Metallotropic Equilibrium and Configurational Stability of 3-Chloro-1-(trimethylsilyl)propargyl and -allenyl Metals: Comparative Study among Lithium, Titanium, and Zinc

A comparative study of the metallotropic equilibrium between 1-chloro-3-(trimethylsilyl)propargyl and -allenyl metals was undertaken by means of lithio-, titano-, and zinco-carbenoids. The lithium and zinc species were shown to react mainly in their allenic metallotropic forms, whereas the titanium species proved to react in both its propargylic and allenic forms. The configurational stability of these organometallics was next examined using a modification of the Hoffmann test. In each case, the organometallic was reacted with a chiral enantiopure electrophile. A comparison of the diastereomeric ratios obtained at low and high conversion rates of the reagent allows assessment of its configurational stability. The lithium species thus exhibited a configurational lability at -125 degrees C in Trapp mixture on the time scale of its reaction with (+)-camphor, while the titanium analogue proved to be configurationally stable at -40 degrees C in THF/Et2O on the time scale defined by its reaction with (S)-N-tritylprolinal. In the context of its reaction with the same electrophile, the zinc bromide species was proven to be partially labile from -80 degrees C in THF and its dynamic kinetic resolution was investigated.