NOTE: Important charges and non-bonding electrons are shown throughout the animation except during the transition phase
Click the structures and reaction arrows in sequence to view the 3D models and animations respectively
Singlet carbenes such as this one here (remember that electron-rich substituents stabilize the singlet spin state), can add to alkenes in an entirely concerted manner, with the curly arrows written as shown. Because the process is concerted we expect that the geometry of the alkenes should be preserved in the product, so the reaction ought to be stereospecific. The two examples above show that this is indeed the case. It is more impressive that the Z-alkene gives the cis cyclopropane as this is less stable compared with the trans cyclopropane, and would change if it could.