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.