Site-Selectivity of Prato Additions to C₇₀: Experimental and Theoretical Studies of a New Thermodynamic Product at the dd-[5,6]-Junction

Abstract

Three Prato monoadduct isomers were synthesized and structurally characterized by 1H, 13C NMR spectra and single-crystal X-ray diffraction, and one adduct on the dd-[5,6]-bond was found as the first example of a Prato [5,6]-adduct of C70. To investigate the mechanism in the generation of this dd-[5,6]-adduct, computational studies were employed to show that it was thermodynamically obtained by sigmatropic rearrangement from the presumed initial kinetic product de-[6,6]-adduct.

Unexpected Disparity in Photoinduced Reactions of C60 and C70 in Water with the Generation of O2•– or 1O2

“…The water-soluble C 60 -PEG 1 and C 70 -PEG 2 , 3 ( Figure 1 a) were prepared by the conjugation of amino-PEG 36 - t Bu ester with the C 60 and C 70 bis-carboxylic acid precursors ( 4 – 6 ) using hexafluorophosphate benzotriazole tetramethyl uronium (HBTU) as a coupling reagent ( Scheme S1 ). 20 , 24 , 25 The C 60 - and C 70 -PEG ( 1 – 3 ) were purified by reversed phase HPLC and were thoroughly soluble in water (>5 mM). The UV–vis-NIR spectra of C 2 n -PEG ( 1 – 3 ) in water ( Figure 1 b) shows that all 1 – 3 had the absorption in the range of the phototherapeutic window (650–860 nm), with a much higher intensity for C 70 -PEG 2 , 3 in comparison to C 60 -PEG 1 .…”

“…The bis-carboxylic acid ester precursors for C 60 and C 70 were prepared by reported methods. 20 , 25 Obtained [6,6]-adduct of C 60 ( 4 ) and ab -[6,6]- and cc -[6,6]-adducts of C 70 ( 5 , 6 ) were converted to C 60 -PEG 1 ( 24 ) and C 70 -PEG 2 , 3 by deprotection and subsequent conjugation with amino-PEG 36 - t Bu ester ( Scheme S1 ). 2 : 1 H NMR (600 MHz, in CDCl 3 ) δ H 1.44 (s, (C H 3 ) 3 , 18H), 2.49 (t, J = 6.6 Hz, CH 2 –C H 2 –CO 2 - t Bu, 4H), 3.62–3.65 (CH 2 – CH 2 –O, 298H); 13 C NMR (150 MHz, in CDCl 3 ) δ C 28.1 (C( C H 3 ) 3 ), 36.3 (CH 2 – C H 2 –CO 2 - t Bu), 39.2 (CH– C H 2 –CO–NH), 39.9 (CH 2 – C H 2 –NH–CO), 54.6 ((CH 2 ) 2 – C H–N), 59.1 (C– C H 2 –N), 61.3 ( C –CH 2 –N), 63.0 ( C –CH 2 –N), 63.2 (C– C H 2 –N), 66.9 (O– C H 2 –CH 2 –CO 2 - t Bu), 70.5 (CH 2 – C H 2 –O), 80.5 ( C (CH 3 ) 3 ), 131.2, 131.3, 131.6, 133.7, 133.8, 137.4, 140.5, 140.6, 142.8, 143.2, 143.3, 145.8, 146.3, 146.7, 146.9, 147.0, 147.1, 147.1, 147.4, 148.8, 149.1, 149.4, 149.5, 149.7, 149.9, 150.2, 150.7, 150.8, 151.0, 151.2, 151.4, 151.4, 155.6 158.1, 170.9; HRMS (MALDI/ESI), matrix: CCA + NA-PFHA) m / z calcd for [C 235 H 325 NaN 3 O 78 ] + : 4460.1449, found 4460.1470 ([M + Na] + ).…”

“…The bis-carboxylic acid ester precursors for C 60 and C 70 were prepared by reported methods. 20,25 Obtained [6,6]-adduct of C 60 (4) and ab- [6,6]-and cc- [6,6]-adducts of C 70 (5,6) were converted to C 60 -PEG 1 24 and C 70 -PEG 2, 3 by deprotection and subsequent conjugation with amino-PEG 36 -t Bu ester (Scheme S1). 2: 1 H NMR (600 MHz, in CDCl 3 ) δ H 1.44 (s, (CH 3 ) 3 , 18H), 2.49 (t, J = 6.6 Hz, CH 2 −CH…”

“…In this study, we used well-defined, water-soluble C 60 -PEG 1 ( 24 ) and C 70 -PEG derivatives 2 and 3 , 20 and tested them for photoinduced DNA damage. The generation of ROS ( 1 O 2 and O 2 •– ) in conjugates 1 – 3 was measured by ESR in the presence of spin-trapping agents.…”

“…This paucity of studies results from the complex generation of multiple C 70 isomers upon derivatization. Even simple monoderivatization methods such as Diels−Alder cycloadditions 19 or 1,3-dipolar cycloaddition reactions (Prato reaction) 20 applied to C 70 produced several isomers (ab- [6,6], cc- [6,6], and de- [6,6] or dd- [5,6]), while C 60 usually generates just one [6,6]-isomer due to the higher double bond character of the [6,6]-bond and I h symmetry of C 60 . 21 Such changes in the addition patterns in fullerenes often induce differences in their UV−vis absorption spectra affecting their photophysical properties, especially in ROS generation.…”