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Two reaction paths, Diels‐Alder type reaction and Michael reaction, involve an iminium ion as a reactive intermediate [37]. These two reaction paths will be discussed in the reactions of cinnamaldehyde and cyclopentadiene catalyzed by diarylprolinol silyl ether (Table 1.1).

The Diels‐Alder reaction was catalyzed by diarylprolinol silyl ether 6, with trifluoromethyl groups on the aryl moiety (Figure 1.6) with a combination of a strong acid such as CF₃CO₂H [38]. It should be noted that the exo‐isomer was obtained predominantly.

On the other hand, when a mixture of cinnamaldehyde and cyclopentadiene was treated with diphenylprolinol silyl ether 5 (Figure 1.6) with a combination of p‐nitrophenol or NaOAc, Michael products were obtained with excellent enantioselectivity without formation of the Diels‐Alder products [39]. Iminium ions are key intermediates in both reactions.

As the pyrrolidine moiety of catalyst 6 is more electron deficient than that of catalyst 5 because of the electron‐withdrawing CF₃ group, the LUMO of the iminium ion generated from 6 is lower than that of the iminium ion generated from 5. The Diels‐Alder reaction proceeds smoothly when the LUMO of the dienophile is lower. Thus, the Diels‐Alder reaction proceeded in the case of 6 (Scheme 1.3). On the other hand, in the Michael reaction (Scheme 1.4), after the formation of the iminium ion, counterion (OH⁻) reacts with p‐nitrophenol to generate phenoxide, which reacts with cyclopentadiene to generate an anion of cyclopentadiene. This anionic nucleophile reacts with the iminium ion to afford the Michael product. For the generation of anionic nucleophile, the nucleophile is limited to the active methylene compounds, and a strong acid cannot be employed.

TABLE 1.1. The two reaction paths in the reaction of cinnamaldehyde and cyclopentadiene

			3	4
Catalyst	Solvent	Additive	Yield/%	a:b	ee/%	Yield/%	exo/endo	ee/%
5	MeOH	p‐nitrophenol	84	70 : 30	92	0
5	MeOH	NaOAc	81	64 : 36	92	0
5	toluene		0			0
6	toluene	CF3CO2H	0			80	85 : 15	97/88

Figure 1.6. Diarylprolinol silyl ether in this study.

Scheme 1.3. The reaction mechanism of the Diels‐Alder reaction.

Scheme 1.4. The reaction mechanism of the Michael reaction.