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Metallotropic Equilibrium and Configurational Stability of 3-Chloro-1-(trimethylsilyl)propargyl and -allenyl Metals: Comparative Study among Lithium, Titanium, and Zinc
journal contribution
posted on 2012-07-09, 00:00 authored by Joseph Bejjani, Candice Botuha, Fabrice Chemla, Franck Ferreira, Sarah Magnus, Alejandro Pérez-LunaA 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
°C in Trapp mixture on the time scale of its reaction with (+)-camphor,
while the titanium analogue proved to be configurationally stable
at −40 °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
°C in THF and its dynamic kinetic resolution was investigated.