Iron-catalyzed, atom-economical, chemo- and regioselective alkene hydroboration with pinacolborane

Article abstract of DOI:10.1002/anie.201210347

Precious iron: A new PNN iron complex has been developed for use in an iron-catalyzed alkene hydroboration reaction under mild conditions. The environmentally friendly and earth-abundant iron catalyst system is superior to precious-metal systems in terms of efficiency and selectivity for α-olefin hydroborations with pinacolborane. Copyright

Full text of DOI:10.1002/anie.201210347

Angewandte
Chemie
DOI: 10.1002/anie.201210347
Organometallic Catalysis
Iron-Catalyzed, Atom-Economical, Chemo- and Regioselective
Alkene Hydroboration with Pinacolborane**
Lei Zhang, Dongjie Peng, Xuebing Leng, and Zheng Huang*
Dedicated to Professor Maurice Brookhart on the occasion of his 70th birthday
Alkylboronic acid derivatives are now widely used as
son complex is extremely sensitive to impurities and the
[
1]
[6l–n]
intermediates in organic synthesis. For example, Suzuki–
reaction should be handled with extra care.
In many cases
3
Miyaura reactions can efficiently couple C(sp ) organoboron
relatively high loadings of expensive Rh and Ir catalysts were
[2]
[6g,l,n]
species with aryl and alkyl halides. An advantage of
necessary for good conversion.
3
alkylboronic acid derivatives over other common C(sp )
The low abundance, high cost, and environmental con-
cerns over precious metals has motivated the investigation of
earth-abundant and inexpensive base-metal alternatives in
organometallic nuclophiles is their unique stability: many of
them can be readily isolated, purified, and even stored in
[
1a]
[8]
air.
There are several methods for the preparation of
recent years. Well-defined iron complexes have received
alkylboronic acid derivatives. One commonly used method
involves the conversion of an alkylhalide into a lithium or
Grignard reagent, followed by the reaction of the organome-
tremendous attention over the past decade in the field of
[
9]
homogeneous catalysis. Recently, Ritter et al. presented an
iminopyridine iron catalyst for selective 1,4-hydroboration of
[
3]
[10]
tallic reagent with boron compounds. However, the syn-
thetic value of this method is limited, owing to poor func-
tional-group compatibility and the formation of waste inor-
ganic salts. Recently, Hartwig et al. have developed a Rh-
catalyzed direct borylation of alkanes with B Pin to form
1,3-dienes. However, to date the hydroboration of widely
available and commercially relevant alkenes using iron
catalyst has remained unknown. Herein, we report the
synthesis of an electron-rich iron pincer complex and its
application to the first example of iron-catalyzed alkene
hydroborations. This system exhibits unprecedented high
efficiency for selective a-olefin hydroborations using pina-
colborane.
2
2
alkylboronate esters, albeit under relatively harsh reaction
[
4]
conditions. More recently, Liu, Marder, Steel, and co-
workers reported a convenient approach for the synthesis of
alkylboronate esters through Cu-catalyzed borylation of alkyl
We commenced our studies by examining the known iron
complexes for catalytic hydroboration of 4-methyl-1-pentene
1a (2 equiv) with pinacolborane (HBPin). The results are
summarized in Table 1. The Ritter complex (iminopyridine)-
FeCl2 3 acts as a catalyst precursor for 1,3-diene hydro-
[
5]
halides. However, the method requires the use of an excess
of B Pin as the borylation reagent, and waste inorganic salts
2
2
were also generated.
Alternatively, alkylboronic acid derivatives can be pre-
[1b,6]
[10]
pared by Rh- or Ir-catalyzed alkene hydroborations.
boration. However, complex 3 (5 mol%), upon addition of
Although dialkylboranes add readily to alkenes, dialkoxybor-
anes react sluggishly with alkenes in the absence of catalyst.
Metal-catalyzed alkene hydroboration is a synthetically
useful method, because the reaction is highly atom econom-
ical and typically occurs under mild conditions. For example,
the Wilkinson catalyst has been extensively used for the
NaBHEt (15 mol%), did not effect a-olefin hydroboration
3
[
11]
(entry 1).
The reaction using FeCl2 alone (5 mol%) or
a combination of FeCl /2,2’-bipyridine (5 mol% each) in the
2
presence of NaBHEt also gave no hydroboration product
3
(entries 2 and 3). Furthermore, iron complex 4, which
contains a bidentate PN ligand (see Table 1), was prepared
(see Supporting Information). The reaction using 4 (5 mol%)
as the precatalyst in THF at 258C gave the anti-Markovnikov
hydroboration product 2a in 5% yield after one hour
(entry 4). Iron complexes of tridentate terpyridine and bis-
(imino)pyridine ligands have been successfully applied to
[
6h–o]
hydroboration of a wide range of alkenes.
Unfortunately,
side reactions such as dehydrogenative borylation and alkene
hydrogenation, can compete with alkene hydrobora-
[
1b,6n,7]
tion.
the hydroboration of vinylarenes, especially with pinacolbor-
ane.
Another drawback is the low regioselectivity in
[
6g,j,k]
[12]
[12,13]
Moreover, the catalytic performance of the Wilkin-
alkene hydrogenation and hydrosilylation.
Encourag-
ingly, the reactions using (terpy)FeCl2 5 (5 mol%) and
iPr
(
PDI)FeCl 6 (5 mol%) afforded 2a in 9 and 61% yields,
2
[
*] L. Zhang, D. Peng, Dr. X. Leng, Prof. Dr. Z. Huang
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
respectively (entries 5 and 6).
The above data suggest that the use of a tridentate ligands
is essential for iron-catalyzed alkene hydroboration. We
envisioned that catalyst development for this transformation
would benefit from the use of a tridentate electron-donating
phosphine ligand. Such a ligand would induce a less electro-
philic iron center and may allow easy access to an olefin-
345 Lingling Road, Shanghai 200032 (China)
E-mail: huangzh@sioc.ac.cn
[
**] We gratefully acknowledge financial support from the National
Natural Sciences Foundation of China (21272255, 21121062), the
“
1000 Youth Talents Plan”, and the Chinese Academy of Sciences.
II
ligated Fe boryl hydride species, which is a potential key
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201210347.
[
14]
intermediate in catalytic alkene hydroboration. To this end,
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[
a]
Table 1: Iron-catalyzed hydroboration of 1a with HBPin.
ment without the precatalyst 7, but with NaBHEt , gave no
3
product 2a (entry 9). The addition of NaBHEt was necessary
3
for catalysis (entry 10).
As a comparison, hydroboration of 1a with the Wilkinson
catalyst, [Rh(PPh ) Cl] (5 mol%), gave 2a in only 68% yield
[
b]
[c]
Entry
Precat.
NaBHEt3
mol%]
t [min]
Yield [%]
3
3
[
[6g,k,16]
after one hour (entry 11).
borylation product (18%) was also detected by H NMR and
Notably, the dehydrogenative
1
1
2
3
4
5
6
7
8
9
3
15
15
15
15
15
15
15
0.75
15
0
60
60
60
60
60
60
60
10
60
60
60
60
<2
<2
<2
5
FeCl2
GC/MS analysis. Finally, a combination of [{Ir(cod)Cl} ]/dppe
2
FeCl /bpy
2
(5 mol% of Ir and dppe; cod = 1,5-cyclooctadiene, dppe =
4
5
6
7
1
,2-bis(diphenylphosphino)ethane) gave 2a in 75% yield
9
[6g]
after one hour (entry 12). These results indicate that the
PNN iron pincer complex is more efficient than the well-
known Rh and Ir catalysts for a-olefin hydroboration with
HBPin.
61
>99
99
0
[d]
7
none
7
10
11
12
0
68
75
To evaluate the scope of the iron-catalyzed alkene
hydroboration, the reactions of a variety of alkenes with
HBPin were carried out using complex 7 as the precatalyst
(Scheme 1). All reactions were selective for anti-Markovni-
kov hydroboration. The alkylboronate ester products are
[
e]
[Rh(PPh ) Cl]
[{Ir(cod)Cl} ]/dppe
0
0
3
3
2
[
(
a] Reaction conditions: HBPin (0.5 mmol) and 1a (1 mmol) in THF
2 mL) at 258C. [b] Unless otherwise specified, 5 mol% of precatalyst
was used. [c] Unless otherwise specified, yields of 2a were determined by
GC analysis using an internal standard. [d] 0.25 mol% of precatalyst was
used. [e] Dehydrogenative borylation product (18%) was also observed.
1
Yield was determined by H NMR spectroscopy using an internal
standard. Bpy=bipyridine, cod=1,5-cyclooctadiene, dppe=1,2-bis(di-
phenylphosphino)ethane.
we sought to develop an electron-rich iron complex with good
thermostability by using a bipyridyl-based phosphine ligand
(
PNN).
Scheme 1. Iron-catalyzed hydroboration of alkenes (1a–p) with HBPin.
Yields shown are of isolated products. Reaction conditions: HBPin
(0.5 mmol) and alkenes 1a–p (1 mmol) in THF (2 mL) at 258C.
[
a] With 7 (0.25 mol%) and NaBHEt (0.75 mol%), 10 min. [b] With 7
3
(
1 mol%) and NaBHEt (3 mol%), 30 min. [c] With 7 (2 mol%) and
3
NaBHEt (6 mol%), 30 min. [d] With 7 (5 mol%) and NaBHEt3
3
(15 mol%), 30 min. Bn=benzyl, Cy=cyclohexyl, Pin=pinacol, Ts=p-
toluenesulfonyl.
Treatment of the Milstein PNN ligand bearing phosphino-
[
15]
tert-butyl groups with FeCl in THF afforded iron pincer
2
complex 7 in 72% yield [Eq. (1)]. The complex was charac-
1
terized by H NMR spectroscopy, elemental analysis, and
moisture- and air-stable, and can be readily purified by flash
single crystal X-ray diffraction. The solid structure of 7
reveals a distorted square pyramidal geometry around the
metal center (see the Supporting Information).
Complex 7 was then tested as the precatalyst for alkene
hydroboration. Gratifyingly, the reaction of 1a with HBPin
catalyzed by 7 (5 mol%) upon addition of NaBHEt3
chromatography. Hydroborations of simple acyclic a-olefins
[
17]
1a–c (2 equiv)
with HBPin occurred within 10 min and
formed products 2a–c in 90–95% yields of isolated products.
The substrates g-phenylpropene (2d, 91%) and cyclohexyle-
thene (2e, 93%) underwent hydroboration in useful yields.
The hydroboration of the sterically demanding tert-butyle-
thene afforded the product 2 f in relatively low yield (62%).
The iron catalytic system is compatible with alkenes bearing
various functional groups. Hydroborations of 1g–k with
HBPin afforded alkylpinacolboronates containing silyl (2g,
93%), ether (2h, 83%), acetal (2i, 82%), tosylate (2j, 82%),
and amine (2k, 96%) functional groups. Furthermore, 1,1-
disubstituted a-olefins, such as a-methylstyrene 1l and 2-
(
15 mol%) in THF at 258C gave the anti-Markovnikov
hydroboration product 2a in quantitative yield in one hour
Table 1, entry 7). The system is so active that the amount of
(
the precatalyst could be reduced to 0.25 mol% and the
reaction still afforded 99% of 2a in 10 min (entry 8). No
branched or dehydrogenative borylation products were
1
detected by GC/MS and H NMR analysis. A control experi-
2
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Chemie
[
a]
methyl-1-pentene 1m, were hydroborated to give 2l and 2m
in 72 and 81% isolated yields, respectively.
Table 2: Iron-catalyzed hydroboration of vinylarenes (8a–d) with HBPin.
Notably, the PNN iron pincer system is inactive for
hydroboration of internal alkenes, such as trans-3-octene and
cyclooctene. The sensitivity of this catalytic system to steric
effects enables the chemoselective hydroboration of a termi-
nal alkene within a molecule containing multiple C=C bonds.
[
b]
[b]
[b]
9
/10/11
9/10/11
9/10/11
Entry
(
no MeCN) (20 mol% MeCN) (40 mol% MeCN)
For example, the hydroboration of the terminal olefin
proceeded without hydroboration of the internal olefin in 4-
vinyl-cyclohexene 1n, to produce 95% yield of isolated 2n.
The selective hydroboration of 1,1-disubstituted olefin in (Æ)-
limonene afforded 76% yield of isolated 2o. Moreover, the
reaction of 1,4-hexadiene 1p (trans/cis = 12:1) gave 90%
yield of isolated 2p (trans/cis = 12:1) without hydroboration
or isomerization of the disubstituted olefin.
1
2
3
4
H, 8a
51:45:20
52:41:21
51:43:29
39:55:17
86:–:–
95:–:–
80:5:–
82:10:–
78:–:–
83:–:–
88:–:–
81:–:–
Me, 8b
MeO, 8c
F, 8d
[a] Reaction conditions: HBPin (0.5 mmol) and 8a–8d (1 mmol) in
toluene (2 mL) with complex 7 (2 mol%) and NaBHEt (6 mol%) at 258C
for 30 min. [b] Data shown are yields [%] relative to HBPin, and were
determined by H NMR spectroscopy using an internal standard.
3
1
The iron-catalyzed chemoselective hydroboration of an a-
olefin relative to 1,2-disubstituted olefin is remarkable. As
a comparison, [Rh(PPh ) Cl]-catalyzed (5 mol%) hydrobo-
ration of 1n afforded six hydroboration products after one
hour, including 2n (51%) and the products arising from
internal olefin hydroboration (14% total by-product yield;
see the Supporting Information for product distributions).
Furthermore, hydroboration of 1,4-hexadiene 1p (trans/cis =
trile (20 mol%), reactions of styrene and 4-methylstyrene
with HBPin in toluene exclusively formed linear boronate
esters 9a (86%) and 9b (95%) after 30 min (Table 2, entries 1
and 2), whereas reactions of 4-methoxylstyrene and 4-
fluorostyrene gave linear boronate esters as the major
products (80 and 82%, respectively) with the formation of
a small amount of vinylboronate esters (5 and 10%). To our
delight, doubling the amount of acetonitrile (40 mol%
relative to HBPin) resulted in the exclusive formation of
the addition products 9c (88%) and 9d (81%; Table 2,
entries 3 and 4).
3
3
1
2:1) catalyzed by [Rh(PPh ) Cl] (5 mol%) gave trans-2p in
3 3
only 5% yield after one hour (10% conversion of HBPin),
together with multiple other hydroboration products. After
1
5 h, at least six hydroboration products, including trans-2p
(
51%), were detected by GC/MS (see the Supporting
On the basis of our preliminary data and precedent
regarding related iron-catalyzed alkene addition processes,
we propose the mechanism of iron-catalyzed alkene hydro-
borations shown in Scheme 2. Following the coordination of
Information). We attribute the poor chemoselectivity of the
Rh-catalyzed alkene hydroboration to facile internal olefin
hydroboration and olefin isomerization.
In contrast to the exclusive formation of hydroboration
products in the reactions of alkene substrates 1a–p, iron-
catalyzed hydroboration of styrene 8a (2 equiv) with HBPin
in THF not only formed the linear hydroboration product 9a
(
1
34% yield), but also the dehydrogenative borylation product
0a in 65% yield after 30 min. Ethylbenzene 11a (21% yield
relative to HBPin) formed by styrene hydrogenation was also
obtained. It should be noted that dehydrogenative boration is
a common side reaction accompanying metal-catalyzed vinyl-
[
6k,7a,18]
arene hydroboration with HBPin.
The selectivity of iron-catalyzed styrene hydroboraion
appeared to be solvent sensitive. With toluene as the solvent,
the yield of the hydroboration product 9a (51%) was
somewhat improved versus the reaction in THF (34%), but
the reaction still gave a moderate amount of 10a (45%) and
1
1a (20%) (Table 2). Similarly, reactions of 4-methylstyrene
Scheme 2. Proposed mechanism for the iron-catalyzed alkene hydro-
boration and dehydrogenative borylation reactions.
(8b), 4-methoxystyrene (8c), and 4-flourostyrene (8d) with
HBPin in toluene gave a mixture of linear hydroboration
products 9b–d, dehydrogenative borylation products 10b–d,
and hydrogenation products 11b–d (Table 2). No side product
resulting from dehydrogenative borylation was detected when
using acetonitrile as the solvent, although the reaction of
styrene with HBPin gave the hydroboration product 9a in
alkene to the iron center, oxidative addition of HBPin to A
forms the formal 18 electron Fe boryl hydride species B,
II
which is similar to those believed to be involved in alkene
hydrogenation and hydrosilylation, and in 1,3-diene hydro-
[
19]
[14]
only 5% yield after 30 min.
boration and hydrosilylation catalyzed by iron complexes.
We speculated that acetonitrile plays a role in inhibiting
both the alkene hydroboration and dehydrogenative boryla-
tion processes, but the suppressing effect of acetonitrile on the
latter is more significant. Indeed, in the presence of acetoni-
Terminal alkenes, except for vinylarenes, undergo selective
1,2-insertion into the FeÀH bond to form linear alkyl complex
[
6l,o]
C.
Reductive elimination then generates the anti-Markov-
nikov product.
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.
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Communications
For vinylarenes without an a substituent, however, dehy-
drogenative borylation presumably proceeds by insertion of
vinylarene into the FeÀB bond rather than the FeÀH bond in
[6] a) N. Miyaura in Catalytic Heterofunctionalization (Eds: A.
Togni, H. Grꢀzmacher), Wiley-VCH, Weinheim, 2001, pp. 1 – 46;
b) C. M. Vogels, S. W. Westcott, Curr. Org. Chem. 2005, 9, 687;
c) I. Beletskaya, A. Pelter, Tetrahedron 1997, 53, 4957; d) K.
Burgess, M. J. Ohlmeyer, Chem. Rev. 1991, 91, 1179; e) D. R.
Edwards, Y. B. Hleba, C. J. Lata, L. A. Calhoun, C. M. Crudden,
Angew. Chem. 2007, 119, 7945; Angew. Chem. Int. Ed. 2007, 46,
[
6n,7b]
3
complex B;
this may be facilitated by the formation of h -
II
[20]
benzyl Fe species D (Scheme 2). The 18 electron complex
3
1
1
D then undergoes an h –h rearrangement to form h -benzyl
Fe complex E, in which b-hydride elimination occurs to
generate the vinylboronate ester. Both h -benzyl and h -
benzyl Fe complexes
et al. have demonstrated that the 18 electron Fe h -benzyl
II
7799; f) C. M. Crudden, Y. B. Hleba, A. C. Chen, J. Am. Chem.
3
[21]
1
Soc. 2004, 126, 9200; g) Y. Yamamoto, R. Fujikawa, T. Ume-
moto, N. Miyaura, Tetrahedron 2004, 60, 10695; h) C. M. Vogels,
P. E. OꢁConnor, T. E. Phillips, K. J. Watson, M. P. Shaver, P. G.
Hayes, S. A. Westcott, Can. J. Chem. 2001, 79, 1898; i) S. Colin,
L. Vaysse-Ludot, J.-P. Lecouve, J. Maddaluno, J. Chem. Soc.
Perkin Trans. 1 2000, 4505; j) P. V. Ramachandran, M. P.
Jennings, H. C. Brown, Org. Lett. 1999, 1, 1399; k) S. Pereira,
M. Srebnik, J. Am. Chem. Soc. 1996, 118, 909; l) D. A. Evans,
G. C. Fu, A. H. Hoveyda, J. Am. Chem. Soc. 1992, 114, 6671;
m) D. A. Evans, G. C. Fu, B. A. Anderson, J. Am. Chem. Soc.
1992, 114, 6679; n) K. Burgess, W. A. Van der Donk, S. A.
Westcott, T. B. Marder, R. T. Baker, J. C. Calabrese, J. Am.
Chem. Soc. 1992, 114, 9350; o) D. Mꢂnnig, H. Nçth, Angew.
Chem. 1985, 97, 854; Angew. Chem. Int. Ed. Engl. 1985, 24, 878.
7] a) M. Murata, S. Watanabe, Y. Masuda, Tetrahedron Lett. 1999,
[
21a,22]
have been reported, and Wrighton
II
3
II
1
species could be in equilibrium with the 16 electron Fe h -
benzyl species.
[
21a]
For a-methylstyrene 1l, the formation of
1
II
the h -benzyl Fe complex E is presumably difficult owing to
steric hindrance. Thus, the reaction of a-methylstyrene is
unlikely to form vinylboronate, which is consistent with our
experimental data (Scheme 1, 2l). The observation that
acetonitrile inhibits the dehydrogenative borylation process
can be rationalized by the formation of a coordination-
saturated species F through binding of the cyano group to the
Fe center in E. Complex F lacks a vacant site for b-hydride
elimination and thus would not give vinylboronate ester.
In summary, we have developed the first iron-catalyzed
alkene hydroboration using an electron-rich PNN iron pincer
complex as the precatalyst. The new iron system is far more
efficient than known noble metal systems for catalytic a-
olefin hydroborations with pinacolborane, and can be used to
synthesize alkylboronate esters that would be difficult to
access by other methods. Featuring a cost-effective and
environmentally benign iron catalyst, 100% atom efficiency,
mild reaction conditions, simple product isolation, and good
functional group compatibility, this is a practical method for
preparing synthetically important alkylboronic acid deriva-
tives.
[
40, 2585; b) S. A. Westcott, T. B. Marder, R. T. Baker, Organo-
metallics 1993, 12, 975; c) S. A. Westcott, H. P. Blom, T. B.
Marder, R. T. Baker, J. C. Calabrese, Inorg. Chem. 1993, 32,
2175.
[8] Catalysis Without Precious Metals (Ed.: R. M. Bullock), Wiley-
VCH, Weinheim, 2010.
[
9] a) Iron Catalysis in Organic Chemistry (Ed.: B. Plietker), Wiley-
VCH, Weinheim, 2008; b) C. Bolm, J. Legros, J. Le Paih, L. Zani,
Chem. Rev. 2004, 104, 6217; c) P. J. Chirik in Catalysis Without
Precious Metals (Ed.: R. M. Bullock), Wiley-VCH, Weinheim,
2010, pp. 83 – 110; d) V. C. Gibson, G. A. Solan, Top. Organomet.
Chem. 2009, 26, 107; e) B. D. Sherry, A. Fꢀrstner, Acc. Chem.
Res. 2008, 41, 1500; f) K. Junge, K. Schroder, M. Beller, Chem.
Commun. 2011, 47, 4849; g) R. H. Morris, Chem. Soc. Rev. 2009,
38, 2282; h) H. Nakazawa, M. Itazaki, Top. Organomet. Chem.
Received: December 29, 2012
Revised: January 20, 2013
2011, 33, 27.
[10] J. Y. Wu, B. Moreau, T. Ritter, J. Am. Chem. Soc. 2009, 131,
Published online: && &&, &&&&
12915.
[
[
[
11] For the reactions in entries 1–6, HBPin was fully converted into
multiple unidentified species (besides 2a) in one hour.
12] S. C. Bart, E. Lobkovsky, P. J. Chirik, J. Am. Chem. Soc. 2004,
Keywords: alkylboronates · homogeneous catalysis ·
hydroboration · iron · synthetic methods
.
126, 13794.
13] a) A. M. Tondreau, C. C. H. Atienza, K. J. Weller, S. A. Nye,
K. M. Lewis, J. G. P. Delis, P. J. Chirik, Science 2012, 335, 567;
b) K. Kamata, A. Suzuki, Y. Nakai, H. Nakazawa, Organo-
metallics 2012, 31, 3825; c) A. M. Tondreau, C. C. H. Atienza,
J. M. Darmon, C. Milsmann, H. M. Hoyt, K. J. Weller, S. A. Nye,
K. M. Lewis, J. Boyer, J. G. P. Delis, E. Lobkovsky, P. J. Chirik,
Organometallics 2012, 31, 4886.
[
1] a) Boronic Acids: Preparation and Applications in Organic
Synthesis and Medicine (Ed.: D. G. Hall), Wiley-VCH, Wein-
heim, 2005; b) C. M. Crudden, D. Edwards, Eur. J. Org. Chem.
2003, 4695.
[
2] For reviews, see: a) R. Jana, T. P. Pathak, M. S. Sigman, Chem.
Rev. 2011, 111, 1417; b) A. C. Frisch, M. Beller, Angew. Chem.
II
[
14] Similar 18 electron olefin-ligated Fe hydride species have been
proposed as intermediates in catalytic alkene addition reactions;
see: Refs. [10] and [12], and J. Y. Wu, B. N. Stanzl, T. Ritter, J.
Am. Chem. Soc. 2010, 132, 13214.
2005, 117, 680; Angew. Chem. Int. Ed. 2005, 44, 674.
[
[
3] H. C. Brown, Organic Synthesis via Organoboranes, Wiley
Interscience, New York, 1975.
[
[
15] E. Balaraman, B. Gnanaprakasam, L. J. W. Shimon, D. Milstein,
J. Am. Chem. Soc. 2010, 132, 16756.
16] Conflicting results for [Rh(PPh ) Cl]-catalyzed alkene hydro-
4] a) I. A. I. Mkhalid, J. H. Barnard, T. B. Marder, J. M. Murphy,
J. F. Hartwig, Chem. Rev. 2010, 110, 890; b) H. Chen, S. Schlecht,
T. C. Semple, J. F. Hartwig, Science 2000, 287, 1995; c) S.
Shimada, A. S. Batsanov, J. A. K. Howard, T. B. Marder,
Angew. Chem. 2001, 113, 2226; Angew. Chem. Int. Ed. 2001,
3
3
boration with HBPin have been reported; see: Ref. [6g,k] and
C. E. Tucker, J. Davidson, P. Knochel, J. Org. Chem. 1992, 57,
40, 2168.
3482. Hydroboration with Rh(PPh ) Cl is extremely sensitive to
3 3
[
5] C.-T. Yang, Z.-Q. Zhang, H. Tajuddin, C.-C. Wu, J. Liang, J.-H.
Liu, Y. Fu, M. Czyzewska, P. G. Steel, T. B. Marder, L. Liu,
Angew. Chem. 2012, 124, 543; Angew. Chem. Int. Ed. 2012, 51,
the purity of the reagent. See Table S1 in the Supporting
Information for details.
[17] Reaction using only 1-nonene (1.05 equiv) with 7 (0.5 mol%)
528.
and NaBHEt (1 mol%) gave 83% yield of isolated 2b.
3
4
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Angewandte
Chemie
3
[
[
18] a) I. A. I. Mkhalid, R. B. Coapes, S. N. Edes, D. N. Coventry,
F. E. S. Souza, R. L. Thomas, J. J. Hall, S.-W. Bi, Z. Lin, T. B.
Marder, Dalton Trans. 2008, 1055; b) R. B. Coapes, F. E. S.
Souza, R. L. Thomas, J. J. Hall, T. B. Marder, Chem. Commun.
[20] For examples of the formation of h -benzyl species by insertion
of vinylarene, see Ref. [7b], and M. Brookhart, F. C. Rix, J. M.
DeSimone, J. C. Barborak, J. Am. Chem. Soc. 1992, 114, 5894.
[21] a) J. P. Blaha, M. S. Wrighton, J. Am. Chem. Soc. 1985, 107, 2694;
b) R. S. Herrick, R. R. Duff, A. B. Frederick, J. Coord. Chem.
1994, 32, 103.
2003, 614; c) P. Nguyen, R. B. Coapes, A. D. Woodward, N. J.
Taylor, J. M. Burke, J. A. K. Howard, T. B. Marder, J. Organo-
met. Chem. 2002, 652, 77; d) J. Takaya, N. Kirai, N. Iwasawa, J.
Am. Chem. Soc. 2011, 133, 12980.
[22] a) T. J. J. Sciarone, A. Meetsma, B. Hesssen, J. H. Teuben, Chem.
Commun. 2002, 1580; b) D. H. Hill, M. A. Parvez, A. Sen, J. Am.
Chem. Soc. 1994, 116, 2889.
19] HBPin was fully converted into 9a and other unidentified
species.
Angew. Chem. Int. Ed. 2013, 52, 1 – 7
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Organometallic Catalysis
L. Zhang, D. Peng, X. Leng,
Z. Huang*
&&&& — &&&&
Iron-Catalyzed, Atom-Economical,
Chemo- and Regioselective Alkene
Hydroboration with Pinacolborane
Precious iron: A new PNN iron complex
has been developed for use in an iron-
catalyzed alkene hydroboration reaction
under mild conditions. The environmen-
tally friendly and earth-abundant iron
catalyst system is superior to precious-
metal systems in terms of efficiency and
selectivity for a-olefin hydroborations
with pinacolborane.
Organometallkatalyse
L. Zhang, D. Peng, X. Leng,
Z. Huang*
&&&& — &&&&
Iron-Catalyzed, Atom-Economical,
Chemo- and Regioselective Alkene
Hydroboration with Pinacolborane
Edles Eisen: Ein Eisenkomplex mit
P,N,N-Ligand fꢀr katalytische Alken-
hydroborierungen unter milden Bedin-
gungen wurde entwickelt. Das System
mit dem umweltvertrꢁglichen und leicht
verfꢀgbaren Katalysatormetall ist Edel-
metallsystemen bei a-Olefin-Hydroborie-
rungen mit Pinakolboran hinsichtlich
Effizienz und Selektivitꢁt ꢀberlegen.
6
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 7
These are not the final page numbers!

Angewandte
Chemie
Communications
Organometallic Catalysis
L. Zhang, D. Peng, X. Leng,
Z. Huang*
&&&& — &&&&
Iron-Catalyzed, Atom-Economical,
Chemo- and Regioselective Alkene
Hydroboration with Pinacolborane
Precious iron: A new PNN iron complex
has been developed for use in an iron-
catalyzed alkene hydroboration reaction
under mild conditions. The environmen-
tally friendly and earth-abundant iron
catalyst system is superior to precious-
metal systems in terms of efficiency and
selectivity for a-olefin hydroborations
with pinacolborane.
Angew. Chem. Int. Ed. 2013, 52, 1 – 7
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7
These are not the final page numbers!