Hydroxyl-directed stereoselective diboration of alkenes

Article abstract of DOI:10.1021/ja504228p

An alkoxide-catalyzed directed diboration of alkenyl alcohols is described. This reaction occurs in a stereoselective fashion and is demonstrated with cyclic and acyclic homoallylic and bishomoallylic alcohol substrates. After oxidation, the reaction generates 1,2-diols such that the process represents a method for the stereoselective directed dihydroxylation of alkenes.

Full text of DOI:10.1021/ja504228p

Communication
pubs.acs.org/JACS
Open Access on 06/18/2015
Hydroxyl-Directed Stereoselective Diboration of Alkenes
Thomas P. Blaisdell,† Thomas C. Caya,† Liang Zhang, Amparo Sanz-Marco, and James P. Morken*
Department of Chemistry, Merkert Chemistry Center, Boston College, Chestnut Hill, Massachusetts 02467, United States
S
* Supporting Information
compete with high background reaction rates; (c) whether the
directed reaction could indeed enable the selective reaction of
challenging hindered substrate classes.
ABSTRACT: An alkoxide-catalyzed directed diboration
of alkenyl alcohols is described. This reaction occurs in a
stereoselective fashion and is demonstrated with cyclic and
acyclic homoallylic and bishomoallylic alcohol substrates.
After oxidation, the reaction generates 1,2-diols such that
the process represents a method for the stereoselective
directed dihydroxylation of alkenes.
To begin probing the ability of alcohol functional groups, or
their derived alkoxides to promote selective diboration of
adjacent alkenes, substrate 1 was examined under a range of
reaction conditions. As depicted in Table 1 entry 1, homoallylic
alcohol 1 was first heated with B2(pin)2 in the absence of base
and was found to be unreactive. When the derived sodium
alkoxide of substrate 1 was employed (entry 2) the substrate
was also left unchanged. Inclusion of 5 equiv of methanol in the
reaction of the alkoxide resulted in a significant improvement
he olefin diboration reaction is a useful strategy for the
T
conversion of alkenes into a range of important building
blocks.1 In this connection, we2 and others3 have developed
strategies for the catalytic enantioselective addition of diboron
reagents to prochiral isolated alkenes. While these reactions
occur selectively with monosubstituted, trans-1,2-disubstituted,
and trisubstituted alkenes, the minimally biased prochiral π-
faces of cis-1,2-disubstituted and 1,1-disubstituted alkenes
render selective reactions of these substrate classes much
more difficult to engineer. As one strategy to address this issue,
we considered that a directing group located proximal to the
reacting olefin might provide an additional organizational
element and enable a solution to these more challenging
substrate classes.4,5 Herein, we describe highly efficient and
selective directed diboration of cyclic and acyclic homoallylic
and bishomoallylic alcohols.6 In addition to providing a
stereoselective route to synthetically versatile 1,2-bis-
(pinacolboronates), this process provides an alternative to
directed OsO4-promoted dihydroxylations that typically require
stoichiometric amounts of osmium and that do not reliably
extend to homoallylic alcohol substrates.7
a
Table 1. Catalytic Enantioselective Diboration of 1
B2(pin)2
(equiv)
conv
(%)
entry base (equiv)
additive (equiv)
dr
1
2
3
4
none
1.05
1.05
1.05
1.05
0
<5
<5
87
83
na
na
NaH (1.0)
NaH (1.0)
0
CH3OH (5)
CH3OH (5)
12.5:1
7.7:1
NaOtBu
(0.15)
5
Na2CO3
(0.15)
1.05
CH3OH (5)
68
7.9:1
6
7
KOtBu (0.15)
1.05
1.05
CH3OH (5)
CH3OH (5)
90
87
12.3:1
12.5:1
Cs2CO3
(0.15)
8
3
4
5
6
Cs2CO3
(0.15)
1.05
1.05
1.05
1.05
1.05
iPrOH (5)
tBuOH (5)
H2O (5)
64
42
9
7.4:1
9.5:1
3.8:1
13.2:1
5.6:1
In lieu of transition-metal catalysts, Lewis bases have been
shown to activate diboron reagents for addition to unsaturated
substrates. While N-heterocyclic carbenes8 and phosphines9
have been shown to catalyze the addition of B2(pin)2 to
activated olefins,10 metal alkoxides11 used in conjunction with
alcohols have been shown to promote the addition of B2(pin)2
to isolated unactivated alkenes. While computational studies
performed by Fernandez suggest that the mechanism of the
alkoxide-catalyzed reaction occurs by alkoxide-induced transfer
of a boryl anion to the alkene providing a transient three-
membered boracycle, mechanistic experiments have not
detailed precise features of the reaction. None the less,
development of a directed process that involves association of
an alkoxide to B2(pin)2, followed by delivery of the diboron
reagent to a tethered alkene, appeared plausible. Issues central
to a selective directed reaction include (a) whether the
geometry required for addition of the diboron to the alkene
could be maintained concurrently with intramolecular alkoxide-
diboron association; (b) whether the directed process could
Cs2CO3
(0.15)
Cs2CO3
(0.15)
Cs2CO3
(0.15)
PhOH (5)
51
45
Cs2CO3
(0.15)
CF3CH2OH (5)
9
Cs2CO3 (0.3)
Cs2CO3 (0.3)
Cs2CO3 (0.3)
Cs2CO3 (0.3)
Cs2CO3 (0.3)
2.0
2.0
2.0
2.0
2.0
CH3OH (5)
CH3OH (17)
CH3OH (25)
CH3OH (50)
>95
>95
>95
>95
>95
9.9:1
12.9:1
12.1:1
9.5:1
b
10
11
12
13
CH3OH
(solvent)
5.7:1
a
Percent conversion determined by 1H NMR analysis, stereoselectivity
b
(dr) determined by SFC. Reaction time is 1 h.
Received: May 1, 2014
Published: June 18, 2014
© 2014 American Chemical Society
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Communication
Figure 1. Directed diboration of alkenyl alcohols.
providing the product in 87% conversion and 12.5:1
diastereomer ratio favoring the syn isomer of product.
Reasoning that the substrate alkoxide likely participates in
reversible acid−base equilibrium with methanol, a catalytic
amount of NaOtBu in the presence of methanol was examined
and found to provide the product in good yields and moderate
stereoselection. Examination of other bases in the presence of
methanol showed that KOtBu and Cs2CO3 provided higher
levels of selectivity and also very good conversion to product.
In an effort to achieve complete conversion and shorter
reaction times, the amounts of Cs2CO3 and B2(pin)2 were
increased. While the stereoselection was slightly diminished
with 30 mol % Cs2CO3 and 5 equiv of methanol, use of 17
equiv of methanol restored the selectivity to a high level and
provided complete conversion in 1 h (entry 10). That the
selectivity improves with added methanol, and is high even with
50 equiv (entry 12), is remarkable for a reaction that appears to
occur by an alkoxide directing effect; indeed, even when the
reaction is run in methanol as solvent, a medium that should
favor the generation of methoxide at the expense of substrate
alkoxide, good levels of stereocontrol are still observed in the
reaction (entry 13).
was nonselective. Lastly, an allylic alcohol was found to
participate (19) but with low selectivity under the present
conditions, and a tetra-substituted alkene was found to be
unreactive (data not shown).
The observations in Table 1 demonstrate the importance of
an alcohol additive as well as the requirement of a Bronsted
base in the reaction. To gain insight into the manner in which
the resident alcohol functional group influences the stereo-
selective diborations in Figure 1, several other experiments
were undertaken. To probe the role of the hydroxyl functional
group, the derived methyl ether was examined in the diboration
reaction. As depicted in Scheme 1, diboration of 20 did indeed
occur; however, an equal mixture of diastereomers 21 was
produced. This observation suggests that the hydroxyl group
Scheme 1
The scope of the directed diboration reaction was examined
with a panel of substrates. Unless otherwise noted, the alkenyl
alcohol was subjected to B2(pin)2 in the presence of cesium
carbonate and methanol and subjected to oxidative work up
with basic hydrogen peroxide (see Supporting Information (SI)
for details). Under these conditions, the probe substrate in
Table 1 was converted to the derived diol 2 in 74% isolated
yield (Figure 1). In addition to terminal alkene substrates, the
reaction extends to 1,1-disubstituted alcohols (products 5 and
6) and also applies to prenyl- and anti- and syn-crotyl-derived
substrates as well (products 7−10). That the reaction occurs by
stereospecific directed syn addition could be ascertained by
observing selective diboration of both trans- and cis-olefin
substrates (products 11 and 12).12 Also of note, cyclic alkenes
participate (products 13−15) and are converted to the syn
diboration product in useful yields. Reaction product 13 is
particularly notable as the corresponding OsO4-catalyzed
dihydroxylation strongly favors the opposite stereoisomer of
reaction product (see the SI). The hydroxyl group could be
moved one carbon further away from the alkene and still have
significant impact on the reaction selectivity as exemplified by
product 16; however, with a trishomoallylic alcohol the reaction
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Journal of the American Chemical Society
Communication
does not function as a neutral Lewis basic chelating group but
either participates in hydrogen bonding or participates as the
derived alkoxide. If one of these modes applies and results in
delivery of the diboron reagent to the adjacent alkene, then a
rate acceleration is anticipated for the directed versus the
nondirected process. To probe this detail, an equimolar mixture
of a homoallylic alcohol and 1-tetradecene was subjected to
diboration with a deficiency of B2(pin)2. After 6 h of reaction, it
was found that the homoallylic alcohol had been consumed to a
greater extent (49% conversion, see Scheme 1, eq 2,
competition 1) than tetradecene (12% conversion). This
experiment establishes a lower limit for the reaction rate ratio
of >∼4. In line with observed selectivity, a similar experiment
showed that a bishomoallylic alcohol reacts at a rate comparable
to the homoallylic alcohol (competition 2), whereas a
trishomoallylic alcohol reacts slower (Scheme 1, competition
3).
allylation methods.15 As depicted in Scheme 3, TBDPS
protected 3-buten-1-ol (22) was subjected to cascade
Scheme 3
Combined, the experiments in Scheme 1 are highly
suggestive of a directed process involving association between
the substrate and the diboron reagent. While more detailed
investigation is clearly warranted, the observation that the
reaction is stereoselective even when run in methanol solvent,
suggests that hydrogen bonding between the substrate alcohol
and B2(pin)2 may not be the dominant effect and that
coordination of the substrate alkoxide with the diboron reagent
is more likely a critical feature. Although the precise mechanism
and a role for methanol still remain to be determined, structure
A that involves alkoxide-B2(pin)2 association rationalizes the
stereochemical outcome of most reactions in Figure 1 and is in
line with the mechanistic features delineated by Fernandez.
To probe the utility of the directed diboration in preparative
organic synthesis, a larger scale reaction was conducted with an
aim on evaluating practical aspects. In this experiment, the
diboration of 1 was conducted on 5.1 g scale, and all reagents
were handled in the open atmosphere (Scheme 2). Moreover,
asymmetric diboration/cross-coupling to furnish substituted
homoallylic alcohol 23. Directed diboration was followed by
catalytic cross-coupling and oxidation to provide 24 with
moderate yield but in excellent levels of stereocontrol.
In conclusion, the diboration of alkenyl alcohols benefits
from substrate directing effects to control the product
configuration. Examination of other substitution patterns and
directing groups should further expand these processes, and
such studies are in progress as is a critical analysis of operative
mechanisms.
ASSOCIATED CONTENT
■
S
* Supporting Information
Procedures, characterization, and spectral data. This material is
available free of charge via the Internet at http://pubs.acs.org.
Scheme 2
AUTHOR INFORMATION
Corresponding Author
morken@bc.edu
■
Author Contributions
†These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
the reaction was conducted in a typical round-bottom flask
fitted with a reflux condenser but was exposed to the open
atmosphere and without a drying tube attached. To minimize
the amount of B2(pin)2 employed but still obtain complete
conversion in a reasonable time frame, the reaction was run at a
increased substrate concentration (0.5 M). After 12 h at 70 °C,
followed by oxidative workup, triol 2 was obtained in 84% yield
and as a 17:1 mixture of syn:anti diastereomers.
Also in connection to synthesis utility, the ability to address
more complex targets with directed diboration was examined.
In this regard, vicinal bis(boronates) participate in effective
cross-coupling reactions that can provide a number of useful
functional group arrays.13 When these cross-couplings are
applied to directed diboration products, they may provide new
strategies for constructing important and challenging structural
motifs. To examine this possibility in the context of directed
diboration, we targeted the C6−C13 portion of spongistatin,14
a subunit that has been prepared previously through carbonyl
■
The NIH (GM-59417) is acknowledged for financial support,
and AllyChem is acknowledged for donations of B2(pin)2. We
thank Ms. Elisa Bushee for experimental assistance.
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