Hexatriene cyclization to 1,3-cyclohexadiene

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). 

The cyclization of 1,3,5-hexatriene (6) to 1,3-cyclohexadiene (7) is predicted to proceed more rapidly in an electrostatic field. 

The ergosterol system mentioned earlier falls in this category. Before the system is discussed, it is worthwhile to point out that, in general, substitution of even a simple group such as an alkyl seems to change the details of the photochemical primary processes in a startling manner among the dienes and trienes. Thus, in solution, 1,3,5-hexatriene cannot be converted to 1,3-cyclohexadiene photochemically, but 2,4,6-octatriene does cyclize photochemically to (rans-5,6-dimethyl-l,3 Cyclohexadiene under the same conditions. ... 

Other related cyclic olefins, 1,3-cyclohexadiene and vinylcyclohexene, are reported to be formed as main products from butadiene over tungsten oxide catalysts [24]. Under the reaction conditions, hexatriene, resulting from the intermolecular metathesis of butadiene, underwent subsequent cyclization to 1,3-cyclohexa-diene ... 

Another example is provided by the cyclization of 1,3-cii-5-hexatriene to 1,3-cyclohexadiene. The reaction proceeds via a cyclic transition state (Figure 1) that may bind singly or doubly charged cations (e.g., Na" ", Li" ", Mg " ", Be " "). 

Figure 1 Reaction scheme for the cyclization of l,3-cir-5-hexatriene to 1,3-cyclohexadiene...

Electrocyclic reaction (Section 30.3) A unimolecular peri-cyclic reaction in which a ring is formed or broken by a concerted reorganization of electrons through a cyclic transition state. For example, the cyclization of 1,3.5-hexatriene to yield 1,3-cyclohexadiene is an electrocyclic reaction. 

Since, in a fluid medium, a molecule undergoes collisions with surrounding molecules at time intervals of sec. or less, photolysis of a compound in the condensed phase should produce the same results as photolysis at very high pressures in the gas phase. In line with this reasoning, photolysis of 1,3-cyclohexadiene in solution leads to very little benzene or C2 hydrocarbons. 1,3,5-Hexatriene is reported as the major product. The reverse reaction, viz. the cyclization of 1,3,5-hexatriene, is not observed in the condensed phase.In the gas phase, the net reaction... 

Consider the thermal reaction 1,3,5-hexatriene — 1,3-cyclohexadiene. (a) Use the symmetry of the polyene HOMO to predict whether the reaction path is conrotatory or disrota-tory. (The HMOs need not be found explicitly all that is needed is the signs of the AOs in the HOMO and these can be found by noting that the HMO nodal pattern is the same as the FE MO nodal pattern.) (b) Do the same as in (a) when the reaction occurs photochemically. (c) State the general rules for the cyclization of the polyene (16.9) with n ir electrons. 

In an entirely analogous way it would be possible to analyze the mechanism of any other pericyclic reaction. From a number of possible examples we confine ourselves to only several ical reactions, the selection of which was motivated both by the effort to cover the basic types of pericyclic processes and also to demonstrate the regular changes in the preferred mechanism with the increasing size of the system. The simplest example in this respect is the thermal cyclization of 1,3,5 hexatriene to cyclohexadiene. 

The thermal cyclization of cM-l,3,5-hexatriene to yield 1,3-cyclohexadiene is an example of an electrocychc reaction. C-1 and C-6 of the original polyene are linked by a O bond in the cyclic product. This reaction is known as the Cope rearrangement. 

The product of the photochemical cyclization of a polyene has a different stereochemistry from the product of the thermal cyclization process. This difference results from the motion of the terminal 2p orbitals of a different frontier molecular orbital. Ultraviolet radiation promotes an electron from the HOMO to the LUMO of the molecule. The LUMO of a 1,3,5-hexatriene is 71 it is antisymmetric. Because the signs of the terminal 2p orbitals on the same side of the molecule are opposite, the required motion of a bond formation is conrotatory. Let s compare the result of this motion for (2 , 4Z,6 )-octatriene with the thermal reaction. Conrotatory motion of the orbitals causes simultaneous conrotatory motion of the C-1 and C-8 methyl groups and yields rra 77r-5,6-dimethyl-1,3-cyclohexadiene (Figure 25.6b). 

Whereas inter- and intramolecular Diels-Alder reactions normally require electron-deficient dienophiles, the 67r-electrocyclization proceeds with a large vaiiety of substituents on a hexatriene. In one such approach, the intramolecular Heck-type reaction of a 2-bromo-1 -en-(ft> — l)-yne 66 is used as a trigger to initiate an intermolecular Heck coupling with an alkene to form the conjugated 1,3,5-hexatriene 67 which eventually cyclizes in a 67T-electrocyclic process (Scheme 3-21) [173]. In many cases, aromatization of the cyclohexadiene 68 formed primarily occurs to yield carbo- and heterobicyclic compounds of type 70 [173a,b]. But with alkyl ethenyl ethers the cyclohexadienes 69 can be obtained in moderate yields [173b]. 

The intramolecular carbopalladation starting from a 2-halo-1,6-enyne leads to a terminal aUcenylpalladium halide relay that can be trapped by an external alkene or alkyne. With the former, the cascade process leads to a 1,3,5-hexatriene that undergoes rapid 67T-electrocyclization to a flve-ring-annelated cyclohexadiene and this, in turn, is easily dehydrogenated to the corresponding aromatic compound (Scheme 15). With an external alkyne trapping the intermediate, a 1,3,5-hexatrienylpalladium intermediate will be formed and can either cyclize by intramolecular carbopalladation or 67r-electrocycliza-tion before termination by dehydropalladation will occur. ... 

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