I have been very interested in your contribution, particularly the
formation and properties of the cyclic trioxo ylides 6a and 6b. From
our extensive work in this area [1] I would suggest that the 13C
signals for 6b are mis-assigned. The signal at 191.09 is in fact the
4-CO with the smaller coupling constant of 7 Hz. The assignment of the
other two carbonyls is uncertain between C2 and C5. A good example for
comparison is MeO2C-C(=PPh3)-CO-CO2Me where the keto CO comes at 184
ppm (J6) and the two esters at 167.5 and 167.8 (J 14 + 15) [2]. We
were also initially surprised that the two bond J(C-P) is smaller than
the three bond value but this seems to be a general trend. We have
also recently obtained a series of 3-triphenylphosphoranylidene
-pyrrolidine-2,4-diones and find J 7-8 for the 4-(keto)CO and J 16-17
for the 2-(lactam)CO. It is notable that Schmidt et al have not
committed themselves in assigning the COs of 6a but I am sure their
signal at 174 (J 8) is also the ketone CO.
In studies of the pyrolysis of a wide range of oxo ylides we have
found a correlation such that examples with 2J(P-CO)<10 Hz eliminate
Ph3PO while those with J>10 do not. It would thus be of great interest
to examine the pyrolysis behaviour of 6a and 6b. If you are not
familiar with the technique we would be happy to try it for you. The
expected products- didehydromaleic and -thiomaleic anhydrides would be
of the greatest interest. In unpublished work we have found that
2-triphenylphosphoranylidenecyclohexanone (J P-CO = 4) does lose Ph3PO
to give cyclohexyne.
I look forward to hearing from you
Alan Aitken
[1] J Chem. Soc., Perkin Trans. 1, 1996, 485, and previous parts in
this series.
[2] J. Chem. Soc., Perkin Trans. 1, 1994, 2467.
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