On the Role of Pre- and Post-electron-transfer Steps in the SmI2 /amine/H(2)O-mediated Reduction of Esters: New Mechanistic Insights and Kinetic Studies

Overview

Journal Chemistry

Specialty Chemistry

Date 2014 Mar 12

PMID 24615806

Citations 2

Authors

Michal Szostak

Malcolm Spain

David J Procter

Affiliations

Soon will be listed here.

Abstract

The mechanism of the SmI2 -mediated reduction of unactivated esters has been studied using a combination of kinetic, radical clocks and reactivity experiments. The kinetic data indicate that all reaction components (SmI2 , amine, H2 O) are involved in the rate equation and that electron transfer is facilitated by Brønsted base assisted deprotonation of water in the transition state. The use of validated cyclopropyl-containing radical clocks demonstrates that the reaction occurs via fast, reversible first electron transfer, and that the electron transfer from simple Sm(II) complexes to aliphatic esters is rapid. Notably, the mechanistic details presented herein indicate that complexation between SmI2 , H2 O and amines affords a new class of structurally diverse, thermodynamically powerful reductants for efficient electron transfer to carboxylic acid derivatives as an attractive alternative to the classical hydride-mediated reductions and as a source of acyl-radical equivalents for CC bond forming processes.

Citing Articles

Diastereoselective Radical 1,4-Ester Migration: Radical Cyclizations of Acyclic Esters with SmI.

Morrill C, Peter A, Amalina I, Pye E, Crisenza G, Kaltsoyannis N J Am Chem Soc. 2022; 144(30):13946-13952.

PMID: 35858251 PMC: 9377304. DOI: 10.1021/jacs.2c05972.

Samarium Iodide Showcase: Unraveling the Mechanistic Puzzle.

Hoz S Acc Chem Res. 2020; 53(11):2680-2691.

PMID: 33063516 PMC: 7676297. DOI: 10.1021/acs.accounts.0c00497.

References

Upadhyay S, Hoz S . The effect of proton donors on the facial stereoselectivity in SmI2 reduction of norcamphor. J Org Chem. 2011; 76(5):1355-60. DOI: 10.1021/jo1023206. View

Szostak M, Sautier B, Procter D . Structural analysis and reactivity of unusual tetrahedral intermediates enabled by SmI2-mediated reduction of barbituric acids: vinylogous N-acyliminium additions to α-hydroxy-N-acyl-carbamides. Chem Commun (Camb). 2014; 50(19):2518-21. DOI: 10.1039/c3cc48932a. View

Szostak M, Spain M, Procter D . Uncovering the importance of proton donors in TmI2-promoted electron transfer: facile C-N bond cleavage in unactivated amides. Angew Chem Int Ed Engl. 2013; 52(28):7237-41. PMC: 4265963. DOI: 10.1002/anie.201303178. View

Szostak M, Spain M, Eberhart A, Procter D . Highly chemoselective reduction of amides (primary, secondary, tertiary) to alcohols using SmI2/amine/H2O under mild conditions. J Am Chem Soc. 2014; 136(6):2268-71. PMC: 3982934. DOI: 10.1021/ja412578t. View

Collins K, Oliveira J, Guazzelli G, Sautier B, De Grazia S, Matsubara H . Selective reductions of cyclic 1,3-diesters by using SmI(2) and H(2)O. Chemistry. 2010; 16(33):10240-9. DOI: 10.1002/chem.201000632. View

Gansauer A, Klatte M, Brandle G, Friedrich J . Catalytic hydrogen atom transfer (HAT) for sustainable and diastereoselective radical reduction. Angew Chem Int Ed Engl. 2012; 51(35):8891-4. DOI: 10.1002/anie.201202818. View

Szostak M, Spain M, Parmar D, Procter D . Selective reductive transformations using samarium diiodide-water. Chem Commun (Camb). 2011; 48(3):330-46. DOI: 10.1039/c1cc14252f. View

Jensen C, Lindsay K, Taaning R, Karaffa J, Hansen A, Skrydstrup T . Can decarbonylation of acyl radicals be overcome in radical addition reactions? En route to a solution employing N-acyl oxazolidinones and SmI2/H2O. J Am Chem Soc. 2005; 127(18):6544-5. DOI: 10.1021/ja050420u. View

Gansauer A, Behlendorf M, Cangonul A, Kube C, Cuerva J, Friedrich J . H2O activation for hydrogen-atom transfer: correct structures and revised mechanisms. Angew Chem Int Ed Engl. 2012; 51(13):3266-70. DOI: 10.1002/anie.201107556. View

10.

Ankner T, Said Stalsmeden A, Hilmersson G . Selective cleavage of 3,5-bis-(trifluoromethyl)benzylcarbamate by SmI2-Et3N-H2O. Chem Commun (Camb). 2013; 49(61):6867-9. DOI: 10.1039/c3cc41642a. View

11.

Masson G, Cividino P, Py S, Vallee Y . SmI2-induced umpolung of the C=N bond: first reductive conjugate addition of nitrones to alpha,beta-unsaturated esters. Angew Chem Int Ed Engl. 2003; 42(20):2265-8. DOI: 10.1002/anie.200250480. View

12.

Amiel-Levy M, Hoz S . Guidelines for the use of proton donors in SmI2 reactions: reduction of alpha-cyanostilbene. J Am Chem Soc. 2009; 131(23):8280-4. DOI: 10.1021/ja9013997. View

13.

Taaning R, Lindsay K, Schiott B, Daasbjerg K, Skrydstrup T . Importance of C-N bond rotation in N-acyl oxazolidinones in their SmI2-promoted coupling to acrylamides. J Am Chem Soc. 2009; 131(29):10253-62. DOI: 10.1021/ja903401y. View

14.

Sautier B, Lyons S, Webb M, Procter D . Radical cyclization cascades of unsaturated Meldrum's acid derivatives. Org Lett. 2011; 14(1):146-9. DOI: 10.1021/ol2029367. View

15.

Tanko J, Gillmore J, Friedline R, Chahma M . Cyclopropylcarbinyl --> homoallyl-type ring opening of ketyl radical anions. Structure/reactivity relationships and the contribution of solvent/counterion reorganization to the intrinsic barrier. J Org Chem. 2005; 70(10):4170-3. DOI: 10.1021/jo047917r. View

16.

Molander G, Harris C . Sequencing Reactions with Samarium(II) Iodide. Chem Rev. 1996; 96(1):307-338. DOI: 10.1021/cr950019y. View

17.

Hansen A, Lindsay K, Sudhadevi Antharjanam P, Karaffa J, Daasbjerg K, Flowers 2nd R . Mechanistic evidence for intermolecular radical carbonyl additions promoted by samarium diiodide. J Am Chem Soc. 2006; 128(30):9616-7. DOI: 10.1021/ja060553v. View

18.

Parmar D, Price K, Spain M, Matsubara H, Bradley P, Procter D . Reductive cyclization cascades of lactones using SmI2-H2O. J Am Chem Soc. 2011; 133(8):2418-20. DOI: 10.1021/ja1114908. View

19.

Parmar D, Matsubara H, Price K, Spain M, Procter D . Lactone radical cyclizations and cyclization cascades mediated by SmI2-H2O. J Am Chem Soc. 2012; 134(30):12751-7. DOI: 10.1021/ja3047975. View

20.

Chahma M, Li X, Phillips J, Schwartz P, Brammer L, Wang Y . Activation/driving force relationships for cyclopropylcarbinyl --> homoallyl-type rearrangements of radical anions. J Phys Chem A. 2006; 109(15):3372-82. DOI: 10.1021/jp050193i. View