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Organic & Biomolecular Chemistry
Page 3 of 5
DOI: 10.1039/C8OB01339J
Journal Name
COMMUNICATION
(Table 3, entries 7,8). However, the less stable
phenylcyclohexyl cation generated from substrate 9i gave the
allylated product 10i in good yield (Table 3, entry 9). Benzylic
ether 9j gave product 10j in poor yield while no expected
product 10k was obtained from adamantyl ether 9k (Table 3,
entries 10, 11). Poor performance of these substrates can be
Conflicts of interest
There are no conflicts to declare.
attributed to a slow fragmentation of the oxonium ion
carbenium ion due to the decreased stability of the latter.
Oxonium ion allylation by-products 11 13 were isolated in the
2 to
3
Acknowledgements
–
The work was supported by an internal grant from the Latvian
Institute of Organic Synthesis.
case of substrates 9b,d,k providing less stable carbenium ions.
Notably, in the case of adamantyl ether 9k, homoallylether 13
formed as the major product.
Notes and references
1
2
3
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Figure 2. Isolated oxonium ion allylation products
Hydrogen bonding properties of HFIP22,23 can potentially promote
the ionization of ethers 7 and 9 to give carbenium ions without
electrochemical activation. Therefore, several control reactions
were performed in the absence of electric current (Table 4). Indeed,
product 8a and 10a,f,h formation was observed, however, the
reaction was very slow and lower yielding. These results indicate
that electrochemical activation of the substrates is crucial for
efficient carbenium ion generation.
4
5
6
7
8
9
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Table 4. Product
8 and 10 formation without electrochemical
activation of the substrate
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Entry
time
18 h
product, NMR yield %
8a, 25
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1
2
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18 h
10a, 5
18 h
10f, 60
3 days
10h, 33
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a
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Conclusions
In summary, we have demonstrated that carbenium ions can
be formed in the presence of nucleophiles by fragmentation of
electrochemically generated oxonium ions. To achieve an
acceptable oxidation potential of the substrate, the
tributylstannylmethyl group was found as an efficient
electroauxiliary for the oxonium ion formation. The application
of this concept was demonstrated for allylation of carbenium
18 R. Francke, D. Cericola, R. Kötz, D. Weingarth and S. R.
Waldvogel, Electrochimica Acta, 2012, 62, 372–380.
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20 The oxidation potential of allylsilane 7a in 0.1 M LiBF4/MeCN
MeCN has been reported (1.65 V vs Ag/Ag+; 1.85 V vs NHE)
which is slightly higher than for substrate 1c. T.Takanami, A.
Abe, K. Suda, H. Ohmuri, M. Masui, Chem. Pharm. Bull. 1990,
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ions electrochemically generated from
a
range of
stannylmethyl ethers in the presence of allylsilanes. Additional
investigations are in progress to expand the scope of this
reaction with other nucleophiles compatible with the oxidation
potential needed for substrate activation.
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22 I. Colomer, A. E. R. Chamberlain, M. B. Haughey and T. J.
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