Allosteric Differentiation of Al(I) Reactivity

Abstract

The dimeric potassium alumanyl, [{SiN^Dipp}AlK]₂ ({SiN^Dipp} = {CH₂SiMe₂N(Dipp)}₂; Dipp = 2,6-i-Pr₂C₆H₃), reacts with two equivalents of PhC≡CR (R = Ph or SiMe₃) with the exclusive formation of the aluminacyclopropene derivatives, [{SiN^Dipp}Al{η²-C₂(R¹)(R²)}]. In contrast, reactions with an equal stoichiometry of both alumanyl and alkyne reagents provide the products of not only alkyne cycloaddition but also para-C-H activation of a phenyl substituent. Supported by a theoretical study, this outcome is judged to result from a sequence of cooperative steps and the introduction of a modicum of kinetic discrimination that is suggested to be allosteric in origin.

Keywords: alkyne; aluminum; arene activation; cycloaddition; density functional theory.

Figure 1

The N,N′‐(diamido)alumanyl anions, I–III, the C,C′‐(dialkyl)alumanyl anion, IV, and the structure of compound V.

Figure 2

(a) The [2 + 1] cycloaddition of diphenylacetylene induced at IV^K ; (b) the C–H activation of benzene by dimeric III^M and the gradation of reactivity resulting from variation of M (M = K, Rb, Cs).

Scheme 1

Synthesis of compounds 1 and 2 and their subsequent lack of reactivity toward III^K .

Figure 3

(a) Plot depicting the molecular structure of 1. Ellipsoids are depicted at 30% probability. Hydrogen atoms have been omitted and peripheral substituents are depicted as wireframes, for clarity. Symmetry operations: ¹ 1 + y − x, 2 − x, ¹∕₃ + z; ² 2 − y, 1 + x − y, − ¹∕₃ + z. (b) Plot depicting the structure of 2. Ellipsoids are depicted at 30% probability. Minor disordered components have been omitted as have hydrogen atoms (those attached to C18 excepted), for clarity. Peripheral substituents are depicted as wireframes, also for visual ease. Symmetry operations: ^{1 3}∕2 − x, − ½ + y, − ½ + z; ^{2 3}∕₂ − x, ½ + y, ½ + z.

Scheme 2

Synthesis of compounds 3 and 4.

Figure 4

Plot depicting the structure of 3. Ellipsoids depicted at 30% probability. Solvent, minor disordered components and hydrogen atoms (H1, H2, H3, H4, and H111 excepted) omitted for perspicuity. Peripheral substituents are depicted as wireframes, also for clarity. Symmetry operations: ¹ 1 − x, 1 − y, − z.

Figure 5

Plot depicting the structure of 4. Ellipsoids are depicted at 30% probability. Hydrogen atoms (H1, H2 and those attached to C18 and C35 excepted) have been omitted and peripheral substituents are depicted as wireframes, for clarity. Symmetry operations: ¹ 1 − x, ½ + y, 1 − z.

Figure 6

Computed free energy profile (BP86‐D3^BJ(PCM = C₆H₆)/BS2//BP86/BS1 level, energies quoted in kcal mol⁻¹) for the formation of the dialuminated alkyne product, F (3 and 4), through initial aluminacyclometalation, followed by para C–H cleavage of Ph. Free energies when R = Ph are in red; free energies when R = SiMe₃ are in black.