38: N-Fluoro reagents

Nicholas Lawrence (N.Lawrence@umist.ac.uk)
Fri, 23 Jun 95 15:26:18 0000

Dear Dr Bohlmann,

It was good to see your poster "New N-fluoro reagents". Nice to see the
N=D0F bond getting some positive exposure. We have found some of the
points you make interesting and useful. However, I feel that I should
defend, or try at least, Selectfluor; especially against the criticism
of it being a poor reagent in terms of atom economy. It is true that
the reagent, F-TEDA-BF4
bis(tetrafluoroborate)}, delivers only 5.4% active fluorine (F+).
However, the reagent was not designed, with atom-efficiency as the main
goal. What it does provide is a crystalline, water-stable, air-stable
source of electrophilic fluorine and importantly, since it has been
commercialised by Air Products Inc., is available off-the-shelf at a
modest cost. [Academics may note that samples are available free !!].
Also, once the fluorine atom has been delivered to the substrate, it is
very easy to recover the mono-quat salt, since it is water soluble.
This mono-quat is simply the precursor to Selectfluor. So the effective
atom economy can be increased by such a recovery process; I accept that
there is an obvious cost for doing so.

You do not give details of the 19F nmr of compound 2. Could you please
send us details of this, as we have made similar compounds ? It would
be useful to make a comparison.

One way to make the sulfonamide reagents more efficient would be to
incorporate more N=D0F bonds into the same molecule. One interesting
possible candidate would be a six-membered ring with alternate SO2 and
NF groups, giving a possible active fluorine content of 19.6%. Is this
a reasonable approach ? It is worth bearing in mind that even this
figure is still a long way off the 50% offered by fluorine itself.
Recent papers from the Chambers group, among others, have indicated that
good control can be achieved in certain reactions by using fluorine.

Keep up the good work, all the best

Nick Lawrence

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