Catalytic Enantioselective Hydrostannation of Cyclopropenes

Article abstract of DOI:10.1021/ja0496928

The first examples of catalytic enantioselective hydrostannation of the C=C double bond of cyclopropenes has been demonstrated. This method allows for the efficient synthesis of 2,2-disubstituted cyclopropylstannanes with high degrees of diastereo- and en

Full text of DOI:10.1021/ja0496928

Published on Web 03/06/2004
Catalytic Enantioselective Hydrostannation of Cyclopropenes
Marina Rubina, Michael Rubin, and Vladimir Gevorgyan*
Department of Chemistry, UniVersity of Illinois at Chicago, 845 West Taylor Street, Chicago, Illinois 60607-7061
Received January 16, 2004; E-mail: vlad@uic.edu
Table 1. Optimization of Enantioselective Hydrostannation of 1a,b
Hydrostannation of the carbon-carbon double bond is an
important process,1 which allows for the straightforward preparation
of various types of useful building blocks for organic synthesis.2
While enantioselective versions of related hydrometalation pro-
cesses, such as hydrosilylation3 and hydroboration,4 are well-known,
no precedents on enantioselective hydrostannation have been
reported to date. Although diastereoselective radical hydrostannation
employing chiral auxiliaries on either the substrate5 or tin hydride
moiety6 have been reported, the obtained de’s were below 40%.
Herein we report the first example of catalytic highly enantio-
selective hydrostannation of the double bond of cyclopropenes,
which allows for easy access to valuable optically active cyclo-
propylstannanes.7
entry
R
catalyst
T,
°
C
time
trans/cis
ee,%a
1
2
3
4
5
6
7
8
CO2Me
CO2Me
CO2Me
Ph
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
Ph
[Pd]b/(R)-MOP
[Pd]/ligand A
[Rh]c/ligand A
[Rh]/ligand A
[Rh]/ligand A
[Rh]/ligand B
[Rh]/ligand C
[Rh]/ligand D
[Rh]/ligand D
[Rh]/ligand D
[Rh]/ligand D
-85
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
0
5 min
10 min
20 min
20 min
5 h
95/5
98/2
12
47
62
65
0d
42
0
80
84
94
90
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
1 day
20 min
20 min
30 min
45 min
45 min
9
10
11
We have recently reported the highly diastereoselective transition
metal-catalyzed hydrostannation of cyclopropenes.8 This method
allowed for efficient introduction of up to five different substituents
in the cyclopropyl ring. Obviously, we were interested in achieving
asymmetric hydrostannation of cyclopropenes en route to nonra-
cemic cyclopropylstannanes. Encouraged by the remarkable ef-
ficiency of the [Pd(π-allyl)Cl]2/(()-MOP catalyst system in the
hydrostannation of a series of multisubstituted cyclopropenes,8 we
naturally attempted enantioselective hydrostannation of 1a in the
presence of optically active (+)-R-MOP ligand. This catalyst system
was previously shown by Hayashi to be very effective in the
enantioselective hydrosilylation of olefins.3 However, this combina-
tion provided disappointingly low enantiomeric induction (12% ee)
in the hydrostannation of 1a (Table 1, entry 1). Screening a number
of commercially available ligands revealed that the Trost ligand
(A) in combination with the Pd catalyst smoothly effected the
reaction, exhibiting a moderate ee (47%, entry 2), while all other
chiral ligands tested provided either no reaction or decomposition
of the starting material. Further improvement of enantioselectivity
(62% ee, entry 3) was achieved when Rh catalyst9 was employed
instead of Pd in combination with ligand A. Moreover, switching
to Rh allowed for complete suppression of the undesired formation
of ditin,10 significant quantities of which were observed in all the
Pd-catalyzed reactions. The hydrostannation of 1b under these
conditions provided comparable degrees of enantiomeric induction
(entry 4). Inspired by the promising results obtained with ligand
A, we decided to screen a series of different diphenylphosphi-
nobenzoic acid-derived ligands, analogues of A, which were
previously demonstrated by Trost to efficiently catalyze asymmetric
allylic alkylation reactions.11 Employment of naphthyl-based ligand
B, however, significantly impeded the reaction and afforded lower
enantioselectivity (42% ee, entry 6). Furthermore, anthracene-based
ligand C provided completely racemic product (entry 7). Gratify-
ingly, hydrostannation of 1b in the presence of stilbene-derived
ligand D proceeded smoothly, affording cyclopropylstannane 2a
with respectable enantioselectivity (80% ee, entry 8). With this result
in hand, we performed further optimization of the reaction condition.
Expectedly, the enantiomeric induction was significantly improved
at lower reaction temperatures. Thus, a slightly higher ee was
-30
-30
a Enantiomeric excess was determined by chiral GC analysis. b [Pd(π-
allyl)Cl]2. c [Rh(COD)Cl]2. d Bu3SnH was employed.
obtained at 0 °C (entry 9), whereas a dramatic improvement to up
to 94% ee was achieved at temperatures as low as -30 °C (entry
10). Hydrostannation of cyclopropene 1b under these conditions
also displayed a very high enantiomeric induction (90%, entry 11).
Notably, the replacement of trimethyltin hydride with Bu3SnH, in
this reaction, resulted in the totally racemic product (entry 5).
Next, the optimized conditions were applied to the hydrostan-
nation of a series of 3,3-disubstituted cyclopropenes (Table 2). We
were pleased to find that preparative hydrostannation of 1a,b
reproduced the high ee’s and allowed for the synthesis of cyclo-
propylstannanes 2a,b in high isolated yields (entries 1, 2). Likewise,
hydrostannation of MOM-protected cyclopropenyl carbinol 1c
proceeded smoothly to give 2c with high yield and enantioselectivity
(entry 3). Hydrostannation of allyl ester 1d similarly to its methyl
analogue 1a, proceeded uneventfully to give 2d with very high
enantiomeric excess (97% ee, entry 4). Esters 1e,f,i,j and MOM-
ethers 1g,h of differently substituted cyclopropenyl carbinols were
also smoothly hydrostannated under these reaction conditions to
afford optically active cyclopropylstannanes 2e-j with high yields
and ee’s (entries 5-10).
Remarkably, facial selectivity of the Rh-catalyzed hydrostanna-
tion was perfectly controlled by steric effects of substituents at C-3
of cyclopropenes, affording cyclopropylstannanes (1R,2S)-212 as
9
3688
J. AM. CHEM. SOC. 2004, 126, 3688-3689
10.1021/ja0496928 CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Table 2. Rh-Catalyzed Enantioselective Hydrostannation of
In conclusion, we believe that the chemistry described herein is
not only fundamentally important as the first example of catalytic
enantioselective hydrostannation of a CdC double bond, but it also
has high potential in synthesis as it allows for the very efficient
and straightforward approach to optically active cyclopropylstan-
nanes, invaluable building blocks for organic synthesis.
Cyclopropenesa
Acknowledgment. The support of the National Science Foun-
dation (CHE-0354613) is gratefully acknowledged.
Supporting Information Available: Experimental details (PDF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
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a All reactions were performed in 1 mmol scale. b Isolated yield.
c Enantiomeric excess was determined by chiral GC. d (c 1.00, CH2Cl2).
single diastereoisomers. This observation is in a striking contrast
with the previously reported Rh-catalyzed enantioselective hy-
droboration of cyclopropenes,9 which was governed by a requisite
directing effect of ester or alkoxymethyl substituents. As can be
seen from Table 2, the reaction is very general with respect to
substituents at C-3 and displays good functional group compatibility.
Thus, we feel that synthesis of optically active cyclopropylmetal
synthons via enantioselective hydrostannation of cyclopropenes has
a more general scope compared to enantioselective hydroboration,9
as it does not require directing groups for achieving high degrees
of enantioselectivity. Furthermore, this method allows for easy
access to optically active trans-stannyl derivatives of cyclopropy-
lcarboxylates, complimentary to the earlier reported cis-boronyl
derivatives. It should be mentioned that tri- and tetrasubstituted
cyclopropenes did not undergo the hydrostannation reaction at all
under these reaction conditions.
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(12) For determination of absolute configuration of 2, see Supporting Information.
JA0496928
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