Alkynes regioselective iodination

A valuable feature of the Nin/Crn-mediated Nozaki-Takai-Hiyama-Kishi coupling of vinyl iodides and aldehydes is that the stereochemistry of the vinyl iodide partner is reflected in the allylic alcohol coupling product, at least when disubstituted or trans tri-substituted vinyl iodides are employed.68 It is, therefore, imperative that the trans vinyl iodide stereochemistry in 159 be rigorously defined. Of the various ways in which this objective could be achieved, a regioselective syn addition of the Zr-H bond of Schwartz s reagent (Cp2ZrHCl) to the alkyne function in 165, followed by exposure of the resulting vinylzirconium species to iodine, seemed to constitute a distinctly direct solution to this important problem. Alkyne 165 could conceivably be derived in short order from compound 166, the projected product of an asymmetric crotylboration of achiral aldehyde 168. 

A regioselective iodoperfluoroalkylation of terminal alkynes (R—C = CH) has been reported, and is based on photolysis ofthe C—I bond in perfluoroalkyl iodides (Rp-I). Addition of the thus-formed RF" radical onto the alkyne afforded a vinyl radical that in turn abstracts an iodine atom from the starting Rp—I to form the end olefin R-C(I)= CH-Rf. A xenon lamp through Pyrex (hv > 300 nm) was used for the reaction, where aliphatic alkynes gave a better alkylation yield with respect to phenylacetylene [81],... 

Addition of IC1 and IBr to 1 -methyl-4-r-butylcyclohexene has been studied. The higher stereoselectivity of the IBr addition is interpreted on the basis of the higher softness of Br relative to Cl-512. Addition of IC1, IBr and t-BuOI to 1,3-butadiene results in a mixture of 1,2- and 1,4-addition according to Markovnikov513. Addition of IC1 and ICN to (perfluoroakyl)alkyne proceeds regioselectively with the iodine attached to the terminal carbon514. 

Efficient and regioselective iron-catalyzed aerobic oxidative reactions afforded 3,5-disubstituted isoxazoles 5 from homopropargylic alcohols 4, r-BuONO as the nitrogen source, and H2O under mild conditions (140L6298).A transition metal-free one-pot synthesis of 3,5-disubstituted isoxazoles used terminal alkynes by treatment with -BuLi, then aldehydes and iodine to afford intermediate a-alkynyl ketones 6 converted into isoxazoles 7 with hydroxylamine (14JOC2049). 

Interestingly, only BiBrj gave quantitative yields of the products 416, while other bismuth(III) halides and Bi(OTf)3 had absolutely no effect for the reported transformation. The aUcenylbismuth derivatives could also be trapped in sUu with iodine or coupled with acyl chlorides in the presence of a palladium catalyst [119]. Addition of organogold compoimds to activated carbon-carbon triple bonds has also been reported to proceed under palladium catalysis [120]. Thus, (PhjPljPdClj or Pd2(dba)j complexes successfully promoted a regioselective syn carboauration of alkynes at ambient temperature (Scheme 10.143). 

Keywords Alkynes, iodine/hydrophosphine binary system, CDCI3, room temperature, regioselective hydroiodination, Markovnikov-type addition, vinyl iodides... 

Fig. 8 Mechanistic rational for the regioselective Cu-catalyzed 3+2 cycloadditions with iodinated or aluminated alkynes...

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