[zephy]

Hi. I've been trying to figure out a way to do these four reactions.

The first two involve a starting reagent as 2-butyne (4 carbon chain with a triple bond in the middle). Convert this to get a butane, with Br on a wedge at location 2, and Cl on the dash at location 3. I forgot my nomenclature, I know, I'll learn it again. The one way I could see doing this was to halogenate but then there's no way to get the Cl on in that particular stereochemistry. The second reaction is to convert 2-butyne to get both Br and Cl on wedges. So if I halogenate with two Br in anti, then Cl could do an SN2 reaction to get the correct stereochemistry, but is this correct?

The third and fourth reactions involve changing a pentene > 1-methyl, 2,4 pentdiene (a 5-carbon ring, methyl at 1, and two double bonds at 2 and 4). First reaction is to change the above so that all double bonds are removed and a Br ends up at the 4 position. I'm assuming I should start off by hydrogenating with H2, Pt to remove all the double bonds because otherwise any additions will add to one or both double bonds. But I'm unsure how to preceed next. The second reaction adds a second methyl group to the same spot as the methyl that already exists, and I am not seeing a solution. -OMe would seem to do something, but I'm not sure if it will attack at that exact spot.

Thank you.

[movies]

Have you learned how to reduce alkynes to alkenes yet?

I think that is the key to the first problem.

For the second problem, do you mean CYCLOpentadiene and CYCLOpentene?

[zephy]

Oh, yes, I forgot to mention cyclo in there, sorry.

An alkyne can be reduced to an alkene via hydrogenation and using a posion catalyst to prevent it from turning into an alkane, right? I've learned that method. But there is no way to add something like to bromines in syn addition...

[movies]

But if you had the trans alkene and the cis alkene, then made the bromohydrin (Br₂ + H₂O or NBS + H₂O) the two alkene isomers would give the opposite relative relationship between the Br and the OH, right?

So you identified the Lindlar reduction to make the cis olefin from the alkyne, do you know how to make the trans olefin from the alkyne?