Problem+2



DIFFICULTY: med-high, 7/10 LEVEL: Third semester undergraduate org chem. 1-2 year graduate student

Back to HOME page

__SOLUTION & DISCUSSON__

To analyze this problem, first compare the components of 1 still present in the three products. Nitrogen atoms are gone, likely as N2. All four skeletal C's are there, and so are the two aryls and the methoxycarbonyl in 2 & 3. But in 4 this latter group is concealed within the furan ring. Another oddity of 4 is the CH3 on C4 of furan. Conceivably, 2 and 3 are probably products of a similar reaction intermediate that has two outcomes from a given point. After all they are E/Z isomers. But 4 results from a diverging route.

Let’s discuss first compounds 2 & 3. The cyclopropane suggests the intermediacy of radical species. In fact, the parent heterocycles of 1 such as model compound 5 are known to yield diradical species 6 and/or 7 by photolysis or heat. These diradicals evolve further to furnish a series of cyclopropanes: Diradical 7 may have three different outcomes a, b, c depending on electron rearrangement and orbital overlap. Direct coupling (a) furnishes cyclopropane 8. Obviously the exocyclic C=C is not involved. Either 6 or 7 are delocalized allyl radicals meaning that the C-R3 carbon has C **. ** character. This is shown in resonance species 9 whose coupling yields 10. By the same token, 9 is an allyl radical in the in the right hand side direction, which justifies the construction of 11. In short, thermolysis or photolysis of heterocycle 5 with the exo C=C bond has been observed to furnish cyclopropanes 8, 10 and 11 by the said diradical mechanisms.

Now, the introduction of an electron withdrawing group (Egw, replacing R3) may drive the C-N bond breaking step towards polarization. Without necessarily reaching the stage of a zwitterion such as 13, a dipolar structure (14) drives the reaction preferentially to 15

Note that 15 resembles products 2 & 3 of this problem, where the COOCH3 would be the Ewg, which it is, no doubt. So this solves this part of the problem.

As for furan 4, two points need consideration. 1.- The furan oxygen must proceed from the carboxylic ester, there is no other around. So there ought to be a O-alkylation step on a d + C atom. Note that this is the benzylic carbon of the p-methoxyphenyl unit. 2.- Hamaguchi et al observed that electron donors on the p-position of this arene group promote the formation of furan 4 from traces to the major component of the mixture. This fact suggests that the p-CH3O- group stabilizes the said benzylic d + C.

This allows us to propose that 4 is the result of the sequence shown below. Note that in dipolar intermediate 17 the oxygen anion is five atoms removed from the benzylic C+ conveniently resonance stabilized by the p-CH3O. A [1,3] hydrogen shift in 17 reintroduces the C=C to the furan ring under the driving force of aromaticity. This is our ‘misterious’ methyl.

Did you like it? You still have two things to work out: 1.- While cyclopropanes 2 and 3 are obtained in a 1:1 ratio in p-Ewg-aryls, compound 3 predominates in p-Edg-aryls (electron donating groups there) 2.- Compound 3 rearranges to 4 under heat. So there is the possibility that 4 is a secondary product, not a primary one.

Back to HOME page