Events

Mechanistic Investigations of Gold(I)-catalyzed Carbene Transfer and Hydrofunctionalization Reactions
Speaker: 
Robert Carden, Ph.D. candidate
Ross Widenhoefer, advisor
Thursday, June 13, 2019 - 1:00pm to 3:00pm
Location: French Family Science Center 2237
Contact: 
Avery, Meg
919-660-1503

Cationic gold(I) complexes have been recognized as efficient catalysts for wide array and transformations, including carbene transfers to form cyclopropanes and hydrofunctionalization reactions. While there have been great strides made in the development of these reactivities, far less is understood about the mechanisms by which these transformations occur. Here are reported studies related to further understanding the mechanisms of gold(I)-catalyzed reactions through kinetic and mechanistic analysis as well as the interrogation of the properties of cationic gold(I) carbene complexes, which are commonly proposed intermediates in gold(I)-catalyzed transformations.

            A series of gold(I) benzylide complexes were synthesized by nucleophilic substitution of a-chloro gold(I) carbenoid complexes with sulfides. These complexes reacted efficiently with alkenes and dimethylsulfoxide to form cyclopropanes and benzaldehyde, respectively. Kinetic analysis of these reactions is consistent with the intermediacy of cationic gold(I) benzylidene complexes. Further mechanistic analysis revealed that alkene stereochemistry is preserved during cyclopropanation and a Hammett analysis of the reaction suggest a concerted mechanism for cyclopropanation.

            To evaluate the electron donor ability of (L)Au fragments in cationic gold(I) carbene complexes, a series of cationic gold (β,β‐disilyl)vinylidene complexes and cationic gold (fluorophenyl)methoxycarbene complexes were synthesized. 29Si and 19F NMR analysis of these complexes compared to organic model compounds revealed that (L)Au fragments are significantly more inductively donating and comparable π‐donors to p-substituted aryl groups. A comparison of various ligands showed that (P(t-Bu)2-o-biphenyl)Au fragments are nominally stronger electron donors than (IPr)Au fragments, both of which are significantly more electron donating that (PPh3)Au and [P(OMe)3]Au fragments.

            Kinetic and mechanistic analysis of the gold(I)-catalyzed hydrofunctionalization of 3-methyl-1,2-butadiene with alcohols and anilines was performed. Experimental data suggest a mechanism for the gold(I)-catalyzed hydroalkoxylation involving endergonic allene displacement of triflate, followed by an outer-sphere attack of alcohol on gold(I)-p-allene complex, followed by rapid protodeauration. In contrast, for the gold(I)-catalyzed hydroamination, the active catalyst is the gold(I) bound nucleophile complex and buildup a bis(gold) vinyl complex suggests a slow protodeauration.

            A brief study evaluating student learning the classroom is also presented. A series of team-based learning applications based on the spiropyran to merocyanine transformation were developed and assessed relative to a series of control application problems. There was no statistically significant difference in student outcomes based on the applications used in class, but students feedback suggests that the interconnected, and real-life examples were more engaging.