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Development of New Chiral Brønsted Acid Catalysis Development of New Chiral Brønsted Acid Catalysis Shin ─ ichi Hirashima ＊ and Hisashi Yamamoto ＊ Molecular Catalyst Research Center, Chubu University 1200 Matsumoto ─ cho, Kasugai 487 ─ 8501, Japan (Received June 26, 2013; E ─ mail: [email protected]; [email protected]) Abstract: Brønsted acid catalysis has received considerable attention in modern organic synthesis. However, the utility of these Brønsted acid catalysts are somewhat limited toward applicable substrates due to its relatively lower reactivities of these Brønsted acids. With these perspectives, it is highly desirable to develop Brønsted acids demonstrating both high reactivities and selectivities. In this feature article, we will describe our achieve- ment in the design and development of Lewis acid assisted Brønsted acid catalysts (LBA), Brønsted acid assisted Brønsted acid catalysts (BBA), strong Brønsted acid catalysts, and their applications. activity of silica ─ supported AlCl 3 arose from Lewis acid acti- 1． Introduction vation of the Brønsted acidic site on the silica surface (Fig- Brønsted acids have long been utilized in organic chemi- ure 2). 3 stry. However, their early application was rather limited to fundamental types of reactions, such as hydrolysis and forma- tion of esters and acetals. The synthetic utility of Brønsted acids as catalysts for carbon ─ carbon bond formation reactions has been limited until quite recently. 1 This limited application Figure 2. Brønsted acidity arising from Lewis acid complex on a silica surface. of Brønsted acids in carbon ─ carbon bond formation reactions was probably due to the unavailability of Brønsted acids with Classic acid ─ catalyzed reactions, such as the addition of suitable reactivity, which results in poor functional group toler- HCl to alkenes, often show a second ─ order dependence on the ance and unexpected side ─ reactions. acids. In that sense, the involvement of dimeric acids such as To address this challenging issue, we started research in the HCl…HCl might be speculated (Figure 3). 4 The same principle design and development of strong Brønsted acids. We pro- of activation of Brønsted acids plays an important role in posed a “combined acid system” between Brønsted acid/Lewis many cases, a number of which are regarded as Brønsted acid acid and Brønsted acid/Brønsted acid catalysis both intermo- assisted Brønsted acid systems. lecularly and intramolecularly, which is now the general con- cept for designing acid catalysts for asymmetric reactions. Moreover, we developed new strong Brønsted acids, which are chiral N ─ tri yl oxo ─ , thio ─ , and seleno ─ phosphoramides. Herein we wish to describe our efforts towards the develop- ment of strong Brønsted acids and their applications. 2． Combined Acid Catalysis Figure 3. Plausible involvement of dimeric acids in acid ─ catalyzed Coordination of a ketone or aldehyde to a Lewis acid can addition of HCI to alkenes. promote enolization by virtue of the enhanced acidity of the α ─ hydrogen atoms in the complex. The practical utility of These combined acid catalysts can be classied as shown in these types of Lewis acid activation is well developed. For Table 1. It should be emphasized that we anticipated a more or example, changes in acidities of up to 24 pK a units are observed less intramolecular assembly of such combined systems rather by combining the measured gas ─ phase acidities with the calcu- than intermolecular arrangements. Thus, a correct design of 2 lated solvation energies for these species (Figure 1). This the catalyst structure is essential for success. example can be described as a typical case for a Lewis acid activation of a weak Brønsted acid. Table 1. The general classications of combined acid catalysis. Figure 1. Lewis acid activation of a weak Brønsted acid. Similar activations can be readily found in various reports of combined acid complexes. Clark showed that the catalytic 1 116 （ 8 ） J. Synth. Org. Chem., Jpn. 有機合成化学71-11＿04論文_Hirashima.indd 8 2013/10/21 10:56:36 2.1 Lewis Acid Assisted Brønsted Acid Catalysts (LBAs) We demonstrated that the synthetic utility of LBAs applied The combination of Lewis acids and Brønsted acids gives to the regio ─ and stereoselective isomerization of a “kinetic” Lewis acid assisted Brønsted acid catalysts and provides an silyl enol ether to a “thermodynamic” one catalyzed by an opportunity to design a “unique proton”, 5 that is, the coordina- achiral LBA (Scheme 3). 8 “Kinetic” TBS enol ethers were tion of a Lewis acid to the heteroatom of the Brønsted acid isomerized to “thermodynamic” species by the coordinate could increase the acidity of the latter. complexes of SnCl 4 catalyst and the monoalkyl ethers of In 1994, we reported that chiral LBAs can be generated in biphenol. For the various structurally diverse substrates, the situ from optically pure BINOL and SnCl 4 in toluene and is isomerization proceeded cleanly in the presence of 5 mol% of stable in solution, even at room temperature. 6 The protonation the achiral LBA. of the silyl enol ether derived from 2 ─ phenylcyclohexanone afforded the S isomer with 97% ee in the presence of a stoi- Scheme 3. Isomerization of silyl enol ethers catalyzed by achiral LBA. TBS = tert butyldimethylsilyl. chiometric amount of (R) ─ LBA (Scheme 1). This LBA reagent ─ could be also applicable to various ketene bis(trialkylsilyl)ace- tals derived from α ─ aryl carboxylic acids. The sense of stereo- induction can be understood in terms of the proposed transi- tion ─ state assembly shown in Scheme 1. The trialkylsiloxy group is directed opposite to the binaphthyl moiety to avoid any steric interaction, and the aryl group is stacked on this naphthyl group. In further studies, enantioselective proton- ation with a stoichiometric amount of an achiral proton source and a catalytic amount of the chiral LBA was possible (Scheme 2). 6b,7 In 2003, optically active 1,2 diarylethane 1,2 diol·SnCl Scheme 1. Enantioselective protonation with chiral LBA generated ─ ─ ─ 4 derivatives were designed as a new type of LBA for enantiose- from (R) ─ BINOL and SnCl 4. lective protonation (Scheme 4). 9 A variety of optically active 1,2 ─ diarylethane ─ 1,2 ─ diols could be readily prepared by asym- metric syn dihydroxylation, which is advantageous to the use of BINOL for the exible design of a new LBA. The most sig- nicant nding is the identication of the conformational direction of the H─ O bond of LBA by X ─ ray diffraction analy- sis (Figure 4). The stereochemical outcome of the enantiose- lective protonation of silyl enol ethers with this LBA could be controlled by a liner OH ─ π interaction with in the initial step (Scheme 4). In 1999, we successfully developed the rst enantioselective biomimetic cyclization of polyprenoids catalyzed by chiral LBA (Scheme 5). 10 We found that the tricyclic ethers were obtained from geranyl phenyl ethers, o ─ geranylphenols, and geranylacetone derivatives using (R) ─ BINOL derivatives and SnCl 4. The reaction proceeded smoothly even in catalytic Scheme 2. Catalytic enantioselective protonation with chiral LBA Scheme 4. Enantioselective protonation with chiral LBA generated from chiral hydrobenzoin derivative and SnCl . generated from monoprotected BINOL and SnCl 4. 4 Vol.71 No.11 2013 （ 9 ） 1117 有機合成化学71-11＿04論文_Hirashima.indd 9 2013/10/21 10:56:37 amounts of the chiral LBA and often took place via [1,3] ─ Claisen ─ type rearrangements using geranyl phenyl ethers as substrates. The best result of 90% ee was observed for the cyclization of p ─ bromophenyl geranyl ether. This LBA approach was also applied to the enantioselective cyclization of homo(polyprenyl)arenes by (R) ─ BINOL ─ o ─ F ─ Bn·SnCl 4 11 (Scheme 5). Several optically active podocarpa ─ 8,11,13 ─ tri- ene diterpenoids and (-) ─ tetracyclic polyprenoids of sedimen- tary origin were synthesized by the enantioselective cyclization of homo(polyprenyl)benzene derivatives induced by this LBA and subsequent diastereoselective cyclization induced by BF 3·Et 2O or CF 3CO 2H·SnCl 4 (75 ─ 80% ee). Moreover, the syn- thetic utility of LBA catalysts was demonstrated by very ef- cient route to (-) ─ 11’ ─ deoxytaondiol methyl ether. Figure 4. X ─ ray structure of LBA generated from monomethylated Very recently, Corey and co ─ workers reported the enanti- chiral hydrobenzoin and SnCl 4 (the O1 ─ H act bond distance shown here is not certain, but its direction could be deter- oselective proton ─ initiated polycyclization of polyenes by the 12 mined). Selected distances ( Å ): O1 ─ H act = 0.72(3), Sn1 ─ 1:1 complex of o,o’ ─ dichloro ─ BINOL and SbCl 5 (Scheme 6). Cl3 = 2.40185(5), Sn1 ─ Sl4 = 2.3522(6), intermolecular This LBA reagent afforded the polycyclized products in high H act…Cl3 = 2.402. Torsion angles (deg): Cl3 ─ Sn1 ─ H act = yields (up to 89%) and enatioselectivity (up to 92% ee). -64, Cl1 ─ Sn1 ─ O1 ─ H act = 30. Bond angles (deg): Sn1 ─ O1 ─ 2.2 Brønsted Acid Assisted Brønsted Acid Catalysts (BBAs) H ac = 119(20, C2 ─ O1 ─ H act = - 64(2), intermolecular O1 ─ H act…Cl3 = 171.11. Hydrogen bonding plays a crucial role in the organization of the three ─ dimensional structure of enzymes and is often Scheme 5. Biomimetic cyclizations catalyzed by chiral LBAs. 1 118 （ 10 ） J. Synth. Org. Chem., Jpn. 有機合成化学71-11＿04論文_Hirashima.indd 10 2013/10/21 10:56:38 Scheme 6. Enantioselective polycyclization by Alder reactions of a wide range of aliphatic and aromatic 17 (R) ─ o ,o’ ─ dichloro ─ BINOL·SbCl 5. aldehydes (Scheme 7). This BBA catalyst can share with TADDOLs the bis(diarylhydroxymethyl) functionality, in which the steric and electronic properties are readily tunable. The axial chirality in BAMOL provides further opportunity for tweaking the chiral environment. The X ─ ray crystal struc- ture not only shows a 1:1 complex of BAMOL and benzalde- hyde, but also reveals the presence of an intramolecular hydro- gen bond between the two hydroxyls and an intermolecular hydrogen bond to the carbonyl oxygen of benzaldehyde (Fig- ure 7).
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