Temahedmn Letters, Vol. 35. No. 39, pp. 7209-7212, 1994 Elsetier Science Ltd

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Chiral Lewis Acids Derived from 1,8-Naphthalenediyibis(dichloroborane)

Michael Reilly and Taeboem Oh*

Depertmentotchrmistry. StateUniversity ofNew Yak, Bii,NY I~W~&~OO

Lewis acid complexation to carbonyl compounds is Imown to have a dmmatic effect on xeactivity and

selectivity iu a variety of reactions.1 One mt development has been in the area of aqmmet& catalysis by

chiral Lewis a15ds.~ There are two major variables that influence highly selective Lewis acid catalyzed

reactionssuchastbeDiels-Aldermsction. TbefirstistheshrmgthoftbeLewisacid,whicl~mustuWchthe

speciik functional group and the reaction at hand, and the second is the highly ordaed transition state

assembly. WhilcthestrcngthdtheLcwisacidcanbcoonaollcdthtoughthcprapcrchoiocofchiralLigand,bre

confamationandthetransition~assesnblyoftencannotbecoatrdlcdprrdictablywithcllnat~

Lewisacids.TbeapplicalionofchiralbimcMicLewisacidstothe _ .- Diels-Alderreauionprpmisesto

offerexcellentamtmloverbothaftheseaqects.

As part of the development of chiral Lewis acids, Lewis acid-carbohyl group interactions have ken

under continued investigation. Equation 1 illustrates some of the m WhiChnctdtobCddE3d.FOT

example, in a conjugated enon*Lewis acid complex, there are two nonbonded lone pair Lewis basic sites

available fur cwrdination to Lewis acids. Solution NMR studies have shown that chelation to one of the

nonbonded lone pairs of electrons by a Lewis acid affects the molecule distktly differently than if the Lewis

acidisc~~~mtheothaMnbondedelectronpair.3 Rotomeasamundtbecoordinationsitecanalsoaffectthe

rcactivityand~l~~~ofthereactiondependingonthe~~aFoundthe~acid. Theconfommticnof

the dienophile (s-cis. s-rruns) is another factor which must be addressed. Simultaneous coordination of a

uubonyl group by two Lewis acidic metal centQF is a potential way to imxease the eonal role played

bytheLewisacidaswellasdoublyactivatethesubsuate.4~ Ina~tinvestigation,Wucsthasshownthatin

an inuamokcular case, when the carbonyl gmup was tethemd by nvo aluminum Lewisacids;botbLewisacids

coordinated simults.tkeously and symmetrically to the central ketone (Structme 1, Figure).6*7 Hine has repmred

that the two hydroxyl groups of biphenylenediol can simultaneously hydrogen bond to a carbonyl group

(srmcture 21.8 K&Y has also shown that biph~~~~i~~~edi~~ pmmotesDicls-Alder reactions. ouriIm3tigation

intothisaTeafocusesanthepaoeivedabilityofabimtallic~LewioecideosimPlltaneouPly~~a

carbonylgroup(Stfuctme3) apiayan~~rolcinooadinntingachiralligandaada~ylgroup

(Strucmre 4) with each Lewis acidic metal center. This behavi~ should lessen the de- of confcamational

ambiguity found in the Diels-Alder transition state snd eventually lead to the development of highly efkient

asymmerlic catalysts for this bqKatant mulsfcmmaticm.

7209

7210

(1)

Figme

Our initial investigation consisted of screening ligands and dicnophiler in the Diels-Alder reaction

catalyzed by chiral Lewis acids derived from 1,8-naphthalenediylbis(dichloroborane), (~).to The following

procedures were typical: The catalysts were prepared by adding a solution of the Lewis acid (5) in

dichloromethane to a solution of the chlral ligand in dichlomtnethane at room tetnpemtute. The mixture was

allowed to stir for 30 minutes and the solvent and HCl were removed under vacuum to give an oily solid which

was used without further manipulations. To a solution of the above catalyst in dichlomttmthane at -78’C was added the dienophile dropwise followed by cyclopentadiene. The solution was stirred for sevemI hours at this

temperature and after the usual work-up procedures, afforded the desired Dlels-Alder adducts. Table I fillmmarizcs the results obtained with various ligands in our semening with a- bmmoaemlein as the dienophlle

and cyelopentadiene. In all cases, the catalysts were active as Uicated by the ma&on aempaatunS,tim.aad

yields. Of the three amino acids we have screened, N-toluenesulfonylated tryptophan gave the highest

enantiomerlc excess of the exe-adduct (entries 1-3) and of the two diols. R(+)-blmaphtbol gave 36% and 28%

ee (entries 5 and 6) while R,R-hydrobenzoin gave a disappointing 0% ee (entxy 4). It is interesting to note that

when ~JI equimolar amount of binaphthol to bidentate Lewis acid was utiked, 36% ee was obtained for (+>

enantiomer while the use of two equivalents of binaphthol to Lewis acid produces the complimentary (-)-

enantiorzr with an cc of 28%. The potential the&ore &s&s to select the dwircd emntiomer by controlling the

equivalence of the same bituqMw1 l&and.

We have screened further the tryptophanderived ligand with methacrolein sod acrolein dienophiles and

have noted some interesting outcomes (entriesir- 11). In the case of methacmlein, with one or two equivalents

of Iigand to Lewis acid, low ee’s were observed for the exe-isomers. However, only one enantiomer was observed for the endo isomer (entries 7 and 8). Also, the exo-endo ratio was not as high as expected from the literature precedent. This Lewis acid, in conjunction with mthacmlein at least, tends to be more selective for

the et&-isomer as indicated by the high enantioselectivlty for the ettdtGsomer and low enautioseleetlvity for

the exo-isomer. The selectivity for the endo isomer in the reaction of aemlein wlttt cyclopentadiene ~8s high

and gave modexate enantiomeric excess (entries 9 and 10). Addition of 4A molecular sieves was detrimental to

the enantioselectlvity (entry 11).

7211

Bmly R Equival~tsa Ligand exo:aldc@ %cccxo:ulcw yickl(%)d

1 Br 1.0

2 Br 2.0

3 Br 2.0

4 Br 2.0

5 Br 1.0

6 Br 2.0

7

8

9

10

11

f=3

a3

H

H

H

1.0

2.0

1.0

2.0

1.0

NH FL TaH

“rg TsHN H

H

+4AM!3

92:8 44: - 84

89Zll 8:- 83

88:12 a- 98

8om &- 83

86~14

80~20

6337

68~32

6s

892

694

36(+): -

28(-): -

24kloo

24: 100

-:62

-:50

-:o

81

81

46

50

53

60

68

aEqui~uofL~oD5. b.lhatioe~obhalbyin~ofths~~~HNMRresoarrre. c. ec’swenchcamedby(+)-Kumhshiftreagenr disdacdykAdsofDicls-Aldsad&as.

7212

Ihesauctundthesu~-caedypt~~Lmodedcoadierdea-lrsmc~u~pw#lt

time. lk-e is some evidence that 5 can simulmueously cootdi~te through buth metal centers to suongly

Lewis basic carbony groups; however, it is not kuowtt’&&er this mode of coa&n&n plays an impcmant

loleinthepmsentcase.” %“ * .i oftheprestlat&*systantorhcDiels-Aldcrrerrctioadlrtrtmpts

to elucidate the act@ solution stmctum&theactivecatslystamcuneutlyiupmgmss.

ACKNOWLEDGMENT: This ~hwassuppartedinpartbytheSuNyResc;achFoundatioaand~e

Dr.NualaMcGnnn-Awd

REFERENCES AND NOTES

1.

2.

For review of this ama, see; Shambayati, S.; Sch&#;r, S. L. In “Comprehensive Organic Synthesis:

Sekctivily, Strategy & Eflciency in Modern Or&a& Chemistty”, B. M. Trost. L Fleming snd L- A.

Paquette, &Is., Pergamon Pnss. New York, 1991, Vol 1, Chapter 1.10, p.283-324.

For reviews and leading references, see: a) Kagan, H. B.; Riaut, 0. Ckau. Rev. 1992.92. 1007-1019.

b) Narasaka, K_ Synrhesis 1991, l-l 1. c) Oh, T.; Reilly. M. Org. Prep. Proced. ht. 1994, 26, 129-

158.

3. (a) Hcnriksson, U.; Forsen, S. Ckem. Comm. 1970.1229. (b) FratieEo, A.; Vidulich. G. A.; Chow. Y.

J. Org. Chem. 1973. 2309-2314. (c) Stilbs. P.; Pots& S. Tetrahe&on Lea. 1974, 31853186. (d)

Hartman, J. S.; Stilbs, P. Tetrahedron Len. 1975.3497-3500. (e) Pmtiello, A.; Kubo, R; Chow, S. 1.

Ckem. Sot. Perkin II 1976. 1205-1209. (f) Toui. J.; Axxam. M. Bull. Sot. Chim. Fr. 1978.11, 283-

291. (g) Wrsctiyer. B. Progress in NMR Spectrosco~ 1979.12. 227-259. (h) Fratiello, A.;

Strover, C. S. J. Org. Chem. 1975,40, 1244-1248.

4.

5.

6.

7.

Beauchamp, A. L.; Ohvier, M. J.; Wucst, J. D.; 2Iacharic. B. /. Am. Ckm. Sot. 1986,208,73-77.

For a theoretical study of formaldehyde complexed to two equivalents of borane, see: Lepage, T- J.;

Wiberg, K. B. J. Am. Chem. Sot. 1988,110. 6642-6650.

Sharma, V.; Simard. M.; Wuesi 3. D. J. Am. Ckcm. Sot. 1992,114. 7931-7933.

For related complexes, see: a) Adams, R. D.; Chen, 0.; Chen. L.; Wu, W.; Yin, J. J. Am. Ckm. Sot.

1991, 213. 9406-9408. b) Waymouth, R. W.; Potter, K. S.; Schaefer, W. P.; Grubbs, R. H.

Organometallics 1990, 9. 2843-2846. c) Erkcr. G.; Dorf, W.; Cxisch, P.; Petersen, J. L.

Organometallics 1986.5, 668-276. d) Adams, H.; Bailey, N. A.; Gauntlett. J. T.; Winter, M. J. /.

Chem. Sot., Chem. Conunun. 1984, 1360-1361.

8. a) Hine. J.; Anh. K. J. Org. C&m. 1987,52. 2083-2086. b) Hint, J.: Anh, IL J. Org. Ckcm. 1987,

52.2089-2091.

9. Kelly, T. R.; Meghani, P.; Ekkundi, V. Tetrahedron L&t. 1990.31,3381-3384.

10. a) Katz, H. E. Organometallics 1987,6, 1134-1136, b) Katz, H. E. J. Org. Ckm. 1985, 50, 2575-

2576.

11. a) Reilly, M.; Oh, T., 207th American Chemical Society National Meeting, San Diego, CA. Match 13-

17,1994. Paper no. 447 in Division of Organic Chemistry.

(Received in USA 24 June 1994; revised 5 August 1994, accepted 8 August 1994)