Catalyst-free room-temperature iClick reaction of molybdenum(II) and tungsten(II) azide complexes with electron-poor alkynes: structural preferences and kinetic studies

摘要

Two isostructural and isoelectronic group VI azide complexes of the general formula [M(eta(3)-allyl)(N-3)(bpy) (CO)(2)] with M = Mo, W and bpy = 2,2'-bipyridine were prepared and fully characterized, including X-ray structure analysis. Both reacted smoothly with electron-poor alkynes such as dimethyl acetylenedicarboxylate (DMAD) and 4,4,4-trifluoro-2-butynoic acid ethyl ester in a catalyst-free room-temperature iClick [3 + 2] cycloaddition reaction. Reaction with phenyl(trifluoromethyl) acetylene, on the other hand, did not lead to any product formation. X-ray structures of the four triazolate complexes isolated showed the monodentate ligand to be N2-coordinated in all cases, which requires a 1,2-shift of the nitrogen from the terminal azide to the triazolate cycloaddition product. On the other hand, a F-19 NMR spectroscopic study of the reaction of the fluorinated alkyne with the tungsten azide complex at 27 degrees C allowed detection of the N1-coordinated intermediate. With this method, the second-order rate constant was determined as (7.3 +/- 0.1) x 10(-2) M-1 s(-1), which compares favorably with that of first-generation compounds such as difluorocyclooctyne (DIFO) used in the strain-promoted azide-alkyne cycloaddition (SPAAC). In contrast, the reaction of the molybdenum analogue was too fast to be studied with NMR methods. Alternatively, solution IR studies revealed pseudo-first order rate constants of 0.4 to 6.5 x 10(-3) s(-1), which increased in the order of Mo > Wand F3C-Cu=C-COOEt > DMAD.