FLASH VACUUM PYROLYSIS OF TERT-BUTYL BETA-KETOESTERS - STERICALLY PROTECTED ALPHA-OXOKETENES

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-degrees-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-degrees-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-degrees-C gave the alpha-oxoketenes 4b and 4d, respectively, unsubstituted dimethyl malonate (1a) and methyl 4,4-dimethyl-3-oxopentanoate (1c), respectively, due to retro-ene reactions with elimination of isobutene, as well as pyrolysis products derived from 1a and 1c, respectively. FVP of alpha-unsubstituted beta-ketoesters 1a and 1c at ca. 500-degrees-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-degrees-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.