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Introduction
Over the last 20 years an explosive growth of research in the field of asymmetric synthesis has occurred 1 -^3. The aim of enantioselective synthesis or catalysis is to produce chiral products (a single enantiomer as the ultimate goal) starting from achiral substrates by exploiting the presence of chiral reagents. The role of these latter is to generate diastereomeric transition states leading to the two enantiomers so that one of them is preferentially formed. In such a schematic representation, only two competitive pathways are present, one leading to the R enantiomer and the other giving rise to the S one. Actually, the situation can be more complex. For instance, both the substrate and the reagent can exist as a mixture of conformational isomers, several conformations can be significantly populated, and they can also exist in different states of aggregation or solvation, each of these species showing its own reac- tivity. The final result is a weighted average depending on the distribution and reactivity of the species involved, and a low stereoselectivity is generally obtained. A rational approach to the control of stereo-selectivity is based on the use of molecules possessing only symmetry elements of pure rotation and belonging to the Cn or Dn symmetry groups, allowing the prediction of the enantioselectivity due to the presence in solution of only a single defined reactive species. Under these assumptions, interest in the applications of chiral atropoisomers, especially binaphthalene systems, has blossomed.4,
BINOL Synthesis
Properties and Synthesis of Racemic BINOL:-
2,2′ - Disubstituted derivatives of 1,1′ - binaphthyl have been widely used in organic synthesis. The stability of the enantiomers, with barriers of rotation ranging from 23.8 kcal/mol for 1,1′ - binaphthyl to more than 46 kcal/mol for 2,2′ - diiodo- 1,1′ - binaphthyl, enables their use as chirality inducers in asymmetric reactions. The most important compound of this type is 1,1′ - binaphthyl- 2,2′ - diol (BINOL, C20H14O2, mp 215-217 °C) the chiral atropo isomers (R)- 1 ([R]20D ) +35.5 (THF, c = 1), mp 205-211 °C and (S)-1 ([R]20D) - 34.5 (THF, c =1), mp 205-211 °C of which are stable at high temperature 6 and allow numerous asymmetric reactions under various experimental conditions.(Scheme 1)
Scheme 1
BINOL 1 is the best known representative of axially chiral molecules^7 and was first prepared as a racemate in 1873 by von Richter.^8 Since this date,the preparation of racemic BINOL 1 has been widely studied, and a well-established method is the oxidative coupling of 2-naphthol using FeCl3,9,10^ K3Fe(CN) 6 ,^11 Mn(acac) 3 (acac = acetylacetone),^12 Cu-amine complexes,^13 or TiCl 4 14 as coupling agents with chemical yields up to 90% being commonly reached. A mechanistic rationale implying that the formation of one molecule of 1 requires 1 equiv of Fe3+ has been proposed. This suggests that the radical species 2•, resulting from a one - electron oxidation of 2 with Fe3+, adds to another neutral 2 to form a new C-C bond and generates carbinyl radical 3•, which eliminates H• and is further oxidized by O2 to release H+ and regain aromaticity.(Scheme 2) These coupling reactions are not catalytic processes and require more than stoichiometric amounts of the metal salts. A few exceptions are the coupling that proceeds catalytically under ultrasonic irradiation of aerated powder mixtures of 2-naphthol and Fe Cl3‚6H2O ( mol %) at 50 °C, FeCl3/Al2O3, the use of Cu(II)-amine complex (1 mol %), VO(acac)2,methyltrioxo rhenium,38 an alumina-supported copper(II) sulfate catalyst ( mol %) under aerated conditions,a vanadyl phosphate (VOPC), and mesoporous molecular sieves (Scheme 3).^15
Scheme 2
(b) An enzymatic resolution of rac-BINOL based on a monomethyl etherification reaction promoted by a transport protein, bovine serum albumin (BSA).^17 In this case, enantiomeric excesses only up to 59% and 41% have been encountered, respectively, for (R)-BINOL 1 and the methylated (S)-product.(Scheme 5) 18
ii)Chemical Resolution of Racemic BINOL:-
The chemical resolution of rac-BINOL has been extensively reported in the literature, and in all cases the methods are based on the easy separation of the pair of diastereomers derived from the reaction of rac-BINOL with a chiral auxiliary. Thus the synthesis of cyclic binaphthyl phosphoric acids from rac-BINOL and their successful resolution via their cinchonine salts. The (R)- and (S)-enantiomers of BINOL were obtained in, respectively, 24% (96% ee) and 26% isolated yield (90% ee).(Scheme 6)^19 Nevertheless, the overall resolved yield is only moderate, and the enantiomeric purity is not satisfactory (respectively, 96% and 90% ee for (R)-BINOL and (S)-BINOL). Moreover, cinchonine is expensive and often recovered in contaminated form. In 1990, a similar method
Scheme 5
Scheme 6
was reported by Miyano et al. by replacement of the cinchonine salt with the less expensive (R)- 2 - aminobutanol. In this case, although both enantiomers are pure, they are obtained in even lower yield (respectively, 30% and 15% for (R)-BINOL and (S)-BINOL .(Scheme 7)^20 Another efficient, practical, and inexpensive method described in 1993 by Brunel et al.^21 involves the tricoordinated compound , easily prepared from phosphorus trichloride and L- menthol. This methodology has been recently reproduced and improved by Tang et al.(Scheme 8)^22 This Compound reacts with rac-BINOL leading to a 1:1 mixture of two diastereomers in quantitative yield. Complete separation of the two diastereomers was achieved in a single recrystallization from diethyl ether. Oxidation with 30% hydrogen peroxide led to the expected compounds which were reduced with LiAlH4 to afford enantiomerically pure (R)- 1 and (S)-1 in 81% and 85% overall yield, respectively.(Scheme 9)
Scheme 7
Scheme 8
iii) Enantioselective Synthesis of BINOL 1 by Oxidative Coupling of 2-Naphthol:-
Although enantioselective oxidative coupling of 2-naphthol with chiral catalysts provides one of the simplest routes to optically active BINOL. An oxidative coupling of 2-naphthol by stirring a mixture of cupric-(S)-phenylethylamine complex and 2-naphthol in equimolar quantities at room temperature under a nitrogen atmosphere.(Scheme 11)^26 BINOL DERIVATIVE Aside from the starting materials derived directly from the chiral pool, (R)- and (S)-BINOL in high enantiopurity (>99%enantiomeric excess) are two of the most inexpensive sources of chirality for organic synthesis. As a consequence, it serves as an important starting material for other sources of chirality for stereoselective synthesis, both stoichiometric and substoichiometric (catalytic).Many important chiral ligands are constructed from the binaphthyl scaffold and ultimately derived from BINOL as a starting material, BINAP being one of the most well known and important. The compound aluminium lithium bis(binaphthoxide) (ALB) is prepared by reaction of BINOL with lithium aluminium hydride. In a different stoichiometric ratio (1:1 BINOL/LiAlH4 instead of 2:1),the chiral reducing agent BINAL (lithium dihydrido(binaphthoxy) aluminate) is produced.(Scheme 12)27,
Scheme 11
It has been employed in an asymmetric Michael reaction with cyclohexenone and dimethyl malonate.(Scheme 13) BINOL MEDIATED REACTIONS
BINOL-Mediated Asymmetric Oxidation and Reduction Reactions:-
Enantioselective Reduction Reactions i)The asymmetric reduction of acylstannanes to prepare enantiomerically enriched alpha- alkoxystannanes^29 in enantiomeric excesses up to 94% ee.(Scheme 14)
Scheme 12
Scheme 13
iv)Recently, an optically active lithium-alkoxide-catalyzed asymmetric reduction of imines with trimethoxyhydrosilane was reported by Hosomi et al. affording the expected amines in moderate ee (up to 72% ee) (Scheme 17)^32 Enantioselective Oxidation Reactions:- Oxidation of Sulfides The catalytic asymmetric oxidation of sulfides to chiral sulfoxides in moderate yield with tert- butylhydroperoxide and renewable furylhydroperoxide was achieved using titanium complex produced in situ from a titanium alkoxide and (R)-BINOL. The highest enantioselectivities (upto 93% ee) were obtained with 2.5 mol % of catalyst. The presence of more than 1 equiv of water with respect to sulfide was essential for the oxidation, and it was found that the water was necessary not only to produce an effective catalyst for a highly enantioselective oxidation but also to maintain the catalytic activity of complex for a longer period of time. A moderate level of asymmetric amplification was observed with this catalytic system. From a mechanistic standpoint, it was revealed that the initial asymmetric oxidation to the chiral sulfoxide is followed by the kinetic resolution of the sulfoxide. The nature of the active species involved in this process has not yet been clearly established.(Scheme 18)33,34,
Scheme 16
Scheme 17
Epoxidation of Olefins The asymmetric epoxidation of olefins is one of the most useful and challenging reactions in modern organic chemistry. In this area, chiral BINOL has been extensively used to develop catalytic or stoichiometric methods of enantioselective epoxidation. Recently, Shibasaki et al. used lanthanum- or ytterbium modified BINOL derivatives as catalysts in the asymmetric epoxidation of enones using hydroperoxides such as tert-butyl hydroperoxide (TBHP).36,67,38, It appears that the enones were best converted to the corresponding epoxides by using the ytterbium.(Scheme 19)
Scheme 18
Scheme 19
Aldol and Related Reactions:-
Titanium-Catalyzed Vinylogous and Homoaldol Reactions- BINOL complexes have also been used in various vinylogous aldol reactions involving acyclic or cyclic enol ethers. Thus, in the presence of BINOL-Ti catalyst, silyoxydioxine and benzaldehyde led to the corresponding alcohol with very good yield and enantiomeric excess. Yields obtained with cinnamaldehyde and pentanal were lower although the enantiomeric excess remained good.(Scheme 22)45, Catalytic Asymmetric Nitroaldol Reaction- The concept of heterobimetallic chiral catalysts has been applied in asymmetric nitroaldol reactions. In this case, the most efficient catalyst was found to be the lanthanum-lithium- BINOL complex (LaLi BINOL (LLB)) leading to enantiomeric excesses varying from 40% to 96% ee .(Scheme 23)47,48,
Scheme 22
Scheme 23
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