Alkenes halo- from alkynes

An alternative synthesis of (Z)-l-halo-l-alkenes involves hydroboration of 1-halo-l-alkynes, followed by protonolysis (246,247). Disubstituted ( )-and (Z)-a1keny1 bromides can be prepared from ( )- and (Z)-a1keny1 boronic esters, respectively, by treatment with bromine followed by base (248). 

The Zweifel trans-alkene synthesis from dialkyl-1-alkenylboranes involves the hy-droboration of 1-halo-l-alkynes with dialkylborane (Eq. 69) 126). It is of interest that the base produces a transfer with inversion 126 127). 

Nitrile oxides (R-C=N -0"), ° which can be generated by base-catalysed elimination of hydrogen halide from halo-oximes (RC(Hal)=NOH), or by dehydration of nitro componnds (RCH2NO2), readily add to alkenes and to alkynes, generating tive-membered heterocycles. Addition to an alkene produces an isoxazoline, unless the alkene also incorporates a group capable of being eliminated in a step after the cycloaddition, as shown below. However, isoxazolines can also be dehydrogenated to the aromatic system. ... 

The synthetic utility of the alkyne/B-X-9-BBN adducts has also been explored.152 (Z)-l-Alkynyl-2-halo-1-alkenes are the result of lithium alkynide action on these latter boranes (Scheme 76).157 The adducts resulting from the treatment of alkynes with B-X-9-BBN have been shown to be labile under protic conditions. However the decomposition has been harnessed and is an efficient preparation of 2-halo-1 -alkenes.158... 

The amide ions are powerful bases and may be used (i) to dehydrohalogenate halo-compounds to alkenes and alkynes, and (ii) to generate reactive anions from terminal acetylenes, and compounds having reactive a-hydrogens (e.g. carbonyl compounds, nitriles, 2-alkylpyridines, etc.) these anions may then be used in a variety of synthetic procedures, e.g. alkylations, reactions with carbonyl components, etc. A further use of the metal amides in liquid ammonia is the formation of other important bases such as sodium triphenylmethide (from sodamide and triphenylmethane). 

The intramolecular carbopalladation starting from a 2-halo-1,6-enyne leads to a terminal aUcenylpalladium halide relay that can be trapped by an external alkene or alkyne. With the former, the cascade process leads to a 1,3,5-hexatriene that undergoes rapid 67T-electrocyclization to a flve-ring-annelated cyclohexadiene and this, in turn, is easily dehydrogenated to the corresponding aromatic compound (Scheme 15). With an external alkyne trapping the intermediate, a 1,3,5-hexatrienylpalladium intermediate will be formed and can either cyclize by intramolecular carbopalladation or 67r-electrocycliza-tion before termination by dehydropalladation will occur. ... 

The reaction proceeds through the following pathway (Scheme 13.2). The results are summarized in Table 13.2 [16]. Table 13.2 The synthesis of [Z]-l-alkynyl-2-halo-l-alkenes from 1-alkynes [16] ... 

Zirconocene j -imine complexes formed by a C-H activation route from a variety of amines can be trapped by halo-alkenes or alkynes to afford functionalized adducts (Scheme 12.32) [43]. 

---