Chiral Organometallic Reagents, II. Equilibration of Diastereomeric Carbenoids, the tert‐Butyl 2‐Bromo‐2‐lithio‐1‐methylcyclopropanecarboxylates

The diastereomeric carbenoids 6 are generated at ‐125°C from the dibromocyclopropane 5 by halogen‐metal exchange and trapped by electrophiles. Since the resulting carbenoids 6 equilibrated by halogen‐metal exchange with their precursor dibromo compound 5, the product ratios depend strongly on the reaction conditions used. Normal addition of n‐butyllithium favors equilibration giving “thermodynamic ratios” of the carbenoids. Inverse addition of 5 to tert‐butyllithium approaches the “kinetic ratio” of carbenoids, which is revealed in reactions in which the carbenoids are trapped in situ by alcohols. The carbenoids 6 have been found to be configurationally stable at ‐125°C. They undergo rapid equilibration at temperatures above ‐78°C before decomposition sets in at ‐60°C.