# Method for regioselective hydroalkylation of terminal alkyne

I'm trying to figure out a method to add a linear alkyl chain R and a hydrogen atom H across the triple bond of a terminal alkyne. The alkyl chain should end up on the internal carbon (C-2):

My attempt :

First, we can hydrolyse the alkyne to produce a methyl ketone. Addition of a Grignard reagent produces the tertiary alcohol. But, now I'm stuck as I can't treat it with $$\ce{H2SO4}$$, because then ring-expansion will ruin the whole matter.

How to move on?

• You want to add R–H across an alkyne? (you need to specify two things for an addition reaction; you can't just add R to one carbon and not do anything to the other carbon, well, unless you're aiming for a radical product. :-) ) – orthocresol Jul 8 '20 at 4:34
• @orthocresol: Yes, I want to add R-H across an alkyne, directly or indirectly. – Rahul Verma Jul 8 '20 at 4:57
• Maybe from the ketone make an enol triflate, then do some kind of Pd cross-coupling. But I'm sure you will get better suggestions from others. – orthocresol Jul 8 '20 at 5:27
• @orthocresol: I'll take a look at it (I heard triflates for the first time actually), and thanks for the edit ;) – Rahul Verma Jul 8 '20 at 5:36
• The simplest route is to treat the alkyne with RCuMgX2 (Normant, 1976) followed by aqueous work up. Schwartz (Zr) and Negishi (Zr, Ti) methodology is also applicable. – user55119 Jul 8 '20 at 16:16

Here is a potential solution using old-school chemistry, though with several steps.

1. Hydroborate the alkyne and work up with $$\ce{H2O2}$$ to give the aldehyde.

2. Alkylate the aldehyde enolate with $$\ce{RI}$$, either directly (LDA or LiHMDS) or via enamine/silyl enol ether

3. Reduce aldehyde to alcohol ($$\ce{NaBH4/EtOH}$$)

4. Form triflate/mesylate and eliminate (DBU/Toluene, reflux)

If you wish to explore @orthocresol's suggestion of enol triflate coupling then coupling the triflate with the cuprate of your alkyl chain looks to be viable according to this Tet. Lett. article from McMurry and Scott1

References

1. John E. McMurry, William J. Scott, “A new method of olefin synthesis. Coupling of lithium dialkylcuprates with enol triflates,” Tetrahedron Letters 1980, 21(45), 4313-4316, DOI: 10.1016/S0040-4039(00)77845-2
• This is the better method, which eliminate rearrangement possibilities. – Mathew Mahindaratne Jul 8 '20 at 8:16
• Also, the last step can be achieved by converting alcohol to halide followed by $\mathrm{E2}$ elimination (using $\ce{t-BuK}$). – Mathew Mahindaratne Jul 8 '20 at 8:25
• Alkylation of primary, saturated halides with enamines does not work well. Stork and Dowd made an improvement by enolizing t-butylimines with Grignard reagents. pubs.acs.org/doi/pdf/10.1021/ja00897a040 – user55119 Jul 8 '20 at 16:31