Tetrazol-5-ylidene Gold(III) Complexes

Tetrazol-5-ylidene Gold(III) Complexes from Sequential [2 + 3] Cycloaddition of Azide to Metal-Bound Isocyanides and N4 Alkylation

Full text: Organometallics, 2017, 36, 3974–3980

We established that gold(III) complexes with rare NHC species, i.e. 1,4-disubstituted tetrazol-5-ylidenes, can be accessed directly from gold(III)-isocyanides. Herein, the [2 + 3] cycloaddition between equimolar amounts of the isocyanide complexes [AuCl₃(CNR)] and tetrabutylammonium azide furnished the corresponding tetrazolate derivatives [n-Bu₄N][AuCl₃(CN₄R)] in good to excellent yield. 

[2 + 3] Cycloaddition of azide to Au^III-bound isocyanides 

This reaction is gold(III)-mediated insofar as no reaction between free azide and uncomplexed isocyanide was observed. Results of DFT calculations for a model system reveal that the mechanism for the cycloaddition of an azide to the isocyanide ligand in [AuCl₃(CNMe)] is stepwise and involves nucleophilic attack of N₃^– on the N atom of CNMe followed by ring-closure. The addition is both kinetically and thermodynamically favorable and occurs via the formation of an acyclic NNNCN-intermediate, whereas the cyclization is the rate-determining step.

Proposed stepwise (A and C) and concerted (B) mechanisms for the formal
CA of N₃^– to [AuCl₃(CNMe)]

In the next step, selective N4-methylation of [n-Bu₄N][AuCl₃(CN₄R)] with equimolar amount of MeOTf leads to the corresponding gold(III) complexes with 1,4-disubstituted tetrazol-5-ylidene carbene ligands, that were obtained in good yield after purification. 

N4-alkylation leading to gold(III)-complexes with 1,4-disubstituted tetrazol-5-ylidenes

With a representative example we demonstrated that the sequence of cycloaddition/alkylation can be undertaken in one-pot manner without the separation of intermediate compounds [n-Bu₄N][AuCl₃(CN₄R)] giving target gold(III)-NHCs in yields comparable to the sequential  procedure. A sequence of gold(III)-mediated [2 + 3] cycloaddition between isocyanide and azide followed by the N4-methylation of the intermediate tetrazolate formed represents a unique approach to gold(III) complexes with 1,4-disubstituted tetrazol-5-ylidene carbene ligands that can hardly be accessed by other routes. Further studies aiming at preparation of gold(III)-NHCs via [2 + 3] cycloaddition are underway in our group.