Magnetic non-equivalence in the methylene group of an ethyl ester

✍ Scribed by G.E. Hall; D. Hughes; D. Rae; A.P. Rhodes

• Publisher: Elsevier Science
• Year: 1967
• Tongue: French
• Weight: 241 KB
• Volume: 8
• Category: Article
• ISSN: 0040-4039
• DOI: 10.1016/s0040-4039(00)90524-0

✦ Synopsis

At room temperature in deuterochloroform solution, ethyl 2-(2,&dinitrophenyl) acetoacetate exists in the enol form, as shown* by a one proton singlet at 13.18 6 in the proton magnetic resonance (p.m.r.) -1 spectrum (l), and the absence of any strong 3 C-0 band above l'i'O0 cm in the infra-red. The methylens protons show magnetic non-equivalence (mE), and the ethyl ester group comprises an ABX 3 p.m.r. spectrum. This was confirmed by double resonance experiments (Fig. 1). Spin-decoupling proved that the multiplet centrad about 4.14 6, end assigned to the methylene protons, is coupled to the three proton triplet at 1.14 6.

Spin tickling experiments (3) showed that JAx (4,) is of opposite sign to JAB, consistent'with JAB being a geminal and JAx a vicinal coupling cons-&ant. This information and the parameters deduced for the ethyl grouping of the enolic form by spectral analysis (4), are given in Table 1.

If the compound were in the keto form, i'ME might have been anticipated (5), although It has not yet been observed in the keto form of an ethyl acetoacetate. Inspection of Dreiding molecular models showed that the observed WE can readily be explained (cf 6) if the enolic *All chemical shifts (s ) are in p.p.m. downfield from internal * tetramethylsilane.