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The fantastic variation of the Diels–Alder reaction: Why do we get different products at low temperatures?
In chemical reactions, either thermodynamic control or kinetic control can affect the composition of the products, especially when there are competing pathways leading to different products, the reaction conditions will affect the selectivity or stereoselectivity. This distinction is particularly important when product A is formed faster than product B, because product A has a lower activation energy than product B, yet product B is more stable. In this case, A is the kinetic product and is more favored under kinetic control, while B is the thermodynamic product and is more favored under thermodynamic control. Reaction conditions such as temperature, pressure or solvent can influence which reaction pathway is preferentially chosen: whether it is kinetically controlled or thermodynamically controlled.
Each chemical reaction operates as if on a continuum between kinetic control and thermodynamic control.
In the Diels–Alder reaction, cyclopentadiene and furan react to produce two isomeric products. At room temperature, kinetic control dominates the reaction, with the less stable endo isomer being the major reaction product. However, when the temperature is raised to 81°C and the reaction time is prolonged, chemical equilibrium begins to take effect and the more stable exo-isomer (exo-isomer) is formed. The exo-isomer is more stable due to its lower steric crowding, while the endo-isomer is favored due to orbital overlap during the transformation.
In 2018, a very rare and outstanding global Examples of kinetic and thermodynamic reaction control. At low temperatures, the reaction selectively generates pincer-type [4+2] cycloaddition products, whereas at high temperatures, exclusive formation of domino products is observed. Theoretical DFT calculations performed on these reactions indicate that the activation barriers for the rate-limiting steps in the drilling process are between 23.1 and 26.8 kcal/mol.In enol chemistry, when the enolate group is protonated, the kinetic product is an enol and the thermodynamic product is a ketone or an aldehyde. In the deprotonation of asymmetric ketones, the kinetic product is the most deprotonated enol, while the thermodynamic product is the more substituted enol. Low temperatures and sterically demanding bases will improve kinetic selectivity. When the deprotonation reaction of the enolate proceeds, the type of product produced is also closely related to the reaction temperature and reaction time.
In the electronucleophilic addition reaction, hydrobromic acid added to 1,3-butadiene will mainly form the more thermodynamically stable 1,4 addition product at room temperature, but if the reaction temperature is lowered to Below room temperature, the kinetic 1,2 addition product is favored. Although the two products were generated from the same source, the exact inter-product selection was shown to be strongly dependent on the reaction conditions.
The reaction context can often influence the choice of products formed, so it is important to understand these controlling factors.
It is important to note that, in theory, each reaction is a continuum between kinetic control and thermodynamic control. As time goes by, the state that each reaction eventually reaches will be close to thermodynamic control. At low times and lower reaction temperatures, kinetic control usually dominates. Therefore, optimizing reaction conditions to improve product selectivity and yield is an important research direction.
In 1944, R.B. Woodward and Harold Baer first reported the relationship between kinetic control and thermodynamic control, as reflected in the reaction process and the composition ratio of the products. In subsequent studies, scientists conducted more in-depth exploration and revelation of this phenomenon and found that different reaction conditions and time courses would significantly affect the distribution of the final products of the reaction.
A deeper understanding of these reactions not only has an impact on chemical synthesis, but also has an increasingly wide application in catalytic reactions. From selective catalysis to serving as a guiding principle for new reaction mechanisms, the relationship between kinetics and thermodynamics is undoubtedly one of the important topics in chemical research.
In future chemical explorations, when we are faced with inherent reaction choices, can we derive more precise mechanisms and methods to control product formation?
