Why is it possible to transmetalate cyclic secondary a-amino
stannanes but not acyclic ones?
Robert E. Gawley,a Jeffrey D. Evanseck,a William H. Pearsonb and Erland P. Stevensb
aDepartment of Chemistry, University of Miami, Coral Gables, Florida, USA 33124-0431
bDepartment of Chemistry, University of Michigan, Ann Arbor, Michigan, USA 48109-1055
In the early 1970s, Peterson showed that (N,N-dialkylamino)methylstannanes
could be efficiently transmetalated to (N,N-dialkylamino)methyllithiums.
But for over 20 years, no reports have emerged of the use of this method
to generate secondary a-aminoorganolithiums of this type. Several groups,
including those of Pearson and Stevens, have found that acyclic 1-(N,N-dialkylamino)alkylstannanes
are resistant to tin-lithium exchange. However, Gawley and coworkers recently
found that cyclic a-aminoorganolithiums can be generated smoothly
from 2-(tributylstannyl)-pyrrolidines and -piperidines. Thus,
secondary a-aminoorganolithiums are available by tin-lithium exchange
only if the carbanionic carbon is in a ring. We do not know why this should
be so. The purpose of this poster is to describe the details of these studies
and see if anyone has any reasonable explanation for this dichotomy. Two
hypotheses are presented to get the discussion going.
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