1 Leibniz-Institut für Katalyse e. V. an der Universität Rostock SYTESIS AD CATALYTIC FUCTIALIZATI F BILGICALLY ACTIVE IDLES Dissertation zur Erlangung des akademischen Grades doctor rerum naturalium (Dr. rer. nat.) der Mathematisch-aturwissenschaftlichen Fakultät der Universität Rostock vorgelegt von icolle Schwarz, geb. am in Schwerin Rostock, 01. Juli 2008 urn:nbn:de:gbv:28-diss
2 Die vorliegende Arbeit entstand in der Zeit von ktober 2005 bis Juni 2008 am Leibniz-Institut für Katalyse e. V. an der Universität Rostock. Einreichung der Dissertation: 01. Juli Gutachter Prof. Dr. Matthias Beller, Leibniz-Institut für Katalyse e. V. an der Universität Rostock 2. Gutachter Prof. Dr. Armin Börner, Leibniz-Institut für Katalyse e. V. an der Universität Rostock Rigorosum: 06. ktober 2008 Prüfungsvorsitzender: Prüfer auptfach (rganische Chemie) Prüferin ebenfach (Toxikologie) Prof. Dr. Udo Kragl, Institut für Chemie, Universität Rostock Prof. Dr. Matthias Beller, Leibniz-Institut für Katalyse e. V. an der Universität Rostock PD Dr. Birgit Tiefenbach, Zentrum für Pharmakologie und Toxikologie, Universität Rostock Wissenschaftliches Kolloquium: 28.ktober 2008
3 Für meine Eltern Siegfried und Kerstin Schwarz
4 erzlicher Dank gebührt meinem hochgeschätzten Lehrer Prof. Dr. Matthias Beller für sein großes Interesse an meiner Arbeit, für sein Vertrauen und Wohlwollen in fachlicher und persönlicher insicht und für die stets gewährte wissenschaftliche Freiheit.
5 in besonderer Dank gilt Dr. Annegret Tillack für die vielen anregenden Diskussionen, die stete ilfsbereitschaft und Zuverlässigkeit, sowie für den gemeinsamen Weg während meiner Diplom- und Doktorandenzeit. in weiterer Dank gilt: Dr. Anahit Pews-Davtyan für die vielen Anregungen und die Kooperation auf dem Gebiet der Indolsynthesen. Dr. Thomas Schareina für die stete Kooperativität und angenehme Laboratmosphäre. Dr. Vivek Khedkar für die Einführung und Zusammenarbeit auf dem Gebiet der ydrohydrazinierung von Indolen. Dr. Dirk Michalik für die Aufnahme zahlreicher MR-Spektren und dessen ilfe bei den Auswertungen. Dr. Ralf Jackstell für die Synthese und Bereitstellung zahlreicher Liganden. anns Martin Kaiser für die stetige ilfsbereitschaft und Vermittlung zur Universität Durham. Dr. Christine Fischer für das schnelle Anfertigen und Auswerten zahlreicher Massenspektren sowie der gesamten Aminierungsgruppe, Dirk ollmann, Karolin Alex und Sebastian Bähn für die gute Zusammenarbeit. Der gesamten Beller Arbeitsgruppe für die freundliche Aufnahme, die kollegiale Atmosphäre im Arbeitskreis und die Kooperativität. Kathleen vius, Susanne Buchholz, Susanne Schareina, Kathrin Reincke, Astrid Lehmann, Karin Buchholz, Andreas Koch und Dr. Wolfgang Baumann für das ssen zahlreicher analytischer Daten. Prof. Dr. Todd B. Marder für die freundliche Aufnahme in seine Arbeitsgruppe und die stete ilfsbereitschaft. Der Firma Esteve, insbesondere Dr. José Luis Díaz und Dr. Jörg olenz für die Kooperation und interessanten Diskussionen. Dem Graduiertenkolleg 1213 für die finanzielle Unterstützung. in besonderer Dank gebührt meinen Eltern, meiner Schwester Kathrin Schwarz und meinem Freund Falko Wahnschaff für ihr immer währendes Vertrauen und ihre Unterstützung.
6 II Abstract Universität Rostock SYTESIS AD CATALYTIC FUCTIALIZATI F BILGICALLY ACTIVE IDLES by icolle Schwarz Leibniz-Institut für Katalyse e. V. an der Universität Rostock The indole ring system constitutes one of the most important heterocycles in nature. This thesis presents the synthesis of new indole derivatives and their further functionalization to potential 5T 6 -receptor ligands. Based on our prior synthesis to tryptamines and tryptopholes we investigated a novel synthetic strategy to innovative indole scaffolds via Ti- or Zn-catalyzed hydroamination and hydrohydrazination of terminal alkynes. Thereby we developed the pharmaceutically interesting class of 3-silyloxyindoles and their application in several palladium-catalyzed reactions. Both Buchwald-artwig amination, C- cross-coupling, nucleophilic substitution and sulfonylation were utilized and optimized. All pharmaceutical relevant final products and their intermediates were tested to be biologically active. Additional achievements in the field of indole synthesis are included.
7 III Table of Contents Preface...VII 1 Palladium-catalyzed Coupling Reactions of Indoles Introduction Suzuki-Miyaura Coupling Reaction eck Coupling Reaction Sonogashira Coupling Reaction Buchwald-artwig Amination Diaryl Ether Coupling Reaction Summary References bjectives of this Work Publications Titanium-Catalyzed ydroamination of Propargyl Alcohol Derivatives: Synthesis of 3-Silyloxy-2-methylindoles via ydrohydrazination Zinc-Promoted ydrohydrazination of Terminal Alkynes: An Efficient Domino Synthesis of Indoles A ovel Palladium Catalyst for the Amination of Electron-Rich Indole Derivatives Palladium-Catalyzed C- and C-C Coupling Reactions of Electron-Rich Indoles Synthesis of 3-(2-,-Diethylaminoethoxy)indoles as Potential 5-T 6 Receptor Ligands Palladium-Catalyzed Amination and Sulfonylation of 5-Bromo-3-[2- (diethylamino)ethoxy)indoles to Potential 5T 6 Receptor Ligands A Convenient and General thod for the Synthesis of Indole-2,3- dicarboxylates and 2-Arylindole-3-carboxylates Selective Reduction and Functionalization of Diethyl 1-alkyl-1indole-2,3-dicarboxylates
8 Table of Contents IV 3.9 General Zn-Catalyzed Intermolecular ydroamination of Terminal Alkynes Zn-Catalyzed Synthesis of Pyrazolines and Pyrazoles via ydrohydrazination First Synthesis of 4,5-Dihydro-3(2)-pyridazinones via Zn-mediated ydrohydrazination
9 V List of Abbreviations Ac aq Ar BIAP Bn Boc Bu nbu tbu tbu (tbu 2 ) BP-phos Bz Cat. Conv. Cy dba DBU DCM DME DMF DMS DPEphos dppf dppm EM EWG equiv Et et al. etc. Et EP GC h L LiMDS m Acetyl Aqueous Aryl 2,2 -Bis(diphenylphosphino)-1,1 -binaphthyl Benzyl tert-butyloxycarbonyl Butyl normal-butyl tert-butyl tert-butanol 2-(Di-tert-butylphosphino)bi-phenyl Benzoyl Catalyst Conversion Cyclohexyl Dibenzylidene acetone 1,8-Diazobicyclo [5.4.0]-undec-7-ene Dichlormethane Dimethoxyethane Dimethylformamide Dimethylsulfoxide Bis(2-diphenylphosphinophenyl)ether Bis(diphenylphosphino)ferrocen Bis(diphenylphosphino)methane Ethoxymethyl Electron-withdrawing group Equivalent Ethyl et alii, et aliae or et alia et cetera Ethanol Receptor for E series ofpprostaglandins Gas chromtography our Ligand Lithium hexamethyldisilazide ta
10 List of Abbreviations VI M min MM f MR Ac o p Ph Ph Ph pin Piv ipr R rt SEM Sphos TBAB TBAI TBDMS TBDPS TBS Tf TF TIPS T Tos TPABr X Xantphos Xphos Y Molar thyl thanol Minute thoxymethyl ether onaflate uclear Magnetic Resonance Acetate thoxy rtho Para Phenyl Benzene Toluene Pinacol Pivaloyl iso-propyl rganic rest Room temperature (Trimethylsily)ethoxymethyl Dicyclohexylphosphino-2',6'-dimethoxybiphenyl Tetrabutylammonium bromide Tetrabutylammonium iodide tert-butyldimethylsilyl tert-butyldiphenylsilyl tert-butyldimethylsilyl Triflate Tetrahydrofuran Triisopropylsilyl Turnover number Tosyl Tetrapropylammonium bromide alide 4,5-Bis(diphenylphosphino)-9,9-dimethylxanthene 2-Dicyclohexylphosphino-2,4,6 -triisopropylbiphenyl alide
11 VII Preface The search for drugs is an old human dream and constitutes still a real challenge for scientists. Already alchemists sought for the one drug, the Elixir healing all human ailments. Today, the growing knowledge in pharmacy and biology makes the search for novel drugs complex and difficult. Therefore it is not surprising that organic chemists are interested in the development of easy accessible synthesis to new potential drugs. An interesting target for drug design is the serotonin (5-hydroxytryptamine 5-T) receptor ligand. It is one of the oldest known neurotransmitter in the central nervous system and responsible for controlling our body temperature, mood, sexual drive and sleep rhythm. This hormone is implicated in many diseases like schizophrenia, anxiety, obesity and Alzheimer s disease. Based on the indole skeleton it is an interesting building block which is of importance to the pharmaceutical industry. The application of new 5-T receptor ligands against these diseases demanded an easy access to indole derivatives and wide scope of functionalization. tal-catalyzed cross-coupling reactions have profoundly changed the protocols for the construction of natural products, building blocks of supermolecular chemistry and lead compounds in medicinal chemistry from simpler entities. The development of efficient new carbon-carbon bond forming reactions by metalcatalyzed cross-coupling has continued to progress dramatically and some of the reactions are finding industrial application on the multipleton scale. Furthermore new protocols for C- and C- couplings have been introduced and find particular use in the synthesis of biologically active compounds for pharmaceutical application. Cross-coupling reactions play an important role in the development of new 5-Treceptor ligands as well. Finding the optimized conditions in C-- or C-- bond forming processes is one of the key steps in the synthesis of new indole derivatives.
12 1 1 Palladium-catalyzed Coupling Reactions of Indoles 1.1 Introduction Indoles represent an essential class of heterocyclic compounds in nature. Based on the various biological actions, this ring system has become a privileged building block for the pharmaceutical industry. owadays, a variety of drugs with significant structural diversity and different biologically activity belong to the indole family.  In Figure 1 few selected examples of known pharmaceutical agents based on the indole scaffold are shown. The corresponding medical indications are listed in Table 1. S 2 a b c 2 S d e f C C 2 g Ac C 2 C h i j Figure 1. Selected biologically active compounds with indole skeleton. In general, indole derivatives are used for diseases related to the central nervous systems (CS), e.g. against migraine headaches like sumatriptan a, lisurid e, and ergrotamin h, or vincristine g against Parkinson s disease.
13 1 Palladium-catalyzed Coupling Reactions of Indoles 2 Clearly, the development of novel methods for the synthesis of indoles is stimulated by their widespread utility in life science.  ver the last century the preparation and functionalization of indoles continued to be an important object of research. Today, a range of well-established classical methods are available. Typical examples include the Fischer indole synthesis, the Gassman synthesis, the Madelung cyclization, the Bischler indole synthesis and the Batcho-Leimgruber synthesis.  Table 1. Selected indole drugs and their medical indication. ame Domain Drug a Sumatriptan migraine headaches, hypertonia Imigran b latonin hormone, primary insomnia Circadin c Tryptophan epilepsy, depression L-Tryptophanratiopharm, Ardeydorm d Pergolid Parkinson s disease Parkotil e Lisurid Parkinson s disease, migraine headaches Dopergin f Reserpin hypertension Briserin g Vincristine cancer chemotherapy ncovin h Ergotamin migraine headaches Ergo-Kranit I Ajmalin cardiac arrhythmia Gilurytmal j Yohimbin hypertension, aphrodisiac Yocon-Glenwood Yocon, Yohimbin Spiegel In addition, a variety of more modern transition metal-based syntheses and domino reactions have been developed. In general, the availability of starting materials and the functional group tolerance defines the suitability of the respective indole synthesis.  In the last two decades transition metal-catalyzed coupling reactions have dramatically improved the synthesis of biologically active molecules. 
14 1 Palladium-catalyzed Coupling Reactions of Indoles 3 R- 2 R- Diaryl Ether Coupling Buchwald-artwig Amination R eck Reaction X R-B() 2 Suzuki-Miyaura R Sonogashira C/ 2 Carbonylation R Arylation Figure 2. Catalytic functionalizations of indoles (X = I, Br, Cl, Ts, Tf, etc.). Especially palladium-catalyzed carbon-carbon, carbon-nitrogen, and carbonoxygen bond forming reactions have become powerful tools for their synthesis. [6,7] Striking features of these methods are their tolerance towards a wide range of functional groups on both coupling partners and their ability to construct efficiently complex organic building blocks in few steps. In Figure 2 several palladiumcatalyzed coupling reactions of indoles are illustrated. In this review the more recent developments from 2003 to 2008 in palladium-catalyzed coupling reactions of indoles are highlighted and summarized. Emphasis is given on those reactions leading to new substituted indole derivatives and less on coupling reactions of azaindoles, carbazoles, and oxindoles.
15 1 Palladium-catalyzed Coupling Reactions of Indoles Suzuki-Miyaura Coupling Reaction The cross-coupling reaction of organoboron reagents with aryl or vinyl halides in the presence of base and palladium catalysts is commonly known as the Suzuki- Miyaura reaction.  After the first application in natural product synthesis in 1981 by Rossi and co-workers [9e] it has been extensively applied over the past 20 years. [9,10] As a key step in the mechanism the coupling reaction of organoboron compounds involves transmetallation from boron to the corresponding arylpalladium(ii) halides. In the presence of (aqueous) base the reaction proceed smoothly and offers a wide range of selective C-C bond formations. rganoboron compounds are convenient reagents, generally thermally stable and inert to water and oxygen. Typically, these transformations can be performed without special precautions. Thus, it is not surprising that Suzuki-Miyaura coupling reactions of indolyl-based boronic acids and indolylboronates have found widespread application in the functionalization of the indole ring. Using different -protecting groups, all positions of the indole skeleton can be functionalized applying this cross-coupling protocol. For example, a large-scale preparation of -methyldihydro-indazoledien 4 is reported by udkins and co-workers.  As outlined in Scheme 1 the synthesis of indazole 4 is realized in 56% yield via palladium-catalyzed Suzuki-Miyaura coupling using 5-bromo-1-methyl-6,7- dihydro1-indazol 3 and indole-2-boronate 2. Conveniently, the 1-carboxyprotected indole-2-boranate 2 is produced in situ by sequential reaction of indole 1 with carbon dioxide and trimethylborate.  1 Br [P(PPh) 3 ] 2 PdCl 2 B TF, a 2 C 3 15 h, reflux 2 Li % Scheme 1. Palladium-catalyzed Suzuki-Miyaura cross-coupling reaction of indole-2-boranate.
16 1 Palladium-catalyzed Coupling Reactions of Indoles 5 Kumar and co-workers developed an attractive method for the substitution of indoles with novel 7-bromo-5,6-dihydroindole-2-ones.  This type of coupling partner is isosteric to indoles and is believed to possess similar biological activity.  B PdCl 2 (PPh 3 ) 2 Ph TF, 2 KF, Bu 4 I Br Ph % Scheme 2. Suzuki-Miyaura cross-coupling reation of 5-pinacol-indole with 7-bromo dihydroindole- 2-one. Scheme 2 shows an example of the coupling of the non-protected indole pinacol ester 5 by using a dual-solvent system, KF as base, and Bu 4 I as phase-transfer catalyst. ere, the desired product 7 is obtained in good yield (68%). batoclax 11 is known to be biological active and shows potent anticancer activity in several animal tumor models. This novel indolylprodigiosin derivative has the ability to antagonize multiple members of the B-cell lymphoma family of antiapoptotic proteins.  The three-step synthesis involves a haloformylation reaction [16, ] followed by a Suzuki cross-coupling reaction with an indole-2-boronic acid 9 to give compound 10a (Scheme 3). The Suzuki reaction proceeded rapidly in the presence of 3 mol% Pd(Ac) 2 and 12 mol% PPh 3. owever, an excess of boronic acid is required for an acceptable yield. The mixture of different solvents slowed down the decomposition of indole-2-boronic acid 9 and raised the yield additionally. Subsequent hydrolysis of both the enamine and the -methoxy carbonyl group afforded the indolylpyrrole aldehyde 10b. The final step includes the acid mediated condensation of 10b with 2,4-dimethyl-1-pyrrole to provide the Cl-salt of batoclax 11. otably, this three-step synthesis was recently applied to the production of batoclax on kg-scale to support clinical studies. Sapi and co-workers described a short synthesis of fused indole heterocycles via palladium-catalyzed cross-coupling. 
17 1 Palladium-catalyzed Coupling Reactions of Indoles 6 Br 8 Pd(Ac) 2 (3 mol%) PPh B() 3 (12 mol%) 2 dioxane,2, toluene C 10a hydrolysis 11 batoclax 10b (48 % over two steps) Scheme 3. Suzuki-Miyaura coupling key step in the synthesis of batoclax. Therefore the authors investigated the reaction of 5-bromoindole 12 and 2-(2,2- dimethylpropionylamino)phenyl boronic acid  14 (Scheme 4). Using Pd(PPh 3 ) 4 as catalyst system and 10% aqueous ac 3 solution or a 2 C 3 as base in DME, the corresponding diaryl compound 17 is formed in 66% yield. Because of problems in the deprotection of the pivaloyl- protecting group, the authors also performed the cross coupling of 12 with the -Boc-protected 2-aniline boronic acid 15 under the same conditions with a yield of 55%. They got higher yields of the same coupling product, using 1-indole boronic acid 13 with -Boc-protected 2- bromoaniline 16 (66%). Y X Pd(PPh 3 ) 4 (3-6 mol%) 10% ac 3 aq or a 2 C 3 (2.4 equiv) Y R R X DME, R R 12 Br 13 B() 2 14 Piv B() 2 15 Boc B() 2 16 Boc Br 17 Piv 18 Boc Si 2, microwave from Scheme 4. Palladium-catalyzed Suzuki-Miyaura reaction to indole derivatives.
18 1 Palladium-catalyzed Coupling Reactions of Indoles 7 After deprotection the desired compound 19 is obtained with a varying amount of the tert-butyl-substituted derivative 20. This type of side product is well known in peptide synthesis using Boc-strategies. Furthermore, the same group reported Suzuki-type coupling reactions with azidesubstituted arenes.  Among the different coupling reactions also 1-azido-2- bromobenzene  21 reacted with 1-indole-5-boronic acid 13 to the desired azido-diaryl compound 22 in 88% yield (Scheme 5). Decomposition of 22 by heating gave the annulated aromatic polycycles 23 and ( 2 ) 2 B Pd(PPh 3 ) 4 (3-6 mol%) 10% ac 3 aq or a 2 C 3 (2.4 equiv) Br DME, 100 C 3 22 (88%) 24 (0-5%) 23 (65%) Scheme 5. Suzuki-Miyaura coupling reaction of 1-azido-2-bromobenzene with 1-indole-5-boronic acid. Different strategies for the synthesis of annulated indoles such as cryptaustoline and cryttowolin  were performed by de Koning and co-workers.  These alkaloides have been reported to possess anti-leukemic and anti-tumor activities. Thus, indole-2-yl boronic acid 25, typically prepared with butyl lithium and borate, was dissolved in ethanol and reacted with naphthalene 26 in the presence of Pd(PPh 3 ) 4 (10 mol%) and a 2 C 3 as base (Scheme 6).  The coupling intermediate 27 is obtained in a yield of 96% over two steps. oteworthy, deprotection of the Boc-group and formation of the aromatic ring  gave instead of the expected naphtho[a]carbazole 29 the isoquinoline 28. In addition, other syntheses towards the cryptaustoline nucleus involving Suzuki-Miyaura couplings of indoles as key step have been mentioned. 
19 1 Palladium-catalyzed Coupling Reactions of Indoles 8 Br C Pd(PPh 3 ) 4 (10 mol%) a 2 C 3 aq B() 2 DME 25 Boc C Boc (96% two steps) Ph 29 Ph 28 Scheme 6. Suzuki-Miyaura coupling reaction of indole-2-yl boronic acid and dihydro-naphthalene. When using indoles in Suzuki-Miyaura reactions this chemistry involves in general the utilization of indolyl halides. In this respect, Larock and co-workers analyzed novel 1,4-palladium migrations.  More specifically, they observed 1,4-palladiummigration in organopalladium intermediates derived from o-halobiaryls. As an example they examined the reaction of 2-iodo-1-methyl-3-phenylindole 30 and boronic acid 31 (Scheme 7). To avoid that the base activates the aryl boronic acid prior to palladium migration, the system was buffered using a combination of cesium pivalate and pivalic acid. ence, components 30 and 31 were stirred at 100 C in the presence of 5 mol% Pd(Ac) 2, 5 mol% (PPh 2 ) 2 C 2 (dppm), 2 equiv of cesium pivalate and 2 equiv of pivalic acid in DMF containing 20 equiv of water. After 3 h product 32 is obtained selectively in 67% yield. Interestingly when 3-(2- iodophenyl)-1-methylindole 34 is allowed to react with 31 under the same conditions, product 32 is produced in a yield of 57% along some traces of isomer 33. I Pd(Ac) 2 (5 mol%) Ar Pd(Ac) 2 (5 mol%) dppm (5 mol%) dppm (5 mol%) Ar-B() 2 Ar Ar-B() 2 32 (67%) 32 (57%) 33 (0%) 33 (traces) Ar = p- 2 CC I Scheme 7. 1,4-Palladium-migration of an o-halobiaryl with an arylboronic acid.
20 1 Palladium-catalyzed Coupling Reactions of Indoles 9 This indicated a clear preference for palladium-migration from the phenyl ring to the indole ring.  Under non-migration conditions  adduct 33 is obtained exclusively in 79% yield when 34 is allowed to react with boronic acid 31. Apparently, palladium migration could be activated or suppressed by manipulation of the reaction conditions. Systematic investigations of potential racemizations in aryl-aryl Suzuki couplings were disclosed by Williams, Giralt and co-workers.  The authors focused on a model reaction where the aryl halide coupling partner was a racemizeable probe molecule 37 and a simple arylboronic acid (13, 35, 36) as its counterpart (Scheme 8). [28,29] The authors were interested in employing the indolyl boronic acids in particular because the indolylphenylglycines produced are models of the biaryl bisamino acids found in the chloropeptins.  In the presence of Pd(PPh 3 ) 4 as catalyst, all couplings were carried out in a mixture of toluene-- 2. Using a 2 C 3 (1.5 equiv) as base in couplings between 13, 35, 36 and hydroxyphenylglycine 37, the corresponding biaryls are produced in 85%, 68%, and 77% yield, respectively. In the case of compound 38 they indicated that 29% of the enantiomer had been produced, analyzed by PLC. Changing the base to (C 2 Ca) 2 the yields dropped to 52%, 46%, and 42% but led to optically pure compounds. As a non-typical base for Suzuki-Miyaura coupling reactions, (C 2 Ca) 2 could be an excellent test case in screening reactions for racemisation of sensitive substrates. 5 6 () 2 B 13 indol-5-yl boronic acid 35 indol-6-yl boronic acid 36 indol-7-yl boronic acid 7 Boc Br Pd(PPh 3 ) 4 a 2 C 3 (1.5 equiv) or (C 2 Ca) 2 toluene, Boc indol-5-yl (85%, 52%) 39 indol-6-yl (68%, 46%) 40 indol-7-yl (77%, 42%) Scheme 8. Suzuki-Miyaura coupling reaction of a racemic molecule and indole-5 (6 or 7)-yl boronic acid.
21 1 Palladium-catalyzed Coupling Reactions of Indoles 10 A wide-ranging study for Suzuki reaction of nitrogen-containing cross-coupling partners has been performed by Fu and co-workers.  The authors reported on a new catalyst system that achieved Suzuki-Miyaura cross-couplings of an array of nitrogen-containing boronic acids and aryl halides. The presence of amino groups and nitrogen heterocycles, which are pervasive in medicinal chemistry, led often to lower reactivity in coupling reactions. For that reason the development of a general method for substrates including nitrogen heterocycles  continues to be an important issue. Table 2 shows the general method with one single procedure for all indole examples. In all cases under an argon atmosphere the heteroarylboronic acid, Pd 2 (dba) 3 and PCy 3 were added to a Schlenk flask and dissolved in dioxane. Finally the (hetero)aryl halide and aqueous K 3 P 4 were added. After 18 h at 100 C the coupling products were isolated in good yields. Thereby the indole was always posed as the boronic acid and led to interesting new indole structures, even with unprotected nitrogen.  This seems to be an efficient method and was used with inactivated aryl chlorides as well. Table 2. Suzuki-Miyaura cross-couplings of heteroarylboronic acids with inactivated aryl halides. Ar X heteroaryl B() 2 Ar = aryl, heteroaryl Pd 2 (dba) 3 (1mol%) PCy 3 (2.4 mol%) K 3 P 4 (1.7 equiv) dioxane/ C heteroaryl Ar Entry eteroaryl boronic acid Ar-X Yield [%] [a] 1 () 2 B Cl 78 B() 2 2 Ts Cl 85 3 () 2 B Cl 89 4 Boc B() 2 Br 30 [b] [a] Isolated yield. [b] Boc-protecting group was removed during the Suzuki reaction.
22 1 Palladium-catalyzed Coupling Reactions of Indoles 11 Buchwald and co-workers developed a similar access to heteroaryl compounds via Suzuki-Miyaura reaction.  They reported on active catalysts composed of palladium and dialkylbiphenylphosphino ligands A or B as a general system for the Suzuki reaction of challenging heterocyclic substrates. As shown in Scheme 9 an excess of indole boronic acid in the case of 41, 42, 44 and 45 reacted with various heteroaryl halide in the presence of Pd(Ac) 2 and K 3 P 4 in n-butanol (s-butanol for 45) at 100 C (43, 44) or at 120 C (41, 42, 45) to give the coupling products in high yields. In the case of 43 the indole was used as the heteroaryl halide component and Pd 2 (dba) 3 instead of Pd(Ac) 2. Unprotected amino groups are traditionally problematical in cross-coupling reactions. The free 2 -group retards the catalytical cycle by binding to the metal center. Therefore the basicity of the aminoheteroaryl moiety should directly associate to the efficacy of the palladium-catalyzed process. [33a, 35] For the success of the transformation previous article reported that it is essential to protect the 2 -group in one hand or the need of chelating ligands to prevent competitive binding of the substrate on the other hand.  This protocol presented a highly stable and active palladiumcatalyst system for the Suzuki-Miyaura coupling of pyridine, pyrrole, and indole boronic acids/esters with non chelating ligands. Pd(Ac) 2 /Ligand A or B R 1 -B() 2 R 2 -halide K 3 P 4 nbutanol, 100 C R 1 -R 2 R 1 = heteroaryl R 2 = heteroaryl ipr PCy 2 ipr PCy 2 ipr 2 ligand A ligand B TIPS 41 (91%) 42 (77%) 43 (97%) S 44 (96%) 45 (90%) Scheme 9. Suzuki-Miyaura coupling of indoles.
23 1 Palladium-catalyzed Coupling Reactions of Indoles 12 Further work applying the same catalytic system to a wider array of heteroaryl substrates was published one year later by the same workers.  Buchwald and co-workers used the same reaction conditions for various indole coupling products. Indole-derived boronic acids were investigated by several groups,  but only a few protocols considered the cross coupling of heteroaryl chlorides with these substrates. [31,34] A wide variety of heterobiaryls in good to excellent yields were developed by the authors (Table 3). Activated heteroaryl chlorides were successfully coupled with indol-5-yl boronic acids by utilization of the palladium-catalyst system. -thyl-5- indolyl boronic acid 46 smoothly reacted with 3-chloro-2,5-dimethylpyrazine, 3- chloropyridine and 5-chloro-2-thiophenecarb-aldehyde in 90%, 77% and, 96% yield, respectively (Table 3, entries 1-3) in the presence of Pd(Ac) 2 (2 mol%, 0.25 mol% for entry 3) and ligand A (see Scheme 9). The reaction mixture was solved in s-butanol and heated up to 100 C for h. Indol-5-yl boronic acid 13 was converted with inactivated heteroaryl chlorides 47d-g (Table 3, entries 4-7) and led to desired biaryls 48d-g in 71-91% yield. owever, for reaction completion it was necessary to increase the reaction temperature to 120 C in the presence of 2 mol% Pd 2 (dba) 3 and ligand B (see Scheme 9) in n-butanol. This method presented an efficient Suzuki-Miyaura reaction of indole boronic acids with heteroaryl chlorides. Table 3. Suzuki-Miyaura reaction of indole boronic acids. () 2 B R 1 13 R 1 = 46 R 1 = R 2 -Cl 47a-g R 2 = heteroaryl Pd-catalyst/ligand A or B K 3 P 4, nbu, 100 C R 2 48a-g R 1 Entry Ar-Cl 47 Ligand Product 48 Yield[%] 1 a Cl A 90
24 1 Palladium-catalyzed Coupling Reactions of Indoles 13 Table 3. Suzuki-Miyaura reaction of indole boronic acids, continued. Entry Ar-Cl 47 Ligand Product 48 Yield[%] 2 b Cl A 77 3 c Cl S A S 96 4 d Cl B 91 5 e S Cl B S 90 6 f Cl B 71 7 g Cl 2 B 2 77 Another catalyst system for palladium cross-coupling was published by Yu and colleagues.  The authors developed a microwave promoted Suzuki coupling reaction of aryl chlorides and boronic acids in an aqueous media in the presence  of PPd 2 shown in Scheme 10. Microwave irradiation has been reported to promote metal-mediated reactions like Suzuki-Miyaura cross-coupling.  They chose an aqueous solvent system (DMF/ 2, 5:1) based on Leadbeater s work.  The use of TBAI as an additive was beneficial in this mixed solvent system, similar to previous reports employing neat water as the solvent.  After optimization of the initial reaction conditions by varying temperature, ratio of reactants, and percentage of catalysts the method was applied to the synthesis of 5-arylindoles 50a-g (Scheme 10). Different aryl chlorides 49a-g were converted with indol-5-yl boronic acid 13 via palladium-catalyzed cross-coupling under special conditions, mentioned before, and resulted the desired compounds in good yields (61-76%).
25 1 Palladium-catalyzed Coupling Reactions of Indoles 14 Cl () 2 B PPd 2 (3 mol%) TBAI (12 mol%) Cs 2 C 3 (4 equiv) R R microwave, 150 C 49a-g 13 DMF/ 2 (5/1) 50a-g 15 min 50 a b c d e f g R: 4-4-F CF 3 4-C Yield [%] Scheme 10. Palladium-catalyzed Suzuki-Miyaura reaction of indol-5-yl boronic acid. The next example investigated the synthesis of indoles via palladium-catalyzed Suzuki-Miyaura reaction on a polystyrene solid phase. Combining solid phase synthesis with high throughput screening has made a significant contribution for lead discoveries and optimization in pharmaceutical research. For producing combinatorial libraries, solid phase synthesis is regarded as a very efficient method.  Suzuki reactions have been already applied to solid phase synthesis but a limited number of examples were reported for the coupling reaction of immobilized arylmetals with free aryl halides in solution.  T. Kasahara and Y. Kondo found a method where immobilized 3-pinacol-substituted indole reacted with different aryl halides under Suzuki standard conditions.  The paper describes the preparation of immobilized indolylboron and the subsequent arylation using palladium-catalyst (Scheme 11). The immobilized 3-iodoindole 51 was easily prepared from indole via iodonation followed by immobilization using chlorosulfonylated polystyrene. Continued along palladium-catalyzed borylation  via Murata s protocol  the indolylboron 52 was investigated. Palladium-catalyzed cross-coupling was  examined subsequently with Pd[P(tert-Bu) 3 ] 2 as catalyst and led to the according coupled products 53a-e in good to very good yields.
Lecture otes Chem 51B S. King Chapter 18 Electrophilic Aromatic Substitution I. Electrophilic Aromatic Substitution The most characteristic reaction of aromatic compounds is electrophilic aromatic substitution,
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