Interconversion, 1,3-diene-cyclobutene

The 1,3-diene—cyclobutene interconversion can even be applied to benzene rings. For example,379 photolysis of l,2,4-tri-/-butylbenzene (92) gives l,2,5-tri-/-butyl[2.2.0]hexadiene (93, a Dewar benzene).380 The reaction owes its success to the fact that once 93 is formed,... 

Fig. 18.4. The 1,3-diene-cyclobutene interconversion. The orbitals shown are not molecular orbitals, but a basis set of p-atomic orbitals, (a) Disrotatory ring closure gives zero sign inversion, (b) Conrotatory ring closure gives one sign inversion. We could have chosen to show any other basis set (e.g., another basis set would have two plus lobes above the plane and two below, etc.). This would change the number of sign inversion, but the disrotatory mode would stiU have an even number of sign inversions, and the conrotatory mode an odd number, whichever basis set was chosen.

The 1,3-diene-cyclobutene interconversion can even be applied to benzene... 

The interconversions of the corresponding dimethylcyclohexadienes and the 2,4,6-octatrienes are also stereospecific (Fig. 29.13). Here, too, thermal and photochemical reactions differ in stereochemistry. If we examine the structures closely, we see something else the stereochemistry of the triene-cyclohexadiene interconversions is opposite to that of the diene-cyclobutene inlerconversions. For the thermal reactions, for example, cis methyl groups in the cyclobutene become cis and trans in the diene cis methyl groups in the cyclohexadiene are trans and trans in the related triene. 

Remarkably, it was discovered that the stereochemical outcome of the reaction was dependent on the energy source. Heat gave one stereochemical result and light another. Figure 20.7 sums up the results of the thermal and photochemical interconversions of cyclobutenes and butadienes. In the thermal reaction, a cis 3,4-disubstituted cyclobutene yields the cis,trans diene. By contrast, in the photochemical process the same cis cyclobutene yields a pair of molecules, the cis,cis diene and the trans,trans diene. 

FIGURE 20.9 In the interconversion of cyclobutene and butadiene, the orbitals at the end of the diene must rotate so as to form a bond, not an antibond. 

An electrocyclic reaction is as easy to analyze as that. Identify the HOMO, and then see whether conrotatory or disrotatory motion is demanded of the end carbons by the lobes of that molecular orbital. All electrocyclic reactions can be understood in this same simple way. The theory tells us that the thermal interconversion of cyclobutene and 1,3-butadiene must take place in a conrotatory way. For the cyclobutene studied by Vogel, conrotation requires the stereochemical relationship that he observed. The cis 3,4-disubstituted cyclobutene can only open in conrotatory fashion, and conrotation forces the formation of the cis,trans diene. Note that there are always two possible conrotatory modes (Fig. 20.12), either one giving the same product in this case. 

The reverse reaction is also conrotatory. In contrast, the photochemical cyclobutene 1,3-diene interconversion is disrotatory in either direction." On the other hand, the... 

This time it is conrotatory movement that results in a bonding situation, and formation of the trans dimethylcyclobutene (18). For the photochemical interconversion (which tends to lie over in favour of the cyclobutene), irradiation of the diene will result in the promotion of an electron into the orbital of next higher energy level, i.e. 3, and the HOMO to be considered now therefore becomes i/r3 (23) ... 

Heinze, MacNeil and coworkers developed the first methodology for the preparation of isomerically pure E- and Z-perfluoro-l,3-pentadienes, as described in equations 56 and 5749,50. Similar methodology was employed for the preparation of related perfluoro-3,5-octadiene systems49. These dienes were key components for studying the kinetic and thermodynamic electrocyclic interconversions of perfluorinated dienes and cyclobutenes,... 

If the motion had been disrotatory, this would still have been evidence for a cyclic mechanism. If the mechanism were a diradical or some other kind of noncyclic process, it is likely that no stereospecificity of either kind would have been observed. The reverse reaction is also conrotatory. In contrast, the photochemical cyclobutene—1,3-diene interconversion is disrotatory in either direction.368 On the other hand, the cyclohexadiene—1,3,5-triene interconversion shows precisely the opposite behavior. The thermal process is disrotatory, while the photochemical process is conrotatory (in either direction). These startling results are a consequence of the symmetry rules mentioned in Chapter 15 (p. 846).Vl,As in the case of cycloaddition reactions, we will use the frontier-orbital and Mdbius-HQckel approaches.37"... 

Consequently, it is not surprising that the rate ratio for solvolysis of 99 100 was found to be greater than 2.5 x 106 and that at 150°C 100 did not solvolyze at all.395 This evidence is kinetic. Unlike the cases of the cyclobutene—1,3-diene and cyclohexadiene—1,3,5-triene interconversions, the direct product here is a cation, which is not stable but reacts with a nucleophile and loses some of its steric integrity in the process, so that much of the evidence has been of the kinetic type rather than from studies of product stereochemistry. However, it has been shown by investigations in super acids, where it is possible to keep the cations intact and to study their structures by nmr, that in all cases studied the cation that is predicted by these rules is in fact formed.396... 

Cyclization of 1,3,5-hexatriene occurs only when the central double bond has the cis configuration. The reaction is reversible at elevated temperatures because of the gain in entropy on ring opening (see Section 4-4B). The cyclobutene-1,3-butadiene interconversion proceeds much less readily, even in the thermodynamically favorable direction of ring opening. However, substituted dienes and cyclobutenes often react more rapidly. 

How can we account for the stereoselectivity of thermal electrocyclic reactions Our problem is to understand why it is that concerted 4n electro-cyclic rearrangements are conrotatory, whereas the corresponding 4n + 2 processes are disrotatory. From what has been said previously, we can expect that the conrotatory processes are related to the Mobius molecular orbitals and the disrotatory processes are related to Hiickel molecular orbitals. Let us see why this is so. Consider the electrocyclic interconversion of a 1,3-diene and a cyclobutene. In this case, the Hiickel transition state one having an... 

An example of a pericyclic reaction that illustrates the rather amazing stereoselectivity that these reactions often exhibit is provided by the thermal and photochemical interconversions of dienes and cyclobutenes. When (2A,4fi )-hcxadicnc is heated, it cyclizes to form /rans-3,4-dimethylcyclobutene. None of the cis-isomer is produced. In the reverse reaction the cyclobutene opens to produce only the ( , j-isomcr of the hexadiene. The reaction is completely stereospecific in both directions ... 

Let s begin by considering the simplest electrocyclic reaction, the thermally induced interconversion of a diene and a cyclobutene. As illustrated in the following example, the reaction is remarkably stereospecific, occurring only by a conrotatory motion ... 

The analysis in Section 22.3 indicates that the thermal interconversion of a diene with a cyclobutene should occur by conrotation. The reaction is allowed in both directions, as long as a conrotatory motion is followed. However, usually only the conversion of the cyclobutene to the diene is observed because the cyclobutene is destabilized by angle strain and is present only in trace amounts at equilibrium. An example of the opening of a cyclobutene to form a diene is provided by the following equation ... 

Only the singlet state interconversions of conjugated dienes and cyclobutenes are taken into consideration here, because the triplet state is generally not involved in this process.596 There are two important factors that affect electrocyclization (1) reaction... 

MO correlation diagram for conrotatory interconversion of 1,3-buta-diene and cyclobutene (C2 symmetry). (Adapted from reference 14.)... 

A further well-known elect rocydie process is the interconversion of cyclohexa-1,3-dienes and hexa-l, 3,5-trienes, of which reaction (4.7) is the prototype. As in the cyclobutene-butadiene transformation, reaction will... 

Similar behavior was exhibited by the bicyclo[ . 2.1] alkenes 81, except that the highly strained meth-ylenecyclopropane 83a was not formed. Surprisingly, 81b and 81c afforded an E,Z-mixture of the 1,3-dienes 82b and 82c under conditions in which the dienes do not undergo interconversion. Thus, ring opening of cyclobutenes is not stereospecific. 

---