Magnesium-catalyzed hydroboration of esters: Evidence for a new zwitterionic mechanism

Article abstract of DOI:10.1039/c3sc52793j

A magnesium-catalyzed ester hydroboration reaction rapidly and efficiently (<0.5 mol% catalyst) provides alkoxy borane products via ester cleavage. Oxidized functional groups, such as cyano-, nitro-, cyclopropyl- and conjugated olefins, are unaffected by the ester reduction. Moreover, metal-catalyzed hydroboration reactions have been previously suggested to involve second-order interactions of hydroborane reagents and M-X (X = R and OR) for B-X bond formation. Catalytic kinetic studies rule out this traditional σ-bond metathesis mechanism for B-O bond formation, and instead a pathway involving a zwitterionic alkoxyborate is proposed. The Royal Society of Chemistry 2014.

Full text of DOI:10.1039/c3sc52793j

Chemical
Science
View Article Online
View Journal | View Issue
EDGE ARTICLE
Magnesium-catalyzed hydroboration of esters:
evidence for a new zwitterionic mechanism†
Cite this: Chem. Sci., 2014, 5, 959
*
Debabrata Mukherjee, Arkady Ellern and Aaron D. Sadow
A magnesium-catalyzed ester hydroboration reaction rapidly and efficiently (<0.5 mol% catalyst) provides
alkoxy borane products via ester cleavage. Oxidized functional groups, such as cyano-, nitro-,
cyclopropyl- and conjugated olefins, are unaffected by the ester reduction. Moreover, metal-catalyzed
hydroboration reactions have been previously suggested to involve second-order interactions of
hydroborane reagents and M–X (X ¼ R and OR) for B–X bond formation. Catalytic kinetic studies rule
out this traditional s-bond metathesis mechanism for B–O bond formation, and instead a pathway
involving a zwitterionic alkoxyborate is proposed.
Received 7th October 2013
Accepted 2nd December 2013
DOI: 10.1039/c3sc52793j
www.rsc.org/chemicalscience
conversion.11 The suggested pathway for aldehyde reduction
involved insertion of a C]O into a [Mg]–H bond to give a
magnesium alkoxide [Mg]–OCH2R.3b
Introduction
Catalytic hydroelementation reactions, such as hydrosilylation
and hydroboration, are key transformations in commodity and
ne chemical production, materials synthesis, and prepara-
tions of complex molecules. With substrates such as esters and
amides, efficient catalytic reduction processes that avoid
traditional aggressive, functional-group intolerant reagents
such as LiAlH4 are highly desirable for the construction of
complex molecules and in the transformation of bio-derived
oxygenates.1 Furthermore, inexpensive and environmentally
friendly main-group-metal based catalysts provide advantages
compared to precious metals complexes. In this context,
catalytic hydrosilylation of esters and amides using zinc salts
or ligand coordinated zinc complexes have been reported.2
Group 2 (Mg and Ca) complex-catalyzed hydrosilylation and
hydroboration of ketones, aldehydes, and pyridines (involving
dearomatization) were also recently described,3 as well as a
Thus, a fundamental challenge associated with conversion
of oxygenates using highly oxophilic early metal catalysts
involves breaking the M–OR bond to complete the catalytic
cycle. Notably, the calcium-catalyzed hydrosilylation shows an
interesting feature that stoichiometric reactions of [Ca]–OR
compounds and hydrosilanes do not provide catalytic products,
and an ionic mechanism was suggested.3a Alternatively, the
proposed mechanism for aldehyde hydroboration invokes a
pathway related to one suggested for rare earth and d0 transi-
tion metal-mediated hydroborations of alkenes and alkynes.3b,12
The key B–C bond forming step in that pathway is proposed to
involve a s-bond metathesis reaction between metal alkyl and
borane.5e Surprisingly, direct experimental support, such as
kinetic studies, for such a s-bond metathesis mechanism for
either olen or carbonyl hydroborations has not been provided.
Here, we offer unambiguous evidence, in the form of catalytic
rate law determinations, that rules out the s-bond metathesis
mechanism in a selective magnesium-catalyzed cleavage and
hydroboration of esters. Instead, we propose that alkoxide
shuttles between magnesium and boron centers for product
formation through a zwitterionic mechanism. This mechanistic
understanding is critically important for the design of new
selective catalytic sites and for the discovery of useful trans-
formations and reaction pathways.
KOH-catalyzed hydrosilylation.4
A
number of reports
describing rst-row transition-metal-catalyzed ester hydro-
silylation including Ti,5 V,6 Mn,7 and Fe,8 as well as the organic
Lewis acid B(C6F5)3, further highlight the important of selec-
tive ester conversions.9 In contrast to the more extensive ester
hydrosilylation literature, few examples of ester hydroboration
are reported.3c,10 An ester was cleaved and reduced in one
example,3c and this caught our attention because ester reduc-
tion is thermodynamically more challenging than aldehyde
Results and discussion
Department of Chemistry, Iowa State University, 1605 Gilman Hall, Ames IA 50011,
USA. E-mail: sadow@iastate.edu
Esters and two equiv. of pinacolborane (HBpin) are readily
converted to alkoxyboronic acid pinacol esters (ROBpin) in the
presence of 0.5–5 mol% ToMMgMe (ToM ¼ tris(4,4-dimethyl-2-
oxazolinyl)phenylborate) in benzene or toluene at ambient
temperature in micromolar-scale experiments (eqn (1)).
† Electronic supplementary information (ESI) available: A general experimental
description, the preparation and characterization of other ToMMg compounds,
kinetic data, rate law derivation and a crystallographic le (cif). CCDC 955601
and 955602. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c3sc52793j
This journal is © The Royal Society of Chemistry 2014
Chem. Sci., 2014, 5, 959–964 | 959

View Article Online
Chemical Science
Edge Article
thiophenes, cyanoarenes and conjugated double bonds are
compatible with the reaction. Thus, this catalytic system shows
selectivity for the ester group over 1,4-addition, in contrast to
the (uncatalyzed) reaction of ester-substituted olens and
diborane.13 The untouched cyclopropyl functionality supports a
2-electron pathway rather than 1-electron mechanism. The
advantage of a magnesium-based catalyst is evident in the sys-
tem's tolerance of a thiophene-containing substrate. Further-
more, the nitro group is unaffected, in contrast to
stoichiometric LiAlH4 reductions.14 In the absence of
ToMMgMe, mixtures of EtOAc and HBin in benzene-d6 do not
provide any conversion even aer heating at 333 K for 24 h. The
lack of background reaction contrasts the interaction of alde-
hydes and HBpin, which occurs slowly without a catalyst.3b
Lactones, including rac-lactide, are readily converted to
dialkoxyboronic acid pinacol esters. For comparison, the tita-
nium-catalyzed hydrosilylation of lactones provides lactols
instead of diols.5e In the current system, polylactide is depoly-
merized to the same 1,2-di(boronate ester) obtained from
rac-lactide, providing a facile catalytic route for a controlled
polymer degradation (Scheme 1).
(1)
The conversions are complete within 15 min (5 mol%) and
0.5–2 h (0.5 mol%) for all esters tested. Molar-scale conversions
are efficient down to 0.5 mol% catalyst loading with respect to
the ester substrate at r.t. (Chart 1). Esters containing a range of
functional groups including alkyl halides, nitroarenes,
The initiation pathway and the speciation of the active
catalyst were probed through separate reactions of the magne-
sium methyl catalyst precursor and the two substrates.
ToMMgMe and one equiv. of HBpin react rapidly in benzene-d6
or toluene-d8 to generate methylboronic acid pinacol ester
(MeBpin) and a black precipitate resulting from decomposition
of the unstable magnesium hydride.15 In contrast, the isolable
magnesium product ToMMgH2Bpin is obtained from slow
addition of a benzene solution of ToMMgMe to 15 equiv. of
HBpin at ambient temperature (eqn (2)).
(2)
1
The oxazoline groups are equivalent in the H and 13C{1H}
1
NMR spectra of ToMMgH2Bpin. In addition, the H NMR spec-
trum (toluene-d8) contained a broad quartet at 4.19 ppm (2H,
1JBH ¼ 93.5 Hz) assigned to ToMMgH2Bpin. A broad singlet at
1.42 ppm (12H, 23 Hz at half-height at 600 MHz) was assigned
to the pinacolate moiety, and this signal sharpened below 263 K
(1.5 Hz at half-height) and remained a singlet even at 193 K.
Likewise, the oxazoline groups are equivalent at 193 K. In the
11B NMR spectrum, a triplet resonance at 3.44 ppm (1JBH ¼ 93.5
Hz) and a singlet at ꢀ18.4 ppm were assigned to the H2Bpin and
ToM groups, respectively. The solid-state IR spectrum showed a
Chart 1 Alkoxylboronic acid pinacol ester products from catalytic
hydroboration. Reaction conditions: RCO2CH2R0 and 2 equiv. of HBpin
in the presence of 0.5–5 mol% ToMMgMe at r.t. Isolated yields of the Scheme 1 Depolymerization of poly(lactic acid) and ring-opening of
corresponding alcohols are given in parenthesis.
lactide by ester cleavage and hydroboration.
960 | Chem. Sci., 2014, 5, 959–964
This journal is © The Royal Society of Chemistry 2014

View Article Online
Edge Article
Chemical Science
single broad peak at 2309 cmꢀ1 corresponding to a nBH stretch
and a single nCN at 1581 cmꢀ1 from the oxazoline groups.
A single crystal X-ray diffraction study revealed that the
pinacolborate unit is coordinated to the Mg center through one
of the two oxygen atoms, and the two hydrogens are bonded to
boron (Fig. 1). Although neither of the two H2B hydrides is
directly pointing towards the Mg atom, the Mg–H distances are
Scheme 2 Pathway for ToMMgOEt formation.
˚
˚
inequivalent (Mg1–H1d, 2.18(3) A and Mg1–H1e, 3.00(3) A).
Likely, the [H2Bpin]ꢀ structure is maintained in solution, rather
than ToMMgH and HBpin dissociating, because ToMMgH2Bpin
is persistent in solution. The equivalence of oxazoline groups
(pseudo-C3v) and the pinacol methyls suggests a rapid exchange
of the H2Bpin coordinating moieties.
1629 and 1594 cmꢀ1, corresponding to non-coordinated and
coordinated oxazoline.
A single crystal X-ray diffraction study reveals that the pre-
cipitate's structure is the bridging ethoxide dimer [(k2-ToM)-
Mg(m-OEt)]2 (Fig. 2). The Mg–O distances of 1.953(1) and
2
M
Alternatively, the reaction of ethyl acetate and ToMMgMe
quantitatively produces ToMMgOEt and acetone within 5 min. A
likely sequence based on elementary organometallic steps
involves insertion of the carbonyl into the Mg–Me bond to give
an b-ethoxy alkoxide, followed by b-ethoxide elimination
(Scheme 2). This transformation could be a model for the
reduction and C–O bond cleavage steps during catalytic
hydroboration, where Me acts as a surrogate for H. Surprisingly,
the kinetic studies of the catalytic reaction presented below rule
out this step as part of the catalysis.
˚
1.957(4) A in [(k -To )Mg(m-OEt)]2 are nearly identical, con-
trasting the non-equivalent distances in [ToMMg(m-OMe)]2
15b
˚
(1.965(1) and 2.037(1) A).
ToMMgOEt is catalytically competent for ester hydro-
boration. Furthermore, treatment of ToMMgOEt with excess
HBpin provides ToMMgH2Bpin and EtOBpin (eqn (3)) as a
possible route to the product of catalysis. Importantly, this step
is also ruled out by catalytic kinetic measurements (see below).
(3)
ToMMgOEt is independently prepared by the reaction of
ToMMgMe and EtOH. The spectroscopic data for this species in
benzene-d6 were acquired immediately aer its formation at
room temperature because a material precipitated within 1 h as
X-ray quality crystals. That precipitate dissolves and remains
soluble in methylene chloride-d2 to give identical spectral data
as in situ generated ToMMgOEt. The 1H NMR spectra of
ToMMgOEt, acquired in methylene chloride-d2 at temperatures
from 296 to 195 K, contained one set of oxazoline resonances
and were consistent with a C3v-symmetric, monomeric solution
structure. This assignment is further supported by the solution
IR spectrum, acquired in methylene chloride, which contained
only one nCN band at 1598 cmꢀ1. However, this geometry is not
maintained in the solid state; an IR spectrum of the crystals
(KBr) precipitated from benzene contained two nCN bands at
Direct competition between HBpin and EtOAc in a 3 : 1 ratio
for reaction with ToMMgMe (5 mol%) gives MeBpin. This
species was also detected in all other catalytic reactions per-
formed at room temperature. Similarly, reactions of HBpin,
EtOAc, and ToMMgMe at 193 K in toluene-d8 give MeBpin as the
exclusive methyl-containing product (eqn (4)). At this low
temperature, ester hydroboration catalysis is slow.
(4)
Apparently, the second-order interaction of ToMMgMe and
HBpin is more than 20ꢁ faster than the reaction of ToMMgMe
and EtOAc. Thus, methyl group transfer to give MeBpin and
ToMMgH2Bpin is responsible for the rst step of catalyst
Fig. 1 Pov-ray rendered thermal ellipsoid plot of ToMMgH2Bpin (35%
probability). Only the H atoms bonded to the B2 atom, located Fig.
2
Pov-ray rendered thermal ellipsoid plot of [(k2-ToM)Mg-
objectively on the difference Fourier map and refined isotropically, (m-OEt)]2. Ellipsoids are plotted at 35% probability. H atoms are not
were included.
included.
This journal is © The Royal Society of Chemistry 2014
Chem. Sci., 2014, 5, 959–964 | 961

View Article Online
Chemical Science
Edge Article
initiation. Based on the above observations, we initially experiments. Thus, the treatment of
a
1 : 1 mixture of
hypothesized that B–O bond formation could result from the PhCO2CH2Ph and CyCO2CH2Cy with ToMMgMe at 353 K for
interaction of ToMMgOR and HBpin involving a concerted four- 2 h provides a mixture of esters PhCO2CH2Ph (19.4%),
centered electrocyclic transition state as proposed for magne- CyCO2CH2Cy (28.8%), PhCO2CH2Cy (25.9%), and CyCO2CH2Ph
sium-catalyzed aldehyde hydroboration.3b Related reactions of (25.9%) resulting from exchange (eqn (6)). Note that the higher
titanium, zinc, gold, and copper alkoxides with organosilanes, temperature given here, in comparison to hydroboration catal-
as part of catalytic silylations, are also proposed to involve ysis, is needed simply to accelerate the approach to equilibrium.
concerted M–O/Si–H cleavage and Si–O/M–H bond formation In the reaction of eqn (6), the crossover products are detected at
through four-center electrocyclic transition states.16–20
room temperature upon mixing the esters with ToMMgMe.
Thus, kinetic studies of the catalytic conversion were
pursued. Within the constraints imposed by the insertion/s-
bond metathesis pathway, either B–O bond formation or
insertion would be turnover-limiting. The former possibility
would be supported by rst-order [HBpin], as suggested by the
stoichiometric experiments. Alternatively, EtOAc insertion
could be turnover-limiting, and rst-order [EtOAc] dependence
would be observed. A third possibility is that one of the two
steps is reversible and the other is turnover-limiting, and then
both reagents would appear in the rate law.
(6)
In a related observation, the reaction of ToMMgOEt and
CyCO2CH2Cy produces a mixture of the starting materials,
ToMMgOCH2Cy and CyCO2Et in a ratio of ꢄ22 : 27 : 25 : 26 aer
6 h in benzene-d6 at 333 K.
The results are surprising and dramatic: half-order [EtOAc]
and zero-order [HBpin] dependencies are observed. Plots of
[EtOAc]1/2 vs. time are linear for >3 half-lives ([EtOAc]ini ¼ 0.30
M, [HBpin]ini ¼ 0.85 M; [ToMMgMe] ¼ 25.2 mM; toluene-d8, 287
K). Zero-, rst- and second-order plots detailing EtOAc conver-
sion are not linear. To determine the order in [HBpin], its
concentration was varied from a slight excess (0.85 M) to
pseudo-rst order conditions (2.4 M). The slope of [EtOAc]1/2 vs.
time curves provides identical kobs values for all [HBpin],
demonstrating zero-order dependence. The observed rate
Then, the speciation of the ToMMg-based catalyst during
catalytic conversion was assessed. A catalytic hydroboration
reaction mixture ([Mg] ¼ 54 mM, [EtOAc]ini ¼ 0.58 M, [HBpin]ini
¼ 1.18 M) was prepared at 193 K and slowly warmed up to 273 K
inside the NMR probe to identify the ToMMg-containing species
present during catalysis. Surprisingly, H and 11B NMR signals
1
for ToMMgOEt or ToMMgH2Bpin were not detected at the
expected positions. Instead, new ToMMg–, –OEt, –BH, and
–Bpin groups were identied along with the substrate starting
materials and catalytic products. Only one set of ToM reso-
nances was observed (1.14 ppm, s, 18H, Me; 3.39 ppm, s, 6H,
CH2). Two new 1H NMR singlet resonances at 1.33 and 1.36 ppm
(6H each vs. ToM) were assigned to inequivalent methyl groups
in the BO2C2Me4 moiety, and new triplet and quartet reso-
nances at 1.46 (3H) and 3.80 (2H) ppm, respectively, were
assigned to an OEt group. A H–B unit is also present, as a 1H–11B
constant varies linearly with [ToMMgMe]. The plot of kobs vs.
(EtOAc)
[ToMMgMe] provides k0
¼ 0.0181(2) Mꢀ1/2 sꢀ1 (at 287 K) for
the experimental rate equation (eqn (5)).
ꢀd½EtOAcꢂ
1
1=2
0
¼ k0½Mgꢂ ½EtOAcꢂ ½HBpinꢂ
(5)
dt
Several mechanistic possibilities are ruled out by this rate
law. First, the reaction of a catalytic magnesium species and
ester (i.e., ToMMgH and EtOAc) is not turnover-limiting because
that pathway requires an overall second-order rate law (rate f
[Mg]1[EtOAc]1). Second, the interaction of a catalytic species and
HBpin is not turnover-limiting, because that would also give a
second-order rate law (rate f [Mg]1[HBpin]1). Thus, neither step
of the s-bond metathesis mechanism is turnover-limiting, and
the empirical rate law of eqn (5) unambiguously rules out that
insertion/s-bond metathesis pathway.
1
HMQC experiment contained a crosspeak between a H NMR
signal at 4.13 ppm and a new broad 11B NMR resonance at 7.2
ppm. From this data, we assign the species observed in the
catalytic reaction mixture as ToMMg{EtO(H)Bpin}. Attempts to
isolate and further characterize this compound by the reaction
of ToMMgOEt and HBpin were unsuccessful, however, it likely
contains bridging Mg–O–B interactions based on the structures
of ToMMgH2Bpin and {DPI}Mg{(RO)2Bpin}.3b ToMMg{EtO(H)-
Bpin} is proposed as the catalytic resting state because it is the
only magnesium-containing substance detected in the reaction
mixture and its reaction with EtOAc to provide product explains
the zero-order [HBpin] kinetic dependence. That is, the hydride
reducing equivalent is already present in the catalyst resting state.
Furthermore, the catalytic reaction proceeds readily in
methylene chloride. In fact, catalytic conversions of EtOAc in
methylene chloride-d2 are qualitatively faster than in toluene-d8
(approximately twice as fast). ToMMgH rapidly decomposes in
benzene;15 even the isolable and stable ToMZnH, which is
persistent in benzene-d6 solution, is converted to ToMZnCl in
methylene chloride over a few days. In contrast, ToMMgH2Bpin
Half-order [EtOAc] dependence indicates a reversible inter-
action of catalyst and ester to give ester cleavage prior to the
turnover-limiting step. The thermodynamics associated with
ester decomposition to two equivalents of aldehyde were
therefore considered; however we are not aware of reports of DG
values for the decomposition reaction of EtOAc into 2 equiv. of
MeCHO.21 The DHꢃ for the conversion of EtOAc into two equiv of
MeCHO is estimated to be 21 kcal molꢀ1 from DHform values.11
Thus, even a catalytic process is likely to produce only small
quantities of aldehyde, at best.
The ability of ToMMgX species to break the C–O bond in
esters is further supported by cross-ester metathesis
962 | Chem. Sci., 2014, 5, 959–964
This journal is © The Royal Society of Chemistry 2014

View Article Online
Edge Article
Chemical Science
electrophilic HB(C6F5)2 gives the zwitterionic hydridoborate
Cp2ZrCHRCH2BH(C6F5)2 through apparent abstraction, rather
than the B–H addition product.24 Admittedly, HB(C6F5)2 and
HBpin are electronically dissimilar.
Comparisons between carbonyl hydroborations and hydro-
silylation (i.e., Si–O bond formation) are also revealing. Only
starting materials are observed in mixtures of ToMMgOEt and
organosilanes. As noted above, s-bond metathesis for Si–O
bond formation is also ruled out in a calcium-catalyzed hydro-
silylation of ketones by stoichiometric experiments,3a and a
copper-catalyzed ketone hydrosilylation is also suggested to
involve a copper–silane complex as the reductant even though
copper alkoxides and silanes afford silyl ethers.25 Similarly, the
s-bond metathesis mechanism was ruled out in Re26 and Mo27
catalytic hydrosilylations.
Scheme 3 Proposed turnover sequence and catalytic cycle for ester
The electronegativity differences for Si–H and B–H
bonds, in fact, suggests that the B–H bond is less polar than
the Si–H bond.28 This suggested polarity is counter to expec-
tations based on the observed reactivity of hydrosilanes and
hydroboranes with high-valent metal alkoxides and further
suggests that the Lewis acidity of the borane signicantly
contributes to the B–O bond forming pathway. In contrast to
the inert nature of ToMMgOEt in the presence of hydrosilanes,
ToMMgNHR and silanes react readily to give Si–N bond
formation, although the pathway involves more polar inter-
mediate species than is typically associated with s-bond
metathesis.15,22
In fact, the present studies do not rule out s-bond
metathesis for the interaction of ToMMgOR and HBpin in
general; we only show that the concerted mechanism is not
likely for B–O bond formation in the catalytic transformation.
Despite this, the discovery of this alternative, non-s-bond-
metathesis pathway for hydroboration highlights other bond-
making and bond-breaking pathways available in main group
systems.
cleavage and hydroboration from the catalyst resting state to product.
is readily characterized by NMR spectroscopy in methylene
chloride-d2.
Thus, the classical s-bond metathesis-type reaction of
ToMMgOEt and HBpin to give ToMMgH and EtOBpin is not
occurring in the catalytic reaction. Furthermore, rate f [Mg]-
[EtOAc]1/2 implies that the resting state ToMMg{EtO(H)Bpin}
does not directly give the EtOBpin product and ToMMgH
because that would give zero-order dependence on ester
concentration. Instead, the catalyst resting state interacts
reversibly with either EtOAc or with MeCHO formed via
reversible EtOAc cleavage. This reversible cleavage and revers-
ible catalyst/0.5(EtOAc) interaction is required by half-order
dependence on [EtOAc] in the catalytic rate law. Product
formation occurs by a unimolecular conversion of a species that
contains Bpin, OCH2R, and ToMMg moieties. A sequence from
the catalyst resting state to product that is consistent with the
catalytic rate law and the observed catalyst speciation is shown
in Scheme 3.
The connectivity of the unobserved intermediate ToMMg
{(RCH2O)2Bpin} is not established, although the related {DPI}
Mg{k2-(RCH2O)2Bpin} was crystallographically characterized,3b
Crystal structure determination of
ToMMgH2Bpin
as was ToMMgH2Bpin. In addition, the 0.5 equiv. of ester is Crystal data
likely provided by a fast reversible cleavage catalyzed by a
C27H43B2N3O5Mg, M ¼ 535.57, monoclinic, a ¼ 9.290(3), b ¼
ToMMg-based complex, as supported by the ester metathesis
studies above.
3
˚
˚
24.265(8), c ¼ 13.856(5) A, V ¼ 2999(2) A , T ¼ 173 K, space
group P21/c, Z ¼ 4, 24 456 reections measured, 5418 unique
(Rint ¼ 0.0901), R ¼ 0.0538, wR2 ¼ 0.1117 [I > 2s(I)], R ¼ 0.1058,
wR2 ¼ 0.1361 (all data).
Conclusions
Our conclusions, based on the kinetic experiments, contrast the
proposed mechanism for early-transition metal and rare earth Crystal structure determination of
element-catalyzed hydroboration of alkenes and alkynes.12
These mechanisms invoke the concerted, 4-center electrocyclic
transition state associated with s-bond metathesis.22 However,
[ToMMgOEt]2
Crystal data
a computational study of Cp2SmH-catalyzed hydroboration of
C
52H74B2N6O8Mg2, M ¼ 981.41, monoclinic, a ¼ 10.4348(11), b
3
˚
˚
ethylene with the model borane HB(OH)2 nds that the ¼ 19.441(2), c ¼ 14.2620(15) A, V ¼ 2849.6(5) A , T ¼ 173 K,
stationary state Cp2Sm{k2-H(OH)B(C2H5)OH} resulted from the space group P21/c, Z ¼ 2, 22 855 reections measured, 5030
addition of Cp2SmC2H5 and HB(OH)2.23 In addition, the reac- unique (Rint ¼ 0.0454), R ¼ 0.0452, wR2 ¼ 0.1178 [I > 2s(I)], R ¼
tion of Cp2Zr(h2-C2H3R)(PMePh2) and 2 equiv. of the highly 0.0643, wR2 ¼ 0.1263 (all data).
This journal is © The Royal Society of Chemistry 2014
Chem. Sci., 2014, 5, 959–964 | 963

View Article Online
Chemical Science
Edge Article
10 A. Y. Khalimon, P. Farha, L. G. Kuzmina and G. I. Nikonov,
Chem. Commun., 2012, 48, 455–457.
Acknowledgements
11 For the reaction EtOAc(l) ! 2 MeCHO(l), DHrxn ¼ 21 kcal
The authors gratefully thank the National Science Foundation
(CHE-0955635 and MRI-1040098) for nancial support. We also
thank Prof. George Kraus for generously providing many of the
substrates used in this study.
molꢀ1
. See: (a) K. B. Wiberg, L. S. Crocker and
K. M. Morgan, J. Am. Chem. Soc., 1991, 113, 3447–3450; (b)
K. B. Wiberg and R. F. Waldron, J. Am. Chem. Soc., 1991,
113, 7697–7705.
12 (a) K. N. Harrison and T. J. Marks, J. Am. Chem. Soc., 1992,
114, 9220–9221; (b) S. Pereira and M. Srebnik,
Organometallics, 1995, 14, 3127–3128; (c) X. He and
J. F. Hartwig, J. Am. Chem. Soc., 1996, 118, 1696–1702.
Notes and references
1 (a) J. March, Advanced organic chemistry: reactions,
mechanisms, and structure, Wiley, New York, 4th edn, 1992;
(b) P. G. Andersson and I. J. Munslow, Modern reduction 13 H. C. Brown and K. A. Keblys, J. Am. Chem. Soc., 1964, 86,
methods, Wiley, New York, 2008; (c) J. Seyden-Penne, 1795–1801.
Reductions by the Alumino and Borohydrides in Organic 14 (a) R. F. Nystrom and W. G. Brown, J. Am. Chem. Soc., 1947,
Synthesis, Wiley, New York, 2nd edn, 1997.
2 (a) H. Mimoun, J. Org. Chem., 1999, 64, 2582–2589; (b) S. Das,
69, 1197–1199; (b) R. F. Nystrom and W. G. Brown, J. Am.
Chem. Soc., 1948, 70, 3738–3740.
¨
K. Moller, K. Junge and M. Beller, Chem.–Eur. J., 2011, 17, 15 (a) J. F. Dunne, S. R. Neal, J. Engelkemier, A. Ellern and
7414–7417; (c) S. Das, D. Addis, S. Zhou, K. Junge and
M. Beller, J. Am. Chem. Soc., 2010, 132, 1770–1771.
3 (a) J. Spielmann and S. Harder, Eur. J. Inorg. Chem., 2008,
1480–1486; (b) M. Arrowsmith, T. J. Hadlington, M. S. Hill
A. D. Sadow, J. Am. Chem. Soc., 2011, 133, 16782–16785; (b)
D. Mukherjee, N. L. Lampland, K. Yan, J. F. Dunne,
A. Ellern and A. D. Sadow, Chem. Commun., 2013, 49,
4334–4336.
¨
and G. Kociok-Kohn, Chem. Commun., 2012, 48, 4567–4569; 16 H. Mimoun, J. Y. de Saint Laumer, L. Giannini, R. Scopelliti
(c) M. Arrowsmith, M. S. Hill, T. Hadlington, G. Kociok- and C. Floriani, J. Am. Chem. Soc., 1999, 121, 6158–6166.
Kohn and C. Weetman, Organometallics, 2011, 30, 5556–5559. 17 J. Yun and S. L. Buchwald, J. Am. Chem. Soc., 1999, 121, 5640–
¨
´
4 J. A. Fernandez-Salas, S. Manzini and S. P. Nolan, Chem.
5644.
Commun., 2013, 49, 9758–9760.
5 (a) S. C. Berk, K. A. Kreutzer and S. L. Buchwald, J. Am. Chem.
18 J. Gajewy, J. Gawronski and M. Kwit, Eur. J. Org. Chem., 2013,
307–318.
Soc., 1991, 113, 5093–5095; (b) S. C. Berk and S. L. Buchwald, 19 H. Ito, A. Watanabe and M. Sawamura, Org. Lett., 2005, 7,
J. Org. Chem., 1992, 57, 3751–3753; (c) K. J. Barr, S. C. Berk 1869–1871.
and S. L. Buchwald, J. Org. Chem., 1994, 59, 4323–4326; (d) 20 H. Ito, K. Takagi, T. Miyahara and M. Sawamura, Org. Lett.,
M. T. Reding and S. L. Buchwald, J. Org. Chem., 1995, 60, 2005, 7, 3001–3004.
7884–7890; (e) X. Verdaguer, M. C. Hansen, S. C. Berk and 21 DG at 300 K for EtOAc ! 2MeCHO is estimated to be 15 kcal
S. L. Buchwald, J. Org. Chem., 1997, 62, 8522–8528.
molꢀ1; Knovel Critical Tables, 2nd edn.
6 L. Pehlivan, E. Metay, S. Laval, W. Dayoub, D. Delbrayelle, 22 R. Waterman, Organometallics, 2013, DOI: 10.1021/
G. Mignani and M. Lemaire, Eur. J. Org. Chem., 2011, 7400–7406. om400760k.
7 Z. Mao, B. T. Gregg and A. R. Cutler, J. Am. Chem. Soc., 1995, 23 S. A. Kulkarni and N. Koga, J. Mol. Struct.: THEOCHEM, 1999,
117, 10139–10140. 461–462, 297–310.
8 (a) D. Bezier, G. T. Venkana, L. C. Misal Castro, J. Zheng, 24 Y. M. Sun, W. E. Piers and S. J. Rettig, Organometallics, 1996,
T. Roisnel, J.-B. Sortais and C. Darcel, Adv. Synth. Catal., 15, 4110–4112.
2012, 354, 1879–1884; (b) K. Junge, B. Wendt, S. Zhou and 25 B. H. Lipshutz, K. Noson, W. Chrisman and A. Lower, J. Am.
M. Beller, Eur. J. Org. Chem., 2013, 2061–2066; (c) Chem. Soc., 2003, 125, 8779–8789.
A. J. Ruddy, C. M. Kelly, S. M. Crawford, C. A. Wheaton, 26 G. Du, P. E. Fanwick and M. M. Abu-Omar, J. Am. Chem. Soc.,
´
O. L. Sydora, B. L. Small, M. Stradiotto and L. Turculet,
Organometallics, 2013, 32, 5581–5588.
9 D. J. Parks and W. E. Piers, J. Am. Chem. Soc., 1996, 118, 9440–
9441.
2007, 129, 5180–5187.
27 O. G. Shirobokov, L. G. Kuzmina and G. I. Nikonov, J. Am.
Chem. Soc., 2011, 133, 6487–6489.
28 L. C. Allen, J. Am. Chem. Soc., 1989, 111, 9003–9014.
964 | Chem. Sci., 2014, 5, 959–964
This journal is © The Royal Society of Chemistry 2014