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Relevant academic research and scientific papers

Phenylene-bridged bis(benzimidazolium) (BBIm2+): a dicationic organic photoredox catalyst

A dicationic photoredox catalyst composed of phenylene-bridged bis(benzimidazolium) (BBIm2+) was designed, synthesised and demonstrated to promote the photochemical decarboxylative hydroxylation and dimerisation of carboxylic acids. The catalytic activity of BBIm2+ was higher than that for a monocation analogue, suggesting that the dicationic nature of BBIm2+ plays a key role in these decarboxylative reactions. The rate constant for the decay of the triplet-triplet absorption of the excited BBIm2+ increased with increasing concentration of the carboxylate anion with a saturated dependence, suggesting that photoinduced electron transfer occurs within the ion pair complex composed of the triplet excited state of BBIm2+ and a carboxylate anion.

Mechanistic investigation of a novel vitamin B12-catalyzed carbon-carbon bond forming reaction, the reductive dimerization of arylalkenes

In the presence of catalytic vitamin B12 and a reducing agent such as Ti(III)citrate or Zn, arylalkenes are dimerized with unusual regioselectivity forming a carbon-carbon bond between the benzylic carbons of each coupling partner. Dimerization products were obtained in good to excellent yields for mono- and 1,1-disubstituted alkenes. Dienes containing one aryl alkene underwent intramolecular cyclization in good yields. However, 1,2-disubstituted and trisubstituted alkenes were unreactive. Mechanistic investigations using radical traps suggest the involvement of benzylic radicals, and the lack of diastereoselectivity in the product distribution is consistent with dimerization of two such reactive intermediates. A strong reducing agent is required for the reaction and fulfills two roles. It returns the Co(II) form of the catalyst generated after the reaction to the active Co(I) state, and by removing Co(II) it also prevents the nonproductive recombination of alkyl radicals with cob(II)alamin. The mechanism of the formation of benzylic radicals from arylalkenes and cob(I)alamin poses an interesting problem. The results with a one-electron transfer probe indicate that radical generation is not likely to involve an electron transfer. Several alternative mechanisms are discussed.

Organomolybdenum and Organotungsten Reagents, V. - On the Additive, Reductive Carbonyl Dimerization (ARCD Reaction)

Reagents of the type R4(PrO)4(μ-PrO)2W2 (3; R = Me, Et, Pr, Bu, sBu, iBu, Hex, Me3SiCH2, Ph) prepared in situ by the action of organolithium or Grignard compounds on (PrO)4(μ-PrO)2W2Cl4 (2), react with aromatic aldehydes or ketones and with the α,β-unsaturated ketone benzylidene acetone in a novel reaction, called the ARCD reaction, to give products 4 of the type RR'R''C-CRR'R'' with mostly good yields.In the case of benzylidene acetone and furfural besides the ARCD products the rearranged products 27 and 29 are formed.With the reagent Ph4(PrO)4(μ-PrO)2W2 (3b) ARCD reactions are also possible in moderate yields with aliphatic aldehydes and ketones.The more closely investigated reagent Me4(PrO)4(μ-PrO)2W2 (3a; decomposition at about -45 deg C) tolerates the aromatic bound functional groups Cl, F, OH, OMe, and NMe2 in the substrates, but not the NO2 and CO2Et substituents.It reacts with PhCOX (X = OEt, Cl, OCOPh) via acetophenone to give PhCMe2-CMe2Ph. - For the ARCD reactions a radical mechanism (Scheme 7) is postulated. - Key Words: Tungsten complexes/ Carbonyl dimerization