π -Conjugated carbocycles and heterocycles via annulation through C-H and X-Y activation across CC triple bonds

,

Why have -conjugated carbo/heterocycles become important in recent years?As mentioned above, those compounds are increasingly important as materials for optoelectronics.There are two major synthetic methods for those substrates; one is Suzuki-type cross-and homo-coupling of aromatic and hetero-aromatics, and the other is annulation with alkynes.In this account, we summarize our own efforts on annulations across alkynes; mono-annulation, cascade annulation, and crossover annulation.What kinds of -conjugated materials are needed and useful for optoelectronic materials?Quations and Answers regarding why, how, and what are summarized in Figure 2. Regarding what kinds of materials are needed, firstly device construction using a conjugated system is made, and secondly performance data of the device are obtained.Evaluation of those data is carried out, and the feedback from the results is utilized in molecular design for next experiments (Figure 2).Before proceeding to the synthesis of -conjugated carbo-/heteroaromatics, a brief summary on the research process of OFET is given in Figure 3, to help clarify what kinds of molecule are needed.
Single crystalline -conjugated carbo-/hetero-cycles are placed between source (Au) and drain (Ca), as shown in the device structure.Both positive charges (holes) and negative charges (electrons) are injected from Au and Ca, respectively, and hole-accumulation layer leads to electroluminescence from the LE-OFETs.High carrier mobility of both holes and electrons, high luminescence efficiency, and robustness of the organic materials are required.To accomplish such requirements, for example, well-controlling HOMO-LUMO gap, as well as heteroatom contacts and - interactions to control of -aggregation, should be considered for the molecular design of -conjugated aromatics and hetero-aromatics.
14 15 Neutral Pd(II) catalyst, such as PdCl 2 (CH 3 CN) 2, was effective for converting orthoalkynylaryl acetals 16 to 1,2-dialkoxyindenes 17 via 1,2-alkyl (R) shift and migration of OR′ of acetal to alkynyl carbon (Scheme 5). 85Not only the neutral Pd(II) but also PtCl 2 -olefin, such as -pinene or 1,5-COD, was active for this type of transformation. 82A similar catalytic system, PtCl 2 -benzoquinone, was also active for the cyclization of related acetals, whereas the optimal catalyst for the reaction of thioacetals was PdI 2 ; ortho-alkynylaryl thioacetals, which did not undergo a 1,2-alkyl shift, gave 1,3-di(alkylsulfonyl)indenes. 86Contrary to the neutral palladium(II), ortho-alkynylaryl acetals 16 was converted to 1,1-dialkoxyindenes 18 with cationic palladium(II) catalyst system such as Pd(CH 3 CN) 4 (BF 4 ) 2 -PPh 3 (Scheme 5). 87echanistic study using DFT calculations suggested that -coordination of cationic Pd + to the benzene ring would be a key step; a stepwise delivery of two methoxy groups would take place via the palladium cationic center coordinated to the aromatic ring.This mechanistic consideration helps to understand why many useful transformations of ortho-alkynylaryl amines, acetals, imines, etc., mentioned above, take place rather readily with Pd and transition metal catalysts, but those of the corresponding aliphatic analogues do not occur easily.

Cascade annulation with two alkynes, activated by -electrophiles
In the above section, annulation with a mono-alkyne of ortho-alkynylaryl system (Y=NR′, O, S, CH, or O=CH in the case of Scheme 7), which leads to indoles, benzofurans, benzothiophenes, indenes, or naphthalenes is mentioned.In this section, cascade annulation of two alkynes of 1,2dialkynyl substituted aromatics (Y=NR, S, O, or O=CH in the case of Scheme 7) is shown (Scheme 8).Other examples for this type of cascade reactions are shown in Scheme 10.Thiophene is incorporated into -conjugated heterocycles; for example, 28 vs. 30.Other thienocarbazole ring 32 and more complicated -conjugated system 34 can be synthesized in good to high yields.] Scheme 10.Cascade annulation to carbazole -conjugated system.
Cascade annulation of thioanisole-substituted aryldiynes 35 using iodine gave iodosubstituted benzo[b]naphtho[2,1-d]thiophenes 36 in high chemical yields (Scheme 11). 102Here, CH 3 NO 2 was the best solvent, but CH 3 CN and CH 2 Cl 2 also gave good results.A wide range of substituted thiophenes can be synthesized in high yields.This type of benzonaphthothiophene -conjugated system was also applied to organic dyes for DSSC.Scheme 12.A proposed mechanism for cascade iodo-cyclization.
As mentioned above, cascade annulation of ortho-alkynyl aniline and thioanisoles (Y=NR and/or S in Scheme 11) proceeded smoothly by the use of gold catalysts and/or iodine.However, in the case of anisoles 40 (Y=O in Scheme 8), iodine-mediated and -electrophilic transition metal-catalyzed cyclization did not give the desired -conjugated product (Scheme 13). 103nstead, use of 2 equivalents of TfOH in CH 3 CN afforded the indenochromene -conjugated system 41 in a very high yield.

Scheme 13. TfOH-mediated cascade annulation to indenochromene.
The scope of this TfOH-mediated cascade annulation is shown in Scheme 14.Many interesting -conjugated heterocycles containing chromene framework can be synthesized in good to high chemical yields.Some chromene-dyes, synthesized by this method, were applied to DSSCs, and POE (, power conversion efficiency) was in a range of 5.07-6.71%.In Scheme 7, the gold-and copper-catalyzed intermolecular benzannulation between orthoalkynyl aldehydes and alkynes is shown.Intramolecular version of this type of benzannulation affords polynuclear aromatic derivatives 104 (Scheme 16).For example, the reaction of enantiomerically pure 45 with catalytic amounts of AuCl 3 gave 46 in a high chemical yield with high ee, which was converted to (+) Rubiginone B 2 and (+) Ochromycinone. 105First naphthopyrylium ion 47 is formed, and secondly intramolecular [4+2] cycloaddition gives intermediate 48, and finally bond rearrangement affords 46.Further, gold-catalyzed intramolecular carbocyclization of alkynyl ketones gives fused tri-and tetracyclic enones, [106][107] though they are not π-conjugated aromatics but aliphatic carbocycles.

Crossover annulation across alkynes through C-X and C-H activation
The dibenzopentalene framework, having two five-membered rings and two external benzene rings, attracts the keen interest of organo-materials chemists because of its increased stability and more extended -conjugation than simple pentalene itself.For example, OFET performance data of dinaphthopentalene 49, shown in Scheme 20, were obtained in recent years. 108  We have found that cascade crossover annulation of two different alkynes, orthoalkynylarylhalides 57 and diarylacetylenes 58, proceeded very well using Pd(OAc) 2 catalyst together with appropriate ligand and base. 112Optimization of reaction conditions are shown in Scheme 18.The use of P(t-Bu) 3 ligand (15 mol%) and 3 equivalents of DBU/CsOPiv in the presence of 5 mol% of Pd(OAc) 2 afforded the crossover annulation product 59 in a high chemical yield.This is an intermolecular C-X/C-H crossover annulation.Differently substituted dibenzopentalenes can be synthesized very smoothly as shown above, but in that case the partners of ortho-alkynylhalides are diarylacetylenes.If dialkylacetylens are used as a partner, a different type of annulation takes place 113 (Scheme 21).The reaction between diethylacetylene and ortho-alkynylarylbromide 65 afforded 66 as a mixture of E-and Z isomers, and optimization of the reaction conditions is shown in the Scheme. 113Here, the use of P(n-Bu) 3 ligand gave a better result than the use of P(t-Bu) 3 ligand and the other bulky ligands.Besides C-X/C-H activation mentioned above, if crossover annulation between arenes and alkynes via C-H/C-H activation becomes feasible, such a reaction is very desirable and ideal for constructing π-conjugated aromatics.Some representative previous reactions are shown in Scheme 24.Pd-and Rh-catalysts are effective for aromatization between biaryls (73 and 75) and alkynes, 114,115 giving the aromatization compounds 74 and 76, respectively.Also, the catalysts are useful for the annulation between 77 and diphenylacetylene, and between 79 and alkynes, to give the annulation products 78 and 80, respectively. 116,117Recently, our group reported Rh(III)catalyzed regioselective annulation between naphthylen-1-ylcarbamates and alkynes (RC≡CR); 118  were not effective at all.The use of one equivalent of PdCl 2 together with CsOPiv gave 9,9′BF, 89 and 90, in 89 % yield, but the yield decreased dramatically to 5 % by the use of 10 mol% of PdCl 2 .This result clearly indicated that the use of appropriate oxidant would be needed for the catalytic reaction.We found that the use of 3 equivalents of MnO 2 oxidant together with 10 mol% of PivOH in the presence of 10 mol% PdCl 2 catalyst is the best condition, leading to 98 % yield of 9,9BF, as a mixture of cis-89 and trans-90 (Scheme 26). 120In general, a mixture of cis-and trans-isomers was obtained, and the reaction at low temperatures (for example, room temperature) gave the cis-isomer predominantly, whereas at higher temperatures the ratio of the trans-isomer increased.Detailed experiments using NMR indicated that isomerization from cisto trans-isomer took place at higher temperatures or even at room temperature by keeping for prolonged time of period.

Conclusions
 -Conjugated carbo-and hetero-cycles are synthesized through annulation across alkyne of ortho-alkynylaryl substrates, using iodine-mediated and/or transition metal-catalyzed, as well as Brønsted acid-catalyzed, reactions.Cascade annulation across two alkynes of 1,2-dialkynyl substituted substrates gives carbo-and heterocycles having more complicated and congested system.Intermolecular crossover annulation of two different bis-arylalkynes via palladiumcatalyzed C-H bond activation affords dibenzo[a,e]pentalenes having two different substituents at C-5 and C-10 positions.Intramolecular crossover annulation of bis-biarylalkynes via palladium-catalyzed dual C-H bond activation gives 9,9′-bis-fluorene derivatives.

Scheme 3 .
Scheme 3. Indole synthesis via N-C and N-S bond addition across an alkyne.

Scheme 26 .Scheme 27 .
Scheme 26.Optimization of the reaction conditions for dual C-H activation.

Scheme 28 .
Scheme 28.A plausible mechanism for dual C-H activation of bis-biarylalkyne.
111il now, a number of synthetic methods for dibenzo[a,e]pentalene derivatives have been reported.A few representative examples for catalytic homo-and crossover-annulations of alkynes are shown in Scheme 17. Homo-annulation of 50 via C-Br/C-Br cascade gave low yields of dibenzopentalenes 51.109Homo-annulation of 52 via C-H/C-H cascade afforded good yields of the products 53, but in a certain case the reaction did not proceed very well.110Most of the previous methods provide dibenzopentalenes symmetrically substituted at 5-and 10-positions, and only a limited number of approaches have been reported for the synthesis of unsymmetrically 5,10-disubstituted dibenzopentalenes, which are available through crossover annulation of two different alkynes.Crossover annulation between 54 and 55 via C-Br/C-Sn cascade of two different alkynes gave good yields of dibenzopentalenes 56 having two different substituents at C-5 and C-10 positions.111However,it is highly desirable to synthesize such differently substituted dibenzopentalenes via crossover annulation, which proceeds efficiently through C-H bond activation, instead of C-Sn bond activation.