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DOI: 10.1039/C6CC03947B
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elimination now substantially more favourable (-16 kJ/mol).
The dimerization of two allene molecules is well known to
proceed as a spontaneous and exothermic reaction to give 1,2-
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4
5
6
7
8
2
7,28
dimethylenecyclobutane.
The corresponding free energies
for this product are also shown. It should be noted that while
the NMR experiment described above reveals evidence for a
C
6
H
8
molecule (two signals 6.23 ppm and 1.56 ppm),
unresolved splitting means it is difficult distinguish between
,2-dimethylenecyclobutane, hexa-1,3-diene or hexa-1,4-
diene.
1
9
1
1
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0
1
1
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1
1
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Scheme 2 DFT calculated Li bond lengths to allyl and N/P 14
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atoms.
15
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1
6
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These calculations strongly support the hypothesis that the
position of the Li atom has implications for the cleavage
mechanism. For instance, when PMDETA is used as a donor,
the Li-N distance is calculated to be 4.46 Å (Li cation is closer
to C3), and experimentally N-C bond cleavage is not observed.
In contrast, in the P-containing analogue the Li-P distance is
significantly shorter 2.55 Å, and in this case the elimination of
7
509.
1
1
1
2
7
8
9
0
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C
3
H
4
is readily observed (Scheme 2).
In conclusion we have shown that the choice of Lewis donor in
these lithium mediated metallation or cleavage reactions of 21
two analogous N-allyl and P-allyl systems dictates the
2
2
2
2
3
4
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experimental outcome. Structural and DFT calculations suggest
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2
011, 15, 2362.
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3
684.
Acknowledgements
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R. E. Mulvey, Dalton Trans., 2013, 42, 6676.
A. R. Kennedy, R. E. Mulvey, D. L. Ramsay and S. D.
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This work was supported by the Australian Research Council
(DE140101137) and Monash University. The authors would like
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S. L. Skraba and R. P. Johnson, J. Org. Chem., 2012, 77,
to thank Prof. Phil Andrews for his valuable input through
many discussions into this research.
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7
E. C. Border, M. Koutsaplis and P. C. Andrews,
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Notes and references
‡
Footnotes relating to the main text should appear here. These
might include comments relevant to but not central to the
matter under discussion, limited experimental and spectral data,
and crystallographic data.
1
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