A. Guirado et al. / Tetrahedron Letters 43 (2002) 1405–1408
1407
of the electrogenerated species shows a remarkable
dependence on the heteroaryl groups supported by the
oxygenated carbon atoms. Thus, furil 1 provides nucle-
ophilic intermediates which undergo electrophilic trap-
ping, whereas those intermediates generated from
diketones 5 show a dramatic change of activity into
electron transfer agents. Therefore, it is understandable
that a less efficient stabilization of the negative charge
supported by a electrogenerated intermediate could
enhance a reducing character. Hence, it can be pre-
sumed that furyl groups are actually providing a better
stabilization over the electron excess of such intermedi-
ates than the other heteroaryl substituents (5g–i). The
mp 138–140°C; 4b (acetonitrile) mp 211–213°C; 3c (ace-
tonitrile) mp 132–134°C; 4c (acetonitrile) mp 193–
195°C; 3d (pet. ether) mp 135°C; 4d (acetonitrile) mp
225–227°C; 3e (acetonitrile) mp 139–141°C; 4e (acetoni-
trile) mp 238–240°C; 3f (acetonitrile) mp 118–120°C; 4f
(acetonitrile) mp 174–176°C. All compounds gave satis-
1
13
factory IR, H, C NMR, mass spectra and elemental
analyses.
Acknowledgements
1
3
acidity of furan-2-carboxylic acid (pK =3.15) is con-
We gratefully acknowledge the financial support of
the Ministerio de Ciencia y Technolog ´ı a (project
BQU2000-0222).
a
sistently higher than both thiophen-2-carboxylic acid
(
pK =3.5) and pirrol-2-carboxylic acid (pK =4.4).
a a
To conclude, it has been established that cathodic
reductions of difuryl-1,2-diketones in the presence of
carbonimidoyl dichlorides have a specific use for
preparing previously unattainable 2-arylimino-4,5-di-2-
furyl-1,3-dioxoles and (E)-1,2-di-2-furylvinylene bis(N-
arylchloroformimidates). Also of significance is the
discovered ability of diketones 5 to act as electron
transfer agents in order to permit the electrochemical
generation of isocyanides by indirect reduction of car-
bonimidoyl dichlorides under a relatively low cathodic
potential.
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(
entries a, f); −1.05 V (entry e). The electricity con-
sumption was 2 F/mol for all cases. All electrolysis
products were isolated by dropping the catholyte solu-
tion into cold brine (200 mL) and filtering or extracting
the mixture with ether. The ether layers were washed
with cold water and dried on anhydrous magnesium
sulfate. A solid mixture of crude products 3 and 4 was
obtained by removing ether under reduced pressure.
These were then isolated in a high purity state by
fractional crystallization from acetonitrile. Crystalliza-
tion of products 4 occurred at room temperature,
whereas products 3 were obtained by crystallization
at−10°C and recrystallization from acetonitrile or
petroleum ether. Compound 3a (pet. ether) mp 125–
1
27°C; 4a (acetonitrile) mp 173–175°C; 3b (acetonitrile)