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Oxazaborolidines and Dioxaborolidines in Enantioselective Catalysis.

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Oxazaborolidines and Dioxaborolidines in Enantioselective Catalysis **
By Braj B. Lohray*, and Vidya Bhushan
The success of enantioselective catalysts depends on the
nature of the metal and the structure and electronic properties of the chiral ligands. A number of highly efficient and
useful catalysts are now available for various reactions.[']
Boranes serve as chiral reagents for a number of asymmetric
reactions. One of the earliest examples of borane catalysts
for the reduction of ketones (or ketoxime ethers) to secondary alcohols (or amines) was reported by Itsuno et al.,[']
in which (S)-valinol was used as chiral ligand. Simultaneously, Corey et al.[3a1investigated the enantioselective reduction
of ketones with THF . BH, and (S)-diphenylprolinol borane
adduct as catalyst. Later they also employed other oxazaboroles, for example, 1, R = Me, nBu, as catalysts and used
other boranes as reducing agents.[3b' (The catalysts 1,
which are easily accessible in both the R and the S form, are
termed CBS reagents.) These afforded excellent yields and
enantioselectivities (80-99 Yo ee) under very mild reaction
conditions (0-25 "C). The reaction (Scheme 1) requires both
of substituted 2-pyranones with THF . BH, in the presence
of oxazaborolidines.
Brown et al.['I achieved the enantioselective addition of diethylzinc to several aldehydes in high yield and
optical purity (up to 96 YOee) by using (4S,SR)-3,4-dimethyl5-phenyl-I ,3,2-oxazaborolidine. Because B-0 and B-N
bonds are shorter than metal-oxygen and -nitrogen bonds,
there is a better chance that the boron catalysts will be more
Yamamoto et a1.[*]suggested that acyloxyboranes might
behave as mixed anhydrides because of the electronegative
trivalent boron atom and could serve as effective catalysts
for various reactions. The chiral acyloxyborane (CAB) 3
prepared by the reaction of monoacylated tartaric acid with
one equivalent of THF . BH, in dichloromethane at 0 "C is
an excellent catalyst for the asymmetric allylation of aldehydes by allylsilanes (Scheme 2).[81The reaction proceeds at
' *o
CH SiMe,
R' &R2
OH R '
3 a (2R,3R)
3b (2S.39
Scheme 2. Asymmetric allylation with allylsilanes in the presence of 20 mol %
3a. R = 2,6-dimethoxyphenyl.
Scheme 1. Possible mechanism of the borane-catalyzed enantioselective reduction of ketones with THF . BH,.
reagents: neither the catalyst (S)-1nor T H F . BH, alone are
able to reduce the ketone in considerable yield. Thus, the
active reducing species, the "chemzyme" has been postulated
to be an intermediate complex 2, which is formed from (S)-1
and BH, . Mathre et al. from industry[4a1have modified the
preparation of (S)-1 and used it for the synthesis of MK0471, a carboanhydrase inhibitor.[4b1Martens et al.[4cl reported the use of oxazaborolidines derived from (S)-indole2-carboxylic acid for asymmetric reduction of aromatic
ketones, though in lower selectivity.
Two particularly recent examples of borane-catalyzed reductions will now be described: Corey and Link['] have reported a general method for the synthesis of a-amino acids
using (S)-1 (R = nBu) as the chiral catalyst for the enantioselective reduction of alkyl (trichloromethyl) ketones by
catecholborane. The (2R)-1,1,l-trichIoroalkan-2-ol thus obtained was treated with NaOH (4 equiv) and NaN, (2 equiv)
at 23 "C affording (S)-a-azido acids, which were subsequently reduced to a-amino acids. Bringmann and HartungL6I
have synthesized substituted chiral biaryls by the reduction
Dr. B. B. Lohrdy, Dr. V. Bhushan
Division of Organic Chemistry
National Chemical Laboratory
Pune 41 1008 (India)
N.C.L. communication no. 5394.
Angrw. Chein. Int. Ed. Engl. 31 (1992) N o . 6
- 78 "C to furnish predominantly erythro homoallylic alcohols (erythro:r h o = 80 :20 to 97: 3) regardless of the configuration of the allylsilanes. CAB is an equally effective catalyst for the Diels-Alder reactionstg1of a variety of dienes
with several dienophiles (aJ-unsaturated acids and aldehydes) under very mild reaction conditions ( - 78 "C)
(Scheme 3). Good to excellent diastereo- (99: 1 to 88: 12) and
96% ee
O C H O (R)
Scheme 3. Diels-Alder reactions enantioselectively catalyzed with 10 mol % 3.
enantioselectivities (80-97 YOee) have been observed in most
of the cases. The reactive species is assumed to be 1,3,2dioxaborolidine 3 and the dienophile.
Yamamoto et al.['ol have extended these studies to DielsAlder reactions catalyzed by N-arylsulfonyl 1,3,2-0xazaborolidine 4. The stereoselectivity with these catalysts is relatively
inferior to that observed with 3. Similar observations have
been reported by Helmchen et al.["] for the Diels-Alder
reaction of cyclopentadiene and methacrolein (em :endo =
99:1, 64% ee for the exo isomer) or crotonaldehyde
(exo:endo= 3:97; 72% ee for the endo isomer). Use of
bulkier aryl substituents such as 2,4,6-triisopropyl- and
2,4,6-tri-t-butylphenyl in 4 had little influence on the
diastereoselectivity of the cycloaddition.
Verlags~esellschafrmbH. W-6940 Weinheim, 1992
0S70-0833/92/0606-0729 $3.50+ .2S/0
Corey and Loh['21 have reported that the Diels-Alder
reaction of cyclopentadiene and 2-bromoacrolein, which is
catalyzed by oxazaborolidines 4 derived from N-tosyl-(S)tryptophan, in dichloromethane at - 78 "C gives (R)-bromoaldehyde 5 (exo:endo = 97:3, 96 YOee) in 1 h. The observed
stereoselectivity in this case is opposite to that normally observed for oxazaborolidines 4 generated from N-tosyl derivatives of (S)-valine or (S)-hexahydrophenylalanine.These re-
hyde to the reaction mixture proved beneficial (permits
enough time for 8 to undergo ring closure) for improving the
enantioselectivity of the reaction. Thus, a,n-disubstituted Narylsulfonylglycines were used for the preparation of the
oxazaborolidine 7, which resulted in catalytic asymmetric
aldol processes providing fi-hydroxy esters of > 97 YOee
from a-unbranched aldehydes (RCH,CHO) and 84-96 Yoee
with a-branched aldehydes (R,CHCHO). The reaction proceeds smoothly in propionitrile at - 78 "C if the aldehydes
are added slowly over 3.5 h affording high yields (68-89%)
of the adduct.
sults suggest a transition state 6 for the synthesis of 5,[12Jin
which the dienophile assumes an orientation parallel to the
indole ring because of the FTC donor-acceptor interactions.
The CAB 3 not only catalyzes ally1 transformations
(Scheme 2) but also asymmetric aldol reactions between
silylenol ethers and aldehydes (Scheme 4).
Scheme 5. Oxazaborolidine in aldol reactions.
In summary, oxaza- or dioxaborolidines have been found
to be effective catalysts for several reaction types and probably in the future other applications will be discovered for
such catalysts.
German version: Angew. Chem. 1992. 104, 740
Scheme 4. Aldol reactions enantioselectrvely cdtalyzed with 20 mol % 3
Yamamoto et al.[*31 used 20 mol % of CAB 3 in propionitrile at - 78 "C as a highly efficient catalyst for the condensation of several E and Z silylenol ethers and ketene acetals
with a variety of aldehydes (yields 49-97%, 80-97% ee).
Interestingly, regardless of the configuration of the
enol ethers, the erythro isomer always predominated
(erythro : threo = 80/20 to > 95/ < 5).
Aromatic and a,P-unsaturated aldehydes always provided
higher diastereo- (eryfhro: threo 2 94: 6 ) and enantioselectivities (92-97 % ee) than saturated aldehydes. Polar solvents improved the selectivity by decreasing the association
of the catalyst with the formation of oligomers as observed
by Helmchen.["l
Kiyooka et al.1141and Masamune et al.['51 used oxazaborolidine 7 under stoichiometric and ybstoichiometric (ie,
as chiral catalysts) conditions, respectively, as the catalyst
for aldol reactions. Masamune suggested that the initial aldo1 adduct 8 must undergo ring closure (as indicated by
arrow in Scheme 5 to release the final product 9 and to regenerate the catalyst. In many cases, slow addition of the alde-
:C; VCH Verlagsgesellscltuji mbH. W-6940 Weinheim, 1992
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Angew. Chem. Inf. Ed. EngI. 31 (1992) No. 6
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catalysing, enantioselectivity, oxazaborolidine, dioxaborolidines
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