Graduate Program

Chemistry

Degree Name

Master of Science (MS)

Semester of Degree Completion

Spring 2018

Thesis Director

Hongshan He

Thesis Committee Member

Daniel J. Sheeran

Thesis Committee Member

Radu F. Semeniuc

Thesis Committee Member

Zhiqing Yan

Abstract

Sonogashira C-C cross-coupling reaction has been heavily used for its simplicity, flexibility and value. Although the original Sonogashira C-C cross coupling reaction conditions entail the use of 0.5 mol% of Pd catalyst, 2 mol% of Cul as co-catalyst at room temperature, the experimental conditions used in laboratories to prepare some compounds vary substantially, especially with respect to the reaction time, temperature and loading amount of the catalysts. The most significant drawback of the conventional Sonogashira reaction is the use of Cul as it leads to the formation of homocoupling product when the reaction is exposed to air or oxidizing agents.

To address the above-mentioned shortcomings, we designed a photocatalyst based on dipyrrometheneboron difluoride (BODIPY). The BODIPY based dichloro(4,7-di(4',4"-difluoro-1',3',5',7'-tetramethyl-4'-bora-3'a,4'a-diaza-s-indacene)-1,10-phenanthroline)palladiurn(II) (YH2-Pd) catalyst was synthesized and purified using column chromatography. The product and the intermediates were characterized using UV-Vis spectroscopy, 1H NMR, fluorescence spectroscopy and cyclic voltamrnetry.

Coupling reactions between iodobenzene and phenylacetylene were successfully catalyzed by the YH2-Pd catalyst in the absence of Cu(I) co-catalyst at room temperature under inert conditions upon irradiation with 13W LED bulb (1050 Lumens). A yield of 94% was obtained via GC-MS after 24 hours with 0.5% loading of the catalyst. Substrate scope of the photoinduced Sonogashira reaction was investigated with various derivatives of iodobenzene and phenylacetylene. When para-substituted iodobenzenes were used, the desired cross-coupling product yields were lower than the model reaction, in contrast to for the phenylacetylene derivatives as the product yields were not significantly different from the model reaction.

With the intentions of simplifying the catalyst system, a mixture of BODIPY and dichloro(l,10-phenanthroline)palladium(ll) ([Pd(phen)Cl2]) was used for the catalysis. A yield of 87% was obtained via GC-MS after 24 hours with 0.5% [Pd(phen)Cl2] and 1% BODIPY. Use of derivatives of iodobenzene resulted in the same pattern of product yield observed for the YH2-Pd catalyst with low yields in general.

Mechanistic investigations were carried out using 31P NMR, UV-Visible spectroscopy and cyclic voltammetry. 31P NMR and UV-Visible spectroscopies revealed the formation of the PPh3 ligated complex upon reduction of Pd(Il) to Pd(0) and cyclic voltammetry studies provided the evidence, which indicates that the reduction is mediated by Et3N. Experimental studies with various aryl halides revealed the crucial role of BODIPY in the oxidative addition step in the catalytic cycle.

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