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

Cyclic and acyclic dicarbonyl[hydrotris(1-pyrazolyl)borato][η 3 -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 1 H and 13 C NMR spectroscopy and, in the case of [TpMo(CO) 2 [η-(1,2,3)-(±)-(1 R,2 S,3 S )-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) 3 Mo(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)-(±)-(1 R,2 S,3 S )-1-methoxy-2-cyclohexen-1-yl].