A Gold Exchange: A Mechanistic Study of a Reversible, Formal Ethylene Insertion into a Gold(III)–Oxygen Bond

The Au­(III) complex Au­(OAcF)2(tpy) (1, OAcF = OCOCF3; tpy = 2-p-tolylpyridine) undergoes reversible
dissociation of the OAcF ligand trans to
C, as seen by 19F NMR. In dichloromethane or trifluoroacetic
acid (TFA), the reaction
between 1 and ethylene produces Au­(OAcF)­(CH2CH2OAcF)­(tpy) (2). The
reaction is a formal insertion of the olefin into the Au–O
bond trans to N. In TFA this reaction occurs in less
than 5 min at ambient temperature, while 1 day is required in dichloromethane.
In trifluoroethanol (TFE), Au­(OAcF)­(CH2CH2OCH2CF3)­(tpy) (3) is formed
as the major product. Both 2 and 3 have
been characterized by X-ray crystallography. In TFA/TFE mixtures, 2 and 3 are in equilibrium with a slight thermodynamic
preference for 2 over 3. Exposure of 2 to ethylene-d4 in TFA caused
exchange of ethylene-d4 for ethylene at
room temperature. The reaction of 1 with cis-1,2-dideuterioethylene furnished Au­(OAcF)­(threo-CHDCHDOAcF)­(tpy), consistent with an overall anti addition to ethylene. DFT­(PBE0-D3) calculations indicate
that the first step of the formal insertion is an associative substitution
of the OAcF trans to N by ethylene. Addition
of free –OAcF to coordinated ethylene
furnishes 2. While substitution of OAcF by
ethylene trans to C has a lower barrier, the kinetic
and thermodynamic preference of 2 over the isomer with
CH2CH2OAcF trans to C accounts for the selective formation of 2. The
DFT calculations suggest that the higher reaction rates observed in
TFA and TFE compared with CH2Cl2 arise from
stabilization of the –OAcF anion lost
during the first reaction step.