Alkynes, halo reagents

In accordance with the Markovnikov rule, terminal alkynes are converted to 2-halo-l-alkenes. The reagents prepared in the reaction of anhydrous HF with organic bases and used in hydrofluorination of alkenes can add to the carbon-carbon triple bond producing Markovnikov products.94-96,98 99 However, 1-bromo-l-alkenes and... 

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). 

Negishi and co-workers also reported in 1978 Pd-catalyzed alkynylation reactions with alkynylmetals containing Mg, B, A1 and Sn. But B and Sn versions have sometimes been known as the Suzuki and Stille alkynylation reactions, respectively. Negishi and co-workers during 1977-1978 reported the Pd-catalyzed alkynylation reactions of alkynylzinc reagents with 1-halo-l-alkynes and aryl halides. 

Terminal and internal (Z)-l-alkenylboronates are prepared from (Z)-(l-halo-l-alkenyl)boronates [23]. which can be readily obtained by hydroboration of 1-halo-1-alkynes (Scheme 16.2). The internal Sv,2-like displacement of the halogen with hydrides [24] or organolithiums [25] takes place wath complete inversion of configuration at the sp carbon. On the other hand, the palladium-catalyzed alkylation of the C—X bond with organozinc reagents provides ( )-l-alkenylboronates [26] which are not available by conventional hydroboration of internal alkynes. 

A thiazole ring may also be constructed from a lactam thiocarbonyl and one of the following reagents DMAD [3177], a 2-bromoacrylic acid or its nitrile [20S1, 3509], an aryl isothiocyanate and an alkyne [2706, 2915], a reactive a-methylene ketone such as dimedone [3944], an acyl halide [2433] or a halo ketone which is prepared in situ [2475]. 

Cyclization of a-halo-l-phenyl-I-alkynes The bromide (1) reacts with this dialkyl cuprate reagent in pentane-ether (10 1) at -30° to reflux to give the... 

Several coupling reactions of 2-substituted cycloalkenyhnetal reagents have been re-ported. " " For example, cyclobutenylmetals, generated in situ via carbometallation of 4-halo-1-metaUo-l-alkynes, are reacted with iodobenzene to give 1,2-disubstituted cyclobutenes, as shown in Scheme 35. 

Unlike 1-alkynes, the hydroboration of 1-haloalkynes with 1 equiv of 9-BBN or ChXjBH does not produce any dibora adducts Moreover, the hydroboration of 1-halo-1-alkynes with these reagents yields exclusively C-1 monohydrobora-tion products. The C-1 monohydroboration products are easily protonolyzed [26] with AcOH to give (Z)-l-halo-l-alkene (Eq. 24.13). 

Brown and Molander [1] have reported the synthesis of conjugated enynes. The reaction involves the synthesis of alkenylcopper intermediates from alkenylbo-ron derivatives of 9-BBN, which undergo coupling with 1-halo-l-alkynes in a stereodefined manner and afford the conjugated trans enynes (>95% pure) in almost quantitative yields. The cuprous iodide is the reagent of choice as compared to cuprous bromide-dimethylsulfide (Eq. 24.21 Table 24.21) [1]. 

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