A classroom activity for learning green chemistry metrics with interlocking building blocks is described. The activity illustrates the strengths and weaknesses of conversion, selectivity, yield, atom economy, reaction mass efficiency, carbon efficiency, E-factor, and effective mass yield by counting, where appropriate, the number of molecular models built, the number of bricks used, or the number of connection points available. The activity is appropriate for students in general chemistry courses through advanced undergraduate green chemistry or industrial chemistry courses.
Hydrosilylation is a valuable approach for the construction of organosilanes, which are precursors to silicone materials that are widely incorporated in our everyday lives. The industry currently relies primarily on Karstedt’s catalyst, Pt2(dvtms)3 (dvtms = 1,3-divinyltetramethyldisiloxane), a precious metal catalyst that exhibits linear selectivity, with regioselectivity favoring the branched product remaining an outstanding challenge. The use of more Earth-abundant, base-metal catalysts has been a recent focus for hydrosilylation reactions, and most reports focus on the development of linear-selective catalysts and are commonly limited to primary and/or secondary silanes. We demonstrate that (NHC)Ni(0) (NHC = N-heterocyclic carbene) complexes are active in the branched-selective hydrosilylation of alkenes with secondary or tertiary silanes, including industrially relevant alkoxy- and chlorosilanes. The scope of alkenes and silanes has been expanded beyond what is currently known for Ni-catalyzed hydrosilylation reactions, including both steric and electronic profiles. In-depth mechanistic studies were also carried out, including stoichiometric and catalytic experiments investigating kinetic and thermodynamic reaction parameters. Radical trap experiments suggest against a one-electron pathway. The rate law of the reaction has a normal dependence on the Ni catalyst and silane and has an inverse dependence on the alkene. Deuterium-labeling studies reveal that hydrosilylation proceeds through a Chalk–Harrod-type mechanism, with the alkene reversibly inserting into a Ni–H bond. Hammett analyses show that the rate of reaction is faster with electron-rich alkenes and electron-poor silanes. Additional mechanistic evidence points to the resting state of the catalyst being a (NHC)Ni(alkene)2 complex, and the rate-determining step being migratory insertion and/or reductive elimination.
Alkenes are used ubiquitously as starting materials and synthetic targets in all areas of chemistry. Controlling their geometry and position along a chain is vital to their reactivity and properties yet remains challenging. Alkene isomerization is an atom-economical process to synthesize targeted alkenes, and selectivity can be controlled using transition metal catalysts. The development of mild, selective isomerization reactivity has enabled efficient tandem catalytic systems for the remote functionalization of alkenes, a process in which a starting alkene is isomerized to a new position prior to the functionalization step. The key challenges in developing isomerization catalysts for remote functionalization applications are (i) a lack of modularity in the catalyst structure and (ii) the requirement of nonmodular and/or harsh additives during catalyst activation. We address both challenges with a modular (NHC)Ni(0)/silane catalytic system (NHC, N-heterocyclic carbene), demonstrating the use of triaryl silanes and readily accessible (NHC)Ni(0) complexes to form the proposed active (NHC)(silyl)Ni−H species in situ. We show that modification of the steric and electronic nature of the catalyst via modification of the ancillary ligand and silane partner, respectively, is easily achieved, creating a uniquely versatile catalytic system that is effective for the formation of internal alkenes with high yield and selectivity for the E-alkene. The use of silanes as mild activators enables isomerization of substrates with a variety of functional groups, including acid-labile groups. The broad substrate scope, enabled by catalyst design, makes this catalytic system a strong candidate for use in tandem catalytic applications. Preliminary mechanistic studies support a Ni−H insertion/elimination pathway.
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