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2016
ВЕСТНИК САНКТ-ПЕТЕРБУРГСКОГО УНИВЕРСИТЕТА
Сер. 4. Том 3 (61). Вып. 1
ХИМИЯ
UDC 541.64:547.78 M. Mqkosza
synthesis of indoles via reactions of nucleophiles with nitroarenes
Institute of Organic Chemistry, Polish Academy of Sciences, 44/52, ul. Kasprzaka, Warsaw, PL-01-224 Poland
It was shown that reactions between nitroarenes and carbon nucleophiles: Grignard reagents, enolate anions, a-halocarbanions etc. are valuable and versatile tools in synthesis of indoles. Reactions of vinylmagnesium halides with o-substituted nitroarenes that proceeds as multistep process initiated by single electron transfer, SET, the Bartoli indole synthesis leads to a variety 7-substituted indoles. A few variants of SnArH reactions in nitroarenes with carbanions open an avenue to almost unlimited variety of substituted indoles and aza indoles. Refs 45. Keywords: nitroarene, carbon nucleophyles, SET, indoles.
М. Макоша
синтез индолов при реакции нуклеофилов с нитроаренами
Институт органической химии Польской академии наук, Польша, PL-01-224, Варшава, ул. Каспржака, 44/52
Показано, что реакция нитроаренов с углерод-нуклеофилами — реагенты Гриньяра, енолят-анионы, a-галогенкарбаниноны — является удобным методом синтеза индолов. Реакции ви-нилмагнийгалогенидов с о-замещёнными нитроаренами проходит как многостадийный процесс, инициируемый электронным переносом. Синтез индолов по Бартоли приводит к 7-за-мещённым индолам. Некоторые варианты SnArH реакции нитроаренов с карбанионами открывают почти неограниченные возможности синтеза замещённых индолов и азаиндолов. Библиогр. 45 назв.
Ключевые слова: нитроарены, углеродные нуклеофилы, SET, индолы.
Indoles are very important heterocyclic systems because the indole rings form structural elements of many natural products, pharmaceuticals, agrochemicals, dyes etc. Construction of the indole ring systems is therefore of great importance and interest and is described in numerous original publications, reviews and monographs [1-4]. In recent years the major approaches to construction of the indole ring systems focus on the use of a variety of transition metal catalyzed reactions [2, 5-7], such as for instance Pd catalyzed intramolecular
© Санкт-Петербургский государственный университет, 2016
cyclization of ortho-ethynyl anilines [8]. In spite of a wide scope and effectiveness of transition metals catalyzed reactions, they have substantial disadvantages — the products contain residual metals and cannot be directly applied hence they need laborious purification before use [9]. Additional disadvantage is use of costly transition metals and ligands as well as complicated starting materials [2, 5-7]. In this short review will be presented simple and efficient alternative approach to synthesis of indoles based on reactions between nucleophiles, mainly carbanions and nitroarenes. Nitroarenes are very interesting and versatile electrophilic compounds able to react with nucleophiles in a variety of ways presented in recent review [10]. The major ways are addition to the ring at positions occupied by hydrogen and halogens, addition to the oxygen and nitrogen atoms of the nitro group and single-electron transfer. These initial reactions between nucleophiles and nitroarenes are shown in scheme 1.
These initial reactions between nucleophiles and nitroarenes are followed by a variety of transformations therefore reactions of nucleophiles with nitroarenes create rich field of organic synthesis. Undisputable the most important reactions between nucleophiles and nitroarenes are initiated by the addition of nucleophiles to the electron deficient rings in positions occupied by halogens X and hydrogen to form intermediate oX and oH adducts respectively. The former process followed by fast departure of X~ from the oX adducts is a well-known reaction of nucleophilic aromatic substitution, SNAr. However addition of nucleophiles to nitroarenes proceeds much faster at positions occupied by hydrogen. Since spontaneous departure of hydride anions from the initially formed oH adducts does not proceed, usually they dissociate hence slower formation of oX adducts and subsequently SN Ar can occure. Nevertheless oH adducts can undergo fast conversion into products of nucleophilic substitution of hydrogen, SN ArH reaction. Three major ways of conversion of these oH adducts are: oxidation with external oxidants — ONSH, elimination of HL when nucleophiles have nucleofugal groups L at the nucleophilic centers — vicarious nucleophilic substitution, VNS and conversion of the oH adducts into substituted nitrosoarenes.
In scheme 2 a general picture of reactions between nucleophiles and nitroarenes exemplified by p-chloronitrobenzene, initiated by the addition to the ring is presented. Fast addition at position ortho to form oH adducts is a reversible process. Dissociation of the oH adducts followed by slow addition at position para and subsequent departure of Cl_ results in SN Ar reaction (2a). On the other hand fast oxidation of the oH adducts with an external oxidants gives products of ONSH (2b). Further conversion of the oH adducts of a-chlorocarbanions e.g. chloromethyl phenyl sulfone proceeds as base induced ^-elimination to give products of VNS (2c). The reaction of p-chloronitrobenzene with aniline carried out at —40°C in the presence of strong base results in the formation of oH adducts that upon protonation give o-nitroso diarylamine (2d).
Detailed discussion of many variants of SN ArH reactions is presented in monograph [11] and reviews [12-14]. Contrary to anionic nucleophiles — the Grignard reagents react with nitroarenes not via direct addition but via initial single electron transfer — SET to form nitroaromatic anion-radicals and radicals. Further fate of these paramagnetic species
Scheme 1
X
X
depends on the structure of the generated radicals. Primary alkyl radicals add to the ring of the nitroaromatic anion radicals in positions ortho- and para- to form anionic oH adducts that can be further oxidized to form products of ONSH- alkylated nitroarenes [15, 16]. On the other hand allyl and aryl radicals add to the nitrogen atom of the nitro group giving substituted hydroxylamines and diaryl amines (scheme 3) [15, 16].
Scheme 2
CI Nu
CI
+ Nu-
NO„
kH > ka
(d)
NO2 CI
H Nu
NO
Nu = PhNH
Nu
+ Cl-
(a)
NO
NO
Scheme 3
Z Z,
+ RMgX-- [(T)
NO
NO2MgX ■;
HR
ZZ
[O]
Z
R
NO.MgX NO2
ho-n^^c^r
The most interesting are reactions of vinyl radicals generated via SET between nitroarenes and vinylmagnesium halides. They add to the oxygen atom of the nitroaromatic anion radicals, subsequent elimination of enolate anion initiate multistep transformation giving, as ultimate products, substituted indoles (scheme 4) [17].
Scheme 4
R1 R
С©л +
NO
R
MgX
r1
N=O
/ MgX
R
R
R
R
R
R
Z MgX
R
NH R
This reaction, the Bartoli indole synthesis proceeds efficiently provided that nitroarene contains a substituent ortho to the nitro group. Due to its steric effect the desired process shown in scheme 4 proceeds selectively. The Bartoli indole synthesis has found wide application in organic synthesis [16].
Much more general and versatile are pathways of syntheses of indoles that consist in introduction of functionalized carbon substituents into nitroaromatic rings followed by further transformations. The most efficient ways of introduction of such substituents into nitroarenes is nucleophilic aromatic substitution of hydrogen SNArH with carbanions via ONSH and VNS reactions. Synthesis of indoles via SN ArH reactions in nitroarenes can be achieved on two ways:
a) Substitution of hydrogen with carbanions in m-nitroaniline and related nitroarenes — the amine group nitrogen atom is in the heterocyclic ring.
b) Introduction of functionalized carbon substituents ortho to the nitro group — the nitrogen atom of the nitro group is in the heterocyclic ring.
Perhaps the simplest synthesis of substituted indoles is reaction of enolate anions of ketones with m-nitroanilines that proceeds via addition of the enolates in vicinity of the amino group. The intermediate oH adducts are oxidized with the atmospheric oxygen whereas the produced o-aminobenzylic ketones undergo intramolecular Baeyer condensation to produce indoles [18].
Although the amino groups deactivate nitroaromatic ring toward nucleophilic addition, formation of the intermediate oH adducts of enolates to m-nitroanilines is promoted by the interaction of the ketone carbonyl and amino groups. Because of this additional interaction the enolate anions add in vicinity (ortho) to the amino group. The reaction is of general character, some examples are given in scheme 5. It is also feasible for practical synthesis [19]. Under similar conditions proceed reactions of carbanions of acetonitrile or phenylacetonitrile with m-nitroaniline giving 2-amino-4- or -6-nitroindoles (scheme 6) [20].
Scheme 5
NO„
CH3COPh
NH
O
i-BuOK DMSO
NO
NH Ph
NO
NH
NO
+ PhCHCN -
NH
i-BuOK
DMSO air
o2n
Ph
NH NH2
Scheme 6
Alternatively m-nitroanilines can be converted into m-nitrobenzoizonitriles that can enter VNS with a variety of carbanions. The produced nitrobenzylic carbanions undergo intramolecular addition to the isocyano group to form indoles (scheme 7) [21].
+
Particularly valuable and general approach to synthesis of indoles from nitroarenes consists in introduction of functionalized carbon substituents in positions ortho to the nitrogroup via VNS or ONSH reactions followed by a variety of further transformations.
The VNS reaction of carbanions of chloroacetonitrile, or more convenient, aryloxyace-tonitriles and chloromethyl phenyl sulfone with a variety of nitroarenes — carbo and heterocyclic, proceeds preferentially in positions ortho to the nitro group to provide o-nitroaryl acetonitriles [22] and o-nitroarylmethyl phenyl sulfones [23, 24] — versatile starting materials for synthesis of substituted indoles. It should be mentioned that the reaction can be directed in the ortho position by the proper conditions [24]. These nitriles and sulfones can be converted into indoles on two major ways:
a) Alkylation of the methylenic groups with allyl or benzyl halides or the Knoevenagel reaction with aldehydes followed by base induced intramolecular condensations. These reactions do not require reduction of the nitro group.
b) Reduction of the nitro group and subsequent reactions.
For instance 2-nitro-5-methoxybenzyl tolyl sulfone, readily available via VNS in p-nit-roanisole, is rapidly alkylated with allyl chloride or benzyl chloride under liquid-solid PTC conditions (K2CO3 anh., Bu4N+Br~) to give expected alkylation products. These products treated with powdered NaOH in DMSO undergo a series of reactions giving as ultimate products W-hydroxy-2-vinyl (or 2-phenyl)-3-tosyl-5-methoxyindoles in excellent yields (scheme 8) [25].
Scheme 8
MeO
MeO
NO
+ Cl^NO Ar ?-BuOK» + Cl so2ar dmf
H Cl
MeO
SO2Ar
-HCl
SO2Ar
NO
Ar = p- MeC.H R = CH=CH Ph
Similar transformations proceeds with o-nitroarylacetonitriles. Thus alkylation of 2-nit-ro-5-halophenyl acetonitriles with allyl chloride followed by treatment of the product with sodium hydroxide in methanol provides W-hydroxy-2-vinyl-3-cyano-5-haloindoles [25]. On the other hand Knoevenagel condensation of these nitriles with acetaldehyde followed by treatment of the produced unsaturated nitriles with K2CO3 in methanol gave W-hydroxy-2-hydroxymethyl-3-cyano-5-haloindoles (scheme 9) [26].
B
The ^-hydroxy functionality can be easily removed by a variety of reagents, thus these reactions provide a general way of synthesis of substituted indoles. Surprisingly, in spite of simplicity and great potential value of these reactions, they have attracted only small interest in chemical community.
Even more attractive and general way of construction of substituted indoles consists in reduction of the nitro group in ortho nitroaryl acetonitriles and ortho nitrobenzyl sulfones.
Hydrogenation of o-nitroarylacetonitriles was recognized as a way of synthesis of indoles as early as 1955 [27]. However the nitriles were not readily accessible thus this synthesis of indoles was without practical value. For instance, o-nitroarylacetonitriles were prepared via SNAr of fluorine in o-fluoronitrobenzenes with carbanion of methyl cyanoacetate followed by partial hydrolysis and decarboxylation [28]. Introduction of VNS reaction into practice of organic synthesis opened possibility of direct cyanomethylation of nitroarenes [22]. Since VNS of hydrogen in halonitroarenes with carbanions proceeds much faster than SNAr of halogens, whereas halogen substituents activates nitroaromatic rings towards nucleophilic addition [29] halogens in halonitroarenes can play double role: protect position they occupied against the reaction and facilitate synthesis of the desired isomers. Further hydrogenation results in formation of indoles and, if desired removal of protecting and activating halogens. Thus VNS cyanomethylation of isomeric chloronitroanisoles gave access to all isomeric 4-, 5-, 6- and 7-methoxyindoles (scheme 10) [30, 31].
Scheme 10
MeO MeO MeO
bLeCHoCNnt ^ H2'Pd(C) " O^N
2 2 H
X = H, CI, Br; L = 4-ClC6H4O, RNCS2: base — NaOH, ¿-BuOK; solvent — DMSO, DMF
Since carbanions of these nitroaryl acetonitriles can be easily alkylated combination of VNS, alkylation and hydrogenation is a simple way to prepare indols having a variety of substituents in position 3 (scheme 11) [30, 31].
Scheme 11
Equally productive is synthesis of indoles via reduction of the nitro group in ortho-nitroarylmethyl aryl sulfones. Contrary to the nitriles, the sulfones can be reduced to ortho-amionarylmethyl sulfones without cyclization (scheme 12) [32, 33].
OCo.
SO2Ph sn, HCl
SO2Ph
CH(OEt)3
NH,
SO2Ph
NaOH _ DMSO
N=CH I
OEt
/SO2Ph
NH
A few ways of conversion of these sulfones into indoles were developed. The base catalyzed condensation of such aminosulfones with aromatic aldehydes provides 2-aryl indoles, probably via formation of imines followed by intramolecular addition of the sulfone carban-ion and subsequent sulfinate elimination (scheme 13) [33]. On the other hand acid catalyzed condensation of the amines with ortho esters produces imidates that enter intramolecular addition of the carbanion followed by elimination of the alcohol to form 3-arylsulfonyl indoles (scheme 12) [32].
Scheme 13
MeO Ж
SO2Tol
OL
MeO Ж
CHO
NH
SO2Tol
>
O-
MeO
NaOH_ DMSO
Yet another way consists in conversion of the amino group into isocyano functionality and further intramolecular addition of the sulfone carbanion (scheme 14) [34].
Scheme 14
П
O
■П
/-1
O
O2Tol
NH
The reductive ways of synthesis of indoles have found wide application in practical synthesis of a variety of indoles [35-40] some examples are given in schemes 15-17.
Scheme 15 [39]
f5s
PhO
CN
■NO,
i-BuOK
f5s-
'yy^cn h2, Pd(C)
f5s
NH NH
f5s
Cl
i-BuOK
■NO,
f5s
SO2Ph
NO,
SO2Ph
AiCHO
NH
f5s
1
NH Ar
Indoles can be also synthesized via reduction of o-nitroarylmethyl ketones, followed by instateneous Baeyer type condensation [41, 42]. However introduction of acylmethyl sub-stituents into nitroarenes via SNAr reaction (VNS or ONSH) is less general, because of moderate nucleophilicity of enolate anions. It is therefore limited to highly active nitroarenes such as dinitrobenzene or nitronaphthalene.
H
SO2Ph
Scheme 16 [38]
MeO.
3 6 2 2
i-BuOK/DMF
2,4,6-Cl3C6H2OCH2CN
B^^-^NO
MeO
YY^CN
MeO
H2/Pd(C)
N \
H
Scheme 17 [40]
OH
OH
MeO
O
SN ArH reactions are also efficient tool in synthesis of oxindoles. Thus intramolecular VNS and ONSH reactions of m-nitroanilides of chloroacetic or alkanoic acids give directly 5-nitrooxidoles [43, 44]. Even more efficient and versatile way to oxindoles is synthesis of alkyl o-nitroarylacetates via VNS in nitroarenes with alkyl chloroacetates followed by reduction of the nitro group and nitromolecular acylation (scheme 18) [45].
Scheme 18
NO
+ Cl'
COOR
i-BuOK DMF
N
N
In this short paper it was shown that reactions between nitroarenes and carbon nucle-ophiles: Grignard reagents, enolate anions, a-halocarbanions etc. are valuable and versatile tools in synthesis of indoles. Reactions of vinylmagnesium halides with o-substituted ni-troarenes that proceeds as multistep process initiated by single electron transfer, SET, the Bartoli indole synthesis leads to a variety 7-substituted indoles [17]. A few variants of SNAr reactions in nitroarenes with carbanions open an avenue to almost unlimited variety of substituted indoles and aza indoles.
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Статья поступила в редакцию 16 сентября 2015 г.
Контактная информация
Makosza Mieczyslaw — Professor; e-mail: icho-s@icho.edu.pl
Макоша Мечислав — профессор; e-mail: icho-s@icho.edu.pl
