cycloaddition is a pericyclic chemical reaction, in which "two or more unsaturated molecules (or parts of the same molecule) combine with the formation of a cyclic adduct in which there is a net reduction of the bond multiplicity."[1] The resulting reaction is a cyclization reaction. Many but not all cycloadditions are concerted. Cycloadditions are usually described by the backbone size of the participants. This would make the Diels-Alder reaction a (4 + 2)cycloaddition, the 1,3- dipolar cycloaddition a (3 + 2)cycloaddition and cyclopropanation of a carbene with an alkene a (2+1)cycloaddition. This type of reaction is non-polar addition reaction. Reaction mechanism Thermal cycloadditions are those cycloadditions where the reactants are in the ground electronic state. They usually have (4n + 2) π electrons participating in the starting material, for some integer n. These reactions occur, for reasons of orbital symmetry, in a suprafacial- suprafacial or antarafacial- antarafacial manner (rare). There are a few examples of thermal cycloadditions which have 4n π electrons (for example the [2 + 2] cycloaddition); these proceed in a suprafacial-antarafacial sense, such as the dimerisation of ketene, in which the orthogonal set of p orbitals allows the reaction to proceed via a crossed transition state. Cycloadditions in which 4n π electrons participate can also occur via photochemical activation. Here, one component has an electron promoted from the HOMO (π bonding) to the LUMO (π* antibonding). Orbital symmetry is then such that the reaction can proceed in a suprafacial-suprafacial manner. An example is the DeMayo reaction. Another example is shown below, the photochemical dimerization of cinnamic acid.[2]The two trans alkenes react head-to- tail, and the isolated isomers are called truxillic acids. Cycloaddition of trans-1,2-bis(4- pyridyl)ethene Supramolecular effects can influence these cycloadditions. The cycloaddition of trans-1,2-bis (4-pyridyl)ethene is directed by resorcinol in the solid- state in 100% yield.[3] Some cycloadditions instead of π bonds operate through strained cyclopropane rings; as these have significant π character. For example, an analog for the Diels-Alder reaction is the quadricyclane- DMAD reaction: In the (i+j+...) cycloaddition notation i and j refer to the number of atoms involved in the cycloaddition. In this notation a Diels-Alder reaction is a (4+2)cycloaddition and a 1,3- dipolar addition such as the first step in ozonolysis is a (3+2)cycloaddition. The IUPAC preferred notation however, with [i+j+...] takes electrons into account and not atoms. In this notation the DA reaction and the dipolar reaction both become a [4+2]cycloaddition. The reaction between norbornadiene and an activated alkyne is a [2+2+2]cycloaddition. ↑Jump back a section Types of cycloaddition
Posted on: Wed, 31 Jul 2013 13:54:59 +0000
Trending Topics
Recently Viewed Topics
© 2015