# Bridged Pyrroloindole‐CAYC‐Gold Complexes: Harnessing Weak Secondary Intramolecular Au···H−C(sp3 ) Interactions in Gold(I) Catalysis

## TL;DR

Scientists designed a new type of gold catalyst with unique properties that could improve chemical reactions in organic synthesis.

## Contribution

A new class of N-heterocyclic carbene ligands (bCAYC) with strong σ-donating ability and steric shielding is introduced.

## Key findings

- bCAYC ligands exhibit a remarkably short Au···H−C(sp3) distance of 2.26 Å.
- bCAYCs are among the strongest σ-donating NHC ligands, as shown by DFT and TEP analysis.
- The ligands enable efficient hydroamination and C–N/C–C bond-forming reactions at room temperature.

## Abstract

Gold(I) catalysis has been established as a tremendously powerful tool in organic synthesis and catalysis. Herein, we introduce a new class of electronically‐ and sterically‐hindered N‐heterocyclic carbene ligands, termed as bridged cyclic amino(ylide)carbene (
b
CAYC), containing 9H‐pyrrolo[1,2‐a]indoles as the carbene core and diaryl sulfonium as the ylide partner. Highly electron‐rich
b
CAYC ligands are suitable for synthesizing the corresponding gold(I) complexes, which exhibit a weak intramolecular Au···H−C(sp3) interaction as a secondary interaction where the gold d‐orbitals act as electron donors and the C–H σ* orbitals act as acceptors. Properties and parameters of Au···H−C(sp3) interactions were investigated through X‐ray, NMR, DFT, AIM, and NBO studies. This study reports a remarkably short Au···H−C(sp3) distance of 2.26 Å, as determined through X‐ray crystallographic analysis. DFT and TEP data analysis revealed that
b
CAYCs are among the most σ‐donating carbene ligands to date, than other widely used NHC ligands. The steric impact of
b
CAYCs has shown excellent potential for steric shielding, with %Vbur reaching up to 42.6%. These unique properties were well‐reflected through the room‐temperature hydroamination and one‐pot C–N and C–C bond‐forming reactions.

We present a new class of electronically and sterically hindered N‐heterocyclic carbenes, referred to as bridged cyclic (amino)ylide carbene (
b
CAYC), where 9H‐pyrrolo[1,2‐a]indole acted as carbene core and diaryl sulfonium as ylide partner. Corresponding
b
CAYC‐Au(I) exhibits a weak secondary intramolecular Au···H−C(sp3) interaction as a secondary interaction. DFT and TEP analysis indicate
b
CAYCs are among the strongest σ‐donating NHCs to date.