Broadening the Scope of Photostimulated SmI 2 Reductions

2015

Chemistry A European J

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The reaction of SmI2 with the substrates 3-methyl-2-butanone, benzyl chloride, p-cyanobenzyl chloride, and anthracene were studied in the presence of water and an amine. In all cases, the water content versus rate profile shows a maximum at around 0.2 M H2 O. The rate versus amine content profile shows in all cases, except for benzyl chloride, saturation behavior, which is typical of a change in the identity of the rate-determining step. The mechanism that is in agreement with the observed data is that electron transfer occurs in the first step. With substrates that are not very electrophilic, the intermediate radical anions lose the added electron back to samarium(III) relatively quickly and the reaction cannot progress efficiently. However, in a mixture of water/amine, the amine deprotonates a molecule of water coordinated to samarium(III). The negatively charged hydroxide, which is coordinated to samarium(III), reduces its electrophilicity, and therefore, lowers the rate of back electron transfer, which allows the reaction to progress. In the case of benzyl chloride, in which electron transfer is rate determining, deprotonation by the amine is coupled to the electron-transfer step.

Section: Resultsmentioning

confidence: 99%

“…8 Their added value stems from the ability to trap efficiently short‐lived radical anions generated by electron transfer from SmI 2 to the substrate before back electron transfer takes place 7a. 9…”

Section: Introductionmentioning

confidence: 99%

Deciphering a 20‐Year‐Old Conundrum: The Mechanisms of Reduction by the Water/Amine/SmI₂ Mixture

Maity

2015

Chemistry A European J

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“…3,4 This paper focuses on the question of whether these two factors are additive or not. 3,4 This paper focuses on the question of whether these two factors are additive or not.…”

Section: Introductionmentioning

confidence: 99%

“…This is why, for example, naphthalene cannot be photoreduced by SmI 2 using TFE as a proton donor 4. A major consequence of the low electrophilicity of PA, NP and CEB is the short life time of their radical anions.…”

mentioning

confidence: 99%

Synergism and Inhibition in the Combination of Visible Light and HMPA in SmI₂ Reductions

Rao

2012

J. Org. Chem.

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The reaction of six substrates (diphenylacetylene, benzonitrile, methyl benzoate, phenylacetylene, naphthalene, and 1-chloro-4-ethylbenzene) with SmI(2) in the presence of MeOH or TFE was studied. The reactions were monitored under three different conditions: (a) irradiation, (b) irradiation in the presence of HMPA, and (c) reactions in the presence of HMPA in the dark. The combination of visible light and HMPA was found in some cases to be synergistic, in others to be additive, and in four cases to be inhibitive. The Marcus theory provides a good understanding of the synergistic and the additivity phenomena. The inhibitive effect is traced to the post electron transfer step in which Sm(3+) plays an important role. Once coordinated to HMPA, Sm(3+) is less capable of assisting in the protonation of the radical anion or the expulsion of the leaving group. Ranking according to the substrate's electron affinity shows that inhibition is manifested for the three least electrophilic substrates: phenylacetylene, naphthalene, and 1-chloro-4-ethylbenzene. Typical of these substrates is the short lifetime of their radical anions. Thus, if a step consecutive to electron transfer is slow and cannot compete successfully with the rapid back electron transfer, the benefit of having the electron transfer step enhanced is much reduced.

“…However, when a proton donor ( t ‐BuOH), which is incapable of forming a complex with SmI 2 is used, the radical anion will not be protonated fast enough and the system will cyclize at a radical anionic level. This understanding enabled us to widen the scope of the substrates reacting with SmI 2 , to substrates whose radical anion was very short lived 10. A unimolecular protonation is thus important because it may efficiently compete with back electron transfer.…”

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confidence: 99%

Physical Organic Chemistry (POC) Makes Chemistry a Science: A Personal Reflection on POC

Israel Journal of Chemistry

2015

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