1-Iodo-l-alkenes

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. 

Vinyl halides may be prepared by halogenation of vinylsilanes but Chan has shown that the stereoselectivi ty of the Lewis acid-promoted iododesilylation of terminal E-vinylsilanes to give E-or Z- 1-iodo-l-alkenes is highly sensitive to the amount of stannic chloride used (Scheme 58). This provides a useful, tunable entry to these iodides and the process has been applied to the synthesis of two insect pheromones with defined E/Z isomeric ratios.169... 

Vinylic iodides. The reagents (2) are converted into 1-iodo-l-alkenes in 65-75% yield by treatment with iodine at -30°. 

A novel and highly efficient synthesis of 1,1-diiodo-, 1,1-dibromo-, and mixed (Z)- or ( )-1-iodo-l-bromo-l-alkenes using the 1,1-heterobimetallic reagents obtained by hydrozirconation of stannylacetylenes has also been described [155], The hydrozirconation and halo-genolysis steps were carried out at room temperature in THF under a nitrogen atmosphere. It is noteworthy that the developed route is compatible with various functionalities... 

A synthesis of potentially useful 1-iodo-l-bromo-l-alkenes has thus been developed that offers complete stereocontrol. These methods have also proven to be highly efficient for the preparation of 1,1-diiodo-l-alkenes and 1,1-dibromo-l-alkenes, which are very useful synthetic intermediates (Scheme 7.26) [155]. 

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. 

Z)-l-Iodo-l-alkenes. Reaction of sodium hexamethyldisilazane with the phos-phonium salt 1 in THF generates iodomethylenetriphenylphosphorane, (C6H5)3P=CHI, which converts aldehydes into (Z)-vinyl iodides (15-62 1) in reactions conducted at —78° in THF/HMPT in 61-91% yield.2... 

For cyclisation, simple vinyl iodides need to be Z, or polymerisation occurs. However, a silyl substituent geminal to the lithium atom promotes causes geometrical instability in the vinyllithium, and even E 1-iodo-l-silyl alkenes 159 and 160 cyclise successfully 76... 

The addition of bromine or iodine to ( )-alkenylboronic acids or esters afforded intermediates, which were converted by base into (Z)-l-halo-l-alkenes.543 On the other hand, ( )-iodo-l-alkenes of high stereochemical purity were prepared by treatment of ( )-l-alkenylboronic acids or esters with sodium hydroxide followed by 1 molar equiv. of iodine.544 Iodine monochloride was used in place of iodine for such a base-induced iodination with retention or inversion of the stereochemistry (Equation (110)).545 Chloramine-T/Nal (Equation (111)),546,547 CuX2 (X = Cl, Br) (Equation (112)),548 and iV-halosuccinimides (Equation (113))549 halogenated the C-B bond with retention of stereochemistry. 

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). 

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

The decisive step in the stereoselective synthesis of ( )-l-bromo-and (Z)-l-iodo-l-alkenes, which were required for the preparation of various pheromones, consists in the hydroboration of different 1-bromo- and 1-iodoacetylenes [107-109] conjugated alkenynes could be readily prepared by metal-catalyzed cross-coupling of 1-alkenylboranes with 1-bromoalkynes [110]. 

Perez-Balado, C. and Marko, I.E., 1-iodo-l-selenoalkenes as versatile alkene 1,1-dianion equivalents. Novel connective approach toward the tetrahydropyran subunit of polycavemoside A, Tetrahedron, 62, 2331, 2006. 

Kadota, I. et ah, A simple and practical method for the stereoselective synthesis of (Z)-l-iodo-l-alkenes from 1,1-diiodo-l-alkenes, Tetrahedron Lett., 44, 8645, 2003. 

Table 13.1 The synthesis of 2-bromo- and2-iodo-l-alkenes [15]...

In contrast to the bromo and iodo analogs, organic chloro compounds are relatively inert toward organolithium reagents. There are only a few classes of chlorinated substrates, notably gm-dichlorocyclopropanes , l,l-dichloro-l-alkenes ° and doubly vicinal oligochlorobenzenes (1,2,3-trichlorobenzene , 1,2,3,4-tetrachlorobenzene , hexachlorobenzene etc.) that are capable of sustaining the halogen/lithium permutation mode. 

The addition of iodine monochloridc to 1,4,4-trifluorocyclobutene and l-chloro-2,3,3-trifluo-rocyclobutene appears to be controlled by similar polarity factors, i.e. the alkene carbons substituted with a fluorine atom are negative with respect to those substituted with either a hydrogen atom or a chlorine atom.13 The products, 3-chloro-2-iodo-l,l,2-trifluorocyclobutane (8a) and l,l-dichloro-2-iodo-2,3,3-trifluorocyclobutane (8b) were isolated in 80 and 45% yield, respectively. Their structures were established by NMR spectroscopy.13... 

In the previous section we have discussed that 1-halo-1-alkenes can be conveniently synthesized. However, with these methods it is not possible to synthesize 2-halo-l-alkenes. Although the halometallation reaction would be a powerful tool for the preparation of 2-halo-1-alkenes, the reaction has not been adequately developed for such purpose 183). Recently, it has been reported that B-bromo-9-borabicyclo[3.3.1]-nonane (B-Br-9-BBN) and B-iodo-9-borabicvclo[3.3.1] nonane (B-I-9-BBN)1841 react with 1-alkynes, stereo-, regio- and chemoselectively, and after protonolysis, 2-halo-1-alkenes are obtained in excellent yields (Eq. 118)1851. 

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