Synergism and inhibition in the combination of visible light and HMPA in SmI2 reductions

Chintada Nageswara Rao, Shmaryahu Hoz

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The reaction of six substrates (diphenylacetylene, benzonitrile, methyl benzoate, phenylacetylene, naphthalene, and 1-chloro-4-ethylbenzene) with SmI2 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 Sm3+ plays an important role. Once coordinated to HMPA, Sm3+ is less capable of assisting in the protonation of the radical anion or the expulsion of the leaving group. Ranking according to the substrates 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.

Original languageEnglish
Pages (from-to)9199-9204
Number of pages6
JournalJournal of Organic Chemistry
Issue number20
StatePublished - 19 Oct 2012


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