Conjugated polyene, electrocyclic

Conjugated polyene, electrocyclic reactions of, 1181-1186 molecular orbitals of, 1179-1180 Conjugated tricne, electrocyclic reactions of, 1182 Conjugation, 482... 

The best way to understand how orbital symmetry affects pericyclic reactions is to look at some examples. Let s look first at a group of polyene rearrangements called electrocyclic reactions. An electrocyclic reaction is a pericyclic process that involves the cycli/ation of a conjugated polyene. One 7r bond is broken, the other 7t bonds change position, a new cr bond is formed, and a cyclic compound results. For example, a conjugated triene can be converted into a cyclohexa-diene, and a conjugated diene can be converted into a cyclobutene. 

Electrocyclic reactions were first described by Woodward and Hoffmann in their classic series of articles. One very interesting aspect of such reactions is, that for a given conjugated polyene photochemical transformation leads to the opposite stereochemical outcome than the thermal one314). 

In electrocyclic reactions of conjugated polyenes, one double bond is lost and a single bond is formed between the terminal C s to give a ring. The reaction is reversible. 

An electrocyclic reaction is the formation of a new a bond across the ends of a conjugated polyene or.the reverse... 

Electrocyclic reactions of conjugated polyenes create chiral molecules through stereospecific conrotatory or disrotatory processes. In solution, the two enantiom-... 

There are several categories of photochromic reactions electrocyclization of conjugated polyenes [3], E-Z isomerization of a double bond [4], radical forma-... 

Electrocyclic reactions involve the cyclization of conjugated polyenes. For example, 1,3,5-hexatriene cyclizes to 1,3-cyclohexadiene on heating. Electrocyclic reactions can occur by either conrotatory or disrotatory paths, depending on the symmetry of the terminal lobes of the tt system. Conrotatory cyclization requires that both lobes rot lte in the same direction, whereas disrotatory cyclization requires that the lobes rotate in oj )posite directions. The reaction course in a specific case can be found by looking at the symmetry of the highest occupied molecular orbital (HOMO). 

Electrocyclic reactions are a class of pericyclic reactions in which a conjugated polyene interconverts with an unsaturated cyclic compound containing one less carbon-carbon double bond than the polyene. " The reactions can be promoted thermally or photochemically and take place with a very high degree of stereoselectivity. 

In an electrocyclic reaction, a cyclic system (ring closure) is formed through the formation of a a-bond from an open-chain conjugated polyene system at the cost of a multiple bond and vice versa (ring opening). These reactions are unimolecular in nature as the rate of reactions depends upon the... 

An electrocyclic reaction is a pericyclic process in which a conjugated polyene undergoes cyclization. In the process, one it bond is converted into a a bond, while the remaining it bonds all change their location. The newly formed a bond joins the ends of the original TT system, thereby creating a ring. Two examples are shown. 

An electrocyclic reaction [23] is the closure of a conjugated polyene to give a cyclic compound with one less n bond, or the reverse. For the first example, consider the thermal cyclization reaction of E, E-2,4,6-octatriene to cis-5,6-dimethylcyclohexadiene (Eq. 5.13). A a bond forms and new n bonds develop concurrently. 

An electrocyclic process is defined as the formation of a single bond between the termini of a conjugated polyene (Iw Iff), and the reverse reaction. Equation (3.16). Reactions (3.3) and (3.4) are specific examples in which k is 4 and 6 ir-electrons respectively. In reaction (3.16) the polyene is shown in the (necessary) all cis configuration. 

With this model, we need only apply the method already used to derive the selection rules for electrocyclic reactions (p. 53). From the Coulson equations, we can deduce that in the in conrotatory cyclization of pentadiene, the MO generates a destabilizing C5-C4 secondary interaction, a stabilizing and Fg a destabilizing interaction. The absolute values of these contributions rise steadily because the terminal coefficients increase from Fg to Fg. Therefore, the sign of their sum is given by the HOMO contribution. If R is an attractor, the HOMO is Fg and rotation inwards is favored. If R is a donor, the HOMO is 4T and rotation inwards is disfavored. As the Coulson equations are valid only for polyenes, these conclusions are correct insofar as R can be modeled by a carbon 2p orbital. It follows that the Rondan-Houk theory works better for conjugative than for saturated substituents. 

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