chiral lewis acids derived from 1,8-naphthalenediylbis(dichloroborane)
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Temahedmn Letters, Vol. 35. No. 39, pp. 7209-7212, 1994 Elsetier Science Ltd
Rimed in Great Britain oo40-4039/94 $7.oo1o.00
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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)
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