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Dixon and co‐workers designed and developed a series of chiral bifunctional iminophosphorane (BIMP) catalysts 32 based on their idea of the introduction of an enhanced organobase functionality relative to tertiary amines into chiral acid–base bifunctional catalysts (Figure 3.12) [86].

Figure 3.12. Chiral bifunctional iminophosphorane (BIMP) catalysts.

The basicity of N‐alkyl triaryliminophosphorane was determined to be comparable to that of guanidines. The attractive feature of the catalyst design is high modularity. The catalysts incorporating a variable hydrogen bond donor, chiral amino acid‐derived scaffold, and triaryliminophosphorane are readily synthesized via a last step Staudinger reaction of a chiral organoazide and a triarylphosphine. The synthetic potential of this class of catalysts was first demonstrated in the enantioselective addition of nitromethane to ketimines catalyzed by 32a in 2013 (Scheme 3.47) [87].

Scheme 3.47. Enantioselective addition of nitromethane to ketimines catalyzed by 32a.

Source: Based on [87].

After that, the BIMP catalysts 32 were successfully utilized in several enantioselective reactions (Scheme 3.48). For instance, a series of enantioselective sulfa‐Michael additions including that with α‐substituted acrylates was developed (Scheme 3.48a) [88]. On the other hand, the enantioselective synthesis of conjugated cyclohexanones through a facial selective 1,3‐proton shift was achieved by using 32c (Scheme 3.48b) [89]. The related enantioselective 1,3‐proton shift was also utilized in the total synthesis of a natural product, (‐)‐himalensine A [91]. Furthermore, 32d having a sterically demanding amide moiety efficiently promoted the direct aldol addition of less acidic acetophenone derivatives to α‐fluorinated ketones to provide the corresponding adducts in high yields with high enantioselectivities (Scheme 3.48c) [90]. Johnson and co‐workers utilized the BIMP catalysts in their development of enantioselective reactions, such as the enantioselective additions of nitroalkanes and alkyl thiols to enone diesters, and the enantioselective three‐component coupling reaction of benzylidene pyruvates, aldehydes, and dialkyl phosphites [92].

Scheme 3.48. Enantioselective reactions catalyzed by 32. (a)

Source: [88].

(b)

Source: Based on [89].

(c)

Source: Based on [90].