Exploiting the confined reactivity of C-2v-symmetrical pentakis-adducts of[60]fullerene: Regioselective formation of hexakis-, heptakis-, and Octakis-adducts with novel addition patterns by addition of diazomethane followedby dinitrogen extrusion

A series of hexakis- to octakis-adducts of C-60 with novel addition pattern s was synthesized by 1,3-dipolar cycloaddition of diazomethane (CH2N2) to p entakis-adducts, whose reactivity is confined to a single 6-6 bond (bond at the intersect between two hexagons), followed by thermal N-2 extrusion and rearrangement. Starting from pentakis-adducts 1a,b or 13, hexaki-adducts ( +/-)-3a,b and (+/-)-17 with one 6-5 open methano bridge (bridge at the junc tion between a hexagon and a pentagon) were obtained in high yield (Schemes 1 and 6). Further conversion with CH2N2 at -80 to -60 provided heptakis-ad ducts 6a,b and 18, respectively. with two 6-5 open methano bridges (Schemes 2 and 6) Upon reacting (+/-)-3a,b at 0 with a large excess of CH2N2, octak is-adducts, (+/-)-5a,b with three 6-5 open methano bridges were obtained (S cheme 2). Oxidation Of the 6-6 double bond, from which the two vicinal meth ano bridges in heptakis-adducts 6a and 18 depart, did not give the desired diketones with an opened fullerene shell, but only led to the 1,2-diols 14 and 19, respectively (Schemes 5 and 6). The nature of the addends in the va rious addition patterns did not affect the regioselectivity of the 1,3-dipo lar cycloaddition of CH2N2 and the subsequent N-2-extrusion process. The re activity was, however, affected by the nature of the addends. and compounds bearing Only fused cyclopropane rings were found to he better dipolarophil es than those bearing both fused cyclopropane and cyclohexene rings. Fronti er-orbital theory provided reliable models for rationalizing both the occur rence and regioselectivity of the observed cycloaddition processes. The reg ioselectivity of the thermal and photochemical N-2 extrusion from CH2N2 add ucts (if highly functionalized C-60 derivatives is identical to that observ ed for CH2N2 adducts of the parent fullerenes C-60 (C61H2N2) or C-70 (C71H2 N2). It is also similar to that previously reported by Klarner et al. for t he thermal and photochemical N-2 elimination from diazoalkane-toluene adduc ts. The experimental results, together with high-level ab initio and densit y-functional calculations, provide strong evidence that thermal N2 extrusio n from all of these pyrazoline derivatives proceeds ria a common mechanism, an eight-electron, orbital-symmetry-controlled [2 pi (s) + 2 pi (s) - 2 si gma (a) + 2 sigma (s)] concerted process ia an aromatic transition state (S chemes 7 and 8).