Hydroboration of internal alkynes catalyzed by FeH(CO)(NO)(PPh3)2: A case of boron-source controlled regioselectivity

Article abstract of DOI:10.1039/c9cy02461a

The structurally defined Fe-H complex FeH(CO)(NO)(Ph₃P)₂ catalyzes the efficient stereoselective hydroboration of a variety of internal alkynes using either pinacolborane (HBpin) or bis(pinacolato)diboron (B₂pin₂) as a boron source. Upon hydroboration of unsymmetrical internal alkynes, a boron-source dependent regioselectivity was observed. Whereas the use of HBpin provides access to products that can be rationalized through a borylferration mechanism, a complementary regioselectivity was observed using B₂pin₂, which is indicative of a hydroferration mechanism.

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Highly dispersed Pt nanoparticles supported on carbon
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DOI: 10.1039/C9CY02461A
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ARTICLE
Hydroboration of Internal Alkynes Catalyzed by
FeH(CO)(NO)(PPh3)2: a Case of Boron-Source controlled
Regioselectivity
Received 00th January 20xx,
Accepted 00th January 20xx
Fabian Rami,a Franziska Bächtle,a and Bernd Plietkera
DOI: 10.1039/x0xx00000x
The structurally defined Fe-H complex FeH(CO)(NO)(Ph3P)2 catalyzes the efficient stereoselective hydroboration of a
variety of internal alkynes using either pinacolborane (HBpin) or bis(pinacolato)diboron (B2pin2) as boron source. Upon
hydroboration of internal unsymmetric alkynes, a boron-source dependent regioselectivity was observed. Whereas the
use of HBpin provides access to products that can be rationalized through a borylferration mechanism, the complementary
regioselectivity was observed using B2pin2, which is indicative for a hydroferration mechanism.
www.rsc.org/
vinylboronates are accessible in good to quantitative yields.
Further, we observed a boron-source dependent preferential
formation of different regioisomers (eq. (4) and (5), Scheme 1).
To the best of our knowledge, control of regioselectivity by
choice of boron source was not reported for iron catalysts so
far, only for Cu-based systems.[14]
Introduction
Vinylboronates are accessible through hydroboration of
alkynes.[1] This process is commonly catalyzed by means of
precious transition metal complexes.[2] In recent years,
Fe-catalyzed hydroboration processes have gained significant
attention.[3-12] In 2013, Enthaler disclosed the first
breakthrough study on hydroboration of mainly terminal
alkynes catalyzed by the readily available Fe-complex Fe2(CO)9
using pinacolborane (HBpin) as boron source (eq. (1),
Scheme 1).[5] Upon using internal alkynes, the corresponding
vinylboronates were isolated in good to excellent yield, albeit
as mixtures of regio- and stereoisomers. In the same year, the
group of Thomas presented an interesting hydroboration using
a bisiminopyridyl-Fe-complex that is converted into the
reactive catalyst species upon reduction using alkyl Grignard
reagents.[6] Remarkably, hydroboration of olefins but also of
two internal alkynes at room temperature were performed
(eq. (2), Scheme 1). Despite the development of alternative
catalytic systems by the groups of Szymczak,[7] Sreedhar,[8]
Webster (eq. (3), Scheme 1),[9] or most recently
Enthaler (2013)
catalysts
O
Fe-cat. [2.5 mol-%]
HBpin
C
CO
OC
OC Fe
OC
Bpin
H
R1
R2
Fe CO
CO
O
(1)
(2)
R1
C
O
C
PhMe, 100 °C
7 examples
R2
Thomas (2013)
Fe-cat. [1 mol-%]
EtMgBr [3 mol-%]
HBpin
Bpin
N
R
R
H
N
N
R
Fe
Ar
Ar
THF, r.t.
2 examples
R
Cl Cl
Webster (2016)
Fe-cat. [5 mol-%]
HBpin
Bpin
H
N
N
R1
R2
Ar
Ar
R1
(3)
Fe
5 examples
R2
C6D6, 90 °C
SiMe3
Nishibayashi,[10] Kirchner[11] and Findlater,[12]
a broadly
Fe-cat. [5 mol-%]
applicable, yet efficient Fe-catalyzed hydroboration of internal
alkynes is still scarce; in most cases only a very limited number
of internal alkynes have been tested. Based on previous work
on Fe-nitrosyl-complexes in catalysis,[13] we report here a
selective hydroboration of internal alkynes using a Fe-H-
nitrosyl complex as catalyst. Both HBpin as well as B2pin2 can
be employed as boron sources, the corresponding
H
Bpin
HBpin
Bpin
H
R1
R1
(4)
PhMe, 80 °C
R2
minor
R2
Ph3P
major
R1
R2
NO
CO
H
Fe
21 examples
Ph3P
Fe-cat. [0.5 mol-%]
NaOMe [5 mol-%]
B2pin2
1a
H
Bpin
Bpin
H
R1
(5)
R1
R2
major
R2
THF/MeOH, 80 °C
minor
Scheme 1. Fe-catalyzed hydroboration of internal alkynes – state of research
and content of this work.
a. Institut für Organische Chemie, Universität Stuttgart, Pfaffenwaldring 55, DE-
70569 Stuttgart, Germany.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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especially in the case of the bis(meta-ester)alkVyineweAr(teicnletOrynlin5e,
DOI: 10.1039/C9CY02461A
Table 2) and the two ortho-substituted starting materials
(entries 6 and 7, Table 2).
Results and discussion
Table 2. Fe-catalyzed hydroboration – scope and limitations of the HBpin
versus B2pin2-protocol using symmetrical internal alkynes.[a]
The
readily
accessible,
acidic
Fe-H-complex
FeH(CO)(NO)(Ph3P)2 1a that showed excellent catalytic
H
H
R
Bpin
performances in hydrosilylation reactions of internal
+
R
R
R
R
alkynes,[13a]
ketones,[13c]
phosphine
oxides[13c]
and
R
Bpin
(Z)
(E)
dehydrogenative silylations,[13b] was chosen for the initial
experiments using similar experimental conditions to those
that proved successful for the Fe-catalyzed hydrosilylation of
internal alkynes (Table 1).
conditions:
I
1a
II
1a
( ) Fe-catalyst
[5 mol-%]
[2.2 eq.]
PhMe, 80 °C, 18 h
( ) Fe-catalyst
[0.5 mol-%]
2 [1.5 eq.]
HBpin
B2pin
NaOMe [5 mol-%]
THF/MeOH (3/1), 80 °C, 9 h
Table 1. Optimization of the reaction conditions.[a]
Fe-catalyst [x mol-%],
boron source [y eq.]
H
H
Entry
R
Prod. (Z): (E)[b]
Yield [%]
85 (I)
92 (II)
87 (I)
94 (II)
80 (I)
44 (II)
86 (I)
94 (II)
25 (I)
87 (II)
54 (I)
68 (II)
39 (I)
71 (II)
54 (I)
64 (II)
69 (I)
86 (II)
68 (I)
87 (II)
Bpin
Ph
Bpin
Ph
Ph
+
1
Ph
3
95:5 (I)
98:2 (II)
97:3 (I)
98:2 (II)
77:23 (I)
98:2 (II)
98:2 (I)
97:3 (II)
77:23 (I)
97:3 (II)
99:1 (I)
99:1 (II)
99:1 (I)
98:2 (II)
98:2 (I)
99:1 (II)
99:1 (I)
99:1 (II)
99:1 (I)
99:1 (II)
Ph
Ph
solvent, 80 °C,
Ph
HBpin
18 h (
20 h (
)
2
3
(Z)-
3
(E)-
B pin
)
2
2
2
p-MeC6H4
p-ClC6H4
p-MeOC6H4
m-MeO2CC6H4
o-MeC6H4
o-BrC6H4
Et
4
PPh2
Ph3P
PPh2
PPh2
3
5
CO
NO
H
CO
NO
CO
NO
CO
NO
Fe
Fe
Fe
Fe
H
Fe
Ph2P
P
Ph2
PPh2
H
H
Ph3P
4
6
1c
1d
1b
1a
5
7
Entry
Cat.
Solv.
B-source
[y eq.]
Yield
[%][b,c]
85
9
26
6
8
[x mol-%]
1a (5)
1
2
3
4
5
PhMe
PhMe
PhMe
PhMe
PhMe
HBpin (2.2)
HBpin (2.2)
HBpin (2.2)
HBpin (2.2)
B2pin2 (1.2)
NaOMe (1.2)
B2pin2 (1.5)
NaOMe (0.05)
7
9
1b (5)
1c (5)
1d (5)
1a (10)
8
10
11
12
67
88
9
n-Pr
6[d]
1a (0.5)
THF
96
10
n-Bu
[a] Reactions were run on 0.3 mmol scale. [b] (Z)/(E) ratio was
determined by 1H NMR or GC integration and was always >95/5. [c]
Determined by 1H NMR integration with 1,3,5-trimethylbenzene as
internal standard. [d] Yield after 9 hours.
[a] Isolated yields for reactions on 0.5 mmol scale. [b] (Z)/(E) ratio was
determined by GC.
This finding caught our attention since it points into the
direction of different modes of action using the same catalyst.
We therefore investigated the hydroboration of non-
symmetrical internal alkynes (Table 3). Indeed, both protocols
allow for the preparation of vinylboronates in good yields, and
we were pleased to find that the regioselective course is
mainly directed through the boron source employed in the
reaction. A general tendency can clearly be delineated from
these results. The use of HBpin (protocol I, Table 3) leads to
the expected Markovnikov-type products in moderate to good
regioselectivities. The use of B2pin2/MeOH on the other hand
provides access to the complementary anti-Markovnikov
products in comparable regioselectivities (protocol II, Table 3).
Even for structurally and electronically similar substituents as
in vinylboronate 22A and 22B a moderate, but orthogonal
regioselectivity was observed. Further, 22 allows for exclusion
of a radical pathway for both of our methods, as ring opening
of the cyclopropyl substituent was not detected. While
attempts to isolate reactive intermediate Fe-complexes were
unsuccessful,[17] the results point into the direction of a boron-
Indeed, those experimental conditions already resulted in
good yields for the hydroboration of diphenylacetylene
(entry 1, Table 1).[15] Among the solvents tested, toluene gave
the highest isolated yield of vinylboronate (Z)-3. Moreover, a
strong ligand effect was observed. The use of alternative Fe-H-
complexes 1b-d with bidentate phosphine ligands did not
improve the overall catalytic activity (entries 2 – 4, Table 1).[16]
Subsequently, we tested B2pin2 as an alternative boron source.
Already initial experiments indicated this protocol to be very
powerful. Excellent conversion and (Z)/(E)-selectivity were
observed with only 0.5 mol-% of catalyst 1a in the presence of
5 mol-% NaOMe (entry 6, Table 1).
Having these two alternative protocols for Fe-catalyzed alkyne
hydroborations in hand, we evaluated their scope and
limitations for symmetrical internal alkynes (Table 2). Various
internal alkynes were successfully reacted into the
corresponding vinylboronates in good to excellent yields and
stereoselectivities. In the majority of investigated substrates
the B2pin2-protocol gave higher yields and better selectivities,
2 | J. Name., 2012, 00, 1-3
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source dependent mechanistic dichotomy. If it is assumed that At the current state of research, we consider the following
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the regioselectivities are the results of either a borylferration working model to provide suitable firstDeOxI:p1l0a.n10a3t9io/Cn9sCYfo0r24t6h1Ae
(Markovnikov-selectivity) or a hydroferration mechanism (anti- observed experimental results (Figure 1).
Markovnikov-selectivity), an alteration of the steric demand on
one of the two alkyne substituents should have a remarkable
H
Ph3P
CO
H
electronically
and
sterically favored
Bpin
influence on the regioselective course of one of two protocols.
To test this hypothesis, arylalkylalkynes with alkyl groups of
different steric demand were subjected to the reaction
conditions. Indeed, while for n-hexylphenylethine a clear
boron-source dependent regioselectivity inversion in the
product 17 from 82 : 18 (HBpin) to 24 : 76 (B2pin2) was
observed, an increase in the steric demand of the alkyl group
results in decreased regioselectivities for the B2pin2-protocol.
Whereas the use of HBpin led to an increase of the
regioselectivity from 82 : 18 (product 17) to 96 : 4 (product
19), a severe decrease of both yield and regioselectivity using
B2pin2 from 24 : 76 (product 17) down to 59 : 41 (product 19)
was observed. These results are indicative for a strong
interaction of the alkyl-substituent with the incoming Fe-
complex.
Fe
NO
R
R
PPh3
V
VI
P
P
NO
H
NO
H
OC
OC
PPh3
Fe
Fe
ON
OC
R
Fe
H
R
P
PPh3
P
1a
I
electronically
favored metal
binding site
II
steric
interaction
HBpin
P
P
NO
B
O
NO
B
O
OC
O
OC
O
PPh3
Fe
Fe
O
ON
OC
R
Fe
B
R
O
P
P
PPh3
III
IV
Table 3. Regioselectivity of Fe-catalyzed hydroboration.[a,b]
Bpin
H
pinB
Ph3P
pinB
H
CO
Bpin
electronically
and
sterically favored
H
R1
R1
R1
R2
+
Fe
NO
PPh3
R2
(Z)/(E)-
R2
R
R
A
B
(Z)/(E)-
VIII
VII
conditions:
I
( )
II
1a
Fe-catalyst [0.5 mol-%]
(
)
1a
Fe-catalyst
[5 mol-%]
HBpin
PhMe, 80 °C, 18 h
[2.2 eq.]
B2pin
2 [1.5 eq.]
Figure 1. Fe-catalyzed hydroboration of internal alkynes - working model.
NaOMe [5 mol-%]
THF/MeOH (3/1), 80 °C, 9 h
Accordingly, we postulate that dependent on the nature of the
boron source, the initial step of the reaction involves either a
hydroferration or a borylferration of the alkyne. In both cases
two different regioisomers I – IV are accessible. In case of the
hydroferration (protocol II) the strong backbinding CO- and
NO-ligands lead to a decrease of electron density at the metal
center favoring transition state I, in which the metal center is
coordinating to the more electron-rich carbon atom of the C-C-
double bond. In addition, unfavorable steric interactions
between the aryl group, in particular when substituted at the
ortho-position, and the bulky metal complex disfavor
transition state II. As a result, formation of the anti-
Markovnikov products VI via vinyl-Fe-complex V is favored.
Increasing the size of the alkyl groups affects the
regioselectivity due to the increasing steric interactions
between the R-group and the metal center in I. As a result, a
decrease in the regioselectivity is observed (protocol II,
products 18, 19, and 20, Table 3). On the contrary, addition of
HBpin (protocol I) leads to the formation of an Fe-B-complex
that undergoes a borylferration via transition states III or IV.
Although electronically favored, transition state III faces
stronger steric interactions between the boryl ligand and the
aryl substituent on the alkyne moiety. Increasing these steric
interactions through ortho-substitution, a significant decrease
in yields might be expected and was experimentally observed
(entries 6 and 7, protocol I, Table 2). Moreover, increasing the
size of the R-group also increases the unfavorable interactions
to the boryl ligand in transition state IV, which ultimately leads
Bpin
Bpin
Bpin
n-hexyl
n-hexyl
n-hexyl
MeO
NC
Cl
13
14
15
I:
A : B
A : B
I:
A : B
I:
A : B
A : B
25%,
64%,
= 87 : 13
= 4 : 96
75%,
85%,
= 84 : 16
83%,
82%,
= 74 : 26
II:
II:
A : B
II:
= 19 : 81
Bpin
= 18 : 82
Bpin
Bpin
n-hexyl
n-hexyl
Me
16
17
A : B
18
I:
A : B
I:
I:
A : B
82%,
= 75 : 25
= 24 : 76
85%,
50%,
= 82 : 18
= 24 : 76
55%,
45%,
= 90 : 10
= 31 : 69
II:
A : B
II:
A : B
II:
A : B
80%,
Bpin
Bpin
Bpin
H
Cl
19
20
21
I:
A : B
A : B
I:
A : B
87%,
51%,
= 94 : 6
I:
A : B
84%,
55%,
= 96 : 4
13%,
35%,
= 0 : 100
= 0 : 100
II:
II:
A : B
= 58 : 42
II:
A : B
= 59 : 41
Bpin
22
I:
A : B
A : B
85%,
90%,
= 61 : 39
= 32 : 68
II:
[a] Isolated yields for reactions on 0.5 mmol scale. [b] Ratio of regioisomers
was determined by GC integration.
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to significantly higher regioselectivities (protocol I, products contrast, the Fe-NO complex 1a showed boron-source
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18, 19, and 20, Table 3).
dependent regioselectivity, yielding eithDeOr I1: 160A.10o3r9/1C69BCYw02it4h61Aa
To benchmark the abovementioned results, different reported regioisomeric ratio of 75 : 25 or 24 : 76 (eq. (5) versus (6),
Fe-catalysts were prepared and subjected to the reaction Scheme 2). Tsuji’s Cu-based system shows superior
conditions using either HBpin or B2pin2 as boron source (eq. (1) regioselectivities, however, yields were significantly lower (eq.
- (6), Scheme 2).[18]
(7) - (10), Scheme 2). Finally, employing Knochel’s catalyst-free
hydroboration using a mixture of pinacol and BH3·Me2S led to
the formation of vinylboronates 16, but in moderate yields and
a regioselectivity of 82 : 18 in favor of the Markovnikov
product 16A (eq. (11), Scheme 2).
Finally, we tested the chemoselectivities of the different
Fe-based catalytic systems using the symmetrical
m,m´-bisallylarylethyne 24 as a test substrate (Scheme 3).
H
Bpin
Ar
n-hexyl
Bpin
H
Ar
Ar
(Ar = p-MeC6H4)
n-hexyl
n-hexyl
16A
23
16B
Ratio
Yield / %
Conditions
Fe-cat. [2.5 mol-%]
HBpin
PhMe, 100 °C
O
(1)
(2)
C
85
complex mixture
CO
OC
OC Fe
OC
Fe CO
C
CO
C
Fe-cat. [0.25 mol-%]
B2pin2
O
O
25
:
75
78
PhMe, 100 °C
Fe-cat. [5 mol-%]
EtMgBr [15 mol-%]
HBpin
pinB
pinB
(3)
(4)
24
27
:
:
76
73
92
28
N
Bpin
THF, r.t.
N
N
Fe
Cl Cl
Ar
Ar
Fe-cat. [0.5 mol-%]
EtMgBr [1.5 mol-%]
B2pin2
27
24
25
26
THF, r.t.
25 26
Ratio (
:
)
Yield / %
Conditions
Fe-cat. [5 mol-%]
HBpin
PhMe, 80 °C
Ph3P
O
75
24
:
:
25
76
82
80
(5)
(6)
C
CO
Fe-cat. [2.5 mol-%]
HBpin
OC
NO
CO
(1)
(2)
27
85
formation of
H
Fe
[1.25 eq.]
OC Fe
OC
Fe CO
C
CO
Fe-cat. [0.5 mol-%]
C
PhMe, 100 °C, 24 h
O
O
NaOMe [5 mol-%]
Ph3P
B2pin2
1a
THF/MeOH, 80 °C
Fe-cat. [5 mol-%]
EtMgBr [15 mol-%]
CuCl [2 mol-%]
ligand (R = 3,5-CH3C6H3)
HBpin
N
95
:
5
52
HBpin
[1.10 eq.]
N
N
92
:
8
45
(7)
Fe
Cl Cl
Ar
Ar
THF, r.t., 45.5 h
PhMe, 0 °C - r.t., 20 h
CuCl [2 mol-%]
ligand (R = 3,5-CH3C6H3)
B2Pin2
Fe-cat. [5 mol-%]
Ph3P
HBpin
[2.20 eq.]
94
88
:
:
6
79
57
(3)
(4)
3
:
:
97
13
24
34
(8)
(9)
ligand
PhMe, 80 °C, 3 h
NO
CO
PhMe, r.t. - 28 °C, 3 h
H
Fe
PR2
PR2
Fe-cat. [0.5 mol-%]
NaOMe [5 mol-%]
CuCl [2 mol-%]
ligand (R = 3,5-CF3C6H3)
HBpin
O
12
Ph3P
1a
B2pin
2 [1.50 eq.]
87
THF/MeOH, 80 °C, 9 h
PhMe, 0 °C - r.t., 20 h
CuCl [2 mol-%]
ligand (R = 3,5-CF3C6H3)
B2Pin2
PhMe, r.t. - 28 °C, 3 h
:
:
98
18
83
42
(10)
(11)
2
Scheme 3. Chemoselectivities in Fe-catalyzed hydroboration of alkynes.
The use of Fe2(CO)9 as a catalyst led to the hydroboration of
the alkyne but, very much to our surprise, also to a -bond
isomerization of the terminal olefins to give vinylboronate 27
in good yield (eq. (1), Scheme 3). On the contrary, Thomas’
Fe-bisiminopyridyl complex catalyzes the chemoselective
hydroboration of the alkyne moiety to 25. Only minor amounts
of the alkylboronate 26 were isolated, isomerization of the -
bond to 27 was not observed (eq. (2), Scheme 3). Employing
the Fe-NO-complex 1a as a catalyst led to the formation of
vinylboronate 25 in moderate to good yields and good to high
chemoselectivites (eq. (3) and (4), Scheme 3).
Based on these results, we propose the following mechanistic
rationale (Figure 2): in presence of HBpin (protocol I), iron
complex 1a reacts under H2-evolution to form the catalytically
active Fe-boryl complex AI. This complex adds to the alkyne in
a borylferration reaction under the constraints outlined in
Figure 1 to give the intermediate vinyliron species AII (please
note: AII corresponds to VII in Figure 1). After oxidative
addition of another equivalent of HBpin, AIII is formed which
undergoes subsequent reductive elimination to furnish the
product in Markovnikov selectivity while AI is regenerated. For
BH3 Me2S
pinacol
82
CH2Cl2, 0 °C - r.t., 8 h
Scheme 2. Fe-catalyzed hydroboration of internal alkynes – a comparative
study.
Moreover, Tsuji’s Cu-based catalyst system[14] was prepared. In
order to cross-check the influence of the ligand, both HBpin
and B2pin2 were employed using two structurally related
Xantphos-ligands (eq. (7) - (10), Scheme 2). We also referenced
these results to the metal-free hydroboration using Knochel’s
in-situ protocol of BH3·Me2S/pinacol (eq. (11), Scheme 2).[19]
Depending on the reaction conditions, different
regioselectivities in the hydroboration of n-hexyarylethyne 23
were observed. Both Enthaler’s and Thomas’ catalytic systems
led to hydroboration, however, depending on the boron
source, either varying yields ranging from 28 to 92 % or
complex product mixtures were observed (eq. (1) - (4), Scheme
2). In all cases, the anti-Markovnikov product 16B was formed
in a maximum selectivity of 24 : 76 (eq. (3), Scheme 2). In
4 | J. Name., 2012, 00, 1-3
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Catalysis Science & Technology
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ARTICLE
B2pin2 as boron source (protocol II), 1a directly adds to the petroleum ether/ethyl acetate mixture) toViewyAiertlicdle Ontlhinee
DOI: 10.1039/C9CY02461A
alkyne in a hydroferration pathway to intermediate BI (which corresponding vinylboronate.
corresponds to V in Figure 1). With B2pin2, NaOMe and MeOH,
complex BII is formed. Under reductive elimination, the
product is released in anti-Markovnikov selectivity, and species
Conclusions
BIII is formed. BIII acts as the catalytically active species in the
following cycles.
Herein we report the use of the structurally well-defined
Fe-NO-hydride complex 1a as a catalyst for the chemoselective
hydroboration of internal alkynes. A variety of aliphatic or
aromatic alkynes were successfully reacted into the
corresponding vinylboronates with very high (Z)/(E)-ratios in
moderate to high yields using both HBpin and B2pin2 as boron
sources. Interestingly, employing the latter boron source
allowed to reduce the catalyst loading to only 0.5 mol-%
without affecting the (Z)/(E)-selectivities or the yields. For
unsymmetrical internal alkynes, different regioisomers were
formed preferentially in dependence of the boron source:
while the use of HBpin led to a preferred formation of the
Markovnikov-products, the selectivities were inverted upon
employing B2pin2 as boron source. This finding is indicative for
the formation of different catalytic species depending on the
boron source. Comparative studies with literature-reported
Fe-based catalytic systems showed that this inversion of
selectivity is only found for the Fe-NO-hydride complex 1a.
Tsuji’s Cu-based system[14] shows a similar boron-source
dependent formation of different regioisomers, however, a
change of the ligand is required in order to obtain synthetically
useful yields. The experimental results presented in this
manuscript are indicative for a boron-source dependent
mechanistic dichotomy. Investigations into the mechanism are
currently ongoing and will be reported separately.
HBpin
H
pinB
[Fe]
Bpin
pinB
Ar
[Fe]
Ar
Alkyl
Alkyl
AIII
AII
pinB
Ar
H
Ar
Alkyl
Alkyl
Bpin
[Fe]
AI
HBpin
Bpin
Alkyl
[Fe]
1a
or
H
H
Ar
Alkyl
Ar
H
[Fe] OMe
BIII
OMe
[Fe]
H
[Fe]
H
H
Bpin
Ar
Alkyl
Ar
Alkyl
BI
BII
B2pin
2 + MeOH + NaOMe
Figure 2. Proposed reaction mechanism for the Fe-H-catalyzed
hydroboration of alkynes.
Acknowledgements
The authors are grateful to the Landesgraduiertenförderung
Baden-Württemberg (Ph.D.-grant for F.R. within the BW-
Experimental Section
Method A (HBpin as boron source): A 10 mL-screw cap Schlenk graduate school “Windy Cities”), and to the Deutsche
tube was briefly dried with a heat gun and charged with 1a Forschungsgemeinschaft (CRC 1333 “Molecular heterogeneous
(16.0 mg, 0.025 mmol). Dry toluene (0.67 mL), the respective catalysis in confined spaces”, Ph.D.-grant for F.B.) for financial
alkyne (0.5 mmol) and pinacolborane (160 μL, 1.1 mmol) were support.
added and the reaction mixture was stirred at 80 °C for
18 hours. After cooling to room temperature, the mixture was
filtered over a pad of silica gel (eluent: diethyl ether). The
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Catalysis Science & Technology
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6 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C9CY02461A
TOC-entry
The structurally defined Fe-H complex FeH(CO)(NO)(Ph3P)2 catalyzes the efficient stereoselective
hydroboration of variety of internal alkynes using either pinacolborane (HBpin) or
a
bis(pinacolato)diboron (B2pin2) as boron source. Upon hydroboration of internal unsymmetric
alkynes, a boron-source dependent regioselectivity was observed. Whereas the use of HBpin
provides access to products that can be rationalized through a borylferration mechanism, the
complementary regioselectivity was observed using B2pin2, which is indicative for a hydroferration
mechanism.
Fe-cat. [5 mol-%]
H
Bpin
HBpin
Bpin
H
R1
R1
PhMe, 80 °C
R2
minor
R2
Ph3P
major
R1
R2
NO
CO
H
Fe
21 examples
Ph3P
Fe-cat. [0.5 mol-%]
NaOMe [5 mol-%]
B2pin2
1a
H
Bpin
Bpin
H
R1
R1
THF/MeOH, 80 °C
R2
major
R2
minor