Iron-Catalyzed Regiodivergent Hydrostannation of Alkynes: Intermediacy of Fe(IV)-H versus Fe(II)-Vinylidene

Article abstract of DOI:10.1021/jacs.0c11448

We report an iron system, Cp*Fe(1,2-R2PC6H4X), which controls the Markovnikov and anti-Markovnikov hydrostannation of alkynes by tuning the ionic metal-heteroatom bonds (Fe-X) reactivity. The sequential addition of nBu3SnH to the iron-amido catalyst (1, X = HN-, R = Ph) affords a distannyl Fe(IV)-H species responsible for syn-addition of the Sn-H bond across the CC bond to produce branched α-vinylstannanes. Activation of the C(sp)-H bond of alkynes by an iron-aryloxide catalyst (2, X = O-, R = Cy) affords an iron(II) vinylidene intermediate, allowing for gem-addition of the Sn-H to the terminal-carbon producing β-vinylstannanes. These catalytic reactions exhibit excellent regioselectivity and broad functional group compatibility and enable the large-scale synthesis of diverse vinylstannanes. Many new reactions have been established based on such a synthetic Fe-X platform to demonstrate that the initial step of the catalysis is conveniently controlled by the activation of either the tin hydride or the alkyne substrate.

Full text of DOI:10.1021/jacs.0c11448

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Article
Iron-Catalyzed Regiodivergent Hydrostannation of Alkynes:
Intermediacy of Fe(IV)−H versus Fe(II)−Vinylidene
Jianguo Liu, Heng Song, Tianlin Wang, Jiong Jia, Qing-Xiao Tong,* Chen-Ho Tung,
and Wenguang Wang*
Cite This: J. Am. Chem. Soc. 2021, 143, 409−419
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ABSTRACT: We report an iron system, Cp*Fe(1,2-R2PC6H4X),
which controls the Markovnikov and anti-Markovnikov hydro-
stannation of alkynes by tuning the ionic metal−heteroatom bonds
(Fe−X) reactivity. The sequential addition of nBu3SnH to the iron−
amido catalyst (1, X = HN−, R = Ph) affords a distannyl Fe(IV)−H
species responsible for syn-addition of the Sn−H bond across the
CC bond to produce branched α-vinylstannanes. Activation of the
C(sp)−H bond of alkynes by an iron−aryloxide catalyst (2, X = O−,
R = Cy) affords an iron(II) vinylidene intermediate, allowing for
gem-addition of the Sn−H to the terminal-carbon producing β-
vinylstannanes. These catalytic reactions exhibit excellent regiose-
lectivity and broad functional group compatibility and enable the
large-scale synthesis of diverse vinylstannanes. Many new reactions have been established based on such a synthetic Fe−X platform
to demonstrate that the initial step of the catalysis is conveniently controlled by the activation of either the tin hydride or the alkyne
substrate.
implemented general Markovnikov hydrostannation of both
aromatic and aliphatic 1-alkynes since the first report on the α-
stannylation of simple aliphatic alkynes with nBu2SnIH.23
Compared to B−H and Si−H bonds, the Sn−H bond is
usually more likely to undergo homolytic cleavage, generating
R3Sn· and H· radicals. This complicates the outcome of the
addition of a tin hydride to alkynes. Prior studies focused on
regioselective hydrostannation using Pd,24 Mo,25 Ru,26 Cu,21,27
and Mg catalysts.28 However, the selective production of α- or
β-vinylstannanes was often sensitive to steric or electronic
factors. Although the oxidative addition of R3Sn−H (R = nBu3,
Me, and Ph) was proposed for the Pd(0)-catalyzed hydro-
stannation,29 the hydrostannation mechanism is probably the
least understood metalloid−hydride addition to alkynes.22a
Exploring fundamental reactions between the transition-metal
complex and R3Sn−H facilitates the development of general
catalytic methods to control the reaction selectivity initially.
Recently, it has been increasingly clear that the outcome of the
reaction can be potentially controlled by using a well-defined
INTRODUCTION
■
Transition-metal complexes featuring ionic metal−heteroatom
bonds (M−X),1 for example, imides,2,3 amides,4−8 thio-
lates,9−11 and alkoxides,12−14 are of particular interest in the
exploration of metal−ligand cooperative catalysis. These M−X
bonds are often highly polarized, allowing them to interact
with either reagents or substrates in homogeneous catalysis or
enzyme-catalyzed synthesis.15−17 In particular, metal-ligated
strategies of this type promote the application of iron18 to
process transformations under mild conditions. The chemistry
involving the Fe−X bond provides not only mechanistic insight
into the catalysis but also an opportunity to control the
reaction selectivity and expand the catalytic strategy. Iron-
catalyzed hydrometalation is emerging as an active field, and
the Fe−X cooperative activation of B−H and Si−H bonds has
attracted rapidly growing attention.19 In contrast, although
hydrostannanes are useful metalloid hydrides, activation of the
Sn−H bond by a synthetic iron system leading to hydro-
stannation has not been reported to date.
Vinylstannanes are practically useful synthetic intermediates
that can couple with a wide range of electrophiles.20
Stannylmetalation of 1-alkynes with an R3Sn−M reagent (M
= Sn, Si, B, Al, Mg) can reveal vinylstannanes upon subsequent
protonolysis.21 Hydrostannation of alkynes with R3Sn−H is
the most straightforward and atom-economic route to
vinylstannanes.22 However, the control of R3Sn−H addition
across alkynes is challenging. In particular, there is no
catalyst, to tune the Sn−H bond activation mode. Fu
̈
rstner et
Received: October 31, 2020
Published: December 29, 2020
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al. reported the [Cp*RuCl]-catalyzed hydrostannation of
functionalized internal alkynes26b and proposed that this
catalysis involved a σ-(H−SnnBu3) intermediate, and the
interaction between the chloride and protic functionality of the
substrate dictates the regioselective hydrostannation. Mankad
et al. reported cooperative Sn−H activation by heterobimetal-
lic (NHC)Cu−[MCO] systems ([MCO] = CpFe(CO)2 or
Mn(CO)5) to generate Fe−H/Cu−SnnBu3 or Cu−H/Mn−
SnnBu3 pairs that control the regioselective hydrostannation of
aliphatic 1-alkynes (Scheme 1a).30 Oestreich and co-workers
iron(II)−amido complex for the Markovnikov hydrostannation
of alkynes and the activation of 1-alkyne by iron(II)−aryloxide
for the anti-Markovnikov hydrostannation (Scheme 1b). Many
new reactions have been established based on such a synthetic
Fe−X platform to demonstrate the rational activating of the tin
hydride and the alkyne for controlling the reaction selectivity.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Cp*Fe(1,2-
R2PC6H4X). Following the recently described procedure,11
complexes 1 and 2 were prepared by treating [Cp*Fe-
(NCMe)3]PF6 in MeCN with a THF solution of 1,2-
Ph2PC6H4NHLi and 1,2-Cy2PC6H4ONa, respectively. The
1H NMR spectra of 1 and 2 in C6D6 show broadened and
paramagnetically shifted signals between −64 and 37 ppm. The
magnetic moments of 1 and 2 in solution were determined to
be 2.52 and 3.21 μB, respectively, consistent with that reported
for the intermediate-spin iron(II) complexes such as [Cp*Fe-
(1,2-Ph2PC6H4S)]2(μ-N2) and Cp*Fe(NHC)Cl.11,33 The
solid-state structures of 1 and 2 were confirmed by
crystallographic analysis (Figure 1), which shows they are
neutral five-coordination iron(II) half-sandwich compounds
with a similar Cp*Fe(1,2-R2PC6H4X) framework.
Scheme 1. Cooperative Sn−H Activation for Alkyne
Hydrostannation
Activation of nBu3SnH by 1. Changing the basic site (X)
from amido to oxide and modifying the coordination sphere
leads to Cp*Fe(1,2-R2PC6H4X) with very different reactivity.
Reflecting by the reactions with nBu3SnH, 1 can activate the
Sn−H bond by stepwise reactions with two molecules of the
tin hydride (Scheme 2), whereas 2 is inert toward the Sn−H
bond activation.
Scheme 2. Stepwise Activation of nBu3SnH by Complex 1
reported that the cooperative Ru−S reactivity of the Ohki−
Tatsumi complexes enables the Sn−H bond to undergo
heterolytic cleavage affording a stannylium ion-like tin
electrophile together with Ru(II)−H species, which achieves
the dehydrogenative stannylation of 1-alkynes.31
We planned to study the activating nBu3SnH reagent and
the substrate, RCC−H,32 by tuning the Lewis acid−base
coordination sphere interactions in the half-sandwich iron(II)
system of Cp*Fe(1,2-R2PC6H4X). The aim was to control the
regioselectivity of catalytic alkyne hydrostannation. We
describe here the selective activation of nBu3SnH by an
Adding 1 equiv of nBu3SnH to 1 in C6D6 caused the orange
solution to turn deep red in a few minutes. A phosphorus
resonance at δ 63.4 (J119
= 636 Hz, J117
= 608 Hz)
Sn−P
Sn−P
appeared in the 31P NMR spectrum of the solution, together
with a stannyl signal at δ 228.8 (d, JP−Sn = 636 Hz) in the 119Sn
NMR spectrum. No hydride signal was observed in the H
1
Figure 1. Solid-state structures of 1 (left) and 2 (right) with 50% probability thermal ellipsoids. For clarity, hydrogen atoms are omitted, and the
two phenyl and cyclohexyl groups bonded at the phosphorus atom are drawn as lines. Selected bond distances (Å) and angles (deg): for 1, Fe−P
2.203(1), Fe−N 1.842(3), P−Fe−N 84.6(1); for 2, Fe−P 2.2409(6), Fe−O 1.900(1), P−Fe−O 86.94(5).
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Figure 2. Structures of 3 (left) and 4a (right) with 50% probability thermal ellipsoids. Selected bond distances (Å) and angles (deg): for 3 Fe−P
2.227(1), Fe−Sn 2.5002(6), N1−Sn 2.099(4), N2−N3 1.099(5), P−Fe−Sn 83.78(3), P−Fe−N2 89.9(1); for 4a Fe−P 2.218 (2), Fe−Sn1
2.540(1), Fe−Sn2 2.666(1), Fe−H 1.28(5), Sn2−H 2.19(5), N−Sn1 2.108(5), P−Fe−Sn1 83.60(5), P−Fe−Sn2 104.52(5), Sn1−Fe−Sn2,
80.44(3).
NMR spectrum. Crystallographic analysis revealed the
structure of an stannyl iron(II) dinitrogen compound,
Cp*Fe(N2)[1,2-Ph2PC6H4NH(nSnBu2)] (3, Figure 2). Ac-
cording to this structure, it appears that nBu3SnH was activated
by eliminating a butane molecule, and the rest of the SnnBu2
motif was inserted into the Fe−N bond, forming an Fe−Sn
bond. The production of butane gas was confirmed by GC
analysis (Supporting Information, Figure S2). The N2−N3
separation is 1.099(5) Å, very close to 1.0977 Å for a free N2
molecule.11a In the IR spectrum of a toluene solution of 3, the
νN≡N band was observed at 2064 cm−1 (Figure S19). These
results suggest that the N2 is weakly coordinated with the iron
center and is replaceable. Indeed, after exposure of a Et2O
solution of 3 to a CO atmosphere, the dinitrogen ligand is
1
Figure 3. Variable-temperature H NMR spectra of 4a in d8-toluene.
substituted by CO, and this is accompanied by the appearance
of a νCO band at 1906 cm−1 in the IR spectrum (Figure S25).
H bond distance in 4a is 2.19(5) Å, comparable to that found
in Cp(CO)FeH(SnEt3)(GeEt3) (dSn−H = 2.16 (13) Å).35d In
the related hydrido iron(IV) complexes, the Fe(IV)−H bond
lengths are between 1.24 and 1.72 Å.38 The Fe−H bond length
of 1.28(5) Å agrees with the assignment of an iron(IV) species,
and the involvement of the η2-coordination mode of the H−Sn
bond can be negligible.
The production of Cp*Fe(CO)[1,2-Ph2PC6H4NH(SnnBu2)]
was fully characterized by X-ray, NMR (1H, 31P, 119Sn, and
13C), and ESI-MS spectroscopy (Supporting Information).
Cp(CO)2FeSnR3 (R = Bu, Ph) has been reported to be
formed by the reaction of [CpFe(CO)2]Na with ClSnR3.34
Compounds of this sort can further react with HSiR3, HSnR3,
or HGeR3 upon photolysis, producing the iron(IV) hydride
containing two group 14 element ligands.35 These reactive
features have attracted considerable attention to the photo-
chemistry of Cp2Fe2(CO)4 or CpFe(CO)2L with metalloid
hydrides for the dehydrogenative dimerization of tBu2SnH2
and hydrosilanes.36 We found that 3 reacts readily with
nBu3SnH to afford a distannyl iron(IV) hydride complex (4a).
Treatment of 3 in d8-toluene with nBu3SnH caused the red
solution to turn brown. The 119Sn NMR spectrum of 4a
displays two sets of doublets, one at δ 195.8 (JP−Sn = 634 Hz)
for the N-stabilized nBu2Sn moiety and the other at δ 54.5
When coordinating the E−H bond to a metal center,
forming a σ-complex is usually prior to the bond oxidative
addition.39 Although complex 4a features the structural
characteristics of an iron(IV) hydride, at low temperatures it
is converted reversibly to the nonclassical σ-stannane complex
(4b), with which it maintains a tautomeric equilibrium
(Scheme 3). Upon cooling a d8-toluene solution of 4a to
1
233 K, the H NMR spectrum indicates that the solution
contains two hydride species. In addition to the signal
displayed at δ −12.3 (d, JP−H = 68 Hz) for 4a, a new hydride
signal appeared at δ −11.2 (d, JP−H = 53 Hz) for 4b (Figure 3),
corresponding to the 31P signal at δ 88.2. At 213 K, this new
signal is well-resolved as a sharp doublet flanked by a tin
1
(JP−Sn = 126 Hz) for the nBu3Sn group. In the H NMR
spectrum, a characteristic hydride signal was observed at δ
−12.3 (d, JP−H = 68 Hz). The hydride resonance is flanked by
tin satellites (Figure 3). The 31P resonance is at δ 65.5 (s), only
2 ppm downfield from the parent complex (3).
Scheme 3. Interconversions between the Iron(IV) Hydride
and the σ-Complex
Combined with the NMR spectra, crystallographic analysis
established the structure of 4a as a neutral complex with the
formula Cp*Fe(H)(SnnBu3)[1,2-Ph2PC6H4NH(SnnBu2)]
(Figure 2). The Fe−Sn(nBu3) bond length of 2.666(1) Å is
about 0.12 Å longer than the 2.540(1) Å observed in Fe−
Sn(nBu2). For comparison, the addition of nBu3SnH to 3
causes the Fe−Sn(nBu2) bond to increase only by 0.04 Å.
These Fe−Sn bond distances all fall in the range of 2.50−2.67
Å reported for the stannyliron(IV) complexes.37 The position
of the hydride ligand was located and refined. The (nBu3)Sn−
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a
satellite (JSn−H = 330 Hz), while the 119Sn resonances were
observed at δ 203.4 (d, JP−Sn = 624 Hz, SnnBu2) and 61.2 (d,
JP−Sn = 132 Hz, σ-H−SnnBu3). The pattern of the hydride
resonance of 4b is very similar to that of the σ-stannane
complex [Cp*RuCl(iPr3P)(η2-H−SnBu3)],26b and the JSn−H
coupling constants lie between 90 and 440 Hz.26b,40
Consequently, it was inferred that a η2-coordinated hydro-
stannane existed at the iron center. After the NMR solution
was warmed to room temperature, only 4a was detected,
implying the completion of the oxidative addition of the Sn−H
bond. The free energy change for the reversible Sn−H bond
cleavage at the iron center was estimated to be −0.46 kcal/mol
at 233 K based on the equilibrium ratio of 4a/4b = 1:2.7.
Complex 4a can hydrostannate alkynes. For instance, the
reaction of 4a with an equimolar amount of phenylacetylene
(5a) in C6D6 completes within several minutes, and 31P NMR
spectroscopic studies suggested the full conversion of 4a to 3.
The organotin product, α-tributyl(1-phenylvinyl)stannane
Table 1. Fe-Catalyzed α-Hydrostannation of Alkynes
1
(6a), formed in 94% yield, was isolated and identified by H
NMR spectroscopy (eq 1). More importantly, both 1 and 3 are
efficient catalysts in the α-hydrostannation of 1-alkynes. Upon
loading 2 mol % of 3 with 1.1 equiv of nBu3SnH in toluene, the
reaction of 5a exclusively produced 6a in 92% isolated yield
(eq 2) within 15 min. Using 1 as a catalyst, the reaction of 5a
under identical conditions also led to fast α-hydrostannation
producing the products with 96% yields.
Catalytic Markovnikov Hydrostannation of Alkynes.
Since the sequential conversions of 1 to 4 through 3 by excess
nBu3SnH are fast, we chose 1 as the catalyst precursor to
examine the alkyne scope. As shown in Table 1, aromatic
alkynes with either electron-withdrawing or electron-donating
substituents at various positions in the aromatic ring of
phenylacetylene allow for hydrostannation under the catalytic
conditions, providing α-vinylstannanes in good yields and with
high regio- and chemoselectivity. In addition to halogen groups
(6d, 6e, 6k, 6m, and 6n), functional groups including −NH2
(6j), −COOMe (6l), and −OH groups (6o) are tolerated well
in the reaction. 4-Ethynyl-N,N-dimethylaniline, which was
difficult to undergo hydrostannation in the trityl-cation-
mediated hydrostannation,41 is efficiently converted to the α-
vinylstannane (6c) in 97% yield by this iron-catalyzed
protocol. In the reaction of p-trifluorophenylacetylene, the α-
product 6g was obtained as a major product with a small
amount of inseparable β-product (α:β = 96:4). Such
regioselective hydrostannation was also found in the case of
1-ethynyl-4-nitrobenzene and 4-ethynylbenzaldehyde, which
provided the corresponding α-vinylstannanes (6h and 6i,
respectively) as major products. Selective hydrometalation of
alkynes containing −CHO and −NO2 groups are extremely
challenging since they are prone to reduction. Both of the
entities were retained, however, in this transformation. 1-
Alkynes functionalized by naphthyl (6p), phenanthryl (6q),
thienyl (6r, 6s), and ferrocenyl (6t) groups underwent
a
Reaction conditions: catalyst 1 (4 μmol, 2 mol %) and nBu3SnH
(0.22 mmol) were dissolved in toluene (0.3 mL), and the alkyne
substrate (0.2 mmol) in 0.2 mL toluene was added dropwise and
stirred for 15 min at room temperature. Isolated yields are given.
b
1
The α/β ratios given in parentheses were determined by H NMR
spectra of the isolated mixtures.
hydrostannation smoothly, giving the α-vinylstannane products
in good yields.
Remarkably, the present iron-catalyzed protocol can also be
used for the transformation of aliphatic alkynes. The reactions
of linear chain alkynes, 1-hexyne and 1-decyne, produced the
aliphatic α-vinylstannanes 6u and 6v in 90% and 85% yield,
respectively. Hydrostannation of 3-phenyl-1-propyne furnished
the α-product (6w) in 89% yield. Aliphatic alkynes with
functional groups such as benzyl ether (6x), iodophenol ether
(6y), and indolyl (6z) were generally well tolerated. The
hydrostannation was also achieved in the presence of an
unprotected alcohol group (6aa). The regioselective Sn−H
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addition was not impeded by the steric hindrance of the
substrate, as exemplified by branched alkynes (6ab−6ae). The
CC triple bond was chemo- and regioselectively hydro-
metalated for a conjugated enyne to afford the organotin-
functionalized diene (6ab) in 94% yield. Importantly, cyclo-
propylvinylstanane (6ac) was obtained in 96% yield in the
reaction with cyclopropylacetylene, and no ring-opening
product was detected. In the case of [Fe−Cu]-catalyzed
hydrostannation, the reaction of 3,3-dimethylbut-1-yne was
found to give (E)-β-vinylstannane in low yield.30 In contrast,
the present iron-based transformation exclusively provides α-
vinylstannanes (6ae).
Addition of nBu3SnH to α,β-unsaturated alkynes by using
catalyst 1 was also examined. Although the production of
vinylstannane was not observed from 1-phenylprop-1-yne and
diphenylethyne, the expected iron-catalyzed Sn−H addition
proceeded successfully with alkynyl ketones and esters, a
generally reliable substrate class (Table 2). In the presence of 2
spectral analysis of the reaction solutions features a peak at m/
z 570.1996 for 7 and m/z 582.2704 for 8, indicative of
incorporation of the alkyne into the corresponding parent
complex (Figures S36 and S41). The aminovinyl and
vinylidene moieties in the resulting complexes were inferred
from the 1H and 13C NMR spectra. The α-vinyl carbon
resonance in 7 was observed as a doublet at δ 196.3 (JP−C = 25
Hz), compared to the doublet at δ 357.3 (JP−C = 38 Hz) for
the α-vinylidene carbon in 8. In the 1H NMR spectra of 8, the
vinyl proton signal was displayed as a doublet at δ 5.62 (d, JP−H
= 5.1 Hz), consistent with the patterns that have been reported
for transition-metal vinylidene complexes.45 In contrast, the
aminovinyl proton signal of 7 at δ 7.31 is obscured by the
1
distinct phenyl proton signals in the H NMR spectrum but
2
was unambiguously observed in the H NMR spectrum of 1
with PhCCD (d-5a, vide infra).
The synergistic Fe−X bond reactivity was further demon-
strated by the deuterated 1-alkyne activation. For the reaction
of 1 with d-5a, deuterium incorporation was observed at the
positions of the amino site (δ 3.05, 51% d-enriched) and β-
vinyl carbon (δ 7.31, 49% d-enriched), in almost a 1:1 ratio
a
Table 2. Hydrostannation of α,β-Unsaturated Alkynes
2
(Figure S37). In the reaction with 2, the H NMR spectrum
2
exhibits only a characteristic vinyl H signal at δ 5.68 (Figure
S42). According to the NMR results, it is proposed that
activation of the C(sp)−H bond at the Fe−X site results in the
formation of the iron−alkynyl intermediate Cp*Fe(C
CPh)(1,2-R2PC6H4XH), which then rearranges to an iron(II)
vinylidene complex. In contrast to the aryloxide in 8, the amino
group of 7 not only serves as a Lewis base to facilitate proton
shuttling but also behaves as a nucleophile which attacks the
vinylidene α-carbon resulting in an intramolecular N−C bond
formation.
a
Reaction conditions were identical to those described in Table 1;
reaction time was 1 h; and isolated yields are provided.
The solid-state structures of 7 and 8 were confirmed by
crystallographic analysis (Figure 4). In 7, the N atom is
attached to the α-vinyl carbon (dN−C29 = 1.477(2) Å), forming
a genuine three-membered Fe−N−C metallic-heterocycle, but
the phenyl vinylidene ligand in 8 is solely coordinated to the
Fe center as an unsaturated carbene with ∠Fe−C29−C30 =
170.3(4)°. The Fe−C29 distances in the two complexes differ
significantly and are 1.851(2) Å for 7 vs 1.737(4) Å for 8. For
complex 8, especially, the Fe−C29 bond length is in the range
of 1.68−1.77 Å reported for the vinylidene complexes,45 and
the C29−C30 bond length of 1.321(6) Å suggests a double
bond character.
As a consequence of the striking structural differences, the
reactive nature of the two organoiron species (7 and 8) toward
nBu3SnH differs greatly. The iron vinyl complex (7) was found
to be stable toward the tin hydride, and no reaction was
observed (eq 3). In contrast, the stoichiometric reaction
between the vinylidene compound (8) and nBu3SnH produced
(E)-tributyl(styryl)stannane (E-9a, >98%) which was identi-
mol % of 1, the reaction of 4-phenylbut-3-yn-2-one with
nBu3SnH at room temperature for 1 h afforded the α,β-
unsaturated organotin compound Z-6af in 76% yield with
excellent regioselectivity and a high degree of diastereoselec-
tivity. Similar anti-hydrostannation was also observed for
alkynoates represented by Z-6ag and Z-6ah, providing the (Z)-
α-stannyl enoate products in moderate yields.
Activation of 1-Alkynes. Activating terminal alkynes by a
transition-metal complex potentially provides access to metal
alkynyl,42 vinylidene,43 or vinyl intermediates,44 which are
widely involved in alkyne functionalization reactions. We
found that both the iron−amido and iron−aryloxide complexes
are capable of activating terminal alkynes, affording iron vinyl
(7) and iron vinylidene (8) intermediates, respectively
(Scheme 4).
Monitored by 31P NMR spectra, the reaction of 1 or 2 in
toluene with 5a led to the clean formation of the
corresponding low-spin iron(II) compounds 7 and 8
respectively after 12 h (Figures S34 and S39). The ESI-MS
Scheme 4. Activation of Phenylacetylene by 1 and 2
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Figure 4. Solid-state structures of 7 (left) and 8 (right). Selected bond distances (Å) and angles (deg): for 7, Fe−P 2.1620(5), Fe−N 2.015(1),
Fe−C29 1.851(2), N−C29 1.477(2), C29−C30 1.335(3), P−Fe−N 84.41(5), C29−Fe−N 44.66(7), Fe−C29−C30 162.7(1); for 8, Fe−P
2.240(1), Fe−O 1.973(3), Fe−C29 1.737(4), C29−C30 1.321(6), P−Fe−O 81.97(9), Fe−C29−C30, 170.3(4).
stereoselectivity and is exemplified by reactions of secondary
and tertiary alkyl alkynes to produce the (E)-isomers as the
major products (9q, 9r). Conjugated enynes are compatible
with the hydrostannation conditions and afford the desired
vinyl-functionalized diene (9s) in 96% yield and excellent
regio- and stereoselectivity. Internal alkynes did not react with
tin hydride under these conditions, consistent with the
observations that the catalysis proceeds via the iron vinylidene
intermediate.
To demonstrate the synthetic utility of the iron-catalyzed
protocol, a 1-alkyne substrate derived from estrone (11) was
subjected to the two reaction conditions by iron−amido and
iron−aryloxide catalysis. Divergent vinylstannanes 12a (495
mg, 87% yield) and 12b (461 mg, 81% yield) were
conveniently synthesized from the scaled-up reactions without
significant loss in yield (Scheme 5). Although terminal
vinylmetal analogues of 11 have been reported by functionaliz-
ing complex organic precusrors,46 the present transformation
provides a general method of synthesizing both α- and β-
vinylstannanes.
1
fied by its H NMR spectrum (eq 4). The ESI-MS spectrum
showed a peak at m/z 480.2206 that indicates complex 2.
Further treatment of the reaction solution with 5a led to the
recovery of 8. The generation of 2 was also trapped by 1-
ethynylferrocene to form a new iron vinylidene complex (10),
which was also structurally characterized by single-crystal X-ray
crystallographic analysis (Figure S4).
Anti-Markovnikov Hydrostannation of Alkynes. Based
on the activation of 1-alkynes and the fundamental conversions
between the iron aryloxide complexes 8 and 2, we achieved the
catalytic anti-Markovnikov hydrostannation of 1-alkynes (eq
5). Using 2 mol % of 2 (or 8) as catalyst, the reaction of 5a
(0.2 mmol) with nBu3SnH (0.22 mmol) for 12 h provided E-
9a in high yield and with excellent regio- and stereoselectivity
(97% yield, β/α > 99:1, E/Z > 99:1). Several 1-alkynes bearing
different functional groups were subjected to the reaction
catalyzed by 2 (Table 3).
Generally, most of the aryl-substituted 1-alkynes examined
in the iron(II)−amido catalysis react smoothly with nBu3SnH
by this iron−aryloxide protocol to afford (E)-β-vinylstannanes
(9a−9m) as the major product. Modifying the substitution on
the aromatic ring with a variety of electron-donating and
electron-withdrawing groups, such as OMe (9b), Me (9g and
9j), NMe2 (9c), F (9e and 9k), Cl (9i), Br (9l), and CF3 (9f),
does not affect the outcome of the reaction much. Notably,
unprotected hydroxyl (9d) and amino (9h) groups were
tolerated in the β-hydrostannation. The reaction of 2-
ethynylnaphthalene provided β-vinylstannanes composed of
two stereoisomers with E/Z in almost a 1:1 ratio. This iron−
aryloxide protocol was also applicable to aliphatic 1-alkynes
with high regioselectivity. Hydrostannation of linear chain
alkynes such as 1-hexyne, 1-decyne, and a benzyl ether
substituted 1-pentyne furnished the β-vinylstannanes 9n, 9o,
and 9p in 86%, 90%, and 85% yield, respectively. Steric
hindrance has no effect on regioselectivity but affects the
Mechanism. To get more insight into the regiodivergent
hydrostannation, deuterium labeling in the hydrostannation of
5a was investigated (Scheme 6a). Using the deuterated
hydrostannane reagent nBu3SnD, the reaction of 5a catalyzed
by 1 led to d-6a with deuterium incorporation at a cis-position
relative to the SnnBu3 moiety. In comparison with 5a, the
reaction of the deuterated alkyne d-5a with nBu3SnH afforded
α-vinylstannane (d-6a′) with deuterium at the trans-position.
These results suggest the iron−amido-catalyzed α-hydro-
stannation proceeds through the syn-addition of the tin
hydride across the CC bond (Figure S5). By contrast, in
the hydrostannation of 5a with nBu3SnD catalyzed by complex
2, the nBu3Sn moiety and D atom were both added to the β-
carbon to give the gem-product d-(E)-9a. This gem-addition
was further confirmed by the reaction of d-5a with nBu3SnH to
form d-(E)-9a′, in which the hydrogen atom at the α-position
is labeled (Figures S6 and S7).
Intermolecular kinetic isotope effects (KIE) experiments
were also performed using nBu3SnH vs nBu3SnD to reduce 5a
and nBu3SnH for the hydrostannation of 5a vs d-5a (Figures
S8−S11). In the iron−amido catalysis, reducing 5a with an
equimolar amount of nBu3SnH and nBu3SnD produces 6a and
d-6a in a 2:1 ratio, giving a KIE value of 2.0. There is no
isotopic H/D competition between 5a and d-5a in the reaction
with nBu3SnH since 6a and d-6a′ were produced in almost a
1:1 ratio. In the iron−aryloxide catalysis, by contrast, the
intermolecular competitions for the 5a/d-5a pair (kH/kD = 2.4)
and nBu3SnH/nBu3SnD pair (kH/kD = 2.1) show mechanis-
tically significant kinetic isotope effects. These results suggest
that the cleavage of Sn−H is involved in the turnover-
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a
Table 3. Fe-Catalyzed β-Hydrostannation of Alkynes
a
Reaction conditions: catalyst 2 (4 μmol, 2 mol %), alkynes (0.2 mmol), and nBu3SnH (0.22 mmol) were dissolved in toluene (0.5 mL) and stirred
at room temperature. Isolated yields are provided. The α/β and E/Z isomers ratios were determined by 1H NMR spectra of the isolated mixtures.
b
c
12 h for aryl-substituted alkynes. 24 h for alkyl-substituted alkynes.
activation of nBu3SnH is much faster than that of the 1-alkyne.
The activation of the first nBu3SnH is probably through an
intermediate of the iron−stannane adduct [1-HSnnBu3] in
which the Sn−H bond is captured at the Fe−N unit
(Supporting Information). Due to the strong nucleophilic
and anionic nature, the amido site can tightly bind the Sn
center, increasing the electron density of stannane and
significantly changing its chemical property. Such an iron−
stannane adduct seems to favor eliminating butane, leading to
the stannyl−iron compound (3). By oxidative addition of the
second stannane to the iron−stannyl species, distannyl
Fe(IV)−H (4a) is formed as a key intermediate for the
hydrometalation reaction. Such iron-based hydrostannation is
proposed through a pathway of alkyne insertion into the Fe−
SnnBu3 bond to process the stannyl moiety at the internal
carbon, producing an β-stannylvinyl iron hydride (Int 1).24,29
Reductive elimination of the vinylstannane product from Int 1
results in regeneration of 3 and completes the cycle. Given that
highly selective syn-addition products are observed from
deuterium-labeling experiments, the outer sphere concerted
mechanism delivering the hydride and the nBu3Sn moiety from
distannyl Fe(IV)−H to the CC bond cannot be ruled out.
In the β-hydrostannation of 1-alkynes, the formation of the
iron vinylidene intermediate (8) is essential to the gem-
addition.47 It is well-known that the α-carbon atom of the
Scheme 5. Controlled Regioselective Hyrostannation of the
Estrone Derivative
determining step of the α-hydrostannation, while the activation
of the C(sp)−H bond of the alkyne and the Sn−H addition are
both involved in the turnover-limiting formation of the β-
vinylstannane.
From a mechanistic viewpoint, α-hydrostannation of 1-
alkynes processed by the iron−amido complex is initiated by
activation of the tin hydride while the β-hydrostannation
begins with the substrate activation (Scheme 6b). Although the
activation of nBu3SnH by 1 appears to compete with 1-alkyne,
the results of the stoichiometric reactions indicate that the
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Scheme 6. Mechanistic Insights into Regiodivergent Hydrostannation
vinylidene ligand is extremely reactive toward nucleophiles.43
The vinylidene ligand in 8 allows nucleophilic attack at the α-
carbon by the tin hydride. We propose that the oxide site
facilitates the Sn−H bond cleavage for the hydride transfer
through O → Sn interaction (Int 2) and could capture the
stannylium fragment to form the iron vinyl intermediate (Int
3). The subsequent transfer of the stannylium fragment to the
vinyl moiety produces the β-vinylstannanes and regenerates 2.
The steric factors in Int 2 more likely control the outcome’s
stereoselectivity because this, in principle, causes the formation
of the two E and Z stereoisomers. According to the crystal
structure of 8, the Ph group of the vinylidene is close to the O
donor. Upon coordination of nBu3SnH, the vinylidene ligand
undergoes isomerization to reduce steric repulsion between the
Ph group and O−SnnBu3 moiety.
form a new stannyliron species, which, in turn, enables the
oxidative addition of the second stannane. Although the iron−
aryloxide is inactive for the Sn−H bond cleavage, it can gently
activate 1-alkyne through metal−ligand cooperation.
To achieve such iron-based cooperative catalysis, the
chemistry at an Fe−X site and the reactivity of the resultant
organoiron species are of obvious significance. Although
stoichiometric transformations of alkynes to vinylidene
complexes have been known for several decades,43 activations
of 1-alkynes by iron complexes to afford vinylidene species for
catalysis have been only slightly reported.32 The present
catalytically regenerated iron vinylidene intermediate might
support exploring other important transformations involving
alkyne functionalization, and these could expand the scope of
iron catalysis in organic synthesis.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.0c11448.
CONCLUSION
■
■
sı
In summary, we report a new type of Cp*Fe(1,2-R2PC6H4X)
cooperative catalyst for regiodivergent hydrostannation of
alkynes by tuning the Fe−X reactivity. We have demonstrated
that the initial step of the catalysis is conveniently controlled
by activation of either the tin hydride or the 1-alkyne. The α-
hydrostannation originates from the fast Sn−H activation by
the iron−amido catalyst to afford the distannyl Fe(IV)−H
intermediate. In contrast, the β-hydrostannation proceeding by
the iron−aryloxide catalyst arises from transforming RCCH
into the Fe−vinylidene intermediate. The critical feature that
distinguishes the iron−amido complex from the iron−
aryloxide is that the amido nitrogen can strongly bind the
first Sn center, resulting in reductive elimination of butane to
Experimental details, characterization, crystallographic
1
data, and H, 13C, 31P, 19F, and 119Sn NMR spectra
(PDF)
Crystallographic CIF data (CIF)
AUTHOR INFORMATION
Corresponding Authors
■
Wenguang Wang − School of Chemistry and Chemical
Engineering, Shandong University, Jinan 250100, China;
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Article
Ligands with Flexible Bonding Modes and Reaction Profiles. Chem.
Soc. Rev. 2017, 46, 2913−2940.
College of Chemistry, Beijing Normal University, Beijing
100875, China; orcid.org/0000-0002-4108-7865;
Email: wwg@sdu.edu.cn
Qing-Xiao Tong − Department of Chemistry, Shantou
University, Shantou 515063, China; orcid.org/0000-
0002-8125-9684; Email: qxtong@stu.edu.cn
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Authors
Jianguo Liu − School of Chemistry and Chemical Engineering,
Shandong University, Jinan 250100, China
Heng Song − School of Chemistry and Chemical Engineering,
Shandong University, Jinan 250100, China
Tianlin Wang − School of Chemistry and Chemical
Engineering, Shandong University, Jinan 250100, China
Jiong Jia − School of Chemistry and Chemical Engineering,
Shandong University, Jinan 250100, China
Chen-Ho Tung − School of Chemistry and Chemical
Engineering, Shandong University, Jinan 250100, China;
orcid.org/0000-0001-9999-9755
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Sulfur Contributes to the Function of the Non-Heme Iron Enzyme
Superoxide Reductase. Acc. Chem. Res. 2007, 40, 501−509.
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Cooperative Catalysis at Metal−Sulfur Bonds. Acc. Chem. Res. 2017,
50, 1258−1269.
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Iron-Catalyzed 1,2-Selective Hydroboration of N-Heteroarenes. J. Am.
Chem. Soc. 2017, 139, 17775−17778. (b) Song, H.; Ye, K.; Geng, P.;
Han, X.; Liao, R.; Tung, C.-H.; Wang, W. Activation of Epoxides by a
Cooperative Iron−Thiolate Catalyst: Intermediacy of Ferrous
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Tung, C.-H.; Wang, W. Cooperative Molybdenum-Thiolate Reactivity
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Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c11448
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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■
We are thankful for the financial support from NSFC
(22022102, 22071010, and 21871166), Natural Science
Foundation of Shandong Province (ZR2019ZD45), and
Pearl River Scholar Funded Scheme 2019 (GDUPS2019).
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