Palladium catalysis Heck, aryl iodides

The coupling of terminal alkynes with aryl or vinyl halides under palladium catalysis is known as the Sonogashira reaction and is rather like the Heck reaction. It is a catalytic process, requiring a palladium(O) complex it is performed in the presence of base, and generally uses copper iodide as a co-catalyst. One partner—the aryl or vinyl halide—is the same as in the Stille and Suzuki couplings but the alkyne needs no metal to activate it the reaction works with the alkyne itself. 

Aryl iodides are more reactive towards oxidative addition than aryl bromides, and a selective Heck coupling (without phosphine ligands) with an unsaturated side chain left the bromide in place. A second Heck reaction of this bromide with an allylic alcohol was used to introduce a second side chain. Cyclization of the amide on to the allylic alcohol was achieved with palladium catalysis, not as might have been expected with palladium(O) but instead with palladium(ll), to produce the seven-membered ring. Finally, the conjugated double bond was reduced and the sulfonamide removed under photolytic conditions. 

Under palladium catalysis, a,/3-unsaturated ketones can undergo Michael-type Heck hydroarylationsJ Recently, the intermolecular hydroarylation of 19 has been realized in supercritical carbon dioxide as a solvent to furnish the products 21 (Scheme Triethylamine is believed to be the source of hydride in these transformations. With certain combinations of substituents R R on the enone 19 and the aryl iodide, significant fractions of the /3-dehydropalladation products 22 were also formed, and the aryl-substituted unsaturated 22 was the sole product when acrylates 19 (R = OEt) were subjected to these conditions. The highly di-diastereoselective hydroarylation of an a,/3-unsaturated ketone 23 (Scheme has been applied in an intramolecular fashion toward the synthesis of the octahydrophenanthrene derivative 24, a novel biomarker from Brazilian marine evaporitic Carmopolis oil. 

Utilisation of transition metal benzothiazol-2-yMene and especially thiazol-2-ylidene complexes in homogenous catalysis reactions are very rare [42-44], Calo et al. reported the use of bis-(3-methyl-benzothiazol-2-ylidene) palladium diiodide in the Heck reaction of aryl bromides and iodides with a catalyst loading of down to KU mol% and near quantitative yields [42,44],... 

CPG material was used as support instead of silica gel for a Heck/SAPC system [6]. There, iodobenzene was coupled with different olefins. The dependencies of different substrates and different bases on the activity were examined. The system was active for several types of olefins. The reactivity of the aryl hahdes was comparable to that of the homogeneous catalysis. Iodides reacted easily, while bromobenzene was converted inefficiently. No reaction occurred using chlorobenzene. With EtjN as base, the highest conversions were achieved. However, leaching of palladium was observed and an /Z mixture of products was isolated. When the amine was replaced by KOAc, the Heck reaction gave selectively the -isomer. No leaching was observed (detection limit approx. 0.1 ppm), but the conversion dropped to 80%. The catalytic system with KOAc as base was successfully used for five consecutive rims, with an overall TON of > 1,200. In all cases, conversion ranged from 70 to 80% and 100% selectivity of the -isomer was achieved. 

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