Coupling reactions with unsaturated substrates

Alkynes enter into a remarkable variety of metal-promoted coupling reactions with olefins, alkynes, and other unsaturated species leading to a diversity of cyclization, oligomerization, and polymerization products of synthetic value. In many instances alkyne complexes are presumed intermediates in these reactions but often this has not been proven. The reader is referred to other reviews [95-97] for more complete coverage of this topic. We briefly summarize here the most useful of these processes, highlighting those systems in which metal-alkyne complexes have been demonstrated as intermediates. 

With alkynes alone, typical products are those of cyclotrimerization, i.e., arenes, although cyclotetramerization to cyclooctatetraenes has been directed selectively by Ni catalysts [1], providing an early commercial process for cyclooctatetraene. Numerous catalysts are known for the former process and, depending on the catalyst and substrate, moderate to high regioselec-tivity (1 4- or 1,3,5-) can be obtained with unsymmetrical alkynes [95, 96]. Selective mixed 

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Another interesting tandem Michael initiated sequence was developed in our laboratory by combining the conjugate addition of unsaturated alkoxides to alkylidene malonates with a palladium-mediated coupling reaction with an organic halide. In this cydization reaction, an organopalladium species acts as the electrophilic partner of the cydization. This reaction results in the trans addition of the organopalladium species and of the nucleophile across the unsaturation, and therefore, in overall difunctionalization of the unsaturated substrates [66,67]. 

Commercially available Pd(PtBu3)2 is a unique, air-stable 14e Pd° complex, an excellent catalyst for cross-coupling reactions of aryl chlorides. The ability of P Bu3 to stabilize such a coordin-atively unsaturated, extremely reactive, and yet easily manageable form of Pd° is one of the most amazing and fruitful recent findings in Pd-based catalysis. The cross-coupling of arylzinc reagents with aryl or vinyl chlorides can be readily accomplished with as little as 0.03% of this catalyst. Both electron-rich and sterically hindered substrates are welcome in this protocol.404... 

Therefore, surface modification strategies for the formation of direct silicon-carbon bonds require, first, a special pre-treatment of the silicon surface to prevent oxidation and, second, an activation of the silicon surface for subsequent reaction with organic moieties. This has been achieved by treatment of the silicon surface with hydrofluoric acid to generate a hydrogen-terminated Si(lll) surface, which can further react with unsaturated co-functionahzed alkenes in the presence of UV irradiation or by thermal activation [27,44,45]. Using this method, carboxylic acid modified silicon substrates have been successfully generated and coupled to thiol modified ONDs via a polylysine/sulfosuccinimidyl 4-(M-maleimidomethyl)-cyclohexane-l-carboxylate couphng (Fig. 12). 

Ligands are not always innocent spectator groups. Cationic j -diiminate alkyl complexes of aluminium, rather than undergoing insertion in the Al-alkyl bond, show attack of the unsaturated substrate (ethene or acetylene) at the ligand (Scheme 1). Surprisingly, the C-C coupling reaction observed with ethene is reversible [42]. 

Alkenyliodonium salts can be used as highly reactive substrates for Heck-type olefination and similar palladium-catalyzed cross-coupling reactions [63 -65]. In a recent example, a series of dienes 80 were stereo- and regioselectively prepared by a palladium-catalyzed Heck-type reaction of alkenyliodonium salts 79 with a,/3-unsaturated carbonyl compounds (Scheme 37) [64]. 

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