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carbenes, which is necessary for a better understanding of this
topical branch of catalysis research.[21,22] Since the disclosure
of our complex 6a as the first reactive gold carbene to be fully
characterized structurally and spectroscopically,[14,15] a small
set of other well-defined gold carbenes has been published.
Figure 2. Relevant bond lengths in 4; comparison with the computed
distances of two simplified model compounds; see Ref. [9].
and long distances between all partners.[6,9,11] In line with this
À
notion, the Rh1 Rh2 bond in 4 is longer (2.423(1) ) than the
average distance observed in ordinary dirhodium tetracar-
boxylate complexes (ca. 2.35 ) and similar to the one found
in the NHC adduct 1 (2.424 ),[12] but notably shorter than in
the computed structures.[9] Arguably more striking is the
Whereas complex 10 draws its stability mainly from charge
delocalization over the (aromatic) organic ligand frame-
work,[25] complex 11 features an unusual trigonal coordina-
tion geometry about the AuI center,[26] while the carbene unit
in complex 12 is massively shielded.[24,27] Together with other
relevant data,[28–30] these species greatly help in putting the
mechanistic discussions on a firmer basis, but do not allow
certain subtle but important aspects of gold catalysis to be
rationalized. It is well known, for example, that the reactivity
of gold carbenes is tunable by proper choice of the ancillary
ligand.[21,22,31] To better understand this subtle but essential
chemical attribute, it is necessary in the long run to learn how
changes of the trans ligand affect the structure of a given gold
carbene.
The new method described here allows such systematic
investigations to be carried out. This aspect is aptly illustrated
by the preparation of two additional complexes of type 6
which differ from the parent complex 6a (L = PCy3) only in
the two-electron donor L at the trans position (Scheme 2).
Specifically, complex 6b (Figure 3) contains a member of
Buchwaldꢀs phosphine ligand family, the donor capacity of
which is down-regulated by four CF3 substituents on the P-Ar2
groups.[32] In contrast, the Mor-DalPhos ligand[33,34] in 6c is
highly electron donating (Figure 4). The fact that the molec-
ular structures and spectral properties of 6a–c reflect these
differences only to a limited extent is arguably telling:
À
Rh1 C1 bond length of 2.061(6) , which clearly exceeds the
computed 1.906 for 8 or 1.939 for 9.[9] Once again, the
corresponding bond length in the dirhodium-NHC adduct 1 is
comparable (2.057 ), even though this complex shows
different reactivity as mentioned previously. We presume
À
that the remarkably long experimental Rh1 C distances in
both complexes reflect the preference of the carbene center to
engage primarily with the flanking arene rings in 4 or with the
neighboring N atoms of the NHC in 1 rather than with the
d electrons that Rh1 could provide. Since 4 exhibits the
reactivity profile of an ordinary dirhodium carbene, however,
this finding is thought to be an important reference and
calibration point for further experimental and computational
investigations into this flourishing field of catalysis.[1–5]
The exceptionally electrophilic character of this dirho-
dium carbene is also reflected in its reactivity. Thus, complex 4
was found to undergo a remarkably easy Rh!Au carbene
transfer on treatment with [(Cy3P)AuNTf2][19,20] at low
temperature to give the corresponding gold carbene 6a in
good yield (Scheme 1). This result is somewhat counter-
intuitive, since gold complexes themselves have gained
prominence as p-acid catalysts because of their pronounced
electrophilic nature.[21,22] Complex 6a had originally been
prepared by transmetalation of the Fischer chromium car-
bene 5,[23] because attempted decompositions of the diazo
precursor 3 with various gold sources of type [LAuNTf2] had
invariably led to formation of azine 7 as the major product
(Scheme 1).[14,15,24] Since the preparation of 5 is challenging (a
speedy chromatographic purification at low temperature is
the best way to purify the compound), the new entry into gold
carbenes by transmetalation of a dirhodium precursor is
clearly more practical. This is particularly true since the diazo
derivative 3 can be readily prepared on a large scale and is
safe to handle.[16] Thus, this method lends itself to a more
systematic study into structure/reactivity relationships of gold
À
Although the Au C bond lengths are responsive to the
nature of L, they fall into a rather narrow range (6a:
2.039(5) , 6b: 2.028(4) , 6c: 2.020(8) ).[35,36] In all the
cases investigated, the carbene center seems to draw stability
primarily from an interaction with the flanking aryl groups, as
manifested in the short bonds to the ipso-C atoms. Although
the p-MeOC6H4 rings are slightly twisted out of coplanarity in
the solid state, likely to relieve steric repulsion of the ortho
protons, they give rise to a single set of NMR signals, even at
À508C. For the low rotational barrier, their p systems can
engage with the (empty) 2p carbene lobe, and this interaction
prevails over LAu!CD electron back-donation. This inter-
pretation is consistent with the fact that the carbene centers in
ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 15452 –15456
Werl, Christophe
