N-Heterocyclic Carbene Catalyzed 1,6-Conjugate Addition of Me3Si-CN to para-Quinone Methides and Fuchsones: Access to α-Arylated Nitriles

Article abstract of DOI:10.1021/acs.orglett.7b00508

An organocatalytic approach toward α-arylated nitriles using N-heterocyclic carbene (NHC) as a catalyst is described. This protocol comprises an NHC catalyzed activation of Me₃Si-CN followed by 1,6-conjugate addition of cyanide to para-quinone methides (p-QMs) and fuchsones leading to α-diaryl- and α-triaryl nitriles in good to excellent yields.

Full text of DOI:10.1021/acs.orglett.7b00508

Letter
pubs.acs.org/OrgLett
N‑Heterocyclic Carbene Catalyzed 1,6-Conjugate Addition of Me3Si-
CN to para-Quinone Methides and Fuchsones: Access to α‑Arylated
Nitriles
Prithwish Goswami, Gurdeep Singh, and Ramasamy Vijaya Anand*
Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Mohali, Sector 81, Knowledge City, S.
A. S. Nagar, Manauli (PO), Punjab - 140306, India
S
* Supporting Information
ABSTRACT: An organocatalytic approach toward α-arylated
nitriles using N-heterocyclic carbene (NHC) as a catalyst is
described. This protocol comprises an NHC catalyzed
activation of Me3Si-CN followed by 1,6-conjugate addition
of cyanide to para-quinone methides (p-QMs) and fuchsones
leading to α-diaryl- and α-triaryl nitriles in good to excellent
yields.
α-Aryl nitriles have emerged as an essential architectural motif,
often found in many pharmaceuticals and biologically active
natural molecules.1 Specifically, many of the α-diaryl and α-triaryl
nitriles possess interesting biological and therapeutic properties
(Figure 1). For example, darotropium bromide (1) has been
metal or ammonium cyanides5 and dehydration of aldoximes/
amides.6 Alternatively, α-aryl nitriles could be accessed through a
Lewis acid catalyzed addition of trimethylsilyl cyanide (Me3Si-
CN) to diarylcarbinols and their derivatives.7 In the recent past,
another fascinating strategy entailing a transition metal catalyzed
coupling of benzyl cyanides with an appropriate aryl coupling
partner has been realized.8 Besides, some metal-free approaches
have been recognized for the synthesis of α-aryl nitriles.9 In
general, most of the hitherto known methods involve either the
use of expensive metal catalysts or harsh reaction conditions.
Therefore, developing a mild and efficient strategy for the
synthesis of α-arylated nitriles, especially, under metal-free
organocatalytic conditions is highly desired.
Over the past two decades, N-heterocyclic carbene (NHC)
catalysis has emerged as a unique tool for C−C and C−
heteroatom bond forming reactions.10 Apart from the Umpolung
reactivity toward carbonyl compounds, NHCs have been
demonstrated to activate silicon nucleophiles through the
formation of a hypervalent silicon−NHC complex.11 In line
with this concept, many C−C bond forming transformations and
polymerization reactions have been reconnoitered.12 In fact,
NHC catalyzed cyanation of aldehydes and imines using Me3Si-
CN as a nucleophile has been reported.13 However, surprisingly,
NHC catalyzed conjugate addition of Me3Si-CN to enone or
dienone systems has not been reported to date, although there
are a few reports available for metal catalyzed 1,4-conjugate
cyanation to enone systems.14 In fact, only one example is
reported for the 1,6-conjugate addition HCN to puupehenone,
which is a dienone system.15 In our continuing program on the
development of NHC catalyzed transformations,16 we sought to
employ the NHC catalyzed silicon activation concept for the
synthesis of α-arylated nitriles through 1,6-conjugate addition of
Me3Si-CN to para-quinone methides (p-QMs) [Figure 2]. While
Figure 1. Some biologically significant α-arylated nitriles.
recognized as a very potent mAchR antagonist.2a Other α-diaryl
nitrile based drugs such as diphenoxylate (2)2b and piritramide
(3)2c are being used for the treatment of diarrhea and
postoperative pain, respectively. α-Triaryl nitrile derivative 4
shows remarkable inhibition activity toward the growth of
protozoa.2d Besides the therapeutic importance, the nitrile
moiety has also been utilized as an important building block
for the synthesis of aldehydes, carboxylic acids, amides, amines,
and N-containing heterocycles.3 Furthermore, nitriles are
extensively employed as a key precursor for the synthesis of
valuable drugs.4 Owing to the significance of α-aryl nitriles,
considerable effort has been made for their syntheses through
different approaches.
The conventional method for the synthesis of α-aryl nitriles
involves a nucleophilic substitution of benzyl halides with alkali
Received: February 20, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b00508
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
could be isolated in 27% yield (entry 3). Further screening was
performed in a variety of solvents (entries 4−10), and 1,4-
dioxane was found to be the best solvent, as the expected product
7a was isolated in 97% yield in just 1 h under these conditions
(entry 10).20 The optimization studies were then extended using
other imidazolium based NHC precatalysts (9−12) in 1,4-
dioxane (entries 11−14). However, the yield of 7a was found to
be lower in all those cases when compared to that of entry 10.
Surprisingly, thiazolium (13) and triazolium (14) based NHC
precatalysts were found to be ineffective to drive this trans-
formation (entries 15 and 16). No product formation was
observed in the absence of the NHC precatalyst, which clearly
indicates that NHC is actually acting as a catalyst for this
transformation (entry 17).
Figure 2. Our hypothesis for the synthesis of α-arylated nitriles.
the chemistry of p-QMs has been investigated by many research
groups17 including our group,18 this particular NHC catalyzed
vinylogous conjugate cyanation reaction of p-QMs using Me3Si-
CN as a cyanide source has not been examined to date, which
prompted us to investigate this transformation in a detailed
manner.
The preliminary optimization studies were conducted on 5a17a
using Me3Si-CN (6) as a cyanide source and a wide range of
NHC−CO2 adducts (8−14) as precatalysts (Table 1). It is well
Having optimized the reaction conditions successfully, the
scope and limitation of the reaction were investigated using a
wide range of p-QMs (5b−z), and the results can be found in
Scheme 1. It is noteworthy to mention that most of the p-QMs
a
Table 1. Catalyst Screen and Optimization
Scheme 1. Substrate Scope with Different p-Quinone
a
Methides
entry
precatalyst
solvent
DCM
time [h]
yield of 7a [%]
1
8
24
24
24
24
24
3
N.D.
trace
27
2
8
THF
3
8
Et2O
4
8
DCE
33
5
8
PhMe
50
6
8
DMSO
80
7
8
MeCN
24
3
45
8
8
DMF
tBuOH
85
9
8
24
1
60
10
11
12
13
14
15
16
17
8
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
97
9
4
85
10
11
12
13
14
-
6
88
6
90
4
91
24
24
24
trace
trace
0
a
Reaction conditions: All reactions were carried out with 5a (0.062
mmol), 6 (0.081 mmol) in solvent (0.5 mL). Yields reported are
isolated yields.
a
Reaction conditions: All reactions were carried out with 20 mg scale
of 5(b−z) in 0.5 mL of solvent. Yields reported are isolated yields.
documented in the literature that generation of the NHC−CO2
adduct is reversible in solution at ambient conditions; thus, it
could be effectively used as an NHC precursor.19 Moreover, one
can perform the reaction under neutral conditions, as no external
base is required to generate the free NHC. The results of the
optimization studies are shown in Table 1. Our initial attempt in
the presence of precatalyst 8 was disappointing, as no product
was observed when we conducted the experiment in CH2Cl2 at
room temperature (entry 1). Changing the solvent from CH2Cl2
to THF did not help, as 7a was observed only in trace amounts
(entry 2). To our delight, when Et2O was used as a solvent, 7a
utilized for this transformation underwent smooth conversion to
their respective α-diaryl nitriles in excellent yields, irrespective of
the electronic nature of the substituents present in the aryl
moiety of p-QMs. For example, p-QMs 5b−p (bearing electron-
rich and halo substituted aryls) as well as 5q−s (bearing electron-
poor aryls) afforded the products 5b−s in 88−95% yields within
a short period of time (45 min−1.5 h). Sterically hindered p-
QMs (5t−v) also reacted efficiently to afford the successive
products in excellent yields (7t−v). In the cases of p-QMs
derived from fluorene-2-carboxaldehyde (5w) and biphenyl-4-
B
DOI: 10.1021/acs.orglett.7b00508
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Letter
carboxaldehyde (5x), the desired products 7w and 7x were
obtained in 93% and 91% isolated yields, respectively. p-QMs (5y
and 5z), derived from 2,6-dimethylphenol and 2,6-diisopropyl-
phenol, respectively, were also found to be well suited for this
transformation and gave the products 7y and 7z in good yields
(82% and 78% correspondingly).
Scheme 3. Synthetic Elaborations of α-Diaryl Nitrile 7a
The successful implementation of p-QMs as electrophiles
prompted us to explore fuchsones as an 1,6-acceptor (Scheme 2).
a b
,
Scheme 2. Substrate Scope with Different Fuchsones
used as a chemical tool for assessing hemolytic anemia induced
by naphthoquinones.23 Besides, 6-cyano-p-quinone methide 18
was subjected to a nucleophilic addition reaction with indole to
provide a leuconitrile24 dye analogue 19 in 90% yield (Scheme
3).
In summary, we have demonstrated an efficient organo-
catalytic protocol for the synthesis of α-diaryl- and α-
triarylnitriles. To the best of our knowledge, this is the first
example of NHC catalyzed 1,6-conjugate addition of Me3Si-CN
to a dienone system. This transformation occurs under mild
conditions and is tolerant to a variety of functional groups.
Further, this protocol provides an easy and straightforward
access to a set of new compounds in good to excellent yields.
a
Reaction conditions: All reactions were carried out with 20 mg of
b
15(a−f) in 0.5 mL of solvent. Reaction was carried out at 80 °C.
Yields reported are isolated yields.
ASSOCIATED CONTENT
■
S
* Supporting Information
Potentially, this method will allow us to prepare α-triaryl nitriles,
which otherwise are very difficult to access through the reported
general methods. When a model reaction was performed with
fuchsone 15a21 under optimized conditions (i.e., with 10 mol %
of 8), the reaction was found to be sluggish and the product 16a
was obtained only in 48% yield. This observation is obvious, as
the sixth position of fuchsones is sterically more hindered when
compared to that of p-QMs, which apparently makes 15a less
reactive toward Me3Si-CN. However, increasing the catalyst
loading to 15 mol % and also increasing the reaction time (12 h)
helped in improving the yield of 16a to 85%. So, further substrate
scope studies were performed with 15 mol % of 8, and the results
are shown in Scheme 2. Most of the fuchsones (15b−e), derived
from corresponding aryl ketones and 2,6-di(tert-butyl)phenol,
were reacted with Me3Si-CN under the modified reaction
conditions and provided the subsequent products (16b−e) in
good yields (73−89%). In the case of fuchsone 15f, derived from
2,6-dimethylphenol and benzophenone, the corresponding
product 16f was isolated in 87% yield after 12 h.
To further show the significance of the developed protocol,
derivatization reactions of one of the α-diaryl nitriles were
investigated. In an experiment, 7a was treated with 4-
bromophenyl boronic acid under Pd-catalyzed oxidative
conditions to give α,α′-diaryl ketone 17 in 57% yield (Scheme
3). α,α′-Diarylated carbonyl compounds are important building
blocks for the synthesis of many biologically important
molecules.22 In another experiment, 7a was oxidized with
activated MnO2 to afford 6-cyano-p-quinone methide 18 in
almost quantitative yield. Similar types of cyanated p-QMs are
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b00508.
1
Experimental details, H, 13C, and 19F spectra of all new
compounds (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: rvijayan@iisermohali.ac.in.
ORCID
Ramasamy Vijaya Anand: 0000-0001-9490-4569
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge DST-SERB (EMR/2015/
001759) for the financial support and IISER Mohali for the
infrastructure. P.G. and G.S. thanks IISER Mohali for a research
fellowship. The NMR and HRMS facilities at IISER Mohali are
gratefully acknowledged.
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