Scope and mechanism of the Pt-catalyzed enantioselective diboration of monosubstituted alkenes

Article abstract of DOI:10.1021/ja4041016

The Pt-catalyzed enantioselective diboration of terminal alkenes can be accomplished in an enantioselective fashion in the presence of chiral phosphonite ligands. Optimal procedures and the substrate scope of this transformation are fully investigated. Reaction progress kinetic analysis and kinetic isotope effects suggest that the stereodefining step in the catalytic cycle is olefin migratory insertion into a Pt-B bond. Density functional theory analysis, combined with other experimental data, suggests that the insertion reaction positions platinum at the internal carbon of the substrate. A stereochemical model for this reaction is advanced that is in line both with these features and with the crystal structure of a Pt-ligand complex.

Full text of DOI:10.1021/ja4041016

Article
pubs.acs.org/JACS
Scope and Mechanism of the Pt-Catalyzed Enantioselective
Diboration of Monosubstituted Alkenes
John R. Coombs, Fredrik Haeffner, Laura T. Kliman, and James P. Morken*
Department of Chemistry, Merkert Chemistry Center, Boston College, Chestnut Hill, Massachusetts 02467, United States
S
* Supporting Information
ABSTRACT: The Pt-catalyzed enantioselective diboration of
terminal alkenes can be accomplished in an enantioselective
fashion in the presence of chiral phosphonite ligands. Optimal
procedures and the substrate scope of this transformation are
fully investigated. Reaction progress kinetic analysis and kinetic
isotope effects suggest that the stereodefining step in the
catalytic cycle is olefin migratory insertion into a Pt−B bond.
Density functional theory analysis, combined with other
experimental data, suggests that the insertion reaction
positions platinum at the internal carbon of the substrate. A
stereochemical model for this reaction is advanced that is in line both with these features and with the crystal structure of a Pt−
ligand complex.
accomplished with a Pt(0) catalyst in the presence of a readily
available taddol-derived phosphonite ligand.11 These initial
studies revealed an effective transformation useful enough to be
adopted for asymmetric natural product syntheses (vide infra);
however, significant improvements regarding the scope and
selectivity were clearly warranted. Moreover, a lack of
mechanistic details about the inner working of this reaction
hampered the design of improved processes. In this report, we
describe an improved catalyst and conditions for the
enantioselective diboration and its application to a broad
range of olefinic substrates. We also provide experimental
insight that suggests olefin migratory insertion to give an
internal C−Pt intermediate is the rate-limiting and stereo-
controlling step of the reaction. This information is used to
develop an understanding of the stereoselection in this
transformation.
1. INTRODUCTION
The catalytic enantioselective reactions of terminal alkenes offer
singular opportunities for strategic chemical synthesis. Such
reactions can facilitate both hydrocarbon chain extension and
functional group installation simultaneously and in a stereo-
selective fashion. Given these features, it is unfortunate that few
catalytic processes apply to terminal olefins in a highly
enantioselective manner. Indeed, other than substrate-specific
reactions that only apply to electronically biased alkenes (e.g.,
styrenes, dienes, and vinyl acetates) or to those that bear
adjacent directing groups,1 the range of asymmetric trans-
formations that apply to simple aliphatic α-olefins is limited.2
To address this shortcoming, we initiated studies on the
catalytic enantioselective diboration of terminal alkene
substrates.3 From the outset, it was anticipated that one
could engage the diboration product in a range of trans-
formations that apply to organoboronate intermediates4 and
therefore expand the range of strategically useful enantiose-
lective reactions that apply to 1-alkenes.
The Pt-catalyzed diboration of alkynes was first reported by
Miyaura and subsequently studied by Marder and Smith.5 A
general feature of these reactions is that they occur by oxidative
addition of B2(pin)2 to Pt(0) leading to a bis(boryl)platinum
intermediate that then reacts by olefin insertion and reductive
elimination. Further study of these reactions led to the
development of alkene6 and diene diborations6c,7 with the
latter reaction being accomplished in an enantioselective
fashion.8 We reported the discovery of a Pt-catalyzed
enantioselective diboration that applies to terminal olefins9
and a contemporaneous report by Hoveyda10 documented a
similar class of reaction products obtained by catalytic and
enantioselective, copper-catalyzed, double borylation of al-
kynes. The enantioselective alkene diboration reaction was
2. RESULTS AND DISCUSSION
2.1. Identification of the Optimal Ligand. An
optimization of ligand structures for the enantioselective
diboration of 1-octene focused on a set of meta-substituted,
taddol-derived, phosphonite and phosphoramidites.12 A sample
of ligands were examined in tetrahydrofuran (THF) and
toluene solvents and the selectivity was found to be relatively
insensitive to the medium. As depicted in Table 1, 3,5-di-
isopropylphenyltaddol-PPh (L4) was found to be comparable
to the previously reported ligand, 3,5-diethylphenyltaddol-PPh
(L3) under the conditions of Table 1.13 A direct correlation
between ligand size and enantioselectivity can also be observed,
with the selectivity increasing as the size of the meta substituent
Received: April 24, 2013
Published: July 18, 2013
© 2013 American Chemical Society
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a
Table 1. Ligand Optimization for Diboration of 1-Octene
b
c
entry ligand
R
R1
solvent yield (%)
er
1
2
L1
L2
L2
L3
L4
L4
L5
L6
L7
L8
L9
L9
L10
L11
L11
H
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
tol
24
84
81
83
82
84
74
69
68
35
73
77
76
34
40
80:20
95:5
Figure 1. Reaction rates with Pt(nbe)3, Pt2(dba)3, and Pt(dba)3.
Reactions employed 1 mol % platinum, 1.2 mol % (R,R)-L4, and 1.2
equiv B2(pin)2. initial [tetradecene] = 1.0 M.
Me
Me
Et
THF
tol
3
92:8
4
tol
96:4
5
i-Pr
i-Pr
t-Bu
OMe
H
THF
tol
97:3
reactions are complete in 3 h. Considering that Pt(dba)3 is the
most conveniently prepared complex of the three, it was
selected for all further studies.
The effect of the catalyst loading and the ligand−metal ratio
is depicted in Table 2. Relative to initially described conditions
6
97:3
7
tol
90:10
90:10
84:16
70:30
89:11
90:10
91:9
8
tol
9
NMe2
NEt2
tol
10
11
12
13
14
15
H
tol
Me
Me
Me
t-Bu
t-Bu
NMe2
NMe2
N(CH2)4
NMe2
NMe2
THF
tol
Table 2. Impact of Catalyst Loading and Ligand−Metal
a
Ratio on the Diboration of 1-Octene
tol
THF
tol
88:12
78:22
a
Reactions were conducted at 0.1 M substrate concentration for 12 h
b
as described in the text. Yield refers to isolated yield of the purified
reaction product. Enantiomeric ratio determined on the derived
c
acetonide by chromatography with a chiral stationary phase.
is varied (H < Me < Et ≅ i-Pr). However, when the size of the
ligand is enhanced past L4, enantioselectivities and yields
diminish (i.e., selectivity is diminished with t-Bu substituted
ligands). Subsequent experiments revealed that, at lower
catalyst loadings and lower ligand−metal ratio, L4 reproducibly
provides high selectivity, whereas selectivity with L3 is lower
and more variable (i.e., with 1 mol % Pt(dba)3 and 1.2 mol %
L3, reaction with 1-octene occurs in 72−90% ee). Thus, L4 was
employed for the remainder of this study.
2.2. Identification of the Optimal Pt(0) Source,
Catalyst Composition, and Catalyst Activation. Among
the complexes that apply to the Pt-catalyzed diboration, Pt(0)
precursors are employed almost exclusively. In terms of
practical utility, dibenzylideneacetone complexes are simple to
prepare and have the ideal features of being thermally stable
and insensitive to air and moisture. Notably, dibenzylidenea-
cetone−platinum complexes are prepared in a single-step
synthesis from dba and K2PtCl4 in an aqueous medium and
without drybox techniques.14 The identity of the product that
arises from this synthesis depends upon the conditions and the
isolation procedure; simple washing of the solid precipitate with
methanol gives Pt(dba)3, whereas recrystallization from THF/
methanol gives Pt2(dba)3.
Pt(dba)3
(mol %)
(R,R)-L4
[octene]
(M)
time
(h)
yield
(%)
entry
(mol %)
er
1
2
3
4
5
6
7
8
9
3.0
0.5
0.2
3.0
1.0
3.0
1.0
1.0
1.0
6.0
1.0
0.4
3.6
1.2
3.0
1.0
0.75
0.5
0.1
1.0
1.0
0.1
1.0
0.1
1.0
1.0
1.0
3
11
28
1
89
88
60
88
82
75
66
39
30
97:3
96:4
96:4
98:2
97:3
86:14
94:6
80:20
64:36
3
3
3
3
3
a
Yield refers to isolated yield of the purified reaction product.
Enantiomeric ratio determined on the derived acetonide by
chromatography with a chiral stationary phase.
(entry 1), the reaction could be run at higher substrate
concentration (1.0 M versus 0.1 M) and the catalyst loading
could be decreased from 3 to 0.5 mol % without a significant
effect on enantioselectivity (entry 2); however, the yield is
diminished at 0.2 mol % catalyst loading (entry 3). A close
analysis of ligand−metal ratio shows that a 1:1 ratio is also
effective (entry 7), although at the price of decreased yield
relative to higher ligand loading (entry 5). When the ligand−
metal ratio is less than 1:1, enantioselectivity also begins to
suffer (entries 8 and 9), likely from competitive diboration by a
phosphonite-free Pt complex. Considering that 1 mol % of a
1.2:1 phosphonite−Pt complex provided reproducibly high
yield and selectivity (entry 5), this catalyst loading/composition
was adopted for subsequent studies. Importantly, employing a
Using 1-tetradecene as a probe substrate for the
enantioselective diboration, we undertook a comparison of
different Pt(0) precursors (Figure 1). This survey showed that
there is very little dependence of enantioselectivity on the
nature of the precatalyst, with dba complexes proving equally
effective as Pt(nbe)3. Analysis of reaction rates by calorimetry
(Figure 1) also indicated that all three complexes provide
equally reactive catalysts: at 1.0 M concentration, all three
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1.2:1 ligand−metal ratio ensures an optimum outcome even if
the ligand batch is slightly oxidized during the course of its
synthesis (up to 4 mol % ligand oxidation is common).
30 min. The vial was then cooled to room temperature and
charged with the alkene by syringe, purged once more with N2,
then stirred at 60 °C for 3 h. Finally, oxidative workup
(generally treatment with NaOH and H2O2) delivered the
derived 1,2-diol.
Analysis of diboration reactions is most conveniently
accomplished by subjecting the intermediate 1,2-bis(boronate)
intermediate to in situ oxidation to the derived 1,2-diol. The
efficiency of this oxidation step was examined to determine how
well the yield of the 1,2-diol reflects to the overall efficiency of
diboration reactions. As depicted in Scheme 2 (eq 1), when
Experiments by Pringle reveal that addition of phosphites to
Pt(nbe)3 results in rapid displacement of the norbornene ligand
(<1 h at rt).15 In contrast, we have found that addition of either
(R,R)-L3 or (R,R)-L4 to Pt(dba)3 does not lead to a substantial
change in the 31P NMR after 1 h at room temperature. To
understand the chemistry that allows Pt(dba)3 to provide a
catalyst with equal activity to Pt(nbe)3 in alkene diborations,
the chemistry of the activation step was studied. In the
procedure employed above for catalytic alkene diboration, a
catalyst “activation step” precedes addition of substrate and
involves treating 1 mol % of Pt(dba)3 with 1.2 mol % of the
phosphonite ligand in the presence of 1.05 equiv of B2(pin)2 at
80 °C for 20−30 min. Without pre-complexation at this
elevated temperature, lower yields (due to olefin isomerization
and hydroboration byproducts) and reduced levels of
enantioselectivity are observed. As depicted in Scheme 1,
Scheme 2
Scheme 1
isolated, purified 1,2-bis(boronate) 2 is subjected to oxidation
with excess H2O2 in the presence of 3.0 M NaOH, the derived
1,2-diol 3 is obtained in 87% yield. The isolated yield for this
transformation is higher than that obtained for the single-pot
diboration/oxidation (82% yield, Table 3 compound 4) but is
not quantitative. This outcome suggests that the yield for the
diboration step is somewhat higher than that for the two-step
diboration/oxidation sequence. Indeed, when the intermediate
1,2-bis(boronate) 2 derived from the diboration of 1-
tetradecene was isolated and purified by column chromatog-
raphy, it was obtained in 95% yield (Scheme 2, eq 2). Some loss
in yield can be attributed to the oxidative workup sequence.
However, considering the ease of isolation and characterization
of the derived 1,2-diols, the two-step procedure was employed
to survey the reaction scope.
2.2.2. Substrate Scope: Aliphatic Alkenes. To learn about
the scope of enantioselective diborations with ligand L4, a
number of aliphatic alkenes were examined and the collected
data shown in Table 3. In general, high enantioselectivities were
observed regardless of the size of the substituent adjacent to the
olefin: 1-octene, vinylcyclohexane and t-butylethylene furnish
the derived 1,2-diols 3, 6, and 7 in 96:4, 96:4, and 93:7
enantiomeric ratios, respectively. The size of the olefin
substituent does have an impact on reactivity, with large
substituents appearing to diminish reactivity. Although 1-octene
and vinyl cyclohexane react efficiently with 1 mol % catalyst, t-
butylethylene required 3 mol % Pt catalyst to deliver a product
yield of 53%. Substrates bearing a variety of functional groups
were also surveyed. With elevated catalyst loading, TBDPS-
protected allyl alcohol was transformed to 10 efficiently but in
slightly diminished enantioselectivity. Oxygenation at the
homoallylic position appeared inconsequential and high yields
and selectivity for 11 were observed. Potentially reactive and
coordinating functional groups were also surveyed: carbonyls
are known to participate in some diboration reactions20 but
were not affected by the diboration of adjacent alkenes
(product 12). Similarly, olefinic substrates bearing ester and
amide functional groups were tolerated during the construction
of 13 and 14.
1
when a similar “activation” reaction was analyzed by H NMR
in d8−THF, diboration of dba was found to occur in >95%
conversion. This reactivity is in line with the Pt−phosphine
catalyzed conjugate borylation of activated alkenes reported by
Marder.16 Notably, according to studies by a number of
investigators17 on the binding of alkenes to related d10 Pd
complexes, one would anticipate that the conjugate borylation
product 1 would bind less effectively to Pt(0) than does dba
such that the conjugate borylation of dba may provide a
mechanism for removing the coordinating enone from the
metal complex. Analysis of the catalyst activation reaction by
31P NMR shows formation of a new phosphorus-containing
structure with broad resonances and coupling to platinum:
31P{1H} NMR (THF) δ 200.0 ppm (1JP−Pt = 1980 Hz); L4 δ =
158.1 ppm (s). Broadening of the phosphorus resonance is
consistent with the ligand bound to a Pt−boryl complex.18
Addition of 1-tetradecene to this mixture at room temperature
results in disappearance of the resonance at 200 ppm and
formation of a new Pt−boryl complex with broadened
resonances in the 31P NMR: 31P{1H} NMR (THF) δ 152.3
ppm (1JP−Pt = 2715 Hz).
2.2.1. Substrate Scope: Procedures and Oxidation
Efficiency. Purified taddol-derived phosphonite ligands are
stable in air for extended time periods. For example, when
purified solid L4 was exposed to air for >1 month, <3%
decomposition was detected by 31P NMR spectroscopy. Note
that L4 is prone to more rapid oxidation when dissolved in
solution (L4 was 12% oxidized after being dissolved in ether for
1 h). Similarly, Pt(dba)3 and B2(pin)2 are also stable to air and
moisture. Accordingly, the following drybox-free diboration/
oxidation procedure was employed to study the substrate scope
in the following sections: in the open atmosphere, 1 mol %
Pt(dba)3, 1.2 mol % L4, and 1.05 equiv B2(pin)2 were weighed
into a vial that was then sealed with a septum cap and purged
with N2. THF was then added by syringe, the vial was then
sealed, placed behind a blast shield,19 and heated to 80 °C for
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16 wherein the less substituted alkene is oxidized without
detectable transformation of the 1,1-disubstitued olefin (eq 3).
2.2.3. Substrate Scope: Aromatic Alkenes. In many
transition-metal-catalyzed processes, π-benzyl stabilization can
alter the regiochemistry of olefin migratory insertion reactions
for aromatic relative to aliphatic alkene substrates.22 If such a
turnover applied to Pt-catalyzed diboration, it would be
expected to dramatically alter the stereochemical outcome of
diborations of styrenes relative to aliphatic alkenes. To examine
whether such a divergent reaction manifold operates in
diborations, a variety of substituted styrenes were examined
(Table 4). Under the optimized conditions, ortho, meta, and
Table 3. Diboration of Aliphatic 1-Alkenes with Pt(dba)3
and (R,R)-L4
a
a
Table 4. Diboration of Styrenes with Pt(dba)3 and (R,R)-L4
a
Conditions: [alkene] = 1.0 M, reaction time = 3 h. Yield refers to
isolated yield of the purified reaction product. Enantiomeric ratio
determined on the derived acetonide by chromatography with a chiral
b
stationary phase. 3.0% Pt(dba)3 and 3.6% (R,R)-L4 was employed for
c
12 h. Oxidation with H2O2 at pH = 7.
a
Reactions conducted at [alkene] = 1.0 M for 3 h. Yield refers to
isolated yield of the purified reaction product. Enantiomeric ratio
In terms of olefin oxidation transformations, the diboration/
oxidation is complementary to osmium-catalyzed dihydrox-
ylation. The osmium-catalyzed reaction is accelerated with
more electron-rich substrates such that more substituted
alkenes generally react faster than less substituted substrates.
In contrast, Pt-catalyzed diboration is highly sensitive to steric
effects such that added substitution on the alkene leads to
marked diminution in reaction rate. This feature can be used
advantagously with polyolefin substrates. For example, the 1,1-
disubstituted alkene in 15 is selectively oxidized (5:1
regioselectivity) under conditions for Sharpless' asymmetric
dihydroxylation to provide a moderate yield of the derived 1,2-
diol (Scheme 3, eq 4).21 In contrast, Pt-catalyzed diboration/
oxidation leads to selective and high yield conversion to 1,2-diol
determined on the derived acetonide by chromatography with a chiral
b
stationary phase. 3.0% Pt(dba)3 and 3.6% (R,R)-L4 was employed
with reaction at 60 °C for 12 h.
para-substituted styrenes undergo effective diboration, afford-
ing moderate to good yields and enantioselectivities. Electron-
rich styrenes provided high enantiopurity (product 22),
whereas electron-deficient styrenes required longer reaction
times, a higher catalyst loading, and their reactions occurred
with slightly diminished enantiocontrol (products 23 and 24).
Collectively, however, the consistently high levels of
enantioselectivity and identical sense of facial selectivity
suggests that both aliphatic and aromatic alkene substrates
react by similar insertion modes.
2.2.4. Substrate Scope: Chiral Substrates. The impact of
substrate chirality is critical to applications in asymmetric
synthesis. As depicted in Table 5, diboration of alkenes
containing β-hydrocarbon stereocenters resulted in highly
diastereoselective reactions, regardless of the enantiomer of
ligand employed (product 25 and 26). Catalyst control is also
the primary stereochemical influence when a methylated
hydrocarbon stereocenter is situated adjacent to the reacting
alkene (compounds 27 and 28). However, there is a small level
of double diastereodifferentiation such that a mismatched
pairing provided a 12:1 dr of 27, whereas the matched case
provides 28 with 20:1 dr. Substrates bearing oxygenated α-
Scheme 3
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a
alkene diboration, even with increased heating and catalyst
concentration. This effect occurred for both cis-and trans-
alkenes and those that are strained or aromatic. Interestingly,
terminal alkynes do not undergo diboration and the presence of
an alkyne is sufficient to poison the reaction of terminal
alkenes. This effect operates whether the alkyne is tethered to
the alkene (41) or simply present in solution (i.e., a 1:1 mixture
of 1-tetradecyne and 1-tetradecene yielded no diboration of
either substrate). It is reasonable to conclude that the more π-
acidic alkyne may bind tightly to the catalyst, preventing either
oxidative addition of B2(pin)2 or alkene binding. Lastly, it
should be noted that the presence of an unprotected alcohol
caused the reaction to take a different course with the alkene
undergoing hydroboration.
Table 5. Diastereoselective Diboration of Chiral Substrates
2.3. Mechanistic Analysis. To provide a general under-
standing of the outcome of Pt-catalyzed enantioselective alkene
diboration reactions and to provide directions for reaction
modification when the need arises, a study of the reaction
mechanism was undertaken. Our analysis makes the assump-
tion that, like other Pt-catalyzed diborations, the catalytic cycle
includes the following steps: insertion of a Pt(0) complex into
the B−B bond of B2(pin)2,5,25 olefin insertion into a Pt−B
bond, and reductive elimination to furnish the product.26
2.3.1. Reaction Progress Kinetic Analysis. To learn how the
reaction rate responds to changes in reaction conditions,
experiments involving the diboration of 1-tetradecene were
analyzed. The reaction rates were monitored by calorimetry and
followed over the entire course of the reaction.27 Under
standard initial reaction conditions ([substrate] = 1 M,
[B2(pin)2] = 1.05 M, 1 mol % Pt(dba)3, 1.2 mol % L4) the
diboration of 1-tetradecene was complete in 3 h with a heat
flow/time profile as depicted above in Figure 1. Reaction rate
data from the calorimeter was corroborated by GC analysis
versus an internal standard. When the data in Figure 1 is
integrated (see Figure 3A; plot of reaction rate versus substrate
concentration), it becomes apparent that, although not quite
zero order, the rate does not change substantially over the first
∼60% of the reaction. Also depicted in Figure 3A, when the
initial concentrations of both B2(pin)2 and 1-tetradecene are
increased to 1.5 M, the reaction rates were largely unaffected
and critically, the rate of this reaction, when at 1 M substrate
concentration (i.e., at 33% conversion), is nearly identical to the
rate of the reaction initiated at 1 M substrate concentration.
This feature suggests that catalyst decomposition and product
inhibition are not significant complicating features to this
process.
As depicted in Figure 3B, doubling the catalyst loading leads
to a 2-fold increase in the reaction rate indicating first-order
dependence of the rate on catalyst concentration. Heat flow
versus time profiles for reactions that employ excess alkene and
excess B2(pin)2 are depicted in parts C and D of Figure 3. From
the data in Figure 3C, it appears that the reaction rate exhibits
slight positive order dependence on B2(pin)2 concentration.
Increased alkene concentration (Figure 3D) appears to be
inconsequential to the reaction rate at early stages of the
reaction. At later stages of the reaction, the heat flow is
enhanced with excess alkene and the reaction provides higher
overall heat output. This feature may be an artifact of
exothermic isomerization of the terminal alkene to internal
alkenes, a process that begins to occur as B2(pin)2 is consumed
(in the absence of B2(pin)2 isomerization of 1-tetradecene to 2-
tetradecene occurs in >95% conversion). When the initial
alkene concentration is reduced to 0.7 M, the overall heat
a
Unless otherwise stated, reactions were conducted at 1.0 M substrate
concentration for 3 h as described in the text. Yield refers to isolated
yield of the purified reaction product. Diastereomer ratio (dr)
b
1
determined by H NMR analysis. 3.0% Pt(dba)3 and 3.6% (R,R)-
or (S,S)-L4 was employed with reaction at 60 °C for 12 h.
stereocenters exhibited much more pronounced differences
between matched and mismatched stereoisomers (products 29
and 31). However, the influence of oxygenated stereocenters is
diminished as the stereocenter is more remote from the alkene
such that the stereochemistry in products 30 and 32 is largely
controlled by the catalyst. Compounds 33 and 34 depict
examples reported in total synthesis efforts from the
laboratories of Shindo23 and Minnaard,24 respectively, that
indicate that high levels of stereoinduction can be obtained with
other oxygenated chiral substrates.
2.2.5. Substrate Scope: Unreactive Substrates. A list of
unreactive substrates is provided in Figure 2. Addition of a
single additional alkene substituent is sufficient to prevent
Figure 2. Unreactive diboration substrates.
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is close to zero-order in both substrates. This feature of the
reaction suggests that the most significant step kinetically is
likely an elementary transformation that, in some manner,
involves a Pt complex that is already loaded with B2(pin)2 and
the alkene. Although a number of mechanisms would give rise
to these observations, a scenario that is consistent with reported
computational and experimental studies is depicted in Scheme
4. In this proposal, oxidative addition of B2(pin)2 to an LnPt(0)
Scheme 4
complex (A → B) is fast and followed by alkene association to
give C. With this sequence of steps, the reaction would exhibit
near zero-order dependence on substrate concentration if the
turnover-limiting step was either C→D (i.e., reversible alkene
binding strongly favors the olefin complex and migratory
insertion is rate-limiting) or D→A (reversible migratory
insertion precedes rate limiting reductive elimination). An
equally tenable hypothesis would arise from a cycle where
olefin coordination precedes oxidative addition.
2.3.2. Natural Abundance 13C Kinetic Isotope Effects. We
have been unable to gain useful information about catalytic
intermediates by NMR spectroscopy. As an alternative
approach to determine whether migratory insertion might be
rate-limiting and stereochemistry-determining or whether
reductive elimination controls the product configuration, 13C
kinetic isotope effects for the alkene substrate were examined.
These experiments were conducted using methods developed
by Singleton for natural abundance kinetic isotope effect (KIE)
analysis.28 With allylbenzene as a probe substrate, two
diborations were conducted on a 1.5 g scale using standard
conditions and stopped at 81 3 and 83 3% conversion.
After recovery of starting material, 13C NMR analysis with a
125 s relaxation delay was conducted and the integrations
relative to C-4 found to be as depicted in Scheme 5. Although
the 12C/13C KIEs for the allylic carbon and aromatic carbons
are negligible, a substantial isotope effect is observed at both
olefinic carbons. This observation appears most consistent with
olefin migratory insertion being the first irreversible step in the
cycle and suggests that addition of a Pt−boryl across the π-
system is the stereochemistry-determining step of the reaction.
Were migratory insertion reversible29 with the reductive
elimination controlling the rate (and therefore stereochemistry)
of the reaction, a sizable KIE at only one carbon would be
expected. It merits mention that the magnitude of the 12C/13C
KIEs are comparable to those measured by Landis for Ziegler−
Natta polymerization of 1-hexene.30 In the Landis study, it was
argued that olefin migratory insertion into a Zr−alkyl is the
turnover-limiting step.
Figure 3. Reaction progress kinetic analysis of the catalytic
enantioselective diboration of 1-tetradecene. (A) Reaction rate versus
substrate concentrations. [Alkene]= [B2(pin)2] for both 1.0 M (red)
and 1.5 M (blue) reactions, with (R,R)-L4: Pt(dba)3 = 1.2: 1.0. (B)
Absolute reaction rate versus [alkene], employing 1.2 mol % (R,R)-L4
and 1.0 mol % Pt(dba)3 (red), 2.4 mol % (R,R)-L4 and 2.0 mol %
Pt(dba)3 (blue). (C) Heat flow versus time of normal diboration
reaction conditions (red) and with excess B2(pin)2 (blue). (D) Heat
flow versus time of normal diboration reaction conditions (red), with
excess alkene (green), and with 0.7 M alkene (blue).
output is 70% of the stoichiometric experiment whereas the
initial rate is comparable.
Collectively, the kinetic data suggests that over most of the
reaction course, alkene diboration catalyzed by Pt(dba)3 and L4
2.3.2.1. Regioselectivity of Olefin Insertion: Experiments.
The studies above suggest that olefin insertion is likely the
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cyclopropyl organoplatinum complex.31 Cyclopropane ring-
opening pathways involving similar intermediates have also
been previously reported by Lautens in the palladium-catalyzed
hydrostannation32 and by Beletskaya using rhodium-catalyzed
hydrosilation.33 Importantly, the proposed internal insertion
mode is not without precedent in group 10 metal-catalyzed
reactions.34
Scheme 5
2.3.2.2. Regioselectivity of Olefin Insertion: Computations.
Further evidence suggesting that olefin migratory insertion in
the (alkene)Pt[B(pin)]2 complex occurs to give an internal C−
Pt bond was gained by density functional theory (DFT)
calculations on a simplified system. As depicted in Figure 4, the
stereocontrolling step of alkene diborations. Development of a
stereochemical model for this reaction will therefore depend on
understanding the regioselectivity of this step. Although an
olefin insertion that positions Pt at the internal carbon (E → F,
Scheme 6) would appear to be more hindered than the
Scheme 6
alternative pathway (G → H), it explains two features: first, this
insertion mode positions the prochiral carbon of the alkene
much closer to the chiral ligand and can more easily explain the
high enantioselectivity for the diboration process; second, if G
→ H were the favored insertion mode for aliphatic alkenes,
then one might expect significantly different selectivity with
aromatic alkenes where π-benzyl stabilization in F might
increase the extent of reaction through this alternate mode.
An experiment consistent with the conjecture that olefin
insertion positions Pt at the internal position is depicted in
Scheme 7. When vinyl cyclopropane 43 was subjected to
Scheme 7
Figure 4. (A) Relative energetics of migratory insertion involving
bis(boryl)platinumpropene complexes. (B) Energy profile for the
migratory insertion of 45 and 48 to give 49 and 52. (C) Calculated
transition state structures for conversions to 45 → 49 and 48 → 52.
All free energies (computed at 333K) are in kcal/mol and are relative
to 48.
reactions of four isomeric bis(boryl)Pt·propene complexes
(45−48, Figure 4A) were examined using DFT.35 Complexes
45−48 employ an ethylene glycol-derived methyl phosphonite
as a model for ligand L4 and vary by the nature of the alkene
coordination: complexes 45 and 48 are rotational isomers with
the alkene binding through the Re face, whereas isomers 46 and
47 contain the alkene bound through its Si face. Of the four
complexes, there was no strong preference for any alkene
orientation with all four being within 1.3 kcal/mol of each
other. Optimization of transition state geometries of each olefin
conditions for catalytic enantioselective diboration, ring-opened
bis(boronate) 44 was obtained as the exclusive reaction
product. As shown in the inset, a reasonable pathway for
production of 44 involves borylplatination of the alkene in a
manner that gives the internal C−Pt bond (I → J, Scheme 7
inset) subsequent rupture of the adjacent cyclopropane (J →
K) and reductive elimination would deliver 44. An analogous
ring-opening was observed by Miyaura in the diboration of
methylene cyclopropanes and was attributed to rupture of an α-
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complex revealed pathways where olefin insertion occurs
concomitantly with rotation of the boryl ligand; the rotation
allows the pinacol oxygen to begin donating electrons to
platinum such that a coordinatively saturated Pt complex is the
immediate product of the insertion step in each case (49−52).
These features are analogous to those calculated by Morokuma
for the insertion of ethylene.36 As depicted in Figure 4A, it was
found that the two pathways where Pt undergoes bond
formation to the internal alkene carbon (45 → 49 and 46 →
50) are lower energy paths than those where Pt bonds to the
terminal carbon (47 → 51 and 48 → 52). The two lowest
energy paths for each regioisomer are depicted in Figure 4B
along with transition state geometries in Figure 4C. Although
the origin of the regiochemical preference is not clear, it is
tenable that a larger coefficient of the alkene HOMO resides on
the terminal carbon, and overlap between this orbital and the
empty p-orbital on boron is relevant during the insertion.
Calculations suggest that related interactions operate in
palladium-catalyzed alkyne silastannation, a reaction that also
occurs by a migratory insertion that provides the internal C−Pd
arrangement.16a,37
2.4. Model for Stereoselectivity in Alkene Diborations
with Pt(dba)3 and L4. With compelling data indicating that
olefin migratory insertion is stereochemistry-determining and
provides the internal C−Pt bond, efforts were directed to
advance a stereochemical model for this diboration reaction.
These studies have been guided by the crystal structure analysis
of a trans-Cl2Pt[(R,R)-L2]2. This compound was obtained by
treatment of PtCl2 with two equiv of (R,R)-L2 in CDCl3 at
room temperature for 18 h followed by warming to 40 °C for 2
h. 31P NMR analysis showed complete conversion of the ligand
(δ = 156.8 ppm) to a new complex (δ = 105.2 ppm). Slow
crystallization from CDCl3 provided crystals suitable for X-ray
analysis, an ORTEP representation of which is depicted in
Figure 5. The unit cell contains two molecules of the complex
in nearly identical conformations, and within each molecule of
complex, both ligands adopt a similar conformation. Parts B
and C of Figure 5 depict “front” and “top” views of the complex
with the substituents from one phosphorus ligand removed.
From these views it appears that the platinum atom sits directly
on top of one of the taddol aromatic rings.
Figure 5. (A) ORTEP of trans-Cl2Pt[(R,R)-L2]2 with 50% probability
ellipsoids. (B) Front view of complex with substituents from one
phosphorus ligand removed. (C) Top view of complex with
substituents from one phosphorus ligand and the dioxolane removed
for clarity.
Should the conformation of the ligand that is represented in
Figure 5 persist during the course of the catalytic cycle, it is
tenable that subsequent to insertion of Pt into the B−B bond of
B2(pin)2, alkene coordination may occur in a manner depicted
in Figure 6. This orientation is consistent with the hypothesis
that insertion provides the internal C−Pt bond, and if
interactions with the taddol aryl ring are minimized, it would
directly lead to the observed product enantiomer. Additional
studies that probe the nuances of these interactions are clearly
warranted and will be described in due course.
Figure 6. Hypothesis for the stereochemical outcome in enantiose-
lective diborations.
ASSOCIATED CONTENT
* Supporting Information
Procedures, characterization, spectral data, and computational
details and crystal data in CIF format. This material is available
free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
morken@bc.edu
■
3. CONCLUSION
Notes
In conclusion, we have described the optimization of reaction
conditions and the scope of the platinum-catalyzed enantiose-
lective diboration of terminal alkenes. Mechanistic experiments
indicate that this reaction occurs by stereochemistry-determin-
ing olefin insertion that furnishes an internal C−Pt bond.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The NIH (GM-59471) is acknowledged for financial support;
AllyChem is acknowledged for a donation of B2(pin)2. We
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thank Drs. Bo Li and Thusitha Jayasunder of Boston College
for assistance with X-ray analysis and with NMR (kinetic
isotope effect studies), respectively.
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dx.doi.org/10.1021/ja4041016 | J. Am. Chem. Soc. 2013, 135, 11222−11231