Iron-Catalyzed Hydroboration: Unlocking Reactivity through Ligand Modulation

Article abstract of DOI:10.1002/chem.201602818

Iron-catalyzed hydroboration (HB) of alkenes and alkynes is reported. A simple change in ligand structure leads to an extensive change in catalyst activity. Reactions proceed efficiently over a wide range of challenging substrates including activated, unactivated and sterically encumbered motifs. Conditions are mild and do not require the use of reducing agents or other additives. Large excesses of borating reagent are not required, allowing control of chemo- and regioselectivity in the presence of multiple double bonds. Mechanistic insight reveals that the reaction is likely to proceed via a highly reactive iron hydride intermediate.

Full text of DOI:10.1002/chem.201602818

A Journal of
Accepted Article
Title: Iron catalyzed hydroboration: unlocking reactivity through ligand modulation
Authors: Maialen Espinal-Viguri; Callum Woof; Ruth Leiper Webster
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To be cited as: Chem. Eur. J. 10.1002/chem.201602818
Link to VoR: http://dx.doi.org/10.1002/chem.201602818
Supported by

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COMMUNICATION
Iron catalyzed hydroboration: unlocking reactivity through ligand
modulation
Maialen Espinal-Viguri, Callum R. Woof, Ruth L. Webster*
Abstract: Iron catalyzed hydroboration of alkenes and alkynes is
reported. A simple change in ligand structure leads to a remarkable
and extensive change in catalyst activity. Reactions proceed
efficiently over a wide range of challenging substrates including
activated, unactivated and sterically encumbered motifs. Conditions
are mild and do not require the use of reducing agents or other
additives. Large excesses of borating reagent are not required,
allowing for exquisite control of chemo- and regioselectivity in the
presence of multiple double bonds. Mechanistic insight reveals that
the reaction is likely to proceed via a highly reactive iron hydride
intermediate.
effectiveness which complements the rationale behind base
metal catalysis. We herein report the use of two iron(II) β-
diketiminate complexes for the HB of both simple and more
challenging alkenes along with alkynes (Scheme 1). The catalyst
system described does not need exogenous reductant
(activation is facile) and therefore, with so few species present in
solution, Reaction Progress Kinetic Analysis[10] can also be used
to rapidly gain mechanistic insight.
Hydroboration is the addition of a B–H bond across an
unsaturated moiety and is a valuable transformation in the
synthesis of alkyl borane building blocks for use in organic
synthesis,[1] not least in Suzuki-Miyaura cross-coupling.[2] The
use of pinacol- or catecholborane as a source of B–H dominates
the literature due to the stability of the resulting products. Many
are air stable, can be isolated, purified and stored on the bench
for many months. However, the relative inertness of these
boranes renders them essentially unreactive towards
unsaturated bonds and catalysis is necessary under moderate to
forcing conditions. It is also important to note that historically
precious metals have been used for catalytic HB,[1, 3] but due to
the high cost, toxicity and low earth abundance of these metals,
there has been renewed interest in the discovery of catalysts
containing earth-abundant, non-toxic, first row transition
metals.[4] In the recent literature, elegant examples have been
presented using iron pre-catalysts.[5]
One of the most common methods to undertake HB using iron
catalysis is to use a neutral ligand in the presence of FeX2
(where X = Cl, Br, OAc and the pre-catalyst is prepared in situ or
used as a ligated complex) and a catalytic amount of Grignard
reagent or hydride additive.[6] Previously, we have exploited iron
pre-catalysts that are capable of undergoing redox neutral, σ-
bond metathesis type reactivity,[7] avoiding the need for
activation by a Grignard or other reducing agent and we were
intrigued by their potential to undergo catalytic HB. β-
Diketiminate ligands have shown exceptional reactivity when
used in conjunction with a host of main group elements and
transition metals,[8] but the power of this ligand in iron catalysis is
vastly underexplored.[7b, 9] Benefits of the β-diketiminate ligand in
iron catalysis include the simplicity, modular tunability and cost-
Scheme 1. Fe(II) pre-catalysts used to catalyze the HB of alkenes and
alkynes.
We initiated our studies using pre-catalyst 1a which contains the
classical 2,6-diisopropyl motif.[11] Pleasingly, not only do
traditional HB substrates, such as 1-hexene, 1-octene and allyl
benzene, undergo catalysis at room temperature (forming 2a, 2b
and 2c respectively in 100% spectroscopic yield, Figure 1), but
more complex substrates such as 1,2-epoxy-5-hexene can be
functionalized but not to the detriment of the epoxide (2d).[12]
Likewise, a primary phosphinoalkene (2o) does not undergo
competing
coordination/deactivation
or
intramolecular
hydrofunctionalization, instead HB operates exclusively.
Substrates obtained from renewable resources also undergo
facile HB. For example in the presence of two equivalents of
HBpin isoprene, myrcene and β-farnesene with their multiple
double bonds are selectively hydroborated at the terminal
positions (2g, 2h, 2i). Limonene and valencene only undergo HB
at the exocyclic double bond (2j, 2l). Note the use of one
equivalent of borane per double bond: it is not necessary to
manipulate stoichiometries and use a vast excess of olefin in
order to obtain a favorable result with these complex systems,
indeed the benefit of these substrates lies in the retention of
multiple double bonds in the molecule, making them suitable for
further functionalization. This level of chemoselectivity is rare in
the field of iron catalysis; reactions of complementary selectivity
have only been reported by Ritter and Huang.[5a, e, f] In this latter
report, the authors also clearly demonstrate the comparably
poor selectivity of Wilkinson’s catalyst in diene HB, highlighting
the benefits of iron catalysis.
[a]
Dr. M. Espinal-Viguri, C. R. Woof, Dr. R. L. Webster
Department of Chemistry
University of Bath
Claverton Down, Bath, UK. BA2 7AY.
E-mail: r.l.webster@bath.ac.uk
Supporting information for this article is given via a link at the end of
the document.

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[c] 36 h. [d] 90 °C, 2.5 h. [e] 60 °C, 48 h, mixture of 2,5- & 2,6-HB isomers
(1:1). [f] 10 mol% 1a, 60 °C, 16 h. [g] 24 h. [h] 60 °C, 16 h.
Unfortunately, with our iron catalysis, compounds containing
ketones, for example nootkatone, show competing boration of
the carbonyl and the desired product, 2k, could not be isolated.
The chemistry is also extended to HB using catecholborane (2a’,
2c’, 2e’) where good isolated yield can be achieved using inert
atmosphere handling conditions.
When other more challenging substrates are tackled, higher
temperatures are necessary. Heating styrene to 60 °C is needed
to achieve 100% spectroscopic yield of 2p: the rate of reaction
at room temperature is around 45 times slower than that
obtained for 1-hexene.[13] 4-Ph, 4-OMe and 4-CF3 styrene also
give exclusive formation of the anti-Markovnikov product (2q, 2r,
2s). Again, anti-Markovnikov selectivity is rare irrespective of
choice of metal catalyst,[14] but not least in iron catalysis[5e] where
many examples rely on a blocking group in the α-position[5b, g] to
prevent Markovnikov selectivity,[5h] or only anti-Markovnikov
reactivity with an unsubstituted styrene has been provided.[5d, k]
Encouraged by these result under heating, we moved to explore
the reactivity of α- and β-methyl styrene. Using 1a, β-
methylstyrene isomerizes to give allylbenzene, which then
reacts with HBpin affording 2c.[14b, d, 15] We therefore sought to
develop a new iron β-diketiminate complex that has enhanced
electronic and steric properties which may proffer more
favorable reactivity than the simple 2,6-diisopropyl substituted
system. Inspired by Coates’ use of sec-phenylethyl substituted
diimine pre-catalysts, which showed enhanced tacticity,
regiocontrol
and
reduced
chain-walking
in
olefin
polymerizations,[16] we developed a second generation iron β-
diketiminate pre-catalyst (1b, Scheme 1).[17] When 1b catalyzes
the HB of styrene, only 3 h are necessary to give a comparable
yield of HB product, but remarkably, this subtle change in ligand
structure gives a vast change in regioselectivity: good levels of
Markovnikov selectivity are obtained (3a, Table 1, Entry 1). This
trend is mirrored in the other styrenes tested (Entries 4 to 6).
Importantly, dehydrogenative HB and hydrogenation are not
observed during the HB of styrene. α- and β-methyl styrene
(Entries 2 and 3) can be functionalized in good isolated yield
when the reaction is performed with heating. Internal activated
double bonds also react; cis-stilbene gives 75% yield 3g after 16
h at 60 °C, whereas trans-stilbene yields 75% 3g after only 5 h
at 70 °C (Entries 7 and 8). Diphenylacetylene reacts to give the
Z product,
3h,
exclusively (Entry 9). To our delight double HB of
diphenylacetylene generates the unique 1,1-dipinacolborane
product (4) after heating to 90 °C (Scheme 2).[18]
Figure 1: Substrate scope for the HB of simple alkenes. All reactions
performed using 0.5 mmol alkene (0.25 mmol for dienes) and 0.5 mmol HBpin,
100% spectroscopic yield obtained in all cases except n.r. = desired reaction
was not achieved. [a] Reaction performed with 0.5 mmol HBcat. [b] 60 °C, 7 h.
Scheme 2: Double HB of diphenylacetylene yields the 1,1-diborated product.

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Table 1: Vinyl arene and alkyne substrate
HB, catalyzed by 1b.
scope in iron-catalyzed
Entry
1
Alkene
Product
Temp., time
Isolated yield (%) [a]
3a
60 °C, 3 h
60 °C, 2.5 h
92 (65 M:35 AM)
77 (65 M:35 AM)
3a’ [b]
2
3
3b
90 °C, 3 h
84
3c
60 °C, 3 h
98
48
60 °C, 18 h [c]
4
5
6
3d, R = Ph
3e, R = OMe 60 °C, 4 h
3f, R = CF3
60 °C, 18 h
78 (7 M:3 AM)
81 (3 M:2 AM)
71 (7 M:3 AM)
90 °C, 3 h
7
8
9
3g
70 °C, 5 h
75
75
3g
60 °C, 16 h
3h
90 °C, 6 h
90 °C, 3.5 h
86
98
3h’ [b]
10
3i
3j
90 °C, 2.5 h
90 °C, 3 h
95
11
12
99 (85:15)
3k
90 °C, 3 h
97 (1:1)
13
3l
60 °C, 16 h
90 °C, 16 h
79
84
14 [d]
3m
Conditions: 0.5 mmol alkene, 0.5 mmol HBpin, 5 mol% [Fe], 0.4 mL C6D6. [a] Isolated yield obtained by silica gel chromatography (100% spectroscopic yield
obtained in all cases except Entry 14). M = Markovnikov; AM = anti-Markovnikov. [b] Reaction carried out with 0.5 mmol HBcat. [c] Reaction using 1a (5 mol%),
100%
spectroscopic
yield.
[d]
Not
isolated,
0.25
mmol
benzonitrile.
Other internal alkynes also react to give the Z-alkene product
exclusively, heteroatoms are tolerated and high levels of
regioselectivity are obtained (Table 1, Entries 10 to 13).
Unfortunately, catalytic reactivity is not observed with terminal

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COMMUNICATION
alkynes such as phenylacetylene, but in contrast benzonitrile
reacts with two equivalents of HBpin to give the N,N-diborated
benzylamine (Table 1, Entry 14).
Returning to the aliphatic substrates reported in Figure 1, use of
1b also allows for an improvement in reactivity compared to that
obtained with 1a (Table 2). This enhanced reactivity is acutely
observed for 2,3-butadiene, valencene and β-pinene (Entries 1,
In summary, we have developed a new catalytic system for the
HB of alkenes using HBpin and HBcat that does not need
exogenous reducing agents or activators and can be undertaken
with a strict 1:1 ratio of reagents. The chemistry has been
extended beyond classical substrates and includes natural
products, vinyl arenes and alkynes. RPKA demonstrates that the
reaction is first order in substrates and catalyst, and
3, 4) where the reactions are between five and eight times faster. stoichiometric studies provide evidence for a catalytic cycle that
Remarkably, although α-pinene shows no reactivity with 1a, 80%
conversion is observed after 16 h at 90 °C with 1b, however, this
occurs with concomitant isomerization to form 2n, a reaction
previously only reported by Chirik using cobalt catalysis.[14d]
is likely to proceed via an iron hydride. Reactivity of styrenes
demonstrates that a subtle change in ligand structure can lead
to a vast change in regioselectivity, whilst alkynes show much
improved reactivity with this change in pre-catalyst. Double HB
of diphenylacetylene is possible and gives the geminal
dipinacolborane product.
Table 2: Improved reaction conditions and yields of aliphatic substrates are
also obtained when 1b is employed as a pre-catalyst.
Entry Product Conditions to obtain 100% spectroscopic yield
Acknowledgements
1a (5 mol%)
RT, 36 h
1b (5 mol%)
RT, 7 h
We thank the EPSRC UK National Mass Spectrometry Facility at
Swansea University for MS analysis and the EPSRC for funding
(EP/M019810/1).
1
2
3
4
5
2e
2g
2l
60 °C, 7 h
60 °C, 16 h
60 °C, 16 h
RT, 16 h
60 °C, 2.5 h
60 °C, 2 h
60 °C, 2.5 h
RT, 16 h
Keywords: iron • homogeneous catalysis • hydroboration •
alkenes • alkynes
2n
2p
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determined to be first order in 1a, HBpin and 1-hexene,[13] thus
supporting our postulated mechanism.
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COMMUNICATION
Iron nails it.
Maialen Espinal-Viguri, Callum R. Woof,
Ruth L. Webster*
Iron(II) hydroboration of alkenes and
alkynes has been developed.
Page No. – Page No.
Substrates include renewable sources
and a change in ligand shows a vast
change in regioselectivity. Alkynes
have been functionalized and a
unique example of double
Iron catalyzed hydroboration:
unlocking reactivity through ligand
modulation.
hydroboration to furnish the 1,1-
dipinacolborane product is provided.