Microwave-assisted Heck-type

SCHEME 1 Microwave (MW)-assisted Heck-type arylation leading to benzoxepines. 

Procter and coworkers have apphed a microwave-assisted Heck reaction as an intermediate step in the construction of tetrahydroquinolones on sohd phase (Scheme 52) [137]. Optimization of the reaction conditions showed DMF to be the optimal solvent for the sohd phase Heck reaction, and a small hbrary of tetrahydroquinolones was prepared using this approach. Additional studies showed that intermediates of the type 66 could be cyclatively cleaved with Sml2 in one step if the sulfur hnker was replaced by a sulfoxide group (i.e., the HASC hnker). 

Further application of the powerful intramolecular reductive Heck reaction has been realized in the efficient microwave-assisted synthesis (Pd(PPh3)4, 3 mol%, HCOONa, 1.5 equiv, DMF HzO 3 1, 110 °G) of the important 3-benzazepine system. The precursors were A-ethynoyl derivatives of N-substituted n-bromophenethylamines, which afforded, via a type c ring construction, 3-benzazepin-2-ones with a substituted exocyclic methylene group at the Cl position <20070L3017>. 

Eberlin also studied the Heck reaction of aryltellurides [45] and Svennebring [19] reported the identification of three types of cationic, catalytic intermediates (Fig. 4A-C) in the microwave-assisted, phosphine-containing Heck arylation of electron rich olefins. In the latter case the authors support a Pd(0)/Pd(II) cycle as opposed to a Pd(II)/Pd(IV) cycle based on ESI-MS evidence for reduction of the palladium(II) precatalyst to Pd(0) by the ligand, and oxidative addition of the aryl substrate to a Pd(0) species. None of the expected Pd-bound olefin intermediates were observed however, this is often the case either because the olefin-bound species is neutral or because OA (oxidative addition) is the turnover-limiting step and the subsequent steps occur too quickly to be observed by the sampling method (in this case sampling included quenching and dilution of samples from a reaction vessel). 

Scheme 10,5 Microwave-assisted copper-catalysed Mizoroki-Heck-type reaction.

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