The present invention relates to compounds of the formula I in which the parameters have the respective meanings given in the description, to the use thereof for high-frequency components, to liquid-crystalline media comprising the compounds, and to high-frequency components, in particular antennae, especially for the gigahertz range, comprising these media. The liquid-crystalline media serve, for example, for the phase shifting of microwaves for tunable ‘phased-array’ antennae.