Communication
ChemComm
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Based on these studies and prior knowledge in this field, we
propose that the retro-Claisen reaction occurs through the
mechanistic pathways depicted in Scheme 4 (top). Again,
enolization could happen for intermediates 10–13 during the
course of reaction. We cannot rule out the possibility that
tropylium ion can coordinate to the alcohol/water reagent itself
to generate a strong Brønsted acid (14), which can in turn
facilitate the retro-Claisen reaction. However, comparative reac-
tions with strong Brønsted acid such as HBF4 or TfOH
(see Table 1, entries 19–22) showed much lower efficiency than
the tropylium-promoted protocol, even at longer reaction
times. Therefore, we believe that the hidden Brønsted acid
catalytic pathway, if indeed exists, is not the predominant
process to mediate the retro-Claisen reaction.
In conclusion, we have developed a new convenient method
for C–C bond cleavage of 1,3-diketone compounds using
tropylium tetrafluoroborate as an organic Lewis acid promoter.
A wide range of carboxylic acid, ester and amide products were
efficiently obtained in batch or flow using this approach with
water, alcohols and amines, respectively. Replacement of these
solvolytic reagents with mercaptans led to the formation of a
range of new alkenyl thioethers.
The authors thank the Australian Research Council (grant
DE150100517) for financial support. MAH thanks the Iraqi
HCED for sponsoring his PhD scholarship.
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F. Li, Chem. Commun., 2014, 50, 12181–12184.
16 See the ESI† for more details.
17 Diketone 2b predominantly formed enamine adducts with amine
substrates, see the ESI† page S18 for more details. Diketone 2c only
formed enamine adducts with primary amines.
18 Reactions between acyclic 1,3-diketone substrates (dibenzoyl-
methane, benzoylacetone and 1-benzoyl-3,3,3-trifluoroacetone) and
mercaptans (cyclohexyl thiol and benzyl thiol) did not work, giving
mainly starting materials back and a mixture of unidentifiable
minor products.
Conflicts of interest
There are no conflicts to declare.
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