Diels-Alder reactions with Ti2C2@C78, M3N@C78, and C78

In a recent paper in Chemistry – a European Journal researchers from Girona and LA have studied the Diels-Alder reactivity of the endohedral metallofullerene Ti₂C₂@C₇₈. Functionalized fullerenes have potentially important applications in non-linear optics, molecular magnets, radioimmunotherapy and many more.
The challenging task to find transition states for the reaction of s-cis-butadiene with any of the double bonds on the carbon sphere, has been achieved with QUILD’s efficient saddle-point searching techniques:

• TSRC enforcing the Hessian to have only 1 negative eigenvalue, along the user-specified reaction path
• adapted delocalized coordinates to deal with mixed strong and weak coordinates

Comparing the activation barriers with those for M₃N@C₇₈ and C₇₈ revealed the empty fullerene to be the most reactive and the M₃N@C₇₈ least reactive, with significantly differing regio-selectivity between the (endohedral) fullerenes.

Most favorable transition states for the Diels-Alder reaction of s-cis-butadiene with Ti₂C₂@C₇₈ (left) and free C₇₈ (right), with different regioselectivity.
The activation barrier for the free fullerene (12.2 kcal/mol) is lower than that of the endohedral metallofullerene (17.4 kcal/mol).

The effect of van der Waals interactions was assessed with Grimme’s D3 corrections and found to strongly affect the potential energy landscape. The dispersion interaction favors the formation of a collision complex between the diene and Ti₂C₂@C₇₈ and furthermore lowers the activation barriers by about 50%.

Related paper, combined experiment and theory on endohedral C-80:

F.-F. Li, A. Rodriguez-Fortea, P. Peng, G. A. Campos Chavez, J. M. Poblet, L. Echegoyen, Electrosynthesis of a Sc3N@I-h-C-80 Methano Derivative from Trianionic Sc3N@I-h-C-80 J. Am. Chem. Soc., 134, 7480-7487 (2012).

transition states, dispersion-corrected DFT (GGA-D3), QUILD

M. Garcia-Borràs, S. Osuna, J. M. Luis, M. Swart, and M. Solà, The Exohedral Diels-Alder Reactivity of the Titanium Carbide Endohedral Metallofullerene Ti2C2@D3h-C78: Comparison with D3h-C78 and M3N@D3h-C78 (M=Sc and Y) Reactivity Chem. Eur. J., 18, 7141-7154 (2012).

S. Osuna, M. Swart, J. M. Campanera, J. M. Poblet and M. Solà, Chemical Reactivity of D3h C78 (Metallo)Fullerene: Regioselectivity Changes Induced by Sc3N Encapsulation J. Am. Chem. Soc., 130, 6206-6214 (2008).