Synthesis and reactivity of chromium α-diimine complexes and 1,4-dimethyl-oxa-norbornene
Date
2018
Authors
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Publisher
University of Delaware
Abstract
This dissertation describes research on α-diimine chromium complexes and 1,4-dimethyl-7-oxabicyclo[2,2,1]hept-2-ene (aka: 1,4-dimethyl-oxa-norbornene). Specifically, the first part focused on synthesis of chromium complexes in low formal oxidation state (+I/+II) and investigation of their reactivity and electronic structures. In the second part, 1,4-dimethyl-7-oxabicyclo[2,2,1]hept-2-ene was synthesized and it's kinetic and thermodynamic data were measured. ☐ Chapter 1 continues the exploration of the reactivity of quintuply bonded compound (µ-ƞ1:ƞ1-HLiPr)2Cr2 by describing the results of its exposure to several small molecules. Careful treatment of (µ-ƞ1:ƞ1-HLiPr)2Cr2 with 1 equivalent of CO forms quadruply bonded dinuclear chromium CO adduct [HLiPrCr]2(µ-CO) (1), which is the precursor for previously reported compound [HLiPrCr](CO)4. When exposed to CO2, (µ-ƞ1:ƞ1-HLiPr)2Cr2 activates the carbon on CO2 gas, leads to electrophilic attack of CO2 on the backbone carbons of the α-diimine ligands and forms a symmetric dinuclear chromium complex (µ-ƞ1:ƞ1-L`)2Cr2 (2). Upon exposure to CS2, (µ-ƞ1:ƞ1-HLiPr)2Cr2 was found to break one of the CS double bonds and to form an asymmetric dinuclear chromium complex [HLiPrCr]2(µ-ƞ2:ƞ2-CS)(µ-S) (3). ☐ Chapter 2 describes the synthesis of a series of alkyl(aryl)/hydride bridged chromium dinuclear complexes. (HLiPr)2Cr2(µ-H)2 (4), the precursor of alkyl(aryl)/hydride complexes, was synthesized by treating [HLiPrCr](CH2TMS)(THF) with hydrogen gas. Alkylations(arylations) of 4 with various lithium alkyls (LiMe, LiPh, LiCH2TMS, LiCH2tBu, LiCH2Ph) produce the corresponding alkyl(aryl)/dihydride complexes: [(HLiPrCr)2(µ-Me)(µ-H)2]-[Li(THF)4]+ (5), [(HLiPrCr)2(µ-Ph)(µ-H)2]-[Li(THF)4]+ (6), [(HLiPrCr)2(µ-CH2TMS)(µ-H)2]-[Li(THF)3(Et2O)]+ (7), [(HLiPrCr)2(µ-CH2tBu)(µ-H)2]-[Li(THF)3(Et2O)]+ (8) and [(HLiPrCr)2(µ-CH2Ph)(µ-H)2]-[Li(THF)4]+ (9) in a solvent mixture of pentane with a few drops of THF. In contrast, the reactions of 4 with LiMe and LiPh in THF leads to dialkyl(diaryl)/hydrido complexes [(HLiPrCr)2(µ-Me)2(µ-H)]-[Li(THF)3(Et2O)]+ (10) and [(HLiPrCr)2(µ-Ph)2 (µ-H)]-[Li(THF)]+ (11). Alkyl(aryl)/hydrido complexes bearing different alkyl(aryl) ligands show significant differences in the stability of reductive elimination which was monitored by 1H-NMR. Treating compound 7 with Me3O+BF4- gave an unusual ligand activated complex (HLiPrCr) (Me,H2LiPrCr)(µ-CH2TMS)(µ-H)2 (12). ☐ Chapter 3 presents the synthesis, purification and characterization of 1,4-dimethyl-7-oxabicyclo[2,2,1]hept-2-ene (15) , which is the presumed intermediate of the Diels-Alder reaction of 2,5-dimethylfuan and ethylene to para-xylene. The activation parameters of retro-Diels-Alder reaction of dimethyl-oxa-norbornene in d6-benzene were measured and compared to computational studies. The rate of dehydration with Bronsted acid and Lewis acid was also investigated by 1H-NMR spectroscopy. Several inorganic salts (AgOTf, Sc(OTf)3, Cu(OTf)2, PdCl2, AgNO3, Pd/C, Rh4(CO)12, W(CO)6, RhCl(PPh3)3, Pt(PPh3)3, Mo(CO)6 and Ar3N+SbCl6-) were tested as catalysts for retro-Diels-Alder reaction. By the principle of microscopic reversibility, such catalysts should also facilitate the forward-Diels-Alder reaction.
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Keywords
Pure sciences, Alkyl-hydride, Chromium, Diels-Alder, Diimine, Oxa-norbornene