Srinivas
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Grignard in presence of free OH
Is it possible to generate Grignard reagent in presence of free OH. Like 2-bromoethanol or 3-bromopropanol
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kavu
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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]
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Nicodem
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Thread Moved
14-8-2015 at 07:04 |
Srinivas
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reports are available where OH is masked with EtMgBr and then generted Grignard on alkyl halide. But not working in my hands any clue
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kavu
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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.
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byko3y
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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.
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Sciencemad
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Grignard reaction of 2-alkoxysilanes
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) 
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kavu
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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]
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Nicodem
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Threads Merged
19-8-2015 at 08:25 |
Sciencemad
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Thanks for reply. Can you please provide the reference details of BuLi reaction
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