Potassium (1-methoxycarbonyl-2-methylprop-2-en-2-ylidene)azinate

In the title compound, K+·C6H8NO4 −, the K+ cations have a coordination number of seven and are surrounded by four bidentate azinate anions. The methylene groups of the anions are always directed towards the coordinated potassium cations. The N—C—C—C torsion angle is 101.2 (2)°. The orthogonal non-conjugated nature of the salt confirms the supposed geometry and reactivity of this compound.

In the title compound, K + ÁC 6 H 8 NO 4 À , the K + cations have a coordination number of seven and are surrounded by four bidentate azinate anions. The methylene groups of the anions are always directed towards the coordinated potassium cations. The N-C-C-C torsion angle is 101.2 (2) . The orthogonal non-conjugated nature of the salt confirms the supposed geometry and reactivity of this compound.

Comment
In the last decade, interest in peptidomimetics has lead to a fast growing research field within organic chemistry (Grauer et al., 2009). Artificial peptide-like compounds are used to explore the principles of protein-protein interactions and their modulation (Vagner et al., 2008;Wu et al., 2008). In the synthesis of different peptidomimetics, amino acid based building blocks play a key role in the assembly of these structures (Kemp, Curran et al., 1991;Kemp, Boyd & Muendel, 1991;Beal et al., 2000;Kühne et al. 2008). In the context of our work we used an already known deprotonation/protonation sequence (Bouveault et al., 1901) to synthesize our compound. The protonation of the title compound occurs exclusively at the α-position whereas no protonation of the methylene group was observed (Baldwin et al., 1977).

Experimental
The title compound, C 6 H 8 KNO 4 , was prepared in good yield from methyl 3-methyl-2-nitrobut-2-enoate by deprotonation with potassium hydride. In a dry, argon-flushed 50 ml flask, 30.1 mmol of potassium hydride where suspended in 15 ml of dry THF. The suspension was cooled to 0 °C and a solution of 30.1 mmol methyl-3-methyl-2-nitrobut-2-enoate in 5 ml of THF was added via syringe over 30 min. After stirring for 5 h at room temperature a small amount of n-octanol was added at 0 °C to destroy the excess of potassium hydride. The paste was filtrated, washed three times with THF and dried in vacuo to give 4.950 g (25.1 mmol, 83%) of an ochre powder. A portion of the salt was recrystallized from MeOH/THF to give colourless prisms.

Refinement
Hydrogen atoms were located in difference Fourier maps and refined at idealized positions (C-H = 0.98 Å for methyl H atoms and 0.95 Å for all other H Atoms) using a riding model. The U values of the hydrogens are constrained relative to U eq of the parent carbon atom (1.2 x U eq (C) for C-H 2 and 1.5 x U eq (C) for methyl H). Fig. 1. A view of (I). Displacement ellipsoids are drawn at the 50% probability level.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.