The claim that 1,4-dimethylcyclohexane contains a chiral center in certain stereoisomers is a fundamental fact of stereochemistry based on the molecular symmetry of the cis and trans isomers (specifically, the trans isomer possesses a center of inversion, while the cis isomer possesses a plane of symmetry and is achiral, but neither possesses a stereogenic center in the traditional sense as no carbon is bonded to four different groups). Because this is a matter of basic chemical definition and structure, no citation is needed.
The user's claim relates to the basic stereochemical classification of 1,4-dimethylcyclohexane. While the retrieved papers discuss organic synthesis, stereoselective hydroxylation, and chiral catalysts, none specifically evaluate whether 1,4-dimethylcyclohexane contains chiral centers. More importantly, basic structural properties of small organic molecules are direct consequences of chemical definitions (constitution, symmetry, and stereocenters) rather than empirical findings requiring literature support. The retrieved literature is entirely tangential to this structural chemical fact.