Gold Difluorocarbenoid Complexes: Spectroscopic and Chemical Profiling

Article abstract of DOI:10.1002/anie.201903957

Gold carbenes of the general type [LAu=CR₂]⁺ are sufficiently long-lived for spectroscopic inspection only if the substituents compensate for the largely missing stabilization of the carbene center by the [LAu]⁺ fragment. π-Donation by two fluorine substituents (R=F) is insufficient; rather, difluorocarbene complexes are so deprived in electron density that they sequester even “weakly coordinating” anions such as triflate or triflimide. This particular bonding situation translates into unmistakable carbenium ion chemistry upon reaction with stilbene as a model substrate.

Full text of DOI:10.1002/anie.201903957

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Accepted Article
Title: Gold Difluorocarbenoid Complexes: Spectroscopic and Chemical
Profiling
Authors: Alois Fürstner, Alexander G Tskhovrebov, and Julia B.
Lingnau
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10.1002/anie.201903957
Angewandte Chemie International Edition
Gold Difluorocarbenoid Complexes: Spectroscopic and Chemical Profiling
Alexander G. Tskhovrebov, Julia B. Lingnau, and Alois Fürstner*
Abstract: Gold carbenes of the general type [LAu=CR2]+ are sufficiently long-lived for spectroscopic
inspection only if the substituents R compensate for the largely missing stabilization of the carbene
center by the [LAu]+ fragment. -Donation by two fluorine substituents (R = F) is insufficient; rather,
difluorocarbene complexes are so deprived in electron density that they sequester even “weakly
coordinating” anions such as triflate or triflimide. This particular bonding situation translates into
unmistakable carbenium ion chemistry upon reaction with stilbene as a model substrate.
Cationic gold carbene complexes can be thought as consisting of a singlet carbene fragment, the lone
pair of which interacts with an orbital of appropriate symmetry of the [LAu]+ unit to form a strong -
bond; -back donation from the metal to the empty carbene p-orbital is (very) modest.1,2,3,4 This
particular bonding situation allows the exceptional electrophilicity of gold carbenes to be
rationalized, which surfaces in a plethora of catalytic transformations;4,5,6,7 at the same time, it
explains why spectroscopic characterization and isolation of these highly reactive species in pure
form proved challenging. Known representatives have substituents at the carbene center that
compensate for the largely missing stabilization that a more strongly -donating metal would entail
(Figure 1); if these substituents overdo their job, however, prototypical reactivity of an electrophilic
carbene is internally “quenched” and the garnered structural information arguably loses relevance
for a better understanding of gold catalysis.8 In practice, the right balance has been reached by
conjugation of the carbene with appropriately substituted arenes (1),9 delocalization of the charge
density within an aromatic ring system (2),10 or simply by massive shielding that prevents any
reaction partner from reaching the highly electrophilic site on steric grounds (3).11,12,13
*
Dr. A. G. Tskhovrebov, M. Sc. J. B. Lingnau, Prof. A. Fürstner
Max-Planck-Institut für Kohlenforschung
45470 Mülheim/Ruhr (Germany)
Email: fuerstner@kofo.mpg.de
Supporting Information for this article can be found on the WWW under ##########
1
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10.1002/anie.201903957
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Figure 1. Top: Selected examples of reactive gold carbene complexes characterized by X-ray
diffraction; the escorting counterions are not shown for clarity; bottom: targeted gold
difluorocarbene complexes A stabilized by -donation from the fluorine lone-pairs; this effect is
operative in the carbenium ion 4 but largely offset by the aryl groups in 5
Scheme 1. An unsuccessful previous attempt at detecting a gold difluorocarbene (presumably in form
of the ether adduct); only the corresponding carbonyl complex 8 was found even at very low
temperature
Based on this analysis, we conjectured that gold difluorocarbenes of type A might be potentially
observable entities (Figure 1): though fluorine substituents are strongly electron-withdrawing in the
-framework, they act as decent -donors. This effect is evident from the structures of other metal
difluorocarbene complexes in the solid state;14 the net stabilizing function notwithstanding, most
examples that proved sufficiently stable for isolation in pure form do comprise a fairly strongly -
donating central metal fragment.15 Cationic difluorocarbene complexes,16 in contrast, as well as
difluorocarbene complexes of platinum and gold as the most widely used carbophilic catalysts in
synthesis are exceedingly rare.17,18 Since the poor -backdonation from [LAu]+ in A imparts partial
2
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10.1002/anie.201903957
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carbocation character onto the ground state of the coordinated carbene ligand,19 it is also deemed
relevant in the present context to recapitulate to which extent fluorine substituents can stabilize
carbenium ions (Figure 1).20 Their -donor capability is clearly manifest, for example, in the structure
of cation 4 in the solid state, in which the CF distance is notably shorter (1.285(11) Å) than an
average Csp2F bond ( 1.333  0.005 Å).21 Interestingly, however, the analogous CF distance in 5 is
1.31 Å and hence almost “normal”, whereas the CcarbeneCipso bonds are contracted.21 The comparison
of 4 and 5 suggests that fluorine substituents can stabilize a carbenium center through -donation
but are less efficient in doing so than an ordinary aryl ring.
These data suggest that difluorocarbene complexes of type A are likely at the edge of being
observable.22 This supposition is in line with a previous study which showed that treatment of
compound 6 with BF3Et2O solely afforded the carbonyl complex 8 (Scheme 1).23 The transient gold
difluorocarbene species 7 as the likely intermediate of this reaction could not be detected by 19F
NMR at 80°C; hydrolysis24 must be extremely fast even under such notably mild conditions.
It is against this backdrop that our attempts at characterizing the intermediate formed upon Lewis-
acid mediated -fluoride elimination from dicoordinate LAuCF3 complexes (10) have to be seen. The
required precursors 10 were conveniently prepared from 9 by following a literature procedure and
their structures in the solid state are contained in the Supporting Information (Scheme 2).25 The only
exception was 9e (L = (p-F3CC6H4)3P), which, on treatment with Me3SiCF3 and AgF, underwent ligand
scrambling with formation of 11 in high yield. The structure of this unique complex in the solid state
(Figure 2) suggests that the pairing of the discrete [L2Au]+ and [Au(CF3)2] ions is reinforced by an
aurophilic interaction between them (Au1Au2 2.88 Å);26,27 the axes of the cationic unit and the
bis(trifluoromethyl)aurate entity are almost orthogonal to each other likely for steric reasons
(C1Au1Au2P2 102.9°). This remarkable ensemble can be stored under inert conditions for
extended periods of time; however, it was found to redistribute instantly on treatment with TMSOTf
even at 50°C to give complex 10e, which was identified by the very characteristic coupling pattern
between the P- and F-nuclei.
3
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Scheme 2. Generation and fate of gold difluorocarbenoid complexes
Figure 2. Structure of complex 11 in the solid state
4
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Addition of TMSOTf to a solution of 10a in CD2Cl2 at 78°C followed by short warming to 20°C
resulted in the fairly clean formation of carbenoid 12a,28 provided that strictly inert conditions were
ensured. While this reactive species was sufficiently long-lived at 50°C for spectroscopic inspection,
it rapidly decomposed into purple gold colloid above this temperature. The structure assignment is
based on the characteristic multiplicity of the signals in the 31P NMR (P = 36.1 ppm, t, JP,F = 31.0 Hz)
and 19F NMR spectra (F = 30.7 ppm, d, JP,F = 31.0 Hz); this pattern and the identical coupling
constant show that one phosphorous center and two fluorine atoms are mutually coupled. The 13C
NMR resonance of the carbenoid center itself is also highly informative, which appears as doublet of
triplets centered at C = 169.6 ppm (JC,P = 169.8, JC,F = 359.5 Hz). This shift speaks against a true “metal
carbene”, which one would expect to resonate at much lower field;29 rather it suggests that the
triflate anion is covalently bound.30 This conclusion is in line by the cross peak observed in the 19F
COSY spectrum between the signal of the “carbenoid” CF2 unit (F = 30.7 ppm) and the OSO2CF3
group (F = 76.1 ppm, see the Supporting Information), which implies that the “weakly coordinating
counterion” is at least in close vicinity. Unfortunately, all attempts at isolating complex 10a in pure
form failed because of rapid gold colloid formation.
5
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Figure 3 Strips from the 31P NMR (top) and 19F NMR spectra (bottom) of a crude reaction mixture
formed from 10d and TMSOTf in CD2Cl2 at 50°C at 45% conversion showing the starting complex
and carbenoid 12d
Figure 4 13C NMR signal (100 MHz, CD2Cl2, 50°C) of the carbenoid C-atom of complex
[(Ph3P)AuCF2(NTf2)] (13)
The overall bonding situation does not vary much upon changing the steric and electronic properties
of the donor ligand trans to the carbenoid unit over a wide range. In all cases investigated were the
spectral features of the resulting intermediates similar to those of 12a (Figure 3 and the SI). Likewise,
replacement of TMSOTf by TMSNTf2 as the activating agent led to the same gross picture in that the
triflimide unit seems to be bound to the resulting gold carbenoid 13 as manifest in the diagnostic
spectral fingerprint (Figure 4 and the SI).31 The use of activators with less coordinating counterions,
however, was unrewarding: even though discrete carbene complexes seem to be generated in the
first place, their extremely short lifetime made full characterization by low temperature NMR
impossible; no carbene resonance, which is a multiplet with very low intensity, could be detected in
any of the cases investigated.
6
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In parallel, we probed the character of the gold difuorocarbenoids by chemical means. It is known for
a long time that the chlorinated complexes [R3PAuCCl3] and [R3PAuCH2Cl], which are close relatives of
10, afford (dichloro)cyclopropanes in high yield simply upon heating in the presence of an olefin in an
inert solvent;32 addition of a chloride scavenger allows the reaction to proceed at lower temperature,
although carbene dimerization then seriously competes with cyclopropanation.33 In contrast,
complexes of type [LAu(CF3)] (10) themselves are thermally stable: no reaction was observed by NMR
upon heating of a mixture of 10a or 10b with E-stilbene in [D8]-toluene for 72 h at 100°C. As
expected, addition of TMSOTf or B(C6F5)5 to a solution of (Cy3P)AuCF3 (10b) in CH2Cl2 at 20°C gave
difluorocyclopropane 14 in 40% and 59% yield, respectively;34,35 in all reactions, competing formation
of volatile CO23 and/or tetrafluoroethylene (F = 131.5 ppm, 223K)36 by hydrolysis or dimerization of
the transient carbenoid, respectively, seem to make up for the rest. Surprisingly, however, use of the
analogous gold precatalyst 10a carrying the less donating Ph3P ligand instead of Cy3P furnished not
only cyclopropane 14 (15%) but also difluoro-olefin 15 (26%). Formation of this unanticipated by-
product mandates an 1,2-aryl shift, which in turn suggests that an intermediate of type C with
significant carbocation character has been passed through. Under this premise, it seemed
improbable that cyclopropane formation is a concerted process. Indeed, reaction of 10a/TMSOTf
with Z-stilbene furnished trans-14 (and 15);37 this result is again consistent with a stepwise pathway
in which a carbenium intermediate of type B opens the doorway to the thermodynamically more
stable cyclopropane.38 Such an ionic mechanism is strongly reminiscent of the reactivity of highly
electrophilic cationic iron carbenes vis-à-vis unsaturated compounds, which are known to involve
stabilized (e. g. benzylic) carbocations as key intermediates.39
In any case, formation of the rearranged difluoroolefin 15 lends further credence to the notion that
the cornucopia of gold catalysis is rooted, to a significant extent, in reactive intermediates featuring
considerable carbocationic character. The perhaps most notable manifestations thereof are
cyclization cascades of polyunsaturated substrates that obey the “Stork-Eschenmoser postulate”,40,41
as well as numerous reaction manifolds amalgamating -acid catalysis with classical reactivity of the
Wagner-Meerwein-, Friedel-Crafts-, Nazarov-, or Prins-type, to mention but a few.4-7,42
The first spectroscopic observation and chemical characterization of gold difluorocarbenoid
complexes summarized above suggests that stabilization of such intermediates by electron donation
7
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10.1002/anie.201903957
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from the metal to the empty carbene p-orbital is poor. As this deficiency is only partly compensated
by overlap with the lone pairs of the fluorine substituents, these reactive species are so deprived in
electron density that they retain even “weakly coordinating” anions such as triflate or triflimide. In
accord with this analysis, gold difluorocarbenoids were found to react with stilbene in a stepwise
rather than concerted fashion to generate benzylic carbocations in the first place as can be deduced
from the downstream speciation. Further studies into structure, bonding and reactivity of late
transition metal carbene complexes from this laboratory43 are underway and will be reported in due
course.
Acknowledgements
Generous financial support by the MPG is gratefully acknowledged. We thank G. Seidel for
exploratory studies and Mr. N. Nöthling, Mr. J. Rust and Dr. C. W. Lehmann for solving the X-ray
structures.
8
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For the Table of Contents
(Dis)Integrate: Gold carbenoids generated by Lewis-acid mediated -fluoride elimination from
LAuCF3 are so deprived in electron density that they even retain poorly coordinating counteranions
(triflate, triflimide); they disintegrate, however, on reaction with (E)- or (Z)-stilbene to give a
carbenium intermediate that accounts for the observed downstream chemistry.
Keywords: Carbene Complexes  Carbenium Ions  Cyclopropanes  Gold  NMR  Organofluorine
Chemistry
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Alternatively, constrained bidentate o-carborane diphosphines enforcing a tricoordinate ligand
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10
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10.1002/anie.201903957
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carbenes have so far hardly ever been implied in catalysis, see: M. Joost, L. Estevez, S. Mallet-
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17
The only isolated platinum difluorocarbene complex has a chelate structure in which the
difluorocarbene unit is stabilized by close intramolecular coordination to the N-atom of
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Chem. Int. Ed. 2010, 49, 4286-4289.
18
Indirect evidence suggests that a Au(+3) difluorocarbene accounts for the stoichiometric CC
bond formation observed on treatment of [IPrAu(CF3)2(Me] with B(C6F5)3, see: M. D. Levin, T.
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22
Other highly electrophilic metal carbenes form bridging carbene complexes; however, the high
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23
12
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24
The origin of the O-atom in 8 has not been firmly determined; it was proposed that the Et2O
ligand derived from BF3Et2O is the source of oxygen, although adventitious moisture must also be
considered.
25
26
M. Blaya, D. Bautista, J. Gil-Rubio, J. Vicente, Organometallics 2014, 33, 6358-6368.
The contact seen in 11 is close to the shorter end of Au···Au interactions, which typically range
from 2.75 to 3.4 Å and are stabilizing to about 5-10 kcal·mol–1; for a review see: H. Schmidbaur,
Chem. Soc. Rev. 1995, 24, 391-400.
27
a) A related heterobimetallic complex [(tBu3P)2Ag]+ [Au(CF3)2] is known which served as
substrate for gold/silver alloy nanoparticle synthesis; this complex lacks any notable AgAu
interactions, see: D. Zopes, C. Hegemann, J. Schläfer, W. Tyrra, S. Mathur, Inorg. Chem. 2015,
54, 3781-3787; b) for a related copper complex [(SIMes)2Cu]+[Cu(CF3)2], see: G. D. Dubinina, J.
Ogikubo, D. A. Vicic, Organometallics 2008, 27, 6233-6235.
28
Raising the temperature to about 20°C was necessary to ensure sufficiently rapid -fluoride
elimination; the exact timing is difficult and case-dependent: too short reaction times result in
incomplete conversion but too long stirring at 20°C causes significant decomposition.
R. H. Crabtree, The Organometallic Chemistry of the Transition Metals, 4th ed., Wiley,
Hoboken, 2005.
29
30
31
32
Treatment of complexes of type [LAuCHRCl] with excess AgOTf also leads to carbenoids
[LAuCH2OTf] retaining the triflate, cf. ref. 33a
Although the triflimide unit in 13 is tentatively drawn as being N-bound, this aspect has to
remain open.
a) E. G. Perevalova, E. I. Smyslova, K. I. Grandberg, Isv. Akad. Nauk SSR 1982, 2836 ; b) A. N.
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a) J. M. Sarria Toro, C. Garcia-Morales, M. Raducan, E. S. Smirnova, A. M. Echavarren, Angew.
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10.1002/anie.201903957
Angewandte Chemie International Edition
34
For a general review on difluorocarbene, see: C. Ni, J. Hu, Synthesis 2014, 46, 842-863.
Small amounts ( 10%) of difluoro-olefin 15 were also detected in the crude mixtures.
35
36
Significant amounts of tetrafluoroethylene were detected by NMR and confirmed by GC/MS in
the reaction promoted by TMSNTf2.
37
38
Traces of cis-14 were detected in the crude mixture by 19F NMR .
It has not been firmly established if conversion of B into 14 proceeds via “backside attack”, as
shown in Scheme 2; this drawing rests on the presumed analogy to the well-known behavior of
electrophilic iron carbenes, cf. ref. 39
39
40
a) M. Brookhart, G. O. Nelson, Organometallics 1984, 3, 650-652; b) M. Brookhart, Y. Liu,
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