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Guanidine is present in a variety of natural products and plays a key role in many biological activities. It can also be found as the side chain of arginine, one of natural amino acids. The intrinsic and distinctive property of guanidine is its strong basicity resulting from the resonance stability in its conjugate acid form, namely guanidinium cation, in which the positive charge can be delocalized over the three nitrogen atoms. In addition, guanidinium cation can interact strongly with anionic species through the combination of hydrogen bonding and ionic bonding, which is widely utilized in molecular recognition [55]. Because of such characteristic features of guanidine and guanidinium cation, chiral guanidine has attracted considerable attention as a promising platform for chiral Brønsted base catalysts in the field of asymmetric synthesis. Indeed, a variety of chiral guanidine catalysts has been developed, and, as a result, various types of useful enantioselective transformations including carbon–carbon bond formations and carbon‐heteroatom bond formations have been accomplished over the past two decades, which are comprehensively summarized in the literature based on the types of transformations or those of catalysts [4]. Following are the representative chiral guanidine catalysts with their fundamental and/or remarkable applications.

As a pioneering study of chiral guanidine as a chiral Brønsted catalyst, in 1994, Nájera and co‐workers reported the enantioselective nitroaldol reaction albeit with only a modest enantioselectivity [56]. On the other hand, the first highly enantioselective reaction was reported by Lipton and co‐workers in 1996 [57]. They developed the enantioselective Strecker reaction catalyzed by chiral guanidine 21 (Scheme 3.31).

Scheme 3.31. Enantioselective Strecker reaction catalyzed by 21. Source: Based on [57].

In 1999, Corey and Grogan developed the enantioselective Strecker reaction catalyzed by chiral bicyclic guanidine 22a, which is the important seminal work in the field of chiral guanidine catalysis (Scheme 3.32) [58].

Scheme 3.32. Enantioselective Strecker reaction catalyzed by 22a.

Source: Based on [58].

In the report, the authors proposed the suggestive reaction mechanism (Figure 3.8). First, the deprotonation of hydrogen cyanide (HCN) by the guanidine proceeds to form the guanidinium cyanide complex. The complex can function as a hydrogen bond donor, and thus the activation of the imine electrophile occurs to form the pretransition‐state termolecular assembly. Finally, the attack of the cyanide within the ion pair to the hydrogen bond‐activated imine occurs to afford the adduct.

Figure 3.8. Proposed reaction mechanism.

Tan and co‐workers intensively studied the enantioselective transformations by using this type of chiral bicyclic guanidines 22, and successfully developed a lot of highly enantioselective reactions [59]. For instance, a series of enantioselective reactions of anthrones, such as the Diels‐Alder reaction with maleimide and the addition to Michael acceptors, was developed [60]. The tandem reaction process involving a Michael addition of thiols followed by a highly enantioselective protonation was also established by using tert‐butyl substituted 22b (Scheme 3.33) [61].

Scheme 3.33. Enantioselective protonation catalyzed by 22b.

Source: Based on [61].

Other remarkable application of chiral bicyclic guanidine 22b is the enantioselective synthesis of axially chiral allenoates by the enantioselective isomerization of 3‐alkynoates (Scheme 3.34) [62].

Scheme 3.34. Enantioselective isomerization of 3‐alkynoate catalyzed by 22b.

Source: [62].

Ishikawa and co‐workers developed chiral monocyclic guanidine 23 having a hydroxy group, and applied the catalyst to the enantioselective Michael addition of glycine imines to acrylates (Scheme 3.35) [63]. The control experiments suggested that the matched relative configuration of the three chiral centers on the catalyst and the existence of the hydroxy group are essential for achieving both high conversion and high enantioselectivity. The catalyst was also applied to the enantioselective oxa‐Michael addition for the synthesis of chromane skeletons [64].

Scheme 3.35. Enantioselective Michael addition of glycine imines to acrylates catalyzed by 23.

Source: [63].

Misaki, Sugimura, and co‐workers developed chiral bicyclic guanidine 24 bearing a hydroxy group as a hydrogen bond donor unit. This catalyst was highly effective in a series of enantioselective reactions of 5H‐oxazol‐4‐ones as a pronucleophile, such as direct aldol reaction and the 1,4‐addition to alkynyl carbonyl compounds (Scheme 3.36) [65].

Scheme 3.36. Enantioselective reactions of 5H‐oxazol‐4‐ones as a pronucleophile catalyzed by 24.

Source: [65].

Nagasawa and co‐workers developed a series of guanidine‐(thio)urea bifunctional catalysts 25 having conformationally flexible chiral linkers [66]. The catalyst design was based on the idea of using the formation of a double hydrogen bonding network, where a guanidine and a (thio)urea simultaneously activate a pronucleophile and an electrophile, respectively. The adequacy of the catalyst design was verified by applying them to a variety of enantioselective reactions. For instance, guanidine‐bisthiourea catalyst 25a was successfully utilized in the ortho‐selective alkylation of phenols through the enantioselective addition with nitroalkenes (Scheme 3.37) [67].

Scheme 3.37. Enantioselective addition of phenols to nitroalkenes catalyzed by 25a.

Source: Based on [67].

On the other hand, the use of guanidine‐bisthiourea catalyst 25b enabled the solvent‐dependent enantiodivergent Mannich‐type reaction (Scheme 3.38) [68]. The authors concluded that the origin of solvent‐dependent stereodiscrimination was controlled by the enthalpy–entropy compensation. This type of catalysts was utilized not only in carbon–carbon bond formations but also in carbon‐heteroatom bond formations [69], such as α‐hydroxylation of tetralone‐derived β‐ketoesters [70].

Scheme 3.38. Solvent‐dependent enantiodivergent Mannich‐type reaction catalyzed by 25b.

Source: Based on [68].

Feng and co‐workers designed bifunctional guanidine catalyst 26a featuring a chiral amino amide backbone, in which an amide moiety functions as a hydrogen bond donor unit [71]. The catalytic activity was demonstrated in the enantioselective addition of β‐ketoesters to nitroalkenes (Scheme 3.39).

Scheme 3.39. Enantioselective addition of β‐ketoesters to nitroalkenes catalyzed by 26a. Source: Based on [71].

Liu, Feng, and co‐workers later developed related guanidine‐amide bifunctional catalysts, such as 26b, 26c, and 26d, and successfully utilized them in several enantioselective reactions (Scheme 3.40) [72].

Scheme 3.40. Enantioselective reactions catalyzed by 26.

Source: [72].

Wang, Qu, and co‐workers developed tartaric acid‐derived seven‐membered cyclic chiral guanidine 27, and utilized the catalyst in the enantioselective α‐hydroxylation of β‐ketoesters and β‐diketones with oxaziridine (Scheme 3.41) [73]. This type of chiral guanidine catalyst was also used in the Michael addition of 3‐substituted oxindoles to nitroalkenes [74].

Scheme 3.41. Enantioselective α‐hydroxylation of β‐ketoesters catalyzed by 27.

Source: Based on [73].

Tan and co‐workers developed an aminoindanol‐derived chiral guanidine 28. The catalyst was utilized in the desymmetrization of meso‐aziridines with thiols and carbamodithioic acids as a pronucleophile, providing the ring‐opening products in high yields with high enantioselectivities (Scheme 3.42) [75].

Scheme 3.42. Desymmetrization of meso‐aziridines with thiols catalyzed by 28.

Source: Based on [75].

Aforementioned all chiral guanidine catalysts control the stereoselectivity of the bond‐forming process based on the central chirality of the catalyst molecule. In contrast, Terada and co‐workers introduced, for the first time, the methodology based on axial chirality of the catalyst molecule into the field of chiral guanidine catalysis [76]. Specifically, the group designed two types of axially chiral guanidines having an axially chiral binaphthyl backbone (Figure 3.9). One is the nine‐membered cyclic guanidines 29, in which an N‐C‐N guanidine subunit is involved in the ring structure. The other is the seven‐membered cyclic guanidines 30, in which one nitrogen atom of guanidine is involved in the ring structure.

Figure 3.9. Axially chiral guanidine catalysts.

The high catalytic activity of nine‐membered 29 was demonstrated in the enantioselective Michael addition of β‐dicarbonyl compounds and diphenyl phosphite to nitroalkenes [77]. Both substituents at the 3,3′‐positions of binaphthyl backbone (Ar) and that attached on the guanidine nitrogen (G) had a strong impact on the stereoselectivity of the reactions. Chiral guanidine 29a was also utilized in the enantioselective direct vinylogous aldol reaction of furanones, as well as vinylogous Michael addition of furanones to nitroalkenes, to provide the corresponding adducts in high yields with high enantioselectivities (Scheme 3.43) [78].

Scheme 3.43. Enantioselective reactions of furanones as a pronucleophile catalyzed by 29a.

Source: [78].

On the other hand, the catalytic activity of seven‐membered 30 was confirmed by the highly enantioselective amination of β‐dicarbonyl compounds with azodicarboxylate (Scheme 3.44) [79]. In this catalyst design, the reach of the steric demand exerted by the aromatic substituents (Ar) is important to provide an efficient chiral environment around the substrate recognition site at the guanidine moiety. Therefore, the employment of chiral guanidine 30a with para‐biphenyl substituents having a bulky tert‐butyl groups at the 3,5‐positions of the terminal phenyl ring was essential to achieve the high level of stereocontrol. This type of chiral guanidine catalyst was also utilized in the enantioselective [3+2] cycloaddition of glycine imines with maleate [80].

Scheme 3.44. Enantioselective amination of β‐dicarbonyl compounds with azodicarboxylate catalyzed by 30a.

Source: Based on [79].