Methodology for the Preparation of C1-Monoalkylated 1,2-Dihydro[C₇₀] Derivatives: Formation of the “Other” Regioisomer

Deprotonation of 1,2-C70H2 with TBAOH, followed by alkylation with methyl bromoacetate, results in formation of a C1-monoalkylated 1,2-dihydro-C70 derivative. The position of the alkyl group (C1) was established by NMR spectroscopy and comparison with literature spectra of C2-monoalkylated analogs. Presumably, C1-alkylation is the major process due to selective deprotonation of 1,2-C70H2 at C1. Substitution of benzyl bromide for methyl bromoacetate results in rapid dialkylation, unless the amount of base is carefully controlled, in which case C1-monobenzylation is the major process. This methodology for alkylation at C1 is complimentary to methods for the C2-monoalkylation of C70 with Zn and methyl bromoacetate

Preparation and Characterization of the Fullerene Diols 1,2-C₆₀(OH)₂, 1,2-C₇₀(OH)₂, and 5,6-C₇₀(OH)₂

The simple fullerene diols C60(OH)2 and C70(OH)2 were prepared by addition of RuO4 followed by acid hydrolysis. The 1,2-C60(OH)2 isomer was formed from C60, and two isomers (1,2 and 5,6) of C70(OH)2 were formed in the RuO4 hydroxylation of C70. These compounds are much more soluble in THF and dioxane than the parent fullerenes. More highly hydroxylated materials are formed as well

Monoalkylation of C₆₀ and C₇₀ with Zn and Active Alkyl Bromides

We report a convenient and simple solution-phase electron-transfer reaction of C60 with zinc and α-bromoacetonitrile, α-bromo acetate esters, allyl bromide, benzyl bromide and α-bromo ketones in DMF, with which different types of monoalkylated C60 derivatives can be prepared. When this method is employed with C70, 2-carbomethoxymethyl-1,2-dihydro[70]fullerene (isomer 5a) is produced as one of the two 1,2-monoalkylated C70 isomers, together with the first 5,6-monoalkylated C70 derivative

Monoalkylation of C₆₀ and C₇₀ with Zn and Active Alkyl Bromides

We report a convenient and simple solution-phase electron-transfer reaction of C60 with zinc and α-bromoacetonitrile, α-bromo acetate esters, allyl bromide, benzyl bromide and α-bromo ketones in DMF, with which different types of monoalkylated C60 derivatives can be prepared. When this method is employed with C70, 2-carbomethoxymethyl-1,2-dihydro[70]fullerene (isomer 5a) is produced as one of the two 1,2-monoalkylated C70 isomers, together with the first 5,6-monoalkylated C70 derivative

The Effect of a Peptide Helix Macrodipole on the p<i>K</i>_a of an Asp Side Chain Carboxylate

A study of the effect of a helix dipole on the pKa of a side chain functional group has been undertaken to determine the magnitude of these electrostatic effects in the absence of interfering influences from a protein matrix. Three helical peptides were prepared:  two containing Asp residues at the N- or C-terminus and one with an Asp residue in the middle of the peptide. These peptides have no reactive residues other than the Asp side chain carboxylate group. Circular dichroism confirmed that these peptides adopt helical conformations in aqueous solution over a broad pH range. The pKa of compound 12, where the Asp residue is at the N-terminus of the helix, is 3.81 ± 0.31. This is lower than the pKa of an Asp residue in a short nonhelical model compound (4.09 ± 0.21) and lower than the pKa values of 23, where the Asp residue is at the C-terminus of a helix (4.17 ± 0.24), and 19, where the Asp residue is in the middle of the helix (4.17 ± 0.29). No significant perturbation was observed at the C-terminus of a helix (compound 23), despite this being the negative pole of the dipole. We believe that this carboxylate is drawn toward the N-terminus by electrostatic attraction to the positive pole of the dipole, resulting in positioning of the carboxylate over the middle of the helix rather than over the C-terminus

Synthesis and Isolation of One Isomer of C₆₀H₆

Synthesis and Isolation of One Isomer of C60H6</sub

Intramolecular Excited State Electronic Coupling Along an α-Helical Peptide

Several families of peptides composed of alternating l-alanine (Ala) and α-aminoisobutyric acid (Aib) residues with an appended N,N-dimethylanilino and/or 2-naphthalenyl group exist in MeOH and CDCl3 as α-helices. Steady state and time-resolved fluorescence measurements show that the distance and dihedral angle between the appended donor and acceptor and the alignment of the vectors for intramolecular charge transfer interaction (from donor to acceptor) with or against that of the helical dipole moment significantly influence the efficiency of photoinduced electronic coupling and, hence, of intramolecular fluorescence quenching

Fulleroid Addition Regiochemistry Is Driven by π-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 π-orbital misalignment and not pyramidalization

A ¹³C INADEQUATE and HF-GIAO Study of C₆₀H₂ and C₆₀H₆ Identification of Ring Currents in a 1,2-Dihydrofullerene

The hydrofullerenes C60H2 (1) and C60H6 (2) have been prepared in 13C-enriched form and 2D INADEQUATE NMR spectra were measured. These spectra have provided unambiguous 13C assignments for 2, and nearly unambiguous assignments for 1. In both cases, the most downfield resonances are immediately adjacent to the sp3 carbons, despite the fact that these carbons are the least pyramidalized carbons in the molecule. Typically, 13C chemical shifts move downfield with increasing pyramidalization (ϑp), but in these systems there is no strong correlation between ϑp and δ. HF-GIAO calculations are able to predict the chemical shifts, but provide little chemical insight into the origin of these chemical shifts. London theory reveals a significant paramagnetic ring current in 1, a feature that helps explain the 1H shifts in these compounds and may contribute to the 13C chemical shifts as well

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