UC San Diego

ABSTRACT OF THE THESIS

Ruthenium-Catalyzed Cyclization/Cycloaromatization of Conjugated Trienes

by

Li-An Wang

Master of Science in Chemistry

University of California San Diego, 2019

Professor Joseph M. O’Connor, Chair

The work describes mechanistic studies on ruthenium-accelerated cyclization/cycloaromatization of trienes. In the beginning, thermal and photochemical reactions of trienes were conducted in order to compare the temperatures and reaction times to those for ruthenium-accelerated analogues. According to the Woodward-Hoffmann rules, cyclization of trienes to cyclohexadienes should proceed in a disrotatory fashion. In the literature, Chaudret and co-workers published that [Cp*Ru(OMe)]2/HOTf could lead to cycloaromatization of (E)-1,3,5-hexatriene, while the mechanism for this reaction was not discussed. And based on previous work from the O’Connor lab, the authors demonstrated that [Cp*Ru(NCMe)3]PF6 mediated the cyclization of conjugated enediynes and dienynes at room temperature, possibly via the formation of hexahapto intermediates, η6-enediyne and η6-dienyne, respectively. Consequently, we proposed that [CpRu(NCMe)3]PF6 would trigger the cycloaromatization of trienes at room temperature via η6-(acyclic triene) metal intermediates, followed by elimination of two hydrogen atoms to generate an arene product.

In the reactions of trienes with [CpRu(NCMe)3]PF6, η4- and η6-acyclic metal complexes were clearly identified by 1H NMR spectroscopy and X-ray crystallography. Subsequently, the isolated η6-acyclic metal complex was heated at 50 ˚C to give 6,7-dimethyl-1,2,3,4-tetrahydronaphthalene. Thus, the hexahapto metal complex was implicated in acceleration of the cyclization. Moreover, the reaction of 1-((E)-prop-1-en-1-yl)-2-((Z)-prop-1-en-1-yl)cyclohex-1-ene with [CpRu(NCMe)3]PF6 sufficiently illustrated that 6π electrocyclization of trienes proceeds in a disrotatory fashion. An agostic interaction was also observed by 1H NMR spectroscopy in a η4-cyclohexadiene ruthenium intermediate, followed by a [1,5]-hydride shift. Most surprisingly, a truly catalytic reaction was found in the reactions of (Z)-1-(2-methylprop-1-en-1-yl)-2-(prop-1-en-1-yl)cyclohex-1-ene and (E)-1-(2-methylprop-1-en-1-yl)-2-(prop-1-en-1-yl)cyclohex-1-ene with [CpRu(NCMe)3]PF6 since the appearance of a free cyclohexadiene derivative was observed and excess triene was consumed. Based on the results of these experiments, several reasonable mechanisms for ruthenium-mediated cyclization of trienes were proposed.