species in many of these re-
actions is assumed to be either a metal allenylidene complex
2 or a metal propargylic intermediate 3 (Scheme 1). Inter-
Within different areas of organic chemistry a frequent final
transformation toward a target molecule is the removal of a
specific hydroxyl moiety, which is a necessary functional
handle at an earlier stage. Due to the high degree of molec-
ular complexity in such advanced substrates, compatible de-
oxygenation processes must be highly selective and the tol-
erance of various functional groups is a prerequisite. The
radical substitution of xanthenes and thiocarbonates follow-
ing the Barton–McCombie protocol (and variations thereof)
is a versatile and widely employed method for the removal
of primary and secondary alcohols.[1,2] However, the deoxy-
genation of tertiary alcohols remains challenging even nowa-
days, due to the poor stability of their thionocarbonyl deriv-
atives, which often results in undesired rearrangement and
elimination reactions. Therefore, only few procedures have
been described.[3] Moreover, from an environmental point of
view, the development of mild and catalytic methods for the
direct deoxygenation is highly desirable, so that the stoichio-
metric derivatization of the hydroxyl moiety with an activat-
ing group is no longer required. Even though some proce-
dures for the direct reduction of alcohols have been de-
scribed, several problems remain to be solved, such as limit-
ed substrate scope and low functional group tolerance, as
most procedures suffer from the necessity of drastic reaction
conditions.[2,4,5]
The reduction of one class of alcohols, the propargylic al-
cohols, is particularly problematic. The Barton–McCombie
[a] V. J. Meyer, Prof. Dr. M. Niggemann
Institute for Organic Chemistry
RWTH Aachen University
Landoltweg 1, 52074 Aachen (Germany)
Fax : (+49)241-8092127
E-mail: niggemann@oc.rwth-aachen.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201103691.
Scheme 1. Mechanistic pathways of propargylic substitution catalyzed by
transition-metal (TM) or Lewis acidic/Brønsted acidic (LA/BA) catalysts.
Chem. Eur. J. 2012, 18, 4687 – 4691
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Table 2. Optimization of the reaction conditions for tertiary alcohols.
conversion of these two organometallic species results in the
predominant formation of the allene derivative 5. By con-
trast, Lewis- or Brønsted acid-catalyzed reactions proceed
via the intermediary formation of carbocation 4,[14] which
leads effectively to the propargylic substitution product 6.
Hence, a preferentially non-transition-metal Lewis acid-cat-
alyzed direct reduction of a propargylic alcohol should yield
the desired product with high selectivity for the acetylene.
Surprisingly, no such protocol has hitherto been described.
Entry[a] Additive
([mol%])
Solvent 14a/14b
t
Yield[b]
[%]
AHCTUNGTRENNUNG
1
2[c]
3[d]
4
5
6
7
8
9
Bu4NPF6 (5)
Bu4NPF6 (5)
Bu4NPF6 (5)
Bu4NBF4 (5)
Bu4NSbF6 (5)
CH2Cl2 1:1.8
CH2Cl2 1:2
1 h
1 h
1 h
1 h
1 h
5 h
4 h
1 h
1 h
5 h
1 h
1 h
74
80
42
75
83
82
88
58
71
30
84
67
CH2Cl2 1:1.8
CH2Cl2 1:2.4
CH2Cl2 1:1.1
PhMe2NH+B
PhMe2NH+B
(C6F5)4À (5) CH2Cl2 1:0.1
ACHTUNGTRENNUNG
(C6F5)4À (5) DCE
1:0.1
toluene 1:8.6
Results and Discussion
Bu4NSbF6 (5)
Bu4NSbF6 (5)
Bu4NSbF6 (5)
Bu4NSbF6 (5)
Bu4NSbF6 (5)
Bu4NBF4 (5)
Bu4NPF6 (5)
DCE
DCE
CHCl3
1:0.7
1:0.6
1:5.5
Encouraged by our recently published results on propargylic
substitution reactions catalyzed by a highly efficacious
Lewis acidic calcium catalyst under very mild reaction con-
ditions,[15] we set out to investigate the suitability of our cat-
alyst system for the deoxygenation of propargylic alcohols.
We were pleased to find that the reduction of a variety of
secondary propargylic alcohols proceeded smoothly at room
10[e]
11
12
13
14
MeNO2 1:0.1
MeNO2 1:0.1
MeNO2 1:0.1
10 min 66
5 min 71
[a] Additive and CaACTHUNTRGNE(UNG NTf2)2 were added at room temperature to alcohol
13 (0.5 mmol) and Et3SiH (1.5 mmol) in solvent (1 mL) and stirred for
the time indicated. [b] Isolated product yield of the mixture of 14a/14b.
[c] 5 equivalents of Et3SiH. [d] 10 equivalents of Et3SiH. [e] Reaction at
08C. DCE=1,2-dichloroethene.
temperature in the presence of CaACHTNUTRGNE(UNG NTf2)2 (5 mol%) and
Bu4NPF6 (5 mol%) in dichloromethane. An excess of of
triethylsilane (three equivalents) was used as an inexpensive
and benign hydride source (Table 1). A range of different
This undesired elimination reaction was an insurmountable
hurdle in ruthenium-catalyzed propargylic reductions.[11]
Increasing the excess of the hydride source to 5 and 10
equivalents did not affect the ratio of reduction versus elimi-
nation product (Table 2, entries 2 and 3), and a high concen-
tration of triethylsilane even led to a decrease in overall
conversion (entry 3). A screening of different additives re-
vealed their major impact on the outcome of the reaction.
In the presence of Bu4NSbF6 (Table 2, entry 5) a moderate
amelioration of both the yield and the selectivity for the de-
sired product were observed, whereas Bu4NBF4 (Table 2,
entry 4) did not enhance the results. Even though N,N-di-
methylanilinium tetra(pentafluorophenyl)borate (Table 2,
entries 6 and 7) slowed down the reaction rate significantly,
from 1 to 5 h, the desired product 14a was obtained in ex-
cellent yield and selectivity. Hence, a first set of optimized
reaction conditions was found. Owing to the high cost of
this rather particular additive we continued the optimization
process. A solvent screening was performed with Bu4NSbF6
as the additive, revealing that reaction in nitromethane gave
excellent results in terms of selectivity, in a reaction time of
just one hour and with an only slightly diminished yield,
compared with the reaction in presence of N,N-dimethylani-
linium tetra(pentafluorophenyl)borate. A re-screen of the
different additives in nitromethane revealed a second set of
optimized reaction conditions for tertiary propargylic alco-
hols. In presence of the original additive, Bu4NPF6, the de-
sired product is obtained in 71% yield after a reaction time
of only 5 min (Table 2, entry 14). With the optimized reac-
tion conditions in hand, a series of differently substituted
tertiary propargylic alcohols were reduced by using both
sets of optimized reaction conditions (Table 3).
Table 1. Propargylic reduction of secondary alcohols.
Entry[a]
R
R1
Product
t
Yield[b]
[%]
AHCTUNGTERG[NNUN min]
1
2
3
Ph
Ph
Ph
Ph
7
8
9
10
5
69
98
85
pMeOC6H4
p-ClC6H4
5
4
Ph
10
5
84
5
6
n-pentyl
Ph
Ph
11
12
5
68
68
10
[a] Bu4NPF6 (5 mol%) and CaACHTNUTRGNE(UNG NTf2)2 (5 mol%) were added at room
temperature to the alcohol (0.5 mmol) and Et3SiH (1.5 mmol) in CH2Cl2
(1 mL) and stirred for the time indicated. [b] Isolated product yield.
secondary propargylic alcohols, were reduced in reaction
times of just 5 to 10 min to give the desired products in
good to excellent yields. Interestingly, little impact of the
electronic properties of the propargylic substituent on the
cation formation was observed; similar results were ob-
tained in the presence of both electron-donating substitu-
ents, such as methoxy or methylenedioxy, and electron-with-
drawing chloro substituents on the propargylic benzene ring.
However, when we turned our attention to the reduction
of tertiary propargylic alcohols, starting with compound 13
as a model substrate, the desired product 14a was obtained
in disappointing 27% yield and the enyne 14b, isolated in
47% yield, was identified as the major product (Table 2).
4688
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Chem. Eur. J. 2012, 18, 4687 – 4691

Highly Chemoselective Calcium-Catalyzed Propargylic Deoxygenation
Table 3. Propargylic reduction of tertiary alcohols.
FULL PAPER
extension of our new methodol-
ogy. Hence we tested the first
direct reduction of methyl-,
allyl- and benzyl propargylic
ethers. Secondary propargylic
ethers 34–36 reacted smoothly
under the standard reaction
Entry[a]
Alcohol
Product
t
Method
Yield[b]
[%]
5 min
15 h
A[c]
B[d]
70
68
1
2
3
4
15
17
19
21
16
18
20
22
5 min
1 h
A
B
74
31
conditions,
with
Bu4NPF6
(5 mol%) as the additive in di-
chloromethane (Table 4).
1 h
20 h
A
B
70
52
The attempted reduction of
the tertiary ethers 37–39 under
the same reaction conditions
once again gave the enyne 14b
as the major product. However,
the same modifications of the
reaction conditions that led to
an efficient reduction of tertiary
alcohols allowed for a success-
ful deetherification of tertiary
propargylic ethers 37–39.
5 min
15 h
A
B
66
69
5 min
6 h
A
B
73
68
5
6
7
8
23
25
27
29
24
26
28
30
5 min
18 h
A
B
76
83
5 min
6 h
A
B
52
52
Triethylsilane-mediated
re-
duction of different carbonyl
compounds has been described
under various reaction condi-
tions.[16] Therefore, the chemo-
selectivity of the newly devel-
oped calcium-catalyzed propar-
gylic deoxygenation was of
5 min
6 h
A
B
65
69
[a] Ca
solvent (1 mL) and stirred for the time indicated. [b] Isolated product yield. [c] Reaction conditions A:
Bu4NPF6 (5 mol%) in MeNO2. [d] Reaction conditions B: PhMe2NHB(C6F5)4 (5 mol%) in DCE.
ACHTUNGTRENNUNG(NTf2)2 (5 mol%) was added at room temperature to the alcohol (0.5 mmol) and Et3SiH (1.5 mmol) in
AHCTUNGTRENNUNG
From previous studies it was known that calcium-cata-
lyzed propargylic substitution reactions are generally accom-
panied by a reversible side reaction. Instead of the desired
nucleophile a molecule of the starting material 31 adds to
the previously formed carbocationic intermediate, owing to
the nucleophilicity of the hydroxyl moiety. Conversion of
the thus formed ether 33 to the final product might occur
through direct attack, or after the addition of a molecule of
water resulting in the regeneration of propargylic alcohol 31
followed by the regular substitution reaction (Scheme 2).
Encouraged by the fact that intermediary-formed ethers
are apparently suitable substrates for calcium-catalyzed
propargylic substitution reactions, the direct reduction of
propargylic ethers was investigated as a synthetically useful
Table 4. Deoxygenation of propargylic ethers.
Entry[a]
Ether
Product
t
Method Yield[b]
[%]
1[a]
34
35
7
7
15 min
15 min
46
56
2[a]
3[a]
36
7
2 h
81
1.5 h
5 h
A[c]
B[d]
59
33
4
37 14a
38 14a
5 min
5 h
A
B
84
77
5
5 min
1 h
A
B
77
66
6
39 14a
[a] Bu4NPF6 (5 mol%) and CaACHTNUTRGNE(UNG NTf2)2 (5 mol%) were added at room
temperature to the ether (0.5 mmol) and Et3SiH (1.5 mmol) in CH2Cl2
(1 mL) and stirred for the time indicated. [b] Isolated product yield.
[c] Reaction conditions A: Bu4NPF6 (5 mol%) in MeNO2. [d] Reaction
Scheme 2. Intermediary formation of self-condensation product 33.
conditions B: PhMe2NHBACHTGUNTERNNU(G C6F5)4 (5 mol%) in DCE.
Chem. Eur. J. 2012, 18, 4687 – 4691
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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4689

M. Niggemann and V.J. Meyer
major interest to us. Hence, different substrates containing
both an aldehyde or a ketone moiety and a propargylic alco-
hol were synthesized and submitted to the reaction condi-
tions (Table 5).
conditions. The primary alcohol in substrate 52 inhibited the
catalyst at room temperature. The reaction proceeded at
508C under microwave conditions, albeit it was rather slug-
gish and the desired product could be isolated in a poor
yield of just 44%.
In summary, we have devel-
oped the first calcium-catalyzed
Table 5. Deoxygenation in the presence of functional groups.
Entry
Alcohol
Product
t
Yield[b]
[%]
direct deoxygenation of secon-
dary as well as tertiary propar-
gylic alcohols. The correspond-
ing hydrocarbons are obtained
under very mild reaction condi-
tions and typical reactions are
completed in reaction times of
5 min to a few hours at room
temperature. Triethylsilane is
used as a convenient, mild, and
inexpensive stoichiometric re-
ductant. As anticipated, this
propargylic reduction, that is, to
our knowledge, the first Lewis
acid-catalyzed reaction of this
type, yields the desired product
with high selectivity and no for-
mation of allene byproducts
was observed. In addition, the
reduction proceeds with extra-
ordinary high chemoselectivity
and carbonyl functions, such as
ketone and ester moieties as
well as additional hydroxyl
groups, remain intact. The re-
markable functional group tol-
erance together with the very
mild reaction conditions and
AHCTUNGTERG[NNUN min]
1[a]
40
42
44
41
43
45
5
68
72
76
2[a]
30
10
3[a]
4[a]
46
48
47
49
5
78
72
5[a]
30
6[c]
50
52
51
53
15
15
72
44
7[d]
[a] Bu4NPF6 (5 mol%) and CaACHTNUTRGNEUGN(NTf2)2 (5 mol%) were added at room temperature to the alcohol (0.5 mmol)
and Et3SiH (1.5 mmol) in CH2Cl2 (1 mL) and stirred for the time indicated. [b] Isolated product yield.
[c] Reaction in MeNO2. [d] Reaction at 508C under microwave conditions.
Different substrates containing benzylic (42) and non-ben-
zylic ketone moieties (44 and 46), were selectively trans-
formed into the desired deoxygenated products with the car-
bonyl function remaining intact. Particularly remarkable is
the high selectivity for the reduction of the alcohol in pres-
ence of the benzylic ketone in 42, as reduction of such ke-
tones with triethylsilanes under acidic conditions is a stan-
dard procedure in organic synthesis. The same excellent che-
moselectivity was observed for propargylic reduction in the
presence of a methyl ester such as in 48. Solely, compound
40 containing a benzylic aldehyde yielded the partially re-
duced compound 41 along with a series of unidentified side
products. A second chemoselectivity study was performed
using substrates 50 and 52 bearing an additional hydroxyl
function. Under the standard reaction conditions, the reduc-
tion of compound 50 proceeded cleanly but ceased at about
40% conversion, probably due to a partial poisoning of the
calcium catalyst by the secondary hydroxyl function. Never-
theless, changing the solvent to nitromethane proved once
more beneficial and the desired product 51 was isolated in
72% yield. Again, no reduction of the secondary hydroxyl
function was observed under these slightly modified reaction
the short reaction times renders our new protocol highly
suitable for an application in the late stages of the synthesis
of a complex molecular target. Furthermore, the same reac-
tion system was applicable to the first direct deetherification
of propargylic ethers to the corresponding acetylenes.
Experimental Section
General procedure: The alcohol (0.5 mmol) and the organosilane
(1.5 mmol) were dissolved in solvent (1 mL). Next, the additive
(5 mol%) and CaACHTNUTRGNEUNG(NTf2)2 (5 mol%) were added at room temperature and
stirred until conversion of the alcohol was complete (monitored by TLC
and/or GC). The product was isolated by adding sat. NaHCO3 solution
(5 mL), extracting the aqueous phase with dichloromethane, and the
combined organic phases were then dried over Na2SO4 and concentrated
in vacuo. The crude product was purified by column chromatography.
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FULL PAPER
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Received: November 23, 2011
Published online: February 29, 2012
Chem. Eur. J. 2012, 18, 4687 – 4691
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www.chemeurj.org
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