Mechanistic Considerations of the Catalytic Guanylation Reaction of

Review pubs.acs.org/Organometallics

Mechanistic Considerations of the Catalytic Guanylation Reaction of Amines with Carbodiimides for Guanidine Synthesis Ling Xu,† Wen-Xiong Zhang,*,†,‡ and Zhenfeng Xi† †

Beijing National Laboratory for Molecular Sciences, and Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, College of Chemistry, Peking University, Beijing 100871, People’s Republic of China ‡ State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, People’s Republic of China ABSTRACT: Catalytic guanylation reactions of amines with carbodiimides have received increasing attention because of the atom-economical preparation of guanidines since 2003. To date, more than 40 catalysts including transition metals, main-group metals, and rare-earth metals have been designed and tested for the guanylation reaction to construct acyclic and cyclic guanidines. In this review, we present a mechanistic consideration on catalytic guanylation reactions of amines with carbodiimides for guanidine synthesis to elucidate its development and importance. Four different types of reaction mechanisms have been well categorized: [2 + 2]-cycloaddition/protonation, insertion/protonation, activation of carbodiimide/nucleophilic addition/intramolecular protonation, and protonation/nucleophilic addition/disassociation. It is useful to understand these reaction processes in order to accelerate the rapid development in related areas to meet the growing need for guanidines.

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INTRODUCTION Metal-catalyzed C−N bond formation by N−H bond activation to construct N-containing compounds is one of the most important chemical transformations in modern organic synthesis.1−5 The catalytic guanylation reaction of amines with carbodiimides (hereafter denoted the CGAC reaction), which is also known as catalytic addition of an amine N−H bond to the CN double bond of carbodiimides (RNCNR′, which has been applied widely in organic synthesis6−26) or hydroamination of carbodiimides, can be a straightforward and atomeconomical route via C−N bond formation to prepare substituted guanidines (RNC(NR′R″)NHR). It is wellknown that guanidine derivatives are an important class of Ncontaining compounds which may be utilized as building blocks in various drugs (Scheme 1),27 natural products, agrochemicals, sweeteners, explosives, etc., as base catalysts in organic synthesis, and also as supporting ligands for various metal complexes.28−53 Although this direct guanylation reaction of primary aliphatic amines with carbodiimides can be performed under rather harsh conditions without catalyst, 54 the guanylation reaction of aromatic amines or secondary amines cannot be achieved without suitable catalysts, owing to their decreased nucleophilicity. An early trial was carried out by tetrabutylammonium fluoride (TBAF) promoted guanylation of some aromatic amines with activated N,N′-diaryl-substituted carbodiimides.55 However, such a catalytic process was hardly explored until Richeson et al. reported the first catalytic guanylation of primary aromatic amines with unactivated carbodiimides using titanium imido complexes in 2003.56 In the catalytic system, secondary amines could not be utilized because the regeneration of “TiN” imido species is required. © XXXX American Chemical Society

Scheme 1. Some Examples of Medicine-Related Molecules Containing Guanidino Groups

Later, Hou et al. reported the first catalytic guanylation of secondary amines with carbodiimides using a half-sandwich rare-earth-metal alkyl complex in 2006.57 In the catalytic process, the regeneration of “Ln−N” amido species met the needs for secondary amines. Thereafter, the CGAC reaction became an attractive area for the atom-economical preparation of guanidines. In the past 10 years, the CGAC reaction has encompassed more than 40 types of compounds acting as suitable catalysts for the preparation of more than 260 guanidines. Very recently, Carrillo-Hermosilla et al. summarReceived: March 25, 2015

A

DOI: 10.1021/acs.organomet.5b00251 Organometallics XXXX, XXX, XXX−XXX

Review

Organometallics ized various guanidines prepared by the CGAC reaction.53 We also summarized recent development in the synthetic methods of guanidines via transition-metal catalysis.58 In this review, we will present mechanistic considerations of the CGAC reaction for guanidine synthesis to elucidate its development and importance (Scheme 2). These catalysts, which cover different Scheme 2. Four Types of Mechanisms for CGAC Reaction

Figure 1. Molecular structure of 2a (R = iPr, Ar = C6F5). Hydrogen atoms are omitted for clarity.

metal complexes, are well categorized by four different types of reaction mechanisms: [2 + 2]-cycloaddition/protonation, insertion/protonation, activation of carbodiimide/nucleophilic addition/intramolecular protonation, and protonation/nucleophilic addition/disassociation. This should provide a deep insight into the processes of CGAC reactions and accelerate rapid development in meeting the needs of the growing interest in guanidines.

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FOUR DIFFERENT TYPES OF GUANYLATION MECHANISMS [2 + 2]-Cycloaddition/Protonation Mechanism. Richeson et al. reported the first transition-metal-catalyzed guanylation of primary aromatic amines with carbodiimides using the titanium imido complexes [(Me2N)C(NiPr)2]2Ti N(2,6-Me2C6H3) (1).56 Many anilines can be added to N,N′diisopropyl carbodiimide (DIC) or N,N′-dicyclohexyl carbodiimide (DCC) to give the corresponding guanidines in good to excellent yields. Some other complexes containing an MN moiety were also tested. Ti(NtBu)Cl2(py)2 and (2,6Me2C6H3)NTaCl3(thf)2 showed lower catalytic activity, while [iPrN(H)C(NiPr)2]2ZrN(2,6-Me2C6H3) could not catalyze this guanylation process. The [2 + 2]-cycloaddition/protonation mechanism was first proposed.53,56 Initially, [2 + 2]-cycloaddition of a carbodiimide (RNCNR) to the TiNAr bond provides the metalbonded dianionic guanidinate complex [(Me2N)C(NiPr)2]2Ti[(NR)(NAr)CNR] (2) (Figure 1). A proton-transfer reaction between an aromatic amine (ArNH2) and 2 releases the neutral guanidine ArNC(NHR)2 and regenerates the TiNR bond. Therefore, secondary amines cannot be used in this system because of the requirement of regenerating a “Ti N” imido moiety. The catalytic addition of hydrazine N−H bonds to carbodiimides, generally referred to hydrohydrazination or aminoguanylation, is an efficient and atom-economical route to prepare substituted N-aminoguanidines. Very recently, Gade et al. reported that the titanium hydrazinediido complex 3 (Figure 2) could serve as a good catalyst for the hydrohydrazination of carbodiimides to provide aminoguanidines and fluoreneiminoguanidines with a broad substrate range.59 The first step of the catalytic cycle proceeds via a [2 + 2]-cycloaddition of the hydrazinediido group of 4 with the carbodiimide, leading to the cycloadduct 5. This establishes a rapid equilibrium with the

Figure 2. Molecular structure of 3a (Ar = p-tol). Hydrogen atoms are omitted for clarity.

hydrazinediido reactant 4. Attack of the hydrazine and subsequent hydrazinolysis yield the corresponding aminoguanidines (Scheme 3). Although the proposed coordinated Scheme 3. Aminoguanylation Mechanism Catalyzed by Titanium Hydrazinediido Complexes

intermediate 6 is not observed spectroscopically, the mechanism is supported by the characterization of the intermediate 5 as well as the kinetics of the catalytic transformation. Insertion/Protonation Mechanism. Rare-Earth-MetalCatalyzed CGAC Reaction. Among the catalysts for the CGAC reaction, rare-earth-metal complexes are found to be B


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Organometallics

These monoanionic guanidinates are bonded strongly to the rare-earth-metal centers because of the chelating effect. Acid−Base Reaction Yielding the Ln−N Active Species. An acid−base reaction between an amine and a rare-earth-metal catalyst precursor is the simplest and the most convenient route to yield the Ln−N active species. In 2006, Hou et al. reported the catalytic guanylation of secondary amines with carbodiimides using the yttrium half-sandwich alkyl complex 12, in which a Y−N species was formed by an acid−base reaction between an amine and an Y−C alkyl bond (Scheme 5).57 The method offers a straightforward, atom-economical route to tetrasubstituted guanidines.

the largest number of guanylation catalysts. To date, almost 20 rare-earth-metal complexes can serve as excellent catalyst precursors to provide the corresponding guanidines. All of these complexes adopt the insertion/protonation mechanism, as shown in Scheme 4. The reaction between the catalyst Scheme 4. Insertion/Protonation Mechanism

Scheme 5. Isolation and Reaction of an Yttrium Guanidinate

The insertion/protonation mechanism was proposed for the first time by Hou et al., as shown in Scheme 4. It was quite different from the [2 + 2]-cycloaddition/protonation mechanism proposed by Richeson et al. The mechanism was based on the isolation, characterization, and stoichiometric reaction of both the yttrium amido complex 13 (Figure 3) and the yttrium guanidinate complex 14 (Figure 4), as shown in Scheme 5. precursor [Ln]−LG (LG = leaving group) and a secondary amine should yield straightforwardly the amido species 7. The nucleophilicity of Ln−N species to carbodiimide increases in comparison with that of the free uncoordinated amines. Nucleophilic addition of 7 to a carbodiimide directly affords the guanidinate 8. Protonation of 8 by another molecule of amine releases the final product guanidine and regenerates 7. The catalytic addition reaction between primary aromatic amines and carbodiimides can take place similarly. In this case, the intramolecular 1,3-proton shift in the initially formed symmetrical guanidinate 10 occurs to give the unsymmetrical guanidinate 11 (path a). Protonolysis of 11 by another molecule of primary amine releases the guanidine and regenerates the amido species 9. Formation of the guanidine through protonolysis of 10 (path b) is also possible. In the case of this mechanism, two critical points are worth mentioning. One is the way that yields the active Ln−N species or Ln guanidinates to start the catalytic cycle. This is significantly dependent on the reactive site of the catalyst precursors. Three main routes yielding the active “Ln−N” species or Ln guanidinates are included: (i) acid−base reaction, (ii) reductive coupling, and (iii) coordination of carbodiimide. The other point is catalytic transformation of a rare-earth-metal guanidinate with an amine via protonation to yield a guanidine.

Figure 3. Molecular structure of 13. Hydrogen atoms are omitted for clarity.

Furthermore, the yttrium half-sandwich alkyl complex 12 could also serve as an excellent catalyst precursor to effect the guanylation of primary aromatic amines with carbodiimides to afford the trisubstituted guanidines in high yields. A wide range of substituted anilines and heterocyclic primary amines such as amino-substituted isoxazoles, pyrazoles, imidazoles, thiazoles, and pyridines can be used for this reaction.60−62 An insertion/ protonation mechanism for the guanylation of primary aromatic C


Review

Organometallics

guanidinate 16 to the unsymmetric guanidinate 17 through intramolecular proton transfer from the aryl amino nitrogen atom to an isopropyl amido nitrogen atom was also observed. Many rare-earth-metal catalyst precursors appeared after the initial report (Scheme 7). Varieties of supporting ligands such as Schiff base, Cp derivatives, bridging indenyl, bridging diamide, and even bridging NHC could be applied. Some homoleptic rare-earth-metal catalyst precursors also appeared. Among all of these catalyst precursors, the active Ln−N species in the catalytic cycle is generally obtained by an acid−base reaction between an amine and an active site of the rare-earthmetal catalyst precursor. The active sites of rare-earth-metal catalyst precursors usually originate from Ln−C alkyl bonds, Ln−N(TMS)2 bonds, or the disassociation of supporting ligands of catalyst precursors. In the case of catalyst precursors having a Ln−N(TMS)2 bond, it is convenient to obtain the active Ln−N species by an acid−base reaction (Scheme 8a). Wang et al. first reported that rare-earth-metal catalyst precursors having Ln−N bonds could act as good catalysts for the guanylation of primary aromatic amines or secondary amines with carbodiimides.63 Other catalyst precursors having Ln−N bonds were also explored.64−72 For homoleptic trivalent rare-earth-metal precursors, the catalytic cycle is initiated by the disassociation of one supporting ligand (Scheme 8b). If there is a bridging ligand in the heteroleptic trivalent rare-earth-metal precursors, an intramolecular acid−base reaction between an amine and a ligand occurs to give a Ln−N species (Scheme 8c). Interestingly, the ytterbium aryloxide 18 supported by a βdiketiminato ligand is an efficient precatalyst for the guanylation of primary aromatic amines with carbodiimides. The mechanism is slightly different from the above insertion/ protonation mechanism, in which the first cycle is initiated by ytterbium guanidinate 20 formed by the reaction of ytterbium aryloxide 18, amines, and carbodiimides (Scheme 9).73 Shen et al. reported the synthesis of a series of heterobimetallic dianionic guanidinate complexes 22 containing both a lanthanide and lithium. These complexes were found to be efficient precatalysts for the guanylation of many amines with carbodiimides to give the corresponding guanidines under mild conditions, especially for biguanidines.74 The proposed mechanism is shown in Scheme 10. The coordination of diamine to lanthanide and lithium metal centers followed by the release of monoguanidine yields the active dual Ln−N and Li−N amido species 24. Nucleophilic addition of the amido species 24 to two carbodiimides affords biguanidinate 25. Protonation of 25 by another molecule of diamine releases the final product biguanidine and 24 is regenerated. The high reactivity of heterobimetallic complexes 22 contributes to the cooperation effect by lanthanide and lithium metals. Although many rare-earth-metal catalysts have been reported for the guanylation reaction, the reaction of secondary amines of the general formula ArRNH or ArAr′NH with carbodiimides is difficult because of the weak nucleophilicity and steric hindrance of these amines. Recently, our group reported the synthesis and structure of heterobimetallic half-sandwich rareearth-metal tris(trimethylsilylmethyl) anionic complexes bearing one 1-phenyl-2,3,4,5-tetrapropylcyclopentadienyl ligand. These soluble anionic compounds can serve as excellent and general catalyst precursors for the addition of different types of amines, including primary aromatic amines (ArNH2), acyclic amines (ArRNH and ArAr′NH; R, R′ = alkyl group, Ar, Ar′ = aromatic group), and cyclic secondary aliphatic amines, to carbodiimides to efficiently yield guanidines.75 The catalyst

Figure 4. Molecular structure of 14. Hydrogen atoms are omitted for clarity.

amines with carbodiimides was proposed, as shown in Scheme 4. The mechanism was also based on the isolation, characterization, and stoichiometric reaction of the yttrium amido complex 15 and two yttrium guanidinate complexes 16 and 17 (Scheme 6, Figure 5). The transformation from the symmetric Scheme 6. Isolation and Reaction of Two Yttrium Guanidinates

Figure 5. Molecular structure of 17. Hydrogen atoms are omitted for clarity. D


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Organometallics Scheme 7. Various Rare-Earth-Metal Catalyst Precursors Yielding the Active Ln−N Species by Acid−Base Reaction

Scheme 8. Formation of Ln−N Species by Acid−Base Reaction

Scheme 9. Guanylation Mechanism by an Ytterbium Aryloxide Supported by β-Diketiminato Ligand

system shows higher activity for guanylation reactions of these types of amines (ArRNH and ArAr′NH) with carbodiimides in comparison to that of the yttrium half-sandwich alkyl complex 12. As 26 is an ate complex with three active Y−C(alkyl) bonds, it is difficult to obtain experimental evidence for the amide intermediate 27 and guanidinate species 28 or 29. However, we are sure that this reaction undergoes the insertion/protonation mechanism (Scheme 11). Reductive Coupling Yielding the Ln−N Active Species. In contrast to the case for many trivalent rare-earth-metal catalyst precursors, divalent rare-earth-metal catalyst precursors were not reported for the CGAC reaction until 2008. Various divalent rare-earth-metal complexes were reported to serve as excellent catalyst precursors for the guanylation reaction of primary or secondary amines with carbodiimides to provide the corresponding guanidines under solvent-free conditions. The activity trends Yb < Eu < Sm for metals and I < MeC5H4< OAr < N(TMS)2 for ligands around the metals were observed. Sm[N(TMS)2]2(thf)3 (30) showed the best catalytic activity among them (Scheme 12).76

The Sm(III) amido intermediate [(TMS)2N]2SmNHAr (32) and Sm(III) guanidinate [(TMS)2N]2Sm[(NR)2C(NHAr)] (33) were proposed to be two active species in the catalytic cycle. However, the formation of 32 is interesting. It proceeds via two steps: (i) the reductive coupling of two carbodiimides (RNCNR) promoted by the Sm(II) complex 30, yielding a bimetallic bisamidinate samarium(III) species {[(TMS)2N]2Sm(NR)2C}2 (31), and (ii) the protonation between 31 and amines (ArNH2), yielding the Sm(III)−N E


Review

Organometallics Scheme 10. Biguanylation Mechanism Catalyzed by RareEarth-Metal Heterobimetallic Complexes 22

Scheme 13. Formation of the Sm(III) Amido Intermediate

Scheme 11. Guanylation Mechanism Catalyzed by 26

Figure 6. Molecular structure of 35a. Hydrogen atoms are omitted for clarity.

C6H4NH2 gave the amide complex 37a by release of one guanidine (Scheme 14). On the basis of the isolation and characterization of 35a−37a, a suitable mechanism was proposed, as shown in Scheme 15. The monoguanidinate 36 and amide 38 are two important intermediates in the catalytic Scheme 14. Isolation and Reaction of Yttrium Intermediates

Scheme 12. Activity Trend of Divalent Rare-Earth Catalysts

active species 32 by release of the biamidine [RNC(NHR)]2 (Scheme 13).76,53 Coordination of Carbodiimide Promoting the Formation of the Ln−N Active Species. Shen et al. demonstrated that the rare-earth-metal tris(aryloxide) complexes Ln(OAr)3 (Ln = Y (34a), Nd (34b), Sm (34c), Yb (34d); Ar1 = 2,6-(tBu)2-4MeC6H2) could catalyze the CGAC reaction.77,78 The influence of the central metals on the activity was observed with the increasing trend Nd < Sm < Yb < Y. The reaction of 34a with carbodiimide in toluene provided the carbodiimide-coordinated adduct 35a (Figure 6) in high yield. The 1:1 reaction of 35a with 4-chloroaniline yielded the corresponding monoguanidinate complex 36a. The protonolysis of 36a with excess 2-MeOF


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Organometallics

(2,6-iPr2C6H3N)V(NR′R″)xCl3‑x (40) is proposed for the insertion of a primary V−N amido bond of R′R″NH to the carbodiimides RNCNR to give the corresponding mixed imido/guanidinate intermediate (2,6-iPr2C6H3N)V[(NR)2C(NR′R″)]xCl3‑x (41), which gives the guanidine compound RR′NC(NR)NHR and regenerates 40 after the amine protonolysis. In this process, the imido unit serves only as an ancillary ligand (Scheme 17). This mechanism was also

Scheme 15. Guanylation Mechanism Catalyzed by 34a

Scheme 17. Favored Insertion of Carbodiimide into a V−N Amido Bond

cycle; however, the first cycle is initiated by yttrium guanidinate 36 formed by the reaction of 34a, amine, and carbodiimide by release of one molecule of ArOH. Therefore, the activation of carbodiimide is crucial for the whole process. Transition-Metal-Catalyzed CGAC Reaction. In 2004, Montilla et al. reported that the VN imido complex (2,6-iPr2C6H3N)VCl3 (39) could catalyze the guanylation process of primary aromatic amines with carbodiimides. Initially, a [2 + 2]-cycloaddition/protonation mechanism, which was similar to that of the TiN imido catalyst 1, was proposed.79 In the following studies, they found that this VN imido catalyst 39 could also perform the guanylation of secondary amines with carbodiimides (Scheme 16). It was

supported by two pieces of experimental evidence: (i) no [2 + 2]-cycloaddition reaction between carbodiimide and 39 was observed and (ii) the complexes V(N-2,6-iPr2C6H3)(S2CiPr)3 and V(N-2,6-iPr2C6H3)[(OCH2CH2)3N], having chelated ligands, showed no catalytic activity. Therefore, the formation of the V−N amido species was critical for the VN imido catalyst 39. Recently the niobium imido complex 42 was reported to catalyze the CGAC reaction (Scheme 18). Similar to the case Scheme 18. Structures of Catalysts 42−44

Scheme 16. Guanylation of Amines with DIC Catalyzed by the Vanadium Imido Complex 39

for the vanadium imido complex 39, the reaction did not follow a [2 + 2]-cycloaddition/protonation mechanism. The active moiety of the precatalyst was a benzyl instead of an imido group.82 Xie et al. reported that the carboranyl−alkoxy-ligated titanium amido complex 43 could catalyze the CGAC reaction of both primary aromatic amines and secondary amines (Scheme 18). The reaction proceeds in a way analogous to that observed in the case of rare-earth-metal amido catalyst precursors.83,84 The oxygen-bridged heterobimetallic complex 44, having a methyl-bound zirconium center and a Ti−amide center, was tested as a catalyst precursor for the guanylation of aromatic amines with carbodiimides (Scheme 18). The results show that the Ti−amide center is primarily responsible for the intermolecular guanylation process.85 The reaction proceeds via the insertion/protonation mechanism. Although the catalytic activity does not rank among the best for known catalysts, this should open a new avenue to design effective multifunctional catalysts for guanylation reactions. ZnEt2 was reported to be a good catalyst precursor for the CGAC reaction, in which the Zn−N amido species formed by

difficult to explain the guanylation of secondary amines with carbodiimides by the [2 + 2]-cycloaddition/protonation mechanism, which required the regeneration of VN imido species.80,81 A DFT analysis reveals that the formation of the model complex [κ2-MeNC(−NMe)NMe]VCl3 through a [2 + 2] process is not favored by either thermodynamics or dynamics. Therefore, the alternative insertion/protonation mechanism was suggested and confirmed by DFT calculations. The reasonable mixed amido/imido intermediate G


Review

Organometallics an acid−base reaction between Zn−C(alkyl) and amines served as the active species in the catalytic cycle.86 Our group has demonstrated that Zn(OTf)2, under an atmosphere of air, serves as an excellent catalyst precursor for the CGAC reaction of various primary and secondary amines, yielding substituted guanidines.87 Main-Group-Metal-Catalyzed CGAC Reaction. Lithium reagents are widely used in organic chemistry. Many lithium reagents, such as n BuLi, PhLi, and LiN(TMS) 2 are commercially available. They are often used in quantitative or even excess amounts as bases to capture the protons of organic substrates. However, organolithium can be utilized as a catalyst precursor as well. Richeson et al. reported that LiN(TMS)2, the first alkalimetal catalyst for the guanylation of aromatic amines with carbodiimides, provided the corresponding guanidines.88 Other organolithium reagents (e.g., nBuLi) and alkali-metal amides (e.g., NaN(TMS)2 and KN(TMS)2) were also shown to be effective precatalysts in this guanylation process. However, these organolithium catalysts are often sensitive to polar functional groups. In the case of anilines or carbodiimides bearing a large substituted group or o-OMe as the coordinatng group, the yields were relatively low in comparison with those of unsubstituted substrates or substrates with little substitution. However, the addition of a catalytic amount of TMEDA could significantly increase the guanylation activity of electrondeficient amines, presumably through modulation of the coordination environment of the cationic Li center (Scheme 19).

Scheme 20. DFT-Calculated Guanylation Mechanism Catalyzed by Lithium Amide/TMEDA System

Scheme 19. Guanylation Catalyzed by LiN(TMS)2

A detailed mechanistic understanding by DFT study was given by the same group. The catalytic cycle of a representative electron-deficient amine with DIC by lithium amide/TMEDA 45 is shown in Scheme 20.89 The monomeric TMEDAcoordinated lithium anilide 46 is used as the putative active catalyst. The calculated mechanism proceeds in two steps: carbodiimide insertion into the Li−N bond and proton transfer from the amine. In the case of carbodiimide insertion, the insertion step is initiated by the coordination of the carbodiimide to the lithium center via the lone pair electrons of nitrogen. The anilide group in 47 migrates to the central sp carbon of the carbodiimide through the transition state TS1 to give the intermediate 48. Then the isomerization of 48 is thermodynamically driven through a simple rotation around the Li−N−C−N dihedral angle to give the guanidinate 49. When the free guanidine products are present in solution, an alternative reaction pathway via the coordination of the incoming guanidine to the metal in 50 is possible, although the activation energy is slightly higher than the unimolecular isomerization mechanism. For the proton transfer stage, the intermediate 51, in which the newly formed guanidine product is coordinated to the Li center via the lone pair of the sp2hybridized nitrogen is formed through the transition state TS4.

Then the guanidine dissociates from the metal in 51 to re-form the monomeric 46. The weak complexation between the metal amide and guanidine should cause guanidine to readily dissociate from the metal center. Alternatively, exchange via coordination of the carbodiimide can occur to give 47. The calculated activation energies for all of the steps in the catalytic cycle are less than 23 kcal mol−1, showing the possibility of this mechanism. Furthermore, Richeson et al. also calculated the catalytic cycle of the guanylation of aniline with DIC using the dichloro aluminum catalyst [Cl2Al−NHPh]. A mechanism parallel to the lithium-catalyzed mechanism was identified, although there are two differences. One is the migratory insertion of carbodiimide into the Al−N bond with a significantly lower barrier than that of carbodiimide into the Li−N bond. The other difference is that a bimolecular pathway with an existing guanidine is more facile because isomerization from the one aluminum guanidinate to the other has a high barrier. Richeson et al. have found that some aluminum complexes, such as Al(NMe2)3 and AlMe2Cl, are effective guanylation catalysts. Our group has found that AlMe3, AlEt3, and AlEt2Cl can serve as more H


Review

Organometallics efficient guanylation catalysts in comparison to aluminum amides. Mechanistic studies have shown that only one Al−Me bond in AlMe3-catalyzed guanylation participates in the catalytic cycle and two other Al−Me bonds remain inert.90 Bergman et al. have reported that the dimethyl aluminum guanidinate complex 52 is an efficient guanylation catalyst (Scheme 21).91 In this process, the active Al−N species is

Scheme 23. Guanylation Mechanism Catalyzed by [(O,OPLY)]AlMe2

Scheme 21. Structures of Catalysts 52−54

formed by the dissociation of the guanidinate ligand, showing the stability of Al−Me bonds toward protonolysis by anilines. In addition, the monomeric Al(NMe2)3 is found to be a good catalyst for the aminoguanylation of hydrazines with carbodiimides to give N-aminoguanidines. The catalytic cycle is proposed to proceed via an insertion/protonation mechanism, which is different from the case for the titanium hydrazinediido catalyst, which proceeds via a [2 + 2]cycloaddition/protonation mechanism to give N-aminoguanidines.59 The κ2-N,N′ coordination ability of the hydrazide ligand likely stabilizes the proposed intermediate 56 (Scheme 22).

examined. After 1 equiv of carbodiimide was added, the CH4 was released and 59 formed. The addition of amine to carbodiimide and subsequent protonation by another amine afforded guanidine as the product. Kinetic studies indicated that the rate-determining step was the generation of 59, which was coordinated with a primary KIE of 3.17. Very recently, we reported various Me3SiCH2Li-catalyzed guanylations of 1,2-diarylhydrazenes and carbodiimides (Scheme 24).95 However, the structures of the final products are quite different from those of aminoguanidines prepared from 1,1-disubstituted hydrazines (Scheme 24). In our system guanidines free of N−N bonds and azo compounds were obtained as the final products after heating for 24 h. A possible mechanism is proposed after detection of several key intermediates in this catalytic cycle. The monodeprotonation of 1,2-diarylhydrazine gives the lithium hydrazide 62. After insertion of a carbodiimide, the guanidino amide 63 can be isolated. The difference in structures between 63 and 61 may be due to the different radii of lithium and aluminum. Then 63 is protonated by 1,2-diarylhydrazine and gives 64 as an intermediate, which is similar to the final product aminoguanidine referred to in Scheme 22. 64 is not stable at elevated temperatures. Its nucleophilicity increases via an intramolecular proton shift, and therefore, the intermolecular nucleophilic attack occurs to afford 66, which decomposes into the two guanidines 68 and 69 and the azo compound 70 after another intramolecular proton shift. Organic Molecule Catalyzed CGAC Reaction. Very recently, Harder et al. has demonstrated that “naked” amides [Me4N]+[Ph2N]− can serve as catalysts for the CGAC reaction.96 Although the efficiency for guanidine synthesis is slightly lower than that for metal-catalyzed processes, this is the first metal-free CGAC reaction. The first step of this process is deprotonation of the amine; the following insertion of CN may go through several transition states (Scheme 25). Due to the lack of metal ion, the transition state is different from those in the aforementioned mechanism. Four possibilities have been proposed by researchers. Considering the solvent effect, the transition state containing either three moieties ([Me4N]+, [Ph2N]−, and carbodiimide) or two moieties ([Ph2N]− and carbodiimide) is possible, but the latter is more feasible.

Scheme 22. Aminoguanylation Mechanism Catalyzed by Aluminum Amido Complex

Sharing a similar mechanism, many IIA group complexes can also catalyze this reaction. These complexes can be simple homoleptic MgBu2, M[N(TMS)2]2(thf)2 (M = Ca, Sr, Ba), ItBu:Mg[N(TMS)2]2 (53; ItBu = 1,3-di-tert-butylimidazol-2ylidene), or heteroleptic β-diketiminato calcium amide 54 (Scheme 21).92,93 However, Mandal et al.94 pointed out that the deprotonation of amine cannot proceed smoothly in the absence of carbodiimide for the phenalenyl-supported Al-containing catalyst [(O,O-PLY)]AlMe2 57 (Scheme 23). Using in situ NMR experiments, the donor−acceptor adduct 58 can be I


Review

Organometallics Scheme 24. Me3SiCH2Li-Catalyzed Reaction between 1,2Diarylhydrazine and Carbodiimide

Scheme 26. Guanylation Mechanism Catalyzed by Lewis Acids

Ln(OTf)3 can act as a Lewis acid catalyst to activate carbodiimides to generate the intermediate 71. Nucleophilic addition of an amine to 71 results in the formation of the intermediate 72. The intramolecular proton transfer releases the guanidines and Ln(OTf)3 regenerates, continuing the catalytic cycle. Then Shen et al. found that the cheap and readily available AlCl3 could act as a good Lewis acid catalyst to effect guanylation under mild and solvent-free conditions. The evidence of the proposed intermediate 72 formed from AlCl3 and a carbodiimide was detected by in situ infrared spectroscopy.98 In addition, commercially available iron catalysts with low price and low toxicity, such as Fe(OAc)2, acted as good Lewis acid catalysts for the guanylation.99 Interestingly, in addition to these homogeneous catalysts, Kantam et al. found in 2012 that nanocrystalline ZnO, CeO2, TiO2, and NiO could act as highly efficient heterogeneous catalysts for the CGAC reaction.100 Aromatic amines bearing diverse polarized functional groups, such as COMe, COPh, CONH2, CO2Et, CN, NO2, etc., could survive the reaction conditions to give the corresponding products with good yields. More importantly, the heterogeneous catalysts can be easily recovered and reused for several cycles with consistent activity. This provides a convenient cost-saving method for the preparation of guanidines. Nano metal oxides act as Lewis acid catalysts to activate carbodiimides to complete the catalytic cycle. In 2014, Frišcǐ ć et al. demonstrated that sulfonyl guanidines could be synthesized in high yield by milling the mixture of sulfonamides and carbodiimide with 5 mol % of CuCl (Scheme 27).101 In this process, substoichiometric amounts of a liquid (LAG, liquid-assisted grinding) is essential. If the mixture is dissolved in CH2Cl2 or acetone, no product is detected after

Scheme 25. [Me4N]+[Ph2N]−-Catalyzed CGAC Reaction between 1,2-Diphenylamine and DIC

Electrophilic Activation of Carbodiimide/Nucleophilic Addition/Intramolecular Protonation Mechanism. Unlike the catalysts referred above, many Lewis acids are found to be good guanylation catalysts. Generally, these Lewis acid catalysts have these main features: (i) easy availability, (ii) tolerance to air and moisture, (iii) broad scope, and (iv) tolerance to many functional groups. In 2009, Shen et al. found that lanthanide triflates (Ln(OTf)3, Ln = La, Nd, Sm, Eu, Er, Yb) could serve as efficient catalysts for guanylation under solvent-free conditions to provide the corresponding guanidines with a wide scope of amines.97 The highest activity for Yb(OTf)3 can be attributed to the strongest Lewis acidity of the Yb metal among these metals. The possible catalytic cycle is proposed in Scheme 26.

Scheme 27. Mechanochemical CuCl-Catalyzed Guanylation of Arylsulfonamides with Carbodiimides

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followed by reductive elimination, and (v) the disassociation of guanidines from Pd(0) species and generation of palladium nanoparticles. This heterogeneous mechanism is based on the isolation and X-ray structures of the tetracoordinate bis(anilino) Pd(II) complex 73a and bis(guanidino) Pd(II) complex 74a in a homogeneous phase (Scheme 30). In addition, 74a thermally

refluxing overnight. Controlled trials indicate that the milling without CuCl leads to incomplete conversion, monitored via utilizing the IR spectrum. The role of CuCl might be as a Lewis acid to largely activate the carbodiimide by coordinating to its π system. The CGAC reaction assisted by mechanochemistry may open a new window to this field. Protonation/Nucleophilic Addition/Disassociation Mechanism. This guanylation reaction under homogeneous Pd(II) catalysis was reported by our group.87 Interestingly, such a CGAC reaction was performed by Garcia et al. using supported palladium nanoparticles, such as nano Pd/MgO, Pd/ C, Pd/CeO2, and Pd/TiO2.102 Unexpectedly, a tandem reaction by combination of guanylation and Hiyama coupling using one solid Pd catalyst could be effected to provide styryl guanidines, which are versatile monomers for the formation of many functionalized and cross-linked copolymers (Scheme 28).

Scheme 30. Isolation of 77a and 78a in a Homogeneous Phase

Scheme 28. Consecutive One-Pot, One-Catalyst Hiyama Coupling/Guanylation

decomposes to form the corresponding N-arylguanidine. For supported Pd catalysts, a basic support such as MgO can contribute to the reaction mechanism by polarization of the N− H bond to increase aniline nucleophilicity. Furthermore, they explored the reaction mechanism for the CGAC reaction using the homogeneous catalyst PdCl2(MeCN)2.103 On the basis of the isolation and characterization of several analogous bis(anilino) and bis(guanidino) Pd(II) intermediates, two reasonable mechanistic pathways for the homogeneous catalytic cycle were proposed (Scheme 31). The first proposal is a concerted protonation/nucleophilic addition/disassociation mechanism. The coordination of anilines to PdCl2(MeCN)2 should give a rapid formation of the bis(anilino) Pd(II) complex 77. Coordination of amine to palladium would decrease the nucleophilicity of amine in comparison to the free uncoordinated amine. The interaction between 73 and DIC should afford the labile intermediate 75, in which coordination of carbodiimide to palladium could overcompensate this negative effect of palladium coordination to the amine. 75 undergoes protonation and nucleophilic addition to give the bis(guanidino) Pd(II) complex 74 (Figure 7). The disassociation of guanidines from Pd(0) species followed by the coordination of anilines to Pd(II) completes the cycle by liberating guanidine and generating 77. Herein the rate-determining step is the attack of 77 at DIC. Alternatively, a stepwise protonation/nucleophilic addition mechanism cannot be excluded. The mechanism is significantly dependent on the acidity of hydrogen atoms bonded to nitrogen in 73 to protonate the nitrogen atom of the carbodiimide. The protonated carbodiimide is activated to accept the nucleophilic attack of the resulting anilide anion or aniline.

A proposed mechanism for the guanylation process is summarized in Scheme 29. The catalytic cycle proceeds by five main steps: (i) coordination of anilines to Pd(0), (ii) oxidative addition yielding Pd(II) species, (iii) coordination and protonation of carbodiimides, (iv) nucleophilic addition Scheme 29. Guanylation Mechanism Catalyzed by Heterogeneous Pd Catalysts

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Organometallics Scheme 31. Two Possible Guanylation Mechanisms Catalyzed by PdCl2(MeCN) in a Homogeneous Phase

Scheme 32. Structural Modes of Aza-Heterocycles Formed by Tandem Guanylation/Cyclization

The tandem guanylation/cyclization reactions can be divided into four types according to the necessity for a catalyst in each step, as shown in Scheme 33.83b,90b,104−111 In the type a reaction, both guanylation and cyclization require catalysts. CuI-catalyzed guanylation/N-arylation cyclization of o-haloanilines with unactivated carbodiimides can provide 2-aminobenzimidazoles, in which CuI serves as a dual catalyst for both guanylation and cyclization.105,106 In the case of a guanylation process, CuI might act as a Lewis acid to activate carbodiimides. However, in the cyclization step, CuI might undergo oxidative addition from Cu(I) to Cu(III) and reductive elimination from Cu(III) to Cu(I). In the type b reaction, catalysts are required only in the CGAC step and the cyclization reaction can occur spontaneously in the presence of additives. We established a one-pot method by combining guanylation and amidation to synthesize cyclic guanidine 77 from amines, carbodiimides, and oxalyl chloride or dimethyl malonyl chloride. Metal catalysts are required to effect the guanylation to yield guanidines. Subsequent acylation of guanidines can occur in the presence of Et3N as base.90b Another example was demonstrated by Xie et al. in 2011. They discovered a tandem reaction between diamine or triamine and carbodiimide catalyzed by a carboranyl−alkoxy-ligated titanium amido complex.83b After the CGAC process is catalyzed by the titanium amido complex, intramolecular nucleophilic addition−elimination gives the final cyclic guanidine 78 or bicyclic guanidine 79. The type c reaction is much richer than those of types a and b because the addition of amines to activated arylcarbodiimides can proceed smoothly without a catalyst.104,108−110 However, the intramolecular cyclization of guanidines with suitable catalysts is critical. For example, the Cu(OAc)2/O2-catalyzed guanylation/N-arylation of amines with nonhalogenated diarylcarbodiimide proceeds a cascade spontaneous addition/C−H activation/C−H functionalization to provide 2-aminobenzimidazoles 76. The catalytic effect of Cu(OAc)2 is mainly in Narylation cyclization. Many tandem three-component reactions involving amines, activated arylcarbodiimides, and CO or isonitrile belong to type c.108−110 The type d reaction, in which no catalyst is needed during the whole tandem transformation, is limited. For example, a tandem guanylation/epoxy ring-opening cyclization between amine and oxiranylcarbodiimide can efficiently yield the dihydroquinazoline 81 without any catalyst.104

Figure 7. Molecular structure of 74b (Ar = 4-OMeC6H4). Hydrogen atoms are omitted for clarity.

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MECHANISTIC ASPECTS OF THE CGAC REACTION IN TANDEM CYCLIZATION Most guanidines resulting from the CGAC reaction involve various functional groups such as N−H, C−N, and CN bonds, which can be decorated in further transformations. When the reactants are well designed, one-pot tandem reactions can occur to give various N-containing heterocyclic compounds (Scheme 32). L


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Organometallics

performed using nanocrystalline ZnO,100 supported palladium nanoparticles,102 and Zn−Al modified hydrotalcite clay.112 Although the efficiency and scope of substrates are limited using heterogeneous catalysts at present, this is a new area and will receive increasing attention in the future. The fourth challenge is how to introduce the guanidine unit to macromolecules and biosystems via CGAC reactions, because the introduction of a guanidine unit might change their properties. In the case of these catalytic systems, the catalysts are often required to be tolerant to water, oxygen, and sensitive functional groups. The design of Lewis acid catalysts will be needed to open this new area. Finally, it is important to find ways to incorporate CGAC reactions into multicomponent tandem reactions to construct some important N-containing compounds.83b,90b,104−113 At present, many catalysts have been designed and synthesized for guanylation reactions; cyclization patterns with or without catalysts remain limited. Research in this area is just beginning and will provide a novel route for the synthesis of various azaheterocycles. In summary, this field of CGAC reactions has received increasing interest because of the efficient preparation of guanidines. Although much progress has been made, many issues remain to be addressed: the design and development of new homogeneous and heterogeneous catalysts, the activity of catalysts, the scope of the substrates, the tolerance to functional groups, and synthetic applications in some important and complex guanidines.

Scheme 33. Four Different Guanylation/Cyclization Modes for the Synthesis of Aza-Heterocycles

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AUTHOR INFORMATION

Corresponding Author

*E-mail for W.-X.Z.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China and the “973” program from the National Basic Research Program of China (2011CB808601).

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REFERENCES

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CONCLUSIONS AND OUTLOOK Although much progress has been achieved in this field of CGAC reactions, there are several remaining challenges. The first challenge is the search for general catalysts for sensitive functional groups (such as OH, COOH, COOR, CHO, and COR) and different types of amines and carbodiimides. It is often difficult for these amines with weak nucleophilicity and steric hindrance to undergo CGAC reactions with steric carbodiimides. The second challenge is the application of the CGAC reaction to the total synthesis of some important natural guanidines. At present, the guanidine unit in many natural guanidines is synthesized by the reaction of an amine compound with a stoichiometric amount of an electrophilic guanylating reagent. The CGAC reaction is expected to induce the guanidine unit. The design and development of heterogeneous catalysts are promising for synthetic chemists because of the advantage of recovery and reuse. Recently, the CGAC reaction has been M


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