Static and Flow Nuclear Magnetic Resonance of Dehydrohalogenation of 2-Chloropropanoyl Chloride

Article abstract of DOI:10.1021/jo01291a008

The dehydrohalogenation of 2-chloropropanoyl chloride (CPC) using diazabicyclooctane (Dabco) or triethylamine (TEA) in chloroform has been studied at temperatures ranging from 30 to -30 deg C by using ¹H nuclear magnetic resonance spectroscopy under static and flowing conditions.In addition to several minor products, three species that have not been found previously are observed : CH3CHClCOCCl(CH3)CClO (DMKC), CH3CHClCOCCl(CH3)CO(NR3)Cl (DMKTA), and 2,4-dichloro-2,4-dimethyl-1,3-cyclobutadione (DDC).For Dabco, the formation of DMKC from DMKTA is fast at 30 deg C, and DMKTA cannot be detected above 0 deg C.On the other hand, for TEA, DMKTA can be detected at 30 deg C, and the formation of DMKC is slow, occurring over a period of several hours (depending on the TEA concentration).This decrease in rate allows the formation of DDC from DMKTA to become competitive.DDC is unstable in the reaction solution.Methylchloroketene appears to be a precursor of DMKTA; however, it is too reactive in the presence of base to be detected even by stopped-flow NMR at -30 deg C.The presence of cyclopentadiene in the reaction mixture does not prevent the formation of DMKC when Dabco is used, but DMKC slowly decays with the resultant formation of the ketene-cyclopentadiene adduct.On the other hand, for TEA, this adduct is formed rapidly, and neither DMKTA nor DMKC is detected.Although the results appear different for the two bases, they can be accommodated by a common mechanism in which the rate of certain steps depends on the nature of the base.