Vinyl diazoacetates decomposition

Diazocarbonyl compounds are especially useful in these reactions because of their ease of formation, relative stability, and controlled reactivity in catalytic reactions [ 1,11 ]. As outlined in Scheme 1, a wide diversity of methodologies are available for this synthesis, with access dependent on the nature of Z. Vinyl- and aryldiazoacetates are accessible by other pathways [2]. The order of reactivity toward diazo decomposition has diazoketones and diazoacetates much more reactive than diazoacetoacetates or diazomalonates. However, the influence of electronic effects on reactivities is more pronounced with phenyl- and vinyl-diazoacetates than with diazoacetoacetates and, especially, diazoacetates [12]. 

As it is known from experience that the metal carbenes operating in most catalyzed reactions of diazo compounds are electrophilic species, it comes as no surprise that only a few examples of efficient catalyzed cyclopropanation of electron-poor alkeiies exist. One of those examples is the copper-catalyzed cyclopropanation of methyl vinyl ketone with ethyl diazoacetate 140), contrasting with the 2-pyrazoline formation in the purely thermal reaction (for failures to obtain cyclopropanes by copper-catalyzed decomposition of diazoesters, see Table VIII in Ref. 6). 

Based on a detailed investigation, it was concluded that the exceptional ability of the molybdenum compounds to promote cyclopropanation of electron-poor alkenes is not caused by intermediate nucleophilic metal carbenes, as one might assume at first glance. Rather, they seem to interfere with the reaction sequence of the uncatalyzed formation of 2-pyrazolines (Scheme 18) by preventing the 1-pyrazoline - 2-pyrazoline tautomerization from occurring. Thereby, the 1-pyrazoline has the opportunity to decompose purely thermally to cyclopropanes and formal vinylic C—H insertion products. This assumption is supported by the following facts a) Neither Mo(CO)6 nor Mo2(OAc)4 influence the rate of [3 + 2] cycloaddition of the diazocarbonyl compound to the alkene. b) Decomposition of ethyl diazoacetate is only weakly accelerated by the molybdenum compounds, c) The latter do not affect the decomposition rate of and product distribution from independently synthesized, representative 1-pyrazolines, and 2-pyrazolines are not at all decomposed in their presence at the given reaction temperature. 

Intermolecular cyclopropanation reactions with ethyl diazoacetate have been employed for the construction of the cyclopropane-containing amino acid 7 (equation 25) Thus, rhodium(II) acetate catalysed decomposition of ethyl diazoacetate in the presence of d-cbz-vinylglycine methyl ester 5 afforded cyclopropyl ester 6 in 85% yield. Removal of the protecting group completed the synthesis of 7. Another example illustrating intermolecular cyclopropanation can be found in Piers and Moss synthesis of ( )-quadrone 8" (equation 26). Intermolecular cyclopropanation of enamide or vinyl ether functions using ethyl diazoacetate has also been used in the synthesis of eburnamonine 9", pentalenolactone E ester 10" and ( )-dicranenone A11" (equations 27-29). 

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