Ring opening of cyclobutenes

Symmetry forbidden reaction (Section 10 14) Concerted re action in which the orbitals involved do not overlap in phase at all stages of the process The disrotatory ring opening of cyclobutene to 1 3 butadiene is a symmetry forbidden reaction... 

There are several general classes of pericyclic reactions for which orbital symmetry factors determine both the stereochemistry and relative reactivity. The first class that we will consider are electrocyclic reactions. An electrocyclic reaction is defined as the formation of a single bond between the ends of a linear conjugated system of n electrons and the reverse process. An example is the thermal ring opening of cyclobutenes to butadienes ... 

Table 13-1. Computed reaction barriers and isomer stabilities [kcal/mol] for the electrocyclic ring opening of cyclobutene (relative to cyclobutene 1, including zero-point vibrational contributions). Except for G2, the results were obtained using the 6-311+G(d,p) basis set.

Ring-opening of cyclobutenes to butadienes is very common a recent example is the formation of the aldehyde 6 in greater than 97% diastereomeric purity from the cyclobutene 5 (R = 4-methoxybenzyl) above —78 °C (equation 5)5. 

The two reactions can be separately carried out. The cyclobutene must be thermodynamically unstable with respect to the diene. Therefore in a thermal process only a ring opening of cyclobutene is observed. But under photochemical conditions dienes are more efficient in absorbing radiant energy than are simple alkenes. This is why ring closure of dienes can be carried out in high yields photochemically. 

We are also carrying out calculations 40) for the electrocyclic ring opening of cyclobutene (27) and bicyclobutane (28) to 1,3-butadiene (29). While.this work is not yet complete, we have established that 27 opens preferentially by... 

In the reverse transformation, the ring opening of cyclobutene to gi butadiene, the conversion involves net transformation of a c-bonding orbital to --bonding orbital. The 4 orbitals of cyclobutene are a, a and tc, tS (Figure 8.6). The 4 orbitals of the product are the familiar MO s of... 

Figure 7.17 Orbital correlation diagram for conrotatory and disrotatory ring openings of cyclobutenes.

The conrotatory movement of groups is typical of thermal ring openings of cyclobutenes and other rings involving 4n electrons. 

Intermediates (6) can yield to the corresponding [3-lactams via conrotatory electrocyclic closures (Fig. 3). The main features of these transition structures closely resemble those found for the thermal electrocyclic ring opening of cyclobutenes [34]. 

---