Recent Progress in the Cross-Coupling Reaction Using Triorgano- Silyl-Type Reagents

SYNLETT0936-52141437-2096 © Georg Thieme Verlag Stuttgart · New York 2017, 28, 1873–1884 account 1873 en

Syn lett T. Komiyama et al. Account

Recent Progress in the Cross-Coupling Reaction Using Triorgano- silyl-Type Reagents

Takeshi Komiyamaa Yasunori Minami*b Tamejiro Hiyama*b 0000-0034-7491-161 a Department of Applied Chemistry, Chuo University, Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan [email protected] b Research and Development Initiative, Chuo University, Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan [email protected]

Received: 25.02.2017 ployed. To assist the silicon–activator interaction, one to Accepted after revision: 27.03.2017 three electronegative heteroatoms such as a halogen or ox- Published online: 04.05.2017 DOI: 10.1055/s-0036-1589008; Art ID: st-2017-a0142-a ygen are often introduced at the silicon center, mainly for the sake of easy formation of the pentacoordinate silicate Abstract The silicon-based cross-coupling reaction has attracted species responsible for transmetalation to a transition-met- much attention over recent decades because there are many advantag- al catalyst. As a result, these silicon reagents become reac- es in using organosilicon compounds. However, the use of reagents with a triorganosilyl group as a key function remains to be established. tive enough to accomplish the cross-coupling, but mean- This account summarizes our recent progress in cross-coupling chemis- while they become air- and moisture-sensitive. Therefore, try with such silyl reagents. careful attention is required in the preparation and han- 1 Introduction dling of the halo- and oxysilanes. 2 Preparation of HOMSi Reagents from Aryl Bromides and Disilanes 3 HOMSi Reagents from Heteroaromatics and Hydrosilanes In contrast, tetraorganosilanes are characterized by 4 Cross-Coupling Polymerization with HOMSi Reagents their robustness and low toxicity. Accordingly, in terms of 5 Cross-Coupling with Aryl(triethyl)silanes stability and ease of handling, they are favorable. However, 6 Amination of Aryl Halides with N-TMS-Amines such reagents generally do not show enough reactivity for 7 Conclusion and Perspective the cross-coupling due possibly to inferior silicate forma- Key words cross-coupling, silylation, amination, organosilicon reagents, tion. In order to find cross-coupling-active alkenyl(triorga- copper, nickel, palladium no)silanes and appropriate activators, many chemists have made efforts to find a solution. Some triorganosilyl-type reagents are summarized in Figure 1. Hatanaka and Hiyama 1 Introduction first discovered that structurally simple vinyl(trimethyl)silanes coupled with aryl iodides upon activation by The cross-coupling reaction has been developed as a tris(dimethylamino)sulfonium difluorotrimethylsilicate common tool to form carbon–carbon and carbon–heteroat- (TASF).3 More than a decade later, Denmark disclosed that om bonds in π-conjugate systems and allows the construc- 1-alkenyl-1-methyl-silacyclobutanes underwent cross-cou- tion of a wide variety of frameworks of potent pharmaceu- pling in the presence of tetrabutylammonium fluoride ticals, agrochemicals, and electronic materials.1 To achieve (TBAF)4 and observed a ring-opening reaction of the four- the bond formation, the use of nucleophilic organometallic membered silacycle by water in TBAF to produce a silanol. reagents such as lithium, magnesium, zinc, aluminum, tin, They considered that this silanol2c was the true active spe- boron, silicon, zirconium, indium and others is essential. cies.4b Similarly, 2-pyridyl,5 2-thienyl,6 benzyl,7 phenyl,8,9 Among them, silicon is attractive as it is an earth-abundant 3,5-bis(trifluoromethyl)phenyl,10 allyl,11 and pentafluoro- element and organosilicon reagents are superior in terms of phenyl12 groups are shown to act as a leaving group to gen- stability, solubility, ease of handling, and accessibility. Now- erate cross-coupling-active fluorosilanes and/or silanols in adays, many types of organosilicon reagents are available situ. A substituent-induced intramolecular activation strat- for alkenylation, arylation, alkynylation, and alkylation of egy is also effective for the coupling. Shindo found that a organic halides.2 For successful reactions, a nucleophilic ac- carboxyl group β-cis to the silicon in alkenyl(trimethyl)sila- tivator such as a fluoride or hydroxide ion is generally em- nes promoted the cross-coupling through intramolecular

Syn lett T. Komiyama et al. Account

13 activation with Cs2CO3. Nakao and Hiyama invented tuted by a heteroaryl, or 3,5-bistrifluoromethylphenyl alkenyl[2-(HydrOxyMethyl)phenyl]dimethylSilanes (alke- group were suggested by Murata,21 Gevorgyan,22 and Murai nyl-HOMSi), which underwent the cross-coupling reaction and Takai.23 14 even in the presence of K2CO3. As described above, organo-HOMSis are examples of Cross-coupling-active aryl(triorgano)silanes were also cutting-edge reagents for silicon-based cross-couplings disclosed as in the case with the alkenyl silanes. Such active (Scheme 1). This type of cross-coupling reagent is applica- arylsilanes are summarized in Figure 2. Arylsilanes bearing ble not only to alkenylation and arylation, but also to al- allyl,15,16 2-(hydroxymethyl)phenyl,14 or (2-hydroxyprop-2- kylation.14g The organo-HOMSi reagents are stable but be- yl)cyclohexyl17 groups can be used to connect a wide range come highly reactive to effect the cross-coupling upon acti- of aryl groups to various aryl halides. Simple 2-pyridyl-18 vation by a weak base such as potassium carbonate.14 and 2-benzofuryltrimethylsilanes19 were employed for Moreover, the silicon residue, cyclic silyl ether 1, is recover- cross-coupling with aryl iodides in the presence of not only able and recyclable to any organo-HOMSi reagents by treat- a palladium catalyst, but also a stoichiometric transition- ment with an organolithium or organomagnesium reagent. metal mediator. A tert-butyldimethylsilyl group was used in Protection of the proximal hydroxy group makes the the cross-coupling only in the case of using 8-hydroxy-1- HOMSi totally cross-coupling inactive.14e,f Orthogonally re- naphthyl as an electrophile.20 Aryl(triorgano)silanes substi- movable protecting groups such as tetrahydropyranyl

(THP), methoxymethyl (MOM), acetyl (Ac), and silyl (SiR3) are applicable. This flexibility permits the construction of a

Biographical Sketches

Takeshi Komiyama was born metal catalysis. He received a thetic Organic Chemistry, Japan in Saitama in 1991, and is a Poster Award at the 5th CSJ (2016), the Shibuya Ken-ichi Ph.D. student at Chuo University Chemistry Festa (2015), the Ga- Award for Encouragement pre- guided by Professors Tamejiro kuin Kaicho Award from the sented by the Director of Chuo Hiyama and Yasunori Minami. Head of Chuo University Alumni University (2017), and the Pre- His research interest is the ex- Association (2016), the Student sentation Award (Industry Divi- ploitation of novel synthetic Presentation Award at the 71th sion) at the 97th CSJ Annual transformations of organosili- Kanto Branch Symposium orga- Meeting (2017). con compounds by transition- nized by the Association of Syn-

Yasunori Minami was born in ing student in 2009, before re- professor. His current research Mie prefecture in 1982. He ceiving his Ph.D. from Osaka interests are synthetic organic spent three months at the Labo- University in 2010 under the su- chemistry using transition-met- ratories of Professor John F. pervision of Professor Nobuaki al catalysts and main group ele- Hartwig (University of Illinois at Kambe. He then joined RDI at ments. Urbana-Champaign) as a visit- Chuo University as an assistant

Tamejiro Hiyama was born in he started his research as a Prin- Kyoto University in 2010, he has Osaka in 1946. He was appoint- cipal Investigator at the Sagami been at Chuo University as an ed Associate Professor at Kyoto Chemical Research Center. In RDI Professor. His research University in 1972, completed 1992, he moved to the Research stems from the invention of his Ph.D. under the supervision Laboratory for Resources Utiliza- synthetic methods for the syn- of Professor Hitosi Nozaki at tion, Tokyo Institute of Technol- thesis of biologically active Kyoto University in 1975, and ogy, as a full professor and then agents and liquid crystalline and spent a postdoctoral year at to the Graduate School of Engi- light-emitting materials. Harvard University working with neering, Kyoto University, in Professor Yoshito Kishi. In 1981, 1997. Since retirement from

Syn lett T. Komiyama et al. Account

HO R Me N Si R Ar SiMe SiMe3 (Het)Ar Si 3 R' Si Me2 EWG N Ar' 3 (Het)Ar SiMe2

with TASF with TBAF with TBAF with TBAF with K2CO3 with TBAF and CuI 15a 14a Hatanaka, Hiyama (1988)3 Denmark (1999)4a Itami, Yoshida (2001)5a Nakao, Hiyama (2003) Nakao, Hiyama (2005) Gros (2005)18a F R'' S Me Si O R R Si Si SiMe Me SiMe 3 Me2 2 SiMe R' R' 2 O N N with TBAF with TBAF with CsF with Ag2O with TBAF with TBAF (cat.) and Ag2O Hiyama (2002)6 Trost (2003)7 Hanamoto (2003)8 Itami, Yoshida (2006)16 Murata (2006)21 Whittaker (2008)18b CF3 R' O R'' R R SiMe3 OH R Si O N Me2 HO O SiMe3 Si CF3 R' SiMe (t-Bu) Me2 2 O Si(i-Pr)2 with TBAF with KOSiMe3 and 18C6 with Cs2CO3 10 9 13 Katayama, Ozawa (2003) Anderson (2004) Shindo (2005) with Cs2CO3 with KF and AgNO3 with NaOH Kita (2008)20 Mori (2009)19 Gevorgyan (2010)22 OH F F3C R F F R'' Si R' HO R Me2 R CF3 Si R' Si F Me2 Me2 R' F SiMe (Het)Ar SiMe2 2

with K2CO3 with TBAF with TBAF with Cs CO or K CO with TBAF Nakao, Hiyama (2005)14a Navasero (2006)11 Spring (2010)12 2 3 2 3 Nakao, Hiyama (2011)17 Murai, Takai (2015)23 Figure 1 Cross-coupling-active alkenyl(triorgano)silanes and their acti- Figure 2 Cross-coupling-active aryl(triorgano)silanes and their activa- vators. Blue units are utilized as coupling partners tors. Blue units are utilized as coupling partners

HO Pd cat. weak base [Pd(allyl)Cl] (0.5 mol%) O HO 2 + X R3 R1 R3 + O O Si CuI (3 mol%) 2 R1 SiR2 R RuPhos (2.1 mol%) + O O R1 HOMSi 1 K2CO3 (2.5 equiv) S Si THF/DMF, 75 °C Me2 S S 1 Br SiMe2 SiMe2 R m (m = MgX or Li) cross-coupling S coupling active Scheme 1 HOMSi reagents as cross-coupling nucleophiles coupling inactive

Cy2P Oi-Pr variety of oligoarenes easily. For example, 2-thienyl-HOMSi HO undergoes cross-coupling with THP-protected 2-(5-bromo- PPTS S MeOH thienyl)-HOMSi in the presence of a catalyst consisting of Oi-Pr SiMe2 S palladium, CuI, and RuPhos to give the protected bithienyl- RuPhos deprotection HOMSi, which can be deprotected to give cross-coupling- coupling active active bithienyl-HOMSi (Scheme 2). Scheme 2 A HOMSi as a cross-coupling nucleophile A similar idea of intramolecular activation was also developed by Tamao, who noted that ortho-oxybenzyl-substi- tuted phenylsilanes generated in situ from cyclic silyl ether 2, phenyllithium, and copper iodide, undergo cross-cou- As discussed before, some tetraorganosilanes are active pling with iodoarenes (Equation 1).24 Smith applied this ba- for the cross-coupling but lack the generality. Herein, we sic concept to cyclic silyl ether 3, which was easily convert- briefly review our recent progress in the development of ed into aryl-(lithioxy-methylphenyl)diisopropylsilanes synthetic methods for triorganosilyl-type reagents and upon treatment with an aryllithium and then cross-coupled their cross-coupling chemistry, taking advantage of such with aryl chlorides (Equation 2).25 silicon reagents.

Syn lett T. Komiyama et al. Account

1) PhLi (1.1 equiv) available for this reaction. This method allows the prepara-

THF I-C6H4-p-CN (1 equiv) tion of phenyl-HOMSi reagents containing any electron- O 0 °C, 10 min PdCl2(PPh3)2 (2 mol%) donating or electron-withdrawing reactive functional Me2Si Ph CN 2) CuI (1.1 equiv) DMF, 80 °C, 3 h group. RuPhos was effective for arenes with an electron-do- 0 °C, 10 min 2 (1.1 equiv) 87% nating aryl group and DavePhos for those with an electron- via withdrawing group. When RuPhos was used, HOMSi re-

agents with OMe, NPh2, and NHBoc groups at para positions CuO (5ba–da) were obtained in good yields, whereas the syn- Ph SiMe2 thesis of a HOMSi reagent with a CF3 group did not proceed Equation 1 effectively; the example with a bulky 2-tolyl aryl group re- duced the reactivity to afford low yields of the product.

Cl-C6H4-p-CF3 (1.0 equiv) CF F C 3 Xphos-Pd-G4 (3 mol%) O(PG) [Pd(allyl)Cl]2 (2.5 mol%) O(PG) 3 RuPhos (10 mol%) PhLi (1.3 equiv) XPhos (3 mol%) CuI (10 mol%) R O Ph CF3 THF THF, r.t., 24 h R Br + Si –78 °C to r.t., 2 h K2CO3 (2.2 equiv) i-Pr2 Si Si 88% H2O (0–4.0 equiv) Me2 Me2 2 dioxane/NMP, 100 °C 3 (1.4 equiv) via CF3 4PG (1.2 equiv) 5 F3C Pd NHMe MsO O(PG) O(THP) LiO XPhos Me R Ph Si(i-Pr)2 Xphos-Pd-G4

Equation 2 Si Si Me2 Me2

PG = THP (5aa) 73% R= OMe (5ba) 75% MOM (5ab) 80% NPh2 (5ca) 67% Ac (5ac) 60% NHBoc (5da) 74% 2 Preparation of HOMSi Reagents from Aryl Sit-BuPh2 (5ad) 54%

Bromides and Disilanes Scheme 3 Pd/Cu-catalyzed synthesis of protected electron-rich Ar- HOMSi reagents The reaction of the cyclic silyl ether 1 with organometallic reagents in Scheme 1 is a typical procedure to prepare any HOMSi reagent. However, it is not suitable in the case of In the case of DavePhos as a ligand, Ar-HOMSi reagents reagents containing a reactive functional group such as 5ea–ia with an electron-neutral or electron-deficient sub- formyl or cyano. To overcome this drawback, transition- stituent such as CF3, CN, Cl, Ac, and CHO were obtained in metal-catalyzed silylation of aryl electrophiles with hydro- moderate to good yields (Scheme 4), whereas the reagent 26 27 28 silanes, disilanes, or silylboranes is feasible. Indeed, with an NO2 group was hard to prepare by the present Kondo reported that the Pd-catalyzed silylation of aryl bro- method. mides using hydrido[2-(hydroxymethyl)phenyl]dimethylsi- lane (H-HOMSi) gave Ar-HOMSis containing acetyl and cya- O(THP) [Pd(allyl)Cl]2 (2.5 mol%) O(THP) DavePhos (10 mol%) 29 no groups, although the yields are moderate in the two CuI (10 mol%) R examples shown. We decided to scrutinize the catalytic R Br + K2CO3 (2.2 equiv) Si Si H2O (0–4.0 equiv) conditions for the synthesis of Ar-HOMSi and found that a Me2 Me2 2 dioxane/NMP, 100 °C Pd/Cu dual catalytic system was effective for the silylation 4THP (1.2 equiv) R= CF3 (5ea) 63% of aryl bromides with protected HOMSi-type disilanes 4 CN (5fa) 26% PG Cy2P NMe2 and could be used to prepare various protected aryl-HOMSi Cl (5ga) 63% Ac (5ha) 35% 30 reagents 5 (Scheme 3). For example, 4-bromotoluene re- CHO (5ia) 27% acted with THP-protected disilanes 4THP in the presence of DavePhos

[Pd(allyl)Cl]2 (2.5 mol%), RuPhos (10 mol%), CuI (10 mol%), Scheme 4 Pd/Cu-catalyzed synthesis of protected electron-neutral K CO (2.2 equiv), H O (4 equiv), and dioxane/NMP co-sol- 2 3 2 and electron-deficient Ar-HOMSi reagents vents to give THP-protected tolyl-HOMSi 5aa in 73% yield.

The disilane 4THP was prepared easily by the reaction of di- chlorotetramethylsilane with the corresponding aryllithi- Bis-HOMSi reagents were also successfully prepared. um. Based on the fact that CuI as a co-catalyst is particularly For example, 2,7-dibromo-9,9-dioctylfluorene was doubly effective for the silylation, a silyl copper reagent is probably silylated smoothly under conditions similar to those in produced as an active nucleophile by the reaction of 4, CuI, Scheme 3 and gave THP-protected bis-HOMSi 6a (Equation and K2CO3. Other protecting groups such as methoxymethyl 3), which was employed for polyarylene synthesis (vide in- (MOM), acetyl, and t-butyl(diphenyl)silyl (TBDPS) are also fra, see section 4).31

Syn lett T. Komiyama et al. Account

n-C8H17 n-C8H17 [Pd(allyl)Cl]2 (5 mol%) O O RuPhos (20 mol%) BrBrCuI (20 mol%) O O + 4THP n-C8H17 n-C8H17 K CO (4.4 equiv) (2.2 equiv) 2 3 H2O (8 equiv) Si Si dioxane/NMP (4:1) Me2 Me2 100 °C, 48 h 6a, 65%

Equation 3

3 HOMSi Reagents from Heteroaromatics [Ir(OMe)(cod)] (5 mol%) 2 (PG)O (PG)O Me4phen (10 mol%) and Hydrosilanes norbornene (1.5 equiv) + (i-Pr) O, 80 °C X 2 SiMe2 In contrast to the preparation of Ar-HOMSi reagents H SiMe2 X starting with aryl halides, catalytic C–H silylation of aro- 7PG, (1.5 equiv) 5 matic hydrocarbons is more attractive in view of green 32 chemistry. This type of transformation is made possible NN by C–H silylation of arenes catalyzed by a transition-metal Me4phen 33 34 35 36 complex, t-BuOK, B(C6F5)3, or a Brønsted acid. (PG)O (THP)O (THP)O HOMSi-type hydrosilanes, H-HOMSi 7, accessible by hy- dride reduction of 1 followed by protection of the resulting n-H25C12 OH group, are an appropriate form for the straightforward SiMe2 SiMe2 SiMe2 S X S synthesis of Ar-HOMSi. The total transformation is consid- Br PG = THP (5ja) 89% X = O (5ka) 93% 5ma, 93% ered a sustainable silicon cross-coupling protocol (Scheme Ac (5jb) 54% NH (5,a) 44% 5). Therefore, we decided to develop the straightforward Scheme 6 Straightforward syntheses of protected Ar-HOMSis via Ir- 37 synthesis of Ar-HOMSi reagents. catalyzed C–H silylation

XR3 R1 R3 R1 H (PG)O (PG)O Pd cat. + Using three equivalents of 7 and norbornene, disi- catalyst weak base THP O lylation was successfully effected to give Ar(HOMSi)2. In- H SiR2 C–H Silylation Deprotection R1 SiR2 Cross- Si deed, Ar(HOMSi) reagents 6b–e were readily accessible, coupling Me2 2 7 originating from 3-bromothiophene, benzodithiophene, H HOMSi 1 3,4-ethylenedioxythiophene, and 4,7-dithienylbenzothiadi-

1) Reduction with LiAlH4, 2) Protection azole (Figure 3) under the conditions in Scheme 6. The re-

sulting Ar(HOMSi)2 compounds are potential monomers for Scheme 5 A HOMSi-based sustainable cross-coupling cycle the synthesis of π-conjugated polymers used for organic electronics. According to Falck’s protocol,33c we first examined the Br reaction of benzothiophene with MOM-protected hydro- S Si Si silanes 7MOM using the [Ir(OMe)cod]2/4,7-di-tert-butyl-by- THP THP S pyridyl (dtbpy) catalytic system and isolated the desired SiTHP S SiTHP 6b, 88% 6c, 86% product in <10% yield. Optimization of the conditions led to (THP)O the isolation of THP-protected benzothienyl-HOMSi 5ja in SiTHP = 89% yield: [Ir(OMe)cod]2 (5 mol%), Me4phen (10 mol%), nor- SiMe2 bornene (1.5 equiv) in (i-Pr)2O (2 M) at 80 °C (Scheme 6). S N N An acetyl group is also a pertinent protecting group. Five- O O membered heteroaromatics such as benzofuran, indole, SiTHP SiTHP SS and 2-bromo-3-dodecylthiophene gave C2-silylated prod- S SiTHP SiTHP ucts in good to excellent yields. The carbon–bromine bond 6d, 98% 6e, 86% (dtbpy, t-BuCHCH ) did not interfere with this silylation, whereas pyridine, 2 pentafluorobenzene, and 1,4-dimethoxybenzene failed to Figure 3 Bis-HOMSi reagents via Ir-catalyzed double C–H silylation undergo silylation.

Syn lett T. Komiyama et al. Account

4 Cross-Coupling Polymerization with Pd[P(o-tolyl)3]2(2 mol%) HO DPPF (2.1 mol%) HOMSi Reagents CuBr⋅SMe2 (3 mol%) + Br Ar2 Br Ar1 Ar2 Ar1 1 Cs2CO3 (4.2 equiv) Polyarylenes play a significant role in organic electron- Ar SiMe2 3 Å MS 8 THF/NMP, 50 °C ics. To construct such π-conjugated molecules, cross-cou- (2.1 equiv) pling using organoboron or organotin reagents is the stan- 38 PPh2 dard approach at present. When organoboron monomers Fe PPh suffer from low solubility owing to their planar and rigid 2 skeletons, organotin reagents containing alkyl chains on the DPPF tin center are preferably employed. However, organotin re- n-C H 6 13 n-H C n-C H agents are toxic in general so that they are not suitable for 17 8 8 17 Ph Ph Ph Ph mass production in industry. In contrast, organosilicon PhS Ph compounds are in general stable and easy to handle with- n-H13C6 out such toxicity problems. However, silicon-based polym- 8a, 94% 8b, 92% 8c, 90% erization leading to polyarylenes has remained almost un- S S explored except for a few efforts.39 To achieve successful N N N N high-molecular-weight-polymer synthesis, we applied the Ph2N NPh2 HOMSi-based cross-coupling. However, the reaction condi- SS tions shown in Scheme 2 require strictly stoichiometric or- 8d, 90% 8e, 76% ganometallic reagents based on organic halides, whereas n-H17C8 n-C8H17 n-C8H17 S n-H17C8 the original HOMSi-based coupling requires loading of sili- N N con regents in a slight excess. Therefore, we pursued optimization of the reaction conditions to achieve cross-coupling polymerization using Ar-HOMSi reagents and exam- 8f, 87% ined double cross-coupling reactions using stoichiometric Scheme 7 Pd-catalyzed double cross-coupling of aryl dibromides with amounts of Ar-HOMSi and dibromoarenes. As a result, 2.1 Ar-HOMSi equivalents of phenyl-HOMSi turned out to doubly couple with dibromoarenes in the presence of Pd[P(o-tolyl)3]2 (2 mol%), DPPF (2.1 mol%), CuBr·SMe2 (3.0 mol%), Cs2CO3 (4.2 With optimized coupling conditions in hand, we next equiv), 3 Å MS, and THF/NMP co-solvent in high efficiency studied the synthesis of polyarylenes. Cross-coupling po- (Scheme 7). For instance, 1,4-dibromo-2,5-dihexylbenzene, lymerization of 9 with 4,7-dibromobenzothiadiazole pro- 2,7-dibromo-9,9-dioctylfluorene, and 2,5-dibromothio- duced in a quantitative yield poly(9,9-dioctyl-fluorene-co- phene were applicable to the coupling of Ph-HOMSi and benzothiadiazole),40 also known as light-emitting material gave products 8a–c in yields higher than 90%. Moreover, F8BT (Equation 5). 4,7-dibromobenzothiadiazole smoothly reacted with 2- Pd[P(o-tolyl)3]2 (2 mol%) thienyl-HOMSi, 4-diphenylaminophenyl-HOMSi, and 9,9- n-H C n-C H S 17 8 8 17 N N DPPF (2.1 mol%) dioctyl-2-fluorenyl-HOMSi to furnish teraryls 8d–f in good CuBr⋅SMe2 (3 mol%) to excellent yields. Si Si + Br Br Cs2CO3 (2.1 equiv) Bis-4,7-(fluorenylene)HOMSi 9 underwent double cou- 3 Å MS THF/NMP, 50 °C pling with 4-bromobenzothiadiazole to give teraryl 8g in an 9 (1.00 equiv) S excellent yield (Equation 4). N n-H17C8 n-C8H17 N HO

Si = n Pd[P(o-tolyl) ] (2 mol%) S 3 2 SiMe n-H17C8 n-C8H17 DPPF (2.1 mol%) 2 N N 99%, PDI = 2.9 CuBr⋅SMe (3 mol%) 2 M = 7900, M = 23000 Si Si + n w Br Cs2CO3 (4.2 equiv) 3 Å MS Equation 5 9 (2.1 equiv) toluene/DME, 50 °C Combined with this double-coupling protocol and the HO S S NN N n-H17C8 n-C8H17 N straightforward disilylation described in Section 3, the lin- Si = ear synthesis of an oligo(arylene)-based bis-HOMSi was SiMe2 easily achieved via a net C–Br/C–H coupling (Scheme 8). Double silylation product 6c (see Figure 3) was deprotected 8g, 91% to give cross-coupling-active benzodithienylene-bis-HOMSi Equation 4 (9), which underwent double cross-coupling with THP-pro-

Syn lett T. Komiyama et al. Account

1) 7THP, Ir cat. 86% 2) TsOH⋅H2O OH HO S MeOH/CH2Cl 85% S H H Si Si S Me Me C–H Silylation 2 S 2 Deprotection 9b

5ma Pd[P(o-tolyl) ] (5 mol%) (THP)O O(THP) 3 2 n-C12H25 n-H25C12 DPPF (5.3 mol%) ⋅ S CuBr SMe2 (7.5 mol%)

Cs CO (2 equiv), 3 Å MS Si SSS Si 2 3 toluene/glyme, 50 °C Me2 Me2 6f, 54% Cross-Coupling

Scheme 8 Linear synthesis of an oligoarylene-Bis-HOMSi by a sequential protocol involving Ir-catalyzed C–H silylation and double cross-coupling tected 5-bromo-4-dodecyl-2-thienyl-HOMSi 5ma (see benzothiadiazole (10) with 4-iodoanisole and 4-iodobro-

Scheme 6), withstanding the steric repulsion between the mobenzene took place in the presence of CuBr2 (10 mol%), dodecyl groups, to produce the coupled product 6f. This oli- Ph-DavePhos (10 mol%), CsF (2.5 equiv) in 1,3-dimethyl-2- goarene unit is a part of a polymer for a solar cell.41 imidazolidinone (DMI) to give teraryls 8h and 8i in excellent yields (Scheme 9). The coupling of 10 with 3-iodocar- bazole produced 8j in a good yield using KF, 18-crown-6 5 Cross-Coupling with Aryl(triethyl)silanes (18C6), and cyclopentyl methyl ether (CPME) instead of CsF and DMI. Double cross-coupling of 2,5-bis(triethylsilyl)- During the studies described in Sections 2, 3, and 4, we 3,4-ethylenedioxythiophene and 1,4-bis(triethylsilyl)tetra- were challenged repeatedly to prepare 4,7-benzothiadi- fluorobenzene, both prepared by the Ir-catalyzed C–H si- azole-based bis-HOMSi reagents by lithiation followed by lylation, proceeded to give 8k and 8l. the reaction with 1, Pd/Cu-catalyzed silylation of bromo- benzothiadiazoles as described in Section 2, and Ir-cata- CuBr2 (10 mol%) Ph-DavePhos (10 mol%) 1 2 2 1 2 lyzed silylation of benzothiadiazoles, but all attempts failed. Et3Si Ar SiEt3 + I Ar Ar Ar Ar A great deal of effort led to the Ir-catalyzed C–H silylation of CsF (2.5 equiv) (2.1 equiv) DMI, 150 °C 8 5,6-difluorobenzothiadiazole using triethylsilane as a si- Ph2P NMe2 O lylation reagent (Equation 6). Since the cross-coupling of aryl(triethyl)silanes had no precedent, we decided to find N N appropriate catalytic conditions for the cross-coupling of Ph-DavePhos DMI aryl(triethyl)silanes. S N N S S N N N N [Ir(cod)OMe]2 (3 mol%) R R Me4phen (6 mol%) + H SiEt H H 3 Et Si SiEt norbornene (5 equiv) 3 3 FF (i-Pr) O, 120 °C (5 equiv) 2 R = OMe (8h) 94% FF FF Br (8i) 91% 10, 94% S N N Equation 6 NN

As discussed previously, the coupling with trialkylsilyl- FF type reagents has been a long-standing synthetic challenge; 8j, 65% a successful outcome would be anticipated to enhance the (KF, 18C6, cyclopentyl methyl ether) availability of not only organosilicon chemistry but also FF synthetic organic chemistry. Thus, we focused on the devel- O O opment of the cross-coupling reaction with trialkylsily- MeO OMe S larenes with aryl halides. We were soon pleased to find that MeO OMe FF 42 CuBr2 was a crucial catalyst for the targeted reaction. For 8k, 83% 8l, 74% example, the reaction of 4,7-bis(triethylsilyl)-5,6-difluoro- Scheme 9 Cu-catalyzed double cross-coupling of bis(triethylsilyl)arenes

Syn lett T. Komiyama et al. Account

The results of the mono-coupling of aryl(triethyl)silanes We propose the catalytic cycle shown in Scheme 10, with 4-iodoanisole are summarized in Figure 4. 2-Benzoth- which consists of three steps: (1) single-electron transfer 1 ienyl, 2-indolyl, 2- and 3-pyridyl-, 3,5-bis(trifluorometh- from Ar (fluoro)silicate to CuX2 (X = Br, I) to generate radi- yl)phenyl-, and 4-cyanophenyl(triethyl)silanes gave the cal species II and/or III and Cu(I)X, (2) oxidative addition of corresponding biaryls in moderate to excellent yields. Ar2–I to Cu(I)X to form Ar2–Cu(III), and (3) coupling of II Whereas 2-pyridylboronic acids and esters were not suffi- and/or III with Ar2–Cu(III) to form the coupled Ar1–Ar2 ciently stable to effect the Suzuki–Miyaura coupling, 2-pyr- product and reproduce Cu(II)X2. idyltrimethylsilane is stable and easy to handle, and 43 smoothly participates in the cross-coupling. We should CuX2 note that electron-neutral and electron-rich arylsilanes F Et Et Si such as 2-naphthyl-, 4-biphenyl-, and 4-diphenylamino- F F SiEt Et phenyl-(triethyl)silanes failed to react, probably due to the 1 2 3 Et Ar1 Ar Ar and/or Et Si + I low electrophilicity of the silicon center. Ar1 Et Ar1 III CsF p-An II 1 2 Ar SiEt3 p-An X2Cu Ar CuX S N p-An N Cu = CuLn X = Br, I 99% 72% 71% I Ar2 F3C Scheme 10 A possible mechanism for the Cu(II)-catalyzed cross-cou- p-An NC p-An pling

F3C 56% 28% 6 Amination of Aryl Halides with N-TMS- Figure 4 Biaryls synthesized by Cu-catalyzed coupling of aryl(trieth- Amines yl)silanes with 4-iodoanisole. p-An = 4-MeOC6H4

The concept of the silicon-based cross-coupling is appli- In addition to 4,7-bis(triethylsilyl)-5,6-benzothiadiazole cable to formation of not only C–C bonds, but also carbon– (10), its phenyldimethylsilyl analog 10′ cross-couples with heteroatom bonds including C–S,46 C–O,47 and C–N48 bonds. 4-iodoanisole to give 4,7-(di-4-anisyl)benzothiadiazole 8h As for C–N bond-forming reactions, the Buchwald–Hartwig in 63% yield along with 4-methoxybiphenyl (4% yield) amination is well described, which generally requires the (Equation 7). Probably a more electron-deficient aryl group use of a strong base.49 On the other hand, Pd-catalyzed ami- on the silicon center was transmetalated to the Pd center nation using an N-silylamine takes place upon activation faster than an electron-rich one. This reactivity order con- with a base such as a fluoride ion. Indeed, Smith observed trasts sharply to that in transmetalation in the Pd-catalyzed that N-TMS-amines couple with aryl bromides in the pres- 44 cross-coupling of arylsilanes. Because the coupling of 10 ence of a palladium catalyst and Cs2CO3 or CsF in supercriti- with 4-iodoanisole is not catalyzed by CuI or CuBr, the cal carbon dioxide.48b,c This observation is seminal but it is a mechanism of the Cu(II)-catalyzed reaction is definitely dif- drawback to use a pressure bottle for supercritical CO2. In ferent from the recorded Cu(I)-catalyzed silicon-based addition, the reactivity of silylamines remains to be im- cross-coupling, which is considered to proceed through a proved. Thus, we decided to improve the conditions for the Cu(I)/Cu(III) cycle.45 C–N bond-forming cross-coupling reaction using N-TMS- amines.50 N-TMS-amines can be prepared by lithiation of S N-H-amines followed by reaction with chlorotrimethylsi- N N CuBr2(10 mol%) Ph-DavePhos (10 mol%) lane. + Me Si SiMe I OMe 2 2 CsF (2.5 equiv) After screening the reaction conditions, we found that DMI, 150 °C amination of 4-bromotoluene took place with N-TMS-di- FF (2.1 equiv) 10' phenylamine (1.1 equiv), Pd(dba)2 (1 mol%), XPhos (2 S mol%), CsF (1.5 equiv), and DMI as a general organic solvent N N (Scheme 11) to give 4-methyltriphenylamine in 97% yield. + OMe MeO OMe Various bromoarenes such as 4-BrC6H4NPh2, 2-BrC6H4Me, and 1- and 2-bromonaphthalenes were applicable to the FF 4% 8h, 63% reaction, whereas 2- or 3-bromothiophene, 3-bromopyri- dine, and bulky 1-bromo-2,6-dimethylbenzene proved un- Equation 7 reactive. It is noteworthy that 4-chlorotoluene undergoes the amination without any serious problems. Other N-TMS-

Syn lett T. Komiyama et al. Account

Pd(dba)2 (1 mol%) R R XPhos (2 mol%) R R N SiMe3 + Br N R' CsF (1.5 equiv) R' DMI, 100 °C

Cy2P i-Pr

i-Pr

i-Pr XPhos

Ph2N R Ph2N Ph2N Ph2N R =Me 97% (77% from p-tolyl-Cl) 99% 94% 99% NPh2 99%

Ph Ph N Me N Me O N Me N R Me H

75% 96% 70% R =CF3 99%* Me 0% *Pd/2XPhos 10 mol%, KF (2.5 equiv) Scheme 11 Pd-catalyzed C–N bond-forming cross-coupling of aryl bromides with N-TMS-amines

amines such as N-TMS-N-methylaniline, N-TMS-aniline, N- The present C–N bond-forming coupling was applied to TMS-morphine, and N-TMS-carbazole are applicable to this poly(arylene-imine)53 synthesis (Scheme 12). Under the amination. Although an N-arylcarbazole unit is an import- optimized conditions, 2,7-dibromo-9,9-dioctylfluorene re- ant motif in organic electronics,51 N-TMS-carbazole could acted with N,N-bis(TMS)-anilines or N-TMS-N′-TMS-para- only be applied to the reaction with electron-deficient aryl phenylenediamine to produce the corresponding polymers, bromides under the palladium catalytic conditions. which might be employed as organic electron-conducting We compared the reactivity of N-TMS-amines with re- materials.54 spect to the rate of amination of 4-bromotoluene using n-H C n-C H TMS-NPh2, TMS-NMePh, TMS-NHPh, and N-TMS-morpho- 17 8 8 17 line. The results are shown in Figure 5,[50b which shows that BrBr the reaction rates are in the order of TMS-NPh2 > TMS- Pd(dba)2 (1 mol%) n-H17C8 n-C8H17 NMePh ≈ TMS-NHPh > N-TMS-morpholine. In contrast, di- XPhos (2 mol%) + N arylamines are less reactive in the Buchwald–Hartwig ami- CsF (3 equiv) Me3Si N SiMe3 DMI, 100 °C nation than alkyl(aryl)amines and monoaryl amines.52 The n opposite reactivity order of N-TMS-amines is attributed to (1.00 equiv) Ph the basicity of amines, which may contribute to the smooth n-C8H17 n-H17C8 n-C8H17 n-H C 17 8 N activation of silicon by a fluoride ion. Ph N N n Ph n 85%, PDI = 2.2 90%, PDI = 2.0 Mn = 4400, Mw = 9900 Mn = 6000, Mw = 12000

Scheme 12 Poly(arylene-imine) synthesis

As mentioned above, the Pd-catalyzed amination protocol does not work well with N-TMS-carbazole and electron- rich aryl bromides. We were pleased to find that nickel catalysis is especially effective for the amination of aryl bro- Figure 5 Time courses for the formation of 4-tolyl-NPh2 (red), 4-tolyl- 55 NMePh (blue), 4-tolyl-NHPh (green), and N-(4-tolyl)morpholine (pink) mides with N-TMS-carbazole. In fact, N-TMS-carbazole by Pd-catalyzed amination of 4-bromotoluene with the corresponding coupled with both electron-rich and electron-deficient aryl silylamines

Syn lett T. Komiyama et al. Account

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