Seven-Membered Cyclic Diamidoalumanyls of Heavier Alkali Metals: Structures and C-H Activation of Arenes

Abstract

Like the previously reported potassium-based system, rubidium and cesium reduction of [{SiN^Dipp}AlI] ({SiN^Dipp} = {CH₂SiMe₂NDipp}₂) with the heavier alkali metals [M = Rb and Cs] provides dimeric group 1 alumanyl derivatives, [{SiN^Dipp}AlM]₂. In contrast, similar treatment with sodium results in over-reduction and incorporation of a formal equivalent of [{SiN^Dipp}Na₂] into the resultant sodium alumanyl species. The dimeric K, Rb, and Cs compounds display a variable efficacy toward the C-H oxidative addition of arene C-H bonds at elevated temperatures (Cs > Rb > K, 110 °C) to yield (hydrido)(organo)aluminate species. Consistent with the synthetic experimental observations, computational (DFT) assessment of the benzene C-H activation indicates that rate-determining attack of the Al(I) nucleophile within the dimeric species is facilitated by π-engagement of the arene with the electrophilic M⁺ cation, which becomes increasingly favorable as group 1 is descended.

Figure 1

Exemplary diamidoalumanyl derivatives: compounds 1–12.

Figure 2

Displacement ellipsoid (30% probability) plot of the structure of compound 14. For clarity, solvent has been omitted, and hydrogen atoms have been removed. Dipp iso-propyl substituents are also presented as wireframes for visual ease. Symmetry operation: ¹1 – x, 1 – y, and 1 – z.

Scheme 1. Synthesis of Compounds 16 and 17

Figure 3

Displacement ellipsoid (30% probability) plots of the structures of (a) compound 16 and (b) compound 17. For clarity, the solvent has been omitted, and with the exception of methyl groups that display close C–H···M contacts, hydrogen atoms are not shown. Most Dipp iso-propyl substituents are presented as wireframes, also for visual ease.

Scheme 2. Synthesis of Compounds 18–21

Figure 4

Displacement ellipsoid plots (30% probability) of (a) the monomeric unit and (b) the polymeric structure of compound 18. Symmetry operations: ¹3/2 – x, – 1/2 + y, 1/2 – z; ² 3/2 – x, 1/2 + y, and 1/2 – z. For clarity, in both plots, hydrogen atoms have been removed with the exception of those which are aluminum-bound. Dipp iso-propyl substituents are represented as wireframes in (a), and disorder has been omitted throughout. In (b), entire Dipp substituents are denoted in wireframe mode, also for visual ease.

Figure 5

Displacement ellipsoid plots (30% probability) of the asymmetric units of (a) 19 and (b) 20. (c) Polymeric structure of compound 21. Symmetry operations (21): ¹x, 1 + y, z;²x, – 1 + y, z. For clarity, hydrogen atoms have been removed with the exception of those which are aluminum-bound or involved in C–H···M interactions. Most Dipp iso-propyl substituents are presented as wireframes, and disorder has also been omitted from 19 for visual ease.

Figure 6

Computed free energy profile (BP86-D3BJ(PCM = C₆H₆)/BS2//BP86/BS1 level, energies quoted in kcal mol^–1) for the formation of the dimeric rubidium (hydrido)(phenyl)aluminate E^Rb.

Figure 7

Computed free energy profile (BP86-D3BJ(PCM = C₆H₆)/BS2//BP86/BS1 level, energies quoted in kcal mol^–1) for the formation of the monomeric rubidium (hydrido)(phenyl)aluminate, J^Rb.