tert-Butyl 2-methyl-2-(4-methylbenzoyl)propanoate

The title compound, C16H22O3, is bent with a dihedral angle of 75.3 (1)° between the mean planes of the benzene ring and a group encompassing the ester functionality (O=C—O—C). In the crystal, the molecules are linked into infinite chains held together by weak C—H⋯O hydrogen-bonded interactions between an H atom on the benzene ring of one molecule and an O atom on the ketone functionality of an adjacent molecule. The chains are arranged with neighbouring tert-butyl and dimethyl groups on adjacent chains exhibiting hydrophobic stacking, with short C—H⋯H—C contacts (2.37 Å) between adjacent chains

The title compound, C 16 H 22 O 3 , is bent with a dihedral angle of 75.3 (1) between the mean planes of the benzene ring and a group encompassing the ester functionality (O C-O-C). In the crystal, the molecules are linked into infinite chains held together by weak C-HÁ Á ÁO hydrogen-bonded interactions between an H atom on the benzene ring of one molecule and an O atom on the ketone functionality of an adjacent molecule. The chains are arranged with neighbouring tertbutyl and dimethyl groups on adjacent chains exhibiting hydrophobic stacking, with short C-HÁ Á ÁH-C contacts (2.37 Å ) between adjacent chains

Comment
Treatment of 2,2-disubstituted t-butyl beta-keto esters with trifluoroacetic acid at room temperature quantitatively generates the corresponding 2,2-disubstituted β-keto acids, which were used to probe the nature of the transition state for the thermal decarboxylation of β-keto acids (Logue et al., 1975). Structurally similar indanone-derived β-keto ester derivatives have been prepared recently (Mouri et al., 2009;Noritake et al., 2008;Rigby & Dixon, 2008). The directing nature of weak C-H···O H-bonds has been noted to be of importance to afford the three dimensional structure observed in these kinds of molecules (Karle et al., 2009).
In this contribution we present the solid state structure of one such 2,2-disubstituted β-keto acid, i.e. the title compound being the tolyl derivative. This is the second paper in a series of four dealing with substituted derivatives (H-, CH 3 -(this paper), Cl-and NO 2 -on the para-position of the phenyl ring) of the title compound. A more detailed comparison of all four substitution compounds will be given in the fourth paper of this series ( 7, 24.1, 27.8, 54.1, 81.8, 129.2, 132.9, 143.5, 174.4, 198.9.

Refinement
All H atoms were placed at calculated positions, with C-H = 0.93 Å (aromatic) or 0.96 Å (methyl) and refined using a riding model with U iso (H) constrained to be 1.5 U eq (C) for methyl groups and 1.2 U eq (C) for all other C atoms. The quality of the data as reflected by only 48% of the reflections observed, large ADP's and inaccurate C-C bond lengths is low. The data had been collected on a 30 year old single point detector instrument not equipped with a low temperature device as part of a class project with undergraduate students. Due to the time constraints imposed by the class schedule a maximum exposure time of 60 s had to be alloted for measuring each reflection.
supplementary materials sup-2 There are close contacts (i.e., <2.4 Å, (Alkorta et al., 2008)) between an H atom on C11 and one on the C18 atom of an adjacent molecule, Fig. 2. These contacts remain present irrespective of if all the H atoms are refined freely (which generates reasonable parameters) or if they are refined generated either with the AFIX 33 or AFIX 137 constraints (used here) as implemented in the Shelxtl software (Sheldrick, 2008). Fig. 1. ORTEP-3 (Farrugia, 1997) drawing of the title compound with displacement ellipsoids drawn at the 50% probability level.

Special details
Experimental. Number of psi-scan sets used was 6. Theta correction was applied. Averaged transmission function was used. No Fourier smoothing was applied.
Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.