thermal alkyl [1,3] shifts, similar to [1,3] hydride shifts, must proceed antarafacially. the geometry of the transition state is prohibitive. but an alkyl group, due to the nature of its orbitals, can invert its geometry and form a new bond with the back lobe of its sp3 orbital. this reaction will result a suprafacial shift. these reactions are still not common in open chain systems because of the highly ordered nature of the transition state. so, the reactions work better in cyclic molecules.