The mechanism and the origins of the stereocontrol observed in the reaction between differently substitutedalkenes and stable (phosphino)(silyl)carbenes giving cyclopropanes have been studied computationally.These cyclopropanation reactions proceed via asynchronous concerted mechanisms involving earlytransition structures with a significant charge transfer from the carbene to the alkene moiety. The geometricfeatures of these transition structures preclude a significant overlap between the orbitals required forsecondary orbital interactions between the reactants. The stereoselectivity observed experimentally stemsfrom favorable electrostatic and steric interactions between the reactants leading to the stereoisomers inwhich the phosphanyl and carbonyl or aryl groups are cis to each other.