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From scientist to synthesis master! Why did Kurt Alder's Eni reaction explode the chemical world?
Through a chemical change called the "ene reaction," Kurt Alder opened a new chapter in organic chemistry in 1943. The ene reaction is a chemical reaction that occurs between an alkene with a double bond and another molecule with multiple bonds (an enophile, the acceptor molecule for the alkene reaction). Such a transformation not only creates new σ bonds, but is also accompanied by the transfer of ene double bonds and the 1,5 shift of hydrogen. The characteristic of this reaction is that it is a group transfer pericyclic reaction, which usually requires highly activated substrates and/or high temperatures to promote the reaction. However, this reaction is compatible with a variety of functional groups, greatly improving the flexibility of organic synthesis.
What is igniting the chemistry world is not just the reaction itself, but also the theory and application behind it.
Reaction of ene component and enophile
The basic components of the ene reaction are divided into "ene" and "enophile". The former is usually a π-bonded molecule with at least one active hydrogen atom. Common ene components include alkenes, alkynes and certain aromatic compounds. If these ene components have additional C=O or C=N bonds, they are very important reaction substrates. For example, the enol or enol anion plays a special role in the ene reaction and corresponds to the Conia-ene reaction. At the same time, enophiles are π-bonded molecules containing electron-attracting groups, which serve as acceptor molecules in the ene reaction to reduce the energy of LUMO. This undoubtedly makes the reaction easier to carry out, which is also one of Kurt Alder's contributions.
Mechanism of ene reaction
The mechanism of ene reaction basically has two main paths: one is the synergistic mechanism, and the other is the free radical mechanism. In a cooperative mechanism with high activation energy, the interaction between HOMO and LUMO of ene and enophile promotes the reaction. In some special cases, when the geometry is not conducive to cooperative mechanisms, it may proceed through spontaneous radical processes. The molecules of alkynes and nitrogen produced by this process also show the diversity of reactions when viewed from one side.
The success of the ene reaction involves a variety of factors, including the three-dimensional accessibility of the structure and the properties of the reactants.
Local selection and stereoselectivity
As with many chemical reactions, the efficiency of the ene reaction depends on the accessibility of the hydrogen atoms of the alkenes in the ene. In general, secondary hydrogen atoms are more easily extracted than tertiary hydrogen atoms. In addition, ene reactions can not only carry out invasive reactions, but also exhibit extremely high selectivity. For example, by using certain imine catalysts, the reaction products can be optimized through exquisite catalyst design.
Lewis acid-catalyzed ene reaction
Past research has shown that Lewis acid catalysis can significantly improve the efficiency of ene reactions. The researchers used a series of Lewis acids to speed up the reaction rate and improve product selectivity. Moreover, these reactions can be carried out at lower temperatures, allowing synthesizers to avoid side reactions caused by high temperatures, successfully achieve harmonious combinations of compounds, and thereby enhance the diversity of products.
Influence on modern synthesis
As the ene reaction pioneered by Kurt Alder has become an important tool in organic synthesis, many contemporary synthetic scientists have widely used it in the synthesis of complex molecules and natural products. At the heart of this reaction is the synthetic flexibility and diversity it offers, allowing scientists to explore more esoteric areas of organic chemistry.
Kurt Alder's ene reaction is not only a chemical process, it also promotes the progress of the entire synthetic chemistry.
When analyzing the history of this reaction and its status in today's chemistry, we cannot deny the influence of Kurt Alder. The ene reaction has shown great potential in terms of simplifying the synthetic route and expanding the scope of application of reaction substrates. This makes people think: What other innovations and challenges can chemical reactions lead to in the future?
