Carbene-activated stannylenes to access selective C(sp3)-H bond scission at the steric limit

Abstract

The ubiquity of N-heterocyclic carbenes (NHCs) in diverse areas of chemical research typically arises from their potent stabilising capabilities and role as innocent spectators to stabilise otherwise non-bottleable compounds and complexes. This has, until now, been particularly true for NHC-stabilised stannylenes, with no exceptions reported thus far. Herein, we demonstrate that the combination of heteroleptic terphenyl-/amido-based stannylenes and tetra-alkyl substituted NHCs renders the corresponding NHC-ligated stannylenes highly reactive, yet isolable. In solution, this induces sterically controlled inter- and intramolecular C(sp³)-H bond scissions, resulting in the selective formation of stannylene metallocycles that depend on both the NHC source and the meta-terphenyl ligand coordinated to tin.

Fig. 1. Notable literature examples and research scope.

I: Landmark NHC-stabilised dihalosilylenes; II: first examples of NHC-ligated stannylenes.

Fig. 2. Synthesis and structures of 2a-c, 3a, and 4b (polymorph a), and 4 c. A.

Reactivity of 1a-c, towards IMe₄ to yield 2a-c, the tetramer 3a, and the dimers 4b,c. Conditions (i) benzene, r.t.; (ii) benzene, r.t.–80 °C. B–D: Molecular structures of 2a, 3a, and 4a determined by single crystal X-ray crystallography. Anisotropic displacement parameters are drawn at the 50% probability level. Hydrogen atoms have been omitted where necessary for clarity, and in the case of 3a terphenyl substituents have also been omitted for clarity.

Fig. 3. Computed ring expansion mechanism.

DFT-calculated mechanism for the tin amide mediated C–H activation cascade forming 3a at the BP86-D3BJ/Def2-TZVP (PCM = Benzene) level of theory. Gibbs free energies (ΔG₂₉₈, kcal•mol^-1) are given relative to the starting materials. Natural Population Analysis (NPA) charges and Wiberg Bond Indices (WBI) given for 1a and 2a were calculated using NBO-7.

Fig. 4. Synthesis and structures of 5a, and 6b,c. A.

Reactivity of ^MesTerSn{N(SiMe₃)₂} (1a), ^DippTerSn{N(SiMe₃)₂} (1b) and ^TippTerSn{N(SiMe₃)₂} (1c) towards IⁱPr₂Me₂ to yield the C–H activation product 5a, and doubly C–H activation products 6b,c. Conditions (i) benzene, r.t. –60 °C; (ii) benzene, r.t.–90 °C. B: DFT-Calculated mechanism for the tuck-in formation of 5a from 1a and IⁱPr₂Me₂ at the BP86-D3BJ/Def2-TZVP (PCM = Benzene) level of theory. Gibbs free energies (ΔG₂₉₈, kcal•mol⁻¹) are given relative to the starting materials. C,D Molecular structures of 5a and 6b determined by single crystal X-ray crystallography. Anisotropic displacement parameters are drawn at the 50% probability level. Hydrogen atoms have been omitted where necessary for clarity, and in the case of 6b terphenyl substituents have also been omitted for clarity.