Lewis Acids - American Chemical Society

Chapter 2

Lewis Acids

Downloaded by COLUMBIA UNIV on December 6, 2016 | http://pubs.acs.org Publication Date (Web): November 30, 2016 | doi: 10.1021/bk-2016-1236.ch002

Manabu Hatano and Kazuaki Ishihara* Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya, 464-8603, Japan *E-mail: [email protected]; Tel.: +81-52-789-3331; Fax: +81-52-789-3222

This chapter reviews recent progress on boron(III) Lewis acids. Due to the chemical stability and easy molecular design of boron(III) compounds, boron(III) Lewis acids have been used in organic synthesis for more than 80 years both stoichiometrically and catalytically. In particular, this chapter focuses on the recent application of boron(III) Lewis acid catalysts in asymmetric reactions. Recent advances in supramolecular cooperative catalysts involving chiral boron(III) Lewis acids are also reviewed. Moreover, recent interesting examples of acid or base cooperative boronic acid catalysts and recent synthetically important advances with electron-deficient triarylborane(III) are described.

Introduction Boron(III) compounds are some of the most useful Lewis acids among main group elements. Boron(III) Lewis acids have been used in organic synthesis for more than 80 years both stoichiometrically and catalytically. In particular, due to the chemical stability and easy molecular design of boron(III) compounds, boron(III) Lewis acid catalysts have been shown to be valuable in asymmetric catalysis. Many excellent textbooks and reviews of chiral boron(III) Lewis acid catalysts have been published (1–8). Moreover, over the past decade there have been outstanding advances in frustrated Lewis pair (FLP) chemistry with the use © 2016 American Chemical Society Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


of bulky boron(III) compounds (e.g., B(C6F5)3) and silanesa and bulky boron(III) compounds and hosphines/amines/N-heterocyclic carbenes (NHC) to activate small molecules such as hydrogen. Therefore, very comprehensive information is now available (9–22). This chapter will not address these topics and instead will focus on a recent trend for the application of boron(III) Lewis acid catalysts in an asymmetric manner. This chapter reviews recent advances with supramolecular cooperative catalysts involving chiral boron(III) Lewis acids based on acid–base combined chemistry (23–26). Moreover, recent interesting examples of acid or base cooperative boronic acid catalysts and synthetically important advances with electron-deficient triarylboranes(III) are also described.

Preface to Cooperative Chiral Boron(III) Lewis Acid Catalysts The concept of ‘combined chiral acid catalysts’ was established by Yamamoto, and these can be classified into Brønsted acid-assisted chiral Lewis acids (chiral BLA), Lewis acid-assisted chiral Lewis acids (chiral LLA), Lewis acid-assisted chiral Brønsted acids (chiral LBA), and Brønsted acid-assisted chiral Brønsted acids (chiral BBA) (27, 28). In early asymmetric boron(III) Lewis acid catalysis, some chiral BLA catalysts were predominantly used in the enantioselective Diels–Alder reaction, hetero Diels–Alder reaction, Hosomi–Sakurai reaction, and Mukaiyama aldol reaction (Figure 1). The first outstanding chiral boron(III) catalyst 1 reported by Yamamoto was based on tartaric acid ligands (29–38). The high reactivity of the chiral acyloxyborane (CAB) catalysts 1 might be caused by intramolecular hydrogen bonding of the terminal carboxylic acid to the alkoxy oxygen atom. Later, Ishihara and Yamamoto developed BLA catalyst 2, which was highly effective for the Diels–Alder reaction (39, 40). The coordination of a proton of the 2-hydroxyphenyl group would trigger hydrogen-bonding, through which the Lewis acidity of the boron(III) center should increase. Moreover, introduction of a further electron-deficient moiety such as in catalyst 3 reported by Ishihara and Yamamoto, which was derived from 3,5-bis(trifluoromethyl)benzeneboronic acid, compensated for the narrow substrate scope of catalyst 2 (41). Moreover, Ishihara and Yamamoto developed a quite simple BLA 4 from a 1:2 molar ratio mixture of a trialkylborate and optically pure simple 1,1′-bi-2-naphtol (BINOL), which was highly effective for the enantioselective aza-Diels–Alder reaction of aldimines (42).

28 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 1. Early chiral BLA catalysts in the enantioselective Diels–Alder reaction.

Intermolecular chiral BLA systems were developed alongside these early intramolecular chiral BLA systems. Mukaiyama developed a combined catalyst consisting of chiral prolinol 5 and BBr3 for a catalytic enantioselective Diels–Alder reaction (Scheme 1) (43). Later, Aggarwal investigated whether the corresponding catalysts 6 and 7 would be prepared in situ due to the release of HBr, which can coordinate nitrogen of the prolinol-ligand and effectively activate the boron(III) center (44).

Scheme 1. Chiral Prolinol-Derived Boron(III) Catalyst for the Diels–Alder Reaction.



As one of the most powerful and useful chiral BLA catalysts to date, Corey developed chiral oxazaborolidine catalysts 8 and 9 with trifluoromethanesulfonic acid (TfOH) and bis(trifluoromethane)sulfonimide (Tf2NH) for the catalytic enantioselective Diels–Alder reaction (Scheme 2) (45–47). The generation of cationic boron(III) Lewis acids is the key to greatly enhancing the catalytic activity for a variety of substrates, such as poorly reactive but synthetically useful quinones, which are scarcely usable with other conventional Lewis acid catalysts.

Scheme 2. Brønsted Acid-Assisted Cationic Chiral Oxazaborolidine Catalysts for the Diels–Alder Reaction.

Oxazaborolidines alone are weak bases in principle, and their full protonation can only be achieved by using a very strong Brønsted acid such as TfOH or Tf2NH. In place of these strong Brønsted acids, Corey used the very strong Lewis acid AlBr3 for the oxazaborolidine as a Lewis acid-assisted Lewis acid (LLA) catalyst (Scheme 3) (48, 49). As a result, LLA catalyst 10 showed a greater turnover efficiency than BLA catalysts 8 and 9. The high catalytic activity of catalyst 10 might be the result of greater steric screening of various Lewis acids, and the activated boron(III) site by the adjacent AlBr3 subunit would also diminish product inhibition more effectively than other possible Lewis acids. In this regard, Sakata reported a quantum-chemical study which used DFT calculations in an AlBr3-activated oxazaborolidine-catalyzed Diels–Alder reaction (50). The calculations clearly showed that the attachment of AlBr3 to the nitrogen atom of oxazaborolidine would enhance the Lewis acidity of its boron(III) center and enable it to coordinate to methacrolein. Moreover, the calculation supported 30 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


the notion that AlBr3 would also facilitate the reaction by reducing the overlap repulsion between the diene and the dienophile.

Scheme 3. AlBr3-Assisted Cationic Chiral Oxazaborolidine Catalyst for the Diels–Alder Reaction.

Very recently, Corey developed much more active acid-assisted cationic chiral oxazaborolidine catalysts for the Diels–Alder reaction (51). The catalytic activity could be further enhanced by the judicious placement of fluorine substituents in the chiral ligand, and a Tf2NH-assisted chiral oxazaborolidine catalyst 11 and an AlBr3-assisted chiral oxazaborolidine catalyst 12 were designed (Scheme 4). A probe reaction between cyclopentadiene and ethyl crotonate clearly showed an order-of-magnitude increase in catalytic power when β-CH2 was replaced by CF2 in the chiral catalysts 11 and 12. Moreover, catalyst 13, in which a 2,5difluorophenyl moiety replaced the o-tolyl moiety, showed much higher catalytic activity than 12. For a variety of substrates, excellent reaction rates, product yields, and enantioselectivities have been achieved through the use of 1−2 mol% of these cationic chiral fluorinated oxazaborolidine catalysts (Figure 2).

Scheme 4. Cationic Chiral Fluorinated Oxazaborolidines. 31 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 2. Scope of substrates with the use of 1 mol% of cationic chiral fluorinated oxazaborolidines.

As overviewed in these pioneering studies by Yamamoto, Corey, and Mukaiyama, the combined use of suitable Brønsted acids or Lewis acids and chiral boron(III) Lewis acids might make it possible to achieve high reactivities and high enantioselectivities, particularly in catalytic enantioselective Diels–Alder reactions. Activation by the further attachment of a strong proton or strong Lewis acid to a complex provides a way to overcome the deactivating effect of a chiral ligand. In this context, this chapter will review very recent advances by using mostly intermolecular acid-assisted–acid catalyst systems, and particularly with B(C6F5)3-assisted chiral boron(III) Lewis acid catalysts and boron(III) Lewis acid-assisted chiral phosphoric acid catalysts.

B(C6F5)3-Assisted Chiral Boron(III) Lewis Acid Catalysts General Properties of the Diels–Alder Reaction The Diels–Alder reaction is one of the most fundamental higher-ordered stereoselective reactions that involve the formation of two carbon–carbon bonds through [4 + 2] cycloadditions. The corresponding cyclohexane skeletons with possibly four successive chiral carbon centers can offer synthetic versatility, particularly in natural products, pharmaceuticals, and agrochemicals (52–57). In several studies to date, enantioselectivity in the Diels–Alder reaction has been successfully controlled by a variety of chiral catalysts or chiral auxiliaries in the substrates. On the other hand, endo/exo-selectivity in the Diels–Alder reaction strongly depends on the substrates, based on the Woodward–Hoffmann rule and Fukui’s conservation rule of orbital symmetry interactions and steric interactions between dienes and dienophiles via pericyclic transition states under thermodynamic or photoreaction conditions (Figure 3) (58–64). 32 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 3. Substrate-controlled endo/exo-selectivity in the Diels–Alder reaction.

Therefore, it is quite difficult to control not only enantioselectivity but also substrate-independent anomalous endo/exo-selectivity, since, while most conventional chiral catalysts can discriminate the enantiofaces of dienophiles, they cannot discriminate the approach of dienes. Many combinations of dienes with dienophiles allow a well-known endo-rule that is based on second-order orbital interactions (Figure 3, left). However, when steric interactions between dienes and dienophiles overcome the second-order orbital interactions in endo-transition states, less-familiar exo-adducts are often predominantly obtained in opposition to the endo-rule (Figure 3, right). For example, in the reaction between cyclopentadiene 15 and acrolein 16, an endo-preference is observed with regard to second-order orbital interactions without significant steric interactions (Scheme 5). In sharp contrast, in the reaction between 15 and methacrolein 18, an exo-preference is observed with regard to steric interaction between the methylene moiety of 15 and the methyl moiety of 18. Thermodynamically more stable optically active exo-17, which has a chiral tertiary carbon center, can also be generated by the epimerization of endo-17, since optically active endo-17 has been synthesized using many conventional chiral catalysts. In contrast, optically active endo-19, which has a chiral quaternary carbon center, cannot be generated by the epimerization of easily available exo-19.

Scheme 5. Second-Order Orbital Interactions and Steric Interactions on endo/exo-Selectivity in the Diels–Alder Reaction.



Anomalous Endo/Exo-Selective Diels–Alder Reaction with Chiral Supramolecular Boron(III) Catalysts To directly address the anomalous endo/exo control in the Diels–Alder reaction, catalysts must be able to accurately discriminate a chiral transition state by recognizing not only the re/si-face of dienophiles but also the endo/exo-approach of dienes. However, it might be easier to design promising catalysts for anomalous exo-control than for anomalous endo-control, since the external exo-approach can be realized when the internal endo-approach would be effectively prevented even with single-molecule catalysts with a relatively small structure. In this regard, anomalous exo-selective Diels–Alder reactions of α-non-substituted acroleins against the original endo-rule have been investigated by a few research groups through the use of single-molecule catalysts. The landmark anomalous exo-induced bulky aluminum(III) Lewis acid catalyst ATPH was developed by Yamamoto (65). Later, anomalous exo-induced chiral Lewis acid catalysts were independently developed by Kündig (Ru(III) catalysts) (66), Sibi (Yb(III) catalysts) (67), Maruoka (diamine catalysts) (68, 69), and Hayashi (prolinol silyl ether catalysts) (70). In sharp contrast to these anomalous exo-induced Diels–Alder reactions with α-non-substituted acroleins, a reaction intermediate in the anomalous endo-induced Diels–Alder reaction with an α-substituted acrolein should have a folding structure regardless of considerable steric repulsion between the reactants. Therefore, a deep and narrow cavity in the catalyst is needed to hold a diene and a dienophile together throughout the transition states. To realize this strategy, the rational design of conformationally flexible chiral supramolecular catalysts, similar to natural enzymes, might be possible. Moreover, according to Lehn’s original definition of a ‘supramolecule’ (71), which contains more than two molecules with non-covalent intermolecular bonds, supramolecular catalysts might be a simple extension from single-molecule catalysts. A useful coordination bond to generate a chiral supramolecular catalyst, PO···B(C6F5)3 (72), is highly attractive since Shibasaki pioneeringly used phosphine oxides as functionalized Lewis base moieties based on acid–base combination chemistry (73–84). Ishihara developed a conformationally flexible, highly active, chiral supramolecular catalyst based on well-designed single-molecule components (85, 86). A chiral supramolecular catalyst 24 was readily prepared in situ from three components such as chiral (R)-3,3′-((RO)2PO)2-BINOL 20 (87–94), 3,5-bis(trifluoromethyl)phenylboronic acid 21, and tris(pentafluorophenyl)borane 23 (Scheme 6). 31P NMR analysis in CD2Cl2 showed a corresponding peak shift of phosphoryl moieties at 19.16 ppm for 20, 17.91 ppm for 22, and 9.42 ppm for 24. Compound 23 would act as a bulky functional group to make a chiral, narrow and deep cavity around the Lewis acidic boron(III) center. Moreover, the strong electron-recipient ability of Lewis acid 23 would increase the Lewis acidity of the central boron(III) through conjugate bonds, which would take advantage of Lewis acid-assisted chiral Lewis acid (chiral LLA) catalysts.



Scheme 6. Preparation of a Chiral Supramolecular Boron(III) Catalyst.

In the presence of chiral supramolecular catalyst 24, a probe Diels–Alder reaction between 15 and 18 was conducted (Scheme 7) (85, 86). As a result, anomalous endo-(2S)-19 was obtained as a major product (99% yield, endo:exo = 83:17) with excellent enantioselectivity (99% ee). In sharp contrast, 20 and incomplete complexes 22 (i.e., [20 + 21]) and 25 (i.e., [20 + 23]) showed almost no catalytic activity (0–2% yield). Catalyst 22 was also not reactive, since it lacks conjugated activation by Lewis acid 23. Moreover, 21 and 23 gave the normal exo-19 as a major product. According to a working model to explain the anomalous stereoselectivity, a chiral, narrow, and deep cavity was assumed (85, 86). A theoretical study for a complex of 18 and 24 at the B3LYP/6-31G* level supported the notion that the two non-covalent P=O···B(C6F5)3 moieties have a syn-conformation (syn-26) on one hand and an anti-conformation (anti-26) on the other hand (Figure 4). As a result, syn-26 was more stable than anti-26 by 3.86 kcal/mol, since significant steric repulsion would be observed among the C6F5 moieties and the central 3,5(CF3)2C6H3B moiety in anti-26.



Scheme 7. Anomalous endo-Selective Diels–Alder Reaction of Methacrolein.

Figure 4. Theoretical calculations for supramolecular boron(III) complexes.

In syn-26, the formyl moiety of 18 with a favored s-trans geometry was doubly coordinated with the B-O(Naph) moiety at the C(=O)H and C(=O)H parts (Figures 4 and 5). In a possible transition state 27, an endo-approach inside the cavity via a re-face attack would be relevant, while an exo-approach via a re-face attack would be unlikely because of the bulkiness of another C6F5 group.



Figure 5. A possible transition state for anomalous endo-selectivity.

Other anomalous endo-selective Diels–Alder reactions of α-haloacroleins are shown in Scheme 8 (85, 86). In the reaction of α-bromoacrolein, supramolecular catalyst 24 was ineffective, and normal exo-28 was obtained as a major product with low enantioselectivity (>99% yield, endo:exo = 16:84, 10–11% ee). In contrast, another supramolecular catalyst 29 with chiral biphenol in place of chiral binaphthol 24 was extremely effective, and the anomalous endo-selectivity was dramatically improved (94% yield, endo:exo = 93:7) with excellent enantioselectivity for endo-(2R)-28 (>99% ee). Another supramolecular catalyst 30 was effective for the reaction of α-chloroacrolein, and an anomalous endo-(2R)-31 was obtained (>99% yield, endo:exo = 88:12) with excellent enantioselectivity (99% ee). Moreover, another optimum supramolecular catalyst 32 was used in the reaction of α-fluoroacrolein, and anomalous endo-(2R)-33 was obtained (>99% yield, endo:exo = 82:18) with high enantioselectivity (96% ee). As with an enzymatic methodology, fine-tuning of the conformationally flexible supramolecular catalysts for each α-haloacrolein was essential for establishing anomalous endo-selectivity as well as excellent enantioselectivity. As the halogen in α-haloacrolein become smaller, larger components at the central aryl borane moiety and the biphenyl moiety were effective. The greater bulkiness may directly or indirectly create a smaller cavity that could suitably recognize a smaller substrate (Figure 6).



Scheme 8. Anomalous endo-Selective Diels–Alder Reaction of α-Haloacroleins.

Figure 6. Optimum size of a suitable chiral cavity for each substrate. As a possible explanation for why the anomalous endo-selectivity of 28 was significantly improved when biphenyl catalyst 29 was used in place of binaphthyl catalyst 24, there might be a slight difference in the dihedral angle of the binaphthyl or biphenyl skeleton. As another possible explanation, the electron-donating ability of the 6,6′-ether moieties in 29 through a resonance 38 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


effect in the conjugate system might induce a stronger intermolecular acid–base coordination of non-covalent P=O···B(C6F5)3 (Figure 7). This coordination bond stabilization might reduce the adventitious dissociation of achiral B(C6F5)3 23. Consequently, normal Diels–Alder reactions with low enantioselectivity by incomplete supramolecular catalysts and/or 23 would be prevented.

Figure 7. A resonance effect in chiral biphenol catalysts.

Anomalous exo-selective Diels–Alder reaction of α-non-substituted acrolein 16 was also established by another optimal chiral supramolecular catalyst 34 (Scheme 9) (85, 86). In general, the reaction of 15 with 16 was endo-selective under substrate-control (endo:exo = 80:20 under thermal conditions). In sharp contrast, supramolecular catalyst 34 with amido moieties in place of phosphoryl moieties was highly effective for the anomalous exo-induced Diels–Alder reaction of 16 with high enantioselectivities (94% ee for exo-(2S)-17, endo:exo = 20:80).

Scheme 9. Anomalous exo-Selective Enantioselective Diels–Alder Reaction. 39 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Similar to catalyst 24 with phosphoryl moieties, a possible transition state for catalyst 34 with amide moieties is shown in Figure 8 (85, 86). Unlike the pseudo-tetrahedral phosphorous structure, the amide has a less-hindered planar structure, and the non-covalent amide–B(C6F5)3 moiety may turn outside in the transition states. As a result, a shallow and wide cavity, which would promote the anomalous exo-approach, would be provided in 35.

Figure 8. A possible transition state for anomalous exo-selectivity.

Moreover, molecular recognition was performed under substrate-competitive Diels–Alder reaction conditions (85, 86). For a 1:1:1 equimolar mixture of 15, 16, and 18, exo-inducing supramolecular catalyst 34 promoted the reaction of 16 exclusively, and anomalous exo-(2S)-17 was obtained as a major product (endo:exo-17 = 20:80, 95% ee for exo-(2S)-17) (Scheme 10). In sharp contrast, achiral catalyst 23 gave a mixture of endo-17 and exo-19 with low substrate-selectivity (16:18 = 63:37) and normal endo/exo-selectivity (endo:exo-17 = 87:13, endo:exo-19 = 9:91). This result strongly suggests that the catalyst exhibits an induced-fit to adapt to a specific substrate.

Scheme 10. Molecular Recognition in the Substrate-Competitive Reaction. 40 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


As another useful coordination bond to generate a new type of chiral supramolecular catalyst, CN···B(C6F5)3 (95) is highly attractive. Ishihara developed chiral Lewis acid catalyst 36, which was prepared in situ from chiral 3-substituted-BINOL, (2-cyanophenyl)boronic acids, and tris(pentafluorophenyl)borane based on CN···B and PO···B coordination bonds, for the enantioselective Diels–Alder reaction (Scheme 11) (96). A probe reaction between cyclopentadiene and methacrolein clearly showed the effectiveness of 36. Catalyst 37 without the CF3 moiety in aryl boronic acid decreased the enantioselectivity. Moreover, the PO···B(C6F5) moiety was also important for inducing high enantioselectivity, and the significantly bulky electron-deficient aryl moiety as seen in catalyst 39, unlike less bulky 38, was needed to keep the high enantio-induction in place of the PO···B(C6F5)3 moiety. Optimization of bulkiness of the aryl moiety might still be needed to further improve the enantioselectivity. Therefore, the concise in situ-construction of the bulky PO···B(C6F5)3 moiety that is achieved by mixing the PO moiety and B(C6F5)3 would be very advantageous by reducing the need for catalyst optimization.

Scheme 11. A Probe Diels–Alder Reaction that Uses Supramolecular Catalysts with CN···B and PO···B Coordination Bonds. 41 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Various acroleins could be used, and the corresponding normal products from acrolein, ethylacrolein, tiglic aldehyde, and ethyl trans-4-oxo-2-butenoate were obtained with high enantioselectivities (Scheme 12). Moreover, acyclic diene 40 could be used in place of cyclopentadiene, and 41 was obtained in 97% yield with 85% ee (Scheme 13).

Scheme 12. Reactions of Various Acroleins through the Use of the Supramolecular Catalyst with CN···B and PO···B Coordination Bonds.

Scheme 13. A Reaction with the Acyclic Diene through the Use of the Supramolecular Catalyst with CN···B and PO···B Coordination Bonds.

Overall, in these Diels–Alder reactions with the use of catalyst 36, anomalous endo/exo-selectivities were not observed, unlike the results with previous supramolecular catalysts, such as 24. Although catalyst 36 would have a chiral cavity in a possible transition state 42 in Figure 9, the structure might be too flexible to control anomalous endo/exo-selectivities. Therefore, a moderately rigid conformationally flexible supramolecular catalyst such as 24 might be essential for inducing anomalous endo/exo-selectivities. Instead, more flexible catalyst 36 showed a relatively wide scope for substrates to induce high enantioselectivities, whereas more rigid catalyst 24 showed narrower substrate specificity to induce high enantioselectivities with anomalous endo/exo-selectivities. 42 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 9. Possible transition state with catalyst 36.

Boron(III) Lewis Acid-Assisted Chiral Phosphoric Acid Catalysts Direct Boron(III) Lewis Acid-Assisted Chiral Phosphoric Acid Catalysts Chiral phosphoric acids are highly useful acid–base cooperative organocatalysts for a variety of asymmetric catalyses (97–101). However, their Brønsted acidity is generally not strong enough to activate less-basic aldehydes compared to more-basic aldimines. To overcome this serious issue, the addition of an achiral Lewis acid to the chiral phosphoric acid would be a highly promising option since the conjugate acid–base moiety of the phosphoric acid is suitable for the Lewis acid-assisted Brønsted acid (LBA) catalyst system (Figure 10). Ishihara developed a BBr3-assisted chiral phosphoric acid catalyst, which was highly effective for the catalytic enantioselective Diels–Alder reaction toward a concise synthesis of isoquinuclidine alkaloids (102).

Figure 10. BBr3-Assisted chiral phosphoric acid catalysts. A probe reaction between cyclopentadiene and methacrolein was promoted in the presence of chiral phosphoric acid (R)-43 and BBr3, and normal exo-product 19 was obtained with 89% ee (Scheme 14). The reaction did not proceed in the absence of BBr3, and the reaction proceeded in the presence of BBr3 alone. Therefore, the cooperative catalyst BBr3-(R)-43 was much more active 43 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


than either BBr3 or (R)-43. Moreover, BBr3 was essential for inducing high enantioselectivity, and other similar boron(III) compounds, such as BF3·Et2O, BCl3, BI3, and B(C6F5)3, were less effective. When cyclohexadiene was used in place of cyclopentadiene, endo-product 44 was also obtained with 87% ee. Therefore, this catalyst was effective for both normal endo- and exo-control.

Scheme 14. BBr3-Assisted Chiral Phosphoric Acid catalyst for Diels–Alder Reactions.

The cooperative catalyst BBr3-(R)-43 showed low catalytic activity (16% ee) for much less reactive acyclic diene 40 (Scheme 15). In sharp contrast, another cooperative catalyst BBr3–N-sulfonyl phosphoramide 45 was effective, and endo46 was obtained in 82% yield with 89% ee (102). Although only specialized examples have been reported to date, the possibility of using this catalytic system with phosphoric acids and N-sulfonyl phosphoramides (103) might be attractive for catalyst optimization in general asymmetric catalysis. To demonstrate the synthetic utility of this approach, Ishihara performed a formal total synthesis of (+)-catharanthine, which is an important indole alkaloid that forms vinblastine (Scheme 16) (102). A key Diels–Alder reaction between 1,2-dihydropyridine 47 and α-bromoacrolein proceeded successfully with the use of cooperative catalyst BBr3-(R)-43, and the corresponding product endo-48 was obtained with 98% ee. Subsequent transformations ultimately provided the desired key intermediate 49 (104) without a loss of optical purity. 44 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Scheme 15. BBr3-Assisted Chiral N-Sulfonyl Phosphoramide Catalyst for the Diels–Alder Reaction.

Scheme 16. Formal Total Synthesis of (+)-Catharanthine.

Moreover, Ishihara performed a transformation to the key intermediate of (+)-allocatharanthine, which is another component of vinblastine (Scheme 17) (102). The key enantioselective Diels–Alder reaction of ethyl-substituted 1,2-dihydropyridine 50 gave the desired endo-51 with 97% ee with the use of BBr3–(S)-43. After some transformations of the isoquinuclidine structure, condensation with 3-indoleacetic acid gave the desired key intermediate 52 (105) without a loss of optical purity. 45 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Scheme 17. Formal Total Synthesis of (+)-Allocatharanthine.

Remote Boron(III) Lewis Acid-Assisted Chiral Phosphoric Acid Catalysts Based on previous study about B(C6F5)3-assisted chiral boron(III) Lewis acid catalysts in the Diels–Alder reaction, Ishihara developed a remote B(C6F5)3-assisted chiral phosphoric acid catalyst 53 for the enantioselective Diels–Alder reaction of α-substituted acroleins with cyclopentadiene as a probe reaction (106). Introduction of the phosphoric acid to the center of supramolecular catalysts might provide additional opportunities for versatile molecular recognition according to the size and/or substitution pattern of the acroleins. Unlike a previous simple boron(III) Lewis acid center such as 24, the highly conjugated Brønsted acid–Brønsted base bifunction of chiral phosphoric acid 53 should be able to doubly coordinate (107–109) to acroleins (Figure 11, left). Moreover, the addition of an achiral Lewis acid (MLn) should provide bifunctional Lewis acid–Brønsted base catalysts (Figure 11, right). ESI-MS analysis and 1H, 19F, and 31P NMR analysis of catalyst 53 suggested that coordination to B(C6F5)3 at the carbonyl groups of the 3,3′-substituents would proceed via coordination at the central P=O moiety, probably due to steric constraints at the narrow inner space. As a result, with the use of catalyst 53, the Diels–Alder product 19 was obtained from methacrolein in 92% yield with 90% ee (Scheme 18). However, the scope of substrates was narrow, and the Diels–Alder products from α-ethylacrolein, α-isopropylacrolein, and α-bromoacrolein showed lower enantioselectivities (84% ee for exo-54, 23% ee for exo-56, and 18% ee for exo-29, respectively). Moreover, anomalous endo/exo-selectivities were not observed in these probe reactions, probably due to the poor cavity effect. 46 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 11. Possible reaction mechanism in remote boron(III) Lewis acid-assisted chiral phosphoric acid catalysts.

Scheme 18. Diels–Alder Reaction Catalyzed by Remote B(C6F5)3-Assisted Chiral Phosphoric Acid. For a much less reactive α,β-disubstituted acrolein such as tiglic aldehyde, catalyst 53 showed low reactivity, and the product was obtained in 38% yield with 56% ee (Scheme 19). To overcome this issue, the Brønsted acid–Brønsted base catalyst system was changed to a Lewis acid–Brønsted base catalyst system through the use of an additional achiral Lewis acid partner. After acid sources were screened, catecholborane (110) was found to be a highly effective boron(III) Lewis acid center, and the product was obtained in 71% yield with 75% ee with the use of catalyst 57 (106). 47 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Scheme 19. Remote B(C6F5)3-Assisted Chiral Boron(III) Phosphate Catalyst for the Diels–Alder Reaction.

Acid or Base Cooperative Arylboronic Acid Catalysts Intramolecular Cooperative System with Brønsted Base-Assisted Arylboronic Acid A variety of boronic acid catalysts have been developed to activate carboxylic acids as mixed anhydrides through dehydrative condensation (111–113). More recent research has focused on the rational design of acid or base cooperative arylboronic acid catalysts to promote various reactions using carboxylic acids as substrates. Ishihara developed the first successful method for the catalytic dehydrative self-condensation of carboxylic acids with the use of the Brønsted base-assisted boronic acid catalyst 58 (Scheme 20) (114, 115). An arylboronic acid bearing bulky (N,N-dialkylamino)methyl groups at the 2,6-positions can catalyze the intramolecular dehydrative condensation of aromatic and aliphatic di- and tetracarboxylic acids. As Whiting pioneeringly reported in dehydrative amide condensation with (2-((N,N-diisopropylamino)methyl)phenyl)boronic acid catalyst (116–119), steric hindrance of the (N,N-dialkylamino)methyl groups might prevent the intramolecular interaction between the boronic acid group and the (N,N-dialkylamino)methyl groups (N → B chelation) that causes inactivation of the boronic acid group. Moreover, the introduction of two bulky substituents at the 2,6-positions might prevent the formation of less active species such as triarylboroxines (120, 121). 48 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Scheme 20. Catalytic Dehydrative Condensation of Aromatic and Aliphatic Dicarboxylic Acids. A model plausible reaction mechanism is shown in Figure 12. The condensation of phthalic acid would occur via monoacyl boronate 59 as an active intermediate. The N,N-dialkylamino nitrogen and the N,N-dialkylammonium proton of 59 would then synergistically promote intramolecular cyclization to 60. The former would act as a Brønsted base to activate the carboxyl group, and the latter would act as a Brønsted acid to activate the carbonyl group through two consecutive hydrogen-bonding interactions supported by the B-hydroxyl group. The subsequent elimination of phthalic anhydride from intermediate 60 would also be synergistically promoted by the N,N-dialkylamino nitrogen and the N,N-dialkylammonium proton of 60.

Figure 12. Possible reaction mechanism in catalytic dehydrative condensation. Intermolecular Cooperative System with Arylboronic Acid and Nucleophilic Base The catalytic dehydrative condensation reaction between carboxylic acids and amines is one of the most ideal methods for synthesizing the corresponding amides. To date, some excellent arylboronic acid catalysts have been developed, as shown in Scheme 21; 61 and 62 by Ishihara and Yamamoto in 1996 (122–126), 49 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


63 by Whiting in 2006 (116–119) 64 by Hall in 2008 (127–131). In addition to these arylboronic acids, boric acid (132, 133), benzo[1,3,2]dioxaborol-2-ol (134–136), and methylboronic acid (137) have been reported to be useful as amidation catalysts. In arylboronic acid-catalysis, a mixed anhydride intermediate 65 is generated at the initial stage from the carboxylic acid and arylboronic acid under azeotropic reflux conditions or in the presence of drying agents (Schemes 21 and 22). As the second activation stage, if a nucleophilic additive (Nu) reacts with 65 to generate a more active cationic intermediate 67 via a tetrahedral intermediate 66, the amide condensation may proceed more rapidly.

Scheme 21. Dehydrative Condensation of Carboxylic Acids with Amines Catalyzed by Arylboronic Acids, and Representative Examples of Catalysts.

Scheme 22. The Second Activation by Nucleophilic Additives in the Presence of Arylboronic Acids. 50 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


In this context, Ishihara recently found that arylboronic acids and N,N-dimethylaminopyridine N-oxide (DMAPO) cooperatively promote dehydrative condensation between various carboxylic acids and amines (138). A probe reaction between 2-phenylbutyric acid and benzylamine was examined in the presence of 5 mol% each of arylboronic acid 62 and an additive (Scheme 23). As a result, arylboronic acid 62 alone did not promote the reaction. N,N-Diisopropylethylamine, 4-(N,N-dimethylamino)pyridine (DMAP), and 4-methoxypyridine N-oxide (MPO) were not effective. In contrast, a weak but more nucleophilic base, DMAPO, was quite effective for amide condensation. A more nucleophilic additive such as 4-(pyrrolidin-1-yl)pyridine N-oxide (PPYO) was less effective than DMAPO, since the strong nucleophilicity of PPYO might reduce the activity of intermediate 67.

Scheme 23. Effects of Additives on the Dehydrative Condensation between 2-Phenylbutyric Acid and Benzylamine. Both the nucleophilicity of the additive and the Lewis acidity and steric effect of the boronic acid are important in the cooperative catalysis. Actually, the cooperative effects of boronic acids were compared in the condensation reaction between bulky 2-phenylbutyric acid or less bulky benzoic acid and benzylamine (Scheme 24). Both catalysts 62–DMAPO and 64b–DMAPO efficiently promoted the reaction of 2-phenylbutyric acid. Interestingly, 62–DMAPO was much more effective than 64b–DMAPO for the amide condensation of benzoic acid. Moreover, Whiting’s catalyst 63–DMAPO was still effective for the reaction of benzoic acid, while the catalytic activity of 63–DMAPO was almost suppressed in the reaction of 2-phenylbutyric acid. In these mismatch situations among the catalysts, additives, and substrates, a less active species 68 would be generated according to the X-ray analysis of an inert species (138).

Scheme 24. Cooperative Effects of Arylboronic Acid and DMAPO. 51 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Overall, 62–DMAPO could be used with both types of these substrates. Notably, not only aliphatic primary amines but also sterically hindered aliphatic secondary amines, less nucleophilic anilines and alkoxyamines reacted with these carboxylic acids (Scheme 25) (138). Moreover, this cooperative method can be scaled to practical volumes of up to an 80 mmol scale. The utility of the cooperative catalyst 62–DMAPO was also demonstrated for the selective amide condensation of β-substituted acrylic acids, and the corresponding amides were obtained in high yields. In general, the reactions without DMAPO gave the corresponding products in low yields (data in brackets in Scheme 25).

Scheme 25. Catalytic Dehydrative Condensation between Carboxylic Acids and Amines. Data in Brackets are the Results without DMAPO.

Intermolecular Cooperative System with Arylboronic Acid and Chiral Aminothiourea Takemoto developed the intramolecular aza- and oxa-Michael reactions of α,β-unsaturated carboxylic acids for the first time with the use of a bifunctional aminoboronic acid 63 (Scheme 26) (139). Not only pyrrolidines but also piperidines, imidazolidinone, dihydrobenzofurans, and chromanes could be synthesized successfully. A plausible mechanism is similar to the previous proposal by Whiting and Ishihara regarding the dehydrative condensation of dicarboxylic acids and amines. The trigonal planar boron(III) species would form an acyloxyborane complex 70 bearing the carboxylic acid moiety of the substrate with the aid of the Lewis acidity of the boron(III) atom and the Brønsted basic moiety of 63. 52 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Scheme 26. Intramolecular Aza-Michael Reaction of α,β-Unsaturated Carboxylic Acid. As shown in the proposed mechanism, the amino group of the aminoboronic acid 63 would not be directly involved in the activation of the nucleophilic moiety of the substrate. Therefore, the reaction would be further facilitated by the addition of an external base, and the use of a chiral base catalyst would induce enantioselectivity. The combined use of an arylboronic acid 62 and a chiral aminothiourea 71 allowed the reaction to proceed in an enantioselective manner, and the desired heterocycles were obtained in high yields with high enantioselectivities (Scheme 27) (139).

Scheme 27. Intramolecular Michael Addition to α,β-Unsaturated Carboxylic Acids.

Synthesis of Electron-Deficient Triarylboranes(III) for Catalysis Preparation of Tris[3,5-bis(trifluoromethyl)phenyl]borane Traditionally, tris(pentafluorophenyl)borane (B(C6F5)3) has been recognized as an excellent co-catalyst in homogeneous Ziegler–Natta olefin-polymerization reactions (140–143). The special properties of B(C6F5)3 have made this strong boron(III) Lewis acid increasingly used as a catalyst and/or a stoichiometric reagent in organic and organometallic chemistry (144–148). Over the 53 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


past decade, B(C6F5)3 has been considered to be one of the most effective Lewis acid components in frustrated Lewis pairs (FLPs) (9–22). For the development of further promising FLPs, some novel homo- and hetero tri(aryl)boranes(III) were recently synthesized, as shown in Figure 13, and their properties have been evaluated with respect to their NMR spectra, X-ray crystal structures, electrochemical studies, etc. (149–151). In this regard, tris[3,5-bis(trifluoromethyl)phenyl]borane(III) 72 would be favored due to the very strong electron-withdrawing ability of its six trifluoromethyl moieties (8). Nevertheless, the synthesis of 72 had not been reported before Ashley and Tamm independently and almost simultaneously reported a preparative procedure in 2012 (Scheme 28) (152, 153). In fact, the synthesis of 72 had been considered to be difficult, since 72 can easily transform to tetra[3,5-bis(trifluoromethyl)phenyl]borate during its preparation due to its strong Lewis acidity. Therefore, stoichiometric control of the starting borane, such as BF3·Et2O, and an extremely pure Grignard reagent should be essential.

Figure 13. Preparation yields of novel homo- and hetero tri(aryl)boranes(III).

Scheme 28. Preparation of Tris[3,5-bis(trifluoromethyl)phenyl]borane(III). As a result, their final optimum procedures turned out to be quite similar, as shown in Scheme 28. According to the Knochel method (154), the corresponding Grignard reagent was prepared from 3,5-bis(trifluoromethyl)phenyl bromide and i-PrMgCl in THF. Next, BF3·Et2O was added to give tris[3,5bis(trifluoromethyl)phenyl]borane(III) as a white crystal. Subsequent sublimation and recrystallization provided 72 in 62–79% yields. The obtained 72 was investigated analytically and used in some probe FLP reactions. As a result, 72 was estimated to be slightly more Lewis acidic (6%) than B(C6F5)3 in a 31P NMR 54 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

analysis with Et3PO, and studies of its FLP features are anticipated in the near future.


Hydrogenation of Unactivated Aldehydes Uozumi developed a good method for the hydrogenation of unactivated aldehydes by using a Hantzsch ester in the presence of an electron-deficient triarylborane(III) 72 as a strong Lewis acid catalyst (Scheme 29) (155). As an initial screening of boron(III) Lewis acid catalysts for the hydrogenation of cyclohexanecarboxaldehyde in toluene at 60 °C, catalyst 72 showed much higher catalytic activity than BF3·Et2O and B(C6F5)3. This result strongly suggests that 72 is more Lewis acidic than B(C6F5)3, as Ashley demonstrated (152).

Scheme 29. Screening of Boron(III) Catalysts in the Hydrogenation of Cyclohexanecarboxaldehyde.

After further optimization of the reaction conditions, tris[3,5bis(trifluoromethyl)phenyl]borane 72 efficiently catalyzed the hydrogenation of unactivated aliphatic aldehydes with a Hantzsch ester in 1,4-dioxane at 100 °C to give the corresponding aliphatic primary alcohols in high yield (Scheme 30). Unactivated aromatic aldehydes also undergo hydrogenation even at 25 °C to furnish the corresponding aromatic primary alcohols in up to 100% yield.

Scheme 30. Scope of Unactivated Aliphatic Aldehydes and Aromatic Aldehydes. 55 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

Conclusion


In conclusion, this chapter reviewed the recent progress on boron(III) Lewis acids. Although boron(III) Lewis acids have been used more than 80 years ago, the synthetic potential of boron(III) Lewis acids has still continued to increase, due to the chemical stability and easy molecular design of boron(III) compounds. In particular, the developments of acid–base cooperative catalysts involving boron(III) Lewis acids are remarkable in this field. Based on innovative scientific and new technological approaches, continued exploratory research will be expected to provide more efficient and practical methods for advanced molecular transformation in the near future.

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