Mechanisms by which alkynes react with CpCr(CO)3H. Application to radical cyclization

Article abstract of DOI:10.1021/ja306120n

The reaction of CpCr(CO)₃H with activated alkynes in benzene has been examined. The kinetics of these reactions have been studied with various alkynes, along with the stereochemistry with which the alkynes are hydrogenated. The hydrogenation of phenyl acetylene and diphenyl acetylene with CpCr(CO) ₃H has been shown to occur by a hydrogen atom transfer (HAT) mechanism. The reaction of CpCr(CO)₃H with dimethyl acetylenedicarboxylate (DMAD) produced hydrogenated products as well as phenyl substitution from reaction with solvent. On the basis of kinetic data, it is thought that the reaction of DMAD may proceed via a single electron transfer (SET) as the rate-determining step. The radical anion of dimethylfumarate was observed by EPR spectroscopy during the course of the reaction, supporting this claim. The aromatic 1,6 eneyne (8) gave cyclized products in 78% yield under catalytic conditions (35 psi H₂), presumably by the 5-exo-trig cyclization of the vinyl radical arising from H? transfer. Using a cobaloxime catalyst (12) hydrogenation was completely eliminated to yield 100% cyclized products.

Full text of DOI:10.1021/ja306120n

Article
pubs.acs.org/JACS
Mechanisms by which Alkynes React with CpCr(CO)3H. Application to
Radical Cyclization
Deven P. Estes, Jack R. Norton,* Steffen Jockusch, and Wesley Sattler
Department of Chemistry, Columbia University, 3000 Broadway, New York, New York 10027, United States
S
* Supporting Information
ABSTRACT: The reaction of CpCr(CO)3H with activated alkynes
in benzene has been examined. The kinetics of these reactions have
been studied with various alkynes, along with the stereochemistry
with which the alkynes are hydrogenated. The hydrogenation of
phenyl acetylene and diphenyl acetylene with CpCr(CO)3H has
been shown to occur by a hydrogen atom transfer (HAT)
mechanism. The reaction of CpCr(CO)3H with dimethyl
acetylenedicarboxylate (DMAD) produced hydrogenated products
as well as phenyl substitution from reaction with solvent. On the
basis of kinetic data, it is thought that the reaction of DMAD may proceed via a single electron transfer (SET) as the rate-
determining step. The radical anion of dimethylfumarate was observed by EPR spectroscopy during the course of the reaction,
supporting this claim. The aromatic 1,6 eneyne (8) gave cyclized products in 78% yield under catalytic conditions (35 psi H2),
presumably by the 5-exo-trig cyclization of the vinyl radical arising from H• transfer. Using a cobaloxime catalyst (12)
hydrogenation was completely eliminated to yield 100% cyclized products.
used to effect hydrogenation, this reaction has also been used in
the catalysis of chain transfer during radical polymerizations,4
and in the initiation of radical cyclizations.5 However, H•
transfer to CC, forming vinyl radicals, has seldom been
INTRODUCTION
■
A variety of mechanisms have been proposed for the reactions
of alkynes with transition-metal hydrides. The most common of
these (eq 1) involves coordination of the alkyne to the metal,
observed.2h,i Ungvar
́
y reported, on the basis of CIDNP
evidence, the transfer of H• to phenylacetylene during
hydroformylation by HCo(CO)4.2i Gridnev and Wayland also
observed trans selectivity in the reaction of cobalt porphyrin
hydrides with various alkynes and assumed a radical
mechanism.2h One expects HAT to CC to be slower than
transfer to CC because alkynes react with radicals more
slowly than olefins do.6
We have recently shown that, under H2, CpCr(CO)3H
catalyzes the transfer of H• and effects the cyclization of various
dienes by a radical mechanism.5a We next considered the
possibility of using vinyl radicals generated by HAT to alkynes.
Despite the fact that alkynes react more slowly, vinyl radicals
are less stable, and should cyclize faster, than the RO2C(Me)-
CR• radicals we have been making (by H• transfer to
acrylates), minimizing the formation of hydrogenated products.
We have demonstrated here that it is possible to form vinyl
radicals from HAT to some alkynes, while others react by
different mechanisms. We have also shown that we can cyclize
an appropriate enyne by a radical mechanism.
followed by its insertion into the M−H bond.1 This mechanism
results in syn addition of the M−H to the triple bond, and gives
a Z-alkene after protolytic cleavage of the M−C bond.
However, there are a number of reports of the anti addition
of M−H to alkynes, resulting in E-olefins after protonolysis.2
Several mechanisms have been offered to explain this, one of
which involves radicals. Clark has proposed that the platinum
dihydride in eq 2 transfers first an electron and then a proton to
EXPERIMENTAL SECTION
■
an electron-poor alkyne, producing a vinyl radical; isomer-
ization of that radical, coordination, and reductive elimination
give an E-olefin.2b
All manipulations were performed under an argon atmosphere using
standard Schlenk or inert atmosphere box techniques. NMR spectra
Transition-metal hydrides with weak metal−hydrogen bonds
have been shown to donate H• to appropriately activated
CC bonds, giving carbon-centered radicals.3 While originally
Received: June 28, 2012
Published: August 17, 2012
© 2012 American Chemical Society
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were taken on either a Bruker 300, 400, or 500 MHz spectrometer. IR
spectra were taken with a Perkin-Elmer Spectrum 2000 FT-IR
spectrometer. Gas chromatography was performed on a HP-5890A gas
chromatograph utilizing a DB-5 column with He as the carrier gas. X-
ray diffraction data were collected on a Bruker Apex II diffractometer.
Crystal data, data collection and refinement parameters are
summarized in the Supporting Information (SI) (Table S1). The
structures were solved using direct methods and standard difference
map techniques, and were refined by full matrix least-squares
procedures on F2 with SHELXTL (Version 6.1).7 Benzene and THF
were distilled from Na/benzophenone ketyl and stored over 3 Å
molecular sieves. Benzene-d6 (Cambridge Isotope Laboratories) was
dried by distillation from CaH2 and then deoxygenated by three
freeze−pump−thaw cycles. All liquids were dried by distillation from
CaH2 and then deoxygenated by three freeze−pump−thaw cycles and
stored under an argon atmosphere. Phenylacetylene, phenylacetylene-
d1, methyl propiolate, diphenylacetylene, dimethyl acetylenedicarbox-
ylate, and di-tert-butyl acetylenedicarboxylate were purchased from
commercial sources.
129.04, 128.40, 128.28, 128.12, 127.96, 127.36, 127.17, 127.02, 126.40,
125.99, 33.12, 25.70, 14.96. IR (neat) 3060, 3023 (sp2 C−H stretch),
2962, 2922, 2852 (sp3 C−H stretch), MS (APCI) 299 [M + 1].
1
2-(Diphenylmethylene)-1-methyl-2,3-dihydro-1H-indene (11). H
NMR (300 MHz, CDCl3, δ) 7.37−7.14 (m, 14H, ArH), 4.14 (q (J =
7.0 Hz), 1H, C−H), 3.81 (AB system (J = 21 Hz, Δν = 263.7 Hz), 2H,
inequivalent CH2 pair) 1.08 (d (J = 7.0 Hz), 3H, CH3). 13C NMR
(125 MHz, CDCl3, δ) 147.6, 144.3, 143.0, 142.5, 141.1, 136.3, 129.5,
129.0, 128.5, 128.2, 126.71, 127.66, 126.60, 124.4, 123.8, 43.6, 37.9,
21.4 IR (neat) 3079, 3056, 3020 (sp2 C−H stretch), 2962, 2922, 2860
(sp3 C−H stretch) cm−1. MS (FAB) 295 [M − 1]
Kinetic Measurements. Stock solutions (C6D6) of both CpCr-
(CO)3H and the appropriate alkyne were added separately to a J-
Young NMR tube and frozen in two layers with appropriate
concentrations to achieve a 10-fold excess of alkyne. This was kept
frozen until the experiment began, when it was quickly melted, mixed,
and inserted into the probe of a 500 MHz NMR spectrometer which
had been equilibrated to 50.0 ( 0.5)°C using an ethylene glycol
chemical shift thermometer. Spectra were taken every 3 min, and the
intensity of the hydride resonance (δ −5.6) was compared with that of
an internal standard (hexamethylcyclotrisiloxane). Reactions were
monitored through at least three half-lives and fit to a first-order
exponential. The product peaks from the reaction with DMAD were fit
to the model in Scheme S1 (SI) using Kintecus kinetic modeling
software.12 Reported rate constants other than that for phenyl-
acetylene are the average of three kinetic runs.
Materials. CpCr(CO)3H and CpCr(CO)3D were synthesized by
known procedures and sublimed prior to use.8 Methyl propiolate-d1,9
dimethylphenylmaleate,10 and the cobaloxime 1211 were also
synthesized by literature procedures.
(3-(2-Ethynylphenyl)prop-1-ene-1,1-diyl)dibenzene (8). To a
solution of 2-bromo-phenylethynyltrimethylsilane (1.58 mmol) in
THF (16 mL) was added nBuLi (1.74 mmol, 1.1 equiv) at −78 °C and
stirred for 20 min. CuCN (71 mg, 0.79 mmol) was then added to the
solution and the temperature raised to −20 °C for 20 minutes after
which time the solution turned from yellow to pale pink. The solution
was returned to −78 °C, and 3-bromo-1,1-diphenyl-prop-1-ene (431
mg, 1.58 mmol in 1 mL THF) was added dropwise, and the mixture
was brought to room temperature slowly and stirred overnight. The
reaction was quenched with NH4Cl, extracted with ether (50 mL, 3
times), and dried with MgSO4 and the solvent was removed.
To this crude mixture in 5 mL of 10:1 THF/H2O was added
dropwise a solution of 0.45 g of tetrabutylammonium fluoride
dissolved in 2 mL of the same solvent mixture at 0 °C. This was
stirred for 12 h and worked up in the same manner as before. This was
then isolated (clear oil, 199 mg, 43% yield) from a silica column using
a 4% toluene in hexane mixture as mobile phase (Rf = 0.19). 1H NMR
(300 MHz, CDCl3, δ), 7.49−7.13 (m, 14H, ArH), 6.27 (t (J = 7.5 Hz),
1H, C−H), 3.66 (d (J = 7.5 Hz), 2H, CH2) 3.19 (s, 1H, C−H).
13C NMR (100 MHz, CDCl3, δ) 143, 142.7, 142.4, 139.8, 132.9, 130.0,
129.0, 128.6, 128.3, 128.1, 127.4, 127.1, 127.04, 127.00, 126.0, 121.6,
82.2, 81.3, 34.5. IR (neat) 3293 (C−H stretch), 2104 (CC
stretch), 1598 (ring stretch), 700 (C−H bend). MS (APCI) 295 [M
+ 1]+.
EPR Measurements. X-band EPR spectra were taken on a Bruker
EMX spectrometer at ambient temperature in benzene in 4 mm J-
Young style tubes (concentrations of organic reactants were ∼1 M). g
values were calculated by comparison with an internal sample of
TEMPO (g = 2.00623).13 EPR simulations were done with Bruker’s
Simphonia software.14 EPR spectra not included in the text can be
found in the SI.
RESULTS AND DISCUSSION
■
Unactivated alkynes such as TMSCCH and 1-octyne proved
unreactive. However, aryl alkynes, such as phenylacetylene, are
readily consumed by CpCr(CO)3H (1). The reaction of
phenylacetylene (0.044 M) with 2 equiv of 1 (0.088 M) yields
the products shown in eq 3. The major products are styrene
Procedure for Cyclizations. Caution! All reactions under gas
pressure should be properly shielded! A solution of (3-(2-ethynylphenyl)-
prop-1-ene-1,1-diyl)dibenzene (8) (0.187 mmol) and CpCr(CO)3H
(0.014 mmol) in benzene (0.5 mL) was added to a Fischer−Porter
reactor that was subsequently charged with an appropriate pressure of
H2. This reaction was then heated with vigorous stirring until the
reaction was over, during which time the green color changed to
brown and then to greenish-gray once the starting material had been
consumed (no color change observed with cobaloxime 12). For details
of compound isolation see SI.
(44%), ethylbenzene (5%), and the organometallic complex 4
(51%). Crystals of 4 were identified by X-ray crystallography as
the fulvene complex (Figure 1); its structure is similar to that of
known fulvene complexes15 (see SI for structural parameters).
The observation of 4 made us question whether H• transfer
was occurring. In order to determine whether the addition of
M−H was syn or anti we treated phenylacetylene-d1 with 1. We
obtained cis- and trans-styrene-d1 in a 1:1 ratio, consistent with
the mechanism in Scheme 1. Donation of an H• to the
terminal carbon of phenylacetylene will give the vinyl radical 5,
which is linear.16 Transfer of a second H• from 1 will not be
affected by the position of the deuterium, so there will be a 1:1
E:Z (7:6) ratio for the resulting styrene-d1. The same result has
2-Benzhydryl-3-methyl-1H-indene (10). 1H NMR (300 MHz,
CDCl3, δ) 7.30−7.16 (m, 14H, ArH), 5.53 (s, 1H, CPh2H), 3.19 (q (J
= 1.9 Hz), 2H, CH2), 2.08 (t (J = 1.9 Hz), 3H, CH3) (long-range
coupling confirmed by COSY). 13C NMR (75 MHz, CDCl3) 146.9,
143.75, 143.0, 135.2, 129.2, 128.4, 128.0, 126.4, 124.4, 123.4, 118.8,
50.2, 39.8, 10.8 IR (neat) 3056, 3024 (sp2 C−H stretch), 2962,
2918(sp3 C−H stretch) cm−1 MS (APCI) 167 [CH(Ph)2]+, 295 [M −
1]+, 329 [M + 33]+
(3-(2-Ethylphenyl)prop-1-ene-1,1-diyl)dibenzene (9). 1H NMR
(300 MHz, CDCl3, δ) 7.41−7.16 (m, 14H, ArH), 6.20 (t (J = 7.5
Hz), 1H, C−H), 3.47 (d (J = 7.5 Hz), 2H, CH2−CH), 2.54 (q (J
= 7.5 Hz), 2H, CH2−CH3), 1.12 (t (J = 7.5 Hz), 3H, CH3). 13C NMR
(100 MHz, CDCl3, δ) 142.50, 142.25, 142.19, 139.88, 138.59, 129.90,
́
been reported by Ungvary for the reaction of phenylacetylene-
d1 with HCo(CO)4.2i
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for example, back transfer of H• competes not only with back
transfer of D• but also with addition of a second D•; both
styrene and phenylacetylene-d1 are products. Addition of excess
CpCr(CO)3• favors exchange over hydrogenation and allows
estimation of kH/kD as 0.2substantially inverse.
In view of the fact that 1 does transfer H• to phenyl-
acetylene, we designed a substrate that could be cyclized by that
reaction. The ease of transfer to phenylacetylene suggested the
enyne 8. (Our previous work3b showed that HAT to 1,1-
diphenyl-1-propene was very slow.) When we treated 8 with a
stoichiometric amount (2 equiv) of 1, we obtained the indene
product 10 in 42% yield, with the balance being the
hydrogenated product 9.
Figure 1. X-ray structure of 4 (20% ellipsoids, hydrogens omitted for
clarity).
Under H2 pressure in the presence of a catalytic amount of 1,
10 was formed in 43% yield along with 22% of 9 and 35% of
the double bond isomer 11, for a total of 78% cyclized material.
Figure 2. Kinetics of HAT from 1 to phenyl acetylene.
With our recently discovered cobaloxime catalyst 12, 8 gives
only 10 and 11 in 48% and 52% yields, respectively (eq 7).17
This is expected since the cobalt hydride is created in situ in
very small concentrations, making this catalyst good at
suppressing hydrogenation.
Scheme 1
Further evidence for a radical mechanism is offered by
comparing the rate constant for phenylacetylene/1 with that for
styrene/1. When we treat 1 (0.012 M) with a 10-fold excess of
phenylacetylene (≥0.124 M), the amounts of 3 and 4 that form
are very small, i.e., the yield of styrene is greater than 95%. The
disappearance of 1 is first order, and kobs is linear in [PhC
CH] (Figure 2), showing the rate law to be that in eq 4.
The mechanism is probably that shown in Scheme 2. The
vinyl radical produced by the initial H• transfer cyclizes to
make 13. A second H• transfer, from 1 to the methylene in 13,
then isomerizes the double bond in 13 to an internal position,
d[1]
dt
−
= 2k1[1][PhC CH]
Scheme 2
(4)
The resulting rate constant (k1) ((2.5 0.1) × 10−3 M−1
s−1) for HAT to PhCCH is 6 times slower than kH for HAT
from 1 to styrene.3b The ratio is very similar to that (1:6) for
6
t
the addition of the Bu radical to PhCCH/PhCHCH2.
The similarity in the rate constant ratio strongly supports our
description of eq 3 as a H• transfer process.
The CrH/CrD isotope effect cannot be measured accurately
because of the competition between back transfer and
hydrogenation after the initial H• transfer. With CpCr(CO)3D,
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giving 10; the fact that no 13 accumulates implies that
isomerization to 10 is fast. The same vinyl radical which made
13 can also react with 1 to form styryl compound 14, which can
further hydrogenate to form 9 or cyclize to form 11.
In order to study the effect of substitution we next studied
diphenylacetylene. Treating PhCCPh with 1 gave only the
expected hydrogenation products, E- and Z-stilbene, as
observed by GC, with no further hydrogenation.
REACTION OF CPCR(CO)3H WITH
ELECTRON-POOR ALKYNES
Treating methyl propiolate (0.0365 M) with stoichiometric 1
(2 equiv) produced methyl acrylate (39%) as well as small
oligomers (confirmed by mass spectrometry and H NMR) of
the methyl propiolate. Larger concentrations (0.58 M) of
propiolate gave lower yields of acrylate, presumably because the
amount of oligomerization increases.
■
1
When we treat 1-d1 with excess methyl propiolate, only
hydrogenation occurs. When we carry out this reaction in the
presence of excess CpCr(CO)3• (15) (in equilibrium with the
dimer [CpCr(CO)3]219) H/D exchange is observed at the start
of the reaction, showing the intermediacy of a vinyl radical
(Scheme 4). (We have found the addition of the metalloradical
Scheme 4
15 to be a useful general method for observing H/D exchange
in the absence of other reactions.) The vinyl radical will not be
linear, but will invert rapidly,16a and both cis and trans acrylate
esters will be formed (a cis/trans of 1.4:1 is observed).
In the presence of a large excess of methyl propiolate, the
reaction is first order in 1. Given the small chain length of the
oligomers and assuming rapid chain propagation, the rate law
for Scheme 5 will be the same as that for phenyl acetylene (eq
Figure 3. Time dependence of cis/trans ratio in diphenyl acetylene
hydrogenation.
The rate constant for HAT (k1 = (1.4 0.3) × 10−4 M−1
s−1) was approximately 20 times smaller than that of PhC
CH, similar to the pattern seen for substituted alkenes.3b This
trend strongly suggests that the key step is HAT.
The cis/trans ratio was not constant over the course of the
reaction, beginning with a slight preference for trans, then
favoring cis formation, but ending with predominantly trans
(Figure 2).
Scheme 5
We propose the mechanism in Scheme 3 to explain this
variation over time. The initial product of the HAT from
Scheme 3
4), with k1 = (3.2
0.3) × 10−4 M−1 s−1. The overall
mechanism is that in Scheme 5; after rate-determining HAT,
the vinyl radical either hydrogenates by reaction with 1 or adds
to propiolate to make an oligomer.
Treating dimethyl acetylenedicarboxylate with 1 (eq 8) gave
16 as the major product (76%) in addition to 17 (17%), 18
CpCr(CO)3H to PhCCPh is presumably a radical cage, with
the Cr remaining near the hydrogen it has just transferred; the
second CpCr(CO)3H will prefer to approach the caged vinyl
radical on the side away from the first Cr (and the H), and will
give predominantly trans (E) product. Later, when the lower
[1] permits cage escape, the free vinyl radical will prefer
approach on the same side as the first hydrogen and give largely
Z-stilbene. Eventually cis/trans isomerization, catalyzed by
reversible H• transfer, will move toward the largely E
equilibrium mixture (the thermodynamic ratio is known18 to
be 99.8:0.2).
(5%), and oligomers as big as tetramers (2%) as minor
products. There was a 4:1 preference for the cis hydrogenation
product 17 over the trans product 18.
The appearance of 16 makes a HAT mechanism seem less
likely. No phenyl-substituted product was seen with methyl
propiolate, implying that the reactivity is very different. While
attack on benzene by vinyl radicals is known,20 it is generally
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intramolecular and occurs more readily for unstabilized vinyl
radicals (although more reactive radicals such as HO• readily
attack benzene).21 A SET mechanism (similar to that of
Clark2b) could explain the presence of 16 (Scheme 6).
Table 1. k1 Values and Reduction Potentials of Alkynes
Scheme 6
The ratio of 17 to 18 remains constant throughout the
reaction, but the ratio of 16 to the sum of 17 and 18 increases
as the reaction continues. This variation in product ratio
excludes a HAT (giving 17 and 18) parallel to SET (giving 16).
Indeed, it proved impossible to simulate the kinetics12 on this
basis.
The initial reaction is probably formation of an ion pair 20/
21 (Tilset and Parker have made cation 20 electrochemically, in
a quasi-reversible fashion, and found it to be very acidic.22)
After escape, the free radical anion 21 (which is probably in a
trans-bent form23) will react with either benzene, 1, or DMAD
(the predominant species). With benzene, 21 will give 19 and
eventually 16. With 1, 21 will be protonated to the vinyl radical
22 (presumably nonlinear, cf. the radical from methyl
propiolate above). With DMAD, 21 will give oligomers (vide
infra). There is literature precedent for the attack of anion
Figure 4. Rate constants (k1) vs alkyne reduction potential.
24
25
−•
and O−•24c) on aromatic
−•
radicals (SO4
,
HPO4
,
compounds (including benzene) and for the formation of
oligomer from 21 and DMAD.26
The mechanism in Scheme 6 was used to fit the
concentration profiles of 16, 17, and 18 using Kintecus kinetic
modeling software12 (see SI for full details and fits). The rate
constants k1, k2, and k3 were optimized respectively as (9.9
0.2) × 10−3 M−1 s−1, (12.5 0.3) s−1, and (850 10) M−1 s−1.
The rate constant k1 for electron transfer to DMAD is over 30
times greater than the rate constant k1 for HAT to methyl
propiolate.
Electron transfer from inorganic reductants such as iodide
and thiocyanate has been observed to be 2000 times faster to
DMAD than to methyl propiolate.26 Irreversible reduction
potentials27 are available for many alkynes, even though they do
not tell us the thermodynamics of electron transfer to these
alkynes. Table 1 shows the reduction potential and the HAT
rate constant for each alkyne studied. A plot of these
irreversible potentials vs the rate constants k1 (Figure 4) offers
a strong argument for a change in mechanism between
phenylacetylene and DMAD.
In order to further study the reaction of DMAD with 1, we
followed the reaction by EPR spectroscopy at room temper-
ature. The EPR spectra produced during the course of the
reaction are shown in Figure 5 as a function of time. The first
signal to appear (radical c) is a triplet of septets (a1 = 1.0 G
Figure 5. EPR spectra from reaction of DMAD with 1. (Top) Stacked
spectra over time. (Middle) Signal attributed to the fumarate radical
anion (spectrum obtained by scaled subtraction of other signals).
(Bottom) Doublet signal attributed to the oligomer spin adduct
(Simulation in blue).
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(6H), a2 = 5.4 G (2H), Figure 5 middle) which has a g value of
2.0044. When the reaction is run with 1-d1, the larger triplet
coupling fades (see SI for EPR spectra). When the methyl
esters are replaced with tBu esters, the septet coupling
disappears. These data match the EPR spectrum of the
dimethyl fumarate radical anion, previously reported in liquid
ammonia at −50 °C.28 Observation of the dimethyl fumarate
radical anion supports our claim that electron transfer from 1 to
electron-poor alkenes and alkynes is possible. This signal is not
observed when 1 reacts directly with dimethyl fumarate, most
likely due to its poor solubility in benzene.
oligomers are one of the major products and thus more likely to
form a spin adduct with 15.
CONCLUSIONS
■
In conclusion we have shown that transition-metal hydrides
with weak M−H bonds can react with activated alkynes by
different mechanisms, (1) H• transfer or (2) electron transfer.
H• transfer can be used to effect the catalytic cyclization of an
appropriate eneyne. Electron transfer gives radical anions
reactive enough to add to benzene.
Two more major EPR signals appear over the course of the
reaction. The first (signal b) is a broad singlet with a g value of
1.9990 whose appearance coincides with the disappearance of
the fumarate radical anion. The second (signal a) is a broad
doublet with a g value of 2.0093 which persists after the
reaction is over. This second signal shows coupling to one
proton (which disappears when the reaction is run with 1-d1)
and satellites due to coupling to one chromium atom (53Cr I =
3/2, 9.55% abundance). Appearance of this radical corresponds
to disappearance of the previous radical. Simulated parameters
for all Cr-centered radicals are given in Table 2.
ASSOCIATED CONTENT
* Supporting Information
Synthetic details, compound characterization, kinetic traces,
crystallographic information file (CIF), X-ray structural
parameters, and EPR spectra. This material is available free of
charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
jrn11@columbia.edu
■
Notes
Table 2. Spin Adducts of CpCr(CO)3• with Alkenes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the U.S. DOE (Grant DE-FG0297ER14807) for
support of this research. We are grateful to Boulder Scientific
and OFS Fitel for additional support. D.P.E. was supported by
the Department of Energy Office of Science Graduate
Fellowship Program (DOE SCGF), made possible in part by
the American Recovery and Reinvestment Act of 2009,
administered by ORISE-ORAU under Contract No. DE-
AC05-06OR23100. The acquisition of NMR spectrometers
was sponsored by the National Science Foundation (CHE-
0840451) and by the National Institutes of Health
(1S10RR25431-1A1). We thank Prof. Gerard Parkin for use
of his X-ray diffractometer and Prof. Nicholas Turro for use of
his EPR spectrometer. We also thank Dr. Jongwook Choi for
preliminary work and John Hartung and Gang Li for helpful
discussions.
a
b
Referenced to TEMPO.13 53Cr satellites were too low in intensity to
c
obtain an accurate a value. Since their multiplicity automatically ruled
them out as candidates for the identity of a and b, their g value was not
determined.
REFERENCES
■
(1) (a) Foo, T.; Bergman, R. G. Organometallics 1992, 11, 1811.
(b) Roddick, D. M.; Fryzuk, M. D.; Seidler, P. F.; Hillhouse, G. L.;
Bercaw, J. E. Organometallics 1985, 4, 97. (c) Frohnapfel, D. S.; White,
P. S.; Templeton, J. L. Organometallics 2000, 19, 1497. (d) Bassetti,
Compound 15 is known to form complexes (so-called “spin
adducts”) with alkenes under ambient conditions (thought to
occur by substitution of CO) that have similar 53Cr coupling
and g values to that of both these radicals.29 Assignment as spin
adducts makes sense given the amount of 15 which is being
produced in this reaction. However, spin adducts with 16, 17,
and 18 do not match the observed spectra (see Table 2).
The assignment of signal b as a spin adduct with solvent,
similar to the one seen with toluene,29 is tentative but
reasonable. An oligomer could displace benzene to create the
final signal. Since only one of the double bonds could be
coordinated at a time, binding to an end group would give rise
to a doublet due to coupling with only one vinyl proton. This
signal was reproduced with low intensity from a mixture of 15
with DMAD laced with trace oligomer impurity (see SI). Under
the conditions of the EPR experiment (1.35 M DMAD),
M.; Casellato, P.; Gamasa, M. P.; Gimeno, J.; Gonzal
Martín-Vaca, B. Organometallics 1997, 16, 5470.
(2) (a) Clark, H. C.; Wong, C. S. J. Am. Chem. Soc. 1977, 99, 7073.
(b) Clark, H. C.; Ferguson, G.; Goel, A. B.; Janzen, E. G.; Ruegger, H.;
Siew, P. Y.; Wong, C. S. J. Am. Chem. Soc. 1986, 108, 6961. (c) Trost,
́
ez-Bernardo, C.;
B. M.; Ball, Z. T.; Joge, T. J. Am. Chem. Soc. 2002, 124, 7922.
̈
(d) Tanke, R. S.; Crabtree, R. H. J. Am. Chem. Soc. 1990, 112, 7984.
(e) Nakamura, A.; Otsuka, S. J. Mol. Catal. 1976, 1, 285. (f) Herberich,
G. E.; Barlage, W. Organometallics 1987, 6, 1924. (g) Selmeczy, A. D.;
Jones, W. D. Inorg. Chim. Acta 2000, 300−302, 138. (h) Gridnev, A.
A.; Ittel, S. D.; Wayland, B. B.; Fryd, M. Organometallics 1996, 15,
5116. (i) Bockman, T. M.; Garst, J. F.; Ungvar
1999, 586, 41.
́
y, F. J. Organomet. Chem.
(3) (a) Sweany, R. L.; Halpern, J. J. Am. Chem. Soc. 1977, 99, 8335.
(b) Choi, J.; Tang, L.; Norton, J. R. J. Am. Chem. Soc. 2007, 129, 234.
(c) Tang, L. H.; Papish, E. T.; Abramo, G. P.; Norton, J. R.; Baik, M.
́
H.; Friesner, R. A.; Rappe, A. J. Am. Chem. Soc. 2006, 128, 11314.
15517
dx.doi.org/10.1021/ja306120n | J. Am. Chem. Soc. 2012, 134, 15512−15518

Journal of the American Chemical Society
Article
(4) (a) Tang, L.; Norton, J. R.; Edwards, J. C. Macromolecules 2003,
36, 9716. (b) Tang, L. H.; Norton, J. R. Macromolecules 2006, 39,
8229. (c) Tang, L. Scope and Mechanism of Chain Transfer Catalysis
with Metalloradicals and Metal Hydrides; Columbia University: New
York, NY, 2005.
(5) (a) Smith, D. M.; Pulling, M. E.; Norton, J. R. J. Am. Chem. Soc.
2007, 129, 770. (b) Hartung, J.; Pulling, M. E.; Smith, D. M.; Yang, D.
X.; Norton, J. R. Tetrahedron 2008, 64, 11822.
(6) Fischer, H.; Radom, L. Angew. Chem., Int. Ed. 2001, 40, 1340.
(7) (a) Sheldrick, G. M. SHELXTL, An Integrated System for
Solving, Refining, and Displaying Crystal Structures from Diffraction
Data; University of Gottingen: Gottingen, 1981. (b) Sheldrick, G. Acta
Crystallogr., Sect. A 2008, 64, 112.
(8) Choi, J.; Tang, L. H.; Norton, J. R. J. Am. Chem. Soc. 2007, 129,
234.
̌
(9) Svenda, J.; Myers, A. G. Org. Lett. 2009, 11, 2437.
(10) Zeng, H.; Hua, R. J. Org. Chem. 2007, 73, 558.
(11) Bakac, A.; Brynildson, M. E.; Espenson, J. H. Inorg. Chem. 1986,
25, 4108.
(12) Ianni, J. C. Kintecus, Windows Version 4.00; www.kintecus.com,
2010.
(13) Rockenbauer, A.; Gyor, M.; Hideg, K.; Hankovszky, H. O. J.
Chem. Soc., Chem. Commun. 1985, 1651.
(14) WINEPR SimFonia, 1.25; Bruker Analytische Messtechnik
GmbH: Rheinstetten, Germany, 1996.
(15) Gleiter, R.; Bleiholder, C.; Rominger, F. Organometallics 2007,
26, 4850.
(16) (a) Korth, H.-G.; Lusztyk, J.; Ingold, K. U. J. Chem. Soc., Perkin
Trans. 2 1990, 1997. (b) Rhodes, C. J.; Roduner, E. J. Chem. Soc.,
Perkin. Trans. 2 1990, 1729.
(17) Li, G.; Han, A.; Pulling, M. E.; Estes, D. P.; Norton, J. R. J. Am.
Chem. Soc. 2012, DOI: 10.1021/ja306037w.
(18) Fischer, G.; Muszkat, K. A.; Fischer, E. J. Chem. Soc. B. 1968,
1156.
(19) McLain, S. J. J. Am. Chem. Soc. 1988, 110, 643.
(20) (a) Mawson, S. D.; Routledge, A.; Weavers, R. T. Tetrahedron
1995, 51, 4665. (b) Montevecchi, P. C.; Navacchia, M. L. J. Org. Chem.
1998, 63, 537.
(21) Fessenden, R. W.; Eiben, K. J. Phys. Chem. 1971, 75, 1186.
(22) Ryan, O. B.; Tilset, M.; Parker, V. D. J. Am. Chem. Soc. 1990,
112, 2618.
(23) (a) Damrauer, R. J. Org. Chem. 2006, 71, 9165. (b) Jordan, K.
D.; Burrow, P. D. Chem. Rev. 1987, 87, 557. (c) Huang, M.-B.; Liu, Y.
J. Phys. Chem. A 2001, 105, 923. (d) Herzberg, G. Electronic Spectra
and Electronic Structure of Polyatomic Molecules. Molecular Spectra
and Molecular Structure; D. Van Nostrand Company, Inc.: Princeton,
NJ, 1966; Vol. 3.
(24) (a) Merga, G.; Aravindakumar, C. T.; Rao, B. S. M.; Mohan, H.;
Mittal, J. P. J. Chem. Soc., Faraday Trans. 1994, 90, 597. (b) Steenken,
S. One-electron redox reactions between radicals and organic
molecules. An addition/elimination (inner-sphere) path [1] Electron
Transfer II. Topics in Current Chemistry; Mattay, J., Ed.; Springer:
Berlin/Heidelberg: 1996; Vol. 177, p 125. (c) Sharma, S. B.; Mudaliar,
M.; Rao, B. S. M.; Mohan, H.; Mittal, J. P. J. Phys. Chem. A 1997, 101,
8402.
(25) Rosso, J. A.; Caregnato, P.; Mora, V. C.; Gonzalez, M. C.;
́
Martire, D. O. Helv. Chim. Acta 2003, 86, 2509.
(26) Dvorko, G. F.; Shilov, A. E. Theor. Exp. Chem 1970, 3, 364.
(27) Utley, J. H. P.; Lines, R. The electrochemistry of the carbon−
carbon triple bond. In The Carbon-Carbon Triple Bond; John Wiley &
Sons, Ltd.: New York, 1978; Vol. 2, Chapter 17, pp 739−753;
published online 2010 , 10.002/9780470771570.
(28) (a) Elson, I. H.; Kemp, T. J.; Greatorex, D.; Jenkins, H. D. B. J.
Chem. Soc., Faraday Trans. 2 1973, 69, 1402. (b) Nelsen, S.; Gillespie,
J. J. Org. Chem. 1975, 40, 2391.
(29) Sokolov, V. I.; Gasanov, R. G.; Goh, L. Y.; Weng, Z.;
Chistyakov, A. L.; Stankevich, I. V. J. Organomet. Chem. 2005, 690,
2333.
15518
dx.doi.org/10.1021/ja306120n | J. Am. Chem. Soc. 2012, 134, 15512−15518