Donor-activated alkali metal dipyridylamides: co-complexation reactions with zinc alkyls and reactivity studies with benzophenone

Previously it was reported that activation of (Bu2Zn)-Bu-t by [(TMEDA) Na(mu-dpa)](2) led to tert-butylation of benzophenone at the challenging para-position, where the sodium amide functions as a metalloligand towards (Bu2Zn)-Bu-t manifested in crystalline [{(TMEDA) Na(dpa)}(2)(ZnBu2)-Bu-t] (TMEDA is N, N, N', N'-tetramethylethylenediamine, dpa is 2,2'-dipyridylamide). Here we find altering the Lewis donor or alkali metal within the metalloligand dictates the reaction outcome, exhibiting a strong influence on alkylation yields and reaction selectivity. Varying the former led to the synthesis of three novel complexes, [(PMDETA)Na(dpa)](2), [(TMDAE)Na(dpa)](2), and [(H-6-TREN)Na(dpa)], characterised through combined structural, spectroscopic and theoretical studies [where PMDETA is N, N, N', N'', N''-pentamethyldiethylenetriamine, TMDAE is N, N, N', N'-tetramethyldiaminoethylether and H-6-TREN is N', N'-bis(2-aminoethyl) ethane-1,2-diamine]. Each new sodium amide can function as a metalloligand to generate a co-complex with (Bu2Zn)-Bu-t. Reacting these new co-complexes with benzophenone proved solvent dependent with yields in THF much lower than those in hexane. Most interestingly, sub-stoichiometric amounts of the metalloligands [(TMEDA) Na(dpa)](2) and [(PMEDTA) Na(dpa)](2) with 1 : 1, (Bu2Zn)-Bu-t-benzophenone mixtures produced good yields of the challenging 1,6-tert-butyl addition product in hexane (52% and 53% respectively). Although exchanging Na for Li gave similar reaction yields, the regioselectivity was significantly compromised; whereas the K system was completely unreactive. Replacing (Bu2Zn)-Bu-t with (Me3SiCH2)(2)Zn shut down the alkylation of benzophenone; in contrast, (BuLi)-Bu-t generates only the reduction product, benzhydrol. Zincation of the parent amine dpa(H) generated the crystalline product [Zn(dpa)(2)], as structurally elucidated through X-ray crystallography and theoretical calculations. Although the reaction mechanism for the alkylation of benzophenone remains unclear, incorporation of the radical scavenger TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl radical) into the reaction system completely inhibits benzophenone alkylation.