Sulfonyl compound and thermal-sensitive recording medium using the same

A thermal recording medium, comprised of a substrate and a thermal developer layer using a developing reaction of an electron donative basic dye precursor and an electron receptive developer, said developer comprising at least one selected from the sulfonyl compounds represented by general formula (1) and the polyvalent metal salts thereof:

• ##STR13## where X represents a hydrogen atom or a lower alkyl group, R.sub.1, R.sub.2 and R.sub.3, which may be equal or different, represent a hydrogen atom, a hydroxyl group, a lower alkyl group or a cycloalkyl group, R.sub.4, R.sub.5 and R.sub.6, which may be equal or different, represent a hydrogen atom, halogen atom, a lower alkyl group or a cycloalkyl group, and R.sub.7 represents a hydrogen atom or ##STR14## where R.sub.4, R.sub.5 and R.sub.6 are defined above, and at least one selected from the diphenyl derivatives represented by general formula (2) below:
• ##STR15## wherein R.sub.8 and R.sub.9, which may be equal or different, represent a hydrogen atom, a lower alkyl group or a lower alkylene group, and R.sub.10 represents a hydrogen atom or straight or branched lower alkyl group, Y represents sulfonyl group, a lower alkylene group or a lower alkylidene group, and n represents 0 or 1.