Reactivity of Aluminum Carbenoid with Unsaturated Substrates

Abstract

Reactivity patterns of the β-diketiminate aluminum(I) complex NacNacAl towards a variety of unsaturated molecules were determined. Reaction of NacNacAl with one equivalent of benzophenone affords η2(C,O) adduct III-2 that undergoes cyclization reactions with benzophenone (III-3), aldimine (III-4), quinoline (III-5), pyridine (III-6), phenyl nitrile (III-7), trimethylsilyl azide (III-8), and a saturated cyclic thiourea (III-9). The latter reacted via unusual C−N cleavage. Analogous η2-coordination products were prepared with p-tolyl benzoate (IV-6), N,N-dimethylbenzamide (IV-9) and (1‐phenylethylidene)aniline (IV-13). Addition of pyridine to such species results in [2+2] cycloaddition products analogous to III-6, except for the case of p‐tolyl benzoate when a migration of the alkoxy group from the ester moiety accompanied by hydrogen transfer from pyridine preserves the aromaticity within the latter. Chemoselective couplings between aliphatic ketones and pyridine were exemplified by reactions with non-enolizable (1R)-(‒) fenchone and enolizable yet sterically encumbered isophorone. The reaction with the CH‐acidic ketone (1R) (+) camphor afforded a hydrido alkoxide (IV-11) as the result of enolization. Whereas the reaction of NacNacAl with (1R)‐(−)‐fenchone in the absence of pyridine led to CH activation in the isopropyl group of the NacNac ligand. NacNacAl demonstrated diverse reactivity in reactions with N‐heterocycles. 4 Dimethylaminopyridine induces rearrangement of NacNacAl by deprotonation of backbone methyl group of the ligand. C−H activation of the methyl group of 4‐picoline produced a species with a reactive terminal methylene. Reaction of NacNacAl with 3,5 lutidine led to the cleavage of the sp2 C−H bond (4‐position). Another reactivity mode was observed for quinoline, which undergoes 2,2′‐coupling. Finally, a reaction of NacNacAl with phthalazine produced a product of the N−N bond cleavage. NacNacAl reacted with a series of polycyclic aromatic hydrocarbons via [4+1] cycloaddition. While a reaction with anthracene was irreversible, with the formation of products of activation of the lateral and central rings, reactions with phenanthrene, triphenylene, and fluoranthene were reversible. Heating reaction mixtures at 90 °C yielded dialuminum hydride VI-6. Mechanistic studies showed that the reaction proceeds via dissociation of polycycles with the release of NacNacAl that undergoes further intermolecular transformations. All novel complexes were characterized by spectroscopic methods and X-ray diffraction analysis for most of them.