C-13-Labeled platinum(IV)-carbohydrate complexes: Structure determination based on H-1-H-1, C-13-H-1, and C-13-C-13 spin-spin coupling constants

The reaction of D-mannose and D-allose with [PtMe3(Me2CO)(3)]BF4 1 in aceto ne affords complexes [PtMe3L]BF4 5 and 6 (5, L = alpha-D-mannofuranose; 6, L = beta-D-allofuranose). The coordination mode and conformation of the car bohydrate ligands in 5 and 6 in acetone-d(6) have been determined from an a nalysis of J(HH), J(CH), and J(CC) in complexes formed using site-specific C-13-labeled D-mannose and D-allose. These coupling data are compared to th ose measured in C-13-labeled complex [PtMe3L]BF4 2 (L = 1,2-O-isopropyliden e-alpha-D-glucofuranose) and 1,2-O-isopropylidene-alpha-D-glucofuranose 3, whose solid-state structures are known, and in 13C-labeled 1,2;5,6rdi-O-iso propylidene-alpha-D-glucofu 4. The preferred furanose ring conformations in 2 and 5 are very similar (E-3/E-4 and E-4/E-o/E-1, respectively; eastern h emisphere of the pseudorotational itinerary), with platinum coordination in volving O3, O5, and O6 of the saccharide. In contrast, the furanose ring of 6 prefers an 4E/E-0/E-1 geometry (western hemisphere of the pseudorotation al itinerary) resulting from altered complexation involving O1, O5, and O6. Couplings within the exocyclic fragments of 2, 5, and 6 also support the. existence of two different platinum coordination modes. In addition to esta blishing the structures and conformations of 2, 5, and 6 in solution, one-, two-, and three-bond J(CH) and J(CC) observed in these complexes provide n ew insights into the effect of structure and conformation on the magnitudes of these couplings in saccharides. Weak platinum(IV) complexation with the carbohydrate conformationally restricts the furanose and exocyclic fragmen t without introducing undesirable structural strain, thereby allowing more reliable correlations between structure and coupling magnitude.