Flash vacuum pyrolysis of tert-butyl β-ketoesters: Sterically protected α-oxoketenes

Article abstract of DOI:10.1016/S0040-4020(01)90376-X

Infrared spectroscopic analysis of the products showed that flash vacuum pyrolyses (FVP) of dimethyl tert-butylmalonate (1b) and methyl tert-butyl(pivaloyl)acetate (1d) at ca. 550°C afforded the corresponding tert-butyl(carbomethoxy)ketene (4b) and tert-butyl(pivaloyl)ketene (4d), respectively, with loss of methanol, together with unreacted 1b and 1d (Ar matrix, 12 K or neat at 77 K; 10^-5 mbar). Monitoring by IR spectroscopy showed that 4b reacted with methanol at ca. -50°C to give 1b. Ketene 4d does not react with methanol at room temperature, but afforded ester 1d on refluxing for 8 h. FVP of 1b and 1d at temperatures above 650°C gave the α-oxoketenes 4b and 4d, respectively, unsubstituted dimethyl malonate (1a) and methyl 4,4-dimethyl-3-oxo-pentanoate (1c), respectively, due to retro-ene reactions with elimination of isobutene, as well as pyrolysis products derived from 1a and 1c, respectively. FVP of α-unsubstituted β-ketoesters 1a and 1c at ca. 500°C (10^-5 mbar) with argon matrix isolation of the products at 12 K afforded the ketenes 4a and 4c as mixtures of s-Z and s-E conformers together with mixtures of unreacted keto (1a, c) and enol forms (2a, c). On warming to temperatures between -90 and -50°C, back-reaction of ketenes 4a, c with methanol resulted in the re-generation of enols 2a, c without increasing the amounts of the keto forms 1a, c.