Twisting the arm: structural constraints in bicyclic expanded-ring N-heterocyclic carbenes.

Katharine R Sampford,Edward B Kidner,Benson M Kariuki,Jamie L Carden,Abigail Berry,Kingsley J Cavell,Paul D Newman,Damien M Murphy

doi:10.1039/c8dt04462g

Katharine R Sampford, Edward B Kidner + Show 6 more

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https://doi.org/10.1039/c8dt04462g

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Abstract

A series of diaryl, mono-aryl/alkyl and dialkyl mono- and bicyclic expanded-ring N-heterocyclic carbenes (ER-NHCs) have been prepared and their complexation to Au(i) investigated through the structural analysis of fifteen Au(NHC)X and/or [Au(NHC)2]X complexes. The substituted diaryl 7-NHCs are the most sterically encumbered with large buried volume (%VB) values of 40-50% with the less flexible six-membered analogues having %VB values at least 5% smaller. Although the bicyclic systems containing fused 6- and 7-membered rings (6,7-NHCs) are constrained with relatively acute NCN bond angles, they have the largest %VB values of the dialkyl derivatives reported here, a feature related to the fixed conformation of the heterocyclic rings and the compressional effect of a pre-set methyl substituent.

Highlights

The examples noted above serve to emphasise a need to understand and appreciate the stereoelectronic features associated with expanded-ring N-heterocyclic carbenes (ER-N-heterocyclic carbene ligands (NHCs)) to provide a holistic appreciation of stereoelectronic character and potential catalytic facility
This is facilitated by the differential reactivity of the two amino groups with the 3-amino position being more readily functionalised than the 1-amino. While this is advantageous for the construction of unsymmetrical frameworks, it is a significant synthetic limitation as, to date, it has not been possible to introduce aryl groups at the 1-position and diarylated derivatives are necessarily absent. This contrasts with the known monocyclic expanded-ring N-heterocyclic carbenes (ER-NHCs) which are predominantly symmetrical, diarylated species as controlled construction of unsymmetrical derivatives is difficult for these compounds
The bicyclic 6,7-NHC core is less rigid than expected with the NCN angle ranging from 117.1 to 121.6°, the largest value is restricted to one complex

Summary

Introduction

The examples noted above serve to emphasise a need to understand and appreciate the stereoelectronic features associated with ER-NHCs to provide a holistic appreciation of stereoelectronic character and potential catalytic facility. The extent to which the wingtips impose upon a metal depends significantly on the NCN bond angle there are other, more subtle, factors that play a part. Six-membered monocyclic derivatives have little conformational freedom precluding structural promiscuity, whereas the larger 7-membered analogues show greater flexibility in the ring leading to less predictable features. Bicyclic NHCs derived from camphor contain fused 6,7-membered rings and are relatively rigid.[6] They contain spatially-set methyl groups that can dictate the adoption of specific orientations of the exo-substituents on the N-atoms. To date, there is insufficient structural data to illuminate the distinctions between these bicyclic derivatives and their monocyclic counterparts

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