Catalytic Ni(II) in reactions of SmI2: Sm(II)-or Ni(0)-based chemistry?

Article abstract of DOI:10.1021/ja204287n

The addition of catalytic amounts of Ni(II) salts provide enhanced reactivity and selectivity in numerous reactions of SmI₂, but the mechanistic basis for their effect is unknown. We report spectroscopic and kinetic studies on the mechanistic role of catalytic Ni(II) in the samarium Barbier reaction. The mechanistic studies presented herein show that the samarium Barbier reaction containing catalytic amounts of Ni(II) salts is driven solely by the reduction of Ni(II) to Ni(0) in a rate-limiting step. Once formed, Ni(0) inserts into the alkyl halide bond through oxidative addition to produce an organonickel species. During the reaction, the formation of colloidal Ni(0) occurs concomitantly with Ni(0) oxidative addition as an unproductive process. Overall, this study shows that a reaction thought to be driven by the unique features of SmI₂ is in fact a result of known Ni(0) chemistry.

Full text of DOI:10.1021/ja204287n

ARTICLE
pubs.acs.org/JACS
Catalytic Ni(II) in Reactions of SmI : Sm(II)- or Ni(0)-Based Chemistry?
2
Kimberly A. Choquette, Dhandapani V. Sadasivam, and Robert A. Flowers, II*
Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 18015, United States
S Supporting Information
b
ABSTRACT: The addition of catalytic amounts of Ni(II) salts
provide enhanced reactivity and selectivity in numerous reactions
of SmI , but the mechanistic basis for their effect is unknown. We
2
report spectroscopic and kinetic studies on the mechanistic role of
catalytic Ni(II) in the samarium Barbier reaction. The mechanistic
studies presented herein show that the samarium Barbier reaction
containing catalytic amounts of Ni(II) salts is driven solely by the
reduction of Ni(II) to Ni(0) in a rate-limiting step. Once formed,
Ni(0) inserts into the alkyl halide bond through oxidative addition
to produce an organonickel species. During the reaction, the
formation of colloidal Ni(0) occurs concomitantly with Ni(0) oxidative addition as an unproductive process. Overall, this study
shows that a reaction thought to be driven by the unique features of SmI is in fact a result of known Ni(0) chemistry.
2
1
2
’
INTRODUCTION
provides good yields of the Ni(0) complex, Ni(CDT). Addi-
tionally, an examination of methods employed to produce Ni(0)
nanoparticles shows that they are typically formed through
Transition metal salts and complexes are important compo-
1
nents of many reactions involving samarium diiodide (SmI ). In
fact, even the seminal paper of Kagan introducing SmI to the
chemistry community described the use of catalytic amounts of
ferric chloride to accelerate several coupling reactions between
alkyl iodides and ketones. During the past several decades, a
series of transition metal salts or complexes have been utilized in
catalytic amounts in a range of important reactions including the
coupling of alkyl iodides with carbonyls, conjugate addition
reactions of alkyl iodides with R,β-unsaturated esters, amides,
and lactones, coupling of acid chlorides and esters, intramolec-
ular cyclizations and Grob fragmentations, and the coupling of
alkyl halides with nitriles. The most commonly utilized salts are
based on Fe(III) or Ni(II), and Kagan showed that NiI was
superior to other transition metal salts in many of these reactions.
As a result, Ni(II)-based additives have become the additive of
choice in most reactions requiring a transition metal-based cata-
2
13
reductions of Ni(II) starting materials. On the basis of the
2
known chemistry of Sm and Ni, could Ni(0) and Ni(II) inter-
mediates be responsible for the unique chemistry initiated by the
addition of catalytic amounts of NiI to SmI ? Furthermore, is
the process driven by homogeneous chemistry or colloidal
Ni(0)? Herein we show that SmI does in fact reduce the Ni(II)
catalyst to Ni(0), and that a Ni(0) species is then responsible for
carrying out the subsequent chemical reaction.
2
2
2
2
3
4
5
6
’
RESULTS AND DISCUSSION
7
The goal of this work was to determine the mechanistic role
of catalytic amounts of Ni(II) in SmI -initiated reactions. Care-
ful examination of the experimental procedures utilizing Ni(II)
shows that, in most cases, the Ni(II) salt and SmI are pre-
2
2
3
2
3
ꢀ7,14
mixed before the addition of substrate.
To initially study
8
a
lyst. Although the addition of catalytic amounts of Ni(II) salts
provides enhanced reactivity and increased selectivity in many
SmI -based reactions, the mechanistic basis for their effect is
unknown. Several reports have suggested that Ni(II) species are
reduced to Ni(0) by SmI ; however, no detailed mechanistic
the SmI ꢀNi(II) system, SmI in tetrahydrofuran (THF) was
2
2
slowly titrated into solutions of NiI , Ni(II) acetylacetonate
2
2
(Ni(acac) ), and 1,2-bis(diphenylphosphino)ethane Ni(II) chlo-
ride (Ni(DPPE) Cl ). In each case the blue color of the SmI
2
2
2
2
8
2
dissipated immediately producing a brown solution. Over time, a
precipitate gradually formed from the solution. To study the
system in more detail, the UVꢀvis spectra of a series of solutions
studies on these suppositions have been initiated.
Nickel complexes in both the (0) and (II) oxidation states are
extremely useful in a range of synthetically important carbonꢀ
of SmI containing increasing amounts of Ni(II) salts were
2
9
carbon bond forming reactions. Activation of catalytic systems
recorded as shown in Figure 1. As increasing amounts of NiI2
employing Ni can be initiated by the reduction of Ni(II) to Ni(0)
with a stoichiometric reductant. Evaluation of the E value of
the Ni(II)/Ni(0) redox couple clearly shows that SmI is capable
were added to the solution of SmI in THF, the UVꢀvis bands
2
1
0
o
corresponding to SmI decreased, and a broad absorbance
2
2
from 400 to 500 nm consistent with colloidal Ni(0) began to
11
of readily reducing Ni(II) to Ni(0). Recent work by Ogoshi
et al. demonstrated that facile reduction of Ni(II) by SmI in
the presence of trans,trans,trans-1,5,9-cyclododecatriene (CDT)
Received: May 10, 2011
Published: May 27, 2011
2
r 2011 American Chemical Society
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|J. Am. Chem. Soc. 2011, 133, 10655–10661

Journal of the American Chemical Society
ARTICLE
15
emerge. Similar behavior was observed across the range of
Ni(II) salts examined.
To examine the reaction between SmI and Ni(II), transmis-
2
sion electron microscopy (TEM) was used to characterize the
solid precipitate obtained from a freshly prepared solution.
Figure 2 contains a TEM image of the solid obtained from the
addition of SmI to Ni(acac) in THF. Similar data were obtained
2
2
for the reduction of other Ni(II) salts. An examination of the
TEM images shows that nanoparticles in the 5ꢀ10 nm range
were formed. The particles showed evidence of aggregation over
the time period where precipitation was observed in the earlier
UVꢀvis experiments.
With the knowledge that colloidal Ni(0) is being formed, the
rate of reduction of Ni(II) by SmI was examined via stopped-
2
flow spectrophotometry using the initial rates method. Since NiI2
has limited solubility under the conditions required for rate
Figure 1. UVꢀvis spectra of SmI in THF (2.5 mM) containing
2
studies, the reduction of Ni(DPPE) Cl was examined. The
increasing amounts of NiI
2
.
2
2
reaction was monitored at 486 nm to observe the growth of the
plasmon resonance as colloidal Ni(0) is formed, while SmI2
exhibits minimal absorbance at this wavelength, as shown in
Figure 3A. The rate of growth of the plasmon resonance was
determined using the linear region of the absorbance growth
from 0.2 to 0.5 s. A plot of initial rate for the reduction of
Ni(DPPE) Cl versus concentration of SmI is shown in
2
2
2
Figure 3B. The rate constant determined from this approach
ꢀ
1
ꢀ1
was found to be 10 ( 1 M
s . This experiment demon-
strates that, in the absence of organic substrates, Ni(0)
nanoparticle formation is a facile process. The question
remains whether the colloidal Ni(0) is responsible for the
reactivity observed for reactions of SmI containing catalytic
2
amounts of Ni(II) salts.
To further explore the system, the coupling of an alkyl iodide
1) and a ketone (2) was chosen as a model reaction (Scheme 1)
(
Scheme 1. Ni(II) Catalyzed Samarium Barbier Reaction
Figure 2. TEM image of nanoparticles produced upon the addition of
.5 mL of SmI (5 mM) to 1.5 mL of Ni(acac) (2.5 mM) and TPP
1.0 mM).
1
2
2
(
Figure 3. (a) Time-resolved UVꢀvis spectrum of the reduction of Ni(DPPE)
2
Cl
2
by SmI
2
. [SmI
2
] = 5 mM; [Ni(DPPE)
2
Cl
2
] = 1.0 mM;
] = 1ꢀ5 mM;
[
[
iodododecane] = 15 mM; [3-pentanone] = 15 mM. (b) Plot of initial rates vs [SmI
2
] for the reduction of Ni(DPPE)
2
Cl by SmI . [SmI
2
2
2
ꢀ
1
ꢀ1
Ni(DPPE) Cl ] = 1.0 mM. Rate constant = 10 ( 1 M
s .
2
2
1
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Journal of the American Chemical Society
ARTICLE
Table 1. Reaction of 1 and 2 with SmI and/or Various Ni
Additives
Table 2. Same Excess RPKA Reaction Conditions
2
a
2
run SmI (M) 1 (M) 1 excess (M) 2 (M) 2 excess (M) Ni(II) (M)
b
entry
Ni additive
reaction time
3 yield
ꢀ
4
1
2
0.02
0.01
0.040
0.035
0.030
0.030
0.040
0.035
0.030
0.030
2.0 ꢁ 10
2.0 ꢁ 10
c
ꢀ4
1
2
3
4
5
6
7
none
NiI (1 mol %)
Ni(acac) (1 mol %)
Ni(DPPE)Cl
72 h
5 m
69%
2
98%
93%
95%
NR
2
15 m
10 m
15 h
15 h
5 m
2
(1 mol %)
d
Ni(0) nanoparticles
e
SmI
2
ꢀNiI
2
centrifuged solid
centrifuged supernant
NR
f
SmI
2
ꢀNiI
2
96%
a
b
Ni(II) additive. Yields based on NMR ratio between Barbier product
and reduced dodecane. No starting material remained. Isolated yield of
Barbier product. Commercial Ni(0) 20 nm powder. SmI (0.96
c
d
e
2
mmol) was added to a solution of NiI
brown solution. After 5 min of stirring the solution was centrifuged for
min at 5000 rpm. The solid obtained in the reaction was separated and
2
(0.48 mmol) dropwise to form
5
washed with THF before being subjected to the substrates (0.4 mmol).
f
Supernatant obtained from procedure (e) was isolated and reacted with
the substrates. NR = no reaction.
to study the impact of Ni(II) salts and colloidal Ni(0) since
the rates of reduction of both substrates by SmI are well-
2
1
6
established. For both substrates, the rate constant for reduction
2
Figure 4. Rate vs [SmI ] for same excess experiments. (0, blue) Run 1
and (left-pointing triangle, black) Run 2. Reaction conditions listed
in Table 2.
by SmI is 5 orders of magnitude slower than the rate constant for
2
Ni(II) reduction by SmI determined above. Initial experiments
2
were conducted for the Barbier reaction employing 1 mol % of
3
Ni(II) salts as described by Kagan et al. The results of these
experiments are shown in Table 1. In the absence of Ni(II), the
reaction was sluggish resulting in a 69% yield of product after 72 h
with the remainder of the reaction mixture consisting of un-
reacted starting material (Table 1, entry 1). The use of 1 mol %
rates of two reactions are monitored: one run set at initial
conditions, and the second run (50% run) set at one-half of
the initial reaction concentrations, with every other reagent in
excess as shown in Table 2.
NiI , Ni(acac) , or Ni(DPPE) Cl led to nearly quantitative
2
2
2
2
The Barbier reaction was carried out using this method with a
yields of product in 5ꢀ15 min (Table 1, entries 2ꢀ4). To test
whether Ni(0) nanoparticles are capable of initiating the reac-
tion, commercially available Ni(0) nanoparticles (20 nm average
particle size) were reacted with 1 and 2 in THF (Table 1, entry 5).
No reaction occurred over a range of Ni(0) concentrations.
catalytic amount of Ni(DPPE) Cl . The reactions were followed
2
2
by stopped-flow spectrophotometry, monitoring the decay of
SmI at 555 nm. Run 1 was performed with a 0.03 M excess of the
2
substrates (with respect to SmI at 0.02 M) and the catalyst set at
2
ꢀ4
1
mol % (2.0 ꢁ 10 M) with respect to SmI (Table 2, Run 1).
2
Since the nanoparticles obtained from the mixture of SmI2
In Run 2, [SmI ] was 0.01 M, one-half of the SmI concentration
2
2
and NiI were smaller in diameter (as determined by TEM),
2
in Run 1. The reagent concentrations were decreased in an
another experiment was initiated in which the SmI and NiI were
2
2
equivalent amount with respect to [SmI ] to maintain the same
2
premixed, the solution centrifuged, and a solid precipitate ob-
tained. These particles were then washed with THF in an inert
atmosphereandmixedwith 1and 2. Noreactionoccurredshowing
that, although Ni(0) nanoparticles are clearly formed during the
course of the reaction, they do not initiate the Barbier reaction,
whichraises the question:is itpossiblethatsoluble Ni(0) is capable
excess concentrations. Identical to Run 1, the [Ni(II)] was
ꢀ
4
maintained at 2.0 ꢁ 10 M (Table 2, Run 2). Under these
conditions, Run 2 mimics the reaction concentrations that would
be present at the midpoint of Run 1.
Ideally in a catalytic reaction system, the concentration of
catalyst remains constant through the course of the reaction. As a
result, if the catalyst is active and constant, the concentration of
Ni should be the same halfway through the reaction. If the
catalyst is not being deactivated during the course of the reaction,
the plots of Runs 1 and 2 will overlay. If catalyst is deactivated
through decreasing [Ni(DPPE) Cl ], then the plots of the two
of initiating the reaction? To test this hypothesis, SmI was mixed
2
with a stoichiometric amount of Ni(II). The colloidal suspension
was filtered, and 1 and 2 were added to the supernatant. Interest-
ingly, a nearly quantitative yield of 3 was obtained after 5 min
(Table 1, entry 7).
2
2
On the basis of the information obtained thus far, it is clear that
runswill not overlay. Runs 1 and 2 were plotted as rate as a function
of concentration, and no overlay was observed (Figure 4). This
observation indicates that the [Ni] decreases over the course of the
reaction and therefore is being deactivated.
Ni(II) salts are acting as catalysts in the samarium Barbier
reaction. To investigate the mechanism by which the catalyst
functions, reaction progress kinetic analysis (RPKA) experi-
17
ments were employed. This method allows kinetic analyses
to be performed on a system under synthetically relevant
conditions. The “same excess” experiment of RPKA enables
one to determine if the catalyst is being deactivated during the
course of the reaction. To run the same excess experiment, the
As described from the experiments in Table 1, both the
purchased Ni(0) nanoparticles (Table 1, entry 5) as well as the
solid Ni(0) obtained from the SmI /NiI mixture (Table 1, entry 5)
2
2
did not initiate the Barbier reaction; however, when the sub-
strates were reacted with the supernatant (Table 1, entry 7),
1
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Journal of the American Chemical Society
ARTICLE
2
Figure 5. Rate vs [SmI ] for same excess experiments. (a) (9, black) Run 3 and (2, red) Run 4. Reaction conditions similar to those listed in Table 2,
with 3 mol % catalyst loading. (b) (b, green) Run 5 and ([, blue) Run 6. Reaction conditions similar to those listed in Table 2, with 5 mol % catalyst
loading.
Table 3. Conditions for Different Excess Experiments to
Determine the Reaction Orders of the Substrates
2
run SmI (M) 1 (M) 1 excess (M) 2 (M) 2 excess (M) Ni(II) (M)
ꢀ
4
4
7
8
9
0.02
0.02
0.02
0.04
0.07
0.04
0.03
0.06
0.03
0.04
0.04
0.07
0.03
0.03
0.06
2.0 ꢁ 10
2.0 ꢁ 10
2.0 ꢁ 10
ꢀ
ꢀ
4
the reaction proceeded. Taking this information together with
the fact that the catalyst is being deactivated with 1 mol % Ni(II),
it is plausible that the deactivation pathway of the catalyst is the
formation of aggregated colloidal Ni(0). As Ni(II) is rapidly
reduced to Ni(0) by SmI , some Ni(0) species could aggregate
2
and precipitate out of solution, while some concentration of
Ni(0) remains suspended and soluble. The remaining soluble
Ni(0) is responsible for catalyzing the reaction.
Even though the Barbier reaction is synthetically success-
ful with 1 mol % Ni(II), deactivation of the catalyst occurs. To
determine if an optimal environment for the catalyst could be
created, thereby enhancing the efficiency of the samarium Barbier
reaction, the [Ni(II)] was increased. Figure 5 displays data result-
ing from the new concentrations of Ni(DPPE) Cl added to the
Figure 6. Turnover frequency of Ni(II) in the samarium Barbier
reaction. [SmI ] = 0.01 M; [1] = 0.035 M; [2] = 0.035 M;
2
ꢀ4
[
1
Ni(DPPE)
2
Cl
2
] = (9, black) 6.0 ꢁ 10
M; (b, red) 8.0 ꢁ
ꢀ4
ꢀ3
0
M, (2, green) 1.0 ꢁ 10 M.
[1] does not change the reaction rate, a finding consistent with a
zero order dependence of 1. A similar overlay is observed when
Runs 7 and 9 are plotted (Figure 8), again indicating a zero order
for 2. These orders were also verified under pseudofirst-order
conditions (see Supporting Information).
2
2
system: 3 mol % (Runs 3 and 4) and 5 mol % (Runs 5 and 6). As
indicated by the same excess experiments, deactivation is decreased
but not prevented with the addition of higher concentrations of
Ni(II). From these results, 3 mol % catalyst loading was deemed
most ideal to employ in further experiments since the least amount
of deactivation was observed at this concentration.
Further, the rate constant of the Barbier reaction with Ni(II)
catalyst was examined usingthe initial rate method, with increasing
amounts of Ni(DPPE) Cl added to the system. A rate constant of
Moreover, using different excess experiments with respect to
Ni], the order of Ni can be determined. The turnover frequency
2
2
ꢀ
1 ꢀ1
[
16.6 ( 1.5 M
s
was obtained from these data (Figure 9). This
of the catalyst is determined by normalizing the rate by [Ni]. A
range of 3ꢀ5 mol % catalyst was examined. The overlay of the
plots indicates that the reaction is first order in the Ni catalyst
value is in good agreement with the rate constant found for the
reduction of Ni(DPPE) Cl by SmI in the absence of substrates
2
2
2
(Figure3b), furtherindicatingthat therate-determiningstep inthe
cycle is the initial reduction of Ni(II).
(Figure 6).
To determine the rate orders of 1 and 2, the effect of the
The order of SmI2 in the reaction was determined by
pseudo first-order kinetic methods. Rates were determined by
stopped-flow spectroscopy with an increasing amount of SmI2.
concentration of each reagent on the rate of reaction was
determined. Table 3 displays the concentrations of each reaction
component. When the decays of Runs 7 and 8 were plotted,
overlay was observed (Figure 7), indicating that an increase in
[Ni(DPPE) ] was kept consistent at 3 mol % with respect to
2
SmI , while both 1 and 2 were set at a 10 equivalent excess. As the
2
1
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Journal of the American Chemical Society
ARTICLE
Figure 7. Plot of rate vs [SmI
2
] for different excess experiments;
Figure 9. Rate constant of Ni(DPPE)
2
Cl
2
2
in the samarium Barbier
(
9, black) Run 7 and (], green) Run 8 to determine 1 reaction order.
reaction. [SmI ] = 5 mM; [Ni(DPPE) Cl ] = 0.5ꢀ4 mol %; [1] =
2
2
Reaction conditions are listed in Table 3. The overlay indicates that the
reaction is zero order in 1.
ꢀ1 ꢀ1
50 mM; [2] = 50 mM. Rate constant = 16 ( 2 M
s .
Figure 10. Rate order of SmI
2
in the Ni(II) catalyzed samarium Barbier
Cl
] = 0.15ꢀ0.60 mM (3
mol % with respect to [SmI ]); [1] = 0.2 M; [2] = 0.2 M. Rate order =
Figure 8. Plot of rate vs [SmI
2
] for different excess experiments;
reaction. [SmI
2
] = 5ꢀ20 mM; [Ni(DPPE)
2
2
(
9, black) Run 7 and (O, red) Run 9 to determine 2 reaction order.
2
Reaction conditions are listed in Table 3. The overlay indicates that the
reaction is zero order in 2.
0.91 ( 0.01.
concentration of [SmI ] increased, the rates increased linearly,
2
Table 4. Reaction Orders for the Reagents in the Ni(II)-
Catalyzed Barbier Reaction
indicating a first-order dependence of SmI (Figure 10). The
2
experimentally determined rate orders for each of the compo-
nents of the Barbier reaction are included in Table 4. Rate orders
of zero were observed for both 1 and 2, while first-order rates
SmI
2
Ni(DPPE)
2
Cl
2
1
2
0.91 ( 0.01
1.0 ( 0.1
0
0
were seen for Ni(II) and SmI . These data suggest the initial
2
reduction of Ni(II) by SmI being the rate-determining step in
the reaction.
2
inserts into the alkyl halide bond through a facile oxidative
addition or aggregates to form colloidal Ni(0), a pathway which
deactivates the catalyst. After oxidative addition, transmetalation
occurs in which Sm(III) releases Ni(II) for reentry into the
catalytic cycle and a more stable organosamarium intermediate
is formed. The organosamarium adds to the ketone, and
final protonation of the Sm(III) alkoxide provides the final
carbinol product. It is important to note that transmetalation
may occur at an alternative stage. For instance, after oxidative
addition the organonickel species may directly add to the ketone
forming a nickel alkoxide. Due to the high oxophilicity of Sm,
The data compiled on the SmI /Ni(II)-initiated Barbier
2
reaction show the following: (1) SmI reduces Ni(II) salts; (2)
2
Ni(0) nanoparticles are formed upon reduction; (3) Ni(II) salts
catalyze the reaction; however, (4) colloidal Ni(0) does not
initiate the Barbier reaction; (5) Ni(II) salts are reduced sig-
18
nificantly faster than alkyl iodides or dialkyl ketones by SmI ; and
2
(6) the reaction is zero order in substrates and first order in SmI2
and Ni. Given these findings, the mechanism in Scheme 2 is
proposed. Ni(II) is initially reduced to Ni(0) through two single
electron transfers from two equivalents of SmI . Ni(0) either
2
1
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Journal of the American Chemical Society
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Scheme 2. Catalytic Cycle of Ni(0)-Mediated Barbier Coupling
transmetalation might occur releasing Ni(II) and forming a Sm
alkoxide, followed by protonation.
and used with no further purification. Ni nanoparticles (aps 20 nm)
were purchased from Sun Innovations Inc.
The Barbier reaction was carriedout in a drybox with degassed reagents.
SmI was prepared as described above. Ni(II) species (0.004 mmol) was
2
’
CONCLUSIONS
added to 2 mL of THF and stirred to dissolve. SmI (4.0 mL, 0.1 M) was
2
added dropwise to the Ni(II) solution, and with the slow addition the
solution turned brown. Separately, iodododecane (1) (49.3 μL, 0.2 mmol)
and 3-pentanone (2) (21.2 μL, 0.2 mmol) were added to 5 mL of THF.
The mechanistic studies presented herein show that the
samarium Barbier reaction containing catalytic amounts of Ni-
II) salts is driven by reduction of Ni(II) to Ni(0). Once reduced,
Ni(0) inserts into the alkyl halide bond through oxidative
addition to produce an organonickel species. Transmetalation
by Sm(III) forms the organosamarium species to couple with the
dialkyl ketone and releases Ni(II) which can be reduced by
(
The substrate solution was added dropwise to the SmI
2
ꢀNi solution. This
reaction was left to stir for 5ꢀ15 min (depending on the Ni(II) additive).
The reaction was quenched by exposing to air and adding 0.1 M HCl. The
reaction mixture was then washed with water and extracted twice with
ether. The organic layer was washed individually with H O, Na S O , and
2
2 2 3
stoichiometric SmI reductant. The formation of colloidal Ni(0)
2
4
brine, dried over MgSO , and then concentrated to obtain the pure barbier
occurs concomitantly with Ni(0) oxidative addition as an un-
productive process.
Catalytic amounts of Ni(II) salts are routinely used in a variety
product.
Stopped-Flow Rate Studies. Kinetic experiments in THF were
performed with a computer-controlled SX.18 MV stopped-flow spectro-
photometer (Applied Photophysics Ltd. Surrey, UK). The SmI and
2
of SmI reductions, improving the reaction time or product ratio,
2
without a clear understanding of the mechanistic role of the
Ni(II) catalyst. This is the first instance where it has been shown
that the reaction thought to be driven by the unique features of
substrates with Ni(II) were taken separately in airtight Hamilton
syringes from a drybox and injected into the stopped-flow system.
The cell block and the drive syringes of the stopped flow reaction
analyzer were flushed a minimum of three times with dry, degassed THF
to make the system anaerobic. Using reaction progress kinetic analysis,
the concentration of SmI2 used for the study was 20 mM. The
concentrations of the substrates were kept at synthetically relevant
conditions with only slight excess (as indicated in text). When using
19
SmI is in fact a result of known Ni(0) chemistry. Given the use
2
of Ni(II) and other transition metal salts in many reactions
initiated by SmI , it is likely that this facet of the reaction
2
chemistry and mechanism is more prevalent than recognized.
We are currently examining the mechanism of other SmI -based
2
reactions containing transition metal salts to determine the
intermediacy of transition metal-based intermediates and the
use of this knowledge to develop alternative methods for
the organic chemists' toolbox. The results of these studies will
be reported in due course.
pseudofirst-order conditions the [SmI
tion of the substrates was kept high relative to [SmI
cases the mole fraction of Ni(II) species were determined with respect to
SmI ]. The reaction rates were determined from the decay of SmI2
2
] was 5 mM, and the concentra-
2
] (125 mM). In all
[
2
absorbance at 555 nm. Under reaction progress kinetic analysis condi-
tions, the rate was determined from the decay of the SmI absorbance at
55 nm: rate = (Δ[SmI ]/Δtime, s). The pseudofirst-order rate con-
stants were determined by using standard methods. The decay of the
SmI displayed first-order behavior over >3 half-lives.
2
5
2
’
EXPERIMENTAL SECTION
2
0
Materials and General Procedures. THF was purified after
2
purging with argon gas and passing over a column of activated alumina
by a Solvent Purification System (Innovative Technology Inc.; MA).
Dried solvents and reagents were stored in an Innovative Technology,
Inc. drybox containing an argon atmosphere and a platinum catalyst for
UVꢀvis spectroscopy experiments were performed on a Shimadzu
UV-1601 UVꢀvis spectrophotometer controlled by UV Probe (version
1.11) software. NMR experiments were carried out on a Bruker 500 MHz
3
instrument using CDCl as the solvent.
2
drying. SmI was prepared by stirring Sm metal and iodine in THF until
2
þ
the characteristic blue color of Sm appeared. The concentration of the
Sm complex (0.10 M in THF) was determined by iodometric titration.
’ ASSOCIATED CONTENT
Supporting Information. General experimental methods
and spectroscopic and rate data. This material is available free of
charge via the Internet at http://pubs.acs.org.
3
-Pentanone (2) was purchased from Aldrich and distilled under
vacuum from CaO before use. Iodododecane (1), Sm metal, and
Ni(acac) were purchased from Acros and used with no further
purification. NiI and Ni(DPPE) Cl were purchased from Aldrich
S
b
2
2
2
2
1
0660
dx.doi.org/10.1021/ja204287n |J. Am. Chem. Soc. 2011, 133, 10655–10661

Journal of the American Chemical Society
AUTHOR INFORMATION
ARTICLE
’
(20) Pedersen, S. U.; Lund, T.; Daasbjerg, K.; Pop, M.; Fussing, I.;
Lund, H. Acta Chem. Scand. 1998, 52, 657.
Corresponding Author
rof2@lehigh.edu
’
ACKNOWLEDGMENT
R.A.F. thanks the National Science Foundation (CHE-
844946) for support of this work. We thank Dr. Joseph A.
0
Teprovich, Jr. for initial work and Mr. James Devery for his useful
comments on the manuscript.
’
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