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These reactions are assumed to proceed via a reaction sequence of [3+2] cycloaddition of the N-ylide with the dichloroalkene and subsequent elimination of two molecules of hydrogen chloride under the reaction conditions (Scheme1). The [3+2] cycloadditions may take place either by direct concerted1,3dipolarcycloadditionbetweentheN-ylideand the alkene25or by a primary Michael addition of the ylide to the alkene26followed by cyclization of the zwitterion intermediate (Scheme 1). The regioselectivity in the formation of 8, 9, 11, and 13 can also be rationalized within the frames of the two mechanisms. Therefore, the frontier molecular orbitals of the ylides derived from 1a and 3a,and of the dichloroalkenes 4 and 5 were calculated by DFT method at the B3LYP 6-31G level27as shown in Figure 4. It is seen that while HOMO(5)–LUMO(pyridinium ylide) interaction is not highly discriminative in differentiating the two possible regioselectivity because of the similar magnitudes of the atomic coefficients at the 1,3-dipolar reactingcenters intheylide, maximum positiveorbitaloverlap
in the dominating LUMO(5)–HOMO(pyridinium ylide) interaction with a smaller energy gap leads to a definite regioselectivity as actually seen in 9. Regioselectivity in product 13a is also in agreement with the FMO interaction consideration, here again the LUMO(4)–HOMO(quinolinium ylide)interaction plays a dominant role in determining the regioselectivity although it has a slightly larger energy gap than in the HOMO(4)–LUMO(quinolinium ylide) interaction,which is not highly discriminative in determining regioselectivity because of the similar atomic coefficient magnitudes at the 1,3-dipolar reacting centers in the ylide. In route 2, as is usually the case in Michael addition to an a,b-unsaturated carbonylcompound, the carbanionic carbon atom in the ylide takes part in an 1,4-addition to the a,b-un-
saturated alkene 5 to give the zwitterion(I) regioselectively,which furnished 9 by cyclization and elimination of two hydrogen chloride molecules.We have further investigated the reactions of 2,3-dichloro-1,4-naphthoquinone 7 with the N-ylides derived from 1–3.However, we found that these reactions did not follow the
normal [3+2] cycloaddition pathway but took place by anentirely different pathway. Therefore, reaction of the ylide from 1a with 7 in MeCN under same conditions as mentioned above afforded the product benzo[f]pyrido[1,2-a]-indole-6,11-dione 15a in 56% yield instead of the expected benzo[f]pyrido[2,1-a]isoindole-7,12-dione. Similar reaction of 7 with the ylides derived from 1b and 1c furnished products15b(58%)and15c(58%),respectively.The structure of 15b is unambiguously established by an X-ray crystallographic analysis (Fig. 5). Similar to compound 10, in the 1H NMR spectra of 15a–15c, the protons at C4 (see formula 15a–15c in Fig. 1) absorb at a remarkably low field with d values in the region 9.75–9.9 ppm due to the deshielding effect of the nearby carbonyl group. Reaction of 7 with the
N-ylides generated from 2 and 3 gave products 16 and 17 in moderate to high yields, respectively (Table 1). Their structure sare further confirmed by an X-raycrystallographic analysis of product 16c |
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