Electron-deficient palladium conversion

Thus reaction occurs at both the electron-deficient and nonactivated olefin moieties of the norbornadiene diester32. Noteworthy also is the much higher exo selectivity compared with the chemoselective TMM-Pd process24. When the olefin is too electron-deficient, as in maleic anhydride, acrolein or acrylonitrile, bonding to the palladium center is so strong that no codimerization occurs. Conversely, if the initial /[-interaction is too weak, then a common side reaction becomes apparent, cyclodimerization of the methylenecyclopropane. 

Transition metal catalyzed processes are useful tools for the synthesis of functionalized thiophenes. Thus for instance, a phosphine-free, palladium catalyzed coupling protocol for the synthesis of 2-arylbenzo[d]thiophenes from various 3-substituted benzo[6]thiophenes and aryl bromides or iodides has been reported <04T3221>. Likewise, 2,2 -bithiophenes have been 5,5 -diarylated directly with aryl bromides in the presence of Pd(OAc)2, bulky phosphine ligands and CS2CO3 <04T6757>. A series of electron-deficient and relatively electron-rich benzo[6]thienyl bromides have been shown to participate in palladium catalyzed amination reactions, as exemplified by the interesting conversion of 63 to the tetracyclic system 64 upon reaction with 2-aminopyridine 65 <04EJO3679>. 

A microwave-assisted fast and convenient route to palladium-catalyzed car-bonylation reactions has been reported. The microwave-assisted amidation of aryl halides 41 was achieved utilizing in situ generated carbon monoxide from [Mo(CO)6] (see Scheme 24) [46]. Aliphatic non-hindered primary amines 42 were observed to couple easily whereas hindered amines or amines with low nucleophilicity, e.g. anilines, afforded low yields and incomplete conversions. No difference in reactivity among electron-deficient, neutral or electron-rich aryl halides was found. In general, iodides could be coupled with solid Pd/C (or Pd(OAc)2) as catalyst, while bromides required a homogeneous catalyst. Ami-dations were studied in a sealed microwave protocol at 150 for 15 min, which could also be performed in a classical oil bath without any loss of yield. 

In 1992 Murahashi, Hosokawa, and co-workers described the anti-Markovnikov oxidative addition of amides and carbamates to electron-deficient olefins by applying a palladium and copper cooperative catalysis under oxygen atmosphere [41]. The proposed mechanism involved a ff-bonded palladium(II) intermediate resulting from the addition of the nucleophile to the olefin, and subsequent ) -palladium hydride elimination to yield the functionalized alkene. Interestingly, both lactams and cyclic carbamates gave predominantly the corresponding E-enamide derivatives. Acyclic amides, conversely, afforded ElZ mixtures of products. The addition of a catalytic amount (5 mol%) of hexamethylphosphoric triamide (HMPA) was found notably beneficial for the reaction of 5-membered lactams and reduced the reaction time of such particular oxidative amidations (Scheme 2). 

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