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

Abstract

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.

Electron Transfer Reduction of Nitriles Using SmI2–Et3N–H2O: Synthetic Utility and Mechanism

“…Over the past 35 years, SmI 2 has been successfully utilized to generate ketyl-type radicals from an impressive range of carbonyl precursors; however, efficient and general reduction of nitriles with SmI 2 has not been reported to date due to the prohibitive redox potential of these substrates, which prevents productive electron transfer from Sm­(II) (Figure ). − …”

Cross-Coupling Reactions Using Samarium(II) Iodide

“…The selectivity of electron transfer from SmI 2 depends on ligands and additives, solvent, and method of preparation of the reagent. − Several major developments in the reduction of common functional groups with Sm­(II) have been reported in the past decade and include: (i) chemoselective reduction of cyclic esters with SmI 2 –H 2 O; (ii) enhanced reduction of common functional groups with SmI 2 –amine–H 2 O; − (iii) the first general reductions of acyclic esters, carboxylic acids, and amides using SmI 2 ; − (iv) reduction of nitriles using SmI 2 –hν and SmI 2 –amine–H 2 O systems; (v) reduction of alkyl fluorides using Sm­(HMDS) 2 in hexanes; (vi) conjugate reductions and eliminations of α,β- and α,β/γ,δ-unsaturated carboxylic acid derivatives; − (vii) reduction of aromatic rings; and (viii) reduction/elimination of benzylic heteroatoms. , Some of these processes have already been successfully applied to the reductive formation of carbon–carbon bonds as summarized in the following sections of this Review. Importantly, in many cases fully chemoselective electron transfer to a selected functional group is possible in the presence of other easily reducible functional groups as a result of thermodynamic control.…”