1.2- dihalo-1-alkene 1-alkyne

Sodium metal in ether has also been used to dehalogenate cyclic 1,2-dihalo-alkenes to cyclic allenes with some cyclic alkyne as a by-product [13]. 

Oxidative addition of alkenyl halides, triflates, and other esters to zerovalent palladium compounds has been long known as a viable route to palladium(ii)-alkenyl complexes. Stereospecific coupling reactions involving mono- and (E)- or (Z)-dihalo-alkenes with palladium-copper catalysis under modified Sonogashira conditions are quite versatile and useful. These proceed through oxidative addition of the alkenyl halide via palladium(ii) alkenyl complexes, followed by coupling with a nucleophile (usually an alkynylcopper reagent obtained in situ with co-catalytic copper(i) from terminal alkynes in the presence of, in this case, a base like piperidine instead of diethylamine). ... 

Zweifel s group has developed two complementary syntheses of 1,1-dihalo-alkenes this permits mixed dihaloalkenes to be prepared. Thus, the hydro-boration of 1-halo-l-alkynes (117) followed by oxidation gives the borinic alkenes (118), which are converted into the bromohaloalkenes (119) stereo-specifically. Alternatively, tra 5-halogenation of ( )-haloalkenylsilanes (120) followed by a/iti-desilicohalogenation gives the ( )-dihaloalkenes (121) [similarly. 

Bromoboration (Z)-/>2-dihalo-l -alkenes1 Reaction of a terminal alkyne (1 -octyne) with BBr3 in CH2C12 results in a p-bromoalkenylborane, which undergoes brominolysis or iodinolysis with retention to provide (Z)-l,2-dihalo-l-alkenes (equation I). 

The currently available methods for the synthesis of the title compounds are confined to the preparation of homo-1,1-dihalo-1-alkenes 180 while only a few reports are available for mixed 1,1-dihalo-1-alkenes of defined stereochemistry 18u. As the hy-droboration reaction proceeds in a stereospecific manner, the hydroboration-oxi-dation-bromination-debromoboration sequence of 1-chloro-l-alkynes produces selectively (Z)-l-bromo-l -chloro-l-alkenes (Eq. 116),82>. The oxidation with anhydrous trimethylamine oxide of the alkenylborane prior to the addition of bromine is necessary to avoid the competing transfer of one of 1,2-dimethylpropyl group from boron to the adjacent carbon atom. Similar reaction sequence provides 1,1-dibromo-l-alkenes (Eq. 117)182). 

In an attempt to synthesize (Z)-l,2-dihalo-l-alkenes from bromoboration adducts of 1-alkynes with B-X-9-BBN, all efforts have been unsuccessful. Fortunately, it has found that the haloboration of 1-alkynes with tribromoborane, followed by the reaction with iodine or bromine chloride in the presence of sodium acetate gives the expected (Z)-l,2-dihalo- 1-alkenes stereospecifically (>98%) in good yields (Eq. 122)187>. 

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