Hydroboration Kinetics of Haloalkynes

Unlike 1-alkynes, which give considerable amount of dihydroboration at the 1 position [1], the 1-halo-1-alkynes alford only monohydroborating products and hydroboration occurring at the 1 position (Eq. 4.17) [2]. 

The kinetic results [2] for hydroboration of haloalkynes and their unsubstituted parent alkynes are shown in Table 4.15. 

Even internal alkynes like 3-hexyne undergoes dihydroboration to yield a gem derivative, whereas 1-haloakynes undergo only monohydroboration at the 1 position. This reveals that the electronic rather than the steric factor is more important in limiting the reaction of 9-BBN with 1-haloalkyne to the monohydroboration product only. 

The relative effects of halogens in 1-halo-1-alkynes are summarized in Table 4.16 [2]. 

in general, replacing a hydrogen or an alkyl group with a group, which reduces electron availability such as halogen or phenyl results in decrease the rate of reaction. The time required for these reactions (0.4 M) to 95% completion are as follows 1-chloro-l-hexyne, 2.5 days 1-bromo-l-hexyne, 11 h 1-iodo-l-hexyne, 8 h. The relative time, thus, indicates that the reaction is useful for synthetic chemist. 

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