Cyclobutene opening

As a first example of an electrocyclic reaction illustrating stereochemistry, let us take the pair of conrotatory cyclobutene openings, showing that the reactions are stereospecific. 

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... 

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 ... 

To explain the increase in the rate of the cyclobutene opening 6.292 —> 6.293, we need to remember that the conrotatory pathway will have a Mobius-like aromatic transition structure, not the anti-aromatic Hiickel cyclobutadiene that we saw in Fig. 1.38. We have not seen the energies for this system expressed in 8 terms, nor can we do it easily here, but the numbers are in Fig. 6.42, where we can see that a... 

Fig. 6.42 7t-Energy changes for a conrotatory cyclobutene opening with a substituent on C-3...

From a synthetic point of view it is useful to know not only the direction of rotation of the substituents, i.e. the stereochemistry of the diene produced, but also the temperatures at which cyclobutenes open at synthetically useful rates. Much of the early work in this area has been done without the synthetic aspect in mind and some of the temperatures quoted below may be of value only for comparison purposes. 

The cyclobutene opens in a four-electron conrotatory electrocyclic reaction. The cyanide can go either way and it will prefer to go out. The diene so produced is unstable and is extremely reactive in 3 cloadditions as it regenerates the benzene ring that way. The regiochemistry is predictably ortho md we should probably use the HOMO of the diene and the LUMO of the unsaturated ester. 

Studies of the asymmetric photocycloadditions of the 1,3-dioxacyclohexenones (-)-20 or (-)-23, respectively with methyl cyclobutene opened a new route for the syntheses of enantiomerically pure (—)- and (-i-)-grandisol (19)81,82. This approach adopts the use of stereofacial differentiation by a rigid spirocyclic arrangement of the auxiliary and the enone. Since the alkene adds preferentially to face a of both enones, (— )-20 and (—)-23, Complementary enantiomeric product channels are available operating from a single enantiomer of the auxiliary in each case. The photocycloaddition reactions proceeded in a head-to-head fashion (HH/HT 7 1 at — 78 °C) to give tricyclic products with cis-anti-cts stereochemistry ( + )-21 and (+)-24, respectively. The auxiliary, ( —)-menthone, was smoothly removed by formic acid treatment of the cycloadduct (+)-21, no epimerization of the acetyl group in (— )-22 was observed under these mild conditions. 

Steric factors would ordinarily be expected to induce a preference for the larger group to move outward, thus generating the F-isomer. It was observed, however, that in the case of l,2,3,4-tetrafluoro-trari5-3,4-bis(trifluoromethyl)cyclobutene, opening occurred with an inward rotation of the trifluoromethyl groups. ... 

Energy Geo- metry 1,5 H shift Cyclobutene opening Diels- Alder... 

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