MECHANISTIC ASPECTS OF MOLYBDENUM-PROMOTED ALLYLIC AMINATION

The mechanism of molybdenum-mediated allylic amination by phenylhydrox ylamine has been probed through a variety of kinetics, trapping, and s toichiometric model reaction studies. Specifically, the amination of 2 -methyl-2-hexene by (dipic)(HMPA)Mo(eta(2)-PhNO) (1a, dipic = 2,6-pyri dinedicarboxylate; HMPA = hexamethylphosphoric triamide) is found to b e first order in la and zeroth order in olefin and HMPA. Evidence for dissociation of nitrosobenzene from la is provided by trapping of the latter as a hetero-Diels-Alder adduct with 2,3-dimethylbutadiene and b y exchange experiments of la with free aryl nitroso compounds. A compe ting pathway involving extrusion of aryl nitrene from la is also impli cated by the production of carbazole from the thermolysis of (dipic)(H MPA)Mo(eta(2)-2-C6H5-C6H4NO) (5) The findings that (1) the ene reactio n of nitrosobenzene with 2-methyl-2-hexene occurs readily (less than o r equal to 70 degrees C) and regioselectively to produce allyl hydroxy lamine 7 and (2) that Mo(IV) complexes (dedtc)(2)MoO (3b, dedtc = N,N- diethyldithiocarbamate) and (dipic)(HMPA)MoO (3a) readily deoxygenate arylhydroxylamines (including 7) support the involvement of these step s in the amination process. Control experiments and model reaction stu dies have identified some of the pathways for the formation of the byp roducts, aniline and azoxybenzene. Together the above results indicate that the primary pathway for Mo-promoted olefin allylic amination inv olves: (1) reaction of LL'Mo(VI)O-2 with RNHOH to form a molybdooxazir idine 1 (and water); (2) dissociation of 1 to form RNO and LL'Mo(IV)O (3);(3) ene-reaction of RNO with the olefin to produce an N-allyl hydr oxylamine; and (4) reduction of the allyl hydroxylamine by 3, yielding the allyl amine and regenerating LL'Mo(VI)O-2 (2).