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Old-school notation versus modern IUPAC notation: How to describe cycloaddition reactions accurately?
Cycloaddition reaction is an important chemical reaction in organic chemistry and often appears in synthetic chemistry. This reaction involves the combination of two or more unsaturated molecules or parts of the same molecule to form a functional cycloaddition product with a reduction in the number of bonds to a specified complexity. Many cycloaddition reactions proceed concertedly, i.e., bond formation and breaking occur in a single process, whereas others proceed stepwise. This reaction provides a means for the formation of carbon-carbon bonds that is independent of nucleophiles or electrophiles.
However, the way these reactions are described has changed over time: from an old notation system to the modern IUPAC standard, this process reflects the chemistry community's pursuit of precision and consistency.
Notation system for cycloaddition reactions
The definition of cycloaddition reactions can be described according to different notation systems. Older notations were based on the number of linear atoms in the reactants, usually expressed in brackets, as in (i + j + …), where the variables indicated the number of linear atoms in each reactant. The products of the reaction will be represented in the form of a ring (i + j + …). For example, the standard Diels-Alder reaction is defined as a (4 + 2)-cycloaddition.
In contrast, the newer IUPAC standard notation places more emphasis on the number of electrons involved in the formation of products rather than just the number of atoms. In this system, the standard Diels-Alder reaction is defined as a [4 + 2]-cycloaddition, which allows different types of reactions to be expressed in a more unified manner.
Thermal cycloaddition reaction and its stereochemistry
Thermal cycloaddition reactions usually involve reactions in the ground state and their reactants usually have (4n + 2) π electrons, and in most cases these reactions are symmetrical in behavior, such as suprafacial-suprafacial (syn/syn stereochemistry). However, the few examples reported indicate that some reactions proceed in an antarafacial-antarafacial (anti/anti stereochemistry) fashion.
It is noteworthy that some thermal cycloadditions, such as the [2 + 2]-cycloaddition, involve 4n π electrons. These reactions proceed in a suprafacial-antarafacial manner. For some stressed olefins, such as trans-cyclobutene derivatives, there are also reports that they proceed in an antarafacial manner in [2 + 2]-cycloaddition reactions.
Photochemical cycloaddition reaction
Photochemical excitation can also lead to cycloaddition reactions. In these cases, one of the components causes an electron jump from the HOMO (pi-bonding) to the LUMO (pi*antibonding), a process that allows the reaction to proceed in a suprafacial-suprafacial manner. For example, the DeMayo reaction falls into this category.
When these reactions are carried out in the solid state, supramolecular effects can also influence the cycloaddition, making the results more variable under specific conditions.
Various types of cycloaddition reactions
Diels-Alder reaction
The Diels-Alder reaction is one of the most important cycloaddition reactions. Formally, it is a [4 + 2] cycloaddition reaction that covers a variety of forms. This includes the inverse electron demand Diels-Alder reaction, the hexadecene metal reaction, etc. In addition, this reaction can be carried out in reverse, namely the retro-Diels-Alder reaction.
Huisgen Cycloaddition
The Huisgen cycloaddition reaction is a (2+3) cycloaddition.
Nitrogen-olefin cycloaddition
Nitrines-olefins cycloaddition belongs to the (3+2) cycloaddition.
Cheletropic reactions
Cheletropic reactions are a subclass of cycloadditions in which both new bonds to one of the reagents in the reaction are to the same atom. A classic example is the reaction of sulfur dioxide with dienes.
Formal ring addition
Formal cycloadditions often involve metal catalysts or similar reactions that generate free radicals stepwise, but these are not strictly considered to be cycloaddition reactions. Some formal [3+3] cycloadditions can be carried out with the aid of catalysts.
The importance of these reactions in different industries and technological advances has led scientists to continuously explore and update their understanding and description. Do you think the way these reactions are labeled will continue to change as new research emerges?
