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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 Followed by Dinitrogen Extrusion
Author(s) -
Haldimann Richard F.,
Fokas Christian,
Diederich François
Publication year - 2001
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/1522-2675(20010613)84:6<1635::aid-hlca1635>3.0.co;2-z
Subject(s) - chemistry , diazomethane , adduct , cyclopropane , reactivity (psychology) , cycloaddition , regioselectivity , methylene , double bond , stereochemistry , medicinal chemistry , ring (chemistry) , organic chemistry , catalysis , medicine , alternative medicine , pathology
A series of hexakis‐ to octakis‐adducts of C 60 with novel addition patterns was synthesized by 1,3‐dipolar cycloaddition of diazomethane (CH 2 N 2 ) to pentakis‐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 , hexakis‐adducts (±)‐ 3a , b and (±)‐ 17 with one 6‐5 open methano bridge (bridge at the junction between a hexagon and a pentagon) were obtained in high yield ( Schemes 1 and 6 ). Further conversion with CH 2 N 2 at −80 to −60° provided heptakis‐adducts 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 CH 2 N 2 , octakis‐adducts, (±)‐ 5a , b with three 6‐5 open methano bridges were obtained ( Scheme 2 ). Oxidation of the 6‐6 double bond, from which the two vicinal methano 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 various addition patterns did not affect the regioselectivity of the 1,3‐dipolar cycloaddition of CH 2 N 2 and the subsequent N 2 ‐extrusion process. The reactivity was, however, affected by the nature of the addends, and compounds bearing only fused cyclopropane rings were found to be better dipolarophiles than those bearing both fused cyclopropane and cyclohexene rings. Frontier‐orbital theory provided reliable models for rationalizing both the occurrence and regioselectivity of the observed cycloaddition processes. The regioselectivity of the thermal and photochemical N 2 extrusion from CH 2 N 2 adducts of highly functionalized C 60 derivatives is identical to that observed for CH 2 N 2 adducts of the parent fullerenes C 60 (C 61 H 2 N 2 ) or C 70 (C 71 H 2 N 2 ). It is also similar to that previously reported by Klärner et al. for the thermal and photochemical N 2 elimination from diazoalkane‐toluene adducts. The experimental results, together with high‐level ab initio and density‐functional calculations, provide strong evidence that thermal N 2 extrusion from all of these pyrazoline derivatives proceeds via a common mechanism, an eight‐electron, orbital‐symmetry‐controlled [2 π s +2 π s +2 σ a +2 σ s ] concerted process via an aromatic transition state ( Schemes 7 and 8 ).