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In organic chemistry, a vinyl iodide functional group (also known as iodoalkenes) is any alkene with an iodide substituent directly bonded to one on the alkene carbons (sp2). Vinyl iodides are versatile molecules that serve as important building blocks and precursors in organic synthesis. They are commonly used in carbon-carbon forming reactions in transition-metal catalyzed cross-coupling reactions, such as Heck reaction, Sonogashira coupling, and Suzuki coupling.〔Xie, Meihua, et al. "Regio-and stereospecific synthesis of vinyl halides via carbozincation of acetylenic sulfones followed by halogenation." Journal of Organometallic Chemistry 694.14 (2009): 2258-2262.〕 Synthesis of well-defined geometry or complexity vinyl iodide is important in stereoselective synthesis of natural products and drugs. == Properties == Vinyl iodides are generally stable under nucleophilic conditions. In SN2 reactions, back-attack is difficult because of steric clash of R groups on carbon adjacent to electrophilic center (see figure 1a).〔Klein, David. Organic Chemistry. John Wiley & Sons, Jun 15, 2011. Google book. Thurs. 28 Nov. 2013. http://books.google.com/books?id=SsX9pbarkQkC&source=gbs_navlinks_s〕 In addition, the lone pair on iodide donates into the ╥ * of the alkene, which reduces electrophilic character on the carbon as a result of decreased in positive charge. Also, this stereoelectronic effect strengthens C-I bond, thus making removal of iodide difficult(see figure 1b).〔Bhupinder, Mehta; Manju, Mehta. Organic Chemistry. PHI Learning Pvt. Ltd., Jan 1, 2005. Google book. Thurs. 28 Nov. 2013. http://books.google.com/books?id=QV6cwXA9XkEC&source=gbs_navlinks_s〕 In SN1 case, dissociation is difficult because of the strengthened C-I bond and loss of iodide will generate unstable carbocation(see figure 1c)〔 In cross-coupling reactions, typically vinyl iodides react faster and under more mild conditions than vinyl chloride and vinyl bromide. The order of reactivity is based on the strength of carbon-halogen bond. C-I bond is the weakest of the halogens, the bond dissociation energies of C-I is 57.6kcal/mol, while fluoride, chloride and bromide are 115, 83.7, 72.1 kcal/mol respectively.〔Blanksby, Stephen J., and G. Barney Ellison. "Bond dissociation energies of organic molecules." Accounts of chemical research 36.4 (2003): 255-263〕 As a result of having weaker bond, vinyl iodide does not polymerize as easily as its vinyl halide counterparts, but rather decompose and release iodide.〔Herman, Jan A., and Pierre Roberge. "X‐ray induced polymerization of vinyl iodide in solution." Journal of Polymer Science 62.174 (1962): S116-S118.〕 It is generally believed that vinyl iodide cannot survive common reduction conditions, which reduces the vinyl iodide to an olefin or unsaturated alkane.〔Zhang, Xing, et al. "An efficient cis-reduction of alkyne to alkene in the presence of a vinyl iodide: stereoselective synthesis of the C22-C31 fragment of leiodolide A." Tetrahedron (2012).〕 However, there is evidence in literature, in which a propargyl alcohol's alkyne was reduced in presence of a vinyl iodide using hydrogen over Pd/CaCO3 or Crabtree's catalyst.〔Denton, Richard W., and Kathlyn A. Parker. "Functional Group Compatibility. Propargyl Alcohol Reduction in the Presence of a Vinyl Iodide." Organic letters 11.13 (2009): 2722-2723.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Vinyl iodide functional group」の詳細全文を読む スポンサード リンク
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