Re: ORGLIST: Re: piperazine et al.

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From: Herb Hedgecock (h.hedgec$##$
Date: Fri Oct 02 1998 - 13:06:35 EDT

On Fri, 2 Oct 1998, Jacob Zabicky wrote (in part):

> However, I may propose you try the following
> long shot schemes, because they can work, and after having had the first
> N-alkylation, the second one would be slowed down by deactivation.
> 1. The first suggestion would be to carry out your reaction without a base.
> You could get with some luck N-propylpiperazonium bromide, and the ammonium
> center will deactivate the second near-by amine, leading preferentially to
> monoalkylation. After you finish you can neutralize and isolate your
> products.

This sounds good, but won't work. The quaternary nitrogen formed by the
first step will not stay protonated, but will quickly redistribute the
proton to some of the unreacted starting material as well as to the
other nitrogen in the same molecule. The alkylation of amines via
alkyl halides is notoriously bad about giving mixtures.

In the same line as was suggested in the quoted paragraph, what if the
piperazine were first reacted with at least one equivalent of HBr or
some other acid. I would think that this should favor tying up one
amine per molecule rather than having a mixture of both amines reacted
in some molecules along with none reacted in others. IF this were true,
then one of the amines indeed would be deactivated, and maybe only the
other one would react when the alkyl bromide was added.

I don't know the cost of piperazine, but the most typical way to favor
monoalkylation is to use a very large excess of the piperazine and a
very limited amount of halide. Then one has the problem of recycling
the unreacted piperazine, and still having to separate some dialkylation
product, albeit less.

I like the idea of someway tying up one amine, maybe with some polymer.

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