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hemistry professor at the University of Maryland, College Park, and another proponent of couplings with organosilicon compounds, concurs. Although many boronic acids are stable and available commercially, highly functionalized ones are difficult to prepare, and some boronic acids are unstable and lose boron readily, he explains. Furthermore, boronic acids tend to couple with each other rather than with the Suzuki partner, forming significant amounts of homocoupling product, he adds. Organosilicon compounds overcome many of these drawbacks. As it happens, silicon is easy to incorporate into organic molecules, and Denmark and DeShong have advanced different silicon reagents for coupling. DeShong has focused on arylsiloxanes of the type C6H5Si(OR)3. He says these are easy to prepare, are not prone to homocoupling, and are reactive with a broad range of coupling partners, including aryl and heteroaryl iodides, bromides, chlorides, and triflates, as well as alkenyl halides. Furthermore, DeShong has uncovered a unique reactivity: When another group, such as fluorine, is added to arylsiloxanes such as C6H5Si(OR)3, the resulting silicate, C6H5Si(OR)3F, can couple with cyclic allylic esters. "There's a patent pending on this chemistry," DeShong says. "Boronic acids can't do it." In work being written up for publication, the chemistry has been applied to a cyclic allylic carbonate in the synthesis of an analog of 7-deoxypancratistatin, a promising antitumor agent. |
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