Reversibility of Ketone Reduction by SmI2-Water and Formation of Organosamarium Intermediates

Article abstract of DOI:10.1021/acs.organomet.7b00392

The reduction of ketones by SmI₂-water has long been thought to proceed through a reversible initial electron transfer with the formation of organosamarium intermediates in a follow-up step. Kinetic experiments on the reduction of two model ketones and structurally similar ketones with a pendant alkene are shown to be consistent with a rate-limiting reduction by SmI₂-water through a proton-coupled electron-transfer (PCET). Literature values for the rates of radical cyclizations and reduction of radicals by SmI₂ and thermochemical data for radical reduction by SmI₂-water further support a rate-limiting initial step for ketone reductions. These data suggest that discrete organosamarium species may not be intermediates in ketone reductions by SmI₂-water.

Full text of DOI:10.1021/acs.organomet.7b00392

Article
pubs.acs.org/Organometallics
Reversibility of Ketone Reduction by SmI2−Water and Formation of
Organosamarium Intermediates
Tesia V. Chciuk, William R. Anderson, Jr., and Robert A. Flowers, II*
Department of Chemistry, Lehigh University, 6 East Packer Avenue, Bethlehem, Pennsylvania 18015, United States
S
* Supporting Information
ABSTRACT: The reduction of ketones by SmI2−water has long
been thought to proceed through a reversible initial electron
transfer with the formation of organosamarium intermediates in a
follow-up step. Kinetic experiments on the reduction of two
model ketones and structurally similar ketones with a pendant
alkene are shown to be consistent with a rate-limiting reduction
by SmI2−water through a proton-coupled electron-transfer
(PCET). Literature values for the rates of radical cyclizations
and reduction of radicals by SmI2 and thermochemical data for
radical reduction by SmI2−water further support a rate-limiting
initial step for ketone reductions. These data suggest that discrete
organosamarium species may not be intermediates in ketone
reductions by SmI2−water.
Scheme 1. Stepwise Reduction of a Ketone by SmI2
Containing a Proton Donor Source
INTRODUCTION
■
The reduction of a carbonyl by samarium diiodide (SmI2) is the
initial step in a range of reactions of synthetic importance.1
Activated carbonyls are typically reduced in the absence of
additives, but alkyl aldehydes, dialkyl ketones, and related
substrates typically require the inclusion of additives such as
Lewis bases, inorganic salts, or proton donors (water, alcohols,
glycols) to accelerate the reactions.2 An early seminal review on
the samarium Barbier reaction used synthetic data available at
the time to deduce the mechanism of ketone reduction. These
limited data were consistent with the reduction of a ketone
being a fast, reversible process with the reaction equilibrium
lying to the side of unreacted ketone and SmI2.3 Other studies
on activated ketones suggested that subsequent steps proceed
through a stepwise House-type mechanism when an alcohol is
introduced into the system as shown in Scheme 1.4 In this
process, electron transfer from SmI2 to a ketone produces a
ketyl radical anion stabilized by SmIII (step 1) in a follow-up
step, and then proton transfer from water or an alcohol
produces a ketyl intermediate (step 2). A second electron
transfer from SmI2 produces an organosamarium intermediate
(step 3), and subsequent proton transfer provides the carbinol
product (step 4).
The hypothesis that the initial step of ketone reduction by
SmI2 is reversible was based on the premise that the presence of
a pendant alkene would drain the intermediate ketyl through
rapid cyclization. Since this seminal hypothesis was presented, a
range of reductions, cross-coupling reactions, and cyclizations
have been examined using HMPA and proton donors in
concert with SmI2.5 Recent elegant cyclizations of unactivated
lactones containing pendant alkenes have been developed by
Procter.6 In these studies, reductions and cyclizations by SmI2−
water were proposed to proceed through a rate-limiting second
ET after cyclization, producing an organosamarium intermedi-
ate (Scheme 1, step 3) that is protonated in the final step
(Scheme 1 step 4).6e More recently we have reported
experimental evidence consistent with a rate-limiting proton-
coupled electron transfer (PCET) from SmI2−water in the
reduction of arenes and carbonyls.7 Since the concept of a
reversible ET arose, we have carried out a great deal of kinetic
studies on a range of functional group reductions.8 Although
Special Issue: Organometallic Actinide and Lanthanide Chemistry
Received: May 25, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.7b00392
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
rate = k1[S]x[SmI2]y[H2O]z
the supposition of a reversible ET in the reduction of a carbonyl
by SmI2−water is reasonable, it has not been directly tested
through rate measurements. Herein we provide experimental
data that are consistent with a rate-limiting PCET in the initial
step, a finding inconsistent with a reversible initial ET followed
by a rate-limiting second ET in the reduction of ketones by
SmI2−water to produce an organosamarium intermediate.
(1)
For the alkenyl-substituted ketone shown in Scheme 3, the
same approach can be used. In the first step of the reduction of
S-1 eq 2 can be derived. Rate expressions for subsequent steps
can be derived to include the rate of cyclization of I-1 to I-2
and the reduction of I-2 to P-1 (see the Supporting
Information). A consequence of including subsequent steps
provides a more complex rate expression that would lead to
different observed rate constants and/or rate orders for SmI2
and water. What is clear from this analysis is that the initial step
of each process provides essentially the same rate expression as
shown in eqs 1 and 2. If the initial step is rate-limiting, kinetic
experiments on a ketone and a structurally similar ketone
containing a pendant alkene should provide a system to test if
the initial step is rate-limiting. If the kinetics for the two
systems are demonstrably different, the data would provide
insight into whether a follow-up step is rate-limiting.
RESULTS AND DISCUSSION
■
When one considers the reduction of a carbonyl by SmI2−
water, it is useful to consider elementary processes for each step
(Scheme 2). To simplify the rate expression, we are showing
Scheme 2. Reduction of a Ketone Through PCET from
SmI2−Water
rate = k1[S‐1]x[SmI2]y[H2O]z
(2)
To analyze the mechanism of ketone reduction by SmI2−
water and to determine the rate-limiting step, a series of kinetic
studies were initiated to elucidate the role of SmI2, water, and
ketone. Chart 1 contains two parent ketones I and III and two
Chart 1. Ketone Substrates Employed in Rate Studies
the transfer of an electron and proton in each step on the basis
of evidence that the initial reduction takes place through
PCET.7 For the reduction of a ketone, the initial step involves
the transfer of an electron from Sm(II) and a proton from
water to produce an intermediate ketyl (I). In the second step,
I is reduced by SmI2−water, affording the alcohol (P).
For a ketone containing a pendant alkene, the same approach
can be used, although the process is somewhat more complex,
as shown in Scheme 3. Initial reduction of substrate S-1 by
SmI2−water leads to intermediate I-1. Cyclization of I-1 leads
to a primary radical (I-2). Reduction of I-2 produces carbocycle
P-1.
related substrates II and IV containing pendant alkenes that
undergo 5-exo-trig cyclizations upon reduction by SmI2−water.9
These substrates were chosen to carefully compare the impact
of a pendant alkene on the rate of carbonyl reduction using a
system with similar steric demands. Rate studies were carried
out under pseudo-first-order conditions with substrate and
water in at least a 10-fold excess by monitoring the decay of the
Sm(II) absorption at 560 nm. Substrates were converted to the
corresponding products without significant loss of SmI2 by side
reactions. All experiments were carried out at least three times
on independently prepared samples to ensure reproducibility.
In all cases, the rate orders of substrate and SmI2 were
approximately 1,10 and the rate order of water up to 1.5 M was
2. The rate orders and constants for the reduction of substrates
I−IV are contained in Table 1. In addition to these studies,
activation parameters were determined for the reduction of
each substrate and the values for the ketone containing a
pendant alkene and the parent ketone were the same within
experimental error (see the Supporting Information). To
further examine the process, kinetic studies were carried out
employing D2O in place of water. The kH/kD values for all
substrates were between 1.7 and 1.8. These data are consistent
with previous studies on the reduction of anthracene and
ketones showing reduction proceeds through a PCET from
SmI2−water.7,11
Scheme 3. Reduction of a Ketone Containing a Pendant
Alkene by SmI2−Water
With this basic mechanistic framework in hand for each
component, rate expressions can be derived for each step. The
rate expression for the initial step of the reduction of a ketone
by SmI2−water (Scheme 2) can be derived as shown in eq 1,
where superscripts x, y, and z are rate orders determined from
kinetic experiments. Another expression was derived for the
second step that would alter the rate orders for SmI2 and water
and provide a different observed rate constant in kinetic studies
(see the Supporting Information).
To further explore the effect of water on the rate of ketone
reduction, the rates of reduction of I−IV were monitored over
B
DOI: 10.1021/acs.organomet.7b00392
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
There are limited studies on the reduction of alkyl radicals by
SmI2, but studies on related systems are known and provide a
great deal of insight into the rate of radical reduction by SmI2.
Fluorescence experiments by Scaiano and co-workers demon-
strated that the bimolecular rate constant for the reduction of a
benzyl radical by photoexcited SmI2 in THF at room
Table 1. Rate Constants and Rate Orders for Substrate,
SmI2, and H2O
rate order
a
b
c
d
substrate
k (M−3 s−1
)
substrate
SmI2
H2O
I
67
73
4
4
1.2 0.1
1.2 0.1
1.4 0.3
1.3 0.1
1
1
1
1
2
2
2
2
temperature is (5.3
1.4) × 107 M−1 s−1.14 In addition to
II
III
IV
150 10
180 10
this work, Curran and Hasegawa employed a hexenyl radical
clock to determine the rate constant for reduction of a primary
radical by SmI2 containing various amounts of HMPA.15
Bimolecular rate constants for the reduction were on the order
of 5 × 105 to 7 × 106 M−1 s−1 employing 2−6 equiv of HMPA,
respectively. Cyclic voltammetry studies on the impact of
HMPA on redox potential of SmI2 demonstrate that 2 equiv of
the additive has only a modest effect on the reducing power of
SmI2, similar to the effect of water at the concentrations
employed in this study.16 While direct kinetic measurements on
the reduction of an alkyl radical by SmI2 alone are unavailable,
the kinetic studies of Curran in concert with previous
voltammetric data are consistent with fast reduction of a
primary radical that is several orders of magnitude faster than
the rate constants observed for the reduction of substrates I−
IV. This analysis demonstrates that ET from SmI2 to the
primary radical formed after formal hydrogen atom transfer
(HAT) and cyclization of II and IV is highly unlikely to be rate-
limiting.
a
b
c
Conditions: 10 mM SmI2, 1 M H2O, and 100 mM substrate.
Conditions: 10 mM SmI2, 1 M H2O, 80−140 mM substrate.
d
Obtained via fractional times method. Conditions: 100 mM
substrate, 10 mM SmI2, 0−1.25 M H2O.
a broad concentration range of water, as displayed graphically in
Figure 1. The results of this study demonstrate two important
It is constructive to examine the initial reduction of a
substrate through PCET and the follow-up reduction of the
intermediate radical through a formal HAT from SmI2−water.
The bond dissociation free energy (BDFE) for the O−H bond
of water bound to Sm(II) was estimated by the limit of
reduction of an arene by SmI2 containing water using our
previously described approach.7a In our hands, trans-stilbene
was found to be reduced to 50% as previously reported.17
Arene substrates such as phenanthrene, which are more difficult
to reduce, are not converted to product by SmI2−water. In the
present case, concerted transfer of a proton and electron from
SmI2−water to trans-stilbene is thermodynamically equivalent
to hydrogen atom transfer between the same reactants. As a
consequence, the reduction can be viewed as one where water
complexation to Sm(II) lowers the BDFE of the O−H of the
bound water, enabling it to donate an H atom to trans-
stilbene.7d To assess the limit of bond weakening of water
bound to Sm(II), the BDFEs in THF for water and the initial
radical formed upon HAT to trans-stilbene were calculated
using density functional calculations employing standard
methods.18 Subtraction of the O−H BDFE from the arene
radical provides an estimate of bond weakening, as shown in
Scheme 4. Using this approach, the bond weakening required
Figure 1. Effect of [H2O] on the rate of reduction of substrates I−IV
by SmI2. Conditions: 10 mM SmI2, 100 mM substrate, 0−3 M H2O,
25 °C.
characteristics: (1) the effect of water on the rate of reduction
of all substrates saturates only at high concentrations of the
additive (see the Supporting Information)12 and (2) the
presence of a pendant alkene has no effect on the rate of ketone
reduction within the error of the experiments.
The kinetic experiments presented above demonstrate that it
is reasonable that the first step in the reduction of a ketone by
SmI2−water is rate-limiting but do not address the rates of
follow-up processes. In particular, we were interested in
determining the viability of organosamarium intermediates
after initial formal hydrogen atom transfer (HAT) to a ketone
from SmI2−water. Organosamarium species are proposed to be
intermediates after reduction of a ketyl radical, as shown in step
3 of Scheme 1. Since a strong C−H bond is formed upon
reduction of an intermediate ketyl or a primary radical formed
after cyclization (I-2 in Scheme 3), we analyzed available data
and calculated thermochemical estimates to determine the
viability of HAT from SmI2−water to intermediate radicals.
The analysis is described below.
Scheme 4. Estimate of the Bond Weakening of Water upon
Coordination to Sm(II)
There are a large number of rate studies on the related 5-exo-
trig cyclizations, and the rate constants for these processes are
fast and typically in the range of 106−107.13 Rate constants are
not available for the cyclization of substrates II and IV through
intermediate ketyls. However, even if they are on the low end
of the known range for 5-exo-trig cyclizations, they are still
several orders of magnitude faster than the values shown in
Table 1.
C
DOI: 10.1021/acs.organomet.7b00392
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
for the reduction of trans-stilbene is 73.9 kcal/mol, which
translates into an O−H BDFE for water bound to Sm(II) of
34.1 kcal/mol.
Scheme 6. Proposed Steps for Reduction of an Unactivated
Ketone by SmI2−water
Using the estimate of 34.1 kcal/mol for the BDFE of water
bound to Sm(II) and values obtained from computational
studies, the thermochemical driving force for each step can be
estimated (Scheme 5). The reduction of a ketone (A) is
Scheme 5. Thermochemical Driving Force for Reduction by
SmI2−Water
of complex processes. We caution that the present study is
limited to the reduction of unactivated ketones by SmI2−water.
Activated ketones can certainly be reduced through a sequential
process initiated by an ET.22 We are currently examining other
functional group reductions to determine the generality for this
class of reactions. The results of this work will be reported in
due course.
EXPERIMENTAL SECTION
■
General Methods. Samarium powder was purchased from Acros
Organics. SmI2 was generated by the standard method of combination
of samarium metal with iodine in THF and stirring for at least 8 h.
Iodometric titrations were performed to verify the concentration of
SmI2. Substrates were synthesized as per the procedures below and
were distilled, degassed, and stored over sieves. 2-Methylcyclohex-
anone was purchased from VWR and distilled and degassed prior to
use. Tetrahydrofuran was purified by a solvent purification system.
H2O and D2O were deoxygenated by bubbling through with argon
overnight. Proton NMR spectra were recorded on a 500 MHz
spectrometer in CDCl3. 13C NMR spectra were measured at 125 MHz
in CDCl3.
endergonic by approximately 17 kcal/mol and is a consequence
of the weak O−H bond formed in the intermediate ketyl
radical. Conversely, the reduction of the intermediate ketyl
radical (B) and after ketyl cyclization (C) are both significantly
exergonic and are consequences of the formation of a strong
C−H bond. Overall, this analysis demonstrates that there is a
substantially greater thermodynamic driving force for radical
reduction to form a C−H bond than for the formation of a
weak O−H bond formed in the creation of the intermediate
ketyl through formal HAT from SmI2−H2O. While one must
be careful when comparing thermodynamic and kinetic
arguments, the Hammond postulate in concert with the
Bell−Evans−Polanyi principle demonstrates that the rate of a
reaction is affected by its driving force.19 As a consequence, this
analysis is consistent with the initial step being rate-limiting.
Furthermore, these data demonstrate that there is a strong
driving force for HAT from SmI2−water to ketyl and primary
radicals, suggesting that organosamarium species may not be
intermediates in these reductions.20
General Reduction/Cyclization Reaction Procedure. In a
round-bottom flask equipped with a stir bar was placed SmI2 (2.5 mol
equivalents vs substrate of 0.1 M solution) in an argon glovebox. To
this was added degassed H2O neat (150 equiv vs Sm) to produce a
deep purple solution of SmI2−H2O. To this solution was added the
desired substrate (1 equiv) neat. Once the purple color was lost and
white precipitate formed, the solution was removed from the glovebox.
The reaction was quenched with 10 vol % HCl (50 mL) and extracted
with ethyl acetate (3 × 50 mL). The organic layers were combined and
washed with DI H2O, followed by saturated Na2S2O3. Once it was
dried with MgSO4 and filtered, the organic solution was concentrated
by rotary evaporation. Reduced and cyclized products were separated
from one another by column chromatography (EtOAc/hexanes).
CONCLUSIONS
■
The combination of kinetic and thermodynamic analyses
provides a compelling argument that the reduction of ketones
by SmI2−water does not proceed through a reversible ET but
likely occurs through an irreversible PCET, as shown in the first
step of Scheme 6. This initial step is rate-limiting for the
reduction of ketones and the intramolecular reductive coupling
of ketones with alkenes examined in this study. Although the
exact speciation of SmI2−water is unclear under these reaction
conditions, there are likely several molecules of water bound to
Sm(II)9,21 and kinetic experiments demonstrate that two waters
are required in the initial reduction step. Additionally, the
strong thermodynamic driving force for HAT from SmI2−water
to ketyls suggests that organosamarium species may not be
intermediates during the course of the reduction. This study
demonstrates that care should be employed when using
product distributions to draw conclusions about the mechanism
1
Structures were verified by H and 13C NMR (see the Supporting
Information).
Kinetic Experiments. 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. The reaction
rates were determined from the decay of SmI2 at 25 °C and 560 nm.
All kinetic experiments were performed under pseudo-first-order
conditions with the concentration of substrate at least 10 times the
concentration of Sm(II). The SmI2−H2O and substrate solutions were
injected independently into the stopped-flow system from airtight BD
syringes prepared in a glovebox. Precipitation or phase separation was
not observed in any cases (even at high concentrations of water) for
any substrates. All concentrations of water provided clean exponential
decays over 3 half-lives. The order of samarium was determined using
the fractional times method (see the Supporting Information).
D
DOI: 10.1021/acs.organomet.7b00392
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(7) (a) Chciuk, T. V.; Li, A. M.; Vazquez-Lopez, A.; Anderson, W. R.,
Jr.; Flowers, R. A., II Org. Lett. 2017, 19, 290−293. (b) Chciuk, T. V.;
Anderson, W. R., Jr.; Flowers, R. A., II J. Am. Chem. Soc. 2016, 138,
8738−8741. (c) Chciuk, T. V.; Anderson, W. R., Jr.; Flowers, R. A., II
Angew. Chem., Int. Ed. 2016, 55, 6033−6036. (d) Chciuk, T. V.;
Flowers, R. A., II J. Am. Chem. Soc. 2015, 137, 11526−11531.
(8) (a) Chciuk, T. V.; Boland, B. P.; Flowers, R. A., II Tetrahedron
Lett. 2015, 56, 3212−3215. (b) Chciuk, T. V.; Hilmersson, G.;
Flowers, R. A., II J. Org. Chem. 2014, 79, 9441−9443. (c) Choquette,
K. A.; Sadasivam, D. V.; Flowers, R. A., II J. Am. Chem. Soc. 2010, 132,
17396−17398. (d) Sadasivam, D. V.; Antharjanam, P. K. S.; Prasad, E.;
Flowers, R. A., II J. Am. Chem. Soc. 2008, 130, 7228−7229. (e) Davis,
T. A.; Chopade, P.; Hilmersson, G.; Flowers, R. A., II Org. Lett. 2005,
7, 119−122. (f) Prasad, E.; Flowers, R. A., II J. Am. Chem. Soc. 2002,
124, 6895−6899. (g) Miller, R. S.; Sealy, J. M.; Fuchs, J. R.; Shabangi,
M.; Flowers, R. A., II J. Am. Chem. Soc. 2000, 122, 7718−7722.
(h) Chopade, P.; Davis, T. A.; Prasad, E.; Flowers, R. A., II Org. Lett.
2004, 6, 2685−2688.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00392.
■
S
Characterization data, rate data, 1H and 13C NMR
spectra of starting materials and products, and computa-
tional methods (PDF)
Cartesian coordinates for calculated structures (XYZ)
AUTHOR INFORMATION
Corresponding Author
*E-mail for R.A.F.: rof2@lehigh.edu.
ORCID
Robert A. Flowers II: 0000-0003-2295-1336
Notes
The authors declare no competing financial interest.
■
(9) Sadasivam, D. V.; Teprovich, J. A., Jr.; Procter, D. J.; Flowers, R.
A., II Org. Lett. 2010, 12, 4140−4143.
(10) The rate orders slightly above unity for ketones are likely a
consequence of a small amount of additional carbonyl coordination
above a 1:1 ratio of carbonyl to Sm. See ref 6f.
ACKNOWLEDGMENTS
■
(11) (a) Warren, J. J.; Menzeleev, A. R.; Kretchmer, J. S.; Miller, T.
We thank Dr. Lawrence Courtney in the Department of
Chemistry at Lehigh University for insightful discussions.
T.V.C. thanks the Department of Chemistry at Lehigh
University for a graduate fellowship. R.A.F. is grateful to the
National Science Foundation (CHE 1565741) for support of
this work.
F., III; Gray, H. B.; Mayer, J. M. J. Phys. Chem. Lett. 2013, 4, 519−523.
(b) Megiatto, J. D., Jr.; Men
E.; Teillout, A.-L.; Llansola-Portoles
́
dez-Hernan
́
dez, D. D.; Tejeda-Ferrari, M.
́
, M. J.; Kodis, G.; Poluektov, O.
G.; Rajh, T.; Mujica, V.; Groy, T. L.; Gust, D.; Moore, T. A.; Moore, A.
L. Nat. Chem. 2014, 6, 423−428. (c) Warren, J. J.; Mayer, J. M. J. Am.
Chem. Soc. 2011, 133, 8544−8551. (d) Schrauben, J. N.; Cattaneo, M.;
Day, T. C.; Tenderholt, A. L.; Mayer, J. M. J. Am. Chem. Soc. 2012,
134, 16635−16645. (e) Tarantino, K. T.; Liu, P.; Knowles, R. R. J. Am.
Chem. Soc. 2013, 135, 10022−10025.
REFERENCES
■
(1) Just-Baringo, X.; Procter, D. J. Acc. Chem. Res. 2015, 48, 1263−
1275 and references cited therein.
(12) The leveling off of the rates is proposed to be a consequence of
saturation of the coordination sphere of Sm(II) at high concentrations
of water. See ref 7d.
(13) Newcomb, M. Radical Kinetics and Clocks. In Encyclopedia of
Radicals in Chemistry, Biology and Materials; Chatgilialoglu, C.; Studer,
A., Eds.; Wiley: Chichester, U.K., 2012; pp 1−18.
(14) Skene, W. G.; Scaiano, J. C.; Cozens, F. L. J. Org. Chem. 1996,
61, 7918−7921.
(15) Hasegawa, E.; Curran, D. P. Tetrahedron Lett. 1993, 34, 1717−
1720.
(2) (a) Choquette, K. A.; Flowers, R. A. Sm and Yb Reagents. In
Comprehensive Organic Synthesis, 2nd ed,; Molander, G. A., Knochel, P.,
Eds.; Elsevier: Oxford, U.K., 2014; Vol. 1, pp 279−343. (b) Szostak,
M.; Fazakerley, N. J.; Parmar, D.; Procter, D. J. Chem. Rev. 2014, 114,
5959−6039. (c) Szostak, M.; Spain, M.; Procter, D. J. Chem. Soc. Rev.
2013, 42, 9155−9183. (d) Procter, D. J.; Flowers, R. A., II; Skrydstrup,
T. Organic Synthesis using Samarium Diiodide: A Practical Guide; RSC
Publishing: Cambridge, U.K., 2010. (e) Nicolaou, K. C.; Ellery, S. P.;
Chen, J. S. Angew. Chem., Int. Ed. 2009, 48, 7140−7165. (f) Flowers, R.
A., II Synlett 2008, 2008, 1427−1439. (g) Dahlen, A.; Hilmersson, G.
Eur. J. Inorg. Chem. 2004, 2004, 3393−3403.
(16) Shabangi, M.; Flowers, R. A., II Tetrahedron Lett. 1997, 38,
1137−1140.
(17) Szostak, M.; Spain, M.; Procter, D. J. J. Org. Chem. 2014, 79,
2522−2537.
(3) Curran, D. P.; Fevig, T. L.; Jasperse, C. P.; Totleben, M. J. Synlett
1992, 1992, 943−961.
(4) Chopade, P.; Prasad, E.; Flowers, R. A., II J. Am. Chem. Soc. 2004,
126, 44−45.
(18) BDFEs were determined using Gaussian09(1) programs
employing the APF-D(2) hybrid DFT method and the 6-
311+g(2d,p) basis set. Solvation values were calculated using the
polarizable continuum model with integral equation formalism
(IEFPCM) with tetrahydrofuran as the solvent. See the Supporting
Information for a complete list of references for the computational
work.
(5) (a) Chciuk, T. V.; Flowers, R. A., II Role of Solvents and
Additives in Reactions of Sm(II) Iodide and Related Reductants. In
Science of Synthesis Knowledge Updates; Marek, I., Ed.; Thieme:
Stuttgart, 2016; Vol. 2, pp 171−266. (b) Edmonds, D. J.; Johnston, D.;
Procter, D. J. Chem. Rev. 2004, 104, 3371−3404. (c) Molander, G. A.;
Harris, C. R. Chem. Rev. 1996, 96, 307−338.
(6) (a) Huang, H.-M.; Procter, D. J. J. Am. Chem. Soc. 2017, 139,
1661−1667. (b) Just-Baringo, X.; Clark, J.; Gutmann, M. J.; Procter,
D. J. Angew. Chem., Int. Ed. 2016, 55, 12499−12502. (c) Huang, H.-
M.; Procter, D. J. J. Am. Chem. Soc. 2016, 138, 7770−7775.
(d) Szostak, M.; Lyons, S. E.; Spain, M.; Procter, D. J. Chem.
Commun. 2014, 50, 8391−8394. (e) Szostak, M.; Spain, M.; Procter,
D. J. J. Am. Chem. Soc. 2014, 136, 8459−8466. (f) Szostak, M.; Spain,
M.; Choquette, K. A.; Flowers, R. A., II; Procter, D. J. J. Am. Chem. Soc.
2013, 135, 15702−15705. (g) Collins, K. D.; Oliveira, J. M.; Guazzelli,
G.; Sautier, B.; De Grazia, S.; Matsubara, H.; Heliwell, M.; Procter, D.
J. Chem. - Eur. J. 2010, 16, 10240−10249. (h) Guazzelli, G.; De Grazia,
S.; Collins, K. D.; Matsubara, H.; Spain, M.; Procter, D. J. J. Am. Chem.
Soc. 2009, 131, 7214−7215. (i) Duffy, L. A.; Matsubara, H.; Procter,
D. J. J. Am. Chem. Soc. 2008, 130, 1136−1137.
(19) (a) Anslyn, E. V.; Dougherty, D. A. Modern Physical Organic
Chemistry; University Science Books: Sausalito, CA, 2006; pp 374−
378. (b) Sun, X. Organic Mechanisms: Reactions, Methodology, and
Biological Applications; Wiley: Hoboken, NJ, 2013; pp 17−19.
(20) A classic review by Mayer notes that it is a common supposition
that stepwise transfers of a proton and electron are favored over the
concerted PCET, but this intuition is incorrect in most cases since ΔG
is always lower for PCET than ΔG for the initial PT or ET. See:
Mayer, J. M. Annu. Rev. Phys. Chem. 2004, 55, 363−390.
(21) Ramirez-Solis, A.; Amaro-Estrada, J. I.; Hernandez-Cobos, J.;
Maron, L. J. Phys. Chem. A 2017, 121, 2293−2297.
(22) Farran, H.; Hoz, S. Org. Lett. 2008, 10, 4875−4877.
E
DOI: 10.1021/acs.organomet.7b00392
Organometallics XXXX, XXX, XXX−XXX