| We have developed procedures for producing reduced fullerenes and have characterized a number of previously unknown examples of this class of compounds. More importantly, we have begun to uncover some of the underlying reactivity of the fullerene core that guides reduction toward specific sites. We have also shown that fullerene C-H bonds provide reactive sites that can be used to form new fullerene-based compounds.
Our Zn(Cu) reduction methods can be optimized for the formation of any one of a number of C60Hn and C70Hn species. We continue to produce new examples of this class of compounds. This method produces high yields of reduced fullerenes from C60 and C70, and produces very simple product mixtures, usually consisting of only 1 to 3 species. There are no other methods that produce simple, tractable mixtures of reduced fullerenes beyond C60H2. This reaction, using Zn(Cu) with water as a proton source, produces a limited range of molecular formulae, and very limited numbers of isomers. For example, of 835,476 possible isomers for C60H6, we obtain only two isomers. This methodology is also superior to catalytic hydrogenation, which tends to produce complex mixtures and highly hydrogenated material depending on the conditions. This procedure is superior in ease and in yield to all other published methods for the preparation of reduced fullerenes.
We are able to isolate C60H2 (1) in greater than 65% yield from the Zn(Cu) reduction of C-60 after only 1 hour of reaction time, making this one of the quickest, simplest, and most convenient methods for the preparation of this important compound. Allowing the reaction to proceed for 2 hours produces a 45% isolated yield of C60H4. We have assigned structures 2a and 2b for the major compounds this mixture. Reactions that proceed for a total of 4 hours produce C60H6 (two isomers in a 6:1 ratio) and C60H6O species (at least 2 distinct compounds). We have identified the major isomer of C60H6 as the D3 species 3a, and the minor isomer is assigned as the C3 isomer 3b.
We continue to extend the range of this reaction and are developing new spectroscopic techniques to characterize these interesting materials.
Bergosh, R. G., Meier, M. S., Laske Cooke, J. A., Spielmann, H. P. and Weedon, B. R. (1997) "Dissolving Metal Reductions of Fullerenes" J. Org. Chem. 62, 7667-7672.
Abstract: Treatment of fullerenes with reducing metals and a proton source leads to the formation of hydrogenated fullerenes (C60Hn and C70Hn). The reaction of C60 with a variety of metals was investigated, using metals that span a broad range of reducing power, from tin (E = -0.14 V) to magnesium (E = -2.36 V). We report useful synthetic routes to a number of reduced fullerenes, including C60H2, C60H4, and C60H6, and we have determined that, despite the huge number of isomers that are possible, a small number are actually formed in these cases. For example, only two isomers of C60H6 are obtained in the Zn(Cu) reduction of C60, in a 6:1 ratio. Three isomers of C70H10 are produced, in ratios that are dependent on reaction time.
Meier, M. S, Weedon, B. R., Spielmann, H. P. (1996) "Synthesis and Isolation of One Isomer of C60H6" J. Am. Chem. Soc. 118, 11682-11683.
We now report a reaction that produces C60H6 with essentially no contamination by adjacent oxidation states. This compound is formed as one dominant isomer and one minor isomer, and we report the assignment of the structure of the major isomer through a comparison of its C-H coupled 13C NMR spectrum with that of 1,2-C60H2. The reaction of C60 with Zn-Cu couple at 50 C in toluene containing a small amount of water proceeds rapidly, with essentially 100% consumption of C60 in about 2 h. After 4 h, virtually all of the products observed are C60H6 isomers or oxides.12 Furthermore, analysis by HPLC indicates that roughly 50% of the soluble material is a single component, one isomer of C60H6. The major isomer of C60H6 produced is the 1,2,33,41,42,50 isomer shown.
Preparation of C70H2, C70H4 and C70H8: Three Independent Reduction Manifolds in the Zn(Cu) Reduction of C70
This article reports the preparation of C70Hn species by Zn(Cu) reduction. The major products were one isomer of C70H2, one major isomer of C70H4, and one major isomer of C70H8. Several minor products were detected by UV-Vis and/or mass spectrometry. The structures of the major products were assigned as 1,2-C70H2, 1,2,56,57-C70H4, and 7,19,23,27,33,37,44,53-C70H8. Interestingly, although the major isomer of C70H4 is generated by reduction of the major isomer of C70H2, the major isomer of C70H8 is not generated by reduction of the major isomer of C70H2 or C70H4. The evidence suggests that there are at least three different reduction manifolds operating. One manifold includes 1,2-C70H2 and 1,2,56,57-C70H4, major products in which highly pyramidalized bonds near the poles of C70 are reduced. A second manifold includes 5,6-C70H2, a minor product. The third manifold consists of a set of C70H2, C70H4, and C70H6 species that are highly reactive and do not accumulate in solution. This third manifold leads to 7,19,23,27,33,37,44,53-C70H8. This third manifold is unique in that these compounds place hydrogens on nonadjacent carbons, a previously unobserved arrangement.
Fulleroid Addition Regiochemistry is Driven by p-Orbital Misalignment
This article reports the first investigation into the regiochemistry of addition to the fulleroid C61H2 by Zn(Cu) reduction and hydroboration. Two major isomers of C61H4 are formed by the reduction with Zn-(Cu) while only one major isomer is formed by hydroboration. The structures of the major isomers formed by reduction with Zn(Cu) were identified as 1,2-C61H4 and 3,4-C61H4. The 1,2-C61H4 isomer is the only dominant isomer formed by hydroboration with no indications of the 3,4-C61H4 isomer being formed. The regiochemistry observed in the formation of 1,2-C61H4 is the same regiochemistry seen in the further reactivity of azafulleroids (C60NR). Strain energies (calculated at the B3LYP-6-31G* level of theory) show that the relief of strain is greater for the hydrogenation of the fulleroid C61H2 than it is for the hydrogenation of C60 itself. This indicates that the twisted, anti-Bredt's rule, double bonds of the fulleroid are a source of greater localized strain than the pyramidalization of the carbons in the rest of the molecule. Thus, the regiochemistry observed for the fulleroid is due to p-orbital misalignment and not pyramidalization.
Preparation
and NMR Characterization of C70H10:
Cutting a Fullerene p-System in Half
H. Peter Spielmann, Brad R. Weedon, and Mark S. Meier*
Department of Biochemistry and
Department of Chemistry, University of Kentucky, Lexington, KY 40506-0055
J. Org. Chem, 65, 2755-2758.
Figure 7. Structure of C70H10
(2) as produced by Zn(Cu) reduction. a) side view; b) top view. Note
the vicinal arrangement of the new hydrogens and the non-adjacent
arrangement of the remaining 8.
Three isomers of C70H10
were prepared by Zn(Cu) reduction of C70.
Three chromatographic bands were identified as C70H10
species by MALDI-FT mass spectrometry, and these compounds were isolated
by repeated HPLC treatments. The major isomer (2) was characterized by 1H
and 13C NMR, while the minor isomers 3 -
4 were isolated in such small quantities that only 1H
NMR analysis was possible. 1H-coupled and
1H-decoupled 13C
NMR of 2 established a 7,8,19,26,33,37,45,49,53,63 - substitution pattern.
This assignment was confirmed by HMBC and DFQ-COSY experiments. This
structure is completely reasonable, as we found that 2 results exclusively
from reduction of the 7,19,23,27,33,37,44,53-C70H8
that is formed in the course of the Zn(Cu) reduction of C70.
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