Alkynes with Halogens

Alkynes can undergo reactions with halogens forming di- and tetra-haloge-nated products. 

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Alkynes undergo addition reactions with halogens. The reaction has been thoroughly examined from a mechanistic point of view. In the presence of excess halogen, tetrahaloalkanes are formed, but mechanistic studies can be carried out with a limited... 

In both oxa- and aza-alkyne Prins cyclization an unexpected halide exchange with halogenated solvents presumably caused by the vinyl cation intermediates was observed [37]. From a synthetic point of view, it is important to use the correct combination of FeXs and X-containing solvent in order to avoid the undesired halide scrambling (Scheme 28). 

Scheme 5.7 illustrates these and other applications of the hydride donors. Entries 1 and 2 are examples of reduction of alkyl halides, whereas Entry 3 shows removal of an aromatic halogen. Entries 4 to 6 are sulfonate displacements, with the last example using a copper hydride reagent. Entry 7 is an epoxide ring opening. Entries 8 and 9 illustrate the difference in ease of reduction of alkynes with and without hydroxy participation. 

The isomerization of the smallest alkynes 80 with halogens in a propargylic position has been described for chlorine [151, 152], bromine [153] and iodine [154] (Scheme 1.35), but often might proceed by an SN2 -type substitution rather than a prototropic rearrangement [155-159]. On the other hand, transformations such as 82 —> 83 [160] or 84 —> 85 [161] are clearly prototropic (Scheme 1.36). This is also true for propargylic halides such as 86 with its additional ester group assisting the prototropic isomerization [162,163] (Scheme 1.37). 

Halogen atoms. The introduction of side-chains on 9-trifluoromethyl-paullone 409 can be accomplished applying a Stille coupling (Scheme 86, Section 5.2.1.1 (2005EJM655)). Similarly, a Heck reaction of iodo 409 with terminal alkenes under standard conditions affords 2-substituted paullones 413 exclusively as E-isomers. The reaction of terminal alkynes with 409 in the presence of cuprous iodide and a palladium catalyst in triethylamine furnishes the 2-alkynyl-paullones 412 (2000BMCL567). 

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]. 

Table 2. Halofluorination of Alkenes and Alkynes with Stoichiometric Equivalents of Halogen Fluorides...

You should know halogenation of an atkene for die MCAT. Notice that alkanes will not react with halogens without light or heat but alkenes will. Alkynes behave just like alkenes when exposed to halogens. 

Cascade reactions using alkyne cyclisations have not been explored, and the isolated example of 416 is unique in a number of ways.145 For a start, 5-exo-dig cyclisation onto the alkyne to give 417 must be sufficiently fast that it competes with halogen-metal exchange of the second iodine atom, which instead traps the product vinyllithium 417, giving the bicyclic product 418 in 47% yield. 

The mildness of these reagents tolerates the presence of various functional groups such as ester, ether, halogen, and nitrile. The stereospecific cis nature of hydroboration gives exclusively the tram alkenylboranes, often also in high regioisomeric purity (Eq. 53). On the other hand, highly pure (Z)-l-alkenyl-dialkylboranes are prepared without any difficulty via the monohydroboration of 1-halo-1-alkynes with disiamyl-borane or dicyclohexylborane, followed by treatment with t-butyllithium (Eq. 55)106). 

Similarly, Takahashi and coworkers reported that treatment of alkynes with zirconocene-ethylene complex (9) and homoallylic bromides gave allylcyclo-propane derivatives [21]. Therefore, the possibility of y-elimination of the halogen atom in zirconacyclopentene intermediates appeared to be more general as expected. The plausible mechanism is shown in Scheme 13. Carbozirconation... 

Alkynes undergo addition reactions with halogens (CI2, Br2) to produce tetrahalo alkanes. To saturate each allqme molecule, two halogen molecules are needed. Alkynes decolorize aqueous Br2 solution, as alkenes do. 

The nnsaturated hydrocarbons include the alkenes, which contain one double bond per molecule, and the alkynes, which contain one triple bond per molecnle. Their systematic names begin as the names of the corresponding alkanes do, bnt they end with -ene or -yne, respectively. The location of the multiple bond may have to be specified in the name by including the address of the mnltiple-bonded carbon atom that is closer to the end of the chain. The alkenes and alkynes are more active than the alkanes for example, they react with halogens to form halogenated hydrocarbons under mnch less severe... 

Hydroalumination of alkenes. Hydroalumination of alkynes is a well-known reaction, but hydroalumination of alkenes has been achieved only recently under catalysis by TiCU or ZrCU, (8, 288). As expected hydroalumination affords a convenient, high-yield route to primary alkanes (by hydrolysis), terminal primary alcohols (by oxygenation), and primary alkyl halides (reaction with halogens, N-halosuccinimides, or CuXa). ... 

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