[60]Fullerene‐Based Macrocycle Ligands

Abstract

Macrocycle ligands have three or more donor sites. Selective replacement of skeleton carbon atoms by heteroatoms and vacancies in C could lead to various macrocycle ligands with a cage-shaped backbone. Theoretical calculations indicate that such C -based macrocycle ligands are as stable as C thermodynamically according to their similar HOMO-LUMO gaps. The synthesis of these ligands is a challenging task. Nevertheless important progresses have been reported. This concept article focuses on the structures of possible C -based macrocycle ligands and related synthetic results.

Expeditious Preparation of Open‐Cage Fullerenes by Rhodium(I)‐Catalyzed [2+2+2] Cycloaddition of Diynes and C60: An Experimental and Theoretical Study

“…In the triplet state, the rds is the formation of the first biradical intermediate with ac losed cyclopropaner ing, and has ab arrier of 22.3 kcal mol À1 .O nce this intermediate is formed, anI SC brings it to the singlet ground state. From here, the closed-shell singlet [5,6] (bis)methanofullerene species containing two cyclopropane rings is formed with ab arriero fo nly 3.3 kcal mol À1 .F inally,athermally allowed retro-[2+ +2+ +2] cycloaddition takes place with ab arriero f2.0 kcal mol À1 to yield the final [5,6] open bis(fulleroid)product. The greater stability of the bis(fulleroid)product compared with the (bis)methanofullerene intermediate can be attributed to a) releaseo fr ing strain upon opening of the two cyclopropylr ings that are fused to C 60 pentagons, and b) retention of the 60 p electron spherical conjugation and homoaromaticity.…”

“…The greater stability of the bis(fulleroid)product compared with the (bis)methanofullerene intermediate can be attributed to a) releaseo fr ing strain upon opening of the two cyclopropylr ings that are fused to C 60 pentagons, and b) retention of the 60 p electron spherical conjugation and homoaromaticity. [22] The latter relates to the fact that open [5,6] isomersa void localizationo f double bondsi np entagonal rings. [25] Ta ken together,t his data shows that neither the di-p-methane rearrangement pathway nor the [4+ +4]/retro-[2+ +2+ +2] reaction mechanism can explain the formationo ft he final bis(fulleroid) product withoutp hotoexcitation.…”

“…At this point, it is convenient to remember that the presence of metals in af ormal cycloaddition can (i)transform pericyclic reactions into stepwise processes, [26] or (ii)within ac oncerted pathway,m odify the formal topologyo f orbitals ymmetry-allowed mechanismsb ym eans of the participation of the orbitals provided by the metallicc enter. [27] With these ideas in mind, we investigated the transformation of A5 to A9 catalyzed by Rh(P9) + .A ss hown in Figure 3, the reaction mechanism that we found follows aRh-catalyzed di-p-methane rearrangement pathway.T hus, conversion of the cyclohexadiene-fused C 60 intermediate A5 to the [5,6] (bis)methanofullerene intermediate A7 is as tepwise process in which,f irst, ac yclopropane ring is formed to yield A6 (Gibbsb arriero f 16.5 kcal mol À1 ,G ibbs reactione nergy of 7.0 kcal mol À1 ). In A6, Rh is h 4 -coordinated to the methanofullerene (Figure 4), thus stabilizing the allyl radical formed.…”

“…[3] From the practical point of view,o pen-cage derivatives of C 60 have proven useful as electron accepting materials in organic solar cells [4] and as ligands for the synthesis of metal complexes. [5] Thep hotoinduced ring-openingr eaction of cyclohexadiene-fused C 60 derivatives by tandem [4+ +4]/retro-[2+ +2+ +2] rearrangement is one of the most well-established synthetic entries to open-cage fullerenes. [6] For all these reasons, the development of efficient syntheticr outes to fullerene derivatives is still an appealing goal in organic synthesis.…”

“…[7] Given that the chemical reactivityo f C 60 is typicalo fa ne lectron-deficient olefin, it acts as av aluable candidate as ac o-substratei n[ 2 + +2+ +2] cycloadditions with alkynes. [7a] Furthermore, cycloaddition reactions with empty fullerenes show ar emarkable preference for the [6,6] ring junction over [5,6] bonds. [1c, 8] To the best of our knowledge,o nly two previous studies have reported transition-metal-promoted [2+ +2+ +2] cycloaddition reactions of two alkynes with C 60 by using stoichiometric amountso ft he transitionm etal species( Scheme 1).…”