Practice Problems in Pericyclic Reactions.
The following are a set of practice problems in Pericyclic Mechanisms,
prepared by Henry Rzepa for the for a second year course in Pericyclic Reactions at the
Department of Chemistry, Imperial College.
Each of these problems is intended to take between 1-3 hours to solve.
They are NOT typical of examination questions! Please note also that the compound numbers are not consecutive between questions.
Qu. 1. Cycloelimination of CO2 from 1 gives the transient species 2 (4n+2 electrons, Huckel transition state, thermally allowed). This is followed by either a [1,5] sigmatropic hydrogen shift (4n+2 electrons, Huckel t.s., thermal) to give 4, subsequently hydrolysed to 5, or by a p2s + p4s intramolecular cycloaddition to give the other product 3. 
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Qu. 2. Formed by cycloelimination of CO2 from A, via a 4n+2 electron Huckel transition state (thermally allowed). B then undergoes a 4n+2 electrocyclisation (Huckel t.s.) utilising either of the two single (alkene like) bonds of the three membered ring to form C or D. These can then undergo a 4n cycloelimination of PhCN (thermal, therefore must proceed via a Mobius transition state with one antrarafacial component) to give E. 
E can undergo an electrocyclisation (4n+2, Huckel, thermal) to give F, which can aromatise by TWO consecutive [1,5] sigmatropic hydrogen shifts (4n+2, Huckel, thermal) to give the final indole product G.
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Qu. 3. a) This is an example of the "Domino Diels Alder Reaction", L. A. Paquette et al, J. Am. Chem. Soc., 1978, 100, 5845.
b) J. Chem. Soc., Chem. Commun, 1988, 777.
Qu. 4. The preparation of "felicene", A. Gilbert and R. Walsh, J. Am. Chem. Soc., 1976, 98, 1606.
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Qu.5. a) The reaction is a [1,3] sigmatropic migration occuring thermally with 4n (n=1) electrons. Hence one antarafacial component must be present, and inversion at the starred atom occurs, resulting in the observed stereochemistry.
b) The absence of both the cyclopropyl and carbonyl groups implies cleavage of the 3-ring, most probably via either [1,3] or [3,5] sigmatropic migration. The presence of only one non-vinylic proton (at d3.76) in the nmr rules against the [1,3] migration and in favour of [3,5] thermal migration (4n electrons and hence one antarafacial component). Models show that this can be easily accomodated on the 3-atom component if bond a forms to the bottomface of the carbonyl p system and bond b breaks from the top face of the C(Ph)-C=O system. Both bonds form/cleave from the top face of the 5-atom component as shown below. This results in the cis ring junction stereochemistry.
See P J Battye, D W Jones and H P Tucker, J. Chem. Soc., Chem. Commun.,1988, 495.
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Qu. 6. DMAD is a reagent that reacts readily with dienes. Two independent 4n+2 electron thermal electrocyclic reactions convert ditropyl into two separate diene units, followed by 4n+2 thermal cycloaddition of the DMAD to give [B]. Four different stereoisomers are possible, but the two unsymmetrical forms are ruled out from the nmr evidence. Distinction between the two symmetrical forms could have been made on the basis of the coupling constant (3Hz), which is known from other examples to be ca 4Hz for the isomer shown and ca 9Hz for the alternative isomer. A double 4n+2 cycloelimination gives rise directly to [C] and [D]. [C] is an isomer of benzene, to which it rearranges via probably homolysis of the cyclopropyl bond.
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Qu 7. a) Comparison between reactant and product suggests they differ only in the position of one hydrogen (by 5 carbon atoms) and a C-C bond;
See valso DOI 10.1016/S0040-4020(01)87607-9.
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Qu. 8. The reversible nmr behaviour suggests an equilibrium between two species, involving the interconversion between an alkene (d 6.3-4.8) and an alkane (d3.3-1.4). At high temperatures, the chemical shift of two hydrogen is averaged to the mean position (d3.82). Hydrogenation 'traps' out the 'alkane', whilst heating 'traps' out the alkene by further (Cope) reaction.
A number of cis/trans isomers for the 10-ring cycloalkene are possible, of which models suggest one in which the new double bonds are formed trans is most likely (see below).The trans form can exist in two different conformations, for both of which the s bond cleaving and the s bond forming are suprafacial on each C3 fragment of the [3,3] sigmatropic rearrangement. The conformation resulting in trans stereochemistry for 6 must be the more stable;
When this species cyclises to 7, this conformation sets the bond up to form from the bottom face of one end of the octatetraene component, and the top face of the other end, to form the trans stereochemistry in the 4-ring. The other possible conformation appears less likely to result in the required stereochemistry;
The conformation where the two double bonds are formed cis looks like this;
A quantitative estimate of the relative stability of these three forms can be obtained by "molecular mechanics" modelling (ie using the MODEL program). Their energies are 114, 139 and 135 kJ mol-1 respectively, which conforms that the stereochemistries of the products are controlled by the conformational stability of the intermediate 10-ring.
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Qu. 9.
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Qu. 10.
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