Non-classical selectivities in the reduction of alkenes by cobalt-mediated hydrogen atom transfer

Abstract

It is shown that the reduction of alkenes by hydrogen atom transfer provides selectivities that are distinct from classical hydrogenation catalysts. The first alkene hydrobromination, hydroiodination, and hydroselenylation reactions that proceed by hydrogen atom transfer processes are also reported.

Mn-, Fe-, and Co-Catalyzed Radical Hydrofunctionalizations of Olefins

“…As discussed above (see Section 5.2 ), Ma and Herzon 354 reported in 2015 the radical hydrobromination and hydroiodination of alkenes using tosyl bromide and diiodomethane as the halogen sources, respectively, in analogy to Carreira’s tosyl chloride. 340 The same method was also applied to the hydroselenation of olefins (Figure 131).…”

“…354 They accomplish these transformations of alkenes to halides with stoichiometric Co(acac) 2 , alongside triethylsilane and 1,4-dihydrobenzene as reductants, tert -butyl hydroperoxide as an additive/oxidant, and tosyl bromide (TsBr) or diiodomethane as sources of halogen. Bromides 274 or 276 as well as iodides 275 or 277 could be generated from the corresponding methallyl- or prenyl-esters.…”

“…27 The Herzon lab has also shown that the alkene-selectivity of a transition metal HAT pathway can be complementary to classical hydrogenation techniques. 354 It is worth noting that identification of the general problems solved by both groups arose in the context of natural products synthesis. We will discuss both contributions in turn.…”

“…395,424 This suggests that steric encumberance (larger phosphine bite angles) on the metal hydride significantly affects rate of HAT to alkenes. 24,354 …”

“…Alkynes and 1,2-disubstituted olefins reacted more slowly whereas trisubstituted alkenes reacted similarly. 354 …”