7t-system and the terminal methyl, although the effect of the alkyl group on the oxygen
atom is included in the parameters of the latter. Calculations on 76 do not take into
account any hyperconjugation of the methylene groups into the 7t-system of the 2,4a,5,6-
tetrahydrophenanthrene substrates. In order to gauge the effect of these neglected
groups, calculations were carried out on the methyl-substituted parent system 77 (figure
2-9 and table 2-2).
(figure 2-9)
Compound
^Pa
£pb
Fa*
Fb*
EF*
77 R = H
0.6887
1.22001.0434
0.5121
1.5555
77 R = CH
30.8643
1.2155
0.8678
0.5166
1.3844
Comparison of these two sets of calculations shows very little difference in the final value of EF* (less than 5%). From these results it can be seen that the potential for the photochemical cyclisation is greatest in the unsubstituted diene 75, where ZF* > 1 .5 0
and so this substrate was chosen for initial studies of this reaction.
The 1-aryl-1,3-butadiene derivative 75 was dissolved in benzene, deoxygenated and irradiated under a nitrogen atmosphere using a medium pressure mercury lamp.55 After 24 hours of photolysis it was found that no reaction had occurred. The addition of 2 equivalents o f a triplet sensitiser to the reaction also resulted in no observable photocyclisation. The lack o f cyclisation when the triplet sensitiser was used is in accordance with studies that have been carried out on the photochemical cyclisations of stilbenes, which have tentatively identified the excited singlet state as the precursor to the cyclised product.68
A possible explanation for this apparent lack of reactivity is that an equilibrium is created between the starting material 75 and the cyclised product 78. If this is the case, the equilibrium greatly favours the starting material 75. Early work carried out on the cyclisation o f 1,4-diphenyl-1,3-butadiene found that the reaction occurred under anaerobic conditions, but greater yields of the cyclised products were formed when the reaction was carried out under aerobic or oxidising conditions.62 The reaction was repeated under aerobic conditions in an attempt to trap any cyclised product as the naphthalene derivative 79 (figure 2-10),
(figure 2-10) but no reaction was observed under these conditions.
Attempts to cyclise the methyl substituted compound 74 were also undertaken. No cyclisation was observed, but a cis-trans isomerisation did occur.* This photochemically induced isomerisation is a common reaction for olefins and is well documented. The cis-
trans isomerisation of stilbene has been studied extensively52 and is thought to proceed
through the triplet excited state. The direct absorption of light by either isomer results in
* The starting olefin was a mixture o f the cis and trans isomers in a ratio o f approximately 1 : 1 . After irradiation the ratio was found to have changed to 2 : 3.
the conversion of the molecule from the singlet ground state (So) to the corresponding singlet excited state (Si). The So —» Si transition is allowed by selection rules. Internal conversion then gives rise to the corresponding cis and trans triplet states (Ti) which may interconvert and subsequently revert back to the ground state species
(figure 2-11).
H H H Ph
}“ (
Sl)= (
SlPh Ph Ph H
(figure 2-11)
The fact that this cis-trans isomerisation occurs implies that excitation of the molecule to the singlet state is occurring, which then gives rise to the triplet excited state through internal conversion. Interestingly, the formation of cyclobutene products was also not observed in the irradiation of the two starting dienes, 74 and 75, probably due to the steric interactions in the molecule. From these results the formation of the tetracyclic ring system from the two tetrahydrophenanthrene compounds 74 and 75 appears to be unlikely.
In order to test this reaction on other substrates, the ester 80 was synthesized in good yield from the unsaturated aldehyde
57
by standard methods69(figure 2-12).
(Et0)2P(0)CH2C02Et NaH, -20°C to R.T.
(Figure 2-12)
Attempts to cyclise the ester
80
were made, as the extra conjugation o f the ester group could favour the required 67t-electrocyclic reaction (cf 1,4-diphenyl-1,3-butadienes). SHMO calculations on the two methyl l-aryl-l,3-butadiene-4-carboxylate analogues81
and82
were carried out to find the free valence indices of the carbons, Ca and Cb, which would become bonded during the cyclisation reaction(figure 2-13
andtable 2-3).
(figure 2-13)
compound ^Pa ^Pb Fa* F b* IF *
81
1.1798 1.2555 0.5523 0.4766 1.028982
1.1845 1.2481 0.5476 0.4840 1.0316(table 2-3)
In these calculations the ester group is taken as a combination of a carbonyl group66 and a methoxy group.67’i- It was found from these calculations that IF * for the two carbons which would be involved in the cyclisation was still greater than 1 (though only just) and so, from the qualitative rules described previously, the cyclisation should still be allowed. Irradiation under anaerobic conditions o f the
trans
-unsaturated ester80
resulted in an inseparable mixture of thecis
isomer83
and the starting material80
in approximately a1 : 1 ratio,
(figure 2-14).
(figure 2-14)
Irradiation of the unsaturated ester
80
under aerobic conditions again resulted in a mixture of thecis
andtrans
compounds with no sign of the naphthalene derivative84
(figure 2-15).
t A SHMO treatment has been performed on the carboxyl groups of unsaturated acids.70 In this treatment the effect of the hydoxyl group on the rc-energies is constant for all the acids. This assumption allowed the author to ignore this group in the calculation as only relative quantities were required. However in our use o f the SHMO a more exact calculation is required.