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Thermolysis converts 1 to (E,E) geometric isomer 2, but 3 to (E,Z) isomer 4.. The Woodward–Hoffmann rules (or the pericyclic selection rules) [1] are a set of rules devised by Robert Burns Woodward and Roald Hoffmann to rationalize or predict certain aspects of the stereochemistry and activation energy of pericyclic reactions, an important class of reactions in organic chemistry.
The Nazarov cyclization reaction is a named electrocyclic reaction converting divinylketones to cyclopentenones. A classic example is the thermal ring-opening reaction of 3,4-dimethylcyclobutene. The cis isomer exclusively yields cis,trans-hexa-2,4-diene whereas the trans isomer gives the trans,trans diene: [2]
The 1969–1970 Woodward–Hoffmann general formulation is seen to be equivalent to the Zimmerman Möbius–Hückel concept. Thus each (4r) a component provides one plus–minus overlap in the cyclic array (i.e. an odd number) for 4n electrons. The (4q + 2) s component just makes certain that the number of electrons in symmetric bonds is 4n + 2.
Since the substrate octatriene is a "4n + 2" molecule, the Woodward–Hoffmann rules predict that the reaction happens in a disrotatory mechanism. Since thermal electrocyclic reactions occur in the HOMO, it is first necessary to draw the appropriate molecular orbitals. Next, the new carbon-carbon bond is formed by taking two of the p-orbitals ...
The best-known example is benzene (C 6 H 6) with a conjugated system of six π electrons, which equals 4n + 2 for n = 1. The molecule undergoes substitution reactions which preserve the six π electron system rather than addition reactions which would destroy it. The stability of this π electron system is referred to as aromaticity. Still, in ...
An electrocyclic reaction is a pericyclic reaction involving the net loss of a pi bond and creation of a sigma bond with formation of a ring. This reaction proceeds through either a conrotatory or disrotatory mechanism. In the conrotatory ring opening of cyclobutene, there are two electrons moving suprafacially (on the pi bond) and two moving ...
To summarize, we are assuming that: (1) the energy of an electron in an isolated C(2p z) orbital is =; (2) the energy of interaction between C(2p z) orbitals on adjacent carbons i and j (i.e., i and j are connected by a σ-bond) is =; (3) orbitals on carbons not joined in this way are assumed not to interact, so = for nonadjacent i and j; and ...
The second step of the Buchner reaction involves a pericyclic concerted ring expansion. Based on Woodward–Hoffmann rules, the electrocyclic opening of norcaradiene derivatives is a 6-electron disrotatory (π 4 s + σ 2 s), thermally allowed process. ring opening. The norcaradiene-cycloheptatriene equilibrium has been studied extensively. [18]