1. Transmetalation of acyclic aminostannanes

Transmetalation of acyclic aminostannanes

In the early 1970s, Peterson showed that a-aminoorganolithium compounds such as (N,N-dimethylamino)methyllithium could be prepared quite easily by transmetalation of the corresponding organostannane (eqn. 1). [1-3] An example from the current work showing the typical conditions is shown in eqn. 2. Note that the carbon bearing the lithium is primary.

We found that the transmetalation of branched stannanes such as 4 and 6 was unsuccessful in generating the secondary a-aminoorganolithiums 5 and 7, respectively (eqns. 3 and 4). Even under drastic conditions, starting material was recovered in good yield. Click here if you want to see how we made the stannane starting materials.

Others have reported similar results. Tsunoda and coworkers did not observe tin-lithium exchange in eqn. 5, although they could generate this organolithium by reductive cleavage of an organosulfur compound [4]. Hence, there is nothing inherently wrong with these organolithiums; tin-lithium exchange is just not the way to go about making them. Chong and coworkers [5] were unable to transmetalate the stannane shown in eqn. (6). Pearson and Stevens (this work) found that recovered starting material was isolated from attempted transmetalations of 8. From the examples in eqns. (5) and (7), it is apparent that stannanes bearing an adjacent cyclic amino group fared no better than those with an acylic amino group in transmetalations.

Does tin-lithium exchange fail because the alkyllithium reagent (e.g. BuLi) does not attack the tin atom for some reason? The answer is no. We have found that while the attempted transmetalation of the trimethylstannyl compound 9 fails, the methyl groups are replaced by butyl groups, thus proving the intermediacy of the stannate 11. Tin-lithium exchange is well-known to be an equilibrium process, with the most stable organolithium being formed preferentially. This experiment is a key one, showing that the desired a-aminoorganolithium is less stable than both methyl- and butyl-lithium! Alternatively, there may be some stereoelectronic reason that the desired a-aminoorganolithium does not dissociate from the stannate. We will return to this point in the Hypothesis Section.

You may be wondering if Bu^sLi or tert-butyllithium would force the transmetalation. They do not. The stannanes are untouched by these reagents, presumably due to steric reasons.

From the failure to transmetalate branched acyclic aminostannanes, one might conclude that secondary a-aminoorganolithiums are inaccessible by this method. Not so! Read on.

References

1. Peterson, D. J. J. Organomet. Chem. 1970, 21, 63. Go back to text.
2. Peterson, D. J. J. Am. Chem. Soc. 1971, 93, 4027. Go back to text.
3. Peterson, D. J.; Ward, J. F. J. Organomet. Chem. 1974, 66, 209. Go back to text.
4. Tsunoda, T.; Fujiwara, K.; Yamamoto, Y.; Itô, S. Tetrahedron Lett. 1991, 32, 1975. Go back to text.
5. Burchat, A. F.; Chong, J. M.; Park, S. B. Tetrahedron Lett. 1993, 34, 51. Go back to text.

Back to the introductory page

1. Transmetalation of acyclic aminostannanes: Primary organolithiums can be formed, but secondary organolithiums cannot. (THIS PAGE)
2. Transmetalation of cyclic aminostannanes: Secondary organolithiums are formed
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.