1-Haloalkynes

Cis-olefins or cis./rjns-dienes can be obtained from alkynes in similar reaction sequences. The alkyne is first hydroborated and then treated with alkaline iodine. If the other substituents on boron are alkyl groups, a cis-olefin is formed (G. Zweifel, 1967). If they are cir-alkenyls, a cis, trans-diene results. The reactions are thought to be iodine-assisted migrations of the cis-alkenyl group followed by (rans-deiodoboronation (G. Zweifel, 1968). Trans, trans-dienes are made from haloalkynes and alkynes. These compounds are added one after the other to thexylborane. The alkenyl(l-haloalkenyl)thexylboranes are converted with sodium methoxide into trans, trans-dienes (E. Negishi, 1973). The thexyl group does not migrate. 

The coupling of alkenylboranes with alkenyl halides is particularly useful for the stereoselective synthesis of conjugated dienes of the four possible double bond isomers[499]. The E and Z forms of vinylboron compounds can be prepared by hydroboration of alkynes and haloalkynes, and their reaction with ( ) or (Z)-vinyl iodides or bromides proceeds without isomerization, and the conjugated dienes of four possible isomeric forms can be prepared in high purity. 

Haloalkynes (R—C=C—X) react with ArSnBu3 and Cul to give R—C= C—Ar. Acetylene reacts with two equivalents of iodobenzene, in the presence of a palladium catalyst and Cul, to give 1,2-diphenylethyne. 1-Trialkylsilyl alkynes react with 1-haloalkynes, in the presence of a CuCl catalyst, to give diynes and with aryl triflates to give 1-aryl alkynes. Alkynes couple with alkyl halides in the presence of Sml2/Sm. Alkynes react with hypervalent iodine compounds " and with reactive alkanes such as adamantane in the presence of AIBN. ... 

In another procedure, the addition of a dialkyIborane to a 1-haloalkyne produces an a-halo vinylic borane (82). Treatment of this with NaOMe gives the rearrangement shown, and protonolysis of the product... 

Nickel-catalyzed carbonylation of a-haloalkynes with carbon monoxide under phase-transfer conditions gave either allenic monoacids or unsaturated diacids.93 The carbonylation initially afforded monoacids, which reacted further to give diacids with high stereoselectivity (Eq. 4.52). 

After preparation from interaction of 3-bromopropyne with copper(I) cyanide and filtration from copper salts, an explosion occurred dining distillation of the evaporated filtrate at 45-60°C/66 mbar. This was attributed to explosion of some dissolved copper acetylide(s). After refiltration the product was again distilled at 45-48°C/53 mbar without incident, and it appeared to be stable, unlike true haloalkynes. However it is undoubtedly an endothemic compound with its two triple bonds. 

The propensity of organotin hydrides for SET reactions has been utilized to initiate radical chain reactions. Anodically promoted oxidation of Ph3SnH to [Ph3Sn] at 0.80 V (vs SCE) initiates the cyclization of several haloalkyne and haloalkene ethers as well as of some fi-lactam derivatives. The catalytic cycle shown in Scheme 1 is based on... 

The reaction of haloalkynes with one equivalent of 1 affords alkynyltitanium compounds by f5-elimination from the (q2-haloalkyne)Ti(OiPr)2 intermediate, as shown in Scheme 9.5, thus providing an easy access to functionalized alkynyltitaniums [26], When this reaction is carried out in the presence of excess 1, a tri-titanated alkene of the type shown in Scheme 9.5 is generated in excellent yield. This is an interesting method for generating the permetallated terminal alkene [27]. 

The direct reduction of haloalkynes using either mercury or vitreous carbon as the cathode has been examined in considerable detail [80-84] one example is portrayed in Eq (77). The influence of reduction potential, current consumption, proton donor, electrode, and substrate concentration on the course of the process has been examined. Vitreous carbon electrodes are preferred, though mercury has been used in many instances. Unfortunately, these reactions suffer from the formation of diorganomercurials. While both alkyl iodides and bromides can be used, the former is generally preferred. Because of their higher reduction potential, alkyl chlorides react via a different mechanism, one involving isomerization to an allene followed by cyclization [83]. 

Copper iodide acts as an efficient reagent for the nucleophilic displacement of 1-haloalkynes. It transforms 1-bromoalkynes (72) into 1-iodoalkynes (73) which, on further treatment with copper(II) bis(arenesulfinate), are converted into the corresponding alkynyl aryl sulfones (74). An electron transfer between 1-haloalkynes and copper(I) salts is believed to take place for the copper-assisted halogen-exchange reaction at the acetylenic carbon atom. 

Addition-elimination (for the chloro compound) and elimination-addition (via an intermediate haloalkyne, for the bromo and iodo compounds) mechanisms account for the activation parameters determined for reaction of 2-(/3,/3-dihalovinyl)-5-nitrothiophenes with MeONa-MeOH. °°... 

The enolates derived from racemic 2-(trimethylsiIyI)- or 2-benzyl-3-ethoxy-2,3,3a<4,7,7a-hexa-hydro-1 //-isoindol-l-one (13) react with some haloalkynes. Attack by the electrophile occurs at the bridgehead carbon (7a) from the least hindered side, to afford only one diastereomer of the alkylation product 14, as judged by spectroscopic properties14,15. In some experiments 2-desilylation is effected before product isolation15. 

Aminocarbonylation has been combined with the Pauson-Khand reaction to construct fused tricyclic alkaloid skeletons (see 00154). The tandem aminocarbonylation/Pauson-Khand reaction of haloalkynes with a chiral allylic amine promoted by Co2(CO)8 gave angular triquinanes as exemplified in Scheme 25. Thus, the reaction of l-chloro-2-phenylethyne 175 with Co2(CO)8 at 0°C gave alkyne-dicobalt complex 176, which was converted to enoyl-dicobalt complex 177 upon warming to 25 °C. The reaction of enoyl-dicobalt complex 177 with cyclopente-nylmethyl(l-phenylethyl)amine 179 yielded Pauson-Khand reaction product, angular triquinane 180, via A -allylic aminocarbonylated alkyne-dicobalt complex 178 (Scheme 25). ... 

SCHEME 85. Cross-couphng of zinc-copper organometallics with 1-haloalkynes... 

A combination of both of the procedures described above results in the preparation of trisubstituted olefins.347 The entire conversion of haloalkyne to 82 can be carried out in one... 

Table 4.16. 13C Chemical Shifts (dc in ppm) and Coupling Constants (nJCH in Hz) of Haloalkanes, Haloalkenes and Haloalkynes [86, 91 a, 255-261],...

The outstanding chemical characteristic of alkenyl halides is their general inertness in SN1 and SN2 reactions. Thus chloroethene fails to react with silver nitrate in ethanol (i.e., low SN1 reactivity), fails to react with potassium iodide in acetone (i.e., low SN-2 reactivity), and only reacts slowly with sodium hydroxide to give ethyne (low E2 reactivity). The haloalkynes, such as RC=C—Cl, are similarly unreactive. 

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