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Issue 38, 2014
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Donor-activated alkali metal dipyridylamides: co-complexation reactions with zinc alkyls and reactivity studies with benzophenone

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Abstract

Previously it was reported that activation of tBu2Zn by [(TMEDA)Na(μ-dpa)]2 led to tert-butylation of benzophenone at the challenging para-position, where the sodium amide functions as a metalloligand towards tBu2Zn manifested in crystalline [{(TMEDA)Na(dpa)}2ZntBu2] (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 [(H6-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 H6-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 tBu2Zn. 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, tBu2Zn–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 tBu2Zn with (Me3SiCH2)2Zn shut down the alkylation of benzophenone; in contrast, tBuLi 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.

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

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Publication details

The article was received on 27 Mar 2014, accepted on 18 Jun 2014 and first published on 18 Jun 2014


Article type: Paper
DOI: 10.1039/C4DT00914B
Dalton Trans., 2014,43, 14409-14423

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

    D. R. Armstrong, E. V. Brouillet, A. R. Kennedy, J. A. Garden, M. Granitzka, R. E. Mulvey and J. J. Trivett, Dalton Trans., 2014, 43, 14409
    DOI: 10.1039/C4DT00914B

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