Homogeneous hydrogenation catalyzed by group 5 metal aryloxide compounds

Valerie M Visciglio, Purdue University

Abstract

Synthetic and mechanistic studies have been undertaken to try and better understand the important steps in homogeneous hydrogenation. The alkylation chemistry of $\rm Ta(OC\sb6H\sb3$-2,6-$\rm Pr\sp{i}\sb2)\sb3Cl\sb2$ has produced a variety of alkyl derivatives. Under high pressure of H$\sb2$ gas, the dialkyl compound undergoes hydrogenolysis of the tantalum alkyl bonds to form a monohydride tetra-aryloxide compound presumably by ligand exchange. In the presence of phosphine, a new series of six-coordinate dihydride compounds: $\rm Ta(OC\sb6H\sb3$-2,6-$\rm Pr\sp{i}\sb2)\sb3H\sb2L$ where L = PBu$\sp{\rm n}\sb3$, PMe$\sb3$, PMe$\sb2$Ph, PMePh have been isolated and characterized by X-ray diffraction for L = PMe$\sb2$Ph. Using $\rm Ta(OC\sb6H\sb3$-2,6-$\rm Pr\sp{i}\sb2)\sb3H\sb2(PMe\sb2Ph),$ chemical exchange of coordinated phosphines can readily occur to form the corresponding phosphine adducts and free PMe$\sb2$Ph. Intramolecular hydrogenation of arylphosphine ligands has been observed when the hydrogenolysis reaction was carried out for extended periods of time ($>$24 h). This new series of tantalum dihydride compounds will catalyze the regioselective hydrogenation of naphthalene and anthracene. Reduction of $\rm M(OC\sb6H\sb3$-2,6-$\rm Pr\sp{i}\sb2)\sb3Cl\sb2$ where M = Nb, Ta with 3 equivalents of Na/Hg in the presence of 1,3-cyclohexadiene results in the formation of a new $\eta\sp4$-cyclohexadiene compound: $\rm M(OC\sb6H\sb3$-2,6-$\rm Pr\sp{i}\sb2)\sb3(\eta\sp4$-$\rm C\sb6H\sb8).$ Both derivatives have been characterized by X-ray diffraction. $\rm Nb(OC\sb6H\sb3$-2,6-$\rm Pr\sp{i}\sb2)\sb3(\eta\sp4$-$\rm C\sb6H\sb8)$ catalyzes the disproportionation of 1,3-cyclohexadiene. $\rm M(OC\sb6H\sb3$-2,6-$\rm Pr\sp{i}\sb2)\sb3(\eta\sp4$-$\rm C\sb6H\sb8)$ where M = Nb, Ta hydrogenates catalytically polynuclear aromatic hydrocarbons, benzene, 1,3-cyclohexadiene and cyclohexene. Hydrogenation of 1,3-cyclohexadiene to cyclohexane does not release cyclohexene as an intermediate and is faster than the hydrogenation of cyclohexene. The reaction of $\rm M(OC\sb6H\sb3$-2,6-$\rm Pr\sp{i}\sb2)\sb3Cl\sb2$ where M = Nb, Ta with cyclohexylmagnesiumchloride generates the monocyclohexyl compound: $\rm M(OC\sb6H\sb3$-2,6-$\rm Pr\sp{i}\sb2)\sb3Cl(C\sb6$-$\rm H\sb{11}).$ An excess of cyclohexylmagnesiumchloride gives rise to the bis-cyclohexyl derivative of Ta. Insertion of tert-butylisocyanide into Ta-alkyl bond was observed, leading to $\eta\sp2$-iminoacyl species: $\rm Ta(OC\sb6H\sb3$-2,6-$\rm Pr\sp{i}\sb2)\sb3Cl(\eta\sp2$-t-$\rm BuNCC \sb6H\sb{11})$ and $\rm Ta(OC\sb6H\sb3$-2,6-$\rm Pr\sp{i}\sb2)\sb3(C\sb6$-$\rm H\sb{11})(\eta\sp2$-t-$\rm BuNCC\sb6H\sb{11}).$

Degree

Ph.D.

Advisors

Rothwell, Purdue University.

Subject Area

Chemistry

COinS