Mechanisms of allene stereoinversion by imidozirconium complexes.
The zirconium-mediated stereoinversion of allenes has been investigated by studying the stereochemical behavior of metallacycles derived from [2 + 2] cycloaddition of enantioenriched allenes with chiral and achiral imidozirconocene complexes. Relative rates of metallacycle racemization were measured by circular dichroism, and intermediates in the selective stereoinversion of diphenylallene with a chiral imidozirconium complex were observed by NMR spectroscopy. Metallacycles derived from dialkylallenes are proposed to racemize via reversible beta-hydride elimination. Stereoinversion of diarylallene-derived metallacycles proceeds much more slowly and is thought to proceed through an eta4-azatrimethylenemethane transition state.
Michael
Forrest E
FE
Department of Chemistry and Center for New Directions in Organic Synthesis, University of California at Berkeley, Berkeley, California 94720, USA.
Duncan
Andrew P
AP
Sweeney
Zachary K
ZK
Bergman
Robert G
RG
D000466:Alkadienes / Q000138:chemical synthesis / Q000737:chemistry*; D007094:Imides / Q000737:chemistry*; D007700:Kinetics; D009942:Organometallic Compounds / Q000737:chemistry; D013237:Stereoisomerism; D015040:Zirconium / Q000737:chemistry*
D000466:Alkadienes; D007094:Imides; D009942:Organometallic Compounds; C025947:propadiene; D015040:Zirconium