Deprotonation and reductive addition reactions of hypervalent aluminium dihydride compounds containing substituted pyrrolyl ligands with phenols, ketones, and aldehydes

Abstract

The reactivities of [C₄H₂N(CH₂NMe₂)₂]AlH₂ (1) with primary and secondary amines, phenols, ketones, and phenyl isothiocyanate were examined. Reactions of 1 with one or two equivalents of 2,6-dichloroaniline in methylene chloride generated [C₄H₂N(CH₂NMe₂)₂]AlH(NHC₆H₃-2,6-Cl₂) (2) and [C₄H₂N(CH₂NMe₂)₂]Al(NHC₆H₃-2,6-Cl₂)₂ (3), respectively, following hydrogen elimination. Similarly, the reactions of 1 with one or two equivalents of carbazole afforded [C₄H₂N(CH₂NMe₂)₂]AlH(NC₁₂H₈) (4) or [C₄H₂N(CH₂NMe₂)₂]Al(NC₁₂H₈)₂ (5) by deprotonating the acidic N–H of carbazole. Reacting 1 with one equivalent of 2,6-diisopropylphenol in diethyl ether formed an aluminium phenoxo compound [C₄H₂N(CH₂NMe₂)₂]AlH(OC₆H₃-2,6-ⁱPr₂) (6), by deprotonation of phenol as well with the elimination of one equivalent hydrogen. Further reaction of 6 with one equivalent of 2,4,6-trimethylacetophenone in methylene chloride generated [C₄H₂N(CH₂NMe₂)₂]Al(OC₆H₃-2,6-ⁱPr₂)[OC(CH₂)(C₆H₂-2,4,6-Me₃)] (7) by deprotonating the methyl proton of the acetophenone. Similar deprotonation occurred when 1 reacted with two equivalents of 2,4,6-trimethylacetophenone in methylene chloride to generate [C₄H₂N(CH₂NMe₂)₂]Al[OC(CH₂)(C₆H₂-2,4,6-Me₃)]₂ (8). Compounds [C₄H₂N(CH₂NMe₂)₂]Al(OCHPh₂)₂ (9), and [C₄H₂N(CH₂NMe₂)₂]Al(SCHNPh)₂ (10) could also be obtained by reacting 1 with two equivalents of benzophenone and phenyl isothiocyanate, respectively through hydroalumination. The ¹H NMR spectra of 10 showed broad signals for the CH₂N and NMe₂ groups, which represent dynamical fluctuations of the molecules in solution state. The estimated energy barrier (ΔG_c^‡) from the coalescence temperature for the fluctuation was estimated at 17.1 Kcal mol⁻¹. The solid-state structures of compounds 2, 3, 5, 7, 9, and 10 have been determined.