It was good to see your poster "New N-fluoro reagents". Nice to see the
N-F 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 {1-Chloromethyl-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane
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-F
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
Dr Nicholas Lawrence
Department of Chemistry
UMIST
PO Box 88
Manchester
M60 1QD
tel 0161 200 4529
fax 0161 236 7677