Hydroboration of alkyne

[bigbutrealdreams]

How come the product is cis? What's the reasoning?
thanks

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[Endeross]

because the bh3 comes in on one side and drops itself and a hydrogen in the mix on that same side. Look at this picture and see if you understand what i mean https://upload.wikimedia.org/wikipe...g/759px-Hydroboration_mechanism_in_alkene.png

Granted, this is an alkene but the idea is the same, It attaches at 2 points simultaneously which forces the methyls to go away from the H and B as they attach, this puts the methyl's cis to eachother

 

[bigbutrealdreams]

because the bh3 comes in on one side and drops itself and a hydrogen in the mix on that same side. Look at this picture and see if you understand what i mean https://upload.wikimedia.org/wikipe...g/759px-Hydroboration_mechanism_in_alkene.png

Granted, this is an alkene but the idea is the same, It attaches at 2 points simultaneously which forces the methyls to go away from the H and B as they attach, this puts the methyl's cis to eachother

EXCELLENT! THANKS