Exploiting the Electronic Tuneability of Carboranes as Supports for Frustrated Lewis Pairs

Abstract

The first example of a carborane with a catecholborolyl substituent, [1-Bcat-2-Ph-closo-1,2-C₂B₁₀H₁₀] (1), has been prepared and characterized and shown to act as the Lewis acid component of an intermolecular frustrated Lewis pair in catalyzing a Michael addition. In combination with B(C₆F₅)₃ the C-carboranylphosphine [1-PPh₂-closo-1,2-C₂B₁₀H₁₁] (IVa) is found to be comparable with PPh₂(C₆F₅) in its ability to catalyze hydrosilylation, whilst the more strongly basic B-carboranylphosphine [9-PPh₂-closo-1,7-C₂B₁₀H₁₁] (V) is less effective and the very weakly basic species [μ-2,2'-PPh-{1-(1'-1',2'-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀}] (IX) is completely ineffective. Base strengths are rank-ordered via measurement of the ¹J ³¹P-⁷⁷Se coupling constants of the phosphineselenides [1-SePPh₂-closo-1,2-C₂B₁₀H₁₁] (2), [9-SePPh₂-closo-1,7-C₂B₁₀H₁₁] (3), and [SePPh₂(C₆F₅)] (4).

Keywords: carborane; catalysis; frustrated Lewis pair; phosphine.

Figure 1

Molecular structure of compound 1. Selected interatomic distances (Å): C1−C2 1.6840(15), C1−B100 1.5703(16), B100−O11 1.3744(14), B100−O12 1.3739(15), and C2−C21 1.5083(14).

Scheme 1

Michael addition of dimethyl malonate and 3-buten-2-one producing dimethyl 2-(3-oxobutyl) malonate.

Scheme 2

Line diagrams of key species.

Scheme 3

Hydrosilylation of dimethylfulvene.

Figure 2

Molecular structure of compound 2. Selected interatomic distances (Å): C1−C2 1.6511(14), C1−P1 1.8869(10), and P1−Se1 2.1037(3).

Figure 3

Molecular structure of compound 3. Selected interatomic distances (Å): B9−P1 1.937(2) and P1−Se1 2.1196(5).

Figure 4

Molecular structure of compound 4. Selected interatomic distances (Å): P1−Se1 2.1047(3), P1−C101 1.8346(11), P1−C111 1.8141(12), and P1−C121 1.8080(12).