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6Porphyrinoids Порфириноиды
Макрогэтэроцмклы
http://macroheterocycles.isuct.ru
Review Обзор
DOI: 10.6060/mhc180483s
Functionalized 2r2r-Bipyrroles: Building Blocks for Pyrrolic Macrocycles
Gonzalo Anguera,ab James T. Brewster II,a David Sanchez-Garcia,b and Jonathan L. Sesslera@
aDepartment of Chemistry, the University of Texas at Austin, A5300 Austin, 78712-1224 Texas, United States
hGrup d'Enginyeria de Materials (GEMAT), Institut Quimic de Sarria, Universitat Ramon Llull, 08017Barcelona, Spain
@Corresponding author E-mail: sessler@cm.utexas.edu
Functionalized N-unsubstituted 2,2'-bipyrroles are basic building blocks for the preparation of pyrrolic macrocycles and natural products, such as prodigiosines. The aim of this review is to provide a description of the most important methodologies used to prepare 2,2'-bipyrroles and their central role as building blocks for the synthesis of porphyrinoids and property-defining structural elements therein.
Keywords: 2,2'-Bipyrroles, macrocycles, expanded porphyrins, oligopyrroles.
Функционализированные 2,2'-бипирролы: строительные блоки для пиррольных макроциклов
Г. Ангуера,^ Д. Т. Брюстер П^ Д. Санчез-Гарсиа^ Д. Л. Сесслер^@
аТехасский университет, А5300 Остин, 78712-1224 Техас, США
ъХимический институт Сарриа, Университет Рамона Лльюлла, 08017 Барселона, Испания ®Е-таИ: sessler@cm.utexas.edu
Функционализированные Ы-незамещенные 2,2'-бипирролы являются основными строительными блоками для синтеза пиррол-содержащих макроциклов и таких природных соединений, как продигиозины. В обзоре представлены наиболее важные методики синтеза и функционализации 2,2'-бипирролов, а также наиболее важных порфириноидов и расширенных порфиринов на их основе.
Ключевые слова: 2,2'-Бипирролы, макроциклы, расширенные порфирины, олигопирролы.
Introduction
Heterocycles represent a privileged class of compounds that have seen utility across a wide range of application areas and which play essential roles in the chemical and medicinal sciences.[1] Of the known heterocycles, pyrrole is one of the simplest. This five-membered heteroaromatic ring compound is characterized by its reactivity; indeed, it is more prone towards electrophilic attack than its related furan or thiophene congeners. It also possesses three sites for potential functionalization (the a- and P-pyrrolic positions and the proton-bearing N).[2-4] Since its discovery in 1834 as a component of coal tar,[5] pyrrole and its derivatives have been found as an integral functionality in numerous natural
products (Scheme 1).[6] Not surprisingly, the preparation and modification of pyrrole has been the focus of many research programs with concomitant attention devoted to the synthesis of pyrrole-incorporated therapeutics,[7] supramolecular constructs,[8] and functional materials.[9]
Acyclic synthetic and natural compounds containing pyrrole typically comprise a modified pyrrole-core integrated within more complex structural motifs, such as 2,2'-bipyrrole. Systems incorporating 2,2'-bipyrrole-based building blocks are now found extensively in the literature (Scheme 2).[70] However, practical applications of these systems rely on compound availability. This, in turn, requires the development of appropriate synthetic methods that furnish the requisite functionalized starting materials, particularly
those with suitable physical properties, such as solubility, controlled light absorption, and stability, among other desirable characteristics. There are a number of reviews focused on the preparation of pyrrole[1112] and pyrrole-based building blocks.[1314] Some that encompass the preparation of 2,2'-bipyrroles, as well as other functional precursors, such as naphthobipyrroles,[1516] benzobipyrroles,[17] or Broring's conformationally restricted 2,2'-bipyrrole,[18] have appeared in recent years.[19-24] Furthermore, in 2017 Setsune published a review highlighting recent advances in 2,2'-bipyrrole-based porphyrinoids with a section devoted to preparative methods.[20] Nevertheless, a dedicated overview of the synthetic literature regarding bipyrroles with an update on recent applications is viewed as something that could complement prior reviews while stimulating further research progress in the area of oligopyrrolic materials.[16-1825] Our goal is to provide such a survey. In providing this overview, we also review the chemistry of synthetic macrocycles wherein bipyrrole is used as a key synthetic building block. Not covered are systems, such as corroles and corrins, that
contain bipyrroles (or bipyrrole derivatives) but whose mode of preparation does not pass through a bipyrrolic intermediate. We view the discussion provided in the present summary as being complementary to that provided by Setsune.[20]
Synthesis of 2,2'-Bipyrroles
First synthesis of a 2,2'-bipyrrole
The first method detailing a purported synthesis of 2,2'-bipyrrole reported by Grabowski and Marchlewski and involved out combining so-called hämopyrrol (10) and 4-methylbenzenediazonium chloride (11) "und längere zeit stehen gelassen"(and leaving to stand for some time) to yield 12 as blue crystals, as determined by elemental analysis (Scheme 3).[26] Reanalysis of this work by Webb and Threlkeld led to the proposal that the blue crystals obtained by Grabowski and Marchlewski were in fact 2 -py rryl-2 -py rrolenine salts.[27,28]
COOH
Scheme 1. Selected natural products containing pyrrole: Protoporphyrin IX (1), prodigiosin (2), indanomicine (3), bilirubin (4), and lukianol (5).
Scheme 2. Synthetic porphyrinoids containing one or more 2,2'-bipyrrole (9) motifs: porphycene (6), sapphyrin (7) and cyclo[n]pyrroles (8). 228 Макрогетероциmbl /Macroheterocycles 2018 11(3) 227-245
Scheme 3. Preparation of the first 2,2'-bipyrrole by Grabowski and Marchlewski.[26]
Syntheses using the Vilsmeier-Haack reaction
Inspired by the fact that the 2,2'-bipyrrole motif is a key structural component of prodigiosin (2), Rapoport and co-workers sought to develop new routes amenable to the preparation of asymmetric 2,2'-bipyrroles.[29] Utilizing methods developed by Fuhlhage and Van der Werf wherein pyrrole was condensed with 1-pyrroline to yield 2-(pyrrolidin-2-yl) pyrrole (15, Scheme 4a),[30] Rapoport and co-workers were able to obtain an asymmetric 2-(pyrrolidin-2-yl)pyrrole (17).[31] The presence of an ester group on the a-position of the starting pyrrole was shown to deactivate the pyrrole and prevent formation of the 2,2'-pyrrolidinylpyrrole. The introduction of a p-methoxy group was able to counteract this latter deactivation effect, allowing for the preparation of intermediate 17 (Scheme 4b). The desired 2,2'-bipyrrole was obtained in a 40 % yield via dehydrogenation of 17 using Pd/C at elevated temperature (Scheme 4b). The synthesis was later modified by way of a Vilsmeier-Haack reaction between 2-pyrrolidinone and pyrrole yielding
5-(pyrrol-2-yl)-3,4-dihydro-2H-pyrrole in improved yields (Scheme 4c).[32] The so-called Rapoport bipyrrole synthesis would later include various pyrrole and 2-pyrrolidinone analogues, thus expanding the substrate scope and allowing access to various asymmetric species.[33] In a continuation of these efforts, Rapoport and Bordner would also develop a route yielding asymmetric 2,2'-bipyrroles directly from 1,5-dihydropyrrol-2-ones of general structure 23 and pyrrole (Scheme 4d).[33]
In pursuit of new methods for the facile construction of prodigiosin-type natural products, Hao and co-workers described a one-pot synthesis of pyrrolyldipyrromethenes from 5-halogenated-2-formylpyrrole (Scheme 5).[34] This synthesis relies on the use of POCl3 to generate an intermediate 9-halodipyrromethene 25, which readily undergoes nucleophilic aromatic substitution to yield the target pyr-rolyldipyrromethenes 26 in 41-79 % yield. Mechanistic studies supported the intermediacy of a 9-halodipyrromethene 25 that undergoes a subsequent SNAr reacton with pyrrole to yield the corresponding pyrrolyldipyrromethenes in 51-90 % yield.
a)
O ' o
13
N
H
14
15
—o
20
H
16
c)
O
17
POCI3
Pd/C
C02Me
^¡^OPOCIj
I
H
CI
e
18
M-S
C02Me
O
X-//
19
CHO
20
21
22
Scheme 4. Dehydrogenation of 2,2'-pyrrolidinylpyrrole 15 (a). Preparation of 2,2'-bipyrrole 17, used as a building block for the synthesis of prodigiosin (b). Synthesis of 2,2'-(1-pyrrolyl)pyrrole 22 by means of a Vilsmeier-Haack reaction (c). Synthesis of 2,2'-bipyrrole 9 (d).
R-i = methyl or H R2-3 = alkyl or H
R5_7 = alkyl, aryl or H
Scheme 5. Synthesis of pyrrolyldipyrromethenes 26 from 5-halogenated 2-acylpyrroles as detailed by Hao, et al.[34]
Syntheses using the Paal-Knorr reaction
With a view to preparing natural products containing 1,2'-, 1,3'-, and 2,2'-bipyrrole subunits, Gribble and co-workers described the preparation of 2,2'-bipyrroles in a three-steps sequence involving the Paal-Knorr reaction (Scheme 6).[35] Treatment of pyrrole ketoaldehyde (28) with benzylamine and acetic acid in methanol led to a series of N-substituted-2,2'-bipyrroles (29) in 30-96 % yield. The resulting bipyrroles could be readily halogenated to give polyhalogenated bipyrrolyl natural products.
a)
EtOzC
b)
'/ w
v
H 30
\ C02Et
-X-
4L
EtOzC N H
32
Br
Et02C.
Cu
Et02C
C02Et
31
C02Et C02Et N
H C02Et
33
14 27, R = CH2OH —| 29
b
28, R = CHO 1
Scheme 6. Synthesis of 2,2'-bipyrrole 29 as described by Gribble et al.[35] a) Butyrolactone, MeMgl, toluene, 89 %. b) PCC, NaOAc, CH2Cl2, 92 %. c) PhCH2NH2, AcOH, MeOH, 81 %.
Synthesis using the Ullmann coupling
Initial attempts to synthesize N,N'-disubstituted and N,N'-unsubstituted-2,2'-bipyrroles were focused on the Ullmann coupling using a mixture of copper powder and 2-halopyrrole without solvent at high temperature (160-200 °C), as reported by Webb and Threlkeld (Scheme 7a,b).[2728] Interestingly, introduction of ethoxy-carbonyl groups attenuated the reactivity of the pyrrole and allowed for preparation of N-substituted iodopyrrole with subsequent coupling furnishing 2,2'-bipyrrole in 58.6 % yield. In the context of N-unsubstituted pyrrole, the authors found that upon switching from a 2-iodo- to 2-bromopyr-role, bipyrrole could be readily prepared in 54 % yield (Scheme 7b). Although limited in terms of substrate scope, Webb and Threlkeld had demonstrated the ability to control pyrrole reactivity while detailing a facile preparation of symmetric 2,2'-bipyrroles.
In pursuit of new methods allowing for the preparation of 5,5'-unsubstituted-2,2'-bipyrroles en route to modified porphyrin rings, Grigg and co-workers optimized the Webb and Threlkeld procedure. The judicious placement of ethoxy-carbonyl groups in the a-position permitted the preparation
Scheme 7. Preparation of 2,2'-bipyrroles by means of an Ullman coupling. Attempted synthesis of 31 (a). Ullmann coupling as used in the synthesis of 33 (b).[27,28]
and use of 2-iodopyrroles in the Ullmann coupling protocol. Furthermore, by carrying out the coupling in N,N-dimeth-ylformamide the temperature could be reduced to 100 °C. In some instances, the reaction could even be completed at room temperature (Scheme 8).[12] The ethoxycarbonyl groups also served as a functional handle allowing for saponification and decarboxylation to yield 5,5'-unsubstituted-2,2'-bipyrrole 36 (Scheme 8). Various substitution patterns were explored in the P-position with the authors noting that alkyl-2,2'-bipyrroles are unstable to air, acid, and turn first green and then blue in a few hours after exposure to air. Upon revisiting this route, Sessler and co-workers demonstrated that BOC-protection of the pyrrole nitrogen improved the yield of the bipyrrole by 20-30 %.[36]
In celebrated chemistry associated with efforts to prepare the corrin core of vitamin B12, Woodward and co-workers sought to cyclize a tetrapyrrolic precursor using formic acid and HBr followed by oxidation with iodine. In pursuing this chemistry, the Harvard group serendipi-tously isolated a deep blue solid that they named sapphy-rin (39). This penta-aza macrocycle may be considered as being a "heterocycle-inserted" expanded porphyrin that contains both 2,2'-bipyrrole and a tripyrrane moieties. In an attempt to study further extensions of the n-system, as well as the coordination chemistry, an optimized synthesis was developed; it involved a 3+2 coupling of 5,5'-difor-mylated-2,2'-bipyrrole 37 with tripyrrane dicarboxylate 38 (Scheme 8).[22]
The Harvard group also reported a new route towards 2,2'-bipyrroles by way of cyanoacrylate protected aldehydes (41, Scheme 9). Here, a copper catalyzed Ullmann
Y—/ Cu, DMF
H
34
"W
NaOH, Et0H/H20
Et02C' W C02Et
35
1. TsOH, EtOH 2. 02
39
Scheme 8. Synthesis of sapphyrin 39.[22]
CF3COOH
OHC
40
45
NaOH -qI_IQ Me0H/H20
OEt
ЕЮ
Scheme 9. Production of a difoimylated 2,2'-bipyrrole by means of an Ullmann coupling-based strategy.[37]
coupling between the cyanoacrylate of the iodopyrrole 43 furnished bipyrrole 44 in about 35 % yield.[37] Although the yield was lower than that afford by the Ullman coupling of the monoester as described separately by Grigg,[22] this newer route afforded the bipyrrole dialdehyde (45) after retro-Knoevenagel reaction of the cyanoacrylate 44. This
modification obviated the need for hydrolysis of the ethyl ester and follow-up decarboxylation and formylation.
As part of their respective efforts to prepare porphyrinoids incorporating 2,2'-bipyrrole subunits, the groups of Vogel and Nonell modified the common approach involving decarboxylation of the 5,5'-carboxylate-2,2'-
Scheme 10. Synthesis of bipyrrole 53 as a precursor in the preparation of porphycene. a) Br2, SO2Cl2, AcOH/HCOOH, 0 °C, 4 h (50-58 %). b) KI/I2, EtOH/H2O, 75 °C (75-77 %). c) Cu, DMF, 20 oC, 17 h (61-71 %). d) NaOH, EtOH/H2O, 15 h, reflux (96 %). e) Sublimation 230 °C, 0.2 torr (89 %). f) POCl3/DMF, NaOAc/H2O (86 %).[2138]
R C02Bn R C02BnC02Bn
JTL TWY ТУТГ
Et02C^H>-, Et02c^N NAC02Et Ею/« «V
54 55 56
R
d, e
— Vwr" r)>^jCr
TsHNHNOC H h CONHNHTs °HC н H CH0
57 53
Scheme 11. Non-decarboxylative synthesis of diformyl-2,2'-bipyrroles. Reaction conditions: a) Cu, DMF, 23 °C. b) Pd/C, HCOOH/HCOONH4, reflux, 4 h (100 %). c) EG, 170 °C, 5 h (93 %). d) NH2-NH2, EtOH, reflux, 48 h (93 %). e) TsCl, pyridine, 23 °C, 1 h (96 %). f) ethylene glycol, Na2CO3, 170 °C (92 %).[39]
О
nc^A,
59 0 R4 /C°2Et , ,ы , R C02Et
4 7 l2/Nal \_/ * Cu, DMF
RCHO -► // \\ -► f~\ —-► 50
DBU, 50 °C Et02C^N/ DCE, 100 °C ЕЮ
58 H h
49
C02Et C02Et
R. JL.R TU'
Et02C' -N N xC02EtEG 180 °C fj W DMF 0HC H H ^HO
50 52 53
Scheme 12. Synthesis of pyrroles from aldehydes and subsequent Ullman coupling to 2,2'-bipyroles.[40]
bipyrrole (51) (Scheme 10).[21,38] Vogel and co-workers switched to a sublimation step to enact decarboxylation, while the Nonell group used sodium hydroxide in ethylene glycol at high temperature. Through these latter modifications, Nonell, et al. were able to carry gram scale quantities of 5,5'-carboxylate-2,2'-bipyrrole through the subsequent decarboxylation and formylation steps to give 5,5'-for-myl-2,2'-bipyrrole in approximately 30 % yield over two steps.
It was also reported by Stockert and co-workers that the unstable a-free bipyrrole could be bypassed by utilizing bipyrrole-2,2'-dicarbonyl tosylhydrazide (57) (Scheme 11).[39] These efforts allowed for improved yields in terms of preparing a-formyl substituted 2,2'-bipyrroles, albeit at the cost of an increased step count.
As a way to expand further the array of substituents that could be incorporated into bipyrroles, Sessler and coworkers implemented a Schollkopf-Magnus-Barton-Zard approach utilizing simple aldehydes and alkyl isocy-anoacetates to furnish pyrrole 60 in 50-70 % yield.[40,41] The simple preparation of a dialkyl pyrrole-2,4-carboxylate allowed subsequent iodination and Ullmann coupling to give 4,4'-disubstituted-2,2'-bipyrrole (50) in only three steps (Scheme 12). The authors noted that the iodopyrrole must contain electron-withdrawing groups for the coupling to proceed in moderate to good yields.
As a culmination of previous efforts involving protecting or adding an a-formyl group at a relatively late step, Jiao and co-workers described a modification of the Ullmann
reaction that allowed for the preparation of 5,5-diformyl-2,2'-bipyrrole (62).[42] These researchers exploited a reductive cross coupling using Pd/C and zinc metal in a biphasic mixture; this allowed highly substituted 2,2'-bipyrroles to be prepared in 26-54 % yield (Scheme 13).
Scheme 13. Synthesis of 2,2'-bipyrrole 62 by Jiao et al.[42]
Syntheses by means of oxidative couplings
The Ullmann coupling of iodopyrrole has remained a mainstay in the preparation of symmetric 2,2'-bipyr-roles. However, routes toward asymmetric constructs have predominately relied on the Rapaport bipyrrole synthesis that proceeds via 2,2'-pyrrolidinylpyrroles. In order to simplify the latter procedure, Patel and co-workers developed a Pd(OAc)2-based oxidative coupling (Scheme 14). This procedure relies on the preparation of 1,1'-carbonyldipyrrole 66, obtained by the coupling of a pyrrole-1-carboxylic acid 63 with the sodium salt of the corresponding pyrrole (64) or the anhydride of the pyrrole-1-carboxylic acid (65).[43]
63 О N
C02H
ОСН3
ft _
N С02СН3 NaH тнр
н
Ol°l"0
COCI2, DMF
TFA
NaH, THF THF, 25 °C
OCH,
rA^C02CH3
\-M
H3CO
>=°
66
Pd(OAo)2
CH3CO2H
C02CH3
H3CO C02CH3
67
LiOCH3
MeOH
68
65
Scheme 14. Synthesis of asymmetric 2,2'-bipyrroles by Patel et al.[43]
R C02RI
1. TOSMIC, n-BuLi V-/
2. п-BuLi, SnMe3CI ^N^Sn(Me)3
H
COzRi C02Ri Cu(N03)2-3H20 'R
~N N-H H
71
69 R = aryl 70
Scheme 15. Synthesis of 2,2'-bipyrroles via a stannylated pyrrole derivative as reported by Sánchez-García et al.[45]
Treatment of the corresponding 1,1'-carbonyldipyrrole (66) with Pd(OAc)2 in acetic acid, followed by hydrolysis, gives the asymmetric 2,2'-bipyrrole (68) in four steps and in 30 % yield (Scheme 14).
Recent efforts to prepare bipyrroles via cross-coupling approaches have served to optimize previously reported strategies. In fact, by modifying the original preparation of a-stannyl pyrrole developed by Van Leussen and co-workers,[44] the Sánchez-García group developed a one-pot synthesis of 2,2'-bipyrroles of general structure 71 from easily prepared or commercially available cinnamates (69) (Scheme 15).[45] This method allows the gram-scale synthesis of dialkyl 4,4'-biaryl-2,2'-bipyrrole-3,3'-carboxylates (71) in up to 50 % yield. The authors made note of what was a short and rapid aqueous extraction for the purification of the stannyl pyrrole crude product before it was subject to Cu-coupling; this generally improves the yield.
Synthetic procedures giving Ж-unsubstituted bipyrroles directly are limited. Advances in hypervalent iodine chemistry, however, have been applied recently to pyrrole precursors and have provided a new route towards 2,2'-bipyrroles.[4647] One of the first demonstrations of a hypervalent iodine-based bipyrrole synthesis was reported by Kita and co-workers. Here, phenyliodine bis(trifluoroacetate) (PIFA) in the presence of a Lewis acid (e.g., TMSBr or BF3-OEt2) was used to effect the oxidative coupling of pyrrole.[48] This new method furnished a range of 3,3',4,4'-tetrasubstituted, 3,3'-disubstituted, and unsub-stituted bipyrroles. Interestingly, this approach proved amenable to electron-rich pyrroles and, upon Ж-protection, could furnish 2,3'-bipyrroles as well. Moreover, when there is only one substituent in the pyrrolic P-position, the reaction can afford a mixture of 3,4'- or 3,3'-substituted 2,2'-bipyrroles. The groups of Waluk, as well as of Panda,
used this generalized strategy to prepare 2,2'-bipyrroles that were carried on to yield meso-substituted porphy-cenes and a P-octamethoxyporphycene (74a), respectively (Scheme 16a).[49,50] A similar oxidative coupling protocol was developed by St^pien and co-workers as part of efforts to develop the chemistry of bis(phenanthropyrroles) (76).[46] Interestingly, when added to FeCl3, the a-free-P-diaryl-monopyrrole (75) was observed to undergo intramolecular ring closure of the P-aryl substituents, as well as an intermolecular a-a coupling of two pyrrole subunits via a Scholl reaction. Unlike the PIFA method, the authors were required to protect the a-position to prevent formation of oligomeric and polymerized by-products (Scheme 16b).
Synthesis via Suzuki-Miyaura coupling
In pursuit of methods that would allow access to natural products such as prodigiosine and tambjamine, Reynolds and co-workers developed Suzuki cross-coupling protocols involving pyrrole-2-boronic acids and bromopyr-roles (Scheme 17).[51] Here, the authors used 1,3-dibromo-5,5-dimethylhydantoin (DBDMH) as a mild bromination reagent to access a series of bromopyrrole derivatives.
Miscellaneous methods
New methodologies devoted to the synthesis of 5-mem-bered heterocycles have allowed for the preparation of 2,2'-bipyrrolic species. Within this context, Pagenkopf and co-workers described a trimethylsilyl trifluorometh-anesulfonate (TMSOTf)-mediated reaction between donor-acceptor cyclopropanes (83) with 2-cyanopyrroles and 2-cyanothiophene (84) as a means of preparing 2,2'-bipyrroles and 2,2'-thienylpyrroles (85, Scheme 18).[5253]
/ \ X X
a)
// w
n'
H
72a: X = О 72b: X = S
1.X = 0, PIFA, TMSBr, DCM X = S, PIFA, BF3 OEt2, DCM
2. POCI3, DMA, DCE
OHC
73a: X = О 73Ь:X = S
CHO
McMurry
MeO
b)
MeO
MeO
74a: X = О 74Ь: X = S
Scheme 16. Synthesis of raeso-substituted porphycenes (a) and synthesis of ß-octamethoxyporphycene 74a and ß-octakis(methylthio) porphycene 74b (b) and synthesis of bis(phenanthropyrroles) 76 via a tandem Scholl reaction (b).
// V
H 77
DBDMH
CHO
THF, -78 °C, rt, 5h 79-85%
К
Вг^м-^СНО
78
92, a)
71-79%
79
CHO
Л1) LDA, triisopropyl borate OMe THF, 0-5 °C, 1 h // ^
-► (HO)2
. 2) aq. KHSO4/NH4CI, rt, 2h H
OMe --------n
R2
CHO
BOC 80
95%
81
N-boc-bromopyrrole
a) t
73-76%
X—//
82
CHO
Scheme 17. Synthesis of 2,2'-bipyrroles 79 and 82 via the Suzuki reaction as reported by Reynolds et al.[51] a) i. Pd^Ph^, dioxane/H2O,
Na2CO3, 100 °C, 3 h, ii. LiOH, THF-MeOH, rt, 30 min.
COzEt R3
AlkylO R, R3_Мвз8ЮТ, R2xi /C/R4 X= orNH
H^/VC02Et + -- )Г /—<f J] Ri = R2 = alkyl or H
iX X CN R1 H X R3 = R4 = alky,orH
83 84 85
Scheme 18. Syntheses of unsymmetrical bipyrroles and thienylpyrroles 85 as reported by Pagenkopf et al.[5253]
The Pagenkopf bipyrrole synthesis allowed for complete regiocontrol, as well as for the preparation of new asymmetric bipyrroles and thienylpyrroles 85. As a general rule, these products could not otherwise be accessed or, if so, only through tedious synthetic means.
Motivated by a desire to prepare bicyclic pyrrole-containing structures, Opatz and co-workers reported a cycliza-tion procedure that allowed for the preparation of 5,6,7,8-tet-
rahydroindolizines, 2,3-dihydro-1^-pyrrolizines, 2,2'-bipy-rroles 88, 6,7,8,9-tetrahydro-5^-pyrrolo[1,2-a]azepines 89, and 5,5'-bis(5-cyano-1-pyrrolines) 90.[54] Starting from readily available 3,4-dihydro-2^-pyrrole-2-carbonitriles 87, this method furnished 2,2'-bipyrroles with a,a'-aryl substituents (Scheme 19).[55]
Trofimov and co-workers reported an elegant example of rearrangement chemistry applied to the preparation
90
Scheme 19. Synthesis of 2,2'-bipyrroles via a two-step route from commercial available substrates as reported by Opatz, et al.[55]
of pyrrolo-bicyclic 1-amino-3-iminopyrrolizines (94). The route involved carbodithioation of 2,3-disubstituted pyrrole 91 followed by synthesis of the cyano-substituted pyr-rolizine ring (93). Benzoylation of the methylated amine followed by pyrrolizine ring opening and an intramolecular cyclization cascade gave 2,2'-bipyrrole 95. This route provided access to purine pyrrole analogues, as well as cyano-substituted bipyrroles. However, it is limited in its application as it requires a-substituents and protection of the pyrrole nitrogen atom (Scheme 20).[56] In a similar vein, Wurthwein and co-workers described the preparation of several pyrrole derivatives, among them 2,2-bipyrroles, via the Aza-Nazarov reaction of 1-aza-1,4-pentadien-3-ones. Unfortunately, in this case the complex preparation of the starting materials and the substitution pattern of the product reduced the applications-related utility of the method.[57]
Functionalization of 2,2'-bipyrroles
Most synthetic methods for functionalization of 2,2'-bipyrroles rely on electrophilic substitution reactions, such as acylations, notably the Vilsmeier-Haack reaction and halogenations. Other approaches preinstall in the starting materials the functional groups that will
appear in the p-positions of the desired p-substituted-2,2'-bipyrrole. Examples of the latter approach include Ullman couplings involving a p-substituted iodo-pyrrole or the copper coupling of a stannyl derivative prepared from the correspondent cinnamate. All these methodologies have in common the fact that the p-substituted groups must be installed from the beginning of the synthesis, and in some cases the preparation of such derivatives can be long and tedious. Borrell and co-workers circumvented what were previously considered to be canonical preparative limitations by means of desulfurization of thienodipyrroles 101 using Raney nickel. This strategy allowed for the modification of the typical 2,2'-bipyrrole substitution pattern as demonstrated by bromination and use of Suzuki cross-coupling procedures to give 4,4'-diaryl-2,2'-thienodipyrroles (101) (Scheme 21).[58]
For the most part preparations of diformyl 2,2'-bipyr-roles have relied on the use of the Vilsmeier-Haack reaction and related electrophilic aromatic substitution procedures. However, recently Sánchez-García and co-workers described the preparation of the first 5,5'-dibrominated-2,2'-bipyrrole (Scheme 22).[59] They studied the reactivity of dibrominated bipyrrole 103 under Suzuki and Stille coupling conditions and found they could produce several new oligopyrroles, such as quater pyrrole or 5,5'-bis(2-thienyl)-2,2'-bipyrrole (104, G=pyrrole or thiophene).[60]
N
H
91
b) Etl
f/S\ a) CS2,KOH/DMSO j/\ a) CH2(CN)2, KOH/DMSO Ri Et
N H
92
S Et
b) Etl
NH2Me
CICH2COPh KOH/DMSO 110-120 °C, 6h
R2
Ri
NC
N-
/
NH3
COPh
Scheme 20. Trofimov's synthesis of 2,2'-bipyrroles 95.[56] Макрогетер0циmbl /Macroheterocycles 2018 11(3) 227-245
N3
k
C02Et N3 N3 xylene JTYJl ^
ohc^s/-CHO Na0Et 115.c H ™
98
96 97
Br2, AcOH AcOEt
.. Br g Br
Y^fi RVYSVYR RB(OH)2'Pd(PPh3)4 JhCK
Et02C^N^AN^C02Et EtOH Eto Ji YA krn Ff ' rn Et02C^x^N^C02Et
I | btU20 nmx C02Et Na2C03, dioxane i i
G G il G G
G G
102: G = SEM 101:G = SEM 99: G = H
56: G = H 100: G = SEM
Scheme 21. Synthesis of 2,2'-bipyrroles from thienodipyrroles as reported by Borell, et al.[58]
Scheme 22. Synthesis of a-substituted 2,2'-bipyrroles by Sánchez-García et al.[59,60]
2,2'-Bipyrroles as versatile building blocks for the preparation of pyrrolyl macrocycles
Tetrapyrrolic porphyrinoids
Pyrrolic macrocycles represent some of the most important prosthetic groups found in nature. Indeed, tetrapyrrolic macrocycles play key roles in many of the biochemical pathways essential for life. This has led to intense study of classic synthetic and semi-synthetic tetrapyrrolic macrocycles, including porphyrin, corrole,[61] and porphycene.[21] These systems have remained a focal point of interest within innumerable research groups and have attracted attention across a wide swath of the chemical and medical communi-
ties. Corroles are related to porphyrins but distinct in terms of lacking a meso-bridge. Formally, they contain a 2,2'-bipyr-role incorporated within a macrocyclic structure. Corrole was first synthesized by Kay and Johnson. Initial efforts by Johnson and co-workers utilized an oxidative cyclization of a,c-biladienes to yield the corrole core.[61] The synthesis of corrole has since been modified to allow for a one pot procedure furnishing gram scale quantities of the desired system (Scheme 23a).[62-64] The mainstay of corrole chemistry continues to rely on the oxidative cyclization to prepare the 2,2'-bipyrrole and yield the macrocyclic core.
Porphycenes (6) were arguably the first reported structural isomers of porphyrin. These systems replace the dipy-rromethene subunit with 2,2'-bipyrrole, a substitution that endows the resultant porphyrinoid with new character-
Scheme 23. Synthesis of corrole 105 (a) and porphycene 6 (b). 236
istics. The original synthesis described by Vogel and coworkers implemented a modified synthetic route towards 5,5'-diformyl-2,2'-bipyrrole (53), followed by cyclization via McMurry coupling and oxidation (Scheme 23b).[21] The authors reported yields ranging from 2-50 % with a correlation between yield and solubility of the 2,2'-bipyrrole. Advances in the synthesis of various substituted 2,2'-bipyr-roles has resulted in improved yields of the porphycene core as well as tunable absorption profiles giving way to new photosensitizers with potential therapeutic applications.[65-72]
Pentaphyrins: sapphyrin
Sapphyrin (39 or 112) is regarded as the first reported expanded porphyrin and is recognized by its penta-aza-heterocyclic core containing tripyrrane and 2,2'-bipyrrole building blocks.[22] Following the initial report by Woodward, the groups of Johnson and Gibbs would explore modification of the heterocyclic constituents through the synthesis of oxa- and thiasapphyrins (Scheme 24).
[73] In a continuation of the efforts, the groups of Sessler and Ibers developed an optimized synthesis of sapphyrin.
[74] This new method improved yields and, most notably, implemented the Schollkopf-Magnus-Barton-Zard pyrrole synthesis allowing for multigram quantities of the ethyl 3,4-dialkyl pyrrole ester to be synthesized. The bipyrrole was prepared using standard copper mediated cross cou-
pling (Scheme 24).[7576] The synthesis of a new expanded porphyrin initiated an entire field of porphyrinoid chemistry while demonstrating the importance of 2,2'-bipyrrole as a central motif in these pyrrolyl macrocycles.[77-84]
Hexaphyrins
Sessler and co-workers continued their initial efforts to expand the chemistry of pyrrolic macrocycles by reporting the synthesis of rubyrin (117).[85] Rubyrin is a hexa-azaporphyrinoid containing two 2,2'-bipyrrole subunits bridged by a central pyrrole. The final compound was prepared utilizing a MacDonald coupling between intermediate tetrapyrrole 116 and bipyrrole 111 (Scheme 25). The first rubyrin to be reported (117) shows physicochemical properties similar to those of sapphyrin (112). In analogy to earlier efforts with sapphyrin, various heterorubyrins (119) were also prepared.[86,87] Several showed promise in terms of biomedical and anion sensor and recognition applications (Scheme 26).[75-86-88-89]
Sessler and co-workers prepared the next relative within the hexaphyrin family, so-called isoamethyrin (121). In analogy to what was true in the case of corroles, the 2,2'-bipy-rrole subunit of this expanded porphyrin was prepared via oxidative cyclization of a linear intermediate (120) (Scheme 27).[90] The development of this new oxidative coupling methodology was further applied to the synthesis
107
y-O.
OrL
N H
108
C02Bn
TsOH, EtOH
109: R = C02Bn \ ^ pd/c 110:R = C02H '
OHC' CHO
111
112
X = Z = NH, Y = O, S, Se or
X = O, S, NH,NCH3 Y = N Z = S,Se
113
Scheme 24. Synthesis of sapphyrin 112 and heterosapphyrins 113.
114
p-TsOH
/PrOH
voJK
108
C02Bn
115: R = C02Bn 116:R = C02H
)h2,
Pd/C
Vf
OHC' N' ^CHO
111
TsOH, EtOH 02
117
Scheme 25. Synthesis of rubyrin 117.
118
X = S, O, Se
1. Pyrrole, TFA 1 eq. DCM
2. p-chloranil
119
X = S, O, Se
Scheme 26. Synthesis of heterorubyrins 119.
Scheme 27. Synthesis of hexaphyrin 121.
a)
122
0,1 M FeCI3
HCI 1M
b)
U02[N(SiMe3)2]2
□CM
123: n = 1 (15%) 124: n= 2 (5%) 125: n= 3 (traces)
123
126
Scheme 28. Synthesis of cyclo[6]pyrrole 123 (a) and synthesis of uranyl complex of cylco[6]pyrrole 126 (b).
of a new class of hybrid expanded porphyrins, the so-called cyclo[n]pyrroles.[23,24'91] Cyclo[6]pyrrole (123) was obtained by oxidative coupling of 2,2'-bipyrrole in a CH2Cl2:HCl (1 M) biphasic system with FeCl3 (Scheme 28a).[23] Interestingly, the acid counteranion was also found to have a templating effect and could be modified to vary the ratio of cyclo[6]pyrrole (123), cyclo[7]pyrrole (124), and cyclo[8] pyrrole (125). It was found that several of the cyclo[n]pyr-roles could be useful in the realm of anion sensing; others, particularly cyclo[6]pyrrole showed promise in the area of uranyl cation coordination chemistry (Scheme 28b).[92]
The development of new methodologies for the synthesis of 2,2'-bipyrrole have provided the starting point for recent efforts by the groups of Sánchez-García and Sessler that led to a convergent approach for the construction of isoamethyrin (130)[93] and naphthoisoamethyrin (131),[94,95] respectively (Scheme 29).
Heptaphyrins
Heptaphyrins, as the name implies, are the next congener within the expanded porphyrin series after hexaphyrins. Building on their previous efforts, Sessler and co-workers used 2,2'-bipyrrole derivatives as building blocks for the synthesis of such compounds.[96,97] Heptaphyrin 134 was prepared in a two-step sequence from 2,2'-bipyrrole 36 and terpyrrole 132 by way of condensation followed by oxidative cyclization of linear precursor 133 (Scheme 30).[97] Osuka and co-workers have also disclosed the preparation of two heptaphyrins,[98] namely 135 and 136; they were prepared by a one pot reaction between pentafluorobenzalde-hyde, pyrrole, dodecyl sulfate, and Sc(OTf)3 under aqueous
conditions. These two heptaphyrins contain a 2,2'-bipyrrole, which results from the in situ formation of the linear precursor followed by oxidative cyclization.
Octaphyrins
Octaphyrins containing two 2,2'-bipyrrole moieties linked through two dipyrromethane subunits can be considered as 2x expanded corroles. The first octaphyrin system of this type was reported by Houk and co-workers and its synthesis relied on an acid mediated condensation between 2,2'-bipyrrole dialdehyde (53) and a dipyrrylmethane dicar-boxylic acid (137) (Scheme 31a).[99 100] More recently, Geier and co-workers have synthesized octaphyrin by coupling a dipyrromethanedicarbinol (140) with an a-unsubstituted 2,2'-bipyrrol 9 or an a-free dipyrromethane with a 2,2'-bypyrroledicarbinol.[101102] Houk and co-workers also reported the synthesis of octaphyrins (142-143) achieved through condensation of the 2,2'-bipyrrole 122 with the diformyl 2,2'-bipyrrole 141 (Scheme 31b).[99] Recent endeavors by Shinokubo and co-workers have also prepared octaphyrins (142 and 143) via Ni-templated intermolecular homocouplings of an a,a'-dibromodipyrrin 144 to yield nickel norcorrole with subsequent ring expansion effected through addition of an aryl Grignard.[103]
Sessler and co-workers have also described a route toward octaphyrins (e.g., 146) containing only two meso-bridges (Scheme 32). This synthesis relied on the acid catalyzed condensation of two 2,2'-bipyrrole subunits, which gives tetrapyrrolic precursor 145, a species that readily undergoes a tandem oxidative coupling and cyclization to give octaphyrin 146.[97]
Scheme 29. Synthesis of isoamethyrin 130 and naphthoisoamethyrin 131.
b)
OHC
142: R = H 143: R = Mes
1. NiCI2/PPh3
2. ArMgBr
Mes
Mes
144: Ni or Pd
Scheme 31. Synthesis of octaphyrins 138-139 (a) and 142-143 (b). Note: P-ethyl chains omitted for clarity in 143.
36
triethyl orthoformate
1) MeOH, TFA
2) NaOH
146
Scheme 32. Synthesis of octaphyrin 146 (a).
Utilizing methods they developed for the preparation of 2,2'-bipyrrole, Sánchez-García and co-workers have also prepared the octaphyrin 149 via a MacDonald condensation of quaterpyrrole 127 with the diformylated quaterpyrrole 148 in 33 % yield, and the cyclo[8]pyrrole 147 via oxidative coupling of quaterpyrrole 127 (Scheme 33).[59]
Expanded porphyrins containing more than eight pyrroles
The controlled synthesis of expanded porphyrins with more than eight pyrrolic constituents remains a chal-lenge.[104-106] Sessler and co-workers are among the first to describe such a compound in the preparation of turcasarin (151) (Scheme 34).[89 104] This porphyrinoid contains ten pyrrole units and exists in a dynamic equilibrium between two figure-eight conformations under normal laboratory condi-
tions, such as room temperature. Setsune and co-workers would continue to expand the porphyrin core by preparing a dodecaphyrin (153) and a hexadecaphyrin (154). Both targets were obtained via the reaction between 2,2'-bipyr-role 122 and benzaldehyde 152 (Scheme 35).[106] Modification of this protocol and using the 2,2'-bis(azafulvene) 155, derived from 2,2'-bipyrrole 156, with 2,2'-bipyrrole afforded the same compounds.[107 108]
Kobayashi and co-workers have also synthesized a cyclo[ 10] pyrrole (158) derived only from 2,2'-bipyrrole 157 by using an oxidative cyclization with croconic acid as a template (Scheme 36).[109] This remains the largest cyclo[n] pyrrole prepared to date.
Recent efforts devoted to expansion of the porphyrin core have led Sessler and co-workers to prepare one of the largest expanded porphyrin yet reported, namely hexadecaphyrin (160) (Scheme 37).[110] This compound
COR1 COR!
FeCI3
dcm-h2so41m
127a: R, = OEt 127b: R, = Met
OHC
148a: R, = OEt 148b: R, = Met
HCI/EtOH
CHO
CHCI3
147a: R1 = OEt 147b: R, = Met
149a: R, = OEt 149b: R1 = Met
Scheme 33. Synthesis of cyclo[8]pyrrole 147 and octaphyrin 149 from quaterpyrrole 127.
Scheme 34. Synthesis of turcasarin 151.
contains eight direct a-pyrrole-to-a-pyrrole linkages and was obtained via the condensation 2,2'-bipyrrole dialdehyde 141 with the hexapyrrolic derivative 159. This expanded porphyrin was shown to support the formation of different metal complexes that interconvert between different conformations as temperature is applied to the solution. The conformational changes produce color differences that may be observed by the naked eye.
Conclusion
The chemistry described in this review is a glimpse into the synthetic efforts devoted to the construction of both 2,2'-bipyrroles and systems containing this subunit within conjugated macrocyclic cores. As the synthetic protocols continue to improve and access to new precursors with varying substitution patterns emerge, the preparation
II Vf II
N'
H H 122
CHO
R J"\ ^R
152
1.TFA. DCM
2. DDQ
3. NaOH
153
154
OHC
1. no catalyst
2. DDQ
PhLi
CHO
li Vf il
-N N' H H
122
Scheme 35. Synthesis of dodecaphyrin 153 and hexadecaphyrin 154.
f-Bu f-Bu
f-Bu
157
f-Bu
Oxidant
croconlc acid CH2CI2
f-Bu f-Bu
f-Bu f-Bu
Scheme 36. Synthesis of cyclo[10]pyrrole 158.
2 x
CO,Et
159
2 x
OHC
CHO
1. HCI/EtOH, CHCI3
2. DDQ
160
Scheme 37. Synthesis of hexadecaphyrin 160 via the condensation of 159 and 141.
of even more complex targets becomes enabled, as gives access to systems of potential real world utility. Indeed, several decades of optimization have already yielded scalable procedures for therapeutically active porphy-cenes and 2,2'-bipyrrole-derived oligopyrrolic functional materials. We thus expect that 2,2'-bipyrrole will remain a keystone in the construction of new and interesting pyrrole-containing macrocyclic species.
Acknowledgements. G.A. would like to thank Fundación Ramón Areces for a postdoctoral fellowship. The work in Austin was supported by the National Institutes of Health (CA 686582 to J.L.S.) and the Robert A. Welch Foundation (F-0018 to J.L.S.). J.T.B. would like to thank Axel Steinbrück (UT Austin) for translating the paper by Grabowski and Marchlewski from German to English.
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Received 17.04.2015 Revised 51.05.2015 Accepted 02.06.2015
