Displacement reactions of trimethylamine-haloalanes, preparation and characterization of dihydroaluminum(1+) salts; Kinetics and mechanism of acid catalyzed hydrolysis of trimethylamine-azidoborane

Abstract

Iodide salts of two novel four-coordinate aluminum cations N,N,N',N'-tetramethylethylenediaminedihydroaluminum(1+) (I) and sparteinedihydroaluminum(1+) (II) were prepared by nucleophilic displacement reactions on trimethylamine-iodoalane and characterized by infrared absorption and conductivity measurements. Compounds I and II exhibited strong absorptions attributed to Al-H stretching at 1890 and 1888 cm^-1 respectively. Trimethylamine-bromoiodoalane was prepared along with N,N,N',N'-tetramethylethylenediamine complexes of AlH2Br (III) and AlHBr2 (IV). The product containing III appears to be the bromide salt of the chelated aluminum(1+) ion while IV appears to yield a molecular complex containing five-coordinate aluminum. The chelated dihydroaluminum(1+) salts are reactive compounds which apparently reduce protic solvents, carbonyls and other functional groups. Trimethylamine-monoazidoborane (V) was prepared in 33% yield by the pyrolysis of bis(trimethylamine)dihydroboron(1+) azide (VI), and characterized by infrared, 11B-nmr and mass spectroscopy. A simplified preparation of VI by direct ion exchange is presented. The pyrolysis of VI also yields trimethylamine adducts of borane (BH3), triazidoborane, and possibly diazidoborane. Trimethylamine-monoazidoborane (V) is hydrolyzed by an acid catalyzed pathway in several mineral acids. The reaction is first-order in [V] and the rate correlates with the Hammett acidity function in 1 - 6 M H2SO4. In this range 0 < H_0 < -2.75, and the rate at 25º is given by the following equation: -d ln [V]/dt = (6.3 x 10^-7 M^-1 sec^-1)h_0. In 8 - 10 M H2SO4, the rate becomes acid independent but correlates with water activity (a_w, molar concentration). In this region, the rate is given by: -d ln [V]/dt = (5.0 x 10^-5 M^-1 sec^-1)a_w. Values of the Hammett acidity function (D_0) for deuterium chloride in deuterium oxide were determined to be equal to previously reported values for H_0 in the range 1 - 4 M HCl. Solvent isotope effects in heavy water were examined at several acid concentrations. The ratio k_H2O/k_D2O =~ 1 was determined at acid concentrations where acid dependent hydrolysis was observed while k_H2O/k_D2O = 2.2 at higher acidities where the rate law is dependent on water activity. No exchange of boron bound hydrogen with solvent was detected. Entropies of activation were determined in both regions of acidity. The adequacies of three common models of acid catalyzed hydrolysis (A-1, A-2, and A-S_E2) are considered. It is suggested that the bimolecular, A-2 scheme is the best representation for this hydrolysis reaction. Assuming the A-2 mechanism, the conjugate acid of V is formed in a rapid pre-equilibrium step which shows a rather small inverse solvent isotope effect. The conjugate acid is then attacked by a water molecule in a bimolecular rate limiting step. This slow step exhibits a normal solvent isotope effect of approximately the same magnitude as the pre-protonation step. A negative entropy of activiation (Delta S ‡ = -20 e.u.) is consistent with such a bimolecular reaction. The possibilities of changes in mechanism with changing acidity are discussed.