Читать книгу Catalytic Asymmetric Synthesis - Группа авторов - Страница 80

In 2012, Lambert and Bandar introduced chiral cyclopropenimines as a powerful class of chiral organobase catalysts for the first time (Figure 3.10) [81].

The basicity of 31 was measured and found to be higher than that of guanidines and comparable to that of P1‐phosphazenes. Their high basicity is attributed to the stabilization of the conjugate acid by three nitrogen lone pairs and an aromatic cyclopropenium ion. The superior catalytic activity of 31 was demonstrated in the enantioselective additions of glycine imines to various kinds of Michael acceptors and imines (Scheme 3.45) [82].

The mechanistic rationale was provided based on the experimental results along with computational study (Figure 3.11) [83]. The lowest‐energy enantiodetermining transition state involves the (E)‐enolate hydrogen‐bonded to the N‐H function of the protonated catalyst, with the acrylate hydrogen‐bonded to the catalyst hydroxy group. Interestingly, an unusual intramolecular C‐H^… O interaction between a hydroxy group and a cyclohexane ring was identified as a key element in transition‐state organization.

Figure 3.10. Chiral cyclopropenimine catalyst.

Scheme 3.45. Enantioselective additions of glycine imines catalyzed by 31.

Source: Based on [82]. Source: Based on [81] and [82].

Figure 3.11. Mechanistic rationale.

Source: [83].

Jørgensen and co‐workers developed the enantioselective [3+2] cycloaddition of glycine imines with 2‐acyl cycloheptatrienes by using 31 (Scheme 3.46) [84]. 31 was also utilized in the catalytic enantioselective [2,3]‐Wittig rearrangement [85].

Scheme 3.46. Enantioselective [3+2] cycloaddition of glycine imines with 2‐acyl cycloheptatrienes catalyzed by 31.

Source: Based on [84].