Palladium-Catalysed Heck Reactions of Alk-1-en-3-ones with Aryl Bromides: A Very Simple Access to (E)-1-Arylalk-1-en-3-ones

Abstract

When appropriate reaction conditions are used, very high yields of (E)-1-arylalk-1-en-3-one derivatives can be obtained by palladium-catalysed reactions of alk-1-en-3-ones with aryl bromides. The tetraphosphine cis,cis, cis-1,2,3,4-tetrakis(diphenylphosphinomethyl)cyclopentane in combination with [Pd(C 3 H 5 )Cl] 2 was found to be a very efficient catalyst for this reaction. In general, higher reaction rates were observed with electron-poor aryl bromides, but the electron-rich aryl bromides 1-bromo-4-(dimethylamino)benzene and 4-bromoanisole also led to the arylated enones in high yields. Even with sterically very congested aryl bromides such as 9-bromoanthracene, 1-bromo-2,4,6-trimethylbenzene or 1-bromo-2,4,6-triisopropylbenzene, the expected (E)-1-arylalk-1-en-3-ones were obtained in moderate to good yields. These enones appear to be unstable under the reaction conditions, and the addition of a small amount of hydroquinone to the reaction mixture was found to be crucial, especially for the vinylation of electron-deficient aryl bromides. A variety of alk-1-en-3-ones has been employed, and better results in terms of substrate/ catalyst ratios were obtained when oct-1-en-3-one or hex-1-en-3-one was used than when but-1-en-3-one or pent-1-en-3-one was used. It should be noted that several reactions can be performed with as little as 0.1-0.001 mol% catalyst.The palladium-catalysed Heck vinylation is one of the most efficient methods for the formation of C-C bonds. [1][2][3][4] The reaction between aryl halides and alk-1-en-3-ones should give an economically and environmentally attractive access to 1-arylalk-1-en-3-one derivatives. In the literature, such reactions are generally performed with very reactive, but generally expensive, aryl iodides. 5-31 The use of aryl bromides would be more interesting both for environmental and economical reasons. In most cases, aryl bromides are cheaper than iodides, and with these substrates a lower mass of waste would be formed. However, so far, the Heck vinylation with alk-1-en-3-ones and aryl bromides, especially under low catalyst loading, has attracted less attention. 32,33 Moreover, surprisingly, most of the results were obtained using electron-neutral or electron-rich aryl bromides. For example, the reaction of 2-bromo-6-methoxynaphthalene with but-1-en-3-one in the presence of 2 mol% [PdCl 2 (PPh 3 ) 2 ] as catalyst gave the expected arylated enone in 45-92% yield. 32,33 The reaction of 2-bromo-1,4-dimethoxynaphthalene and but-1-en-3-one with 20 mol% tetrakis(triphenylphosphine)palladium led to the desired coupling product in 73% yield. 34 The reactions of bromobenzene, bromo(methoxy)benzenes, bromonaphthalenes, a bromophenylcarbohydrate, or a bromoindole with alk-1-en-3-ones in the presence of 10-20 mol% of a palladium(II) acetate/triphenylphosphine catalyst also gave the 1-aryl alk-1-en-3-ones in good yields. [35][36][37][38] On the other hand, in several other cases, moderate yields were obtained. The reaction of 2-bromotoluene or 1-bromo-2,4,5-trimethylbenzene with but...

4-HO-TEMPO-Catalyzed Redox Annulation of Cyclopropanols with Oxime Acetates toward Pyridine Derivatives

“…As enones can be accessed from precursors different from those of cyclopropanols, this 4-HO-TEMPOH-mediated protocol serves as a complement to that described above. However, as the terminal enones are liable to polymerize, using cyclopropanols as substrates is more suitable for the preparation of 4-unsubstituted pyridines which would otherwise involve the annulation of terminal enones with oxime acetates …”