Schrock CatalystEdit

Schrock catalyst refers to a class of high-oxidation-state molybdenum and tungsten alkylidene complexes that catalyze olefin metathesis. Developed in the late 20th century by Nobel laureate Richard R. Schrock and coworkers, these catalysts opened access to a broad range of transformations by enabling the exchange of alkylidene fragments between olefins. They are distinguished from later ruthenium-based systems by their exceptional activity under demanding conditions and their sensitivity to air and many functional groups, which shapes their practical use in synthesis and polymer chemistry.

The term encompasses a family of metal-carbene catalysts whose active centers are metal alkylidenes (Mo=CHR or W=CHR) supported by strongly binding, typically anionic ligands such as alkoxides or related chelating frameworks. This chemistry underpins key processes in which C=C bonds are reorganized, a concept central to the broader field of olefin metathesis and its practical manifestations in the laboratory and the factory.

History

The Schrock catalysts emerged from the early exploration of metathesis catalysts that could operate reliably on challenging substrates. In the 1990s, Schrock and his group demonstrated that high-valent molybdenum and tungsten alkylidenes could mediate metathesis reactions with unusual substrate scope and reactivity, complementing the then-emerging ruthenium-based systems developed by Robert H. Grubbs and others. The results attracted wide attention and contributed to the awarding of the Nobel Prize in Chemistry 2005 to Chauvin, Grubbs, and Schrock for the development of metathesis catalysts and their application in organic synthesis. The Schrock line of catalysts remains a benchmark for understanding the fundamental limits of metathesis, even as other families—particularly ruthenium-based catalysts—have become dominant in many practical settings.

Chemistry

Structure and activation

Schrock catalysts are based on Mo or W centers in high oxidation states coordinated to robust, strongly binding ligands. The active metal alkylidene center (Mo=CHR or W=CHR) drives the metathesis cycle, which proceeds through a sequence of cycloaddition and cycloreversion steps that interconvert olefin substrates and products. The ligands surrounding the metal center are chosen to stabilize the high oxidation state while allowing the alkylidene fragment to engage olefin partners. Substrates bearing coordinating heteroatoms or acidic protons can deactivate or poison these catalysts, a sensitivity that contrasts with the broader operational tolerance seen in some ruthenium systems.

Catalytic cycle and scope

The catalytic cycle for Schrock-type metathesis is rooted in the classic metathesis mechanism, including the formation of a metallacyclobutane intermediate and its fragmentation to yield new alkene fragments. These catalysts excel in certain challenging cases, including substrates with bulky substituents or multifunctional groups that can participate in robust reaction sequences under controlled conditions. The high reactivity of the Mo and W alkylidene centers makes Schrock catalysts particularly effective for certain ROMP and CM/RCM processes when handling demanding substrates, although with a trade-off in air sensitivity and handling requirements.

Substrate scope and limitations

Schrock catalysts often require strictly anhydrous, rigorously dried reaction media and inert atmosphere handling. Their strong reactivity can be an advantage for difficult substrates, but it also means the catalysts are more prone to deactivation by impurities or functional groups that coordinate to the metal center. This has shaped the practical deployment of these catalysts, with many workers leveraging more tolerant systems (notably Grubbs catalyst-type ruthenium catalysts) for routine laboratory and industrial use, while reserving Schrock catalysts for cases where their particular reactivity profile is essential.

Applications

ROMP, RCM, and CM

Schrock-type catalysts have been employed in a range of metathesis applications, including ring-opening metathesis polymerization to make defined polymer architectures from cyclic olefins and various forms of ring-closing metathesis and cross-metathesis to forge or modify carbon–carbon double bonds in complex molecules. Their strong activity can enable transformations that are difficult for other catalysts and can be leveraged to construct macrocycles, cyclic olefins, and polymer backbones with precise stereochemical and regiochemical control.

Industries and research

Beyond academic demonstrations, Schrock catalysts have influenced polymer science and synthetic organic chemistry by providing a complementary option to ruthenium systems. They have contributed to the understanding of metathesis mechanisms and catalysis design, and they remain a reference point in discussions of catalyst performance, selectivity, and the balance between reactivity and stability. Linkages to prominent researchers and institutions are reflected in related organometallic chemistry and catalysis literature.

Strengths, limitations, and comparisons

Reactivity: Schrock catalysts can be extraordinarily active for demanding substrates, making them valuable in specialized synthetic contexts.
Sensitivity: They require stringent handling (air- and moisture-free conditions) and can be deactivated by impurities or coordinating functional groups.
Functional-group tolerance: Generally less tolerant of certain heteroatom-containing substrates compared with some ruthenium-based systems.
Practicality: While offering outstanding reactivity in the right setting, their handling and disposal considerations have influenced the broad adoption of alternative catalysts.

When choosing a catalyst for olefin metathesis, researchers weigh Schrock catalysts against systems such as the Grubbs catalyst family and newer generations of metathesis catalysts. The choice depends on substrate scope, desired reaction conditions, and considerations of operational practicality.

Controversies and debates

Schrock catalysts sit at an interesting intersection of fundamental organometallic chemistry and practical synthesis. Debates have centered on:

Trade-offs between reactivity and robustness: While Schrock catalysts offer high activity on challenging substrates, their air sensitivity and intolerance to certain functional groups limit their general applicability relative to more tolerant catalysts.
Industrial viability and sustainability: The use of heavy metals and the need for stringent handling raise questions about large-scale deployment and environmental considerations, fueling ongoing research into safer, more recyclable catalysts.
Evolution of metathesis catalysis: The rise of ruthenium-based systems (and later generations) has shaped the field, with some researchers emphasizing the broad substrate scope and ease of use of Ru catalysts, while others continue to exploit the unique capabilities of high-oxidation-state Mo/W systems for specific tasks.
Intellectual property and licensing: Patents and licensing histories surrounding metathesis catalysts have influenced how and where particular catalyst families are taught, published, and implemented in industry and academia.