Table 4.
Calculated Activation Energies and Experimental Cycloadduct Ratios for the (4+3) Cycloadditions of 2Ph with 2-Substituted Furansa
| syn-I | anti-I | syn-II | anti-II | |
|---|---|---|---|---|
| Reactions in the absence of ZnCl2 | ||||
| 2-Methylfuran | ||||
| Δ H ‡ | 6.2 | 7.5 | 6.7 | 7.8 |
| Δ G ‡ | 20.4 | 22.0 | 20.9 | 22.2 |
| Experimental | 67% | 23% | 7% | 3% |
| Methyl 2-furoate | ||||
| Δ H ‡ | 5.3 | 8.7 | 3.4 | 9.3 |
| Δ G ‡ | 20.5 | 23.2 | 19.3 | 23.3 |
| Experimental | 100% | |||
| Reactions in the presence of ZnCl2 | ||||
| 2-Methylfuran | ||||
| Δ H ‡ | −2.9 | −1.7 | −1.9 | −0.6 |
| Δ G ‡ | 11.7 | 13.6 | 11.7 | 13.4 |
| Experimental | 80% | 20% | ||
| Methyl 2-furoate | ||||
| Δ H ‡ | −1.1 | 4.1 | −1.7 | 5.4 |
| Δ G ‡ | 14.1 | 18.5 | 13.9 | 18.4 |
| Experimental | 30% | 70% | ||
| 2-Cyanofuran | ||||
| Δ H ‡ | 2.3 | 6.1 | 3.4 | 7.6 |
| Δ G ‡ | 17.0 | 20.9 | 17.7 | 21.4 |
| Experimental | 83% | 17% | ||
a
B3LYP/6-31G(d), 298.15 K, kcal/mol; Experimental isomer ratios given as percentages.