Iron-Catalyzed Hydroborylative Cyclization of 1,6-Enynes

Article abstract of DOI:10.1002/chem.201704401

We report first Fe-catalyzed hydroborylative cyclization reaction. The process provides one C?C and one C?B bond in a single operation and shows a wide scope, allowing the formation of carbo- and heterocycles containing a homoallylic boryl unit that can b

Full text of DOI:10.1002/chem.201704401

A Journal of
Accepted Article
Title: Iron-Catalyzed Hydroborylative Cyclization of 1,6-Enynes
Authors: Diego J. Cardenas, Natalia Cabrera-Lobera, Patricia
Rodríguez-Salamanca, Juan Carlos Nieto-Carmona, and
Elena Buñuel
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To be cited as: Chem. Eur. J. 10.1002/chem.201704401
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Iron-Catalyzed Hydroborylative Cyclization of 1,6-Enynes
Natalia Cabrera-Lobera, Patricia Rodríguez-Salamanca, Juan C. Nieto-Carmona, Elena Buñuel, Diego
J. Cárdenas*[a]
Abstract: We report first Fe-catalyzed hydroborylative cyclization
reaction. The process provides one C-C and one C-B bond in a
single operation and shows a wide scope, allowing the formation of
carbo- and heterocycles containing a homoallylic boryl unit that can
be further functionalized. The reaction takes place in smooth
conditions, with inexpensive catalytic system and full atom economy
since HBpin is the borylation agent, in contrast to our previously
reported Pd-catalyzed reaction. Both aryl and alkyl substituted
alkynes are reactive, revealing a wide reaction scope. Mechanistic
studies suggest the intermediacy of Fe(II)-hydride active catalyst
capable to react with the alkyne group prior to alkene insertion, and
computational studies suggest the occurrence of barrierless –bond
Kumada and Negishi cross-coupling of alkyl electrophiles, and
have recently turned our attention to the possibility of using
these metals for hydroborylative cyclization of enynes. Very
recently Co-catalyzed hydroborylative cyclization of enynes has
been reported by Lu. Both alkyl and alkenylboronates can be
prepared by suitable choice of catalyst.[10a] Ge has developed
the enantioselective version of this reaction.[10b] Herein, we wish
to report our results on the scope and mechanism of the first Fe-
catalyzed hydroborylative cyclization reaction of 1,6-enynes.
This reaction uses inexpensive Fe salts, simple 2,2’-biquinoline
ligand and HBpin as the boryl source, being therefore atom-
economical. It provides alkylboronates, which are usually not as
metathesis involving HBpin and Fe-C bonds along the catalytic cycle. readily accessible as alkenyl- or arylboron derivatives (Scheme
1).
The development of new synthetic methods has to take into
Previous work
account economic and environmental requisites that have
become unavoidable. Accordingly, we are witnessing the
renaissance of first-row transition metal-catalyzed reactions of
synthetic interest.[1] In this respect, Fe has demonstrated to be
extremely versatile.[2,3] On the other hand, boron compounds
(especially boronic acids and boronates) constitute valuable
reagents for their high stability, good reactivity, versatility and
low toxicity. Hydroboration,[4,5] and carboboration reactions,[6]
allow formation of boronates in the absence of highly basic and
nucleophilic reagents, constituting useful synthetic tools widely
applicable in the presence of a variety of functional groups.
During the last years, our group has contributed to the
development of Pd-catalyzed hydroborylative cyclization
reactions,[7] that have allowed to access allylic, homoallylic and
alkenylic boronates with concomitant formation of 1 or 2 rings
from simple poly-unsaturated compounds (enynes, enallenes,
allenynes, enediynes, etc.), avoiding the use of organohalides
and Li and Mg nucleophiles. These reactions provide one or two
C-C bonds along with a new C-B bond, in smooth ligandless
conditions and stereoselectively. However, this methodology
shows some drawbacks, as the use of diboron derivatives
B2(pin)2 (that leads to the loss of one B unit) and the
impossibility to tune the catalysts properties since it takes place
in the absence of added ligands. In the course of our research
to find more convenient catalysts for C-C bond formation, we
have developed Ni,[8] and Fe-based[9] catalytic systems for the
Cárdenas et al.
R1
R1
Pd(OAc)2 (5 mol %)
Z
Z
Z
B2pin2
+
MeOH (1 equiv)
toluene, 50 ºC
Bpin
1.1 equiv
R2
R2
R1
LCoCl2 (5 mol%)
NaBHEt3 (15 mol%)
Lu et al.
toluene, 23 ºC
Bpin
R1
O
iPr
N
L =
Z
N
N
H
Bpin
+
Bn
1.5 or 2.0
equiv
iPr
R1
Co(acac)2 (3 mol%)
Z
Me
THF, 23 ºC
Bpin
86-98% ee
Ge et al.
N
P
tBu
tBu
N
P
(4 mol%)
Me
This work
FeBr2 (5 mol%)
MeMgCl (10 mol%)
dioxane, 23 ºC
R1
R2
Z
R2
R1
R3
Z
+
H
Bpin
Bpin
1.5 equiv
R3
N
N
(5 mol%)
Scheme 1. Synthesis of cyclic homoallylic boronates from enynes.
[a]
Natalia Cabrera-Lobera, Patricia Rodríguez-Salamanca, Juan
Carlos Nieto-Carmona, Dr. Elena Buñuel, Prof. D. J. Cárdenas
Departamento de Química Orgánica, Facultad de Ciencias
Universidad Autónoma de Madrid, Institute for Advanced Research
in Chemical Sciences (IAdChem)
Av. Francisco Tomás y Valiente 7, Cantoblanco,28049-Madrid,
Spain
E-mail:diego.cardenas@uam.es
The search for the optimized reaction conditions involved finding
the most appropriate Fe salt, ligand and additive. Fe-catalyzed
hydroboration reactions usually require the participation of
borohydrides in order to generate Fe-H species. We found that
the most convenient catalyst is generated by reaction of FeBr2 (5
mol%) 2,2’-biquinoline (5 mol%) and MeMgCl (10 mol%).
Supporting information for this article is given via a link at the end of
the document.
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10.1002/chem.201704401
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Table 1. Fe-catalyzed hydroborylative cyclization of 1,6-enynes..
Under the optimized conditions, found after extensive
experimentation (see Supplementary Information), the reaction
could be extended to a wide variety of 1,6-enynes to give
borylated carbo- and heterocycles (Table 1).
R2
Z
R1
R2
FeBr2 (5 mol%)
R1
2,2'-biquinoline (5 mol%)
R3
+
H
Bpin
Z
MeMgCl (10 mol%)
dioxane, 23 ºC, 5 h
1.5 equiv
Bpin
Compared
to
our
previously
reported
Pd-catalyzed
R3
hydroborylative cyclization,[6] much better results were obtained
for the formation of N- and O-containing heterocycles regarding
both the scope and the yields (Table 1, entries 1-10 and 15-19).
In addition, aryl ring on the alkyne may hold a variety of
substituents with very different electronic properties, ranging
from MeO to CN and CF3 (entries 1-10). Carbocycles are also
accessible (entries 11-14) and there is no need for the substrate
to bear a quaternary center to enhance Thorpe-Ingold effect and
provide good results (entries 11-12). Alkynes with substituents
other than aromatic rings were also suitable reaction partners
(entries 15-17). Moreover, additional substitution on the internal
carbon of the alkene also afforded the expected boronate albeit
in moderate yield (entry 18). The reaction also tolerates
substitution in the propargyl carbon, although no sterocontrol on
the formation of the new stereogenic center was observed (entry
19). Nevertheless, the desired products were not observed
when the distal carbon of the alkene is substituted with either Me
or Ph groups for O and NTs tethered substrates. We performed
some transformations on the boronates to illustrate their
reactivity and potential applicability (Scheme 2).
0.4 mmol
Substrate
Product
Yielda (%)
1
1a
2a : Z = O, R = H
77
79
56
76
55
76
60
60
70
48
57b
37b
2
1b
1c
1d
1e
1f
2b : Z = O, R = Me
2c : Z = O, R = OMe
2d : Z = O R = Cl
3
4
5
2e : Z = O R = CF3
2f : Z = NTs, R = H
2g : Z = NTs, R = Me
2h : Z = NTs, R = OMe
2i : Z = NTs, R = Cl
2j : Z = NTs, R = CN
2k : Z = CH2, R = H
2l : Z = CH2, R = Me
6
7
1g
1h
1i
8
NaOH (9 equiv)
H2O2 (30 equiv)
Ph
Z
9
3a
3b
: Z = O (86 %)
: Z = NTs (80 %)
Et2O
Ph
OH
Z
10
11
12
1j
Ph
1k
1l
Z
Bpin
KHF2 (4.5 equiv)
MeOH-H2O
4a : Z = O (84 %)
4b : Z = NTs (72 %)
2a or 2f
BF3K
Scheme 2. Functionalisation of boronates.
13
14
56
55
2m
2n
1m
1n
Thus, two O and NTs heterocycles were oxidized to the
corresponding alcohols 3a-b, or transformed into the
synthetically useful trifluoroborate salts (4a-b), that are
convenient nucleophiles for Suzuki cross-coupling (non-
optimized yields).[11]
HBpin (2 equiv)
FeBr2 (5 mol%)
2,2'-biquinoline (5 mol%)
no reaction
PhO
MeMgCl (35 mol %)
15
16
17
1o
1p
1q
2o : Z = NTs, R = H
2p : Z = NTs, R = OMe
2q : Z = O, R = OMe
62
60
54
THF, 0 ºC to 23 ºC, 3 h
HBpin (1.5 equiv)
FeBr2 (5 mol%)
2,2'-biquinoline (5 mol%)
Me
Ph
Me
MeMgCl (10 mol %)
THF, 0 ºC to 23 ºC, 3 h
Bpin
33%
18
19
27c
Conv = 90%
1r
2r
HBpin (1.5 equiv)
FeBr2 (5 mol%)
MeO
MeO
OMe
Ph
2,2'-biquinoline (5 mol%)
Bpin
72d
Ph
+
MeMgCl (10 mol %)
Bpin
THF, 0 ºC to 23 ºC, 3 h
Ph
2s
1s
12%
22%
Conv = 90%
[a] Isolated yield. [b] Reaction time: 24 h. [c] It contains a minor impurity
that could not be separated nor identified. [d] Diastereomeric ratio = 1:0.9. For
the racemic substrate product was isolated in 80% yield, with the same d.r.
Scheme 3. Reactions of simple alkenes and alkynes.
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We were interested in getting insight into the reaction
mechanism for this novel transformation, and performed some
experiments with this aim. Thus, simple alkenes as
allylphenylether, that contains an electronically similar double
bond, did not react and we recovered the unchanged starting
substrate (Scheme 3).
expected that coordinatively unsaturated III gets stabilized by
solvent. The calculated stabilization energy upon coordination of
two molecules of THF is -12.5 kcal mol-1 (see Supplementary
Information) and therefore it drives the formation of the
alkyl(hydride)Fe complex to completion. We then studied the
interaction of III with the enyne through the alkyne group,
according to the experimental results. No transition state could
be located for alkyne coordination nor for the hydrometalation of
the alkyne. A study of the potential energy surface for the
coordination and insertion of the alkyne into the Fe-H bond
confirmed the absence of activation barrier for this step (Figure
1). The resulting alkenyl-Fe(II) intermediate IV is formed in an
exoergic fashion and the resulting complex is further stabilized
by coordination of the alkene (V), what is necessary for the next
step to occur. Albeit we could not locate a transition state for the
carbometalation of the alkene, calculations indicate that this step
proceeds with low activation energy (ca 5 kcal mol-1, see
Supplementary Information) to provide alkyl-Fe complex VI.
Finally, dissociation of the resulting C-C double bond (VI to VII)
allows gathering of a second HBpin molecule to cleave the alkyl-
Fe bond, furnishing the final compound by means of an exoergic
and again barrierless –bond metathesis reaction, regenerating
the active catalyst upon dissociation of VIII.
Instead, the reaction of alkynes afforded hydroboration
compounds. Therefore, enyne activation seems to proceed
through initial alkyne functionalisation. On the other hand, as we
did in our previous studies on cross-coupling reactions, we tried
to determine the oxidation state of the active species. The
participation of a Grignard reagent in the activation process of
the Fe salt could imply reduction of the Fe(II) precatalyst by
homocoupling of the nucleophile. However, reaction of an
equimolar mixture of FeBr2 and 2,2’-biquinoline with two different
Grignard reagents afforded a low yield of homocoupling non-
volatile compound, or even no coupling at all (Scheme 4),
suggesting that Fe does not get reduced and that the
catalytically active complex is a Fe(II) one.
O
O
MgBr
O
(0.16 or 0.56 mmol)
O
O
dioxane
23 ºC, 30 min
9%
O
FeBr2
(0.08 mmol)
Catalyst generation
(0.007 mmol,already
present in the reagent)
H3C
+
N
CH3
CH3
MeMgCl
L
HBpin
-15.1
N
Ph
Bpin
FeX2
N
Fe
2,2'-biquinoline
(0.08 mmol)
MgCl
H
N
Fe
(0.16 mmol)
CH3
no homocoupling product
I
dioxane
II
23 ºC, 30 min
H3C Bpin
+ 9.1
Catalytic cycle
Scheme 4. Grignard reagent does not reduce Fe(II)-2,2’-biquinoline complex.
N
Ph
N
Moreover, the use of just 1 equiv of MeMgCl did not afford a
catalytically active system, what points to a dialkyl-Fe(II)
complex as a competent catalytic species in the dark green
solution obtained by reaction of FeBr2 and 2 equiv of MeMgCl.
This solution immediately reacts with HBpin as we observed in a
separate experiment and affords MeBpin, that was detected by
GC-MS. Noteworthy, for related Fe-catalyzed reductive
cyclizations of enynes,[3j,3k,12] stoichiometric amounts of alkyl-
Grignard reagents are necessary to provide the putative active
Fe hydride for the catalytic cycle. In contrast, in our case
hydrogen atom is transferred to the final product from
pinacolborane rendering an atom-economical overall process.
With these results, we performed DFT calculations with M06-2X
functional (Scheme 5) in order to explore the reactivity of model
L2Fe(CH3)2 (I, L2 = 2,2’-biquinoline) and to try to find a feasible
reaction pathway from this compound, whose intermediacy is
strongly suggested by the experimental results mentioned above.
Initial calculations using different spin states pointed to singlet
derivatives as the most stable ones. Interestingly, –bond
metathesis reaction of I with HBpin proceeds downhill (G = -
15.0 kcal mol-1) without activation barrier. This could be
demonstrated by computing the energy along the C-B bond
formation.[13] In contrast, C-C reductive elimination to Fe(0)
cannot take place, since it shows a very high activation energy
(32.9 kcal mol-1). The corresponding MeBpin-L2Fe(H)(CH3)
adduct (II) that forms upon –metathesis would dissociate
MeBpin (+9.1 kcal mol-1) to give the active catalyst III.[14] It is
Ph
N
Fe
H
N
Fe
CH3
IV
-25.4
H
CH3
III
Ph
-6.6
+15.1
Ph
Bpin
Ph
N
H
N
N
Fe
N
Fe
Bpin
H
CH3
VIII
CH3
V
-21.3
H Bpin
-18.2
Ph
Ph
N
+9.0
N
N
Fe
N
Fe
CH3
VII
CH3
VI
Scheme 5. Computed feasible reaction pathway. G (kcal mol-1) calculated at
M06-2X/6-31G(d) (C,H,B,N,O), LANL2DZ (Fe) level.
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The proposed reaction pathway shows extremely low activation
energies for all steps, what makes it feasible and moreover,
highly likely. Formation of dialkyl-Fe intermediate, suggested by
experiments, is key to this process, since it readily reacts with
HBpin to afford a hydride complex highly reactive towards the
alkyne moiety. The subsequent steps are very fast and allow a
facile regeneration of the active species. Intermediate diorgano-
Fe complexes are stable towards reductive elimination due to
the high activation energy of this process. Alkyl spectator ligands
have been also found in Zn promoted reactions,[15] and even
more reactive aryl-Fe bonds have been proposed to remain
intact through catalytic cycles in cross-coupling reactions.[16] The
high activation barrier for redictive elimination allows
Keywords: Iron • Carbocycles • Heterocycles • Boron • Reaction
mechanisms
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In conclusion, we have developed the first Fe-catalyzed
hydroborylative cyclization reaction, that affords one C-C and
one C-B bond in a single operation, and have proposed a
reasonable reaction mechanism. The reaction shows a wide
scope and allows the formation of carbo- and heterocycles in a
single step, smooth conditions, with inexpensive catalytic system
and full atom economy. This reaction is far more convenient
compared with the previously reported Pd-catalyzed reaction for
several reasons, including the possibility of tuning the ligand for
further developments, such as asymmetric versions.
Calculations show that alkyl-Fe complexes can be used as
triggering reagents for the activation of main group element
derivatives of synthetic interest. Thus, the extension of the
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Acknowledgements
We thank Spanish MINECO for funding (Grants CTQ2013-
42806-R and CTQ2016-79826-R) and the MECD for a FPU
fellowship to N. C.-L.
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10.1002/chem.201704401
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Title
The first Fe-catalyzed borylative cyclization reaction is reported. The process
affords one C-C and one C-B bond and shows a wide scope, allowing the formation
of carbo- and heterocycles containing a boryl unit that can be further functionalized.
The reaction takes place in smooth conditions, with inexpensive catalytic system
and full atom economy. Both aryl and alkyl substituted alkynes are reactive.
Mechanistic studies suggest the intermediacy of Fe(II)-hydride active catalyst
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