On the cause of low thermal stability of ethyl halodiazoacetates.

Magnus Mortén,Martin Hennum,Tore Bonge-Hansen

doi:10.3762/bjoc.12.155

Abstract

Rates for the thermal decomposition of ethyl halodiazoacetates (halo = Cl, Br, I) have been obtained, and reported herein are their half-lives. The experimental results are supported by DFT calculations, and we provide a possible explanation for the reduced thermal stability of ethyl halodiazoacetates compared to ethyl diazoacetate and for the relative decomposition rates between the chloro, bromo and iodo analogs. We have also briefly studied the thermal, non-catalytic cyclopropanation of styrenes and compared the results to the analogous Rh(II)-catalyzed reactions.

Highlights

The chemistry of diazo compounds has fascinated organic chemists ever since Theodor Curtius synthesized ethyl diazoacetate (EDA, 1) for the first time in 1883 [1]
The synthesis and properties of diazo compounds have been a topic of much interest, relevant are their thermal stability and sensitivity towards Brønsted and Lewis acids
Ethyl diazoacetate (EDA) has a reported half-life of 109 h at 100 °C [3], while we measured 2a to decay with a half-life of 1 h 46 min at 25 °C (Table 1, entry 1)

Summary

Introduction

The chemistry of diazo compounds has fascinated organic chemists ever since Theodor Curtius synthesized ethyl diazoacetate (EDA, 1) for the first time in 1883 [1]. Non-stabilized diazo compounds are thermally labile and usually decompose within hours at room temperature. They are inherently unstable to acid and the diazo carbon has a significant nucleophilic character. The higher reaction rate implies a lower turnover limiting barrier in the catalytic cycle with 2b compared to 1 This example made us wonder whether halodiazoacetates have an increased sensitivity (relative to EDA) towards Brønsted and Lewis acids in general. The halodiazoesters readily decompose at room temperature within hours, but they can be handled in solutions at 0 °C or lower temperatures This significantly reduced thermal stability of halodiazoacetates relative to EDA, in addition to the presumably increased acid sensitivity, inspired us to investigate the properties of the halodiazoacetates in more detail. We set out to study the thermal stabilities of 2a–c by using kinetic data and obtaining information of substituent effects

Results and Discussion

Conclusion

Representative procedure for cyclopropanation of styrenes