Cyclobutenes from butadienes

The forbidden nature of the disrotatory process rests in the crossing of the 7t( 3) and 7t (02) molecular orbitals. The n electrons in cyclobutene occupy an orbital correlated with an antibonding orbital ( 3) of butadiene. To construct butadiene from cyclobutene, electron pairs must flow into the two bonding orbitals, tfii and 2. Similar to cyclobutanation, then, this process could conceivably be rendered allowed if an electron pair were removed from the 1/13) orbital and an electron pair added to the 7r ifiz) orbital as the disrotatory process proceeded. A transition metal could effect these operations through properly ordered dyz and dzx orbitals. This process is illustrated in Fig. 9. 

The photochemical disrotatory closure of butadiene to cyclobutene has been described with a state-correlation diagram, like that shown in Figure 21.4. It is based on the familiar orbital-correlation diagram of Woodward and Hoffmann," from which the intended correlations indicated by the dashed lines can readily be deduced. The solid lines indicate that there is an avoided crossing, which is put in as a result of the quantum mechanical noncrossing rule. It says that two states of the same total symmetry cannot cross. Instead, as they approach each other in energy, they will mix and separate, as the solid lines indicate. 

Fig, 4,18 The stereochemistry of many reactions is easily predicted from the symmetry of molecular orbitals, usually the highest occupied n MO (n HOMO). In the ring closure of 1,3-butadiene to cyclobutene the phase (+ or —) of the HOMO (i//2) at the end carbons (the atoms that bond) is such that closure must occur in a conrotatory sense, giving a definite stereochemical outcome. In the example above there is only one product. The reverse process is actually thermodynamically favored, and the cis dimethyl cyclobutene opens to the cis, trans diene. No attempt is made here to show quantitatively the positions of the energy levels or to size the AOs according to their contributions to the MOs... 

In conrotatory ring opening, the reoriented a orbitals derived from cyclobutene look like part of the butadiene molecular orbitals 1 2 and %. The orbitals derived from the double bond of cyclobutene look like part of the butadiene molecular orbitals and 3. Because the signs of the cyclobutene orbitals can be correlated with bonding orbitals of butadiene by... 

Bicyclo[1.1.0]butane is usually a side product of the photocyclization of butadiene to cyclobutene (Srinivasan, 1963) in isooctane, the quantum yield ratio is I 16 (Sonntag and Srinivasan, 1971). It becomes the major product in systems in which the butadiene moiety is constrained near an s-trans conformation and bond formation between the two terminal methylene groups that leads to cyclobutene is disfavored. An example is the substituted diene 88 in Scheme 30, for which the bicyclobutane is the major product a nearly orthogonal conformation should result from the presence of the 2,3-di-r-bu-tyl substituents (Hopf et al., 1994). 

The orbitals processes described above are quite independent of the electronic state of the reaction system, so can be used without change for the photochemical process. In this case, one electron is promoted from the HOMO to the LUMO of both butadiene and cyclobutene, so that, although the orbitals correlations are not changed, we now have three different types of MOs doubly occupied, singly occupied and unoccupied. This means that, instead of the one border between doubly occupied and unocuppied MOs found for the thermal reaction, we now have two that may not be crossed by an orbital correlation line. This leads to the two diagrams shown in Fig. 4.17. 

The Woodward-Hoffmann (W-H) rules are qualitative statements regarding relative activation energies for two possible modes of reaction, which may have different stereochemical outcomes. For simple systems, the rules may be derived from a conservation of orbital symmetry, but they may also be generalized by an FMO treatment with conservation of bonding. Let us illustrate the Woodward-Hoffmann rules with a couple of examples, the preference of the 4 + 2 over the 2 + 2 product for the reaction of butadiene with ethylene, and the ring-closure of butadiene to cyclobutene. 

Not only must we consider the symmetry properties of 1,3-butadiene orbitals, but we must also consider the s)mtunetry properties of both cyclobutene and the transition structure expected for the conversion of the reactant to product. The two pathways for the closure of 13-butadiene to cyclobutene are illustrated in Figure 11.16. The Cl—C2, C2—C3, and C3—C4 a bonds are shown as solid lines. The p orbitals of 1,3-butadiene and cyclobutene, as well as the sp orbitals of the C3—C4 cr bond of cyclobutene, are represented by the shapes of the atomic p or sp orbitals. This is therefore only a basis set representation, not an illustration of a particular molecular orbital. Although there are many symmetry elements present in the representations of both 1,3-butadiene and cyclobutene, in the conrotatory reaction the only symmetry element that is present continuously from reactant through transition structure to product is the C2 rotation. Similarly, only the a reflection is present from reactant through transition structure to product for the disrotatory pathway. 

State correlation diagram for conrotatory interconversion of 1,3-butadiene and cyclobutene. (Adapted from reference 31.)... 

Extension of the original formalism of the overlap determinant method to photochemical reactions [56] can again be best demonstrated by reactions the course of which is governed by the Woodward-Hofimaim rules. In this case the most important result is the exact reproduction of the reversal of the stereochemical course of the reaction in comparison with analogous thermal processes (see Table 1). As an example let us analyze first the photochemical isomerization of 1,3 butadiene to cyclobutene. The most important modification enters into the formalism at the level of the construction of the irreducible core, where it is necessary to respect the fact that the reactant does not enter into the reaction in the ground, but in the excited state. This circumstance finds its reflection in that one of the n bonds entering into the irreducible core from the part of the butadiene is to be replaced by the... 

The practical exploitation of the proposed criterion can be very simply demonstrated by the example of the electrocyclic transformation of butadiene to cyclobutene, for which the structure of the possible intermediates can be quite reliably estimated from the available results of quantum chemical calculations [123]. This reaction is especially convenient for the demonstration purposes since it displays both possible types of the dissection of the More O Ferrall diagrams [121] as schematically given in Figs. 9 and 10. Especially interesting is, above all, the case of forbidden disrotatory cyclization, for which the special form of the dissection allows the classification of the reaction mechanism even without the knowledge of the reaction path. As can be seen from the Fig. (9) no reaction path coimecting the reactant with the product can avoid the region of the intermediate so that the reaction has to be classified as nonconcerted. 

As can be seen from this comparison, the resulting values are affected by the choice of the critical structure and on going from X(n/4) to X(-7t/4), the systematic shift of the dominant similarity from the zwitterionic state Z + Z2 to the state Zj -Z2 is observed. We can thus see that the predictions for both types of critical structures differ and the problem thus appears which of the above two critical structures should be regarded as a true model of the transition state in forbidden reactions. Similarly as in the case of allowed reactions such a decision does not arise from the approach itself, but some external additional information is generally required. This usually represents no problem since the desired information can be obtained, as in the case of allowed reactions, from the simple qualitative considerations based on the least motion principle [80,81], or from the direct quantum chemical calculations.This is also the case with us here, where the desired information is provided by the quantum chemical study [63] of the thermally forbidden cyclization of the butadiene to cyclobutene. From this shufy it follows that the ground state of the transition state should correspond to the ground state of the cyclobutadiene which is the Zj - Z2 state. 

Electrocyclic reactions are not really different from cycloadditions. Figure 20.27 compares the equilibration of 1,3-butadiene and cyclobutene with the 2 + 2 dimerization of a pair of ethylenes. The only difference is the extra o bond in butadiene, and this bond is surely not one of the important ones in the reaction—it seems to be just going along for the ride. Why should its presence or absence change the level of detail av able to us through an orbital symmetry analysis It shouldn t, and in fact, it doesn t. 

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