However, Hoveyda group have demonstrated using nickel-based catalyst (Ni(dppp)Cl 2), DIBAL-H with N-iodosuccinimide (NIS), selectively favor α-vinyl iodide with little to no byproducts. in addition, the vinyl metal intermediate can be mildly nucleophilic, for example vinyl aluminum, can form C-C bonds under catalytic conditions. Introducing an α-vinyl iodide from a terminal position of an alkyne is a difficult step. As a result, most synthetic methods often involve a hydrometalation step before addition of I+ source. However, this reaction does not happen at good rates or very high stereoselectively. This generally makes 2-iodo-1-alkenes or α-vinyl iodide by Markovnikov's rule. The common and simplest approach to make vinyl iodide is addition of one equivalent HI to alkyne. Below are various means and methods in introducing and synthesizing vinyl iodides. In synthesis, it is useful to introduce vinyl iodide at various positions to be set up for a coupling reaction at the next synthetic step. Stereochemistry such as E-Z notation or cis-trans alkene geometry is important since some transition metal cross- coupling reactions, such as the Suzuki coupling, can retain olefin geometry. Example of regiochemistry is whether the iodide is positioned in either alpha or beta position on the olefin. Vinyl iodides with well-defined geometry ( regiochemistry and stereochemistry) are important in synthesis since many natural products and drugs that have specific structure and dimension. Vinyl iodides are synthesized by methods such as iodination and substitution reaction. In S N1 case, dissociation is difficult because of the strengthened C-I bond and loss of the iodide will generate an unstable carbocation(see figure 1c) Also, this stereoelectronic effect strengthens the C-I bond, thus making removal of the iodide difficult (see figure 1b). 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 positive charge. In S N2 reactions, back-attack is difficult because of steric clash of R groups on carbon adjacent to electrophilic center (see figure 1a). Vinyl iodides are generally stable under nucleophilic conditions. Synthesis of well-defined geometry or complexity vinyl iodide is important in stereoselective synthesis of natural products and drugs. They are commonly used in carbon-carbon forming reactions in transition-metal catalyzed cross- coupling reactions, such as Stille reaction, Heck reaction, Sonogashira coupling, and Suzuki coupling. Vinyl iodides are versatile molecules that serve as important building blocks and precursors in organic synthesis. Chromium metal does not react with nitric acid, HNO 3 and in fact is passivated.In organic chemistry, a vinyl iodide (also known as an iodoalkene) functional group is an alkene with one or more iodide substituents. Similar results are seen for sulphuric acid but pure samples of chromium may be resistant to attack. In practice, the Cr(II) is present as the complex ion 2+. Under still milder conditions, chromium metal reacts with the halogens fluorine, F 2, chlorine, Cl 2, bromine, Br 2, and iodine, I 2, to form the corresponding trihalides chromium(III) fluoride, CrF 3, chromium(III) chloride, CrCl 3, chromium(III) bromide, CrBr 3, or chromium(III) iodide, CrI 3.ĢCr(s) + 3Cl 2(g) → 2CrCl 3(s) ĢCr(s) + 3Br 2(g) → 2CrBr 3(s) ĢCr(s) + 3I 2(g) → 2CrI 3(s) Reaction of chromium with acidsĬhromium metal dissolves in dilute hydrochloric acid to form solutions containing the aquated Cr(II) ion together with hydrogen gas, H 2. Under milder conditions, chromium(V) fluoride, CrF 5, is formed. Reaction of chromium with the halogensĬhromium reacts directly with fluorine, F 2, at 400☌ and 200-300 atmospheres to form chromium(VI) fluoride, CrF 6. Reaction of chromium with waterĬhromium metal does not react with water at room temperature. Chromium metal does not react with air or oxygen at room temperature.
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