Preparation of Dicarbonyl[hydrotris(1-pyrazolyl)borato](η3-allyl)molybdenum Complexes Bearing Electron-Donating Substituents (1-((tert-Butyldimethylsilyl)oxy), 1-Alkoxy, and 1-Acetoxy) via the Nucleophilic Addition of Mo(CO)3(DMF)3 to Enals and Enones
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Abstract
Cyclic and acyclic dicarbonyl[hydrotris(1-pyrazolyl)borato][η3-1-((tert-butyldimethylsilyl)oxy)allyl]molybdenum complexes and a variety of their 1-acetoxy and 1-alkoxy (RO = MeO, i-PrO) analogues were prepared and characterized by IR and 1H and 13C NMR spectroscopy and, in the case of [TpMo(CO)2[η-(1,2,3)-(±)-(1R,2S,3S)-1-methoxy-2-cyclohexen-1-yl], by X-ray crystallography. These complexes were prepared in moderate to excellent yields by the tert-butyldimethylsilyl chloride promoted oxidative addition of α,β-unsaturated aldehydes and acyclic and cyclic ketones to (DMF)3Mo(CO)3 followed by ligand metathesis with potassium hydrotris(1-pyrazolyl)borate. The 1-tert-butyldimethylsiloxy- and the 1-methoxy- and 1-isopropoxy-substituted acyclic complexes were formed solely as the syn isomer; however, the 1-acetoxy analogue underwent isomerization to a thermodynamic mixture of the syn and anti isomers in which the anti isomer predominated (3.7 : 1). The 1-((tert-butyldimethylsilyl)oxy)-3-alkyl- or 1-alkoxy-3-alkyl-disubstituted acyclic complexes were formed with syn-silyloxy/anti-alkyl or syn-alkoxy/anti-alkyl stereochemistry, while the disubstituted allyls bearing a 1-acetoxy substituent existed as mixtures of both possible syn/anti isomers and the syn/syn and anti/anti isomers. The conformation and configuration of the isomers was confirmed through nOe studies on several complexes and by X-ray crystallography in the case of [TpMo(CO)2[η-(1,2,3)-(±)-(1R,2S,3S)-1-methoxy-2-cyclohexen-1-yl].
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