2-naphthol derivatives enantioselective reactions

They next studied the asymmetric oxidative polymerization of achiral 2,3-dihydroxynaphthalene (Scheme 42). The polymerization of this monomer with CuCl2-(-)-sparteine complex resulted in a low yield and gave a low molecular weight oligomer, whereas the polymerization with CuCl-(S)-Phbox quantitatively gave a polymer with Mn of 10 600-15 300. The enantioselectiv-ity attained in this polymerization, however, was estimated to be low, with 43% ee from the model reaction [169]. When vanadyl sulfate (VOSO -Phbox complex was used instead of the copper catalyst system, the enantioselectivity was improved up to 80% ee [170]. Asymmetric cross-coupling polymerization of two kinds of naphthol derivatives was also reported [171,172]. 

Chiral metal alkoxides M(OR)4 have been developed as asymmetric variants of ordinary Lewis acids, such as A1C13 and ZrCU, and are used as catalysts for selective carbon-carbon bond formation. Thus, starting from bidentate l,l -bi-2-naphthol derivatives (BINOL) and SnCU, a series of chiral tin(iv) aryloxides 221 (Figure 7) was prepared and successfully applied to the enantioselective Diels-Alder reaction <2006TL873>. Similar silocanes obtained from menthone- or camphor-derived 2,2 -biphenols have been obtained and their configuration was analyzed by NOE differential spectroscopy (NOEDS) <1997JOC7156>. 

Scheme 16. Comparison of TADDOL with BINOL and Future Goals. Derivatives of (R,R)-TADDOLs and of (P)- or (S)-BINOL, when employed in the same reaction, often give the same product enantiomer preferentially (cf. the Cj-symmetrical structural similarity). While TADDOLs are much easier to prepare and to modify, they are many orders of magnitude less acidic than BINOLs (TADDOLates are less polar ligands to a metal). Some TADDOL derivatives contain CPh2X groups which are labile to (undesired) nucleophilic substitution the dioxolane group of TADDOLs, on the other hand, is surprisingly stable to hydrolysis. BINOL (a naphthol derivative ) can be very sensitive to oxidation and (undesired) electrophilic aromatic substitution, and there are conditions under which it may racemize. Some goals to increase the usefulness of the TADDOL system are shown at the bottom of the Scheme other P derivatives, more acidic derivatives, reagents for enantioselective protonation, deprotonation,...

The ability of chiral binaphthol (BINOL) derivatives as a Br0nsted acid catalyst to facilitate asymmetric transformations was demonstrated by Schaus by the development of the highly enantioselective Morita-Baylis-Hilhnan reaction of cyclohex-enone with aldehydes effectively catalyzed by (R)-octahydro-l,l -bi-2-naphthol derivatives, bearing either 3,5-bis(trifluoromethyl)phenyl (24a) or 3,5-xylyl (24b) groups at the 3,3 -positions, in combined use with triethylphosphine as a nucleophilic promoter (Scheme 7.43) [68]. 

Intramolecular carbon-nitrogen bond formation may result from the Ullmann coupling of l,3-bis(2-iodoaryl)propan-2-amines catalysed by copper. Using (i )-BINOL, l,l -Bi-2-naphthol, ligands, the enantioselective formation of indolines and 1,2,3,4-tetrahydroquinolines was achieved. Copper catalysis has also been used in the intramolecular formation of imidazobenzimidazole derivatives. The reaction is likely to involve the formation of intermediates, such as (16), which on aerobic oxidation yield the product. There is evidence for an intramolecular 0- -N Smiles rearrangement, as... 

Mikami et al. have reported that the chiral titanium reagent 12 derived from bi-naphthol and TiCl2(0-i-Pr)2 catalyzes the Diels-Adder reaction of a-bromoacrolein or methacrolein with isoprene or 1-methoxy-l,3-butadiene to afford the cycloadducts with high enantioselectivity [18] (Scheme 1.25). 

A heterobimetallic BINOL-Ga/Li complex 53 has been developed for the enantioselective ARO of meso-cpoxides (BINOL = l,T-bi(2-naphthol)).278 Using />-methoxyphenol as the nucleophile, this etherification reaction was observed to take place with a high level of asymmetric induction. An improved catalyst 54 has also been reported that exhibits greater stability under the reaction conditions and delivers higher yields and ee s (Equation (78)).279 A simple catalyst derived from Sc(OTf)3 and the chiral bipyridine ligand 52 has been shown to be effective for the ARO of aryl-substituted /// -epoxides with aliphatic alcohols to give high ee s (Equation (79)).280... 

In 1997, Kobayashi and colleagues reported the first truly catalytic enantioselective Mannich-type reactions of aldimines 24 with silyl enolates 37 using a novel chiral zirconium catalyst 38 prepared from zirconium (IV) fert-butoxide, 2 equivalents of (R)-6,6 -dibromo-l,l -bi-2-naphthol, and N-methylimidazole (Scheme 13) [27, 28], In addition to imines derived from aromatic aldehydes, those derived from heterocyclic aldehydes also worked well in this reaction, and good to high yields and enantiomeric excess were obtained. The hydroxy group of the 2-hydroxyphenylimine moiety, which coordinates to the zirconium as a bidentate ligand, is essential to obtain high selectivity in this method. 

In this chapter, we focus on recent achievements in the enantioselective synthesis of chiral amines using 1,1 bi 2 naphthol (BINOL) derived monophosphoric acid (1) or related phosphoric acids as chiral Bronsted acid catalysts 2, 3], The contents are arranged according to the type of bond forming reaction, including carbon carbon, carbon hydrogen, and carbon heteroatom bond forming reactions, followed by specific reaction types. 

In the previous section, lanthanide triflates were shown to be excellent catalysts for achiral aza Diels-Alder reactions. While stoichiometric amounts of Lewis acids are required in many cases, a small amount of the triflate effectively catalyzes the reactions. On the other hand, chiral lanthanide Lewis acids have been developed to realize highly enantioselective Diels-Alder reactions of 2-ox-azolidin-l-one with dienes [89]. The reaction of N-benzylideneaniline with cyclop entadiene was first performed under the influence of 20 mol% of a chiral ytterbium Lewis acid prepared from ytterbium triflate (Yb(OTf)3), fR)-(+)-l,l -bi-naphthol (BINOL), and trimethylpiperidine (TMP). The reaction proceeded smoothly at room temperature to afford the desired tetrahydroquinoline derivative in a 53% yield, although no chiral induction was observed. At this stage, it was indicated that bidentate coordination between a substrate and a chiral Lewis acid would be necessary for reasonable chiral induction. N-Benzylidene-2-hydroxy aniline (31a) was then prepared, and the reaction with cyclopentadiene (32a) was examined. It was found that the reaction proceeded smoothly to afford the corresponding 8-hydroxyquinoline derivative (33a) [90] in a high yield. The enantiomeric excess of the cis adduct in the first trial was only 6% however, the selectivity increased when diazabicyclo-[5,4,0]-undec-7-ene (DBU) was used in-... 

On the other hand, 2-naphthols have been used with different success as Michael donors in conjugate Friedel Crafts reactions with nitroalkenes and related substrates (Scheme 4.53). For example, cinchonine-derived thiourea 72b was identified as an excellent promoter for the reaction of a wide variety of 2-naphthols and nitroolefins, providing excellent yields and enantioselectivities. Remarkably, the more challenging p-alkyl substituted nitroalkenes were also found to undergo the reaction in a highly stereoselective way and with comparable yields to those obtained when nitrostyrene derivatives were employed. 

Another example of organocatalytic AFC alkylation reaction with p,y-unsaturated a-keto esters was reported by Wang and co-workers in 2012. In the presence of 10 mol% rosin-derived tertiary amine-thiourea 96, a variety of p,y-unsaturated a-keto esters reacted with 1-naphthol smoothly to afford the modified chromanes 97 in good yields (79-86%) with up to 96% ee (Scheme 6.39). Again, the hydrogen bonding between substrates and catalyst was proposed to be a key element for the enantioselective control. 

The group of Aleman and Parra reported the AFC reaction of phenol with Z-bromonitroalkene followed by a nucleophilic substitution on the bromide carbon. With 10 mol% of squaramide catalyst 157, the chiral trans-dihydroarylfuran derivatives 158 were efficiently synthesized from Z-bromonitroalkene and naphthol or phenol derivatives with moderate to good yields (28-94%) and excellent enantioselectivity (66-98% ee). The key of this eat-alytic system is the neutralization of the generated HBr by a stoichiometrie eo-base (Scheme 6.73). 

Cinchonidine (99) has extended the substrate scope of the ketone conjugate additions to P-substituted methylidene malononitriles. In particular, the reaction of a-chloromethyl ketones, under very low loading conditions, affords tetrasubstituted cyclopropanes in moderate to good enantioselectivities after intramolecular cycliza-tion (Scheme 2.45) [142], A similar strategy has been followed to synthesize, with moderate to good enantioselectivities (56-90% ee) optically active naphlhopyran derivatives by a conjugate addition/cyclization sequence between 2-naphthol and a,a-dicyanoolefins [ 143 ]. 

On the other hand, Yang etal. [33] have developed an organocatalyzed enantioselective FC-type addition reaction of 2-naphthol 212 with p,y-unsaturated a-ketoesters 209 using a cinchona alkaloid-derived thiourea catalyst 213 (Scheme 2.30). The resulting product 214 is in rapid equilibrium with the cyclic hemiketal 215, which was dehydrated with a catalytic amount of concentrated H SO in a one-pot fashion, providing the naphthopyran derivatives 216 with moderate to good yields (51-91%) and enantioselectivities (57-90% ee). 

Wang elegantly demonstrated the potentiality of chiral diarylprolinol ether 54 in the synthesis of chromanes 56 via enantioselective Michael-type Friedel-Crafts alkylation/cychzation cascade synthetic sequence between 5a and a,p-unsaturated aldehydes 37a [30a]. Under optimal conditions, moderate diastereoselectivity and high enantioselectivity were obtained. Differently, phenol was found unreactive (Scheme 5.17a). The same team years later documented also the activity of a rosin-derived tertiary amine-thiourea 55 in similar process involving 1- and 2-naphthols and P,y-unsaturated a-ketoesters 25 (Scheme 5.17b) [30b]. A proposed model of the enantiodiscrimi-nating step of the reaction is also provided by the authors (58). 

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