Chloroform, diradicals from

In a recently reported synthesis of pyridines, lithiated methoxyallenes react with nitriles in the presence of trifluoroacetic acid (Scheme 107) <2004CEJ4283>. The mechanism is postulated to proceed via initial protonation followed by nucleophilic addition of the trifluoroacetate ion with subsequent intramolecular acyl transfer and aldol condensation to give the pyridine. An additional pyridine formation starting from azaenyne allenes forms a-5-didehydro-3-picoline diradicals, which can be trapped by 1,4-cyclohexadiene, chloroform, and methanol to produce various pyridines <20040L2059>. 

The photochemical Diels-Alder reactions of anthracene with fumarodinitrile and 1,4-benzoquinone have been studied in chloroform solution. Not surprisingly, the addition occurs in competition with dimerization of the arene and proceeds by way of electron transfer from anthracene to the dienophiles. The radical ion pair has been detected by transient absorption spectroscopy, and the resulting diradical precursor of adduct formation from the quinone was observed by ESR at 77 K. 2,7-Dibromotropone is reported to undergo (871+471) photoaddition to 9,10-dicyanoanthracene in benzene-methanol (9 1), giving (25) as the primary adduct which is then proposed to react with methanol and water (solvent contaminant) to yield the final product (26). In contrast, 2-bromotropone and the anthracene in CH2CI2 solution afford the substitution products (27) (62%) and (28) (25%). 

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