Fluorine-Containing Synthons - American Chemical Society

Chapter 16

Polyfluorinated Binaphthol Ligands in Asymmetric Catalysis

Downloaded by UNIV OF PITTSBURGH on August 12, 2015 | http://pubs.acs.org Publication Date: July 21, 2005 | doi: 10.1021/bk-2005-0911.ch016

Yu Chen and AndreiK.Yudin* Davenport Research Laboratories, Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada

Introduction Modern asymmetric synthesis relies on new and improved catalytic transformations. Understanding the balance of steric and electronic factors is a prerequisite tofine-tuninga catalyst to achieve optimal selectivity in a particular reaction. Among the chiral ligands developed so far, BINOL 1 and related molecules with axial chirality have found wide utility in asymmetric catalysis

(1).

Figure 1. BINOL

BINOL was first synthesized in 1926 (2), however, its potential as a ligand for metal-mediated catalysis was left unrecognized until 1979 when Noyori demonstrated its utility in the reduction of aromatic ketones and aldehydes (3). Since Noyori's discovery, many modifications of the BINOL skeleton aimed at changing its steric and electronic properties have been reported (4). For example, partially hydrogenated BINOL was used in enantioselective alkylation of aldehydes (4a); conjugate addition of diethylzinc to cyclic enones (4b); and ring opening of epoxides (4c). Selective bromination at the 6 and 6' positions of 288

© 2005 American Chemical Society

In Fluorine-Containing Synthons; Soloshonok, V.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.


289

the binaphthyl ring was shown to increase the enantioselectivity of the corresponding titanium catalysts in glyoxylate-ene reactions (4d). Bulky triarylsilyl groups at the 3 and 3'positions of the binaphthyl ring led to increased levels of enantiofacial discrimination of aldehydes in asymmetric Diels-Alder reactions (4e). 3,3'-Dinitrooctahydrobinaphthol was successfully applied in titanium-catalyzed asymmetric oxidation of methyl p-tolyl sulfide (4f). Our explorations in the field of binaphthyl catalysis led us to synthesize partially fluorinated species. This interest was driven by the known stability of C-F bond towards oxidation (5); propensity of fluoroaromatics to engage in stabilizing stacking interactions with electron-rich aromatic rings (6), and well documented nucleophilic aromatic substitution chemistry of fluoroaromatic compounds (7). We envisaged that substitution of hydrogens byfluorinesat the selected positions of the binaphthol ring should induce considerable change in the electronic character of the aromatic system. In this review we will discuss our recent efforts in the development of chiral polyfluorinated binaphthols as well as their application in asymmetric catalysis.

Preparation of FgBINOL and F BINOL 4

The racemic form of FgBINOL 2 was prepared according to Scheme 1. Tetrafluorobenzyne 3, generated by treating chloropentafluorobenzene with nbutyllithium at -15°C, was reacted with 3-methoxythiophene (8) in a Diels-Alder reaction. Upon in situ extrusion of sulfur, 2-methoxy-5,6,7,8tetrafluoronaphthalene 4 was obtained in 52% yield. Demethylation with BBr gave 5,6,7,8-tetrafluoronaphthol 5, which failed to undergo the FeCl -catalyzed oxidative coupling, commonly used for the preparation of BINOL from 2naphthol. This lack of reactivity is believed to result from a relatively high oxidation potential of 5 (1.84V vs Ag/AgCl). The reductive Ullmann coupling was therefore chosen through the intermediacy of the 1-brominated precursor 6 to give the desired bis(methoxy) product 7 in 85% yield. Demethylation of 7 produced 2 in 90% yield, which was resolved throughfractionalcrystallization of the bis[(-)-menthoxycarbonyl] derivatives 8 to afford the enantiomerically pure F BINOL 2 (Scheme 2) (P).Our initial attemps to prepare F BINOL 9 through the coupling methods incorporating standard Suzuki and Stille protocols failed. Fortunately, the oxidative cross-coupling using Cu(OH)ClTMEDA as the catalyst was successful (for selected examples on Ullmann and oxidative coupling see (2,10,11% however, only moderate yield (40%) was obtained. The optimal reaction condition involved mixing 1 eq of 2-naphthol, 0.7 eq of 5,6,7,8tetrafluoronaphthol 5, and 10 mol% Cu(OH)ClTMEDA at 140°C for 24 hours. The main by-products of this reaction were BINOL and FgBINOL, which were isolated in 50% yield and less than 10% yield, respectively (Scheme 3). 3

3

8

4


290


3-methoxythiophene

*DMAP=4-(Dimethylamino)pyridine

(R)-2

(R)4

Scheme 2

OH

f

OH

5 (0.7 eq.) Cu(OH)CITMEDA (10mol%) 140'C, 24h

40% Scheme 3

*TMEDA = tetramethyl ethylenediarnine


291

A reductive pathway similar to our approach to F BINOL proved to be even less successful. The Ullmann coupling between 6 and l-bromo-2methoxynaphthol afforded only 5% yield of the desired product, and the major product observed was 2,2'-bismethoxy-F BINOL obtained in 85% yield. Since reductive couplings typically proceed in favor of electron-poor substrates, aldehyde 10 was utilized in place of l-bromo-2-methoxynaphthol (Scheme 4). The desired product 11 was obtained in 48% yield, which was then oxidized through a Baeyer-Villiger reaction, followed by hydrolysis to give alcohol 12 in 64% yield (two steps). Demethylation with BBr afforded racemic F BINOL 9 in 92% yield. The resolution of homochiral F BINOL was accomplished through a similar procedure to that used with FgBINOL (12). 8

8


3

4

4

*DMAP=4-(DimethyIamino)-pyridine Py=pyridine Scheme 4

Regioselective Nucleophilic Aromatic Substitution of Fluorine on Polyfluorinated Binaphthols The electron-deficient nature of the polyfluorinated aromatic rings was found to raise the oxidative stability of compound 2 compared to 1 as well as to increase the acidity of the ring-bound hydroxyl groups. Furthermore, the pKa' of the hydroxyl group in 1 decreases by 1 unit upon fluorine substitution at the aromatic rings (1, pKa'=10.28; 2, pKa'=9.29). The average pKa' of F BINOL 9 is 9.8, just between that of FgBINOL and BINOL (13). Since fluorine atom is 4



292 only 0.27Â larger than hydrogen atom, one could anticipate only a small increase in the barrier to axial torsion upon fluorination. Such a combination of steric and electronic alterations in fact was found to lead to dramatic increase in configurational stability of homochiral 2 with electronic effects playing a decisive role. Contrary to 1, which readily undergoes racemization in both acidic and basic media (14), a dramatic increase in configurational stability of (-)-2 was observed under both acidic and basic conditions. When (-)-2 was subjected to reflux in a 1:1 mixture of THF and 13% aq HCI, no racemization was detected after 24h. In comparison, (-)-l was racemized under these conditions decreasing enantiomeric excess of (-)-l from 99% to 13%. On the other hand, the enantiomeric excess of (-)-l decreased from 99% to 0% after 12h boiling in 5% aqueous NaOH, however, no racemization was observed when (-)-2 was refluxed in aqueous NaOH after 24h. The nucleophilic displacement of aromaticfluorineis a well-known reaction with a wide scope and utility (7). A variety of nucleophiles are known to participate in this chemistry. We thought that this chemistry might provide a versatile and mild route to functionalizedfluorinatedligands with axial chirality. Thus, various alkoxides have been used as nucleophiles, and the corresponding products 15 were obtained in moderate to good yields with no racemization observed in the course of any of these substitution processes (12,15) (Scheme 5, eq 1 and 2). it should be noted that the 2,2'-hydroxyl groups on either F BINOL or F4BINOL require protection since the reaction of sodium alkoxide with unprotected polyfluorinated binaphthols can produce a complicated mixture of polymethoxylated products. Alkyllithium species have also been used as nucleophiles to modify the 7-position of thefluorinatedring system. In this case, the unprotected polyfluorinated binaphthols can be used directly. For instance, the reaction between unprotected F BINOL and 'BuLi occurs at -78°C with the desired product 20 obtained in 40% yield. (Scheme 5, eq 3) 8

4

Scheme 5


293

Application of Polyfluorinated Binaphthols in Asymmetric catalysis


Oxidation of Sulfides to Sulfoxides Enantiomerically pure sulfoxides are important compounds which have provided increasing application in asymmetric synthesis (16). To illustrate the utility of the fluorinated binaphthol-derived catalysts in asymmetric catalysis, we first chose sulfide oxidation as a model reaction (17,12,15). The results of this study are summarized in Scheme 6. Compared with (R)-BWOL, both (R)F BINOL and (K>-F BINOL proved to be more efficient, and the % ee of the sulfoxide product 21 was improved (Scheme 6, entry 1, 2 and 3). Further introduction of steric bulk to the 7 and/or 7'-position(s) of the binaphthol rings resulted in decreased enantioselectivity as well as chemical yield (Scheme 6, entry 4-6). Slightly increased enantioselectivity was observed when lowering the reaction temperature, however, chemical yield was decreased (Scheme 6, entry 7). Similar to the previous results reported by Kagan and co-workers (18), water was essential to ensure rapid turnover in the titanium/diethyl tartrate oxidation system and two equivalents of water with respect to sulfide were found necessary for the turnover to take place in our case. It should also be noted that the most intriguing difference between the fluorinated binaphthols and BINOL systems in this sulfoxidation reaction is that a reversal of asymmetric induction upon fluorine substitution was observed (Scheme 6, entry 1 to entries 2-7) (19). ο t 8

4

a

S

x

Tl(0'Pr) /LM1/2)(5mol%)

^ y

H 0 (2eq), CMHP (1.2eq) CHCI

K^J

4

2

3

Entry

Ligand

8

^

21

Time (h)

Temperature (°C)

Yield (%)

ee (%)

1

(R)-B\mi

42

0

69

3W

2

(fl>F BINOL

4.5

0

77

75 (S)

3

(R)-? BMOL

2

0

78

B0(S)

4

(RH9

3

0

49

28 (S)

e

4

5

(R)-20

2

0

32

27 (S)

6

fRJ-16a

5

0

47

51 (S)

7

(KJ-F BiNOL

18

-20

46

84 (S)

4

Scheme 6


294

Diethylzinc Addition to Aldehydes Enantioselective addition of dialkylzinc to aldehydes is one of the most reliable methods to prepare chiral .sec-alcohols (20). It is also a standard reaction to test the reactivity and enantioselectivity of newly designed chiral ligands. Thus, the efficiency of the fluorinated binaphthol ligands were tested in the asymmetric addition of diethylzinc to naphthaldehyde (21). The catalysts were formed by mixing TiCPrO) with fR>BINOL 1, fR>F BINOL 2, or 7,7'disubstituted (T^-Fg-BINOL 16 under the conditions where formation of monomeric catalyst precursors of 1:1 composition is favored (7:1 Ti('PrO) /Iigand ratio). The results showed that all catalysts give similar enantioselectivities (Scheme 7), which indicates that both steric (substitution at the 7,7'-positions) and electronic effects are relatively insignificant in this case.


4

8

4

1. Τφ'Ρή ίΙ* (7/1) BINOL/Ti('PrO) to the more Lewis acidic (R)ο,ό'-Β^-ΒΙΝΟί 27/Ti('PrO) , the %ee was increased from 69% to 80% ( Scheme 9, entry 1 and 2). Due to the highly electron-defficient nature of fluorinated binaphthols, both tetrafluorinated binaphthol (F BINOL) and octafluorinated binaphthol (FgBINOL) were then investigated in the reaction between aromatic amines and ethyl glyoxylate (Scheme 9) (34). When (R)F BINOL was employed in this reaction, a similar result to (7y-6,6'-Br -BINOL was observed (Scheme 9, entry 2 and 3). However, it was found that further increase in the Lewis acidity of the catalyst system by replacing (7?>F -BINOL with (K>F BINOL caused a decrease in both chemical yield and enantioselectivity of the desired product 28(Scheme 9, entry 4). On the other hand, the product 29 was obtained in 38% yield, which is believed to be formed through dehydration of the initially formed 28 followed by another FriedelCrafts reaction with A^jV-dimethylaniline. A "mixed" catalyst system (R)BINOL/f/^FgBINOL/TiOPrO^ was also tested under the same reaction conditions. The product 28 was obtained in 40% yield with 32% ee, while product 29 was obtained in 51% yield (Scheme 9, entry 5). l

4

4

4


4

4

2

4

8

yield (%)

1 2

ee (%)of28

catalyst

Entry

Ti(0'Pr) (5mol%)/(/?;-BINOL (10mol%) 4

Ti(0'Pr)

,

4

(5mol%)/f/?>6,6 -Br -BINOL(10mol%) 2

28

29

94

-

95

69 80 77

3

Ti(0'Pr) (5mol%)/(K>F BINOL(10mol%)

92

4

TKO^r^iSmoiyoJ/f^-FeBINOLilOmo^)

54

38

53

5

Ti(0'Pr) (5ηιοΙ%)/Γ/?;-Ρ ΒΙΝΟί(5ΓηοΙ%)/^-ΒΙΝΟί(5ΓηοΙ%)

40

51

32

4

4

4

8

Scheme 9


297


Epoxidation The study of asymmetric epoxidation of olefins commenced in 1965 (55). In the 1980s, Sharpless and coworkers developed an efficient system that systematically produced either enantiomer of an epoxide from an allylic alcohol using substoichiometric amounts of titanium isopropoxide and diethyl tartrate.(Jtf) Since then, much progress has been made towards asymmetric epoxidation of other types of olefins (37,38). Due to the fact that chiral α,βepoxy ketones are useful intermediates in the synthesis of a wide range of natural products and pharmaceutically relevant molecules (39), great efforts have been recently devoted to developing efficient methods for enantioselective epoxidation of α,β-enones (40). In 1997 Shibasaki and co-workers reported a general catalytic asymmetric epoxidation of enones using alkali metal-free lanthanide-BINOL complexes (41). A few years later, both triphenylarsine oxide (37) and thiphenylphosphine oxide (42) were reported to be effective additives in the La-BINOL catalyzed asymmetric epoxidation. In the presence of 10 to 15moI% of the additive, the catalyst complex La-(/?)-BINOL-Ph PO or La-(/?)BINOL-Ph AsO exhibited both higher reactivity and enantioselectivity than La(#)-BINOL. Qian and co-workers demonstrated an efficient asymmetric epoxidation reaction of enones catalyzed by Ln( PrO) -6,6 -R -BINOL complexes (Ln=Yb, Gd; R=Br, Ph) (39). Their results showed that the steric bulk of substituents at 3,3'-positions on binaphthol rings is detrimental to the enantioselectivity of the reaction. Although the enantioselectivity was also decreased by introducing electron-donating groups at 6,6'-positions of the binaphthol rings, it was found that excellent enantioselectivity could be achieved by introducing electron-withdrawing substituents at these positions. Since fluorine is more electronegative than bromine, we reasoned that the fluorinated binaphthols might be better suited for the Lanthanide-BINOL catalyzed asymmetric epoxidation. The results showed that both ^-FgBINOL and (R)F4BINOL are superior to (7y-6,6'-Br -BINOL in terms of enantioselectivity in the epoxidation reaction of fnms-chalcone 30a. Whlie the C -unsymmetric F4BINOL dramatically improved the ee to 93% (Scheme 10, entry 3), the best ee value was still generated by the most electron-deficient ligand FgBINOL (Scheme 10, entry 2). In this same reaction, Gd-(7?>F BINOL system proved to be more efficient than Yb-fft>F BINOL in both reactivity and enantioselectivity (Scheme 10, entry 2 and 6). The desired product 31a was obtained in 86% yield with 96% ee under the optimal condition (Scheme 10, entry 2). Due to our observation of the superiority of FgBINOL as an efficient ligand in the asymmetric epoxidation of frcms-chalcone, we further explored the generality 3

3

,

,

3

2

2

2

8

8


298

Ln(/-PrO) /llgand(5mol%)

9

3

/^^p^ PrT

h

"Ph

+

CMHP

P h

A|x 31a

30a Ln(/-PrO)

Entry


P h

MS4A, rt.THF

3

ligand

time

yield (%)

ee (%)

1

Gd(/-PrO)

3

(K)-BIN0L

8h

89

83

2

Gd(/-PrO)

3

(K)-F BINOL

6h

88

96

8

3

Gd(/-PrO)

3

(R)-F BIN0L

8h

84

93

4

Gd(APrO)

3

(K)-3,3'-Br BIN0L

48h

36

27

δ

Gd(f-PrO)

6

Yb(/-PrO)

3

3

4

2

(/?)-6,6'-Br BIN0L

8h

95

78

(R)-F BINOL

6h

60

70

2

8

*CMHP=Cumene hydroperoxide Scheme 10

of this catalyst system by reacting a range of α,β-unsaturated ketones 30b-g under the optimal reaction conditions. Good to excellent ee's and yields were obtained on a broad scope of substrates bearing either electro-withdrawing or electron-donating functionalities (Scheme 11); however, no reaction occurred in the case of #*a«s-4-phenyl-3-buten-2-one 30h and /ra«s-4-methoxy-chalcone 30i (Scheme 12, entry 1 and 4). O R R

Gd(/-PrO) /(R)-F BINOL (5mol%) 3

X^

+

Ο

8

C M H P

M^

R

Rl R

1

2

K

MS4Â, rt,THF

K

z

3 1

30a-fl Entry

R2

1

R

Ri

2

time

Yield(%)

ee(%) 96

1

31a

Ph

Ph

6h

86

2

31b

Ph

2-CI-C H

4

16h

91

90

3

31c

4'-CI-C H4

4-CI-C H

4

3h

93

97

4

31d

Ph

5

31©

4·-ΜβΟ-Ο Η

6

31f

4'-CI-C H

7

31g

Ph

6

6

e

10h

88

99

Ph

12 h

82

97

Ph

7h

91

96

7h

90

93

4-N0 -C H 2

β

6

4

4

6

4-CI-C H e

4

4

(CMHP: cumene hydroperoxide) Scheme 11

Further study indicated that excellent reactivity and enantioselectivity could be achieved by employing Gd-fR)-BINOL-Ph PO (1:1:3) system as the catalyst (Scheme 12, entry 2 and 5). On the other hand, the analogue Gd-{K>F BINOLPh PO (1:1:3) was less efficient than Gd-(7?>BINOL-Ph PO (1:1:3) in terms of both reactivity and enantioselectivity (Scheme 12, entry 3 and 6). 3

8

3

3


299

Gd(/-PrO) /ligand (5mol%) 3

+

CMHP R i ^ ^ R 2

MS4Â, rt.JHF

(1.5eq)

31h-i

30h-i


Entry

R

Ri

ligand

2

Ph PO

time

3

Yield(%)

ee(%)

1

30h

CH

3

Ph

(ff)-F BINOL

-

48h

0

2

30h

CH

3

Ph

(Rhsmoi

15mol%

6h

92

98

3

30h

CH

3

Ph

(R)-F BIN0L

15mol%

20h

65

76

4

30i

Ph

4-MeO-C H

(Rj-F BIN0L

-

48h

0

5

30i

Ph

4-Me0-C H

4

(R)-BmOL

16m ol%

16h

83

90

6

30i

Ph

4-MeO-C H

4

(flj-F BIN0L

15mol%

2days

62

78

8

8

e

8

e

4

8

8


The activity of Gd-fK>BINOL-Ph PO (1:1:3) and Gd-fK>FgBINOL were then compared in the epoxidation of substrate 30j. The latter proved to be more efficient in terms of both reactivity and enantioselectivity (Scheme 13) (43). 3

Gd(/-PrO) /ligand(5mol%) 3

CMHP MS4Â, rt.THF 30j

31j time

yield(%)

ee(%)

48h

63

30

15mol%

48h

40

16

•

16h

85

78

Entry

ligand

Ph PO

1

(Kj-BINOL

-

2

fflJ-BINOL

3

W-F BINOL

3

8

'

-


Conclusions This review discussed the synthesis of highly electron-deficient polyfluorinated binaphthols as well as their applications as chiral ligands in metal mediated asymmetric catalysis. By employing polyfluorinated binaphthols instead of other commercially available BINOL family derivatives, improved chemical reactivity and enantioselectivity were observed in several useful catalytic asymmetric transformation processes. The polyfluorinated binaphthols are believed to be a class of useful binaphthol ligands and a complement to the BINOL family, especially in reactions involving highly acidic and/or oxidative conditions.


300


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