Stoichiometric and catalytic organotitanium chemistry supported by aryloxide ligation. (Volumes I and II)
Abstract
A review of the known Group 4 metallacyclic complexes demonstrates the wide range of bond coupling reactions that can be performed by early transition metals. The vast majority of isolated Group 4 metallacycles were supported by cyclopentadiene ligands. These supporting ligands have proven useful in the synthesis of stable metal complexes, allowing for isolation and characterization. However, a disadvantage of this greater stability might be a lessening of subsequent reactivity of the complexes. In comparison, we investigated the chemistry of titanium organometallic complexes supported by aryloxide ligation. Aryloxide ligands offer a greater range of both steric and electronic control of the metal center compared to cyclopentadiene ligands, and through this we hoped to discover a greater range of reactivity. Preliminary results with low valent titanium complexes indicated that facile carbon-carbon bond coupling reactions could be performed. Structural evidence was obtained which showed a high tendency for titanium to donate the electrons from d-orbitals into the orbitals of unsaturated molecules, which is a key factor for bond activation. However, inconsistent results from initial experiments caused our focus to shift away from direct study of low-valent systems. A key to all subsequent work was the sodium reduction of Ti(OAr$\sp{\prime\prime})\sb2$Cl$\sb2$(OAr$\sp{\prime\prime}$ = 2,6-diphenylphenoxide) in the presence of 3-hexyne. This resulted in isolation of the titanacyclopentadiene complex Ti(OAr$\sp{\prime\prime})\sb2$(C$\sb4$Et$\sb4$) from coupling of two equivalents of the alkyne at titanium. In the presence of an excess of 3-hexyne, catalytic cyclotrimerization was observed to yield hexaethylbenzene. This type of reaction had not been reported previously for Group 4 metallacyclopentadiene complexes supported by cyclopentadiene ligands, providing our first evidence for different reactivity with our aryloxide supported system. That initial result cascaded into a multitude of similar discoveries, and a wide range of stoichiometric and catalytic chemistry centered around the (Ti(OAr)$\sb2$) core. Other examples of unique results include the first single crystal x-ray structural characterizations of a titanacyclopentane complex Ti(OAr$\sp{\prime\prime})\sb2$(CH$\sb2)\sb4$, a mononuclear oxotitanacyclopropane complex Ti(OAr$\sp{\prime\prime})\sb2$(OCPh$\sb2$)(PMe$\sb3$), an azatitanacyclopropene complex (Ti(OAr$\sp{\prime\prime})\sb2$(PhCN)) $\sb2$, and a Group 4 imido complex Ti(OAr$\sp\prime)$(py-4pd)$\sb2$(NPh).
Degree
Ph.D.
Advisors
Rothwell, Purdue University.
Subject Area
Chemistry
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