Oxygen-Substituted Ketenes and Bisketenes
Cyclobutenediones are sources of observable bisketenes and
are the first examples of the family of fully conjugated cyclic
enediones, of which the 1,2- and 1,4-benzoquinones 1 and 2
are the most familiar representatives. The parent 1,2-cy-
clobutenedione (3a)2a-f has long attracted interest for theoretical
reasons, and substituted cyclobutenediones are valuable in
synthesis.2g-k The parent 1,3-isomer (3b), for which no Kekule´
structure without formal charges may be drawn, is unknown,
but substituted derivatives, known as 1,3-squaraines, have many
practical applications.3
constants for ring closure of bisketenes 4b to cyclobutenediones
4a vary by a factor of 1018, and in recent studies of bisketenes
with diamino substituents (R ) R1 N), we found these to be
2
the most reactive that have been observed, a result attributed to
the property of these electronegative but π-donating groups to
destabilize ketenes and stabilize cyclobutenediones.4a However,
calculations indicate that ketenes with OH and MeO substituents
are destabilized even more than by NH2 groups, which is
consistent with the known lower stabilization of ketenes by more
electronegative substituents.4d,e The current study of oxygen-
substituted bisketenes from cyclobutenediones5,6 was undertaken
to establish the net effect of the substituents in this series.
Upon photolysis, 1,2-cyclobutenediones 4a form 1,2-bis-
ketenes 4b (eq 1), which have twisted almost perpendicular
conformations as shown.1j,4 The stabilities of bisketenes 4b
relative to cyclobutenediones 4a cover a vast range, and
bisketenes 4b with two electropositive trialkylsilyl substituents
have been found to be more stable than cyclobutenediones 4a,
but in all other known cases, the latter are preferred.4 Rate
1,2-Dihydroxycyclobutenedione 5a (squaric acid)5a,b upon
photolysis in a matrix at 10 K formed bisketene 6a, as identified
by the IR absorption at 2125 cm-1, and with further photolysis
at 10 K successively forms deltic acid 7 and dihydroxyacetylene
8 (eq 2).5c Other reported photolyses of di(oxygen)-substituted
cyclobutenediones include those with di(EtO),6a-c di(i-PrO),6d
di(t-BuO),4c,6d di(Me3SiO),6c and di(AcO)6e,f substituents. Cy-
clopropenones and alkynes are observed products in some of
these reactions,6a-d as in eq 2, and these have been proposed to
form through further reactions of unobserved bisketene
intermediates.6a,d,e
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The goal of the present investigation was to characterize
examples of oxygen-substituted 1,2-bisketenes in solution and
to obtain quantitative measurements of their reactivity and
stability. The results would increase the understanding of the
properties of oxygen-substituted ketenes and bisketenes and
facilitate their practical application.
Results
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