ECHET96 Search CD [Molecules: 4] [Related articles/posters: 081 032 114 109 043 ]

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

Brief overview

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.

Here we go! Click on the links for the rest of the story.

  1. Transmetalation of acyclic aminostannanes: Primary organolithiums can be formed, but secondary organolithiums cannot
  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