Transmetalation of Acyclic Tungsten Carbenes to Coinage Metals: Distinct Behavior of Silver toward Carbene Transfer and Hydrolysis

The
transmetalation of acyclic monoamino and alkoxo carbenes from
tungsten to group 11 metals has been studied for all three metals
by analyzing the new metal carbenes formed and also their decomposition
products in solution. The ease of transmetalation follows the trend
Au > Cu > Ag, and the gold carbenes can be isolated: a copper
carbene
could be detected, but we found no experimental evidence of the formation
of any silver carbene. The electrophilic character of copper and gold
carbenes is supported by the identity of the products formed upon
hydrolysis of the carbenes involved. Silver shows a distinct behavior,
and the presence of Ag+ ions in an acetonitrile solution
of a tungsten aminocarbene leads to the formal protonation of the
carbene fragment to give an iminium salt. This outcome, a seemingly
nucleophilic behavior, is unexpected for group 11 carbenes, but DFT
calculations show that the protonation of a putative cationic silver
carbene is energetically accessible.