Stereoselective allyl- and crotylboration of alpha-hydroxyketones via neighboring group control

Article abstract of DOI:10.1016/0040-4039(91)85008-S

Allylboronates and crotylboronates react with alpha-hydroxyketones to yield tertiary homoallylic alcohols in a highly stereocontrolled manner. The reaction presumably proceeds via a rigid bicyclic transition state. The alpha-hydroxy substituent exerts a r

Full text of DOI:10.1016/0040-4039(91)85008-S

Tetrahedron Letters, Vol.32, No.17, pp 1945 1948, 1991
Printed irt Great Britain
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Pergamon Press plc
STEREOSELECT1VE ALLYL- AND CROTYLBORATION OF
ALPHA- HYDROXYKETONES VIA NEIGHBORING GROUP CONTROL
Zhe Wang, Xian-Jun Meng and George W. Kabalka*
Departments of Chemistry and Radiology
University of Tennessee, Knoxville, TN 37996 1600
Abstract:
homoallylic alcohols in
rigid bicyclic transition state. The alpha-hydroxy substituent exerts
effect on the diastereoselectivity of the reaetion. Homoallylic alcohols are
diastereoselectivities approaching 100%.
Aliylboronates and crotylboronates react with alpha-hydroxykelones to yield tertiary
highly stereocontrolled manner. The reaction presumably proceeds via
remarkable and predictable
formed with
a
a
a
The development of new methods for acyclic diastereo and enantiomeric control of chiral
centers is of major importance in synthetic organic chemistry 1, The stereoselective allylalion of
prostereogenic carbonyl compounds is one of the most useful stereoselective reactions in thai
several stereogenic centers can be generated in single step2, However, the allylation of ketones is
difficult3; for example, the addition of (E)- and (Z)-pentenylboronates to ketones requires high
pressure and 4 5 days4.
11 is known that alpha-alkoxy substituents accelerate the reaction rate in aldol condensations 5
a
and
organometallic addition reactions6.
It is
also known
that
the
hydroxy group
can
diastereoselectively
direct an incoming reagent by coordinating with it, e.g.
intramolecular
Tishchenko 7 and Sakurai8 allylsilylation reactions. We wish to report that allyboronate 1, (E)-
crotylboronate 2, and (Z) crotyiboronate9 react with a|pha-hydroxyketones to yield tertiary
homoallylic alcohols stereoselectively. The essential tkmctions of the OH group are: (1) activation of
the carbonyl group and (2) stereocuntrol at carbons and (structure 5). The configuration at the
allylic p~)sition (carbon in 5) is determined by the borane reagents (2 or -3). Our results suggest that
the reaction proceeds through rigid bicyclic transition state (Scheme 1). Bulky R2 groups favor
3
4
5
I
2
3
a
7
the transition state 7a over 7b and lead to syn/anti ratios approaching 100%.
o
~ . , . . / B/~oO,iPp ,r
"
P'
~'O[Pr
/ O i P r
3
1
R2
~ , , , , , . , i B ~ i p r
W
B
~
o
i
P
r
R(" TCCH
RE Rz OH "
1945

1946
Scheme~l.
Proposed Transition Stales of the Allyl- and Crotylboration of Hydroxyketones.
H.,., ~,.~__~1RO~'
,'~ LL-~" : ' g - OiPr
H
~,R;
~
3 1 R2
R~-" ~~:'~0
~zR1
~
H
7a
~, 1,2-syn
(1, 2 or 3) + 4
~
,
~
H
R1
1
R~
,,~ L.~" %,B-OiPr
RzR1 Zb
6, 1,2-anti
As illustrated in Table 1, treatment of alpha-hydroxyketone
4a with triethylamine (1.0
equivalent, -78 °C fl~r
5
minutes) tbllowed by the addition of an allyl or crotylhoronate at -78 OC, and
then warming to room temperature, produced tertiary homoallylic alcohols in good yields and with
excellent
stereoselectivities.
Table I.
Reaction of Aceto[ (4a) with Boronates.
/ O j P r
RE~ ' ~ " ~ . ~ ' B ~.Oipr
I~z
0
HO CH3
+
~ /H~ -I Ot
CH3
et3N~
~ 3 . . ~ V OH
RE RZ
Compound
RE
Rz
Rxn Time (hr)
% Yielda
84
S y n / A n t i b
1
H
H
15 (r.t.)
2
CH3
lq
28 (r.t.)
72
97:3c
6:94d
E/Z=98:2
3
H
C143
36 (r.t.)
65
7JE=95:5
a) Isolated yields,
b) The structures were determined by NMR analyses and confirmed by elemental
c) Maximum ratio which could
d) Maximum ratio which could be
analyses and the syn/anti ratios were determined by NMR analyses,
be obtained 98/2 based on the purity of the starting material 2.
obtained 95/5 based on the purity of the starting material 3.
=
=
A
plausible mechanism for the triethylamine initiated allyl and crotylhoration involves
a
ligand exchange 10 to form mixed boronate which then reacts via transition state 7a involving
a

1947
coordination between the boron atom and the carbonyl group. The outstanding stereoselectivity of
the allyl- and crotylboration of hindered alpha-hydroxyketone 4h (benzoin), Table I1, supports this
postulation.
Preliminary solvent investigations revealed that the reactions are faster in methylene
dichloride than in THF which is consistent with recent observationsll. Preliminary studies also
revealed that the isopropyl boronates react faster than pinacol boronates; no differences in
stereoselectivity were observed at room temperature. However, under refluxing conditions, the
stereoselectivity decreased for reactions run in TI-1F.
The stereoehemistry of each of the products was determined by NMR anatysis7,12, The
structure of the product of the reaction of
analysis.
2
and benzoin was confirmed by X-ray crystallographic
Table 11.
Reaction of Benzoin (4b) with Boronates.
~ O [ P r
O
HO .Ph
JL .-Ph
mz
RE RzOH "'
Compound
RE
RZ
Rxn Time (hr)
% Yielda
1,2 Synb
100
2,3 Syn or Anti
2
CH3
H
24 (-10°C - reflux)
83
80
100
I00
98(syn) c
95(anti) d
E/Z=98:2
3
H
CH3
32 ( 10 "C
reflux)
Z/E-95:5
a) Isolated yields,
b) The structures were determined by NMR analyses and confirmed byelemental
analyses and the syn/anti ratios were determined by NMR analyses,
be obtained 98/2 based on the purity of the starting material 2.
95/5 based on the puri~ of the starting material 3,
c) Maximum ratio which could
d) Maximum ratio which could be
-
obtained
-
In summary, alpha-hydroxy substituents exert
a
remarkable effect on the diastereoselectivity
of the allylhorafion and crotylboration reactions. The rigid bicyclie transition state produces
diastereoselectivities approaching 100%.
Acknowledgement:
We wish to acknowledge Dr. C. Woods and Mr. James L. E. Burn for the X ray crystallographic
analysis and financial support from the Department of Energy.

1948
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1 9 8 9 .
(2)
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(Received in USA 15 January 1991)