Sonogashira Coupling

One of the topical research areas dealing with Pd-catalyzed cross-coupling reactions is the development of regenerable catalytic systems through immobilization of highly active palladium complexes, clusters or nanoparticles onto an inert support. In the context of increased environmental requirements, highly active and multiply reusable heterogeneous catalytic systems have a number of practical advantages over homogeneous analogues. In this review, various approaches to immobilization on a polystyrene support of N-heterocyclic carbene and acyclic diaminocarbene palladium complexes have been considered for the first time; these complexes are a promising alternative to the widely used phosphine complexes. Special attention focuses on modern physicochemical methods used for investigation of corresponding heterogeneous systems. In addition, examples are given of using reusable heterogeneous catalysts for Sonogashira–Hagihara and Suzuki–Miyaura reactions, with emphasis on the true nature of the palladium catalytic species.
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An environmentally benign surfactant (TPGS-750-M), a diester composed of racemic α-tocopherol, MPEG-750, and succinic acid, has been designed and readily prepared as an effective nanomicelle-forming species for general use in metal-catalyzed cross-coupling reactions in water. Several "name" reactions, including Heck, Suzuki-Miyaura, Sonogashira, and Negishi-like couplings, have been studied using this technology, as have aminations, C-H activations, and olefin metathesis reactions. Physical data in the form of DLS and cryo-TEM measurements suggest that particle size and shape are key elements in achieving high levels of conversion and, hence, good isolated yields of products. This new amphiphile will soon be commercially available.

Several 2-alkynyl 1,3,4-oxadiazoles have been synthesized efficiently by employing palladium catalyzed cross-coupling transformation under the Sono-gashira reaction conditions. This reaction has been ap-plied here for the first time for the preparation of oxadiazole derivatives. The products were formed in high yields and no side products could be detected.

Pd/C is used as an efficient catalyst for the copper-free Sonogashira coupling of acid chlorides and terminal alkynes to afford ynones in high yields (Tables 1 and 3). Cyclization of (2-methoxyaryl)-substituted ynones induced by I2/ammonium cerium(IV) nitrate (CAN) at room temperature gave 3-iodochromenones (=3-iodo-4H-1-benzopyran-4-ones) in excellent yield (Table 4).

Polyether dendritic wedges have been attached to bis(2-pyridylimino)isoindole (BPI) ligands, and the corresponding PdII complexes have been synthesized and studied as hydrogenation catalysts. The fixation of dendrons at the ligand framework was carried out at the stage of the phthalodinitrile precursor of the BPI ligands by nucleophilic substitution of 4-nitrophthalodinitrile with the in situ generated alcoholates. Reaction of 1,3-dibenzyloxy-2-propanol [G-1] and the coupling of Fréchet's first and second generation arylether dendrons [(3,5)-G-1]-OH, [t-Bu-(3,5)-G-1]-OH and [(3,5)-G-2]-OH with 4-nitrophthalodinitrile gave the corresponding O-coupled phthalodinitrile derivatives. The synthesis of the BPI ligands was achieved by reaction with two molar equivalents of 2-amino-5-bromopyridine to give the protioligands (11-Br)-BPI-[G-1] (4) and (11-Br)-BPI-[R-(3,5)-G-1] (R = H: 5a, tBu: 5b) whereas the reaction of compound 5a with Me3SiCCH, Ph3SiCCH and PhCCH, using Sonogashira's [Pd(PPh3)4]/CuI catalyst, gave the corresponding bis(alkynyl)-derivatives. All the protioligands were cleanly metallated to give the corresponding palladium(II) complexes. In contrast to the unsubstituted complex, its dendrimer functionalized derivatives [(11-Br)-BPI-[R-(3,5)-G-1]-PdCl] (R = H: 9a, tBu : 9b) and [(11-Br)-BPI-[(3,5)-G-2]-PdCl] (11) hydrogenated styrene and 1-octene without decomposition (TOF = 5 h−1 at T = 295 K, p(H2) = 1 bar, 2 mol% cat.) and among these, catalyst 11, carrying the 2nd generation dendron displayed sufficient stability to be isolated and reused several times.

The utilization of Pd nanoparticles stabilized by aggregates of
hetero-oligophenylene derivative 3 as an excellent catalyst in a copper/
amine free Sonogashira coupling reaction under aerial conditions at
room temperature has been demonstrated