Disilene Addition to C 70
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a b C
-
Se -
-c'
d'(=f)
(=g) Sb'(=h)
a' (=I) Figure 1. C70 nomenclature. Addition to the 21,42 positions is predicted to be favorable, but only additions to the 1,9 and 7,8 positions has so far been observed.
363 Mat. Res. Soc. Symp. Proc. Vol. 359 @1995 Materials Research Society
energies of formation that yield insight into the fundamental chemical nature of C70.1 A unique low energy addition pattern -- addition to the 21,42 carbons of C70 -- was first identified by semiempirical calculations on C70 H2 and verified by ab initio Hartree-Fock methods. 2 This addition pattern is 1,4 across a 6-ring that spans the equator of C70 as is shown in Figure 1. However, it is perhaps best described as addition to one carbon in each hemisphere of C70 . These calculations on C70 H 2 prompted the search for methods to functionalize the equatorial region of C70 chemoselectively with applications to both materials and pharmaceuticals. Because the energy of the 21,42 isomer of C70H 2 is estimated to lie 4.5 kcal/mol higher in energy than the lowest energy 1,9-isomer, [2+4] cycloadditions to C70 were proposed as the most direct route to functionalization of the C70 equatorial region. There are many examples of fullerenes reacting as the 2-atom reagent in cycloadditions; this would be the first example of a fullerene reacting as the 4-atom component. SEMIEMPIRICAL
CALCULATIONS
A series of semiempirical calculations were undertaken in order to focus the experimental program. The MNDO/PM3 3 heats of reaction of C7 0 with common dienophiles to yield 21,42addended C 70 [2+4] cycloaddition products are presented in Table 1. With the exception of benzyne, none of the products containing only first row atoms was significantly thermodynamically stable with respect to cycloreversion. The final geometries clearly show high Table 1. Results of semiempirical calculations on [2+4] cycloadditions to C7 0's 21,42 carbons Ene Component Ethylene Acetylene 2-Butyne Maleic anhydride DMADa DEADb HF2Bc Benzyne Fumaronitrile Cyclobutene Corrected C4H6 d Dicyanoacetylene trans- 1,2-C 2H 2 F2 S-acetaldehyde cyclo-C 3 F4 MeOC-=COMe Cyclopropene H 2 Si=SiH 2 e
AHf (C70) kcallmol 884.17 884.17 884.17 884.17 884.17 884.17 884.17 884.17 884.17 884.17 884.17 884.17 884.17 884.17 884.17 884.17 884.17
AHf(Ene) kcal/mol 16.61 50.69 29.77 -90.08 -107.87 -128.36 -253.66 129.89 85.99 37.67
Sum (Reagents) 900.78 934.86 913.94 794.09 776.30 755.81 630.51 1014.1 970.16 921.84
127.99 -71.52 29.17 -102.48 -9.66 68.17 56.81
1012.20 812.65 913.34 781.69 874.51 952.34 940.98
Sum (Product) 908.67 950.67 930.42 806.55 794.48 794.41 643.67 954.32 990.69 922.41 933.03 1026.8 825.98 908.15 779.04 877.32 949.04 865.36
aDimethylacetylene dicarboxylate bDiethylazo dicarboxylate cHexafluoro-2-butyne dHand corrected for relatively poor calculated value of cyclobutene vs. cyclobutane eAccuracy of disilene heat of formation is not known
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AAHf kcallmol 7.90 15.82 16.48 12.46 18.19 38.60 13.16 -59.74 20.53 0.57 12.19 14.69 13.33 -5.19 -2.65 2.81 -3.30 -75.62
bond s
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