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.
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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− © XXXX American Chemical Society
Scheme 1. Stepwise Reduction of a Ketone by SmI2 Containing a Proton Donor Source
water were proposed to proceed through a rate-limiting second ET after cyclization, producing an organosamarium intermediate (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 protoncoupled 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
A
DOI: 10.1021/acs.organomet.7b00392 Organometallics XXXX, XXX, XXX−XXX
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Organometallics rate = k1[S]x [SmI 2]y [H 2O]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.
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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 [SmI 2]y [H 2O]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 To further explore the effect of water on the rate of ketone reduction, the rates of reduction of I−IV were monitored over
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). B
DOI: 10.1021/acs.organomet.7b00392 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
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 demonstrated that the bimolecular rate constant for the reduction of a benzyl radical by photoexcited SmI2 in THF at room temperature is (5.3 ± 1.4) × 107 M−1 s−1.14 In addition to 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 ratelimiting. 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 transstilbene.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
Table 1. Rate Constants and Rate Orders for Substrate, SmI2, and H2O rate order substrate I II III IV
k (M
−3
67 73 150 180
± ± ± ±
−1 a
s ) 4 4 10 10
substrate 1.2 1.2 1.4 1.3
± ± ± ±
b
0.1 0.1 0.3 0.1
SmI2c
H2Od
1 1 1 1
2 2 2 2
a
Conditions: 10 mM SmI2, 1 M H2O, and 100 mM substrate. Conditions: 10 mM SmI2, 1 M H2O, 80−140 mM substrate. c Obtained via fractional times method. dConditions: 100 mM substrate, 10 mM SmI2, 0−1.25 M H2O. b
a broad concentration range of water, as displayed graphically in Figure 1. The results of this study demonstrate two important
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. There are a large number of rate studies on the related 5-exotrig 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.
Scheme 4. Estimate of the Bond Weakening of Water upon Coordination to Sm(II)
C
DOI: 10.1021/acs.organomet.7b00392 Organometallics XXXX, XXX, XXX−XXX
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Organometallics 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. 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 6. Proposed Steps for Reduction of an Unactivated Ketone by SmI2−water
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.
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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-Methylcyclohexanone 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. 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). Structures were verified by 1H 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).
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
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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 D
DOI: 10.1021/acs.organomet.7b00392 Organometallics XXXX, XXX, XXX−XXX
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(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. (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. (11) (a) Warren, J. J.; Menzeleev, A. R.; Kretchmer, J. S.; Miller, T. F., III; Gray, H. B.; Mayer, J. M. J. Phys. Chem. Lett. 2013, 4, 519−523. (b) Megiatto, J. D., Jr.; Méndez-Hernández, D. D.; Tejeda-Ferrari, M. E.; Teillout, A.-L.; Llansola-Portolés, 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. (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. (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. (18) BDFEs were determined using Gaussian09(1) programs employing the APF-D(2) hybrid DFT method and the 6311+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. (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.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00392. Characterization data, rate data, 1H and 13C NMR spectra of starting materials and products, and computational methods (PDF) Cartesian coordinates for calculated structures (XYZ)
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AUTHOR INFORMATION
Corresponding Author
*E-mail for R.A.F.:
[email protected]. ORCID
Robert A. Flowers II: 0000-0003-2295-1336 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS 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.
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REFERENCES
(1) Just-Baringo, X.; Procter, D. J. Acc. Chem. Res. 2015, 48, 1263− 1275 and references cited therein. (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. (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. (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. E
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