Substrate-directable electron transfer reactions. Dramatic rate enhancement in the chemoselective reduction of cyclic esters using SmI2-H 2O: Mechanism, scope, and synthetic utility

Article abstract of DOI:10.1021/ja4078864

Substrate-directable reactions play a pivotal role in organic synthesis, but are uncommon in reactions proceeding via radical mechanisms. Herein, we provide experimental evidence showing dramatic rate acceleration in the Sm(II)-mediated reduction of cyclic esters that is enabled by transient chelation between a directing group and the lanthanide center. This process allows unprecedented chemoselectivity in the reduction of cyclic esters using SmI₂-H₂O and for the first time proceeds with a broad substrate scope. Initial studies on the origin of selectivity and synthetic application to form carbon-carbon bonds are also disclosed.

Full text of DOI:10.1021/ja4078864

Communication
pubs.acs.org/JACS
Substrate-Directable Electron Transfer Reactions. Dramatic Rate
Enhancement in the Chemoselective Reduction of Cyclic Esters Using
SmI2−H2O: Mechanism, Scope, and Synthetic Utility
Michal Szostak,*,† Malcolm Spain,† Kimberly A. Choquette,‡ Robert A. Flowers, II,‡
and David J. Procter*,†
†School of Chemistry, University of Manchester, Oxford Road, Manchester, M13 9PL, United Kingdom
‡Department of Chemistry, Lehigh University, Seeley G. Mudd Building, Bethlehem, Pennsylvania 18015, United States
S
* Supporting Information
ABSTRACT: Substrate-directable reactions play a pivotal
role in organic synthesis, but are uncommon in reactions
proceeding via radical mechanisms. Herein, we provide
experimental evidence showing dramatic rate acceleration
in the Sm(II)-mediated reduction of cyclic esters that is
enabled by transient chelation between a directing group
and the lanthanide center. This process allows unprece-
dented chemoselectivity in the reduction of cyclic esters
using SmI2−H2O and for the first time proceeds with a
broad substrate scope. Initial studies on the origin of
selectivity and synthetic application to form carbon−
carbon bonds are also disclosed.
he use of directing groups is a powerful strategy to control
Tthe enantio- and diastereoselectivity as well as site
selectivity of valuable synthetic reactions by providing highly
ordered transition states through multiple-point substrate−
reagent interactions.1 Since the pioneering work reported by
Henbest in 1957,2 considerable effort has been devoted to the
Figure 1. (a) Directing group strategy toward electron transfer. (b)
development of substrate-directable reactions that proceed via
closed-shell pathways.3 However, considerably fewer studies
have employed directing groups in an alternative mechanistic
manifold involving sequential electron transfers using low-
valent metal reagents, including Sm(II) reductants (Figure
1A).4 In this regard, several elegant approaches to control
stereochemical aspects of electron transfer steps exist;5
however, a strategy based on a chelate-controlled activation
of inert functional groups toward productive electron transfer
by coordination of low-valent, often highly oxophilic, hard
Lewis acidic metals remains vastly unexplored.6,7
Herein, we provide experimental evidence showing dramatic
rate acceleration in the Sm(II)-mediated reduction of cyclic
esters that is enabled by transient chelation between directing
groups and the lanthanide center.8 Using kinetic experiments
we quantify the effect of directing groups on the rate of the
electron transfer in a set of cyclic esters previously unreactive
toward the reagent system (Figure 1B) and establish, for the
first time, that inert functional groups can be activated toward
highly chemoselective electron transfer from a SmI2 complex9
by the use of a directing group effect (Figure 1C).
Previous work: chemoselective reduction of lactones with SmI2. (c)
This study: rate acceleration in the chelation-enabled electron transfer
to cyclic esters using SmI2−H2O.
aliphatic esters were outside the reducing range of SmI2.6,7 The
reaction showed unprecedented chemoselectivity in that the
SmI2−H2O complex was completely selective for six-membered
lactones over other classes of esters and lactones, which was
ascribed to the anomeric stabilization of the ketyl radical
intermediate in a six-membered ring system (Figure 1B).11
However, the reaction was very limited in scope in that it
tolerated only sterically accessible six-membered lactones.
When expanding the scope of this transformation, we
hypothesized that positioning a suitable directing group within
close proximity to the formed radical anion would stabilize this
intermediate by chelation and facilitate the electron transfer in
the rate determining step of the reaction. Importantly, we
recognized that, if successful, this process would constitute one
of the first substrate directable electron transfer reactions,
Recently, we reported the reduction of cyclic esters using
Received: July 31, 2013
Published: September 30, 2013
SmI2.10 Prior to this work it had been thought that simple
© 2013 American Chemical Society
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Communication
which could deliver unique chemoselectivity impossible to
achieve by existing one- or two-electron mechanisms.
In view of the paucity of mechanistic studies on the role of
directing groups in electron transfer processes, we began our
examination by determining the effect of various directing
groups on the rate of the reduction of five-, six-, and seven-
membered lactones (Table 1). Experiments were carried out
that the reaction is sensitive to steric hindrance around the
lactone carbonyl group. Finally, the rate constants measured for
several α-aryl-containing substrates (see SI for details and
additional discussion) confirm the importance of steric factors
in the coordination of Sm(II) and further demonstrate the
unique properties of SmI2−H2O over other Sm(II) reducing
systems.9
The rate orders of substrates and SmI2 were determined by
the fractional times method and/or the initial rates method.
Previous studies demonstrated that in a related reduction of 5-
decanolide under the same experimental conditions the rate
order of H2O is zero.11a In the present study, we found that, in
all cases, the rate order of SmI2 was one. The substrate rate
order studies showed the rate order of one (within
experimental error) for all studied substrates except for the
alcohol-containing ester, which showed the rate order of two
(entry 8, within experimental error). The obtained rate orders
are consistent with the proposed mechanism for the reduction
of cyclic esters using SmI2−H2O.11 The rate order of two for
the alcohol-containing ester most likely results from the
formation of a dimeric SmI2−alcohol complex with the alcohol
serving as one of the Sm(II) ligands.12,13
Having identified the effect of directing groups, we
investigated the synthetic scope of the reduction using SmI2−
H2O (Table 2). Typically a 2-fold excess of reagent was used to
ensure that the reactions were complete. In general, the
substrates were selected to demonstrate the ring-size chemo-
selectivity of the process, emphasize functional group tolerance,
and probe the ease of reduction of five- to seven-membered
lactones that could potentially be used as radical precursors for
stereocontrolled C−C bond formation.11a−f In the event, a
wide range of lactone-bearing ester and amide directing groups
furnished the corresponding diols in good to excellent yields
(entries 1−15). Importantly, over-reduction of acyclic directing
groups was not observed under the reaction conditions, thus
demonstrating the high chemoselectivity of the process.
Moreover, the method tolerates functional groups that can be
reduced under single electron transfer conditions, including
aromatic rings, benzyl ethers, nitriles, acetates, and terminal
olefins.7d Remarkably, under these reaction conditions, the
reductive 5-exo cyclization is not observed (entries 13−15),
showing that the directed reduction of acyl-type radicals is
rapid.14 Overall, these synthetic studies demonstrate that highly
chemoselective electron transfer to lactones of ring sizes other
than six can be readily achieved and open the door to the use of
a wide range of cyclic esters in radical transformations with the
SmI2−H2O reagent.
Table 1. Observed Rate Constants and Reaction Orders for
the Reduction of Lactones Using SmI2−H2O
a
rate order
entry
n
R1, R2
H, H
k [M−1 s−1
]
lactone
SmI2
1
0
1
2
0
1
2
0
0
0
1
1
<1.0 × 10−5
0.014 0.001
3.0 × 10−4
−
−
1.0
2
H, H
0.8 0.03
3
H, H
−
−
4
H, CO2Et
H, CO2Et
H, CO2Et
H, CO2t-Bu
H, C(OH)R
H, CONHCy
H, Ph
419 45
1.1 0.1
1.0 0.1
1.3 0.1
1.0 0.1
2.2 0.2
−
1.0
1.0
1.0
1.0
1.0
−
5
610 10
6
0.91 0.05
0.32 0.03
0.34 0.03
>1.0 × 104
0.012 0.001
0.007 0.001
7
8
9
10
11
0.9 0.1
1.0
C5H11, H
1.1 0.1
1.0
a
Conditions: [SmI2] = 10 mM; H2O = 150 equiv; lactone = 10−50
equiv, T = 30.0
0.1 °C. Entry 8, R = C5H10. See SI for full
experimental details.
using stopped-flow spectrophotometry under pseudo-first-order
conditions. Rates were obtained by monitoring the decay of the
SmI2−H2O complex at 560 nm. Plots of the observed rate
constants against concentration for the reduction of cyclic
esters are shown in the Supporting Information (SI).
The reduction of a simple six-membered lactone (entry 2, k =
0.014 M−1 s−1) was found to be much faster than that of the
corresponding five- and seven-membered lactones (entries 1
and 3, k < 0.1 × 10−5 M−1 s−1, lower limit value, and k = 3 ×
10−4 M−1 s−1, respectively). In agreement with our previous
findings,10,11 these rate constants can be explained by anomeric
stabilization of the radical anions. The slow rate for reduction of
five- and seven-membered lactones prohibits their use in
chemoselective Sm(II)-mediated transformations (the natural
decay of SmI2 of 4.4 × 10−4 s−1).8 Remarkably, dramatic rate
acceleration ranging from 4 to 7 orders of magnitude is observed in
all three ring systems by using an ester directing group in the α-
position of lactone substrates (entries 4−6).12 The acceleration is
consistent with efficient chelation of the SmI2−H2O reductant.
Intriguingly, it appears that a high concentration of water does
not prohibit the Sm(II) chelation (vide infra), although
previous studies have shown that water displaces Lewis basic
oxygens from the coordination sphere of Sm(II).13 A range of
other directing groups results in a very significant acceleration
of the rate of the reduction, from 4 to 9 orders of magnitude,
demonstrating that the effect is not limited to simple esters
(entries 7−9). The reduction of an α-phenyl-substituted
lactone (entry 10) demonstrates that chelation of Sm(II) is
the major factor contributing to the rate enhancement. The low
rate of reduction of 5-alkyl-substituted lactone (entry 11, ca. 2
times slower than that of the unsubstituted analogue) suggests
We have carried out initial studies to elucidate the
mechanism of the reduction (see SI). (1) The reduction of
six-membered lactone 1e with limiting SmI2 gives the
corresponding lactol (86%, 52:48 dr). (2) The reduction of
1e with SmI2−D2O gives the lactol with >98% D1
incorporation (96.0% D2 incorporation in the reduction of
1f), indicating that anions are protonated during a series of
electron transfer steps.11a,15,16 (3) The kinetic isotope effects
for the lactol and diol formation (kH/kD = 1.27 0.1 and 1.24
0.1, respectively) suggest that proton transfer is not involved
in the rate determining step of the reaction. (4) In the
reduction of 5-decanolide under various SmI2 conditions lactol
is not detected, suggesting that α-ester stabilizes the tetrahedral
intermediate. (5) The reaction shows full chemoselectivity over
alkyl iodides and aryl esters7e (favoring the cyclic ester) as well
as aliphatic ketones and aldehydes (favoring the latter).
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Table 2. Chelation-Enabled Reduction of Lactones Using
SmI2−H2O
Scheme 1. Reductive Cyclizations of Lactones Using SmI2−
H2O Enabled by the Directing Group Effect
a
values are in good agreement with the anomeric stabilization of
ketyl-type radicals in these systems and demonstrate that a
plethora of reductive cyclizations (cf. reductions) can be
achieved in these systems.
Finally, we have preliminary results pertaining to the origin
of chemoselectivity in the directed reduction of cyclic esters
using SmI2−H2O (Schemes 2 and 3). The reduction of a
Scheme 2. Investigating the Origin of Selectivity in the
Reduction of Lactones Using SmI2−H2O
Scheme 3. Investigating the Directing Group Effect on the
Stability of Ketyl-Type Radicals
a
b
Conditions: SmI2 (6−8 equiv), THF, H2O, 23 °C. C7H15 derivatives
at C5 were used. See SI for full experimental details.
Importantly, the SmI2−H2O system has been shown to tolerate
other functional groups that are incompatible with Sm(II)-
Lewis bases.6,7 (6) The use of water is critical for the observed
reactivity (no reaction is observed in the absence and at low
concentration of water). (7) Other additives (LiCl, HMPA,
MeOH, amine/Et3N) do not promote the reaction efficiently,
which is consistent with the coordination of water to Sm(II) to
enhance the redox potential of the reagent and to stabilize the
formed radical anion.13
It is particularly noteworthy that the radical intermediates
formed in the first electron transfer can be exploited in radical
cyclizations by simply modifying the reaction conditions (slow
addition protocol, see SI) (Scheme 1). Cyclic esters having five-
to seven-membered rings undergo efficient radical cyclization to
give cyclopentanols containing three contiguous stereocenters
in good yields and in general high diastereoselectivities.17
Moreover, by approximating the rate constant for cyclization of
the 5-hexenyl radical (2.3 × 105 s−1)14 we were able to estimate
the relative rate constant for the reduction of acyl-type radicals
A at the 10 M concentration of H2O (Scheme 1). The obtained
flexible 16-membered lactone under the same reaction
conditions was unproductive, demonstrating that the release
of strain may play a role in the chemoselectivity of the reaction
(Scheme 2). The reduction of activated ester groups in acyclic
diesters and malonate monoamides was not observed under the
reaction conditions despite their well-established capacity to
accept electrons from SmI2−THF systems (Scheme 2).18
Finally, the reaction of a series of radical clocks with SmI2−
H2O points to the crucial role of directing groups to stabilize
the formed radical anions (Scheme 3),19,20 which is in
agreement with the kinetic rate studies and the proposed
mechanism.
In summary, the results presented herein demonstrate that
the presence of a suitably placed directing group dramatically
accelerates the rate of the reduction of cyclic esters with SmI2−
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Journal of the American Chemical Society
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1998, 63, 812. (e) Kan, T.; Hosokawa, S.; Nara, S.; Tamiya, H.;
Shirahama, H.; Matsuda, F. J. Org. Chem. 2004, 69, 8956. Directed
reductions: (f) Keck, G. E.; Wager, C. A.; Sell, T.; Wager, T. T. J. Org.
Chem. 1999, 64, 2172. (g) Keck, G. E.; Wager, C. A. Org. Lett. 2000, 2,
2307. (h) Keck, G. E.; Truong, A. P. Org. Lett. 2002, 4, 3131.
(6) Procter, D. J.; Flowers, R. A., II; Skrydstrup, T. Organic Synthesis
using Samarium Diiodide: A Practical Guide; RSC Publishing:
Cambridge, 2010.
(7) Recent reviews on SmI2: (a) Molander, G. A.; Harris, C. R. Chem.
Rev. 1996, 96, 307. (b) Krief, A.; Laval, A. M. Chem. Rev. 1999, 99,
745. (c) Kagan, H. B. Tetrahedron 2003, 59, 10351. (d) Nicolaou, K.
C.; Ellery, S. P.; Chen, J. S. Angew. Chem., Int. Ed. 2009, 48, 7140.
(e) Szostak, M.; Spain, M.; Procter, D. J. Chem. Soc. Rev. 2013,
DOI: 10.1039/C3CS60223K.
H2O. Remarkably, the reagent system is fully chemoselective
for the reduction of lactones over esters and has been exploited
in reductive carbon−carbon bond formation to rapidly build-up
molecular complexity. Importantly, the current study is one of
the few examples of substrate-directable reactions proceeding
via sequential one-electron mechanisms. We believe that these
findings will have an important impact for the activation of inert
functional groups via open-shell reaction pathways. Application
of this strategy to other substrate-directable electron transfer
reactions is underway in our laboratory and will be described
shortly.
ASSOCIATED CONTENT
* Supporting Information
■
(8) For a seminal mechanistic study on the β-directing effect, see:
Prasad, E.; Flowers, R. A., II. J. Am. Chem. Soc. 2002, 124, 6357.
(9) Reviews: (a) Dahlen, A.; Hilmersson, G. Eur. J. Inorg. Chem.
́
2004, 3393. (b) Szostak, M.; Spain, M.; Parmar, D.; Procter, D. J.
Chem. Commun. 2012, 48, 330.
S
Experimental procedures, compound characterization data, and
details of rate studies. This material is available free of charge
via the Internet at http://pubs.acs.org.
(10) Duffy, L. A.; Matsubara, H.; Procter, D. J. J. Am. Chem. Soc.
2008, 130, 1136.
AUTHOR INFORMATION
Corresponding Authors
michal.szostak@manchester.ac.uk
david.j.procter@manchester.ac.uk
■
(11) For selected studies on SmI2−H2O, see: (a) Parmar, D.; Duffy,
L. A.; Sadasivam, D. V.; Matsubara, H.; Bradley, P. A.; Flowers, R. A.,
II; Procter, D. J. J. Am. Chem. Soc. 2009, 131, 15467. (b) Parmar, D.;
Price, K.; Spain, M.; Matsubara, H.; Bradley, P. A.; Procter, D. J. J. Am.
Chem. Soc. 2011, 133, 2418. (c) Parmar, D.; Matsubara, H.; Price, K.;
Spain, M.; Procter, D. J. J. Am. Chem. Soc. 2012, 134, 12751.
(d) Guazzelli, G.; De Grazia, S.; Collins, K. D.; Matsubara, H.; Spain,
M.; Procter, D. J. J. Am. Chem. Soc. 2009, 131, 7214. (e) Szostak, M.;
Spain, M.; Procter, D. J. Chem. Commun. 2011, 47, 10254. For a
related study on TmI2−H2O, see: (f) Szostak, M.; Spain, M.; Procter,
D. J. Angew. Chem., Int. Ed. 2013, 52, 7237.
(12) Selected recent mechanistic studies on SmI2 reductants:
(a) Miller, R. S.; Sealy, J. M.; Shabangi, M.; Kuhlman, M. L.; Fuchs,
J. R.; Flowers, R. A., II. J. Am. Chem. Soc. 2000, 122, 7718. (b) Prasad,
E.; Flowers, R. A., II. J. Am. Chem. Soc. 2002, 124, 6895. (c) Sadasivam,
D. V.; Antharjanam, P. K. S.; Prasad, E.; Flowers, R. A., II. J. Am. Chem.
Soc. 2008, 130, 7228. (d) Choquette, K. A.; Sadasivam, D. V.; Flowers,
R. A., II. J. Am. Chem. Soc. 2010, 132, 17396.
(13) (a) Chopade, P. R.; Prasad, E.; Flowers, R. A., II. J. Am. Chem.
Soc. 2004, 126, 44. (b) Prasad, E.; Flowers, R. A., II. J. Am. Chem. Soc.
2005, 127, 18093. (c) Amiel-Levy, M.; Hoz, S. J. Am. Chem. Soc. 2009,
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(14) (a) Newcomb, M. Tetrahedron 1993, 49, 1151. (b) Hasegawa,
E.; Curran, D. P. Tetrahedron Lett. 1993, 34, 1717. (c) Curran, D. P.;
Fevig, T. L.; Jasperse, C. P.; Totleben, J. Synlett 1992, 943.
(d) Stevenson, J. P.; Jackson, W. F.; Tanko, J. M. J. Am. Chem. Soc.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge the EPSRC and GSK for financial support.
R.A.F thanks the National Science Foundation (CHE 0844946
and 1266333) for the work at Lehigh University.
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