Selective [1,4]-Hydrovinylation of 1,3-Dienes with Unactivated Olefins Enabled by Iron Diimine Catalysts

Article abstract of DOI:10.1021/jacs.8b00245

The selective, intermolecular [1,4]-hydrovinylation of conjugated dienes with unactivated α-olefins catalyzed by α-diimine iron complexes is described. Value-added "skipped" diene products were obtained with exclusive [1,4]-selectivity, and the formation

Full text of DOI:10.1021/jacs.8b00245

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Article
Selective [1,4]-Hydrovinylation of 1,3-Dienes with
Unactivated Olefins Enabled by Iron Diimine Catalysts
Valerie A. Schmidt, C. Rose Kennedy, Máté J. Bezdek, and Paul J. Chirik
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b00245 • Publication Date (Web): 07 Feb 2018
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Selective [1,4]-Hydrovinylation of 1,3-Dienes with Unactivated Olefins
Enabled by Iron Diimine Catalysts
Valerie A. Schmidt,† C. Rose Kennedy, Máté J. Bezdek, Paul J. Chirik*
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Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
ABSTRACT:
The selective, intermolecular [1,4]-hydrovinylation of conjugated dienes with unactivated α-olefins catalyzed by α-diimine iron
complexes is described. Value-added “skipped” diene products were obtained with exclusive [1,4]-selectivity, and the formation of branched,
(Z)-olefin products was observed with no evidence for alkene isomerization. Mechanistic studies conducted with the well-defined, single-
component iron precatalyst (MesDI)Fe(COD) (MesDI =[2,4,6-Me3-C6H2-N=CMe]2); COD = 1,5-cyclooctadiene) provided insights into the
origin of the high selectivity. An iron diene complex was identified as the catalyst resting state, and one such isoprene complex, (iPrDI)Fe(h -
4
C5H8), was isolated and characterized. A combination of single crystal X-ray diffraction, Mößbauer spectroscopy, magnetic measurements,
and DFT calculations established that the complex is best described as a high-spin Fe(I) center (SFe = 3/2) engaged in antiferromagnetic
coupling to an α-diimine radical anion (SDI = –1/2), giving rise to the observed S = 1 ground state. Deuterium-labeling experiments and kinet-
ic analyses of the catalytic reaction provided support for a pathway involving oxidative cyclization of an alkene with the diene complex to gen-
erate an iron metallacycle. The observed selectivity can be understood in terms of competing steric interactions in the transition states for
oxidative cyclization and subsequent β-hydrogen elimination.
(Scheme 1B).14 Nickel15,16 and ruthenium17 catalysts with NHC or
INTRODUCTION
phosphorus-based ligands are also known to effect the homodimer-
ization of 1,3-dienes15 as well as the [1,2]-hydrovinylation of al-
kenes and 1,3-dienes.16,17
Ethylene and downstream linear a-olefins derived from petro-
chemical sources and biomass number among the most abundant
building blocks for synthesis and constitute core organic base stock
for many products of the chemical industry.1 The development of
vast shale gas reserves has increased domestic ethylene production
to record-setting levels with supply often exceeding manufacturing
polymerization capacity.2 Production of linear a-olefins, specifical-
ly 1-hexene and 1-octene, from selective oligomerization of eth-
ylene has also undergone rapid expansion.3 Catalytic methods that
transform these abundant alkenes into new hydrocarbon architec-
tures and other value-added products are therefore desirable.
Scheme 1. State-of-the-art methods for selective [1,4]-
hydrovinylation of 1,3-dienes rely on in situ catalyst activation.
A. Seminal Report (Hata, 1964)a
Fe(acac)2 (0.5 mol %)
+
+
+
AlEt3 (2 mol %)
PhMe, 30 °C
(~40 atm)
35% yield
olefin isomerization products
• poor chemoselectivity • limited to ethylene • high ethylene pressure required • active catalyst unknown
•
B. Current State of the Art: Hydrovinylation of Unsymmetric 1,3-Dienes
C1 regioselectivity with ethylene (RajanBabu and coworkers)b
Co(dppb)Cl2
(10 mol %)
4
4
Ph
Ph
Ph
Ph
P
P
Co
+
Hydrovinylation reactions are atom-economical methods for
upgrading commodity hydrocarbons.4,5 Structural complexity is
increased in a single step, and the resulting products offer differen-
tiated alkene functionality amenable to subsequent elaboration.5
Key challenges for catalyst development include the control of
chemo-, regio- and stereoselectivity. Examples of metal-catalyzed
reactions for the hydrovinylation of 1,3-dienes date to reports from
Hata6 and from Iwamoto and Yuguchi7 wherein treatment of buta-
AlMe3 (30 mol %)
Cl Cl
1
1
CH2Cl2/PhMe, 0 °C
>95% yield
(1 atm)
Co(dppb)Cl2
>95:5 C1:C4
C1, linear regioselectivity with α-olefins (Hilt and coworkers)c
PPh2
OAc
CoBr2 (5 mol %)
Ligand (5 mol %)
O
4
O
OAc
4
P
β
β
α
+
O
then, Zn (10 mol %)
ZnI2 (10 mol %)
CH2Cl2, rt
Ph
α
1
1
(1 equiv)
79% yield
90:10 C1:C4, 5:95 b:l
C4, branched regioselectivity with α-olefins (Hilt and coworkers)d
Ligand
1a
2a
diene ( ) with high pressures of ethylene ( ), iron salts, and or-
ganoaluminum activators produced low yields of isomeric coupling
products (Scheme 1A).8 Subsequent optimization studies identi-
fied more robust iron9 and cobalt10 compounds with
bis(phosphine) ligands to achieve the same transformation with
improved yield and shorter reaction times. Hilt,11 RajanBabu,12 and
others have independently advanced these reactions with cobalt
bis(phosphine) complexes to promote [1,4]-hydrovinylation of a
range of dienes with alkenes under mild conditions (Scheme 1B).
In some cases, ligand modification altered the sense of regioselec-
tivity with unsymmetric substrates (branched vs. linear, C1 vs. C4
C–C bond formation).11,13 High enantioselectivity has been
achieved for the hydrovinylation of substitutionally varied 1,3-
dienes with ethylene12a–e or acrylate esters.12f Separately, Ritter and
coworkers reported that pyridine-2-imine-supported iron complex-
es promoted linear-selective hydrovinylation with styrenes
Ph
Ph
4
4
nBu
β
α
P
P
nBu
Co(dppe)Br2 (2 mol %)
Ph
Ph
β
α
Co
+
ZnI2 (10 mol %)
Bu4NBH4 (2.5 mol %)
CH2Cl2, rt
Br Br
1
1
Co(dppe)Br2
(1–2 equiv)
84% yield
84:16 C4:C1
C4, linear regioselectivity with styrenes (Ritter and coworkers)e
4
4
H
N
α
α
(
PhMeAPAI)FeCl2 (2 mol %)
+
Fe Cl
Cl
N
β
Mg (4 mol %)
Et2O, rt
β
Ph
Ph
Ph
1
1
(1.1 equiv)
79% yield
84:16 C4:C1
• no access to C1, branched regioisomer • active catalysts unknown •
(
PhMeAPAI)FeCl2
a See ref 6. b See ref 12d. c See ref 11k. The branched (
from C–C bond formation at Cβ; the linear ( ) product arises from C–
C bond formation at Cα. d See ref 11b. e See ref 14.
b
) product arises
l
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Despite these advances, methods for the selective synthesis of insertion generates a diamagnetic iron allyl–alkyl metallacycle
1
2
3
4
5
6
7
8
the various “skipped” dienes (1,4-dienes) available from hydrovi-
nylation of commodity hydrocarbons remain underdeveloped.
Specifically, branched regioisomers arising from C–C bond for-
mation between the Cβ position of an α-olefin and the C1 position
of an unsymmetric diene are inaccessible using current methods.
Rational catalyst evolution and improvement has been limited by a
lack of mechanistic insight. State-of-the-art hydrovinylation meth-
ods (Scheme 1B) typically rely on the in situ activation of metal–
ligand combinations with reducing agents such as Mg, Zn,
NBu4BH4, or MAO (MAO = methylaluminoxane).11–14 RajanBabu
and coworkers have characterized cobalt(I) precatalysts that did
not require activation by a reductant.12f However, these complexes
(Scheme 2B, inset) that undergoes butadiene-induced reductive
elimination to liberate the vinylcyclobutane product.24 The metal-
lacycle, which has been characterized crystallographically, is best
described as a closed-shell Fe(II)–Fe(IV) hybrid with the redox
noninnocent pyridine(diimine) functioning as
a classical p-
acceptor.23,24 The differing ground-state electronic structures ob-
served experimentally for these two systems (alkene–alkene vs.
alkene–diene coupling) have been corroborated by CASPT2/DFT
analysis.25
9
These observations suggested that catalyst control could be used
to leverage the reactivity of metallacycles formed from alkene–
diene coupling to favor hydrovinylation rather than [2+2]-
cycloaddition. Specifically, we reasoned that less sterically encum-
bered coordination environments would disfavor C(sp3)–C(sp3)
reductive elimination and open pathways for b-hydrogen elimina-
tion followed by C–H reductive elimination to yield acyclic prod-
ucts (Scheme 2C). Lending credence to this hypothesis, a pyri-
dine(diimine) iron catalyst bearing a single 2-aryl substituent, (2-
tBuPDI)FeCH3, produced 2,3-dimethylbutene as the major product
of propylene dimerization (Scheme 2A, bottom). 20 This product
likely derives from the same metallacycle that led to 1,2-dimethyl
cyclobutane with more sterically hindered catalysts.
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required the addition of NaB(ArF)4 (ArF
=
3,5-bis-
trifluoromethylphenyl) to promote catalysis, and the isolation of
on-cycle species was not reported.12f Beyond this example, no con-
crete mechanistic insight has been described about the identities,
speciation, and oxidation states of metal complexes responsible for
C–C bond formation in the [1,4]-hydrovinylation of dienes.
Scheme 2. Catalyst-controlled reactivity of metallacyclic interme-
diates allow selective access to value-added cyclic and acyclic hy-
drocarbons.
A. Alkene–Alkene Coupling: Catalyst-Controlled [2+2]-Cyclodimerization or [1,2]-Hydrovinylation a
[Fe]
Scheme 3. Hydrovinylation of butadiene and substituted 1,3-
dienes catalyzed by well-defined diimine iron complexes affords
C1, branched products.a
Via:
Ar
C–C reductive elimination
(R1 = R2 = c-C5H9)
N
N
III
Fe
N
R1
iPr
[Fe’]
R2
N
iPr
iPr
β-H & C–H reductive elimination
R2
(S = 1)
N
Fe
(R1
=
tBu, R2 = H)
R’
B. Alkene–Diene Coupling: Selective Cross-[2+2]-Cycloaddition b
Via:
3aa–3af
iPr
where R = R’’ = H
R’ = H, Me, Et, nBu, nHex, nOct
(
iPrDI)Fe(COD) (5 mol %)
Ar
N
C5H12, C6D12, or C6D6; 23 °C, 2 h
R’’
R
+
N
[Fe]
single isomer, > 98% conv
R’
Fe
R1
N
+
1a–e
(1 equiv)
2a–s
(1 equiv)
C–C reductive elimination
(R1 = R2 = Me or iPr)
N
Fe
4
α
R1
R’’
(S = 0)
N
R
R’
β
C. Alkene–Diene Coupling: Opportunity for Selective, Catalyst-Controlled Cross-[1,4]-Hydrovinylation
1
3aa–3es
where R ≠ H
L
(
MesDI)Fe(COD) (1 mol %)
C5H12, C6D12, or C6D6, 23 °C
high C1, branched selectivity
26 examples
+
L
Fe
[Fe’]
β-H & C–H
reductive elimination
(this work)
• high regioselectivity • accomodates ethylene and α-olefins • 1:1 stoichiometry •
• well-defined active catalyst •
a See refs 20 and 21. b See refs 20 and 24.
a
Linear hydrovinylation products and olefin isomers were not ob-
served. Conversion was determined by 1H NMR spectroscopy. For-
mation of the hydrovinylation cross products 3ab–af (where R = R’’ =
H and R’ ≠ H) was accompanied by formation of 1,5-cyclooctadiene
(up to 43% with respect to butadiene).
Reduced, aryl-substituted pyridine(diimine)iron complexes are
highly effective catalysts for numerous C–C bond-forming reac-
tions with unactivated alkenes as substrates,18 including the regio-
and diastereoselective intra-19 and intermolecular20 [2+2]-
cycloaddition of α-olefins (Scheme 2A, top). Catalytically compe-
tent intermediates have been isolated, and their electronic struc-
tures, which involve participation of the redox-active pyri-
dine(diimine) ligand, have been elucidated.21–23 The reaction pro-
ceeds through an Fe(I)/Fe(III) cycle involving oxidative coupling
of two alkenes to form a five-membered metallacycle (Scheme 2A,
inset). In this mechanism, both the intermediate bis-olefin complex
(not shown) and metallacycle (Scheme 2A, inset) have S = 1
ground states, and the pyridine(diimine) maintains its one-electron
reduced, radical form throughout.21
By virtue of their open coordination sites and analogy to the pyr-
idine(diimine) iron complexes responsible for [2+2]-cycloaddition
reactions, reduced iron complexes bearing redox-active bidentate
ligands were identified as promising candidates for selective cataly-
sis of alkene–diene hydrovinylation. Seminal reports from Takacs26
and tom Dieck27 introduced the use of such ligands for diene cou-
pling reactions, and Ritter and coworkers applied pyridine-2-imine-
supported iron complexes for linear-selective hydrovinylation using
styrenes (Scheme 2A).14 However, extension of the method be-
yond styrenes and allylbenzene has not been realized. Related bi-
dentate a-diimine ligands were targeted for our investigation of
iron-catalyzed hydrovinylation given their ease of preparation,
modularity, well-established redox activity,28 and precedent in iron
catalysis.27,29,30 Here we describe the synthesis and electronic struc-
Selective cross-[2+2]-cycloaddition reactions of alkenes and
dienes have also been observed using pyridine(diimine)iron cata-
lysts (Scheme 2B).20,24 In this case, the sequence initiates from an S
= 0 pyridine(diimine) iron butadiene complex (not shown), with
an unusual s-trans diene configuration.19,24 Reversible ethylene
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myrcene with 1-hexene, the scope of the reaction with various cou-
ture studies of a-diimine iron diene complexes that are highly ac-
1
2
3
4
5
6
7
8
pling partners was explored. Butadiene self-dimerization proceeded
rapidly in the absence of a suitable cross-coupling partner using
tive for the hydrovinylation of dienes with unactivated alkenes with
nearly exclusive [1,4]-selectivity using equimolar ratios of sub-
strates (Scheme 3). Kinetic studies, deuterium-labeling experi-
ments, and the isolation of intermediates allowed identification of a
resting-state iron diene complex bearing a redox-active a-diimine
ligand that supports a mechanism involving oxidative coupling with
alkenes to form on-cycle metallacyclic intermediates. Comparison
of the electronic structure of catalytically relevant diimine iron
complexes to related pyridine(diimine) iron complexes is also pre-
sented, highlighting the prevalence of ligand-centered radicals in
Fe-catalyzed carbon–carbon bond-forming chemistry.18b
(
(
MesDI)Fe(COD),30 but the butadiene hydrovinylation products
3aa 3af
–
) were formed in high yield using ethylene or an α-olefin
with more encumbered (iPrDI)Fe(COD) as the pre-catalyst
(Scheme 3, top). By contrast, substituted 1,3-dienes underwent
only very slow self-dimerization under standard reaction conditions
with (MesDI)Fe(COD), and these substrates engaged readily in
[1,4]-hydrovinylation with ethylene or 1-hexene (Scheme 3, bot-
tom; Scheme 4A). Notably, cross reactivity was observed with
equimolar quantities of diene and a-olefin, highlighting the im-
proved activity of this class of catalysts over pyridine(diimine) iron
complexes, with which excess alkene was required to suppress
diene coupling chemistry.20
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RESULTS AND DISCUSSION
Method Development and Catalyst Optimization.
The devel-
opment of a method for intermolecular, iron-catalyzed diene hy-
Table 1. Iron-catalyzed 1,4-hydrovinylation of myrcene with 1-
hexene.a
1b
drovinylation was explored initially using myrcene ( ) and 1-
R
2d
hexene ( ) as coupling partners (Table 1). These specific sub-
4
4
4
1
α
β
nBu
β
α
β
[Fe] (5 mol %)
α
β
1
strates were selected because both are commodity chemicals avail-
able from petrochemical feedstocks1 or biomass,31 and both are
easily-handled liquids at room temperature.
As described previously, [(MePDI)Fe(N2)]2(μ2-N2) (MePDI =
2,6-(2,6-Me2-C6H3-N=CMe)2C5H3N) promoted efficient cross-
reactivity of myrcene and 1-hexene but furnished a near 1:1 mixture
+
+
+
neat, 23 °C, 20 h
nBu
2d
nBu
R
α
1
R
R
4
1
nBu
R =
1b
3bd
4bd
C4, branched
5bd
(1 equiv) (5 equiv)
C1, branched
[2+2]
Entry [Fe]
Convb
Product Ratiob
3bd
4bc
and ) and [2+2]-cycloaddition
of [1,4]-hydrovinylation (
5bd
(
) products.20,24 To improve selectivity for hydrovinylation, the
3bd
48
<5
73
74
91
95
95
--
4bd
<1
<1
1
5bd
52
>95
21
17
3
a-diimine iron olefin complexes (iPrDI)Fe(COE) (iPrDI = [2,6-iPr2-
cyclooctene) and (iPrDI)Fe(COD)
1
2
3
4
5
6
7
8
[(MePDI)Fe(N2)]2(μ2-N2)
>98%
43%
C6H3-N=CMe]2; COE
=
(COD = 1,5-cyclooctadiene) were explored. These compounds
were first synthesized by our laboratory for application in alkene
hydrogenation,29 and a related example, (MeDI)Fe(COD), was
reported by Ritter as a catalyst for the [4+4] homodimerization of
butadiene with very high turnover numbers.30 Both
(
(
(
(
iPr(TB)PDI)Fe(N2)2
iPrDI)Fe(COE)
iPrDI)Fe(COD)
94%
93%
2
C5H9
iPrDI)Fe(COE) and (iPrDI)Fe(COD) offered improved selectivity
for hydrovinylation relative to the pyridine(diimine)iron catalyst,
3bd
DI)Fe(COD)
98%
4
(
(EtDI)Fe(COD)
MesDI)Fe(COD)
>98%
>98%
<2% d
3
2
generating
as the major product with decreased amounts of
5bd
(
5
<1
--
[2+2]-cycloaddition product
iron compounds are more straightforward to prepare and handle,
these were selected for additional catalyst evaluation studies.
. Because the 1,5-cycloctadiene
FeCl2 + Mg(C4H6)•2THFc
--
iPr
The effect of 2,6-aryl substituents on the a-diimine catalysts was
evaluated for the hydrovinylation of myrcene with excess 1-hexene
(Table 1). The least sterically encumbered iron complex in the
series, (MesDI)Fe(COD), was the most selective, affording com-
plete chemoselectivity for hydrovinylation, excellent regioselectivi-
ty (95:5 C1, branched : C4, branched), and exclusive formation of
the (Z)-olefin isomer. Increasing the size of the 2,6-aryl substitu-
ents decreased selectivity for hydrovinylation; precatalysts bearing
ethyl or cyclopentyl groups produced small but detectable quanti-
Ar
N
Ar
N
N
N
Fe
N
Fe
N2
N
N2
N
N2
iPr
iPr
iPr
N
N
Fe
N
iPr
(
iPr(TB)PDI)Fe(N2)2
iPr
[Fe]
[(MePDI)Fe(N2)]2(µ2-N2)
(
iPrDI)Fe(COE)
R
N
N
R
R
N
Fe
N
Fe
R = iPr
5bd
ties of the [2+2] cycloaddition product
. Treatment of FeCl2
(
iPrDI)Fe(COD)
C5H9
R = C5H9
R = Et
(
DI)Fe(COD)
EtDI)Fe(COD)
with Mg(C4H6)•2THF in the absence of ligand afforded only trace
conversion and yielded a complex mixture of isomeric products.
The regioselectivity obtained with (MesDI)Fe(COD) comple-
ments cobalt-catalyzed methods reported previously that favor the
C4, branched or C1, linear isomers of related products.11 It is also
notable that the catalyst was prepared readily in three straightfor-
ward steps: (i) the condensation of inexpensive reagents (diacetyl
and 2,4,6-trimethylaniline) to form the diimine, (ii) metalation
with an iron(II) halide, and (iii) subsequent reduction with magne-
sium butadiene in the presence of 1,5-cyclooctadiene (see Experi-
mental Section and Supporting Information).
R
MesDI)Fe(COD)
(
(
a Reactions executed with 0.2 mmol myrcene and 1.0 mmol 1-hexene at ambient
temperature. Linear hydrovinylation products and olefin isomers not observed.
b
Conversion (with respect to myrcene) and product ratio determined after 20
c
hours by gas chromatography. 5 mol% Mg(C4H6)•2THF was added as an in
situ reductant. d Complex mixture of products.
Regioselective Diene Hydrovinylation.
Having identified a high-
ly active and selective iron catalyst for the [1,4]-hydrovinylation of
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Scheme 4. Scope of [1,4]-Hydrovinylation.a
1
2
3
4
5
6
7
8
4
4
4
β
4
N
α
R’
α
β
R’
R’’
R’’
+
+
+
R’’
R
Fe
R’’
R
N
α
β
R’
2a–s
R
R’
R
α
β
1
1
1
1
1a–e
3aa–3es
4aa–4es
C4, branched
6aa–6es
C1, linear
(1 equiv)
(1 or 5 equiv)
C1, branched
(
MesDI)Fe(COD) (1 mol %)
(minor regioisomers)
C5H12, C6D12, or C6D6, 23 °C
9
A. Scope of Substituted Dienes:
4
4
4
4
4
4
4
4
4
97% yield, 25 h d
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
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42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
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98:2 b:l
92:8 C1:C4
nBu
nBu
nBu
nBu
1
1
1
1
1
1
1
1
1
81% yield, 4 h e
>98:2 b:l
3aa
3ba
3ca
3cd
3da
3dd
3ea
3ed
92% yield, 4 h b,c
97% yield
15 min b,c
92% yield, 6 h d
>98% yield, 8 h b,c
>98:2 C1:C4f
97% yield, 4 h e
92% yield, 40 h e
96% yield, 24 h b,c
>98:2 C1:C4f
93% yield, 40 h e,g
3bd
92:8 C1:C4
>98:2 C1:C4
>98:2 b:l
>98:2 b:l
80:20 b:l
96:4 C1:C4f
91:9 C1:C4
B. Scope of α-Olefins:
iPr
tBu
Ph
R
nHex
R
nOct
R
R
R
R
R
3bb
3bc
3be
3bf
3bg
3bh
3bi
93% yield, 24 h d
74% yield, 4 h e
95% yield, 25 h d
85% yield, 25 h d
92% yield, 18 h d
64% yield, 42 h d
90% yield, 24 h e,g,h
>98:2 b:l, 93:7 C1:C4
>98:2 b:l, 94:6 C1:C4
98:2 b:l, 92:8 C1:C4
98:2 b:l, 91:9 C1:C4
>98:2 b:l, 94:6 C1:C4
97:3 b:l, 90:10 C1:C4
>98:2 b:l, 70:30 C1:C4
R
R
R
R
R
R
3bn
3bj
3bk
3bl
3bm
3bo
78% yield, 25 h e
>95:5 b:l, >95:5 C1:C4f
88:12 endo:exo i
57% yield, 24 h e,g
34% yield, 30 h e,g
74% yield, 8 h d
85% yield, 24 h e
99% yield, 18 h e,g,h
94:6 b:l, 86:14 C1:C4
98:2 b:l, 73:27 C1:C4
>98:2 b:l, 98:2 C1:C4
>98:2 b:l, 78:22 C1:C4
98:2 b:l, 69:31 C1:C4
R =
N
SiMe3
R
R
R
R
SiMe3
R
BPin
6bt
3bp
86% yield, 4 he,j
3bq
3br
3bs
84% yield, 33 h d
<2:98 b:l, >98:2 C1:C4
70:30 Z:E
85% yield, 12 hd,g,h
72% yield, 20 h e
88% yield, 11 h e
98:2 b:l, 92:8 C1:C4
>98:2 b:l, 97:3 C1:C4
88:12 b:l, 62:38 C1:C4
>98:2 b:l, 92:8 C1:C4
a Reactions monitored for consumption of diene
indicated time. Product ratios determined by gas chromatography unless noted otherwise. See the Supporting Information for a complete tabulation
1
by gas chromatography. Combined yields listed for isomeric product mixtures isolated after the
b
c
of substrates 1a–e and 2a–t
0.25 mmol
f
.
Yield determined by 1H NMR integration relative to an internal standard. Reactions executed with 0.25 mmol
1
and
and 2.5 mmol
2
2
in C6D6 or C6D12. d Reactions executed with 0.5 mmol
1
and 0.5 mmol
g
2
in C5H12. e Reactions executed with 0.5 mmol
1
1
in C5H12. Product ratios determined from relative H NMR integration. Unidentified isomeric products also formed: 7% of product mixture with
h
i
3ed, 8% with 3bi, 3% with 3bj, 3% with 3bk, 5% with 3bo, and 7% with 3bq. Reaction run neat. Using a 70:30 endo:exo mixture of 5-vinyl-2-
norbornene isomers. j With 5% 2:1 myrcene–hexadiene adduct.
2s
and vinyltrimethylsilane ( )—were all suitable substrates for the
iron-catalyzed hydrovinylation reaction, although longer reaction
times (up to 96 h to form
reaction conditions (for
Dienes bearing substitution at the C2 position exclusively, such
as isoprene and myrcene, underwent C–C bond formation at the
C1 termini and formed the branched regioisomers arising from
coupling at Cβ. The iron-catalyzed reactions were also highly ste-
3bo
3bi
and
; see Supporting Information) or neat
3bo
) were required for the most
3aa–ea, 3bd, 3cd
3ed
reoselective; only the (Z)-isomers of
were detected. The preference for C–C bond formation at C1 was
3ea 3ed
, and
hindered cases. Terminal alkenes bearing pendant internal al-
2m
kenes—such as 4-vinylcyclohexene ( ), 5-vinyl-2-norbornene
2n 2o
( ), and citronellene ( )—reacted exclusively at the terminal
retained with additional substitution (e.g. products
and
),
although selectivity for the branched product eroded slightly. In
each of these examples, the stereochemical outcome could be justi-
fied in terms of steric interactions between a postulated metallacy-
cle and the ligand framework during C–C bond formation, wherein
the mechanism for oxidative cyclization would require formation of
the cis-olefin geometry (vide infra).
position and thereby introduced sterically and electronically differ-
entiated synthetic handles into the tetra-olefin products. In a nota-
ble entry,
modest excess (5 equiv) of the bis-olefin generated the [1,4]-
hydrovinylation product cleanly, forming only trace levels of side
products arising from cycloisomerization or sequential hydrovinyl-
ation at both termini.
3bp
2p
was prepared from 1,5-hexadiene ( ). Use of a
The scope of the alkene partner for the iron-catalyzed hydrovi-
nylation reaction was explored using myrcene as a representative
diene. Nearly exclusive C1, branched selectivity was maintained for
2a
a range of linear and branched a-olefins including ethylene ( ),
2b 2f 2g
propylene ( ), 1-decene ( ), allylbenzene ( ), and 3-methyl-1-
2h
pentene ( ), 3-methyl-1-butene ( ), 3-methyl-1-pentene ( ),
2l 2o 2r
pentene ( ). More hindered alkenes—such as 3,3-dimethyl-1-
2i 2j 2k
vinylcyclobutane ( ), citronellene ( ), allyltrimethylsilane ( ),
ACS Paragon Plus Environment

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Journal of the American Chemical Society
A.
B.
C.
D.
1
2
3
4
5
6
7
8
-5.5
-6.0
-6.5
-7.0
-7.5
-8.0
-6.0
-5.5
-6.0
-6.5
-7.0
-7.5
-8.0
0.5
0.4
0.3
0.2
0.1
0.0
with [1b]0 = 0.5 M, [2e]0 = 0.5 M
with [2e]0 = 0.5 M
m = ꢀ0.16
with [1b]0 = 0.5 M
m = 1.0
with [1b]0 = [2e]0 = 0.5 M
m = 1.1
-6.5
-7.0
-7.5
-8.0
-8.5
1b
3be
b = ꢀ7.1
b = ꢀ0.04
R2 = 0.99
b = ꢀ4.5
R2 = 0.97
R2 = 0.98
4be
6be
9
10
11
0
300
600
900 1200 1500
-2.0
-1.5
-1.0
-0.5
0.0
0.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
-6.5
-6.0
-5.5
-5.0
-4.5
-4.0
ln[1b]0
ln[2e]0
ln[Fe]tot
time (min)
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 1
. Kinetic Analysis. A. The reaction time-course, wherein the concentrations of reactants and products are fit to an integrated first-
order rate law, shows well-behaved kinetics with constant product selectivity over time. B–D. The approximately zero-order dependence on
1b
2e
diene [ ], first-order dependence on alkene [ ], and first-order dependence on iron catalyst [Fe]tot were determined from initial rate meas-
urements. All experiments conducted with [Fe]tot = [(MesDI)Fe(COD)]tot = 5 mM unless noted otherwise.
Scheme 5. In situ catalyst activation for multigram-scale, iron-
catalyzed hydrovinylation reactions.a
The C1, branched products of hydrovinylation reactions with al-
2r 2s
lyltrimethylsilane ( ) and vinyltrimethylsilane ( ) offer addi-
3br 3bs
tional synthetic versatility, as the silyl groups in
elaborated to other functionality. Furthermore, the electronically
2t
and
may be
N
biased vinylboronic acid pinacol ester ( ) reacted with myrcene to
Br
+
Fe
N
6bt
afford the C1, linear product ( ). This product is also poised for
Br
diversification by cross coupling or other deborylative transfor-
mations to afford access to a suite of hydrovinylation products with
regioselectivity complementary to that achieved through the direct
reaction of unbiased olefins. The discrepancy in selectivity suggests
that different mechanisms may be operative for the hydrovinyla-
tion reactions with electron-neutral and electron-deficient olefin
coupling partners. Nonetheless, styrene and acrylates, which were
the optimal substrates for the pyridine-2-imine iron catalysts de-
scribed by Ritter,14 were not reactive under these conditions.
1b,c
2b
3bb: 90% yield (4.0 g)
>98:2 b:l, 95:5 C1:C4
3cb: 70% yield (1.9 g)
>98:2 b:l, 97:3 C1:C4
(
MesDI)FeBr2 (1 mol %)
25 mmol
(1 equiv)
30 mmol
(1.2 equiv)
Mg(C4H6)•2THF (1.2 mol %)
10 : 1 C5H12/Et2O, 23 °C, 24 h
N
Br
+
Fe
N
nBu
nBu
Br
1c
2d
3cd: 98% yield (7.4 g)
>98:2 b:l, 94:6 C1:C4
(
MesDI)FeBr2 (0.5 mol %)
50 mmol
(1 equiv)
60 mmol
(1.2 equiv)
Mg(C4H6)•2THF (0.6 mol %)
10 : 1 C5H12/Et2O, 23 °C, 72 h
Use of single-component, reduced iron precatalysts, such as
MesDI)Fe(COD), allowed for the evaluation of intrinsic catalyst
(
a Combined yields listed for isomeric product mixtures isolated after
the indicated time. Product ratios determined by gas chromatography.
activity and selectivity without potential complications from acti-
vators and their byproducts. Nonetheless, select hydrovinylation
reactions were also conducted using the bench-stable iron(II)
halide precatalyst (MesDI)FeBr2 directly with the addition of mag-
nesium butadiene (Mg(C4H6)•2THF) as an in situ reductant. No
erosion of chemo- or regioselectivity was observed with the in situ
activation protocol, and the modified conditions—Et2O was also
added to improve solubility of the iron(II) halide and reductant—
Kinetic Analysis.
Selective 1,4-hydrovinylation promoted by a
well-defined, single-component organometallic iron precursor
offered the opportunity for mechanistic studies to determine what
role the a-diimine played in catalytically relevant intermediates.
To establish the nature of the active species responsible for C–C
bond formation and for dictating selectivity for hydrovinylation, a
kinetic analysis of a representative reaction was conducted. The
2e
hydrovinylation of myrcene ( ) with 1-octene ( ) was selected
as both are easily handled liquid reagents. The progress of the reac-
tion under catalytically relevant conditions (where [ ]0 = [ ]0 =
3bb
on multigram scale (Scheme 5). Altogether, the iron-
allowed for the facile preparation of hydrovinylation products
3cb 3cd
,
, and
1b
catalyzed hydrovinylation represents a highly efficient method for
achieving rapid structural elaboration of minimally functionalized
building blocks to afford value-added “skipped” diene products
with high chemo- and regioselectivity.
1b
2e
1b
2e
0.5 M, see Figure 1A; or where [ ]0 = (1/5)[ ]0 = 0.5 M; see
Supporting Information) was monitored by gas chromatography
over ~5 half lives at 23 °C (Figure 1A). Clean exponential decay of
1b
3be
]
myrcene concentration, [ ] and corresponding growth of [
over time established that the reaction exhibited well-behaved
kinetics with no induction period or change in selectivity over
time. Accordingly, subsequent analyses to determine the order in
each reaction component—approximately zero-order in diene
1b
[
2e
], first-order in alkene [ ], and first-order in catalyst [Fe]tot—
were performed using the method of initial rates (Figure 1B–D;
see Supporting Information for experimental details).32
ACS Paragon Plus Environment

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Table 2. Select Bond Distances (Å) and Angles (°) for (iPrDI)Fe(η4-C5H8) and Related Structures.
1
2
0 e
2– e
iPrDI)Fe(η4-C5H8) a
(
iPrDI)Fe(COD) b
(
MeDI)Fe(COD) d
(RDI(ox)
)
(RDI•)1– e
(RDI(red)
)
C5H9
(
(
c
DI)Fe(COD)
3
4
5
6
7
8
9
Fe–N(1)
1.957(1)
1.966(2)
1.343(2)
1.340(2)
1.419(2)
2.136(2)
2.070(2)
2.091(2)
2.157(2)
1.9964(18)
1.9944(18)
1.344(3)
1.349(3)
1.413(3)
2.096(3)
2.115(3)
1.991(2)
1.997(2)
1.988(2)
1.348(2)
1.346(2)
1.411(2)
2.091(2)
2.107(2)
Fe–N(2)
1.991(2)
N(1)–C(1)
N(2)–C(2)
C(1)–C(2)
Fe–C(29)
1.344(3)
1.29
1.29
1.47
1.35
1.35
1.41
1.38
1.38
1.36
1.3412(17)
1.503(3)
2.0901(14)
2.1235(13)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Fe–C(30)
Fe–C(31)
Fe–C(32)
Fe–C(33)
2.105(2)
2.082(2)
1.375(6)
2.1240(14)
2.0733(14)
1.391(2)
2.105(2)
2.108(2)
1.397(3)
Fe–C(34)
C(29)–C(30)
C(30)–C(31)
C(31)–C(32)
C(33)–C(34)
N(1)–Fe–N(2)
1.404(3)
1.440(3)
1.404(3)
1.397(2)
79.59(7)
1.393(2)
79.82(4)
1.401(2)
79.75(6)
80.31(6)
a See Scheme 6B. b Taken from ref 29. c See Supporting Information. d Taken from ref 30. e Taken from ref 28a where (RDI(ox))0 represents a generic diimine ligand in its neutral
form, (RDI•)1– represents a generic diimine ligand in its one-electron reduced radical monanionic form, and (RDI(red) 2– represents a generic diimine ligand in its closed-shell,
two-electron reduced, dianionic form. Average values obtained from crystallographic analysis of diimine complexes with unambiguous ligand oxidation state assignments.
)
Scheme 6. Independent Synthesis and Structural Characterization
Resting
product mixtures isolated after the indicated time. Product ratios
determined by gas chromatography.
of
the
Probable
Catalyst
State
A. Independent Synthesis of a Probable Catalytic Intermediate
iPr
iPr
Synthesis and Electronic Structure of Catalytic Intermediates.
The rate law for the catalytic hydrovinylation is most consistent
with an a-diimine iron diene complex as the catalyst resting state.
Attempts to observe such a compound directly by H NMR spec-
troscopy during catalytic turnover were unsuccessful due to line
broadening; therefore, independent synthesis of such an interme-
N
N
(20 equiv)
C5H12, 23 °C, 1 h
80% yield
iPr
iPr
iPr
iPr
Fe
N
N
Fe
1
iPr
iPr
(
iPrDI)Fe(COD)
(
iPrDI)Fe(η4-C5H8)
B. Solid-State Structure of (iPrDI)Fe(η4-C5H8) a
diate was pursued. The isopropyl-substituted variant of the a-
diimine ligand, iPrDI, was selected for these studies due to its im-
proved crystallinity and slightly diminished reactivity compared to
the MesDI analog. Treatment of (iPrDI)Fe(COD) with a large excess
(20 equiv) of isoprene afforded, following recrystallization from
pentane, a red–brown paramagnetic solid identified as the isoprene
complex (iPrDI)Fe(η4-C5H8) (Scheme 6A). This complex proved
to be catalytically competent for hydrovinylation; exposure of a 5:1
mixture of 1-hexene and isoprene to 5 mol % (iPrDI)Fe(η4-C5H8)
3cd
3cd 4cd 5cd
: : ) after
yielded
(34% combined yield; 36:32:31
20 hours at 23 °C with effectively identical selectivity to that ob-
tained with (iPrDI)Fe(COD) under the same conditions (Scheme
6C).
The solid-state structure of (iPrDI)Fe(η4-C5H8) was determined
by single crystal X-ray diffraction (Scheme 6B), confirming its
identity. In contrast to the s-trans diene configuration observed
19,24
with pyridine(diimine)iron butadiene complexes, isoprene
adopts an s-cis configuration in (iPrDI)Fe(η4-C5H8). It is likely that
the s-cis geometry, which is typical for transition metal diene com-
plexes,33 is precluded by bulky tridentate pyridine(diimine) ligands
but can be accommodated in the more open coordination envi-
ronment afforded by a bidentate diimine ligand. Like the bis-olefin
complexes (iPrDI)Fe(COD)29 and (MeDI)Fe(COD),30 the coordi-
nation geometry of the iron diene complex is best described as
pseudo-tetrahedral. A solution magnetic moment of 2.5 µB was
measured for (iPrDI)Fe(η4-C5H8) at 25 °C in benzene-d6, con-
sistent with an S = 1 ground state.34
C. Catalytic Competence for Hydrovinylation b
nBu
+
[Fe] (5 mol %)
+
+
nBu
neat, 23 °C, 20 h
nBu
nBu
1c
2d
3cd
4cd
5cd
(1 equiv)
(5 equiv)
with (iPrDI)Fe(COD):
34% combined yield (37 : 32 : 33)
34% combined yield (37 : 33 : 31)
(
iPrDI)Fe(η4-C5H8):
a Representation of the molecular structure of (iPrDI)Fe(η4-C5H8)
with 30% probability ellipsoids. Hydrogen atoms and one isopro-
pyl group omitted for clarity. b Combined yields listed for isomeric
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Journal of the American Chemical Society
A. (iPrDI)Fe(η4-C5H8)
B. (iPrDI)Fe(COD)
1
2
3
4
5
6
7
8
9
10
100
100
98
96
99
98
94
ꢀ = 0.48 mm/s
|ꢀEQ| = 1.30 mm/s
ꢀ = 0.56 mm/s
|ꢀEQ| = 1.68 mm/s
97
92
-4.0 -2.0 0.0
2.0
4.0
-4.0
-2.0
0.0
2.0
4.0
Velocity (mm/s)
Velocity (mm/s)
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 2. Zero-field 57Fe Mößbauer spectra (left) measured at 80 K, and spin-density plots (right) obtained from broken symmetry (3,1) Mulliken
population analysis (red, positive spin density; yellow, negative spin density) calculated at the B3LYP level of theory.
Table 3. Zero-Field 57Fe Mößbauer Parameters for (iPrDI)Fe(η4-C5H8), (iPrDI)Fe(COD), and Related Structures.a
[Fe]
δ
(mm/s)
|ΔEQ|
(mm/s)
C.N. b
Oxidation state
assignment
[Fe]
δ
(mm/s)
|ΔEQ|
(mm/s)
C.N. b
Oxidation
state
assignment
g
(
iPrDI)Fe(η4-C5H8)
0.56
0.49
0.48
0.46
1.68
2.02
1.30
0.98
4
Fe(I)
(
Menacnac)Fe(NCtBu)2
0.72
0.79
1.87
0.59
4
4
Fe(I)
BS(3,1)c
(
Menacnac)Fe
Fe(I)–Fe(II)
hybrid
g
(4-tBu-Py)2
(
iPrDI)Fe(COD)
MesDI)Fe(COD)
MeDI)Fe(COD) d,e
4
4
4
4
4
4
5
Fe(I)
Fe(I)
Fe(I)
Fe(I)
Fe(II)
Fe(II)
(
(
Menacnac)FeBr(THF) g
0.89
0.70
2.36
0.74
4
5
Fe(II)
Fe(I)
Menacnac)Fe
(η6-C6H6) g
(
0.47
0.44
0.84
0.77
0.44
1.37
0.88
2.57
1.81
0.27
(
f
h
(
iPrDI)FeCl2
iPrDI)2Fe f
MeDI)Fe
(
iPr(TB)PDI)Fe(N2)2
0.45
0.73
0.83
1.81
5
5
Fe(0)–Fe(II)
hybrid
(
(
Fe(0)–Fe(II)
hybrid
(
iPr(TB)PDI)Fe
(η2,η2-((CH2CHCH2)2) h
Fe(I)
(η6-C6H5CH3) d
a All measurements recorded at 80 K. Complete ligand structures for all listed complexes depicted in the Supporting Information. b Coordination number. c Calculated using the
broken symmetry solution for the electronic structure of (iPrDI)Fe(η4-C5H8) determined using DFT. d Taken from ref 30. e Two polymorphs characterized independently. f Taken
from ref 28. g Taken from ref 38. h Taken from ref 21; see Table 1 for ligand structure.
Distortion in the metrical parameters of a ligand is an established
reporter of the redox-activity of a chelate.35 Specifically, the imine
N(1)–C(1) and N(2)–C(2) and backbone C(1)–C(2) bond dis-
tances of diimine ligands are most sensitive to changes in to ligand
oxidation state.28,36 The relevant bond metrics for (iPrDI)Fe(η4-
porting Information). These data are compiled in Table 3 along
with the Mößbauer parameters for related diimine,28,30 β-
diketiminate (nacnac),38 and pyridine(diimine)21 complexes for
comparison.
Full-molecule density functional theory (DFT) calculations of
iPrDI)Fe(η4-C5H8) and (iPrDI)Fe(COD) were performed using the
C5H8) are compiled in Table 2, where they are compared with the
(
C5H9
analogous metrics for (iPrDI)Fe(COD),29
(
(
DI)Fe(COD),
B3LYP functional39,40 and either unrestricted Kohn–Sham (UKS)41
and broken-symmetry (BS) possibilities; geometry optimizations
were initiated from the experimental solid-state structure. Use of
these DFT methods, especially when coupled to spectroscopic
data, is widely precedented for determining the ground-state elec-
tronic structures of first-row transition metal complexes with redox-
active ligands.21,22,28,30,38,42 The calculations converged to broken
symmetry (3,1) solutions for both (iPrDI)Fe(η4-C5H8) and
MeDI)Fe(COD),30 and average values characteristic of a generic
diimine ligand in its neutral, radical anion, and closed-shell dianion
forms.28 The imine N(1)–C(1) and N(2)–C(2) bond distances
(1.343(2) and 1.340(2) Å, respectively) for the isoprene complex
are elongated relative to the neutral form of the ligand (1.29 Å),
while the backbone C(1)–C(2) bond is contracted (1.419(2) Å vs.
1.47 Å for neutral ligand), suggesting one-electron reduction of the
diimine ligand. Conversely, the C(29)–C(30) and C(31)–C(32)
bond lengths of the isoprene ligand are minimally perturbed rela-
tive to those typical of free dienes (1.330 Å for C=C, 1.455 Å for C–
C).37
These structural parameters, in combination with the magnetic
data, are indicative of a high-spin iron(I) center (SFe = 3/2) en-
gaged in antiferromagnetic coupling with a diimine ligand radical
(SDI = –1/2) accounting for the experimentally observed S = 1
ground state. The zero-field 57Fe Mößbauer data (δ = 0.56 mm/s
and |ΔEQ| = 1.68 mm/s; Figure 2A, left) for the isoprene complex
are also consistent with this electronic structure description. Zero-
field 57Fe Mößbauer data were also collected for (iPrDI)Fe(COD)
(δ = 0.48 mm/s and |ΔEQ| = 1.30 mm/s; Figure 2B, left) and (Mes-
DI)Fe(COD) (δ = 0.46 mm/s and |ΔEQ| = 0.98 mm/s; see Sup-
(
iPrDI)Fe(COD). The computed structures reproduced the exper-
imental Mößbauer parameters adequately (e.g. δ = 0.49 mm/s,
|ΔEQ| = 2.02 mm/s predicted for (iPrDI)Fe(η4-C5H8)).42c The spin
delocalization depicted in Figure 2 highlights the participation of
the diimine ligand in the electronic structure of the active iron cata-
lyst. The computational results, combined with the spectroscopic
and crystallographic data, underscore the similarities between elec-
tronic structures of (iPrDI)Fe(η4-C5H8) and (iPrDI)Fe(COD); both
16-electron complexes are comprised of high-spin SFe = 3/2 iron(I)
centers coupled antiferromagnetically to diimine ligand radicals
(SDI = –1/2) to afford a net S =1 spin state. These findings are in
agreement with Ritter’s assignment for (MeDI)Fe(COD).30 While
spin-state changes upon alkene coordination may, in principle,
occur en route to product formation, these findings provided com-
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Page 8 of 13
Scheme 7. Evidence for On-Cycle Metallacycle Formation
A.
pelling support for the involvement of ligand redox activity in the
overall catalytic cycle.
1
B.
2
3
Deuterium-Labeling Experiments in Support of a Metallacyclic
Intermediate.
Following identification and electronic structure
4
5
6
7
8
9
determination of the catalyst resting state, the possible intermedia-
cy of a metallacycle in the mechanism of iron-catalyzed hydrovinyl-
ation was explored. Several key observations supported this hy-
pothesis. Only (Z) olefin products were observed (vide supra);
neither the (E) isomers nor structural isomers were detected under
catalytic conditions (Scheme 7A). Furthermore, when 3-methyl-
(Z)-3
(E)-3
iso-3
(E)-1e
(Z)-1e
observed
not observed
not observed
reactive
unreactive
C.
D
N
Fe
N
+
D
nHex
2e-d1
nHex
1e)
1,3-pentadiene ( was subjected to standard catalytic conditions
1e
as a mixture of (E) and (Z) isomers, only (E)- underwent hy-
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1b
3be-d1
75% yield
MesDIFe(COD) (4 mol %)
C5H12, 23 °C, 9 h
(1 equiv)
(1 equiv)
>98:2 b:l, 96:4 C1:C4
3ea
, respectively (Scheme 4); (E)- was not consumed. This reac-
drovinylation with ethylene or 1-hexene to form products
3ed 1e
tivity difference could be attributed to the energetic penalty arising
and
D.
Cossee
100
H
75
50
25
from repulsive A1,3 interactions in the s-cis conformation of (Z)-
1e
N
1c (1 equiv)
(Scheme 7B).43 Taken together, these observations suggested that a
mechanism involving direct [1,4]-addition of an iron hydride
across a diene was unlikely. While this “Cossee” mechanism is
commonly invoked in many alkene oligomerization and diene te-
lomerization reactions,44,45 the iron allyl resulting from such a pro-
cess would be expected to isomerize rapidly prior to olefin inser-
tion, leading to a mixture of isomeric products.
0
3ca
Fe
N
Metallacyclic
100
+
D
75
50
25
2a (1 equiv)
d3
(
MesDI)Fe(COD) (1 mol %)
C6D12, 23 °C
d4
+
3ca-d4
0
Experimental
100
D
H
2a-d4 (1 equiv)
75
50
25
More direct evidence was provided by a series of deuterium-
labeling experiments. Performing the hydrovinylation of myrcene
d3
0
3ca-d1
3ca-d3
2
) with 2-[ H1]-oct-1-ene ( -d1) yielded -d1, where the al-
94 96 98 100 102
1b
(
2e
3be
crossover not observed
m/z
kene β-deuterium was transferred exclusively to the C4 terminus of
the 1,3-diene coupling partner (Scheme 7C). No isotope scram-
bling arising from competitive chain-walking processes was ob-
served. Based on this pattern of isotopic-labeling, an additional
crossover/competition experiment was conducted to distinguish
definitively between the Cossee and metallacyclic mechanistic
possibilities. This experimental design was introduced inde-
pendently by Belov et al.46 and by Bercaw and coworkers47 in the
context of titanium- and chromium-catalyzed ethylene oligomeriza-
tion, respectively. It has since been widely applied to study the
mechanisms accessed with other oligomerization catalysts.48 The
experiment takes advantage of the fact that on-cycle metallacycle
formation results in intramolecular transfer of hydrogen (or deuter-
ium) from one chain end to the other. In contrast, propagation
through a metal hydride, alkyl, or allyl complex in a Cossee mecha-
nism results in the intermolecular transfer of hydrogen (or deuteri-
um), thereby affording a statistical mixture of crossover isotopo-
logues. Treatment of isoprene with a 1:1 mixture of ethylene and
ethylene-d4 under catalytic conditions for hydrovinylation pro-
Given the combined evidence of the kinetic characterization, the
isotope-labeling analysis, and the stereo- and regiochemical course
of the reaction, a mechanism involving on-cycle oxidative cycliza-
tion is proposed, as depicted in Scheme 8A. In addition to account-
ing for the reactivity and selectivity trends observed experimentally,
this mechanism likely proceeds through a two-electron redox cou-
ple enabled by the participation of the redox-active supporting
ligand. An analogous Fe(I)/Fe(III) couple was identified previous-
ly for the [2+2] cycloaddition of alkenes, wherein select examples
of iron(III) metallacycles supported by the iPr(TB)PDI ligand (see
Table 1 for structure) were isolated and characterized spectroscop-
ically.21 Alternatively, oxidative cyclization may result in single-
electron oxidation of both the diimine ligand-centered radical and
the metal center to afford an Fe(II) metallacycle bearing a neutral
diimine ligand.51 Given that metallacycle formation is rate-
determining, these possibilities cannot be distinguished rigorously
from the experimental information available for this system.
3ca
crossover products that would arise from the intermediacy of a
3ca
and -d4 (Scheme 7D). The
duced a mixture of isotoplogues
3ca
3ca
propagating metal hydride ( -d1 and -d3) were not detected
by quantitative 13C NMR analysis, and the distribution of isotopo-
logues measured by high resolution mass spectrometry was most
consistent with that expected for a mechanism involving metallacy-
cle formation (Scheme 7D). The slight deviation of the isotopo-
logue ratio from 1:1 was further indicative of an inverse, secondary,
competition kinetic isotope effect (kH/kD = 0.83(11)).49 Given that
β-hydrogen elimination and C–H bond-forming reductive elimina-
tion would be expected to afford large, primary kinetic isotope
effects, this result supported rate-determining C–C bond for-
mation.50
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Journal of the American Chemical Society
developed to date for the hydrovinylation of 1,3-dienes. An iron
Scheme 8. Mechanistic Proposal
A. Proposed Catalytic Cycle:a
1
diene complex was identified as the resting state and structurally
characterized. Determination of the electronic structures of the
precatalyst and the resting state established the role of the redox-
active α-diimine ligand, wherein the catalytically active species is
best described as an iron(I) center antiferromagnetically coupled to
an α-diimine ligand radical anion. Isotopic-labeling studies, kinetic
analyses, and structure–reactivity–selectivity relationships provided
evidence for a mechanism wherein oxidative cyclization to form an
intermediate metallacycle is followed by β-hydrogen elimination.
The mechanistic parallels between this system and those described
previously for the [2+2] cycloaddition of olefins highlight the gen-
eral efficacy of these modular iron catalysts for elaborating simple
hydrocarbon building blocks to generate value-added structures
with excellent and tunable selectivity.
2
3
4
5
6
7
8
9
(
MesDI)Fe(COD)
R
R
R'
reductive elimination &
ligand exchange
diene
– COD
N
N
FeI
N
Fe
R
N
H
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
R
R'
catalyst resting state
oxidative
β-H
elimination
H
cyclization
R'
r.d.s
EXPERIMENTAL SECTION52
Synthesis of (MesDI)Fe(COD).
N
N
Fe
R
In an N2-atmosphere glovebox, a
20-mL scintillation vial was charged with MesDI (500 mg, 1.55
mmol, 1.0 equiv),53 iron(II) bromide (333 mg, 1.55 mmol, 1.0
equiv), and a PTFE-coated magnetic stir bar. Tetrahydrofuran (15
mL) was added, and the vial was sealed with a PTFE-lined screw
cap. The reaction mixture was maintained with vigorous stirring at
ambient temperature (23 °C) as the reaction mixture developed a
deep red/purple hue. After 16 hours, the solvent was removed in
vacuo. The solid was resuspended in diethyl ether (15 mL) and
collected by vacuum filtration over a glass frit, rinsing with addi-
tional diethyl ether (3 × 5 mL) until the rinses ran clear. The pur-
ple solid (html hex color code #524c59, 758 mg, 91% yield) was
H
R'
B. Selectivity Model:
R''
R'
[Fe]
R
[Fe]
[Fe]
R'
R'
vs.
vs.
R
R
R''
R''
H
H
H
steric encumbrance
remote from the ligand
steric encumbrance proximal
to the ligand
steric encumbrance proximal
to the ligand
gauche butane where R'' ≠ H
gauche butane relieved
gauche butane relieved
R''
R'
R
1
dried in vacuo. H NMR (CD2Cl2, 300 MHz) δ = 116.85 (6H),
15.25 (6H), 7.46 (12H), 2.58 (4H) ppm.
R
R'
R
R'
R''
R''
C1, branched
C4, branched
C1, linear
In an N2-atmosphere glovebox, a 50-mL round bottom flask was
charged with (MesDI)FeBr2 (400 mg, 0.75 mmol, 1.0 equiv) and a
PTFE-coated magnetic stir bar. Diethyl ether (8 mL) and 1,5-
cyclooctadiene (0.3 mL, 2.4 mmol, 3.3 equiv) were added. The
mixture was frozen in a cold well chilled with liquid N2 for >15
minutes before Mg(C4H6)•2THF (196 mg, 0.88 mmol, 1.2
equiv)54 was added in one portion. The flask was sealed with a
greased glass stopper, which was secured with a Keck clamp, and
returned to the cold well. The reaction flask was maintained in the
cold well for 1 hour as it gradually warmed. After 1 hour, the flask
was removed from the cold well, and the reaction mixture was
stirred vigorously at ambient temperature (23 °C) for two addi-
tional hours. The dark purple reaction mixture developed a
green/brown hue within 45 minutes. After 3 hours in total, Celite
(2 g) was added directly to the reaction flask, and the solvent was
removed in vacuo. The dry Celite was transferred to a glass frit and
washed with pentane (60 mL). In the receiving flask, the filtrate
was concentrated to afford a sticky brown solid. The solid was re-
dissolved in pentane (10 mL), and the solvent was removed in
vacuo. This process was repeated to remove excess 1,5-
cyclooctadiene until the resulting residue was a brown solid. The
crude material was recrystallized from pentane at –35 °C overnight
and dried in vacuo to afford an olive solid (html hex color code
#3F3F00, 218 mg over two crops, 60% yield). 1H NMR (C6D6, 300
MHz) δ = 295.65 (4H), 6.96 (6H), 5.26 (12H), –7.00 (4H), –
59.24 (4H), –173.98 (4H), –240.25 (6H) ppm. Elemental analysis
calculated for C30H40N2Fe: C, 74.37; H, 8.32; N, 5.78. Found: C,
74.68; H, 7.98; N, 5.74. Note that the reduction may also be effect-
ed using Na(Hg) as described previously for the synthesis of
major
minor where R'' = H
minor where R'' ≠ H
a The electronic structure, including ligand redox activity, is depicted
explicitly only for the resting state iron diene complex. The neutral
form of the diimine ligand is drawn in other cases to indicate that the
electronic structure, which may or may not involve diimine ligand
redox activity, is unknown. r.d.s. = rate-determining step
While the transition-state electronic structure likewise cannot be
determined unambiguously from the experimental data available,
the general mechanistic pathway provides sufficient context to
appreciate the basis for selectivity. To the extent that the regiose-
lectivity-determining transition structure resembles the postulated
metallacyclic intermediate, the high degree of selectivity observed
for formation of C1, branched products can be justified based on
interactions between the metallacycle and ligand framework
(Scheme 8B). Namely, the metallacycle en route to the major
product minimizes repulsive steric interactions between the cou-
pling partners (where R’’ = H) and the ligand. Reversing the regio-
chemistry of either the diene or olefin coupling partner increases
the steric encumberance proximal to the ligand and metal center.
This energetic difference is attenuated where R’’ ≠ H due to the
introduction of repulsive gauche butane interactions between the
coupling partners, resulting in a modest increase in the formation of
the minor C1, linear product. Nonetheless, selectivity for C1 vs. C4
addition remains high.
CONCLUDING REMARKS
In summary, well-defined single-component iron precatalysts for
the selective [1,4]-hydrovinylation of conjugated dienes with α-
olefins have been discovered. The products obtained from this
method address a major deficiency in regiocontrol among reactions
(
iPrDI)Fe(COD);29 however, this method affords a mixture of (Mes-
DI)Fe(COD) and (MesDI)2Fe.
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Page 10 of 13
ship (PGS-D). Financial support was provided by Firmenich. We thank
Dr. Jonathan Darmon and Dr. István Pelczer for helpful discussion.
General Procedure for [1,4]-Hydrovinylation with Gaseous
Substrates.
1
2
3
4
5
6
7
8
In an N2-atmosphere glovebox, a J-Young NMR tube
was charged with (iPrDI)Fe(COD) (0.006 g, 0.01 mmol, 5 mol %)
or (MesDI)Fe(COD) (0.005 g, 0.01 mmol, 5 mol %) as a solution in
C6D12, C6D6, or dodecane (0.5 mL). The tube was sealed, removed
from the glovebox, frozen in liquid N2, and evacuated under high
vacuum. Butadiene (0.2 mmol, 1 equiv) and ethylene, propylene,
or 1-butene (0.2 mmol, 1 equiv) were added to a separate J-Young
NMR tube by sequential calibrated gas bulb additions. The com-
bined starting materials were then transferred to the J-Young tube
containing the catalyst solution by way of a vacuum transfer tube.
The tube was sealed under static vacuum, and gradually warmed to
ambient temperature (23 °C). Reaction progress was monitored by
1H NMR analysis. Upon consumption of the substrates, the vola-
tiles were transferred by way of a vacuum transfer tube to a new J-
Young tube for product analysis.
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groups (e.g. alkynes, alkenes, ketones, etc.) are known, this work fo-
cuses specifically on the hydrovinylation of 1,3-dienes. This area has
been reviewed: (a) RajanBabu, T. V. In Comprehensive Asymmetric
Catalysis. Jacobsen, E. N., Pfaltz, A., Yamamto, H., Eds.; Springer:
Berlin, 1999, pp. 417–427. (b) RajanBabu, T. V. Synlett, 2009, 853–
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4291.
(8) Contemporaneous work explored transition metal-catalyzed hy-
drovinylation of dienes with acrylates under similar conditions: Mis-
ono, A.; Uchida, Y.; Saito, T.; Uchida, K. Bull. Chem. Soc. Jpn. 1967
40, 1889–1893.
(9) Hata, G.; Miyake, A. Bull. Chem. Soc. Jpn. 1968, 41, 2762–2764.
General Procedure for [1,4]-Hydrovinylation with Liquid Sub-
In an N2-atmosphere glovebox, a 1.5-mL vial was charged
strates.
1
2
with diene (0.5 mmol, 1 equiv), alkene (0.5 mmol, 1 equiv; or
2.5 mmol, 5 equiv), and a PTFE-coated magnetic stir bar. A stock
solution of (MesDI)Fe(COD) (24 mg/10 mL, 0.005 M) was pre-
pared in a 20-mL scintillation vial, and a portion (1.0 mL, 0.005
mmol, 1 mol %) was transferred to the reaction vial. The vial was
sealed with a PTFE-lined screw cap, and the reaction was main-
tained with stirring at ambient temperature (23 °C) in the glove-
box. The reaction was monitored by GC analysis of aliquots re-
moved from the reaction mixture. Upon consumption of the sub-
strates, the vial was removed from the glovebox. The contents were
exposed to air and allowed to rest for >1 hour, over which time the
deactivated catalyst precipitated from solution. The reaction mix-
ture was filtered through a plug of silica, eluting with pentane (5 × 2
mL), and the filtrate was concentrated under reduced pressure.
ASSOCIATED CONTENT
Supporting Information
,
The Supporting Information is available free of charge via the internet
at http://pubs.acs.org.
(10) (a) Hata, G.; Miyake, A. Bull. Chem. Soc. Jpn. 1968, 41, 2443–2446.
(b) Kagawa, T.; Inoue, Y.; Hashimoto, H. Bull. Chem. Soc. Jpn. 1970
43, 1250–1251.
(11) (a) Hilt, G.; du Mesnil, F.-X.; Lüers, S. Angew. Chem. Int. Ed. 2001
,
Crystallographic information for
cypDIFe(COD) (CIF)
(
iPrDI)Fe(η4-C5H8) and
,
Experimental details; characterization data including NMR spectra of
novel compounds; Mößbauer spectroscopic data; kinetic data; compu-
tational methods and results (PDF)
40, 387–389. (b) Hilt, F.; Lüers, S. Synthesis, 2002, 609–618. (c)
Hilt, G.; Lüers, S.; Schmidt, F. Synthesis, 2004, 634–638. (d) Hilt, G.;
Danz, M.; Treutwein, J. Org. Lett. 2009, 11, 3322–3325. (e) Arndt,
M.; Reinhold, A.; Hilt, G. J. Org. Chem. 2010, 75, 5203–5210. (f)
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2011, 662–668. (i) Arndt, M.; Dindaroğlu, M.; Schmalz, H.-G.; Hilt,
G. Org. Lett. 2011, 13, 6236–6239. (j) Erver, F.; Kuttner, J. R.; Hilt,
G. J. Org. Chem. 2012, 77, 8375–8385. (k) Arndt, M.; Dindaroğlu,
AUTHOR INFORMATION
Corresponding Author
* pchirik@princeton.edu
ORCID
2012
, 3534–3542.
M.; Schmalz, H.-G.; Hilt, G. Synthesis,
(12) (a) Sharma, R. K.; RajanBabu, T. V. J. Am. Chem. Soc. 2010, 132,
3295–3297. (b) Page, J. P.; RajanBabu, T. V. J. Am. Chem. Soc. 2012
Paul J. Chirik: 0000-0001-8473-2898
C. Rose Kennedy: 0000-0003-3681-819X
Máté J. Bezdek: 0000-0001-7860-2894
,
134, 6556–6559. (c) Timsina, Y. N.; Biswas, S.; RajanBabu, T. V. J.
Am. Chem. Soc. 2014, 6215–6218. (d) Timsina, Y. N.; Sharma, R. K.;
RajanBabu, T. V. Chem. Sci. 2015, 6, 3994–4008. (e) Biswas, S.;
Present Addresses
Page, J. P.; Dewese, K. R.; RajanBabu, T. V. J. Am. Chem. Soc. 2015
,
†Department of Chemistry and Biochemistry, UC San Diego; La Jolla,
CA 92093, USA
137, 14268–14271. (f) Jing, S. M.; Balasanthiran, V.; Pagar, V.; Gal-
lucci, J. C.; RajanBabu, T. V. J. Am. Chem. Soc. 2017, 139, 18034–
18043.
Notes
The authors declare no competing financial interest.
(13) Cobalt catalysts also effect [1,2]-hydrovinylation of alkenes: (a)
Pillai, S. M.; Tembe, G. L.; Ravindranathan, M. J. Mol. Catal. 1993
,
ACKNOWLEDGMENT
84, 77–86. (b) Grutters, M. M. P.; Müller, C.; Vogt, D. J. Am. Chem.
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M. P. ; van der Vlugt, J. I.; Pei, Y.; Mills, A. M.; Lutz, M.; Spek, A. L.;
V.A.S. thanks the NIH for a Ruth L. Kirschstein National Research
Service Award (F32 GM109594). M.J.B. thanks the Natural Sciences
and Engineering Research Council of Canada for a predoctoral fellow-
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2
3
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must fall at or after the rate-determining step and that C–C bond-
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9
(52) Representative procedures are reported here; for complete experi-
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