Reduction of .alpha.-halo ketones by organotin hydrides. An electron-transfer-hydrogen atom abstraction mechanism

Domino Radical Addition/Oxidation Sequence with Photocatalysis: One‐Pot Synthesis of Polysubstituted Furans from α‐Chloro‐Alkyl Ketones and Styrenes

“…Based on the above experimental results and previous reports, [9,22] ar adicala ddition/oxidation pathway is proposed in Scheme 2. AS ET from the excited photocatalyst to 1a generates an electron-deficient a-carbonyl carbon radical A along with Ir IV (ppy) 3 .I ntermolecular p-addition of the radical A to 2a produces benzylic radical B.AfurtherS ET from radical B (E ox 1=2 = 0.73 V, vs. SCE) [20] to Ir IV (ppy) 3 (E red 1=2 = 0.77 V, vs. SCE) [23] gives carbocation C and regenerates the photocatalyst.S ubsequently, cyclization and deprotonation of C facilitated by the base results in dihydrofuran derivative E.F inally,o xidationo fi ntermediate E by the excited photocatalyst or K 2 S 2 O 8 provides the desired product 3a.…”

Visible Light Photoredox Catalysis with Transition Metal Complexes: Applications in Organic Synthesis

“…The reaction is initiated by single-electron transfer from 9,10-dihydro-10-methylacridine ( 11 , E 1/2 red = +0.8 V vs SCE) 30 to *Ru(bpy) 3 2+ to give the radical cation 13 and the reduced species Ru(bpy) 3 + . The Ru(I) intermediate may then reduce phenacyl bromide ( E 1/2 red = –0.49 V vs SCE), 31 returning the catalyst to its Ru(II) oxidation state. Upon undergoing single-electron reduction, α -bromocarbonyls such as 9 are known to undergo mesolysis, the fragmentation of a radical anion to afford an anion (bromide) and a radical (the α -carbonyl radical 14 ).…”

Visible‐Light Photocatalysis as an Enabling Tool for the Functionalization of Unactivated C(sp3)‐Substrates

“…In 2012, Stephenson and co-workers disclosed an elegant method for the reduction of alkyl C-I bonds aided by visible-light photocatalysis (Scheme 15). [51] Mechanistically, the excited-state fac-Ir(ppy) 3 photocatalyst (29) was believed to in- With use of another Ir photocatalyst -Ir(ppy) 2 (dtbbpy)PF 6 (32) -and DIPEA (20) as sacrificial electron donor, a set of alkyl iodides could undergo the same reductive transformation with an emphasis on the intramolecular cyclization/hydrogen abstraction cascade process (Scheme 16). [52] Through a combination of Garegg-Samuelsson iodination in batch followed by visible-light-mediated reductive deiodination in flow, aliphatic alcohols 33 could be deoxygenated effectively in a useful telescoped reaction (Scheme 17).…”

“…The valorization of these valuable substrates represents a substantially more complex synthetic challenge, due in part to more negative reduction potentials required for efficient radical formation (cf. EtI: E 1/2 red = -1.67 V vs. SCE; [18] BnI: E 1/2 red = -1.31 V vs. SCE; [19] α-bromoacetophenone: E 1/2 red = -0.78 V vs. SCE [20] ) and competing side reactions enabled by the presence of -hydrogen atoms. The discovery of new and selective reactions to harness these substrates efficiently will undoubtedly be of enormous importance for the continued development not only of photocatalysis but also of organic chemistry in general.…”