C-C Cross-Couplings from a Cyclometalated Au(III) C ∧ N Complex: Mechanistic Insights and Synthetic Developments

Abstract

In recent years, the reactivity of gold complexes was shown to extend well beyond π-activation and to hold promises to achieve selective cross-couplings in several C-C and C-E (E=heteroatom) bond forming reactions. Here, with the aim of exploiting new organometallic species for cross-coupling reactions, we report on the Au(III)-mediated C(sp² )-C(sp) occurring upon reaction of the cyclometalated complex [Au(C^CH2 N)Cl₂ ] (1, C^CH2 N=2-benzylpyridine) with AgPhCC. The reaction progress has been monitored by NMR spectroscopy, demonstrating the involvement of a number of key intermediates, whose structures have been unambiguously ascertained through 1D and 2D NMR analyses (¹ H, ¹³ C, ¹ H-¹ H COSY, ¹ H-¹³ C HSQC and ¹ H-¹³ C HMBC) as well as by HR-ESI-MS and X-ray diffraction studies. Furthermore, crystallographic studies have serendipitously resulted in the authentication of zwitterionic Au(I) complexes as side-products arising from cyclization of the coupling product in the coordination sphere of gold. The experimental work has been paralleled and complemented by DFT calculations of the reaction profiles, providing valuable insight into the structure and energetics of the key intermediates and transition states, as well as on the coordination sphere of gold along the whole process. Of note, the broader scope of the cross-coupling at the Au(III) C^CH2 N centre has also been demonstrated studying the reaction of 1 with C(sp² )-based nucleophiles, namely vinyl and heteroaryl tin and zinc reagents. These reactions stand as rare examples of C(sp² )-C(sp² ) cross-couplings at Au(III).

Keywords: NMR spectroscopy; cross-coupling; gold cyclometalated complexes; organometallics; reductive elimination.