Transmetalation of cyclic aminostannanes
From the failure to transmetalate
branched acyclic aminostannanes, one might conclude that secondary
a-aminoorganolithiums are either thermodynamically unstable or kinetically
inaccessible. However, following the maxim 'There's no excuse for not
doing the experiment', the Gawley group found that secondary cyclic
a-aminoorganolithiums 13 are formed readily at low temperature
by tin-lithium exchange on the stannanes 12 (eqn. 9). [1,2]
Now, the question arises: Why is it possible to easily transmetalate
primary and cyclic secondary a-aminoorganostannanes, but not secondary
acyclic ones? The short answer is that we don't know, but this
poster describes some of the possibilities we have considered. Unfortunately,
testing these hypotheses has not led to a definitive answer, so we submit
our thoughts and preliminary data to
the organic community for advice and suggestions. Click
here to see two hypotheses.
Note: The difference cannot be due to the simple fact that the nitrogen
is part of a ring. Click here to see
an example where a tin compound with a pyrrolidine-type nitrogen does not
transmetalate.
References
1. Gawley, R. E.; Zhang, Q. J. Am. Chem. Soc. 1993, 115,
7515.
2. Gawley, R. E.; Zhang, Q.; Campagna, S. J. Am. Chem. Soc. 1995,
117, 11817.
Back to the introductory page
1. Transmetalation of acyclic aminostannanes:
Primary organolithiums can be formed, but secondary organolithiums cannot
2. Transmetalation of cyclic aminostannanes: Secondary organolithiums are
formed. (THIS PAGE)
3. Hypotheses: Conformational effects (i.e.,
a kinetic problem) or thermodynamics may be responsible
4. These studies allow a ranking of the relative
stabilities of organolithiums
5. How the aminostannanes were made.