Removal of benzotriazole moiety from 2-[2-aryl-2-(benzotriazol-1- yl)ethyl]tetrahydro-2H-pyrans and 2-[2-aryl-2-(benzotriazol-1-yl)ethyl]-5- (methyl)tetrahydrofurans using lithium naphthalene radical anion

Article abstract of DOI:10.1016/S0040-4020(99)00120-9

In order to see the effects of non-bonding electrons in oxygen atoms on the cleavage of a bond between N-1 of the benzotriazole moiety and the a- carbon atom bonded to N-1 by lithium (6a) and sodium naphthalenides (6b), 2- [2-aryl-2-(benzotriazol-1-yl)ethyl]tetrahydro-2H-pyrans (4), and 2-[2-aryl- 2-(benzotriazol-1-yl)ethyl]-5-(methyl)tetrahydrofurans (5), and 1- (benzotriazol-1-yl)-1,2-diphenylethane (7) were prepared. The reactions of 4 with 6a in THF at room temperature gave 2-(2-arylethyl)tetrahydro-2H-pyrans (12) in 45 to 62 % yields along with benzotriazole and naphthalene. In addition, 2-(benzoylethyl)tetrahydro-2H-pyran (15) (12 %) was obtained only from the reaction of 2-[2(benzotriazol-1-yl)-2-(phenyl)ethyl]tetrahydro-2H- pyran. Similarly, the reactions of 5 with 6a under the same conditions afforded 2-(2-arylethyl)-5-(methyl)tetrahydrofurans (30) in 59 to 77 % yields along with the foregoing byproducts. Interestingly, the reaction of 7 with 6a under the same conditions gave deoxybenzoin (24) in 41% yield along with the foregoing byproducts. The results suggest that 6a acts as a single electron- transfer agent in the reactions of 4 and 5, and a base in the reaction ofT. It is envisaged that for the former, Li⁺ participates in the formation of a six-membered cyclic intermediate so that cleavage of a-C-N-1 bond is facilitated to give eventually 12 and 22, whereas for the latter, 6a abstracts a proton from a-C bonded to N-1 to generate a carbanion 19, which extrudes a nitrogen molecule to generate a new phenyl carbanion 20. Protonation leading to mono iminobenzoin 21, followed by hydrolysis gives 15. The formation of 24 can be explained based on the same mechanism as for the latter reaction.