1-Iodo-l-alkynes

Halogen shifts have been found for tungsten, with assumed formation of iodovinylidenes in reactions of 1-iodo-l-alkynes with W(CO)5(thf) en route to cyclization of 2-(iodoethynyl)styrenes to naphthalenes and of iodo-alkynyl silyl enol ethers [147], while more substantial confirmation is found in Mn =C=C(I)CH (OR)2 (CO)2Cp [R = Me, Et (OR)2 = 0(CH2)30], of which the XRD structure of Mn =C=C(I)CH(OMe)2 (CO)2Cp was determined [148]. 

A variety of fluorinated alkenyl zinc reagents such as CF2=CFZnCl, CF2=CHZnCl, E and Z-RCF = CFZnCl have been prepared by the first method (Scheme 54). In the presence of palladium catalyst, these fluorinated alkenyl zinc reagents undergo cross-coupling reactions with aryl iodides, vinyl iodides, acid chlorides and 1-iodo-l-alkynes to give the corresponding fluorinated alkenyl derivatives [127, 146-153], which have been utilized in the synthesis of fluorinated codlemones [154]. Typical examples are outlined below (Scheme 55). 

Iodo-l-alkynes undergo an unprecedented head-to-tail dimerization in the presence of 39/HBF4 in CH2C12 (equation 68). In discussing the mechanism, the authors state that the formation of the dimer should require regeneration of an equivalent amount of T+ , and they propose the mechanism in equation 69. Aromatic compounds treated with 39 in the... 

The stereoselective synthesis of the title compounds has been achieved. Thexylborane on reaction with 2 molar equivalents of 1-iodo-l-alkyne at 0 °C proceeds to near completion (88% for 1-iodo-l-hexyne) to form fully substituted organoborane (33), which upon treatment with 2 molar equivalents of sodium methoxide at 0 °C readily produces trans-1,2,3-butatrienes (Eq. 102) 157). The same reaction, however, with either 1-chloro or 1-bromo-l-alkynes is sluggish to form the thexyl-l-halo-l-alkenyl-borane31). 

For the reaction of lithium enolate anions of esters with I2 or CX4, see 12-5. The conversion of terminal alkynes to 1-iodo-l-alkynes was reported using Nal under electrochemical conditions. The reaction of an aryl alkyne with HInCl2/BEt3,... 

Enynes. Vinylic organic cuprates react with 1-bromo- or 1-iodo-l-alkynes in the presence of 1.2—2 eq. of TMEDA to give, after hydrolysis, enynes usually in yields of 75-85%. 

A number of syntheses of di- and polyacetylenes has been reported. 1-Iodo-l-alkynes couple with terminal acetylenes under palladium-copper catalysis to give 1,3-diynes thus y-iodopropargyl alcohol and phenylacetylene afford compound 30. Oxidative coupling of 1 -alkynes to yield symmetrical 1,3-diynes is brought about by air and copper(I) chloride in the presence of N, A -tetramethylethylenediamine (equation Trialkylsilyl sub-... 

Of the various 16-hydroxy analogs clearly the most interesting were the 16-methyl derivatives, and further development of this series was undertaken (55). Among the analogs prepared were compounds featuring a 13-cis double bond (Scheme 33), the vinyl iodide for which was obtained vm diimide reduction (22) of the 1-iodo-l-alkyne 199, a 17-trans double bond (203, Scheme 33)... 

The reaction of potassium salts of 4-pentynoic acids—prepared from the add and KOBu-t or KH—with 1-bromo-l-alkynes affords 6-( )-alkylidene-y-butyrolactonest (Scheme 21). Lithium and sodium carboxylates have proved unsuccessful. Addition of potassium bromide, use of DMSO, and excess 4-pentynoic acid have been found to increase the yield, whereas 1-iodo-l-alkynes lead to the formation of the expected lactone along with major amounts of S-iodo- y methylene-y-butyrolactone. Tri(o-tolyl)phosphine and tri(2-furyl)phosphine can promote an effident transformation. The latter, however, has been found to give the best results. 

Allenylcopper(l) compounds (215) react with 1-iodo-l-alkynes to give allenynes (216) exclusively. ... 

Bis(pyridine)iodine(I) tetrafluoroborate (IPy2BF4) is a mild source of electrophilic iodine. 1 -Bromo- 1-alkynes react stereoselectively with this reagent, in presence of a nucleophile, to give 65-80% of 2-functionalized 1-bromo-1-iodo-l-alkenes (equation 61)515. 

Hydroalumination of 1-chloro-l-alkynes. Lithium aluminum hydride adds to 1-chloro-l-alkynes (1) regio- and stereoselectively to form the a-chlorovinyl alanates 2, which are moderately stable at 0°C. On methanolysis they are converted into (E)-l-chloro-l-alkenes (3). They can also be converted into (Z)-l-bromo-l-chloro-l-alkenes (5) and into (Z)-l-chloro-l-iodo-l-alkenes (6). 

Halogenation of vinylboranes generates vinyl halides. Both cis and trans halides are available by modification of the reaction conditions. When trans-alkenyl boronic acid (73, derived from 1-octyne) was treated with iodine and sodium hydroxide, trans-1-iodo-l-octene (74) was formed in 90%. When the boronic acid was treated with iodine and then with base, the (Z)-alkenyl iodide (75) was produced.Vinylboranes derived from internal alkynes lead to cis-trans mixtures with both of these procedures. Boronic acids derived from alkynes and catecholborane give the (Z)-bromide on addition of bromine followed by sodium hydroxide. ... 

The change in charge polarization across the C C reflected in the C NMR data for haloalkynes indicates that polarization decreases in the order I > Br > Cl. This results in an increase in the amount of negative charge or electron density at C-2 and decrease in that at C-1, as one proceeds from the iodo- to bromo- to the chloro-substituted compounds. This thus correlates with the rate of hydroboration (which occurs at C-1) in the order I > Br > Cl. Further, the reason for the nucleophilic attack at the C-2 position [23] in 1-chloro-l-alkynes is the decrease in the electron density at that position caused by the -M effect of chlorine. Both regioselectivity and rate of hydroboration depend on the electron availability at specific sites in the molecule, and the above reason explains why hydroboration does not occur at the 2 position. 

Before discussing examples of the reactions, it should be pointed out that most types of vinylic halides used in this reaction are easily available. The UV-catalyzed addition of HBr to terminal acetylenes forms the 1-bromo-l-alkenes. The cis isomer is formed almost exclusively if the addition is carried out at dry ice temperature. The 2-bromo or 2-iodo-l-alkenes are obtained from reacting aqueous hydrogen halides with alkynes. The 2-substituted-1-bromo-l-alkenes are available by the bromination-base dehydrobromination reactions. 

The first "tandem reactions forming geminal carbon-carbon bonds" were reported in 1991, and the following example is representative. A mixture of 5-iodo-l-pentyne (1 eq.) and p-fluorophenyl isocyanide (5 eq.) in t-butylbenzene containing hexamethylditin (1.5 eq.) was irradiated under nitrogen in a pyrex flask at 150°C for several hours until all of the alkyne had been consumed (followed by H nmr). Standard work up gave a mixture of the cyclopentaquinolines 1 and 2 in 56% yield and 85/15 ratio. 

Fluoro-l-iodo-l-alkenes. These compounds are prepared from 1 -alkynes by the... 

That surfaces of e.g. alumina play a role in the hydroiodination of alkenes and alkynes had been observed earlier . In refluxing low boiling petroleum ether a mixture of 1 -hexyne, iodine and dehydrated alumina afforded in 2 h a 62% yield of the Markovnikov addition product 2-iodo-l-hexene. The authors assume that iodine reacts with surface hydroxyl groups to form HI. The presence of alumina is required for the reaction to occur. 

Instead of silver nitrate, Pagni, Kabalka and coworkers used activated and even out-of-the-bottle y-alumina to promote electrophilic additions of iodine to alkynes (cf. the alumina-catalysed addition of HI). ( )-l,2-diiodoalkenes were obtained in 50-98% yield. With terminal alkynes and unactivated alumina, moreover, a minor amount of 1,1,2-triiodo-l-alkene was detected it was the product of iodine addition to intermediate 1-iodo-1-alkyne. Still another method of electrophilic addition of iodine to alkynes involves mercury(II) salts in dichloromethane at 0 °C. ... 

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