Is there a reaction mechanism for distilling wine into brandy and what is it?

No. No chemical reaction occurs when wine is distilled into brandy. Distillation is a separation technique that uses differences in volatility, or the inclination a substance has to vaporize, to separate the components of a mixture. 

To distill brandy, the starting wine is heated. The ethanol in the wine (along with other aromatic compounds) evaporates before the water. The evaporated gases are passed through a condenser (usually a chilled glass tube) where they revert to a liquid state. Thus, the wine has become more concentrated and has a higher alcohol content. Distillation may be repeated one or several times to achieve the desired alcohol concentration.

However, there most certainly was a chemical reaction in the process of making the starting wine. Yeast transformed sugar from grape juice into ethanol and carbon dioxide.