Terminal alkyne definition

Although beyond the scope of the present discussion, another key realization that has shaped the definition of click chemistry in recent years was that while olefins, through their selective oxidative functionalization, provide convenient access to reactive modules, the assembly of these energetic blocks into the final structures is best achieved through cydoaddition reactions involving carbon-het-eroatom bond formation, such as [l,3]-dipolar cydoadditions and hetero-Diels-Al-der reactions. The copper(i)-catalyzed cydoaddition of azides and terminal alkynes [5] is arguably the most powerful and reliable way to date to stitch a broad variety... 

Kuniyasu H, Yamashita F, Terao J, Kambe N. Definitive evidence for the insertion of terminal alkynes into arylS—Pt bonds o-halogen effect in stoichimetric and catalytic reactions. Angew. Chem. Int. Ed. 2007 46 5929-5933. 

The above inconveniences are circumvented by the application of substituted borane derivatives, e.g., Sia2BH and 9-BBN (see Fig. 1 for definitions), which react with high regioselectivity and sensitivity to steric factors. Thus, 1-alkenes and 1-alkynes are hydroborated at the terminal position. The internal unsymmetrically disubstituted alkenes are also selectively hydroborated. The clean transformation of 1-alkynes into vinylboranes—not possible with borane—can be achieved with these reagents (Fig. 2). 

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