Iron-catalyzed intermolecular [2π + 2π] cycloaddition

Article abstract of DOI:10.1021/ja202992p

The bis(imino)pyridine iron dinitrogen compounds, (^iPrPDI) Fe(N₂)₂ and [(^MePDI)Fe(N₂)] ₂(μ₂-N₂) (^RPDI = 2,6-(2,6-R ₂-C₆H₃N=CMe)₂C₅H ₃N; R = ⁱPr, Me), promote the catalytic intermolecular [2π + 2π] cycloaddition of ethylene and butadiene to form vinylcyclobutane. Stoichiometric experiments resulted in isolation of a catalytically competent iron metallocycle intermediate, which was shown to undergo diene-induced C-C reductive elimination. Deuterium labeling experiments establish competitive cyclometalation of the bis(imino)pyridine aryl substituents during catalytic turnover.

Full text of DOI:10.1021/ja202992p

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Iron-Catalyzed Intermolecular [2π þ 2π] Cycloaddition
Sarah K. Russell,† Emil Lobkovsky,‡ and Paul J. Chirik*,†
†Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
‡Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853, United States
S Supporting Information
b
Scheme 1
ABSTRACT: The bis(imino)pyridine iron dinitrogen com-
pounds, (iPrPDI)Fe(N2)2 and [(MePDI)Fe(N2)]2(μ2-N2)
(RPDI = 2,6-(2,6-R2-C6H3NdCMe)2C5H3N; R = iPr, Me),
promote the catalytic intermolecular [2π þ 2π] cycloaddi-
tion of ethylene and butadiene to form vinylcyclobutane.
Stoichiometric experiments resulted in isolation of a cataly-
tically competent iron metallocycle intermediate, which was
shown to undergo diene-induced CꢀC reductive elimina-
tion. Deuterium labeling experiments establish competitive
cyclometalation of the bis(imino)pyridine aryl substituents
during catalytic turnover.
ycloaddition reactions are powerful and broadly applied
methods for the construction of cyclic compounds. The
C
reported observation of vinylcyclobutane as a component of the
product mixture obtained from addition of ethylene to butadiene in
the presence of organotitanium catalysts at high temperature (T =
135ꢀ150 °C) and pressure (1200 psi).12 A related iron-catalyzed
[2π þ 2π] cycloaddition of norbornadiene and butadiene has also
been described, but the selectivity for the cyclobutane product was
relatively low (∼20%). Codimerization of ethylene with butadiene or
isopropene has also been reported with Fe(acac)3/AlR3 mixtures to
yield predominantly linear products.13
prototypical example is the DielsꢀAlder reaction, a symmetry-
allowed [4π þ 2π] transformation1 between an alkene and a
diene to yield a six-membered ring. By contrast, the correspond-
ing [2π þ 2π] cycloaddition to form cyclobutane derivatives,
while thermodynamically favorable, is thermally forbidden,1 and
alternative synthetic routes such as photochemistry,2 use of
highly strained substrates3 and activated π-systems,4 and metal
catalysis5 have all been used to overcome the orbital symmetry
constraint. Our laboratory has reported a facile iron-catalyzed
method for the intramolecular [2π þ 2π] cyclization of R,ω-
dienes to the corresponding cyclobutane-containing bicycles.6
One notable feature of the base metal-catalyzed method is the
ability to rapidly cyclize unactivated dienes such as 1,6-hepta-
diene to [0.2.3]bicycloheptane as well as various allyl amines to
the corresponding disubstituted fused pyrrolidines in the absence
of chromophores.6 Extension of the [2π þ 2π] cyclization to
intermolecular examples is necessary if a general base metal-
catalyzed process for the synthesis of substituted cyclobutanes is
to be realized. Here we describe progress toward this objective
with the synthesis of vinyl cyclobutane from the intermolecular
cycloaddition of ethylene and butadiene. Investigations into the
mechanism of this unusual reaction and isolation and character-
ization of a catalytically-competent intermediate are also described.
The reaction of ethylene and butadiene is often cited as a peda-
gogical example of the [4πþ 2π] DielsꢀAlder reaction,7 although in
practice high temperatures (175 °C) and pressures (6000 psi) are
required to promote cycloaddition.8 Addition of metal catalysts
facilitates the addition, but linear hexadienes are the major products.9
Ritter and co-workers10 have reported an iron-catalyzed version of
this reaction for the selective 1,4-addition of various styrene deriva-
tives to dienes such as isoprene and myrcene, an interesting variant of
established cobalt-catalyzed hydrovinylation reactions.11 Cannell has
Our laboratory has recently reported the straightforward
synthesis and catalytic utility of bis(imino)pyridine iron buta-
diene compounds, (RPDI)Fe(η4-C4H6) (RPDI = 2,6-(2,6-R2-
C6H3NdCMe)2C5H3N; R = iPr, Me).6,14 The potential inter-
mediacy of these species in an intermolecular [2π þ 2π]
cycloaddition with ethylene prompted additional studies. Ex-
posure of an equimolar mixture of ethylene and butadiene to a
benzene-d6 solution containing 5 mol % (total [Fe]) of either
(iPrPDI)Fe(N2)2 or [(MePDI)Fe(N2)]2(μ2-N2)16 at 23 °C
15
furnished vinylcyclobutane in 95% yield over the course of 24
and 16 h, respectively (Scheme 1). Only the product from iron-
catalyzed intermolecular [2π þ 2π] cycloaddition was observed;
1
no hydrovinylation products were detected by H NMR spec-
troscopy. The reaction was also performed on a preparative scale
with both iron compounds in decane solvent and exclusively
furnished vinylcyclobutane in >95% yield.
Introduction of a methyl group into the diene substrate altered
the outcome of the catalytic reaction, favoring 1,4-addition over
[2π þ 2π] cycloaddition. A benzene-d6 solution containing
an equimolar mixture of ethylene and isoprene exclusively furn-
ished 5-methylhexa-1,4-diene in the presence of 2.5 mol % of
Received: April 1, 2011
Published: May 20, 2011
r
2011 American Chemical Society
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[(MePDI)Fe(N2)]2(μ2-N2) after 16 h at 23 °C. Performing the
same reaction with 5 mol % of (iPrPDI)Fe(N2)2 also resulted in
1,4-addition chemistry, with (E)-2-methylhexa-2,4-diene identi-
fied as the sole product. This diene likely arises from alkene
isomerization of initially formed 5-methylhexa-1,4-diene to form
the conjugated diene, as (iPrPDI)Fe(N2)2 has been shown to be
an effective iron-based olefin isomerization catalyst.15 The most
substituted diene examined, 2,3-dimethylbuta-1,3-diene, pro-
duced no reaction with ethylene in the presence of either
(iPrPDI)Fe(N2)2 or [(MePDI)Fe(N2)]2(μ2-N2) (Scheme 1).
The observation of hydrovinylation with ethylene and isoprene
expands the scope of this iron-catalyzed carbonꢀcarbon bond-
forming reaction, as the previous example was effective only with
substituted styrenes.10
The mechanisms of the iron-catalyzed intermolecular [2π þ
2π] cycloaddition and 1,4-addition were probed with a series of
deuterium labeling experiments (Scheme 2). Performing the
catalytic cyclization of butadiene with CD2dCD2 in the presence
of 2.5 mol % of [(MePDI)Fe(N2)]2(μ2-N2) exclusively furnished
vinylcyclobutane-d4, with the deuterium atoms solely in the 2 and
3 positions of the four-membered ring (Scheme 2). With
isoprene as the diene substrate, 1,4-addition of CD2dCD2 in
the presence of 2.5 mol % of [(MePDI)Fe(N2)]2(μ2-N2) furn-
ished 5-methylhexa-1,4-diene-d4, with isotopic labels in the 1 and
2 positions of the diene as well as the in the methyl group
(Scheme 2). Other unidentified minor products accompanied
formation of 5-methylhexa-1,4-diene-d4 and were observed only
when CD2dCD2 was used.
hydrovinylation as a precursor to overall [2π þ 2π] cycloaddi-
tion. Likewise, performing the cyclization in presence of 1,4-
hexadiene did not consume the diene. These results are con-
sistent with the deuterium labeling experiments, which also
support exclusive cycloaddition.
Stoichiometric experiments were also conducted in an attempt
to observe and characterize catalytically competent intermedi-
ates. Our laboratory has previously reported that addition of 1,3-
butadiene to the iron dinitrogen compound, (iPrPDI)Fe(N2)2,
yielded the corresponding trans-butadiene derivative, (iPrPDI)Fe(η4-
C4H6).6 The methyl analogue, (MePDI)Fe(η4-C4H6), was prepared
in a similar manner. An analogous isoprene compound was also
spectroscopically characterized using the same synthetic route. In a
related experiment, addition of ethylene to a benzene-d6 solution of
[(MePDI)Fe(N2)]2(μ2-N2) initially yielded a yellow solution, tenta-
tively assigned as the iron ethylene complex, (MePDI)Fe(η2-C2H4),
which is significantly less stable than (MePDI)Fe(η4-C4H6) and
readily forms the latter compound when treated with butadiene.
Treatment of a diethyl ether solution of (MePDI)Fe(η4-C4H6)
with a slight excess of ethylene (∼3 equiv) furnished a new
diamagnetic product, identified as the bis(imino)pyridine iron
metallocycle, containing an allyl and an alkyl ligand, arising from
ethylene insertion into the coordinated diene (eq 1).
One possibility is that the catalytic formation of vinylcyclobu-
tane proceeds via initial hydrovinylation to form 1,4-hexadiene,
followed by cyclization to yield the observed [2π þ 2π] product.
Addition of excess 1,4-hexadiene to a benzene-d6 solution of
[(MePDI)Fe(N2)]2(μ2-N2) produced no reaction, arguing against
The iron metallocycle, (MePDI)Fe((CH2)3(CH)2CH2), was
also cleanly prepared from addition of vinylcyclobutane to a
benzene-d6 solution of [(MePDI)Fe(N2)]2(μ2-N2), demonstrat-
ing the reversibility of the CꢀC bond-forming reaction. This
reaction also represents a rare observation of sp3ꢀsp3 CꢀC
bond activation under mild conditions by an iron compound.17
The diamagnetic bis(imino)pyridine iron metallocycle was
Scheme 2
1
characterized by H and 13C NMR spectroscopy, M€ossbauer
spectroscopy, combustion analysis, and single-crystal X-ray dif-
fraction. Representations of the solid-state structure of the
molecule are presented in Figure 1, along with selected metric
parameters. The ironꢀcarbon distances are consistent with an
η3-allyl ligand tethered to a metal alkyl, giving rise to a rare
example of a six-coordinate bis(imino)pyridine iron compound.18
Figure 1. Representation ofthe solid-state structure of (MePDI)Fe((CH2)3(CH)2CH2) at 30%probability ellipsoids. Bond distances (Å): C(26)ꢀC(27),
1.505(3); C(27)ꢀC(28), 1.453(4); C(28)ꢀC(29), 1.524(7);C(29)ꢀC(30), 1.341(6);Fe(1)ꢀC(26), 2.0797(19); Fe(1)ꢀC(29), 2.155(5); Fe(1)ꢀC(30),
2.096(4); Fe(1)ꢀC(31), 2.149(2). Angles (deg): N(2)ꢀFe(1)ꢀC(26), 84.13(7); N(2)ꢀFe(1)ꢀC(30), 164.03(17); C(26)ꢀFe(1)ꢀC(30), 107.00(17).
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Scheme 3
Scheme 4
in the aryl methyl groups of the bis(imino)pyridine ligand,
suggesting that cyclometalation21 is also competitive with the
insertionꢀdeinsertion reaction. Analysis of the isotopically la-
beled product by 1H NMR spectroscopy in benzene-d6 revealed
proton incorporation into both the C(R) and C(β) positions. In
a control experiment, CD2dCD2 was added to a benzene solu-
tion of [(MePDI)Fe(N2)]2(μ2-N2) at 23 °C, and rapid deuterium
incorporation into the bis(imino)pyridine aryl methyl groups
was observed, demonstrating reversible cyclometalation from the
putative iron ethylene compound as well.21
Converse experiments were also conducted with bis(imi-
no)pyridine iron compounds with deuterated methyl groups on
the aryl substituents. These compounds will be denoted with an
asterisk. Monitoring a benzene solution of (Me*PDI)Fe(η4-C4H6)
by 2H NMR spectroscopy established deuterium incorporation into
the terminal positions of the butadiene ligand. No isotopic label was
observed in the internal positions of coordinated diene over the
course of 16 h. The relevance of the isotopic exchange process to
catalytic cyclization was also examined by monitoring the isotopic
composition of the [(MePDI)Fe] species by 2H NMR spectroscopy
during turnover. At both 20 and 50% conversion to vinylcyclobu-
tane-d4 using CD2dCD2, deuterium incorporation into the aryl
methyl groups was observed, demonstrating that aryl substituent
cyclometalation occurs during catalytic turnover.
On the basis of all of the experimental observations, a mechan-
ism for the bis(imino)pyridine iron-catalyzed intermolecular [2π þ
2π] cyclization and 1,4-addition10 is proposed (Scheme 5).
Catalytic turnover initiates with displacement of the dinitrogen
ligands by butadiene to form the iron η4-butadiene complex.
Insertion of ethylene forms the observed and characterized iron
metallocycle, and turnover-limiting butadiene-induced reductive
elimination furnishes vinylcyclobutane and regenerates the pro-
pagating iron butadiene compound. With isoprene as the sub-
strate, it is likely the analogous intermediate is formed, as
proposed by Ritter.10 The more sterically hindered diene is a
less potent ligand and, as a consequence, decreases the rate of
ligand-induced reductive elimination, allowing β-hydrogen elim-
ination from the metallocycle to become competitive. Subse-
quent CꢀH reductive elimination furnishes the observed 1,4-
addition product. In both pathways, deuterium labeling experi-
ments establish cyclometalation from both methyl and isopropyl
aryl substituents during turnover.
The competency of (MePDI)Fe((CH2)3(CH)2CH2) in the
catalytic [2π þ 2π] cycloaddition process was also evaluated to
gain insight into the mechanism of turnover. Allowing a benzene-
d6 solution of (MePDI)Fe((CH2)3(CH)2CH2) to stand for 24 h
at 23 °C resulted in dissociation of ethylene, with reconstitution
of (MePDI)Fe(η4-C4H6). Under the experimental conditions
employed, a 2:1 mixture of (MePDI)Fe((CH2)3(CH)2CH2) and
(
MePDI)Fe(η4-C4H6) was obtained after 24 h (Scheme 3). This
reaction was suppressed by addition of excess ethylene to the
solution. Attempts to promote CꢀC reductive elimination by
warming a benzene-d6 solution of (MePDI)Fe((CH2)3(CH)2CH2)
to 65 °C were unsuccessful, as decomposition of the iron compound
predominated. No vinylcyclobutane was observed, establishing that
CꢀC reductive elimination does not proceed directly from
(
MePDI)Fe((CH2)3(CH)2CH2).19
The lack of thermal reductive elimination from (MePDI)Fe-
((CH2)3(CH)2CH2) prompted exploration of alternative li-
gand-induced pathways.20 Addition of 4 atm of CO to a ben-
zene-d6 solution of (MePDI)Fe((CH2)3(CH)2CH2) at 23 °C
cleanly and quantitatively induced CꢀC bond formation and
generated vinylcyclobutane along with the iron dicarbonyl deriv-
ative, (MePDI)Fe(CO)2.16 Hydrogenation (1 atm, 16 h, 23 °C)
of (MePDI)Fe((CH2)3(CH)2CH2) produced a similar outcome,
as ethylcyclobutane was observed, consistent with the initial
formation of vinylcyclobutane, followed by rapid alkene hydro-
genation.16 In chemistry more relevant to catalytic turnover,
treatment of (MePDI)Fe((CH2)3(CH)2CH2) with excess buta-
diene also liberated vinylcyclobutane, with concomitant forma-
tion of the iron butadiene compound, (MePDI)Fe(η4-C4H6).
Notably, addition of isoprene to the same iron compound did not
induce CꢀC bond formation. All of the above experiments are
consistent with a ligand-induced reductive elimination process;
however, it is noteworthy that ethylene and isoprene are not
sufficiently potent ligands to promote CꢀC bond formation and
vinylcyclobutane dissociation.
The observation of ethylene loss from (MePDI)Fe((CH2)3-
(CH)2CH2) established the reversibility of olefin insertion. To
further explore this phenomenon, a benzene solution of
(
MePDI)Fe((CH2)3(CH)2CH2) was treated with 3 equiv of
2
CD2dCD2 (Scheme 4). Monitoring the reaction by H NMR
spectroscopy established deuterium incorporation into the ex-
pected C(R) and C(β) positions of the alkyl portion of the
metallocycle. Notably, deuterium incorporation was also observed
In summary, an iron-catalyzed intermolecular [2π þ 2π] cy-
cloaddition between butadiene and ethylene to form vinylcyclo-
butane has been discovered. Isolation of a catalytically competent
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Scheme 5
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’ ASSOCIATED CONTENT
S
Supporting Information. Complete experimental proce-
b
dures, characterization, and crystallographic data for (MePDI)Fe-
((CH2)3(CH)2CH2). This material is available free of charge via
the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
pchirik@princeton.edu
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Int. Ed. 2007, 46, 2269.
’ ACKNOWLEDGMENT
We thank the U.S. National Science Foundation and the
Deutsche Forschungsgemeinschaft for a Cooperative Activities
in Chemistry between U.S. and German Investigators grant.
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