Cyclobutene stereochemistry

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 ... 

The ring closure of a diene to a cyclobutene can occur with rotation of the two termini in the same conrotatory) or opposite disrotatory) directions. For suitable substituted compounds, these two reaction modes lead to products with different stereochemistry. 

Would you expect the following reaction to proceed in a con rotatory or dis-votalory manner Show the stereochemistry of the cyclobutene product, and explain your answer. 

Ring-opening of the frans-cyclobutene isomer shown takes place at much lower temperature than a similar ring-opening of the cis-cyclouiitciie isomer. Explain the temperature effect, and identify the stereochemistry of each reaction as either conrotatory or disrotatory. 

The stereochemistry of the cyclobutene isomerizations and the reverse processes of this type, involving the formation of a bond between the ends of a linear system containing a number of 7i--electrons, has been discussed by Woodward and Hoffmann (1965). They term such processes electrocyclic and consider that their steric course is determined by the symmetry of the highest occupied molecular orbital of the open-chain isomer. In an open-chain system containing 4 7T-electrons (such as butadiene), the symmetry of the highest occupied ground-state orbital is such that bonding interaction between the ends of the chain must involve overlap between orbital envelopes on opposite faces of the system, and this can only occur in a conrotatory process ... 

Cyclobutenes are effective dienophiles.19-21 The stereochemistry of these reactions has been studied by employing 2,5-dimethyl-3,4-diphenylcyclopenta-2,4-dienone as the diene. Thus, r/.v-3,4-dich 1 orocyclobutene reacted with the dienone to give the anri,exo- and the anti,endo- so-mers 15 in a ratio of 4 1.22 Similar cycloaddition of the dienone with dimethyl tricy-clo[4,2,2.02 5]dcca-3,7,9-triene-7,8-dicarboxylatc gave the exo- and the rwfo-isomers 16 in a ratio of 6 1.22 The exo stereoselectivity is attributed to steric factors.22... 

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... 

Electrocyclic reactions, 163, 165 butadienes to cyclobutenes, 164-165 component analysis, 168 stereochemistry, 165 Electron... 

One of the many photoreactions of conjugated dienes is the formation of cyclobutenes,99 and this has been reported both in cyclic and acyclic dienes. The stereochemistry of such photocyclizations has been discussed in terms of orbital symmetry by Woodward and Hoffmann.100... 

This was the first instance in which cyclization of a 1,3-diene to a cyclobutene was observed. This type of cyclization has proved to be of considerable generality and utility (see Sec. IVD). It is a striking fact that in the ergosterol series 9,10-cis-stereochemistry (Formulas 324 and 325) leads to valence tautomers (Formulas 326 and 327), while 9,10-traws-stereochemistry (Formulas 317 and 320) leads to ring fission (Formula 318). 

The stereochemistry of opening of cyclobutenes to butadienes was established some time before the advent of the pericyclic theory.88 It is conrotatory, in accord with the theory, as illustrated by the examples shown in Equations 12.46 and 12.47.89 Because the equilibrium in the monocyclic case favors the diene, the... 

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. 

The fact that the reactions take place in the direction of ring-opening is determined by thermodynamics, but the stereochemistry is most certainly not, for the cyclobutene 4.36 gives the thermodynamically more strained product 4.37 with one of the double bonds cis. Thermodynamics affects the stereochemistry only with the opening of the cyclobutene 4.38, which shows a preference for one of the conrotatory modes, that giving the trans,trans diene 4.39, where the rules could have led to the cis,cis diene equally well. This type of selectivity is called torqueoselectivity. 

In this example the ring system is compatible with the allowed stereochemistry the disrotatory equilibrium between 4.55 and 4.56 has no problems. On the other hand, rings can constrain or even prevent allowed electrocyclic reactions. In the cyclobutene 4.57, for example, the ring fusion... 

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... 

We now know that the picture is not as simple as this, syn Sn2 reactions are preferred but anti S>j2 reactions are also possible and the result found depends on the molecule under observation. Here is a convincing example of Sn2 reactions going with syn stereochemistry. The molecule is a planar cyclobutene, which makes the stereochemistry easy to see. 

Cyclobutene has been reported to undergo 1,3 cycloaddition to benzene under photolysis conditions.355 Although the endo stereochemistry of the cyclobutane ring in this adduct is assumed (see 371), pyrolysis of the hydrocarbon at ca 250 °C gave dihydrotriquinacene 372 as the major product. Using cis-3,4-dimethylcyclo-butene, 373 was obtained and similarly transformed to 374 without loss of stereochemistry by thermolysis.376 ... 

This chiller is restricted to a short, but by no means complete, review of key synthetic routes to cyclobutenes, bmzocyclobutenes and cyclobutenones and a generally qualitative discussion of the way in which substituents control both the ease of ring opening and the stereochemistry of the products obtained. The reader should thus be in position to make useful predictions. Finally we have included pertinent synthetic applications which illustrate in useful and often very imaginative ways the value of the... 

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