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Synthesis, Characterization, and Reactivity of (Fluoroalkyl)- and (Fluorocycloalkyl)cobaloximes:  Molecular Structure of a (2-Fluorocyclohexyl)cobaloxime Complex and Hindered Rotation of 2-Fluorocycloalkyl Ligands

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journal contribution
posted on 24.11.2003, 00:00 authored by Jelena Galinkina, Eduard Rusanov, Christoph Wagner, Harry Schmidt, Dieter Ströhl, Sven Tobisch, Dirk Steinborn
Reaction of Ph3P−[Co]- ([Co] = Co(dmgH)2; dmgH2 = dimethylglyoxime), prepared by reduction of Ph3P−[Co]−Cl with NaBH4 in methanolic NaOH, with BrCH2CH2CH2F resulted in formation of Ph3P−[Co]−CH2CH2CH2F (3). In neutral methanolic solutions py*−[Co]−H/py*−[Co]- (py* = py, 4-(t-Bu)py, 3-Fpy; 4-(t-Bu)py = 4-tert-butylpyridine, 3-Fpy = 3-fluoropyridine) were found to react with BrCH2CHF2, yielding the 2,2-difluoroethyl complexes py*−[Co]−CH2CHF2 (py* = py (4a), 4-(t-Bu)py (4b), 3-Fpy (4c)). Reactions of XCH2CH2F (X = Br, TfO; TfO = triflate) with reduced cobaloximes in alkaline and neutral methanolic solutions resulted in formation of the 2-methoxyethyl complexes py*−[Co]−CH2CH2OMe (py* = py (5a), 4-(t-Bu)py (5b), 3-Fpy (5c)) with ethylene as side product. py*−[Co]−H/py*−[Co]- (py* = py, 3-Fpy) was found to react with TfOCH2CMe2F, yielding py*−[Co]−CHCMe2 (py* = py (6a), 3-Fpy (6b)) and H2CCMe2. All these reactions indicate the formation of the (unseen) intermediate py*−[Co]−CH2CR2F (R = H, Me), which decomposes via nucleophilic substitution (F → OMe), heterolytic fragmentation, yielding olefins, and HF elimination, yielding vinyl complexes, respectively. Analogous reactions of reduced cobaloximes with trans-1-bromo-2-fluorocyclohexane and trans-1-bromo-2-fluorocyclopentane resulted in the formation of (2-fluorocyclohexyl)- and (2-fluorocyclopentyl)cobaloximes, py*−[Co]−C6H10F (py* = py (7a), 4-(t-Bu)py (7b)) and py*−[Co]−C5H8F (py* = py (10a), 4-(t-Bu)py (10b)). All these complexes were fully characterized by 1H, 13C, and 19F NMR spectroscopic investigations. Molecular structures of the cobaloximes 3, 4a, 7b, and 7b·CH2Cl2 were obtained by single-crystal X-ray diffraction analyses, exhibiting complexes with an equatorial pseudomacrocyclic (dmgH)2 ligand as well as axial base (PPh3, py*) and organo ligand in mutually trans positions. The cycloalkyl complexes are the ae isomers, having the sterically demanding Co(dmgH)2 moieties as equatorial substituents. The axially oriented fluoro substituents give rise to hindered rotation of 2-fluorocycloalkyl ligands, as indicated by two distinct sets of signals for dmgH ligands in the 1H and 13C NMR spectra. Further proof for this came from DFT calculations of py−[Co]−C6H10F (11) and, for comparison, of the cyclohexyl complex py−[Co]−C6H11 (12). The conformational energy diagrams of 11 and 12 are discussed.