The electrophile involved in the above reaction is

trichloromethyl anion $\left(\stackrel{\ominus }{\mathrm{C}}{\mathrm{HCl}}_2\right)$

dichlorocarbene $\left(:{\mathrm{CCl}}_2\right)$

dichloromethyl cation$\left({ }^{\oplus }{\mathrm{CHCl}}_2\right)$

formyl cation$\left({ }^{\oplus }\mathrm{CHO}\right)$