We report our finding that by exploiting the synergistic steric effects between substrate and catalyst, an intramolecular Pd-catalyzed alkyne carbohalogenation can be achieved. This operationally simple method uses the bulky Pd/Q-Phos combination and allows access to tetrasubstituted vinyl halides from the corresponding aryl chlorides, bromides, and iodides. Steric effects in the substrate play a key role by promoting C sp 2-halogen reductive elimination and enabling catalytic turnover. Through a reversible oxidative addition mechanism, a thermodynamically driven isomerization reaction is observed at elevated temperatures. Thus by changing the reaction temperature, both stereoisomers of the reaction become readily accessible.

No abstract

No abstract

We report our finding that by exploiting the synergistic steric effects between substrate and catalyst, an intramolecular Pd-catalyzed alkyne carbohalogenation can be achieved. This operationally simple method uses the bulky Pd/ Q-Phos combination and allows access to tetrasubstituted vinyl halides from the corresponding aryl chlorides, bromides, and iodides. Steric effects in the substrate play a key role by promoting C sp 2-halogen reductive elimination and enabling catalytic turnover. Through a reversible oxidative addition mechanism, a thermodynamically driven isomerization reaction is observed at elevated temperatures. Thus by changing the reaction temperature, both stereoisomers of the reaction become readily accessible.Aryl and vinyl halides are versatile building blocks in organic synthesis due to their applicability toward a vast range of transition-metal-catalyzed cross-coupling reactions. Oxidative addition into an aryl or vinyl halide bond constitutes the first step in many traditional Pd-catalyzed cross-coupling reactions and is generally considered to be an irreversible and thermodynamically favorable step. [1] Consequently, carbonhalogen reductive elimination from Pd II complexes (i.e., the microscopic reverse reaction) is rarely encountered when using conventional ligand sets. [2][3][4] This long-standing paradigm was shifted when Hartwig reported the stoichiometric reductive elimination of aryl halides from monomeric ArPd II X species in 2003. [3b] Interestingly, this reductive elimination was promoted by the addition of excess PtBu 3 , an electron-rich and sterically encumbering ligand (Scheme 1 a). Typically, electron-poor Pd II complexes are better suited to undergo reductive elimination compared to electron-rich complexes with similar steric properties. [5] However, these studies reveal that unfavorable electronic effects can be overridden in the reductive elimination of C sp 2 À X bonds from electron-rich Pd II centers, provided that bulky phosphine ligands are employed. In this study, both K eq and yield are increased upon the use of ortho-substituted aryl halide complexes, thus suggesting that substrate sterics may also play an intimate role in promoting this elementary process.Our group has demonstrated that the use of bulky monodentate phosphine ligands, such as PtBu 3 and Q-Phos, can effectively promote C sp 3 À I reductive elimination in the Pd 0 -catalyzed carboiodination of alkenes (Scheme 1 b). [4a-f] To date, the successful realization of an alkyne carbohalogenation reaction has been impeded by a limited library of ligands fitting the criteria for promoting a difficult carbon-halogen reductive elimination step. Furthermore, the reactive nature of the final vinyl halide product presents additional complications, which stem from reinsertion of the Pd 0 catalyst, followed by potential catalyst deactivation, attenuation in turnover numbers and/or the initiation of unwanted coupling or oligomerization processes. Scheme 1. a) Hartwig's stoichiometric studies on aryl halide reductive el...

This article describes the integration of a cocurricular "Community" into a first-year undergraduate chemistry course at the University of Toronto. The Community has been in existence since 2006, with over 700 students being involved. Its broad objectives have been three-fold: to inform course members about departmental resources and their significance, to teach skills and provide tips away from formal class content, and to promote the importance of undergraduate research/highlight available opportunities. The Community has received very positive feedback over a number of years and has resulted in a large increase in the number of students enrolling in second-and fourth-year undergraduate science research courses.