One-pot synthesis of 4?-alkyl-4-cyanobiaryls on the basis of the terephthalonitrile dianion and neutral aromatic nitrile cross-coupling

Article abstract of DOI:10.3762/bjoc.12.153

A convenient one-pot approach to alkylcyanobiaryls is described. The method is based on biaryl cross-coupling between the sodium salt of the terephthalonitrile dianion and a neutral aromatic nitrile in liquid ammonia, and successive alkylation of the long

Full text of DOI:10.3762/bjoc.12.153

One-pot synthesis of 4′-alkyl-4-cyanobiaryls on the basis
of the terephthalonitrile dianion and
neutral aromatic nitrile cross-coupling
Roman Yu. Peshkov1,2, Elena V. Panteleeva*1,2, Wang Chunyan2,3,
Evgeny V. Tretyakov1,2 and Vitalij D. Shteingarts1,§
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2016, 12, 1577–1584.
1Laboratory of the Investigation of Nucleophilic and Radical Ionic
Reactions, N.N. Vorozhtsov Novosibirsk Institute of Organic
Chemistry of Siberian Branch of Russian Academy of Sciences, Ac.
Lavrentiev Avenue, 9, Novosibirsk, 630090, Russia, 2Natural
Sciences Department, Novosibirsk State University, Pirogova St., 2,
Novosibirsk, 630090, Russia and 3Heilongjang University, Xuefu
Road, 74, Harbin, 150080, China
doi:10.3762/bjoc.12.153
Received: 18 April 2016
Accepted: 05 July 2016
Published: 25 July 2016
Associate Editor: L. Vaccaro
Email:
© 2016 Peshkov et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Elena V. Panteleeva* - pantel@nioch.nsc.ru
* Corresponding author
§ Deceased
Keywords:
alkylcyanobiaryls; cross-coupling; cyanoarenes; reactive
intermediates; reductive alkylation
Abstract
A convenient one-pot approach to alkylcyanobiaryls is described. The method is based on biaryl cross-coupling between the
sodium salt of the terephthalonitrile dianion and a neutral aromatic nitrile in liquid ammonia, and successive alkylation of the
long-lived anionic intermediate with alkyl bromides. The reaction is compatible with benzonitriles that contain methyl, methoxy
and phenyl groups, fluorine atoms, and a 1-cyanonaphthalene residue. The variety of ω-substituted alkyl bromides, including an
extra bromine atom, a double bond, cyano and ester groups, as well as a 1,3-dioxane fragment are suitable as alkylation reagents.
Introduction
Alkylcyanobiphenyls are well known largely due to their meso- stage approaches to these compounds, including successive
genic properties, which were discovered by Gray in the 1970th biphenyl acylation–reduction–halogenation–cyanodehalogena-
[1,2]. Alkylcyanobiphenyls and their analogs (e.g., dialkyl and tion (Scheme 1) [15], are currently being replaced with methods
alkoxy alkyl derivatives) are still being used as components of based on transition-metal-catalyzed biaryl cross-coupling [16-
liquid crystal mixtures [3-8], lubricants [9], media for confor- 21]. This methodology is well developed, compatible with dif-
mational analysis using NMR [10-12] and EPR [13] techniques, ferent substituents in the substrate and opens up a new ap-
and nanoparticles-doped liquid crystals [14]. The classic multi- proach to a large number of functionalized biaryls with good or
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Scheme 1: The main synthetic approaches to alkylcyanobiphenyls.
excellent yields. Nevertheless, the methodology has some Because of the above-mentioned shortcomings of cross-cou-
disadvantages: the catalytic systems are quite expensive, the pling methods, we applied the method based on the direct C–H
ligands are often hardly accessible and require special synthetic arylation of unactivated monocyanoarene 2 by the tereph-
efforts, and preactivation of the substrates via introduction of an thalonitrile (12−) dianion [23]. It was shown that the interaction
organometallic group is necessary. The last disadvantage is of 12− alkali metal salt with benzonitrile (2a) and its 2- and
overcome to some extent by the direct C–H arylation protocols 3-CH3-, CH3O-, and F-substituted derivatives (2b–f) in liquid
[18], but usually they are catalytic and thus the resulting cross- ammonia provided 4,4'-dicyanobiphenyls (4, Scheme 2)
coupling products contain traces of transition metals difficult to [23,24]. In the case of 12− cross-coupling with 2- or
remove that limits their use in areas such as pharmacy [22].
3-cyanobiphenyls, the corresponding meta- and para-dicyano-
Scheme 2: Para-cyanophenylation of substituted benzonitriles 2 by dianion 12− with the formation of a long-lived cyanocyclohexadienyl anion 3, fol-
lowed by oxidation or alkylation, leading to dicyanobiphenyls 4 or alkylcyanobiphenyls 5.
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terphenyls [25] formed in good yields. The study of probable entry 1) [23]. In order to expand the scope of the synthetic
pathways for the 12− interaction with benzonitriles 2 demon- utilization of the cross-coupling reaction under investigation, as
strated that the cyanocyclohexadienyl anion 3 was a long-lived well as to work out a short and convenient approach to a num-
reaction intermediate [23,24]. This finding allowed us to obtain ber of universal carbonitrilic structural blocks with different
not only dicyanobiphenyls 4 via anion 3 oxidation, but also both aryl and alkyl moieties [16-19], we varied the nature of the
alkylcyanobiphenyls 5 by treating 3 with an alkyl halide. neutral participant 2 in the coupling with dianion 12– and also
Earlier, we used butyl bromide for anion 3 trapping and ob- the reagent for anion 3 alkylation. The presented results not
tained 4-butyl-4'-cyanobiphenyl (5aa) in 56% yield (Table 1, only support the approach based on anionic forms of cyano-
Table 1: Interaction of dianion 12− with benzonitrile (2a), followed by alkylation of 3 with ω-substituted-alkyl bromides 6a–f.
Entry
1
Benzo nitrile, 2
Alkyl halide, 6
Alkylcyanobiphenyl product, 5
Product yielda
BuBr
6a
56b
2a
2a
2a
2a
2a
2a
5aa
5ab
5ac
5ad
Br(CH2)3CH=CH2
2
3
4
5
6
70 (64)
61 (55)
64 (52)
59 (51)
44 (38)
6b
Br(CH2)5Br
6c
Br(CH2)4CN
6d
Br(CH2)5CO2Et
6e
5ae
5af
6f
aNMR yield, % (isolated yield, %) calculated as mean values of no less than three runs. Deviation does not exceed 5%; bsee [23].
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arenes as effective cross-coupling reagents toward neutral unac- The structures of the obtained products 5ab–af were consistent
tivated substrates [26,27], but also expand the knowledge about with the proposed reaction scheme (Scheme 2). It corresponds
nucleophilic mechanisms such as aromatic substitution of a to the introduction of the cyanophenyl fragment of dianion 12−
hydrogen atom (SNH) [28] and the vicarious substitution [29]. into the para-position of nitrile 2a with the formation of cyclo-
hexadienyl anion 3 after rapid decyanation of the primary cross-
Results and Discussion
coupling dianionic product. Next, the alkylation of intermediate
The suggested one-pot synthesis of alkylcyanobiaryls 5 3 occurs at the position ipso to the cyano group. This orienta-
comprises consecutive generation of terephthalonitrile dianion tion is in agreement with the electronic structure of anion 3,
(12–) [30-32] by the addition of metallic sodium to a suspen- which is characterized by HOMO localization predominantly at
sion of dinitrile 1 in liquid ammonia, treating of the thus formed the ipso position to the cyano group [24]. This type of HOMO
12− salt with a twofold excess of nitrile 2, stirring the reaction is typical for cyclohexadienyl anions generated from mono-
mixture for ca. 1.5 h, which is necessary for cross-coupling, and cyanoarenes [42,43]. The highest yield of 5 (5ab, 70%) was ob-
final quenching by the addition of an excess of alkyl halide 6. tained for 5-bromopent-1-ene (6b). Reactions with ω-substi-
The reaction proceeds under evaporating ammonia at −33 °C, tuted alkyl bromides 6d and 6e provided lower product yields,
and it does not need an additional source of inert atmosphere for which depended on the nature of the alkylating reagent. In the
the overall reaction time, which is ca. 3 h. The reaction mix- reaction with dibromopentane 6c we found no disubstitution
tures thus obtained, along with target alkylcyanobiaryls 5 (aver- product – the corresponding 1,5-bis(4-cyanobiphenyl-4-yl)pen-
age yields 40–70%, Table 1), contain starting materials 1 (up to tane – but the reaction mixture contained traces of compounds
10 mol %), 2 (up to 46% relative to the initial amount of 2), and with molar masses of 325 and 352 (<1.5% according to
several byproducts, such as 4-alkylbenzonitriles 7, 1-alkylcyclo- GC–MS), which can be attributed to 4'-(5-phenylpentyl)-[1,1'-
hexa-2,5-dienecarbonitriles 8, and alkylbenzenes 9 (together, up biphenyl]-4-carbonitrile and 4'-(5-(1-cyanocyclohexa-2,5-dien-
to 25%).
1-yl)pentyl)-[1,1'-biphenyl]-4-carbonitrile, respectively. The
detection of these byproducts indicates that the bromine-con-
According to the total reaction scheme, these byproducts arise taining product 5ac is drawn into consecutive reactions with the
via alkylation of the anionic forms of nitriles 1 and 2 [26]. The anionic forms of 2 as an alkylating reagent and that the rate is
latter appear in the reaction media due to the electron transfer comparable or higher than the rate of the interaction of dibro-
from 12− to neutral cyanoarene 2 (Scheme 2). Of note, 4,4'- mide 6c with anion 3 (compared with [34]). In the case of alky-
dicyanobiphenyl 4 (formed due to anion 3 oxidation) in all lating reagents 6d,e that contain quite acidic protons in the
cases except one (see below) is detected in the reaction mix- α-position to the electron-withdrawing substituent, we suppose
tures only in trace amounts (up to 5%). Thus, the alkylation step that partial protonation of the anionic reaction intermediates
can be assumed to be quite fast, and the target product yield takes place together with the alkylation, thereby reducing the
mostly depends on the efficiency of the cross-coupling stage. yield of the target product. The decreased yield in the reaction
Relatively volatile components 2 and 7–9 are easily distilled off with acetal 6f is probably due to the latter’s lower reactivity
in vacuo, and alkylcyanobiphenyls 5 are additionally purified compared to other alkylating reagents, what was indicated by
by preparative TLC leading to 31–67% isolated yields.
increased yield of dicyanobiphenyl 4 (14% vs 5% formed in
reactions with 6b–e).
The reaction proved to be compatible with alkyl halides con-
taining different synthetically valuable substituents. This was To demonstrate the effects of varying the aromatic moiety, a set
shown by the example of alkylation of anion 3 arising from the of aromatic nitriles containing substituents of different natures
sodium salt of 12− and benzonitrile 2a cross-coupling by alkyl and positions were used in the reaction (Table 2). We chose the
bromides 6b–f that were functionalized at their terminal posi- nitriles that had earlier effectively underwent a cross-coupling
tion with a double bond (6b) [33], an extra bromine atom (6c) with dianion 12− [24,25], in particular, 2- and 3-methyl (2b,c),
[34,35], a cyano (6d) or ester group (6e), and a 1,3-dioxane 2- and 3-methoxy- (2d,e), and 2-fluorobenzonitrile (2f), as well
fragment (6f). The desired 4'-(ω-substituted-alkyl)-biphenyl-4- as 4'-methyl-2-cyanobiphenyl (2h). In addition, two new neutral
carbonitriles 5ab–af formed in these reactions in 44–70% yields carbonitriles – 2,6-difluorobenzonitrile (2g) and 1-cyanonaph-
depending on the nature of the alkylating reagent 6 (Table 1). It thalene (2i) – were tested as cross-coupling participants and
should be noted that alkyl halides containing the above- found to react with dianion 12−. Alkylation of intermediate
mentioned functional groups have already been used in Birch- anions 3b–i in all these reactions was provided by butyl bro-
type reductive alkylations of polynuclear aromatic compounds mide (6a). The composition of the reaction mixtures and the
[36-39], as well as aromatic acids, and their esters and amides yields of butylcyanobiaryls 5ba–ia were similar to those ob-
[40,41].
tained in the reactions of 12− with 2a and alkyl bromides 6a–f.
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The highest yield (70–75%) was obtained from the reactions spatial hindrance in course of the cross-coupling. The yields
with tolunitriles 2b,c. In general, the product yields matched the depended on the nature of the substituent in 2b–f as follows:
previously reported [24] dependence of the cross-coupling effi- yield for methoxy < for fluorine < for methyl. Most likely this
ciency on the substituent identity and position in the neutral order reflects the influence of the substituent electronic effect
reaction participant 2. The reactions of 12− with 2-substituted on the two competing mechanisms within cross-coupling:
benzonitriles provided higher product yields than in the case of heterolytic addition of dianion 12− (SNH) to nitrile 2, and/or
the 3-substituted analogs, apparently due to the decrease in electron transfer from 12− to 2 followed by recombination or re-
Table 2: Interaction of dianion 12– with cyanoarenes 2b–i, followed by alkylation of intermediate anions 3b–i with butyl bromide 6a.
Entry
Substituted benzonitrile, 2
Alkyl halide, 6
Alkylcyanobiphenyl product, 5
Product yielda
75 (67)
BuBr
6a
1
2b
5ba
5ca
5da
5ea
5fa
BuBr
6a
2
3
4
5
70 (65)
66 (56)
35 (31)
55 (47)
2c
2d
BuBr
6a
BuBr
6a
2e
2f
BuBr
6a
BuBr
6a
6
69 (60)
2g
5ga
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Table 2: Interaction of dianion 12– with cyanoarenes 2b–i, followed by alkylation of intermediate anions 3b–i with butyl bromide 6a. (continued)
BuBr
6a
7
68 (62)
66 (50)
2h
5ha
BuBr
6a
8
2i
5ia
aNMR yield, % (isolated yield, %) calculated as mean values of no less than three runs. Deviation does not exceed 5%.
laxation in the primary pair of radical anions 1·− and 2·− to the cyano group providing alkyldihydro product 11, which
(Scheme 2). These probable reaction mechanisms were studied undergoes fast dehydrocyanation in basic reaction media thus
in [23,24] and are beyond consideration of the present paper. In forming final aromatic product 5ia. The high ipso-regioselectiv-
this context, 2,6-difluorobenzonitrile (2g) and 4'-methyl- ity of the bulylation of monoanion 3i is typical for an alkylation
biphenyl-2-carbonitrile (2h) [25] (which can be considered as with primary alkyl halides of cyanocyclohexadienyl anions
benzonitrile, substituted with a p-tolyl fragment at the 2 posi- derived by two-electron reduction of aromatic mononitriles in
tion) show generally the same behavior as all the explored liquid ammonia, which was studied earlier and was found to
benzonitriles.
proceed via an SN2 mechanism [26,35].
The result obtained in the reaction with 1-naphthonitrile (2i) Conclusion
deserves separate discussion because the formation of 4-(4- We propose a one-pot method of the transition-metal-free biaryl
butylnaphthalen-1-yl)benzonitrile (5ia) is the first evidence that cross-coupling for the preparation of 4'-alkyl-4-cyanobiaryls as
dianion 12− can effectively undergo the cross-coupling with an potentially valuable building blocks [3-9,20,44] with a variable
annulated aromatic nitrile (Table 2, entry 8). The structure of structure of aromatic moieties (biphenylic, m-terphenylic or
5ia shows that a new bond forms between the carbon atoms at phenylnaphthylic), as well as an alkyl moiety. The main advan-
position 1 of dianion 12− and position 4 of 2i, leading to the for- tages of this method are experimental simplicity, utilization of
mation of the dimeric dianion 10 (Scheme 3). Further transfor- commercially accessible reagents (terephthalonitrile, metallic
mation of this primary product into butylbiaryl 5ia is via the sodium, substituted aromatic monocarbonitriles, alkyl bro-
pathways described above, i.e., a rapid and irreversible decya- mides), together with a recyclable solvent – liquid ammonia –
nation of dianion 10 with the formation of long-living which is now considered as a green solvent [45-47]. Our ap-
monoanion 3i, and the alkylation of anion 3i at the position ipso proach is compatible with a variety of different synthetically
Scheme 3: para-Cyanophenylation of 1-cyanonaphthalene 5i by dianion 12− with subsequent butylation providing 4-(4-butylnaphthalen-1-yl)benzo-
nitrile (5ia).
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Supporting Information File 1
Experimental section and 1H, 13C and 19F NMR spectra of
all synthesized compounds.
[http://www.beilstein-journals.org/bjoc/content/
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The authors thank the Multi-Access Chemical Service Center
SB RAS for spectral and analytical measurements. Authors are
grateful to Dr. Ilia V. Eltsov (Novosibirsk State University) for
the help in performing NMR measurements. The present work
is supported by grant of RFBR №14-03-00108.
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