Exploring bis-(amino)cyclopropenylidene as a non-covalent Br?nsted base catalyst in conjugate addition reactions

Article abstract of DOI:10.1039/c7ob02882b

Bis-(amino)cyclopropenylidene has been utilised as a non-covalent Br?nsted base catalyst in the 1,6-conjugate addition of carbon nucleophiles to p-QMs. This protocol makes it possible to access unsymmetrical diaryl- and triarylmethanes in good to excellen

Full text of DOI:10.1039/c7ob02882b

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Exploring bis-(amino)cyclopropenylidene as a non-covalent
Brønsted base catalyst in conjugate addition reactions
Received 00th January 20xx,
Accepted 00th January 20xx
Gurdeep Singh, Prithwish Goswami and Ramasamy Vijaya Anand*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Bis-(amino)cyclopropenylidene has been utilised as a non-covalent cyclopropenylidene based carbenes have attracted the
Brønsted base catalyst in the 1,6-conjugate addition of carbon attention as these carbenes are reasonably stable, and also the
nucleophiles to p-QMs. This protocol allows to access reactivity is comparable with that of NHCs.15 Until the isolation
unsymmetrical diaryl- and triarylmethanes in good to excellent of stable bis-(diisopropylamino)cyclopropenylidene by
yields. Further, this catalyst was also explored in 1,4-conjugate Bertrand and co-workers in 2006,16 the chemistry of
addition of carbon nucleophiles to enone systems.
bis(amino)cyclopropenylidenes (BACs) was recognised only by
the organometallic chemists.17 However, after this pioneering
report,16 BAC has been explored as a nucleophilic catalyst in
few organocatalytic transformations18 such as benzoin
condensation,19 aza-benzoin reaction20 and Stetter reaction.21
Recently, we also utilised BAC as a nucleophilic catalyst for the
1,6-conjugate addition of umpolung of aldehydes to p-quinone
methides.22 However, surprisingly, no report is available in the
literature, where BAC has been utilised as a Brønsted base
In recent years, N-Heterocyclic carbene (NHC) catalysis has
been deliberated as one of the most powerful strategies to
effect different organic transformations due to the different
modes of activation of NHCs toward a wide range of functional
groups.1 Primarily, the organocatalytic behaviour of NHCs has
been recognised in the reactions involving umpolung of
aldehydes and also a few Michael acceptors.2 Moreover, NHCs
have been utilised as a nucleophilic catalyst in other important
transformations such as polymerisation3 Morita-Baylis-Hillman
reaction,4 etc. Recently, the catalytic utility of N-Heterocyclic
carbene, as a Brønsted base,5 has been uncovered in a few
enantioselective transformations6 and other transformations
such as, transesterification reaction,7 intra- and inter-
molecular Michael,8 oxa-Michael,9 aza-Michael10 and phospha-
Michael reactions.11 Apart from these, NHC has also been used
as a Brønsted base catalyst in silyl enol ether formation12 and
intramolecular ring opening of isochromene derivatives.13
Recently, we have also developed 1,6-conjugate addition of
dialkylphosphites and 2-naphthols to p-quinone methides (p-
QMs) to access unsymmetrical diarylmethyl phosphonates14a
and triarylmethanes,14b respectively using NHC as a Brønsted
base catalyst.
catalyst. Bertrand and co-workers predicted, based on the
donation ability of the amino groups and the carbene bond
angle (57.2 ), that BAC could act as an extremely strong
donor.23 Since
-donicity has a linear relationship with basicity,
π-
°
σ-
σ
one can expect that the carbene centre of BAC could indeed
act as a Brønsted base. In fact, this is the case with NHCs as
well, and therefore, as discussed earlier, many transformations
involving Brønsted base catalysis have been explored using
NHC as a base catalyst. In line with this concept, while
exploring p-quinone methides (p-QMs)24 as 1,6-acceptors,25 we
envisaged that BAC could act as non-covalent Brønsted base in
conjugate addition reactions using appropriate nucleophiles
having an acidic proton (Scheme 1). Herein, we report 1,6- and
1,4-conjugate addition of carbon nucleophiles using BAC as a
Brønsted base catalyst.
In recent years, there has been a growing interest among the
synthetic community to synthesise other non-heterocyclic
based carbenes in stable form. In this circumstance,
Scheme 1. Our hypothesis
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Table 1. Optimisation studiesa
absence of BAC pre-catalyst, 3a was obtained only in 15%
yield, indicating a catalyst is actually required to drive this
transformation. To show the scalability of this method, an
experiment was performed in 0.5 g scale of 1a using 5 mol%
each of 11 and Cs2CO3 in THF and the product 3a was obtained
in 90% yield after 6 h.
With the optimised conditions in hand, the scope of reaction
was investigated with a variety of p-QMs (Table 2). It is evident
from Table 2, in most of the cases, the respective adducts
were obtained in good to excellent yield within a short span of
time. This protocol was found to be very effective in the cases
of p-QMs (1b-e) derived from electron rich aldehydes, and the
desired products (3b-e) were obtained in excellent yields
Entry
Catalyst
Base
Solvent
Time [h]
Yieid [%]
1
2
4
5
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
K2CO3
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
DCM
Et2O
DCE
DMSO
THF
THF
5
5
50
54
57
61
63
65
75
94
77
81
80
79
72
75
77
15
90
3
6
5
4
7
5
5
8
4
(
≥
90%). In the cases of p-QMs, derived from a simple
benzaldehyde (1f) and 4-tert-butyl-benzaldehyde (1g), the
corresponding products (3f 3g) were obtained in 94% and
93% yields respectively. The haloarenes substituted p-QMs
1h–l) also underwent smooth conversion to the adducts 3h-l
6
9
4
7
10
11
12
11
11
11
11
11
11
-
1
&
8
1
9
1
(
10
11
12
13
14
15
16
17b
1
in excellent yields (91-95%). The p-QM (1m), substituted with
electron-poor arene, provided 3m in 89% yield. However, the
p-QM (1n), derived from 1-naphtaldehyde, gave the adduct 3n
only in 79% isolated yield. Surprisingly, p-QM (1o), substituted
with sterically bulky pyrene unit, furnished the corresponding
product 3o in excellent yield (91%).
DBU
1
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
2
2
2
1.5
12
6
11
a Reaction Conditions: All reactions were carried out with 1a (0.062 mmol) and 2a
Table 2. Substrate scope with p-QMsa
b
(0.074 mmol) in 1 mL of solvent. Reaction was carried out in 0.5 g scale of 1a
using 5 mol% each of 11 and Cs2CO3. Yields reported are isolated yields.
To optimise the reaction conditions, we initiated our
investigation using readily available p-QM (1a) and diethyl
malonate (2a) in the presence of a wide range of carbene
precursors (4-12), and the results are summarised in Table 1.
To our delight, our initial attempt itself, using 10 mol% of
triazolium salt
4 and Cs2CO3 as a base in THF, gave the positive
result; however, the yield of isolated 3a was not so impressive
(only 50%) [Entry 1]. Further screening using different NHC
pre-catalysts (5-9) did not help to a great extent, as 3a was
obtained in the range of 54-65% yield in all those cases (Entries
2-6). However, when a cyclopropenylidene based carbene
precursor 10 was used as a catalyst, 3a was isolated in 75%
yield (Entry 7). Further optimisation experiments were carried
out using other BAC precursors 11
all the three BAC precursors
(diisopropylamino)cyclopropenylidene precursor
& 12 (Entries 8 & 9). Among
(
10
-12),
bis-
was
(
11
)
found to be the best one as, in this case, 3a was obtained at
the maximum of 94% isolated yield in an hour (Entry 8). Then,
we went on to optimise other parameters such as base and
solvent. Though bases such as K2CO3 and DBU were found to
be effective to drive this transformation (Entries 10 & 11), the
yield of 3a was relatively inferior (around 80%). To find a
suitable solvent, a few experiments were carried out in
solvents such as CH2Cl2, Et2O, DCE and DMSO (Entries 12-15).
a Reaction Conditions: All reactions were carried out in 20 mg scale of 1(b-o) in 1
mL of THF. b The reaction time was 1.5 h. Yields reported are Isolated yields.
However, in all those cases, 3a was obtained only in the range Later, we turned our attention to investigate the scope of
of 72-79% yield. This clearly indicates that THF is the most different carbon nucleophiles under the standard reaction
suitable solvent for this transformation (Entry 8). In the
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Table 3 Substrate scope with different carbon nucleophilesa
78% isolated yield; however, the reaction took a long time to
complete (24 h). In another experiment, 1a was treated with
14a in the presence of 10 mol% of Cs2CO3 without 11 in THF at
rt and, in this case, 15a was observed only in traces even after
24 h. This clearly indicates that the catalyst 11 is actually
required for this transformation. Further elaboration of the
OH
O
tBu
tBu
tBu
tBu
11 (10 mol%)
Cs2CO3 (10 mol%)
+
EWG
EWG
EWG
THF, rt, 1 h
1a
2b-e
13a-d
EWG
MeO
----------------------------------------------------------------------------------------------------------------------------------------------
OH
OMe
substrate scope using different p-QMs (1b
naphthols (14a 14b) and, in all those cases, the expected
triarylmethane derivatives (15b ) were obtained in moderate
, 1c and 1g) and 2-
OH
tBu
tBu
tBu
tBu
&
-
e
COOEt
X
yields (71-75%). Unfortunately, phenol (14c) failed to react
with p-QM under the reaction conditions.
13c, Y = Ac, 95% (dr = 1:1.3)c
13a, X = Ac, 94%
13b, X = CN, 98%b
Y
13d, Y = CN, 91% (dr = 1:1)c
X
MeO
MeO
a Reaction Conditions: All reactions were carried out with 0.062 mmol of 1a in 1 To show the generality of this methodology, the same catalytic
mL of THF. Reaction time was 40 min. c Diastereomeric ratios were determined
b
system was also extended for the 1,4-conjugate addition of
from 1H NMR analysis of the crude reaction mixture. Yields reported are isolated
diethyl malonate to different enone systems (Table 5). Initially,
yields.
the chalcone 16a was treated with 10 mol% of Cs2CO3 without
the catalyst 11 in THF. However, in this case, the expected 1,4-
conditions, and the results are summarised in Table 3. As
adduct 17a was obtained only in 15% yield after 24 h.
expected, this method worked very well with different
Interestingly, when the same reaction was carried out in the
symmetrical active methylene compounds, such as
presence of 10 mol% of 11 and Cs2CO3 in THF, 17a was isolated
dimethylmalonate (2b) and malononitrile (2c), and the desired
in 89% yield in 24 h (Table 5). This observation clearly
products 13a
& 13b were obtained in 94 and 98% isolated
demonstrates that the BAC catalyst is actually required to
drive this transformation. Consequently, further substrate
yields, respectively. Reaction of 1a with ethylacetoacetate (2d
)
provided the desired product 13c as a mixture of diasteromers
(dr = 1:1.3) in 95% yield. In the case of ethyl 2-cyanoacetate
scope was investigated using different chalcones (16b
this reaction conditions. It is evident from Table 5 that the
chalcones (16b ), derived from substituted benzaldehydes and
acetophenone, furnished the corresponding 1,4-adducts 17b
in the range of 81-98 % isolated yields. Other chalcones (16k
-n) under
(
2e), the product 13d was isolated in 91% yield with the
-j
diastereomeric ratio of 1:1.
-
j
-
After successfully exploring the active methylene compounds
as nucleophiles, we were interested to investigate the
applicability of this protocol for other nucleophiles having an
acidic proton such as 2-naphthol and phenols under optimised
reaction conditions. When 1a was reacted with 2-naphthol
(14a) using 10 mol% of 11 and Cs2CO3 in THF at rt, the
expected unsymmetrical triarylmethane 15a was obtained in
m
), derived from substituted acetophenones and 4-chloro-
benzaldehyde, also underwent 1,4-addition reaction and
generated the subsequent 1,4-adducts 17k in good yields
(74-79%). The chalcone 16n), derived from 3-bromo-
-
m
(
thiophene-2-carboxaldehyde and acetophenone, also reacted
under the reaction condition and gave the product 17n in 94%.
Table 4 Substrate scope with 2-naphtholsa
Table 5 Substrate scope with different chalconesa
tBu
HO
11 (10 mol%)
tBu
O
R
HO
Cs2CO3 (10 mol%)
tBu
+
R1
THF, rt, 24 h
HO
R
tBu
R1
15a-f
14a-c (1.2 equiv)
1a-c & 1g (1 equiv)
--------------------------------------------------------------------------------------------------------------------------------------
tBu tBu tBu
OMe
HO
OMe
HO
tBu
HO
tBu
tBu
tBu
OMe
HO
HO
HO
15a, 24 h, 78%
15c, 24 h, 73%
15b, 24 h, 74%
tBu
OH
tBu
MeO
OMe
HO
tBu
OMe
tBu
tBu
HO
tBu
HO
HO
OH
Br
MeO
15d, 24 h, 71%
a Reaction Conditions: All reactions were carried out with 20 mg scale of 16(a-n)
15e, 24 h, 75%
15f, 24 h, 0%
in 1 mL of THF. Yields reported are isolated yield.
To further demonstrate the synthetic application of this
protocol, one of the products 13a was treated with hydrazine
and sodium carbonate in refluxing ethanol for 15 h, and as
a Reaction Conditions: All reactions were carried out with 20 mg scale of 1(a-c &
1c) in 1 mL of THF. Yields reported are Isolated yields.
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We have demonstrated, for the first time, the catalytic
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a non-
covalent Brønsted Base in 1,4- and 1,6-conjugate addition
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(EMR/2015/001759) for the financial support and IISER Mohali
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Organic & Biomolecular Chemistry
Page 6 of 6
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DOI: 10.1039/C7OB02882B
Exploring bis-(amino)cyclopropenylidene as a non-covalent Brønsted base
catalyst in conjugate addition reactions
Gurdeep Singh, Prithwish Goswami and Ramasamy Vijaya Anand*
NR2
R2N
Nu
R2
R1
R
Ar
if E = p-QM
if E = Enone
H
O
Nu
HO
E
Nu
R
Nu-H = H2C(CO2R)2, 2-naphthols
25 examples, 71-98% yields
Nu-H = H2C(CO2R)2
14 examples, 74-98% yields
Bronsted base
activation
The catalytic application of Bis-(amino)cyclopropenylidene, as a non-covalent Brønsted base, in the
1,4- and 1,6-conjugate addition of carbon nucleophiles to enones and p-quinone methides,
respectively, is described.