Ligand Effects on the Diastereoselectivities of Samarium Diiodide Promoted Pinacol Coupling

Pedersen, Henriette L.; Christensen, Torben Birk; Enemærke, Rasmus J.; Daasbjerg, Kim; Skrydstrup, Troels

doi:10.1002/(sici)1099-0690(199903)1999:3<565::aid-ejoc565>3.3.co;2-a

Eur. J. Org. Chem.

1999

DOI: 10.1002/(sici)1099-0690(199903)1999:3<565::aid-ejoc565>3.3.co;2-a

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Ligand Effects on the Diastereoselectivities of Samarium Diiodide Promoted Pinacol Coupling

Henriette L. Pedersen

Torben Birk Christensen

Rasmus J. Enemærke

et al.

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“…[1,2] Typically alcohols and glycols are used as proton donor sources, whereas glymes are used as coordinating additives. All of these additives have been shown to exert an influence on the regiochemical [3,4] and stereochemical [5,6] outcomes of numerous SmI 2 -mediated reactions. The seminal work of Hoz et al demonstrated that the interplay between proton-donor coordination and acidity is likely to play an important mechanistic role in reductions.…”

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Mechanistic Studies of Proton‐Donor Coordination to Samarium Diiodide

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Alcohols, glycols, and glymes are commonly used as additives in numerous reductions and reductive coupling reactions of samarium diiodide (SmI 2 ). [1,2] Typically alcohols and glycols are used as proton donor sources, whereas glymes are used as coordinating additives. All of these additives have been shown to exert an influence on the regiochemical [3,4] and stereochemical [5,6] outcomes of numerous SmI 2 -mediated reactions. The seminal work of Hoz et al. demonstrated that the interplay between proton-donor coordination and acidity is likely to play an important mechanistic role in reductions. [7] Studies by DahlØn and Hilmersson have shown that coordinating alcohols enhance the rate of ketone reduction substantially and that the rate increase is proportional to the number of ethereal oxygen atoms in the proton donor source.[8] More recent studies in our group have shown that the high affinity of water for SmI 2 in THF leads to substrate reduction through a unique mechanistic pathway involving a Sm II -water complex.[9] Taken together, these studies show that proton-donor coordination enhances the reactivity of SmI 2 . Unfortunately, the paucity of well-characterized systems for which the structures of the ground-state reductants are known limits our understanding of the mechanistic rationale for this phenomenon. Presented herein are crystallographic, spectroscopic, and kinetic studies on substrate reduction by SmI 2 in solutions containing diethylene glycol (dg), diethylene glycol monomethyl ether (dgme), and diglyme (dgde). These studies show 1) that initial coordination of the additives liberates THF or iodide from SmI 2 , thus providing open coordination sites for substrate, 2) that saturation of SmI 2 with the additive decreases its reactivity, and 3) that replacement of a hydroxyl proton with a methyl group significantly decreases the affinity of the additive for SmI 2 .In searching for appropriate systems to explore, we sought proton donors and related systems that would have a high affinity for Sm II and simultaneously provide stable complexes that could be characterized. Many complexes of SmI 2 with triand tetraglyme provide complex aggregates which would complicate mechanistic analysis.[10] We were drawn to the structure of the [Sm(dgde) 3 ] 2+ cation. [11] This species is unique among Sm II -glyme structures because it is monomeric, thus indicating that an opportunity existed for isolation and characterization of complexes of dg, dgme, and dgde with SmI 2 . Furthermore, this system offers an opportunity to examine the impact of subtle changes to the system through substitution of the proton of the hydroxyl group with methyl substituents.To initially examine the system, the UV/Vis spectrum of SmI 2 (1 mmol in THF) was monitored in the presence of increasing amounts of dg, dgme, and dgde. A subset of these experiments is displayed in Figure 1. Upon addition of two equivalents of dg, the long-wavelength absorption peaks of SmI 2 (l max = 555 and 618 nm) begin to merge, indicating coordination. Upon furt...

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Mechanistic Studies of Proton‐Donor Coordination to Samarium Diiodide

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Ligand Effects on the Diastereoselectivities of Samarium Diiodide Promoted Pinacol Coupling

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Mechanistic Studies of Proton‐Donor Coordination to Samarium Diiodide

Mechanistic Studies of Proton‐Donor Coordination to Samarium Diiodide

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