Free-Radical Chemistry of Lactones:  Fragmentation of β-Lactones. The Beneficial Effect of Catalytic Benzeneselenol on Chain Propagation

Polarity-reversal catalysis of hydrogen-atom abstraction reactions: concepts and applications in organic chemistry

“…32 However, polar effects favour abstraction of hydrogen by PhSe • from the tin hydride and the catalytic cycle involved is shown in eqns. (30) and (31). It was pointed…”

“…Crich and his co-workers have reported the use of benzeneselenol as a polarity-reversal catalyst for the abstraction of hydrogen from tin hydrides by carbon-centred radicals. [29][30][31] The electronegativity of selenium is only marginally less than that of sulfur and PhSe • is expected to exhibit electrophilic properties, like PhS • . The Se-H bond is much weaker than the S-H bond and both enthalpic and polar factors favour abstraction of hydrogen from the selenol by a nucleophilic alkyl radical, a process which is extremely rapid at room temperature and significantly faster than the direct abstraction of hydrogen from the tin hydride.…”

“…29 PRC by benzeneselenol of the overall transfer of hydrogen from tributyltin hydride to relatively unreactive radicals, such as allylic and cyclohexadienyl radicals, has proved useful in increasing the efficiency of chain reactions involving these species. 31 The much greater rate at which a nucleophilic carboncentred radical R • abstracts hydrogen from PhSeH, as compared with Bu 3 SnH, has also been exploited in experiments designed to measure the rate of rearrangement of R • using the so-called radical-clock method. 30 The 'clock reaction' is the trapping of the alkyl radical R • by a constant catalytic quantity of the selenol under pseudo-first-order conditions, 32 obviating the need to work with a large excess of the tin hydride in order to achieve simple reaction kinetics.…”

Organophotoredox/Ni-Cocatalyzed Allylation of Allenes: Regio-, and Diastereoselective Access to Homoallylic Alcohols

“…On the other hand, the light on/off experiment of the reaction was conducted, which suggested that a radical-chain propagation mechanism was not involved in the reaction (Scheme 3d), further supported by the quantum yield 0.39%. [15] To gain insight into interesting diastereoselectivity trends observed with aryl aldehydes, a Hammett plot of its para-substituent was constructed (Scheme 4e). [16] Satisfactory correlations between dr and the electronic effects of the substituents were observed, which was similar to a recently reported allylation reaction catalyzed by cobalt [7d] .…”

Synthesis and Applications of Organic Selenols

“…Densely functionalised organoselenium compounds, bearing labile groups and characterised by high molecular complexity, can be prepared exploiting the reactivity of selenols. Selenols are also employed as catalysts for the synthesis of peptide thioesters or peptide ligation from bis(2‐sulfanylethyl)amido (SEA) peptides [12–13] Catalytic amounts of selenols were demonstrated to have a beneficial effect on the chain propagation step of the radical fragmentation of β‐lactones, thus enabling to improve the efficiency of the process [14] …”