Alkali-metal- and alkaline-earth-metal-mediated C-O activation of an anisole-substituted phosphido-borane ligand

Abstract

The reaction between {(Me3Si)2CH}PH(C6H4-2-OMe) (4) and 1 equiv of BH3·SMe2 yields the phosphine-borane {(Me3Si)2CH}PH(BH3)(C6H4-2-OMe) (5). Subsequent reaction between 5 and 1 equiv of n-BuLi in THF gives the phosphido-borane complex [{(Me3Si)2CH}P(BH3)(C6H4-2-OMe)]Li(THF) (6a), which was isolated as a colorless microcrystalline solid. Treatment of 5 with 1 equiv of PhCH2M yields the corresponding complexes [{(Me3Si)2CH}P(BH3)(C6H4-2-OMe)]ML (ML = Na(THF) (6b), K(pmdeta) (6c); pmdeta = N,N,N′,N″,N″-pentamethyldiethylenetriamine), after crystallization in the presence of the corresponding coligand. While compounds 6b,c are stable toward heat, compound 6a decomposes on heating to 50 °C in toluene to give the cluster [[{(Me3Si)2CH}PH(C6H4-2-O)]Li]6 (7) and the tertiary phosphine-borane {(Me3Si)2CH}P(BH3)(Me)(C6H4-2-OMe) (8). Related C-O cleavage reactions are observed when MgI2 is treated with 2 equiv of 6a and when CaI2 is treated with 2 equiv of [{(Me3Si)2CH}P(BH3)(C6H4-2-OMe)]K in THF, giving [{(Me3Si)2CH}P(BH3)(C6H4-2-O)Mg(THF)2]2 (9) and [{(Me3Si)2CH}P(BH3)(C6H4-2-O)Ca(THF)]4 (10), respectively, along with 1 equiv of 8 in each case. In contrast, the reaction between SrI2 and 2 equiv of [{(Me3Si)2CH}P(BH3)(C6H4-2-OMe)]K in THF yields [{(Me3Si)2CH}P(BH3)(C6H4-2-OMe)Sr(THF)4] (11).