Nitrile Reduction and Carbon Monoxide Replacement in Tungsten(II) Bis(acetylacetonate) Complexes

Nitrile Reduction and Carbon Monoxide Replacement in Tungsten(II) Bis(acetylacetonate) Complexes

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A substitutionally labile tungsten(II) reagent, W(CO)3(acac) 2 (acac = acetylacetonate) has been synthesized and isolated. Initial reactivity investigations include replacement of one CO ligand with two-electron donors, such as phosphines, or replacement of two equivalents of CO with conventional four-electron donor ligands, such as alkynes and nitriles. Aldehydes, imines, and ketones surprisingly bind to tungsten as four-electron donors relying on lone-pair donation from the heteroatom to satisfy the eighteen electron requirement. Nitrile complexes of the form W(CO)(acac)2(eta 2-Na‰iCR) (R = Ph, Me) undergo alkylation at the nitrogen lone pair with MeOTf and EtOTf (OTf = CF3SO3) yielding cationic iminoacyl complexes of the form [W(CO)(acac)2(eta2-MeN=CR)][OTf]. Nucleophilic attack occurs at the iminoacyl carbon with hydride (Na[HB(OMe) 3]) or methyl (MeMgBr) reagents reducing the iminoacyl to a sidebound imine ligand. The attack generates a new chiral center at the imine carbon with diastereomeric ratios on the order of 2:1. Cationic iminoacyl-carbonyl complexes exhibit facile carbon monoxide replacement with a variety of reagents. The CO ligand in the iminoacyl cations can be replaced thermally with isonitrile and bulky phosphine reagents yielding complexes of the type [W(L)(acac) 2(eta2-MeN=CPh)]+ (L = isonitrile or bulky phosphine). Addition of tricyclohexylphosphine (PCy3) to [W(CO)(acac)2(eta2-MeN=CPh)]+ liberates CO to form [W(PCy3)(acac)2(eta2-MeN=CPh)] +, a molecule exhibiting fluxional behavior on the NMR time scale due to rapid and reversible phosphine dissociation. Smaller phosphine reagents, such as trimethylphosphine, attack the iminoacyl carbon to form [W(CO)(acac) 2(eta2-MeN-C(PMe3)Ph)]+. Under photolytic conditions, alkyne reagents replace CO yielding cationic alkyne-iminoacyl complexes of the type [W(eta2-alkyne)(acac) 2(eta2-MeN=CR)]+. Nucleophilic attack with Na[HB(OMe)3] delivers a hydride to the iminoacyl carbon yielding a neutral alkyne-imine complex of the formula W(eta2-alkyne)(acac) 2(eta2-MeN=CHPh), a molecule that is fluxional on the NMR time scale due to rotation of the alkyne ligand. Addition of MeOTf to the neutral alkyne-imine complex results in a second nitrogen-methyl linkage to yield a cationic alkyne-iminium complex of the form [W(acac)2(eta 2-Me2N=CHPh)(eta2-MeCa‰iCMe)] +. Formally, the original sidebound nitrile ligand undergoes three ligand based manipulations while being reduced to a sidebound iminium ligand.The toluene was removed on a rotary-evaporator and the yellow product was washed with pentane to remove excess ... each 6H, acac CH3); ^C^H} NMR ( CD2C12, 298 K, S): 26.6 , 28.1 (acac CH3), 102.0 (acac CH, VW-c = 62 Hz), 127.7, anbsp;...


Title:Nitrile Reduction and Carbon Monoxide Replacement in Tungsten(II) Bis(acetylacetonate) Complexes
Author: Andrew B. Jackson
Publisher:ProQuest - 2008
ISBN-13:

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