TRIPODAL TITANIUM SILSESQUIOXANE COMPLEXES: SELECTIVE CATALYSTS FOR EPOXIDATION REACTIONS
Sarah Peake, PhD Candidate, University of Kentucky
June 19, 2015, 11:00 am, NIA, Rm 101
Abstract:
Regiomeric mixture of HMe2Si(CH2)3(i-Bu)6Si7O9(OH)3 (6), containing a Si-H group in one of the ligands of the silsesquioxane, was tethered onto a vinyl terminated hyperbranched poly(siloxysilane) polymer via a hydrosilation reaction to generate extremely active catalysts, NCP-9 and CP-9. The synthesis of 6, in good yield, was accomplished via hydrosilation of CH2=CHCH2(i-Bu)7Si8O12 (1) to generate ClMe2Si(CH2)3(i-Bu)7Si8O12 (3) followed by the reduction of 3 with LiAlH4 to afford HMe2Si(CH2)3(i-Bu)7Si8O12 (4) where the base-catalyzed excision of one framework silicon was employed to generate a regiomeric mixture of 6.
[Ti(NMe2){Et3Si(CH2)3(i-Bu)6Si7O12}] (7), [Ti(NMe2){HMe2Si(CH2)3(i-Bu)6Si7O12}] (8), [Ti(NMe2){(i-C4H9)7Si7O12}] (9) and [Ti(NMe2){(c-C6H11)7Si7O12}] (10) were synthesized via protonolysis of Ti(NMe2)4 with one equivalent of the trisilanol precursor in order to determine if the presence of isomers would be intrinsically different as compared to the uniformly substituted catalysts. Isomers 8 and 9, demonstrated lower activity as compared to the uniformly substituted catalysts 9 and 10, however the isomers still exhibited extremely high catalytic activity for the epoxidation of 1-octene using tert-butyl hydroperoxide (TBHP) relative to titanium catalysts used in industry. Additionally, 9, 10, NCP-9 and CP-9 were very selective catalysts for the epoxidation of various olefins such as terminal (1-octene), cyclic (cyclohexene or 1-methylcyclohexene), and more demanding olefins (limonene or α-pinene) employing TBHP as the oxidant. Furthermore, NCP-9 and CP-9 were recyclable with minimal loss of titanium however the catalysts could also be repaired if a loss in activity was observed.
Preliminary epoxidation reactions employing NCP-9 and CP-9 along with hydrogen peroxide (H2O2) as the oxidant were also explored using different solvents. NCP-9 degraded quickly due to the hydrolysis of the titanium from the large amount of water present in the reaction mixture however CP-9 showed activity for the epoxidation of cyclohexene. Finally, regiomeric mixture of HS(CH2)3(i-Bu)6Si7O9(OH)3 (12), was tethered onto gold nanoparticles for the conversion of propene to propylene oxide using molecular hydrogen and oxygen. While the catalysts showed low activity under our reaction conditions, numerous improvements can be investigated in order to improve upon the catalysts.
Keywords: Olefin epoxidation; Titanium silsesquioxane; Gold nanoparticles; Heterogeneous catalysis; Immobilized catalysts; Non-activated alkenes
Bio:
Sarah Peak grew up in Owen County, Kentucky. She holds a BS (’08) from Georgetown College and a MS (’10) from Eastern Kentucky University. Her area of study was New Synthetic Routes to Produce Heterocyclic Compounds, Thiophenes. She is currently a PhD candidate, expected to graduate in August 2015 from University of Kentucky. Her thesis work is “The Development Tripodal Titanium Catalysts for the Conversion of Olefins to Their Corresponding Epoxide”.