Synthesis of 1-hexene from 1-octyne

This is my synthetic pathway. For one, is it plausible? For another, is it efficient?

1) Reduction of 1-octyne with hydrogen gas and Lindlar's catalyst to 1-octene.

2) Oxymercuration to install a hydroxyl group at the most substituted carbon. This creates 2-octanol.

3) Reaction with tosyl chloride and pyridine to create a sulfonate ester - a much better leaving group than an alcohol.

4) E2 elimination with sodium hydroxide and strong heating to create the internal, Zaitsev alkene.

5) Ozonolysis to create two aldehydes.

6) Reduction of longer aldehyde with hydrogen gas over transition metal to primary alcohol.

7) Dehydration of primary alcohol to alkene using a Lewis acid (aluminium trichloride in this case).

2 Answers

On the plus side, you've clearly thought about this problem and have worked out a detailed synthetic route. I think most of the steps would work as desired. The two steps that concern me the most are 4 and 7. I suspect your step 4 will give a mixture of products. Perhaps with further experimentation you could find reaction conditions that would give you the desired product without other products from undesired side reaction (elimination in the other direction and perhaps some rearrangement products too). I see your step 7 as the most problematical. Aluminum chloride, heat and the alcohol will produce a carbocation and it will undergo significant rearrangement creating a variety of elimination and addition (polymer formation) products. So while your reaction scheme is plausible, in my opinion it is not efficient.

• Could I dehydrate the alcohol via heating and sulfuric acid? – Dissenter Sep 8 '14 at 22:26
• That would still proceed through a (primary) carbocation intermediate and be subject to rearrangement and polymerization in addition to the desired elimination. Perhaps dehydration under basic conditions might have a better chance of success. – ron Sep 8 '14 at 22:33
• You mean NaOH dehydration? – Dissenter Sep 8 '14 at 22:54
• Any nucleophile that favors elimination over substitution. – ron Sep 8 '14 at 23:00

I guess this is just a paper exercise, since trying to make 1-hexene from octyne means you forget efficiency. A very useful and interesting exercise though. I agree with Ron about potential issues with step 4. I think sulfuric acid is worth trying for step 7 though and if it fails you can try stronger dehydrating agents that are less/non acidic or an indirect method like your tosylate elimination or even selenium chemistry. For step 6 I would use the standard $\ce{NaBH4}$ or $\ce{LiAlH4}$ for reduction.

Alternatively, after step 1, you can apply your ozonolysis-aldehyde reduction - elimination cycle twice to get to the product. Generally you sacrifice 2 of your 8 carbons ie 25% "wasted" from your starting material.