Cyclopropenes, addition

Addition to cyclopropenes. Addition of allylindiums to the more substituted carbon atom of the double bond is observed. Carboxyl and hydroxymethyl groups have directing effect. 

Table 4. The reaction barriers (kcal/mol) for the cyclopropene addition to...

Table 5. Structural parameters for transition state structures for cyclopropene addition to furan computed with ab initio and DFT methods by using 6-31+G(d) basis set.

Table 8. Secondary Orbital Interactions (SOI) presented through bond orders between diene and dienophile moieties of transition state structures for cyclopropene addition to thiophene, thiophene 1-oxide, and thiophene 1,1-dioxide.

All computed transition state structures are for symmetric and synchronous formation of both C-C bonds. Some representations of the series of compounds are presented in Figure 1. The bond distances of the newly-forming C-C bonds varied from 2 to almost 2.2 A. They are characteristic transition state structures for Diels-Alder reactions [32]. The bond distances varied slightly for the two isomeric transition state structures, the exo and endo cyclopropene addition to thiophene 1,1-dioxide (Figure 1), demonstrating the stabilizing influence of steric repulsion interactions or SOI in exo and/or endo transition state structures. 

Let us now evaluate the activation barriers for cycloaddition reactions (Table 9). According to AM 1 computational studies, furan has a lower activation barrier than pyrrole, and pyrrole has a lower activation barrier than thiophene for cycloaddition reactions. This was the exact order of reactivity determined on the basis of the FMO changes going from reactant to transition state structures. For the cyclopropene addition to furan, the exo cycloadduct has a lower activation barrier and therefore, it should be the preferred product. For the cyclopropene addition to pyrrole, the computed activation barriers for two isomeric transition state structures were very close, indicating formation of a mixture of the products. On the other hand, as speculated on the basis of bond orders in the transition state structures between cyclopropene and thiophene, the endo transition state had a slightly lower activation barrier than its exo isomer. 

The transition state with the highest Bond Order Deviation (BOD) also has a higher energy. From the example of cyclopropene addition to benzo-fused furans, we can perfectly demonstrate this approach. The BOD for both exo and endo transition state structures of cyclopropene addition to benzo[6]furan were higher than for addition to benzo[c]furan (Table 24). On the other hand, the transition... 

Figure 6. Four possible transition state structures for cyclopropene addition to benzo[6] and benzo[c]furan computed by the AMI semiempirical method.

Let us now evaluate exo-endo selectivity in cyclopropene addition to benzo[c]heterocycles. On the basis of bond order analysis, the exo addition of cyclopropene to benzo[c]furan was selected over the endo addition while in the case of benzo[c]pyrrole and benzo[c]thiophene, it was suggested that SOI might be responsible for formation of an exo cycloadduct product. The computed activation energies for cyclopropene addition to the benzo[c]-fused heterocycle clearly favors formation of the exo cycloadduct. The activation barrier with benzo[c]furan was... 

Figure 7. Transition state structures for cyclopropene addition to 1,2-oxazole.

Table 31. The AMI and the B3LYP/6-31G(d) computed activation barriers for the acetylene, ethylene, and cyclopropene additions to 1,2- diazole, oxadiazole and thiazole...

The other method to determine reactivity for reactions with synchronous concerted cyclic transition state structures is evaluation of the transition state ring aromaticity through bond order deviation. The results of the exo cyclopropene addition to the heterocycles and to cyclopentadiene are presented in Table 33. The higher the sum of bond order deviation from average bond order (x) is, the lower aromatic character the transition state structure has. The most reactive dienophile was cyclopentadiene, followed by furan, and then heterocycles. The most reactive heterocycle with heteroatoms in 1,3-position was 1,3-oxazole as was predicted on the basis of the FMO energy changes (Table 32). The least reactive was 1,3-diazole, as one would expect on the basis of experimental observations. It is very difficult to rely on the transition state structure bond order deviation to determine the experimental feasibility of a reaction but, because SBOD for furan and 1,3-oxadiazole were very similar, one can conclude that the cycloaddition with 1,3-oxadiazole is also experimentally feasible. 

Transition state structures are for s3mchronous concerted mechanism of Diels Alder reactions and are very similar to each other. The two isomeric structures for the cyclopropene addition to 1,2,5-oxadiazole are presented in Figure 9. Sometimes the AMI semiempirical method can compute as3Tnmetric transition state structures that are, in energy, very close to the symmetric transition state structures presented in Figure 9. 

Figure 10. The AMI computed transition state structures for ethylene, ethylene, and cyclopropene addition to 1,3,4-oxadiazole.

As mentioned above, the cycloaddition reaction with 1,3,4-oxadiazole is predicted to be LUMO diene (heterocycle) controlled. That definitely suggests that with electron-withdrawing substituents in the two and five positions of the heterocycle ring, the heterocycle should become more reactive as a diene for Diels-Alder reactions. To study the usefulness of 1,3,4-oxadiazole and its derivatives as dienes for the Diels-Alder reaction, we are presenting the results of our theoretical study of the cyclopropene addition to 2,5-di(trifluoromethyl)-l,3,4-oxadiazole. The AMI computed FMO energy gap for this reaction pair was only 8.00266 eV in comparison to 9.64149 eV FMO energy gap between LUMO of 1,3,4-oxadiazole and HOMO of cyclopropene. Therefore, the computed activation barrier for the cyclopropene addition to 2,5-bis(trifluoromethyl)-1,3,4-oxadiazole should be very... 

Table 46. Bond order uniformity for six-membered transition state structure of an exo and endo cyclopropene addition to 4,4-dimethyl-[4H]-1,2-diazole computed with the AMI method.

To confirm this assumption, we have computed activation barriers for acetylene, ethylene and cyclopropene addition to 4,4-dimethyl-[4H]-l,2-diazole. To our delight, the B3LYP/6-31G(d)/AMl computed activation barrier for ethylene addition to 4,4-dimethyl-[4//]-1,2-diazole is almost identical (Table 47) to the value obtained with full B3LYP/6-31+G(d) calculation on [4H]-1,2-diazole as dienophile (Table 44). The activation barrier for the acetylene addition is 22.3 kcal/mol indicating that this reaction should be also experimentally feasible. As indicated... 

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