Sciencemadness Discussion Board

Alkynylboronate preparation difficulties

Metacelsus - 17-5-2018 at 06:23

I'm trying to make an alkynylboronate following a literature procedure, but it's not working. Here's what I'm doing:

1) Deprotonate the alkyne with nBuLi (in THF solution, -78 °C)

2) Add the alkyne solution to a solution of triisopropyl borate (-78 °C, let warm to r.t.)

At this point, a thick white precipitate forms, as it should. I think the next step is where it goes wrong:

3) Chill back to -78 °C, then add anh. HCl in ether. (At this point, the precipitate dissolves.)

4) Distill the mixture to isolate product.

When I do the distillation, I get almost all triisopropyl borate, with faint traces of my desired product. I think the HCl is resulting in loss of alkyne from the tetrahedral complex, instead of loss of isopropanol. I trust the literature procedure, but I don't know why it's not working.

For reference, here's an example procedure from the Schreiber lab (this one uses ether instead of THF but there are other examples that use THF):
Quote:
To a –78 °C solution of 1-pentyne (32) (5.55 g, 81.4 mmol) in 35 mL of anhydrous diethyl ether was dropwise added n-BuLi (81 mmol, 2.5M in hexanes). The solution was stirred at –78 °C for 30 min, after which time the solution was transferred via cannula to a –78 °C solution of triisopropylborate (18.7 mL, 81 mmol) in diethyl ether (180 mL). The resulting heterogeneous solution was then warmed to rt over ca. 1 h. The solution was cooled to –78 °C and HCl was added dropwise via syringe (35 mL of a 2M solution in diethyl ether). The solution was warmed to rt, then diethyl ether was removed via distillation, following which, distillation at reduced pressure afforded 9.36 g (68%) of the desired alkynylboronic ester.


Can anyone provide insight into what's going on? I note that the literature procedure uses less than a full equivalent of HCl. How important is this?

[Edited on 5-17-2018 by Metacelsus]

AvBaeyer - 17-5-2018 at 14:24

Do you know the purity of the borate ester? How old is it? If you just ordered it, the vendor should be able to give you some lot information.

More importantly, did you make the HCl/ether solution fresh? Typically, this should be very fresh as it decomposes with time and also absorbs water like a sponge.

Other than that, there appears to be no other reason your reaction should have failed.

AvB

Metacelsus - 18-5-2018 at 04:52

NMR of the borate is good. I did not freshly prepare the HCl/ether. Maybe that is the problem.

Bert - 18-5-2018 at 04:59

Water in your (hopefully!) anhydrous HCl/ether seems the simplest and most likely type of contamination.

Not that I know enough about your system to say how little water it would take to disrupt the desired reaction.

BTW, what is the intended product for?

(Edit)

I am late to the party- Looks like you already thought of that.

Good luck- I finally got all the bits and pieces for the rotovap. Might need to ask YOU a few questions...

[Edited on 5-18-2018 by Bert]

Metacelsus - 19-5-2018 at 06:45

After some more work (better HCl and an improved distillation apparatus), I've managed to get an acceptable yield of product.

Bert — the product is intended for subsequent use to make a boron-based linker for an HDDA reaction.