Group 4 Metalloporphyrin diolato Complexes and Catalytic Application of Metalloporphyrins and Related Transition Metal Complexes Page: 32 of 200
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(TTP)Ti=O and free pinacol. The reactions of other titanium diolates with water are much
slower.
Diolato displacement reactions. The hafnium and zirconium pinacolates undergo
substitution reactions with other diols. For example, when hafnium pinacolate,
(TTP)HfqOC(Me)2C(Me)20] (1), was treated with benzopinacole (3.0 equiv) in C6DS,
(TTP)Hf[OC(Ph)2C(Ph)20] (3) was produced quantitatively in hours. Similarly, treatment of
1 with 2,3-diphenylbutane-2,3-diol (4.7 equiv) generated (TTP)Hf[OC(Me)(Ph)C(Me)(Ph)O]
(2) quantitatively in hours. However, pinacol (15 equiv) was not able to displace 2,3-
diphenylbutane-2,3-diol from (TTP)Hf[OC(Me)(Ph)C(Me)(Ph)O] (2). It took 2 weeks for
benzopinacole (-12 equiv) to displace 2,3-diphenylbutane-2,3-diol from 2. No intermediate
species were observed in the course of any of the substitution reactions. These reactions
could also serve as an alternative route for preparing new diolate complexes.
The titanium pinacolate, (TTP)Ti[OC(Me)2C(Me)20] (5), is less sensitive to
displacement by other diols, presumably due to the smaller size of Ti(IV) compared with
zirconium and hafnium. For example, (TTP)Ti[OC(Me)2C(Me)20] (5) only partially reacted
with excess hydrobenzoin (~5.0 equiv) reaching a steady state with an equilibrium constant
of-0.2. The reverse reaction from (TTP)Ti[OCH(Ph)CH(Ph)O] (7) and pinacol (27 equiv)
produced the pinacolate 5 smoothly in >95% conversion. With more bulky diols, no
exchange reaction was observed.
Thermal reactivity. Although the hafnium and zirconium diolates were generally
robust at ambient temperature, they underwent C-C bond cleavage reactions at elevated
temperature. Heating an aerobic benzene-d6 solution of (TTP)Hf[OC(Ph)2C(Ph)20] (3) with
a sand bath (-120 0C) in a sealed NMR tube resulted in the nearly quantitative formation of
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Du, Guodong. Group 4 Metalloporphyrin diolato Complexes and Catalytic Application of Metalloporphyrins and Related Transition Metal Complexes, thesis or dissertation, December 19, 2004; Ames, Iowa. (https://digital.library.unt.edu/ark:/67531/metadc785649/m1/32/: accessed April 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.