Ligand-tetrahydrofuran coupling in chelated aluminum phosphinates

When the reagents LAlMe (L = N,N'-(alkylene or arylene) bis(3,5-di-tert-butyl)salicylidenelmine (alkylene = ethylene (Salen(tBu)) (1), propylene (Salpen(tBu)) (2), and butylene (Salben tBu)) (3); arylene = phenylene (Salophen(tBu) (4), 3,4-dimethylphenylene (Salomphen(tBu) (5)) are combined with Ph(H)P(O)OH in tetrahydrofuran (thf) the unique aluminophosphinate compounds, [L(tBu)Al{O2P(H)Ph}](n) with L, n = Salen, infinity (6), Salpen, 2 (7), Salben, 2 (8),. Salophen, infinity (9) and Salomphen, infinity (10) are produced. The yields for the latter two reactions are low, and it was subsequently found that the unique thf-coupled compounds appear in the thf filtrates of the original reaction mixture. These compounds are, [L-thf(tBu)Al{O2P(H)Ph}](2), L = Salophen (13) and Salomphen (14). The thf connects through an a-carbon to only one of the two possible imine carbons of the ligand. While trying to determine how this coupling proceeds,,the six-coordinate, solution-state species LAlMe(thf) (L = Salophen (11) and Salomphen (12) were discovered and implicated as intermediates. All of the compounds are characterized by melting point, NMR, IR, and-X-ray analyses for 5-8, 13, and 14, A possible mechanism for the thf coupling event is presented.