Enantioconvergent hydroboration of a racemic allene: Enantioselective synthesis of (E)-δ-stannyl- anti -homoallylic alcohols via aldehyde crotylboration

Article abstract of DOI:10.1021/ja2010187

The enantioconvergent hydroboration of racemic allenylstannane (±)-1 with (^dIpc)₂BH converts both enantiomers of (±)-1 into the enantioenriched crotylborane (S)-E-3. Subsequent crotylboration of aldehydes with (S)-E-3 provides (E)-st

Full text of DOI:10.1021/ja2010187

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pubs.acs.org/JACS
Enantioconvergent Hydroboration of a Racemic Allene:
Enantioselective Synthesis of (E)-δ-Stannyl-anti-homoallylic
Alcohols via Aldehyde Crotylboration
Ming Chen and William R. Roush*
Department of Chemistry, Scripps Florida, Jupiter, Florida 33458, United States
S Supporting Information
b
intermediates provide the same enantiomer of a reaction product
with high enantiomeric excess.
ABSTRACT: The enantioconvergent hydroboration of
racemic allenylstannane (()-1 with (dIpc)2BH converts
both enantiomers of (()-1 into the enantioenriched cro-
tylborane (S)-E-3. Subsequent crotylboration of aldehydes
with (S)-E-3 provides (E)-stannyl-homoallylic alcohols 5 in
good yields and with excellent enantioselectivity.
As part of ongoing studies to expand the utility of the double
allylboration chemistry developed in our laboratory,5 we studied
the hydroboration of racemic 3-methyl-allenylstannane (()-1
with d-diisopinocampheylborane [(dIpc)2BH]. As depicted in
Figure 1, this reaction could lead either to reagent 2 or 3, which
when treated with aldehydes should react to provide 4 or 5,
respectively.6 The homoallylic alcohol products 4 and 5 are
properly functionalized for use in subsequent CꢀC bond form-
ing reactions.7 Yet, because (()-1 is racemic, we assumed at the
outset that the enantioselective hydroboration would need to be
occur in the kinetic resolution manifold.8
symmetric synthesis of chiral, nonracemic molecules is a
A
major objective in organic chemistry. A wide variety of highly
enantiomerically enriched molecules can be generated with
excellent selectivity owing to the many classes of highly enantio-
selective chiral reagents, auxiliaries, and catalysts that have been
developed over the past several decades.1
Treatment of (()-1 with 1.0 equiv of (dIpc)2BH at 0 °C in
diethyl ether followed by addition of 1.0 equiv of hydrocinna-
maldehyde at ꢀ78 °C provided (E)-δ-stannyl-anti-homoallylic
alcohol 5a in 71% yield. Significantly, 5a was obtained with 92% ee
from racemic (()-1. Several other aldehydes were also examined
in this sequence (Table 1). In all cases, (E)-δ-stannyl-anti-
homoallylic alcohols 5 were obtained in 56ꢀ73% yields with
high enantioselectivities (88ꢀ94% ee9); however, the yields
of 5 are 81ꢀ89% when RCHO is the limiting reagent (0.7 equiv)
(Table 1). Each reaction also provided 3ꢀ5% of the (E)-δ-
stannyl-syn-homoallylic alcohol isomer 6 (20ꢀ30% ee).
The prevailing approach in asymmetric synthesis focuses on
introducing chirality in reactions of prochiral substrates using
chiral reagents or chiral catalysts. Other strategies, however, are
available for the synthesis of highly enantiomerically enriched
compounds. Because it is often easier and more cost-effective to
synthesize racemates, resolution remains a valuable tool to access
highly enantiomerically enriched molecules (especially in the
pharmaceutical industry). Kinetic resolution is a well-established
approach that enables partial or complete separation of a
racemate based on the different rates of reaction of each enan-
tiomer with a chiral, nonracemic reagent or catalyst.2 However,
even in an ideal case, the overall efficiency of kinetic resolution is
limited to a theoretical maximum yield of 50%. The other 50% of
enantiomeric material is discarded or recycled. Additional stra-
tegies, such as dynamic kinetic resolution (DKR),3 address this
limitation when applicable. DKR involves rapid racemization of
substrates or symmetrization of reaction intermediates, with
product formation occurring under CurtinꢀHammett control
via a rate-determining enantioselective transition state. In this
way, both enantiomers of the substrate are converted into a single
enantiomerically enriched product with 100% theoretical yield.
We describe here a remarkable example of the enantiocon-
vergent reaction of a racemic allene to give an enantiomerically
enriched product.4 Unlike dynamic kinetic resolution, the en-
antioconvergent process does not involve racemization of
the substrate or the symmetrization of a reaction intermediate
prior to the enantioselective step. Rather, both enantiomers
of the racemate are converted into different enantiomerically
enriched intermediates by chemically distinct, kinetically con-
trolled pathways. Subsequent transformations of the nonracemic
Assuming that the crotylboration proceeds through a chairlike
transition state,6 the results in Table 1 indicate that the inter-
mediate produced in the hydroboration of racemic allene
(()-1 with (dIpc)2BH is (S)-R-tributylstannyl-(E)-crotylborane
[(S)-E-3] (Figure 2). The Bu3Sn group is positioned R to the
boron atom in (S)-E-3 presumably due to the ability of the CꢀSn
bond to interact with the empty p orbital on boron.10,11 Subsequent
crotylboration of aldehydes with (S)-E-3 via the chairlike TS-1
(with equatorial placement of the R-Bu3Sn group) provides 5.
Based on the data in Table 1, it was immediately apparent that
these reactions do not involve the kinetic resolution of (()-1
with (dIpc)2BH, as the maximum yield of the kinetic resolution
would be only 50%. Rather, the efficiency and enantioselectivity
of this process led to the supposition that hydroboration of
(()-1 with (dIpc)2BH converted both allene enantiomers, (P)-1
and (M)-1, into the same nonracemic intermediate (S)-E-3.
Direct evidence in support of this deduction was obtained by
performing the hydroboration of the two enantiomerically en-
riched allenes, (P)-1 and (M)-1, with (dIpc)2BH (Figure 2).12
Hydroboration of (P)-1 (g95% ee)12 with 1 equiv of (dIpc)2BH at
Received: February 1, 2011
Published: March 30, 2011
r
2011 American Chemical Society
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Figure 1. Proposed enantioselective hydroboration of racemic allenyl-
stannane (()-1 and subsequent crotylboration reactions.
Table 1. Synthesis of Homoallylic Alcohols 5 from (()-1a
entry
RCHO
productb
% yield (5)b,c
% ee (5)d
1
2
3
4
5
6
7
8
9
Ph(CH2)2CHO
Ph(CH2)2CHOe
PhCH2CHO
PhCHO
5a
71
92
88
92
89
94
90
92
93
90g
ent-5a
5b
67 (84)
69
Figure 2. Proposed reaction pathway and hydroborationꢀallylboration
of single enantiomeric allenes (P)-1 and (M)-1.
5c
67 (89)
73
BnO(CH2)2CHO
BnOCH2CHO
PhCHdCHCHO
CyCHO
5d
5e
boron to the central allene carbon in this series.8b,c The overall
chemical efficiency and enantioselectivity of the reactions of the
racemic allene 1 summarized in Table 1 are thus approximated
by the weighted average of the efficiencies (yield and enantio-
selectivity) of the matched and mismatched double asymmetric
hydroboration reactions of (P)-1 and (M)-1, respectively.
We considered the possibility that if (P)-1 and (M)-1 equili-
brate under the hydroboration conditions, the results in Table 1
and Figure 2 could be explained by a DKR.4b However, this
possibility was eliminated by experiments in which single en-
antiomer allene (P)-1 (g95% ee) was treated with 0.5 equiv of
(dIpc)2BH or (lIpc)2BH. In all cases the recovered allene (g95%
ee) showed no detectable sign of racemization even when the
hydroboration reactions were extended to 12 h at 0 °C (see Table
S1 in SI). Therefore, the enantioconvergent hydroboration of
racemic 1 does not involve a DKR process.
70
5f
71 (87)
64 (81)
56
5g
t-Buf
5h
a The reactions were performed by treating (()-1 in Et2O (0.1 M) with
(dIpc)2BH (1.0 equiv) at 0 °C followed by addition of RCHO (1.0
equiv) at ꢀ78 °C. The mixture was stirred at ꢀ78 °C for 8 h. The
reactions were terminated by addition of NaHCO3 and H2O2 at 0 °C
prior to product isolation. b Isolated yield of 5. In addition, small
amounts of syn-homoallylic alcohols 6 (20ꢀ30% ee) were also obtained
in each experiment (3ꢀ5%). The diastereoselectivity of these reactions
was typically g15:1 (ratio of 5/6). c Yields in parentheses are based on
RCHO (0.7 equiv) used as the limiting reagent. d Determined by
Mosher ester analysis. e (lIpc)2BH was used. f Reaction was warmed to
g
ambient temperature after the addition of t-BuCHO. Determined by
Mosher ester analysis of the diol obtained after ozonolysis of 5h.
As depicted in Figure 3, we propose that the hydroboration of
allene (P)-1 with (dIpc)2BH occurs on the re-face (bottom face,
as drawn) of the methyl substituted olefin unit of (P)-1, anti to
the Bu3Sn group to give intermediate (R)d-Z-7. The face
selectivity of this step is consistent with the known enantioselec-
tivity of hydroboration of (Z)-olefins by (dIpc)2BH,8b,14 as well
as by the preference of allene hydroboration to occur anti to
bulky substituents at the distal position.5,15h,15i The hydrobora-
tion of (P)-1 by (dIpc)2BH is thus stereochemically matched.
The resulting crotylborane (R)d-Z-7 can undergo a stereochemi-
cally controlled, stereospecific, suprafacial 1,3-boratropic shift,15
to give (S)d-E-3. As noted in the second equation of Figure 3,
hydroboration of (P)-1 on the olefin adjacent to the Bu3Sn group
inexorably leads, via σ bond rotations and the indicated
stereospecific 1,3-boratropic shifts, to the diastereomeric reagent
(R)d-E-3 (which will undergo crotylboration of aldehydes to give
ent-5).16 Thus, the regiochemistry of the enantioselective hydro-
boration (e.g., right- vs left-hand allenyl double bond) deter-
mines the absolute stereochemistry of the 1,1-boryl stannyl
stereocenter in intermediate 3. To explain the enantioconvergent
hydroboration process, we propose that hydroboration of the
0 °C, followed by addition of hydrocinnamaldehyde at ꢀ78 °C,
provided alcohol (R,S)-5a in 81ꢀ88% yield and >95% ee.
Monitoring of this hydroboration reaction by 1H NMR revealed
that (S)-E-3 is the major allylborane species present (see
Supporting Information (SI)). Notably, when (M)-1 (g95%
ee)12 was treated under identical conditions with 1 equiv of
(dIpc)2BH and then 1 equiv of hydrocinnamaldehyde, the
identical alcohol (R,S)-5a was isolated, albeit in reduced yield
(42%) and diminished enantioselectivity (82% ee). Here again,
(S)-E-3 was observed when the hydroboration of (M)-1 with
(dIpc)2BH was monitored by 1H NMR (data not shown). Based
on the mechanism discussed subsequently, the hydroboration of
allene (P)-1 with (dIpc)2BH likely is a matched double asym-
metric reaction, while hydroboration of allene (M)-1 with
(dIpc)2BH is presumed to be the mismatched pair.13 The minor
syn diastereomer 6a and ent-6a from these two experiments
are enantiomeric; thus, an enantioconvergent process is not
dominant in the pathway(s) leading to the minor syn diastereomers
6a/ent-6a. The diminished chemical efficiency of the mismatched
hydroboration of (M)-1 is likely due to the competitive addition of
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enantioselectivity of the crotylboration reactions of the reagent
generated from (M)-1 and (dIpc)2BH.16
This analysis also provides the basis to rationalize that the
dominant pathways that give rise to the minor syn-homoallylic
alcohols 6 (Table 1) are not enantioconvergent (Figure 2): 6a
[from (P)-1] derives from (S)d-Z-8, whereas ent-6a [from (M)-1]
derives at least in part from (R)d-Z-8.
The 1,3-boratropic shifts presented in Figure 3 are concerted,
stereospecific suparafacial sigmatropic rearrangements that in-
volve the transfer of chirality from one center (in the precursor)
to a new center in the product. As such, the stereochemistry at
the new center established in the product of each boratropic shift
[e.g., the R-boryl-R-stannyl center in (S)-E-3] is determined by
the configuration and enantiomeric purity of the stereocenter in
the 1,3-transposed precursor [e.g., (R)-Z-7 in the hydroboration
of (P)-1].15f,g Thus, the success of this method for generation of
(S)-E-3 translates directly to the enantioselectivity of the allene
hydroboration step using (dIpc)2BH. This is in contrast to our
recent report on the hydroboration of the parent monosubstituted
allenylstannane,15k in which the stereochemistry and enantiomeric
purity of the R-boryl-R-stannyl center is induced by the diisopi-
nocampheylborane unit during the 1,3-boratropic shift.
Other racemic allenes are also substrates for the enantiocon-
vergent hydroboration reaction. As illustrated in Table 2, sub-
jection of racemic allene (()-10 to the standard hydroborationꢀ
crotylboration conditions using 1 equiv of (dIpc)2BH and 1 equiv
of aldehyde provides the (E)-δ-stannyl-anti-homoallylic alcohols
12a and 12b in 71ꢀ76% yields with excellent diastereo- and
enantioselectivities (>25:1 dr, 94% ee). Similarly, homoallylic
alcohols 12c and 12d were obtained in 59ꢀ61% yields and
95ꢀ97% ee, along with approximately 10% of (E)-δ-stannyl-syn-
homoallylic alcohols 13c and 13d from racemic allene (()-11.17
The stoichiometries, chemical efficiencies, and enantioselectivity
of these reactions, as for those in Table 1, are consistent with both
enantiomers of racemic allenes (()-10 and (()-11 undergoing
enantioconvergent hydroboration reactions with (dIpc)2BH.
In conclusion, we have documented a remarkable enantio-
convergent and highly enantioselective allene hydroboration
reaction. Hydroboration of (()-1 with (dIpc)2BH converts both
enantiomers, (P)-1 and (M)-1, into the same intermediate, (S)-
E-3. Subsequent crotylboration of (S)-E-3 with a variety of
aldehydes provides (E)-δ-stannyl-anti-homoallylic alcohols 5 in
good yields and high enantioselectivities.
There are a few points worth noting. First, these studies
constitute the first examples of the highly enantioselective
hydroborations of chiral allenes.8 The sense of asymmetric
induction is dictated by the enantioselectivity of the chiral,
nonracemic borane, (dIpc)2BH, which parallels the enantioselec-
tivity of the hydroboration of (Z)-alkenes with this reagent.14
Second, the hydroboration of the two enantiomers of racemic
allene 1 proceed with different modes of allene addition (Figure 3),
a regiochemical divergence also noted by Bergman.4a The
crotylborane reagent (S)-E-3 is then obtained from the initial
hydroboration intermediates, (R)-Z-7 and/or (R)-Z-8, via re-
versible but stereospecific 1,3-boratropic shifts.15 The ability
of both allene enantiomers to converge to a single, highly enantiose-
lective reagent (S)-E-3 via this hydroboration sequence represents a
remarkable example of the enantioconvergent reaction of the two
enantiomers of a racemate. Therefore, synthesis of enantiomeri-
cally pure allenylstannanes (P)-1 or (M)-1 is not necessary to
obtain homoallylic alcohols 5 with high enantiomeric excess, nor
is it necessary to utilize a kinetic resolution in the hydroboration
Figure 3. Proposed hydroboration pathways for the two enantiomers of
allenylstannane (()-1. The (R)- or (S)-descriptor defines the config-
uration of the allylic borane stereocenter in each intermediate; E and Z
denote the double bond configuration, and the “d” superscript denotes
the absolute configuration of the Ipc2B-unit. Thus, (S)d-E-3 and (R)d-E-
3 are diastereomers and not enantiomers.
enantiomeric allene (M)-1 with (dIpc)2BH occurs (preferen-
tially) on the si-face (top face, as drawn in the third equation of
Figure 3) of the Bu3Sn-substituted olefin of (M)-1, syn to the
methyl group, to provide, directly, reagent (S)d-E-3, the same
intermediate as obtained from (P)-1 in the first equation. The
sense of hydroboration in the conversion of (M)-1 to (S)-E-3 is
again consistent with the enantioselectivity of hydroboration of
(Z)-olefins by (dIpc)2BH8b,14 but is mismatched in that the
hydroboration occurs on the sterically disfavored olefin face syn
to the distal methyl substituent. A second possible pathway that
permits (S)-E-3 to be generated from (M)-1 is shown in the
fourth equation of Figure 3. In this case, hydroboration of (M)-1
by (dIpc)2BH on the Bu3Sn-substituted olefin anti to the distal
methyl group requires that (dIpc)2BH interact with the allene in a
manner opposite to that previously documented8b,14 for hydro-
borations of (Z)-alkenes by this reagent (hence, this pathway is
stereochemically disfavored on the part of (dIpc)2BH)). The
resulting product, (R)d-Z-8, can isomerize to (S)-E-3 by way of
(S)d-E-9 via two successive σ bond rotations and suparafacial 1,3-
boratropic shifts.15 These insights indicate that the “top” vs
“bottom” sense of allene hydroboration does not influence the
enantiomeric purity of the 1,1-boryl stannyl stereocenter in 3.
However, as is the case with (P)-1, hydroboration of (M)-1 at the
opposite end of the allene, in this case on the methyl-substituted
allenyl double bond as shown in the fifth equation of Figure 3,
inevitably produces the diastereomeric reagent, (R)d-E-3. The
latter pathway presumably contributes to the reduced
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Table 2. Enantioconvergent Hydroboration and Allylbora-
Bender, C. F.; Widenhoefer, R. A. J. Am. Chem. Soc. 2007, 129, 14148.
(c) One recent example of the enantioconvergent Cu(I)-mediated
borylation of a racemic cyclic allylic ether was reported by Ito and co-
workers: Ito, H.; Kunii, S.; Sawamura, M. Nat. Chem. 2010, 2, 972.
(5) (a) Flamme, E. M.; Roush, W. R. J. Am. Chem. Soc. 2002,
124, 13644. (b) Kister, J.; DeBaillie, A. C.; Lira, R.; Roush, W. R.
J. Am. Chem. Soc. 2009, 131, 14174.
tion Reactions of Racemic Allenes (()-10 and (()-11a
(6) (a) Roush, W. R. In Comprehensive Organic Synthesis; Trost,
B. M., Ed.; Pergamon Press: Oxford, 1991; Vol. 2, p 1. (b) Yamamoto,
Y.; Asao, N. Chem. Rev. 1993, 93, 2207. (c) Denmark, S. E.; Almstead,
N. G. In Modern Carbonyl Chemistry; Otera, J., Ed.; Wiley-VCH:
Weinheim, 2000; p 299. (d) Chemler, S. R.; Roush, W. R. In Modern
Carbonyl Chemistry; Otera, J., Ed.; Wiley-VCH: Weinheim, 2000; p 403.
(e) Denmark, S. E.; Fu, J. Chem. Rev. 2003, 103, 2763. (f) Lachance, H.;
Hall, D. G. Org. React. 2008, 73, 1.
(7) (a) Stille, J. K. Angew. Chem., Int. Ed. Engl. 1986, 25, 508.
(b) Farina, V.; Krishnamurthy, V.; Scott, W. J. Org. React. 1997, 50, 1.
(8) (a) Waters, W. L.; Caserio, M. C. Tetrahedron Lett. 1968, 5233.
(b) Moore, W. R.; Anderson, H. W.; Clark, S. D. J. Am. Chem. Soc. 1973,
95, 835. (c) Brown, H. C.; Liotta, R.; Kramer, G. W. J. Am. Chem. Soc.
1979, 101, 2966.
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(b) Ohtani, I.; Kusumi, T.; Kashman, Y.; Kakisawa, H. J. Am. Chem. Soc.
1991, 113, 4092.
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(c) Wierschke, S. G.; Chandrasekhar, J.; Jorgensen, W. L. J. Am. Chem.
Soc. 1985, 107, 1496. (d) Lambert, J. B.; Wang, G. T.; Finzel, R. B. J. Am.
Chem. Soc. 1987, 109, 7838. (e) Lambert, J. B.; Wang, G. T.; Teramura,
D. H. J. Org. Chem. 1988, 53, 5422.
allene
RCHO
ratio (12/13)
% yieldb
71 (12a)
% ee (12)c
10 Ph(CH2)2CHO
10 PhCHO
>25:1
>25:1
5:1
94
94
95
97
76 (12b)
59 (12c) þ 10 (13c)
61 (12d) þ 11 (13d)
11 Ph(CH2)2CHO
11 PhCHO
6:1
a The reactions were performed by treating (()-10 or (()-11 in Et2O
(0.1 M) with (dIpc)2BH (1.0 equiv) at 0 °C for 5 h followed by addition
of RCHO (1.0 equiv) at ꢀ78 °C. The mixture was stirred at ꢀ78 °C for
8 h. The reactions were terminated by addition of NaHCO3 and H2O2 at
0 °C prior to product isolation. b Isolated yield of the indicated products
(listed in parentheses). c Determined by Mosher ester analysis.
step. Finally, the highly diastereo- and enantioselective stannyl-
crotylboration reaction described here, however, provides anti-3-
alkyl-homoallylic alcohols with an (E)-vinylstannane that can be
used directly in a variety of CꢀC bond-forming reactions.7
(11) Stewart, P. S.; Chen, M.; Roush, W. R.; Ess, D. H. Org. Lett.
2011, 13, 1478. (b) Chen, M.; Roush, W. R. Org. Lett. 2011, 13, in press
(DOI: 10.1021/ol200392u).
’ ASSOCIATED CONTENT
(12) Racemic allenylstannane 1 is prepared in two steps from
commercially available (()-3-butyn-2-ol using the route described for
(M)-1 and (P)-1. The enantiomeric purity of allenes (M)-1 and (P)-1
was determined by Mosher ester analyses of the homopropargyl alcohols
derived from their BF3•Et2O catalyzed propargylation reactions of
hydrocinnamaldehyde. (a) Marshall, J. A.; Lu, Z.-H.; Johns, B. A.
J. Org. Chem. 1998, 63, 817. (b) Marshall, J. A.; Chobanian, H. Org.
Synth. 2005, 82, 43.
S
Supporting Information. Experimental procedures and
b
spectroscopic data for all new compounds. Control experiments,
stereochemistry assignments, and results of hydroboration of
(P)-1 with dicyclohexylborane. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
(13) Masamune, S.; Choy, W.; Petersen, J. S.; Sita, L. R. Angew.
Chem., Int. Ed. Engl. 1985, 24, 1.
Corresponding Author
roush@scripps.edu
(14) (a) Brown, H. C.; Zweifel, G. J. Am. Chem. Soc. 1961, 83, 486.
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(16) The preference for the R-stannyl group to occupy an equatorial
position in the transition state overrides the enantioselectivity of the
(Ipc)2B-unit, as shown in ref 15k. Thus, the reactions of (S)d-E-3 and
(R)d-E-3 with aldehydes will lead to enantiomeric homoallylic alcohols
5a and ent-5a.
’ ACKNOWLEDGMENT
Financial support provided by the NIH (GM038436 and
GM026782) is gratefully acknowledged.
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40, 59. (d) Pellissier, H. Tetrahedron 2008, 64, 1563.
(4) (a) Early examples of the enantioconvergent reactions of racemic
allenes were reported by Bergman and co-workers:Sweeney, Z. K.;
Salsman, J. L.; Andersen, R. A.; Bergman, R. G. Angew. Chem., Int. Ed.
2000, 39, 2339. (b) An example of dynamic kinetic resolution of racemic
allenes was reported by Widenhoefer and co-workers: Zhang, Z.;
(17) The decreased diastereoselectivity with 11 may be due to
decreased si-face hydroboration of the (M) enantiomer of 11.
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