| The dissertation provides first a detailed review on the factors influencing Suzuki cross-coupling for C-C bond formation and research progresses in this field. And then, describes thesis studies on the Suzuki cross-coupling reaction. The studies include two following parts.â… Cyelopalladated ferrocenylimines catalyzed Suzuki cross-coupling reactionThe lab found previously that cyclopalladated ferrocenylimines could be used as palladacycle catalysts in a wide range of homogeneous reaction including Heck reaction, the dimerization of ArHgCl, and more recently the Suzuki reaction in organic solvent. The work described below is a further study of cyclopalladated ferrocenylimines as catalysts in Suzuki cross-coupling reaction. This part of work has two aspects.Cyclopalladated ferrocenylimines catalizing Suzuki cross-coupling reaction in water, and the ultrasonic irradiation effect on the reactionFirst, the effects of conventional heating and ultrasound on the cyclopalladated ferrocenylimines-catalyzed Suzuki cross-coupling reaction of phenylboronic acid with aryl halides were investigated in water solvent. Cat. 1 was found to have good catalysis on reactions of aryl iodides and aryl bromides as coupling partners. The catalyst showed poor catalysis towards aryl chlorides as coupling partners. In contrast, Cat. 2 yielded around 91% product yields for heterogeneous reactions of electron-withdrawing group-bearing aryl chlorides with phenylboronic acid. Ultrasonic irradiation was found able to accelerate the Suzuki reaction dramatically. In case of 4-iodoanisole, reaction time was reduced to 1 hour from 20 hours, with slightly higher yield. (Scheme 1) Cyclopalladated ferrocenylimines catalyzing Suzuki cross-coupling reaction for hetero-biaryl synthesisA. Application of cyclopalladated ferrocenylimines catalysts in the synthesis of aryl-substituted pyridinesThe Suzuki cross-coupling reaction of 3-pyridylboronic pinacol ester with aryl halides (I, Br, Cl) was carried out in DMF/H2O(3/1, v/v) at 110℃, in the presence of Cat. I and K2CO3 or CsCO3 (1.0 equiv.). Reactions were conducted under open air condition. With the current system, 3-pyridyl biaryl compounds could be readily synthesized. Reactivity of aryl halides decreases in the order of ArI＞ArBr＞＞ArCl. Electron-withdrawing substitution on benzene ring enhances the reactivity, while electron-donating substitution reduces the reactivity. The steric hindrances have significant effects on the coupling reaction yields. Using this scheme, a list of 3-arylsubstituted pyridines have been synthesized with high yields (Scheme 2).With an aim to achieve selective mono-arylation and bis-arylation, the Suzuki cross-coupling reactions of phenylboronic acids with 3,5-dibromopyridine, catalyzed by cyclopalladated ferrocenylimines, were further investigated. When Cat. I system was used in DMF/KF·2H2O at 120℃, a 99/1 ratio of bis/mono was obtained with a yield of 54%. In contrast, when Cat.â…¡system was used in toluene/methanol(4/1)/K2CO3(reflux at 75℃), a 63/27 ratio of mono/bis was obtained with a yield of 65%. (Scheme 3). In order to achieve selective mono-arylation and bis-arylation, further studies were conducted to optimize above reaction systems by varying the molar ratios of PyBr2 to ArB(OH)2. Under the reaction conditions of Cat.â… , DMF/KF·2H2O, 120℃, 3 equiv. of ArB(OH)2, ratios of bis-to-mono ranging from 97/3 to 99/1 were achieved with yields ranging from 83% to 100% of bis-arylation products. For mono-arylation, by using the reaction conditions of Cat.â…¡, K2CO3/toluene/methanol (4/1, v/v), reflux, 5.6 equiv, of 3,5-dibromopyridine, ratios of mono-to-bis ranging from 90/10 to 99/1 were attained with product yields ranging from 58% to 99%. Ten new compounds were confirmed by 1HNMR, 13CNMR and HRMS characterization. Substitution and steric hindrances have limited effects on selective arylation under the current reaction systems. (Scheme 4)Similar reactions with phenyl or heterocyclic dihalides were also conducted. Results indicate that oxidative addition is apparently the rate-determine step in these reactions. Mechanism for the selective Suzuki cross-coupling reaction, under condition of 1.0 equiv, ofphenylboronic acid, was proposed. (Scheme 5) B. Application of cyclopalladated ferrocenylimines as catalysts in the synthesis of aryl-substituted thiopheneThe Suzuki cross-coupling reactions of 3-thienylboronic acid with aryl iodides or aryl bromides were carried out in DMF at 80℃in the presence of Cat.â… and K3PO4. By using this method, 3-arylsubstituted thiophenes were readily synthesized with high yields. Electron-withdrawing substitution on benzene ring enhances the reactivity, while electron-donating substitution reduces the reactivity. The reaction system was ineffective for aryl chlorides. Emission spectroscopic analysis found that a number of synthesized compounds using this reaction system were fluorescent. (Scheme 6). â…¡PPh3AuCl/DAB-Cy catalyzing Suzuki coupling of activated arylchlorides with arylboronic acidsUsing a new catalyst of PPh3AuCl/DAB-Cy, Suzuki cross-couplings between activated aryl chlorides and arylboronic acids were investigated under the conditions of DMF/K2CO3/130℃with a ratio of PPh3AuCl: DAB-Cy of 3/10(Scheme 7). The new system was found to be air-stable and could be easily handled. With strong electronwithdrawing substitution on benzene ring, good reaction yields were attained, while with electron-donating substitution no products were obtained. Steric hindrances had marginal effects on the yields.A plausible mechanism for the Au(I)-catalyzing Suzuki cross-coupling reaction is proposed and shown in Scheme 8.
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