Catalytic homogeneous hydrogenation of olefins using N-heterocyclic carbene complexes of rhodium and iridium

Leslie D Vazquez-Serrano, Purdue University

Abstract

A synthesis of a small library of rhodium(I) and iridium(I) complexes that combine different types of N-heterocyclic carbenes with pyridines and phosphines was developed. The complexes were tested for the hydrogenation of the simple olefins, 1-octene, cyclohexene, 1-methyl-1-cyclohexene and 2,3-dimethyl-2-butene. Three major trends were observed: first, complexes having iridium and a combination of a phosphine and a NHC prove to be more effective than those with a combination of a pyridine and a NHC. Second, the complex with the much bulkier IMes ligand is more reactive than those with IMe for unhindered alkenes (1-octene and cyclohexene). For tertiary and quaternary alkenes, however, the less hindered IMe complexes are able to hydrogenate these substrates considerably faster. Lastly, the use of the BARF counterion produced the fastest catalyst from the series. The reactivity of [Ir(cod)(IMe){P(n-Bu)3}]BARF is superior to Crabtree's catalyst for the hydrogenation of simple olefins. Another advantage of this NHC-phosphine series is that the reaction progress can be followed visually. The reactions can be carried out with reagent grade solvents decanted in air, without prior inert gas sparging. In order to provide insight into the mechanism of the hydrogenation reaction, para hydrogen induced polarization (PHIP) 1H NMR studies were carried out using the complexes with [D8] styrene. Enhancements are observed in the PHIP 1H-NMR spectrum, which suggest a dihydride mechanism. Kinetics experiments also revealed a half-order with respect to the catalyst, this suggests that the key transformation on the reaction is the conversion of an iridium dimmer into monomers. This unusual type of intermediate prompted us to use low temperature 1H-NMR and 31P-NMR, and Mass Spectrometry to analyze the structure of the key intermediate. The presence of a dimeric intermediate on a dihydride mechanism for the catalytic homogeneous hydrogenations is proposed.

Degree

Ph.D.

Advisors

Buriak, Purdue University.

Subject Area

Organic chemistry|Chemistry

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