Novel group 13 reagents for the activation of small molecules

This thesis examines the activation of small molecules by aluminyl compounds, as well as by aluminium imides, derived from them, and related gallium compounds.
Chapter 3 details the reactions of nucleophilic aluminyl and gallyl reagents, K2[NONM]2 (M = Al, Ga; NON = 4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethyl-xanthene) with a range of organoazides. Depending on steric bulk, reaction with one or two equivalents per metal centre is observed, affording terminal metal imides or metallo tetrazenes respectively. These imides are shown to be highly reactive species, as exemplified by their reactions with H2, solvent molecules (such as toluene or THF) or intramolecular C(sp3)–H bonds.
Chapter 4 describes the reactivity of the aluminium imide K2[NONAl(NDipp)]2 (Dipp = 2,6- iPr2C6H3) towards a range of unsaturated oxygen-containing substrates. The reaction of this imide with CO2 yields DippNCO and the carbonate complex K2[NONAl(CO3)]2 under dilute conditions. The reaction with N2O yields DippN3 and the cis-hyponitrite complex K2[NONAl(ONNO)]2, while the reaction with benzaldehyde leads to the formation of the hemiaminal complex K[NONAlN(Dipp)OCHPh]; subsequent reaction with CO2 then leads to the liberation of the imine PhCHNDipp and K2[NONAl(CO3)]2. Thus, it is demonstrated that K2[NONAl(NDipp)]2 can act as a transfer reagent for the [NDipp]2– fragment. Experimental and computational investigations reveal that these reactions likely proceed via a series of cycloaddition and cycloreversion reactions, and that depending on conditions alternative products may be obtained.
Chapter 5 details the reactions of a range of group 13 imides with CO, as well as the reactions of boryl- and silyl- substituted group 13 imides with N2O. The reaction of K2[NONAl(NDipp)]2 with CO leads to the uptake of two molecules of CO and affords K2[NONAl{C(=NDipp)CO2}]2, a product that is derived via a combination of C–O bond cleavage and C–C bond formation steps. The analogous reactions with silyl- or boryliv substituted imides instead lead to the formation of heterocarboxylate-cyanide complexes, K2[NONM(O2CR)(CN)]2, or siloxy-cyanide complexes, K2[NONM(OSiR3)(CN)]2, depending on the nature of the group 13 metal and the imido substituent. Heteroatom substituted aluminium imides react readily with N2O affording the corresponding organoazides and the aluminium hyponitrite complex K2[NONAl(ONNO)]2. K[NONGa(NSiPh3)] however reacts only slowly with N2O at elevated temperatures, ultimately leading to the formation of siloxyazide complex K[NONGa(OSiPh3)(N3)]. Thus, the heteroatom substituted group 13 imides act either as a source of a [NR]2–, as in the conversion of N2O to RN3, or as a source of the nitride ion, [N]3–, facilitating the conversion of CO to [CN]–.
Chapter 6 describes the reactivity of aluminyl reagents towards CO. The four-electron reduction of CO affords compounds featuring topologically linear [C4O4]4– chains, which can undergo selective chain growth to afford a branched [C6O6]4– chain, thus demonstrating solution state chain branching via C–O cleavage for the first time. Thermal isomerisation of the [C4O4]4– fragment can be observed at elevated temperatures in the presence of additional potassium ions. Under more reducing conditions a compound featuring a linear [C4O4]6– fragment can be obtained, while a salt metathesis reaction allows access to a compound featuring a cyclic [C4O4]2– unit. The mechanism for CO homologation proceeds via initial coordination of CO, followed by C–C coupling and carbene dimerisation. In the initial coordination complexes, CO primarily acts as a Z type ligand, reflecting the highly electron-rich nature of the aluminyl reagents.

Keywords: imine
group 13
small molecule activation
aluminium
aluminyl
carbon monoxide
low valent
homologation
Advisor: Aldridge, S