Palladium-catalyzed cross-coupling reactions occupy a unique status among methods for the construction of carbon– carbon bonds. The well-established Stille–Migita–Kosugi coupling of organostannanes and the Suzuki–Miyaura coupling of organoboron compounds are two reactions
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چکیده
unique status among methods for the construction of carbon– carbon bonds. The well-established Stille–Migita–Kosugi coupling of organostannanes and the Suzuki–Miyaura coupling of organoboron compounds are two reactions that, until recently, have been the “gold-standard” among various palladium-catalyzed coupling reactions. Their wide use and extraordinary synthetic utility have provided the impetus to develop newer and more effective variants (Fig. 1). More recently organosilanes have also been shown to be competent coupling partners in those reactions that employ shelf-stable reagents. The early work of Hiyama demonstrated that in the presence of a nucleophilic promoter, fluorosilanes could undergo a variety of cross-coupling reactions whose scope rivaled that of organotin and organoboron cross-couplings. Since these reports, many variants of organosilicon crosscoupling have been developed that extend beyond the use of fluorosilanes and that possess many attractive features which now elevate this method to be a viable alternative to other types of cross-coupling. This report will outline recent advances in the development of organosilicon cross-coupling reactions and provide an overview of scope and applicability of this new technology. Organosilanes were previously thought to be too unreactive to be effective cross-coupling partners. The small electronegativity difference between silicon and carbon resulted in a relatively weak nucleophilic reagent for the cross-coupling. The use of fluorosilanes was therefore a significant breakthrough, demonstrating that in the presence of a nucleophilic promoter, these compounds could access a pentacoordinate “ate” species, which was postulated to undergo the coupling reaction due to its enhanced polarization at the carbon–silicon bond. Because silicon-based compounds are generally non-toxic, have low molecular weight, and are easily incorporated into molecules by a variety of methods, the potential for developing other silicon-based cross-coupling systems superior to methods previously employed was recognized. One of the first successful alternatives to fluorosilanes in organosilicon cross-coupling was achieved through the use of siletanes. Predicated on the necessity to generate a pentacoordinate silicon for the coupling process, it was hypothesized that the use of a siletane should enable generation of the pentacoordinate silicate more readily in the presence of a nucleophile due to the enhanced Lewis acidity of the silicon. This property is a manifestation of “strain release Lewis acidity” which has its origins in the difference in coordination geometry between four coordinate (tetrahedral) and five coordinate (trigonal bipyramidal) silicon species (Fig. 2). Therefore, the angle strain in a four coordinate siletane (79° vs. 109°) is partially relieved upon binding a fifth ligand to produce a trigonal bipyramidal species (79° vs. 90°) in which the siletane bridges an apical and basal position. On the basis of this conceptual framework, a cross-coupling system employing both alkenyland arylsiletanes was developed. Alkenylsiletanes (E)and (Z)-1 underwent facile, room temperature cross-coupling to both electron-rich and electron-deficient aryl iodides (Chart 1). In addition to the remarkable mildness and reactivity (ca. 10 min at ambient temperature), high stereospecificity with respect to alkene geometry in the coupling was observed (greater than 98% in most cases). Arylsiletane 4 underwent similarly facile crosscoupling to generate an unsymmetrical biaryl product. Unlike the alkenylsiletanes, this coupling required slightly elevated temperatures. In addition, heteroatom substitution on the silicon atom was necessary to enhance the polarity and thus reactivity of the sp carbon–silicon bond. The use of siletanes in organosilicon cross-coupling achieved many significant milestones that hinted at the poDecember 2002 Chem. Pharm. Bull. 50(12) 1531—1541 (2002) 1531
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