Why do cycloaddition reactions involving C60 prefer [6,6] over [5,6] bonds?

The origin of the experimentally known preference for [6,6] over [5,6] bonds in cycloaddition reactions involving C60 has been computationally explored. To this end, the Diels-Alder reaction between cyclopentadiene and C60 has been analysed by means of the recently introduced activation strain model of reactivity in combination with the energy decomposition analysis method. Other issues, such as the aromaticity of the corresponding transition states, have also been considered. These results indicate that the major factor controlling the observed regioselectivity is the more stabilising interaction between the deformed reactants in the [6,6] reaction pathway along the entire reaction coordinate. Why [6,6]? The preference for [6,6] over [5,6] bonds in cycloaddition reactions involving C60 is experimentally well-established (see figure). However, the reasons (i.e., physical factors) behind this preference are so far completely unknown. By means of the recently introduced activation strain model of reactivity in combination with the energy decomposition analysis method, a definite answer to the question in the title is providedWe are grateful for financial support from the Spanish MICINN (grants CTQ2010-20714-C02-01, Consolider-Ingenio 2010, and CSD2007-00006 to I. F. and CTQ2011-23156/BQU to M. S.), CAM (grant S2009/PPQ-1634 to I. F.), Catalan DIUE (projects 2009SGR637 and XRQTC to M. S.), the FEDER fund for the grant UNGI08-4E-003 to M. S., the National Research School Combination-Catalysis (NRSC-C), and The Netherlands Organization for Scientific Research (NWO/CW and NWO/NCF). Support for the research of M. S. was received through the ICREA Academia 2009 prize of the Catalan DIU