Stable gold(III) catalysts by oxidative addition of a carbon-carbon bond

Abstract

Low-valent late transition-metal catalysis has become indispensable to chemical synthesis, but homogeneous high-valent transition-metal catalysis is underdeveloped, mainly owing to the reactivity of high-valent transition-metal complexes and the challenges associated with synthesizing them. Here we report a carbon-carbon bond cleavage at ambient conditions by a Au(i) complex that generates a stable Au(iii) cationic complex. In contrast to the well-established soft and carbophilic Au(i) catalyst, this Au(iii) complex exhibits hard, oxophilic Lewis acidity. For example, we observed catalytic activation of α,β-unsaturated aldehydes towards selective conjugate additions as well as activation of an unsaturated aldehyde-allene for a [2 + 2] cycloaddition reaction. The origin of the regioselectivity and catalytic activity was elucidated by X-ray crystallographic analysis of an isolated Au(iii)-activated cinnamaldehyde intermediate. The concepts revealed suggest a strategy for accessing high-valent transition-metal catalysis from readily available precursors.

Figure 1. Routes to high-valent metal complexes

a, Standard route to high-valent metal complexes using halogen-based oxidants. b, Hypothetically synthetic pathway for accessing Au(III)-C bond-stabilized Au(III) complexes.

Figure 2. Accessing gold(III) via oxidative addition of a carbon-carbon bond

Attempted access to complex 3 via transmetalation of Sn(biphenyl)(ⁿBu)₂ with IPrAuCl₃ 2. Proposed pathway for the oxidative addition of IPrAu(I) complex with biphenylene/2,3,6,7-tetramethylbiphenylene. Coordination chemistry of IPrAu(III)(biphenyl) complex 3 and 6. X-ray structure of IPrAu(III)(biphenyl)Cl, complex 3 and [IPrAu(III)(biphenyl)(Me₂NC(O)H)][SbF₆] complex 6, SbF₆⁻ anion is omitted for clarity.

Figure 3. Examples of selective Au(III)-catalyzed 1,4-additions

a, Mukaiyama-Michael addition. b, Nitronate Michael-addition. c, In situ generation of IPrAu(III)(Me₄-biphenyl) catalyst for Mukaiyama−Michael addition. d, One-pot tandem Au(I)/Au(III) and Au(III)/Au(III)-catalyzed reactions. [a] NMR yield.

Figure 4. Remote selectivity in gold(III) catalyzed addtions to dienals

a, δ-selective thiol addition and reduction reactions. b, γ, δ-selective Diels−Alder reaction and in situ generation of IPrAu(III)(biphenyl) catalyst for Diels−Alder reactions.

Figure 5. Gold(III) catalyzed [2 + 2] cycloaddition of a allene-aldehyde

Figure 6. A model for the obtained selectivity

X-ray structure of [IPrAu(III)(biphenyl)(η¹-cinnamaldehyde)][SbF₆] complex 22. SbF₆⁻ anion is omitted for clarity.