Sciencemadness Discussion Board

Grignard in presence of free OH

Srinivas - 14-8-2015 at 03:36

Is it possible to generate Grignard reagent in presence of free OH. Like 2-bromoethanol or 3-bromopropanol

kavu - 14-8-2015 at 04:27

No, it is not. Grignard reagents (conjugate bases of alkanes) are much more basic than typical alcohols, so a deprotonation will result. The alcohol needs to be protected (silyls, benzyl, THP etc.) before any such manipulations is carried out.

[Edited on 14-8-2015 by kavu]

Srinivas - 16-8-2015 at 06:42

reports are available where OH is masked with EtMgBr and then generted Grignard on alkyl halide. But not working in my hands any clue

kavu - 16-8-2015 at 07:24

In the case of these deprotonation based "protections", which sometimes work, you have the problem of dealing with a dual nucleophile. Not only do you have a nucleophilic carbon, but the oxyanion can attack as well. Not knowing what the electrophile in this case is per se, it's difficult to pinpoint the exact issue. Considering that you might be using 2-bromoethanol etc. for making the anionic Grignard reagent, the basicity difference between the added Grigard (EtMgBr) functioning as the magnesium source and the halide to be metallated might not be great enough to lead to desired metallation.

byko3y - 16-8-2015 at 11:59

First mole of grignard reagent converts alcohol to alcoholate, second mole actually does grignard coupling. For example, 2-3 moles of phenylmagnesium bromide reacts with 1 mole of 2-chloroethanol leading to 2-phenylethanol in good yield.
You gave too few information, I cannot say what you did wrong.

Grignard reaction of 2-alkoxysilanes

Sciencemad - 19-8-2015 at 04:33

Aim to generate Grignard of 2-alkoxysilanes (silyl protected Bromoethanol) followed by reaction with epoxide. Not observed required product,
It resulted in B-elimination
Is it possible to generate Grignard of 2-alkoxysilanes (silyl protected Bromoethanol) :)

kavu - 19-8-2015 at 05:39

Interestingly enough silyl protected bromoethanols are not very commonly applied in grignard situations, presumably due to this tendency to undergo elimination under Lewis acidic conditions (what you observed basically). Only five hits can be found on reaxys, all without a yield. Reasonable yields have been obtained with corresponding lithium derivatives (formed using nBuLi). More suitable protecting groups for Grignard use include benzyl, MOM and THP.

[Edited on 19-8-2015 by kavu]

Sciencemad - 19-8-2015 at 22:42

Thanks for reply. Can you please provide the reference details of BuLi reaction