In order to further establish the robustness of the improved
methodology, we sought to produce the [Au(NHC)(OH)] complexes
on a larger scale (Table 4). Thus, 2 was successfully synthesised on
1, 5 and 20 g scales (Table 4, entries 1–3). Yields increased as the
scale of the reaction was increased. This observation suggested
that yield losses were intrinsically related to mechanical loss and
work-up processes. Extending the large-scale methodology to
other related NHC ligands also proved successful (Table 4, entries
4–7).
As previously mentioned, the new conditions dictate a different
pathway from previous methodologies. A possible mechanism for
this reaction is depicted in Scheme 2, in which the tert-amyl alcohol
first reacts with NaOH to generate the alkoxide species [14].
Although the equilibrium position surely lies towards the hydroxide,
the (small) amount of the alkoxide would react rapidly with
[Au(NHC)Cl]. The intermediate thus formed, [Au(NHC)(Ot
Am)],
reacts quickly with water in the reaction mixture to form the
[Au(NHC)(OH)] complex, and thus regenerating the tert-amyl
alcohol. To support the proposed mechanism, we isolated and