Evaluation of the electronic properties of a carbodiphosphorane through gold catalysis

Article abstract of DOI:10.1002/adsc.201100287

Carbodiphosphoranes [C(PR₃)₂] are divalent carbon(0) derivatives which can be used as ligands to form either M←C(PR ₃)₂ or (M)₂← C(PR₃)₂ species. They were computationally predicted to be even stronger electron donors than N-heterocyclic carbenes. We have introduced hexaphenylcarbodiphosphorane [C(PPh₃)₂] for the first time in gold catalysis in order to validate this prediction by experimentation. Its mono- and digold complexes were compared to tris(2,4-di-tert-butylphenyl) phosphite, triphenylphosphine, and 1,3-bis(2,6-diisopropylphenyl)imidazol-2- ylidene in representative gold(I)-catalyzed transformations. The advantages and limitations of these ligands are discussed. Copyright

Full text of DOI:10.1002/adsc.201100287

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DOI: 10.1002/adsc.201100287
Evaluation of the Electronic Properties of a Carbodiphosphorane
through Gold Catalysis
Ahmad El-Hellani,a Christophe Bour,a and Vincent Gandona,*
a
ICMMO, UMR CNRS 8182, Univ Paris-Sud 11, 91405 Orsay cedex, France
Fax : (+33)-1-6915-4747; e-mail: vincent.gandon@u-psud.fr
Received: April 16, 2011; Published online: August 4, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adcs.201100287.
Abstract: Carbodiphosphoranes [C
(PR3)2] are
atom with two lone pairs of electrons (Scheme 1).[4]
The first member of this family was hexaphenylcarbo-
divalent carbon(0) derivatives which can be used
!
!
as ligands to form either M
CACHTUNGTRENNUNG
C
(PR3)2 or (M)2
diphosphorane [C
ity to serve as ligand was shown in 1973 after the
characterization of [W(CO)5{C
(PPh3)2}].[6] In the
same paper, C(PPh3)2 was also described as Ph3P-sta-
(PPh3)2], reported in 1961.[5] Its abil-
ACHTUNGTRENNUNG
dicted to be even stronger electron donors than N-
heterocyclic carbenes. We have introduced hexa-
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
phenylcarbodiphosphorane [C
(PPh3)2] for the first
bilized carbon(0) with two lone pairs, suggesting that
such a ligand could bear two metals. The proof of
concept was demonstrated in 1976, with the synthesis
time in gold catalysis in order to validate this pre-
diction by experimentation. Its mono- and digold
complexes were compared to tris(2,4-di-tert-butyl-
phenyl) phosphite, triphenylphosphine, and 1,3-
bis(2,6-diisopropylphenyl)imidazol-2-ylidene in rep-
resentative gold(I)-catalyzed transformations. The
advantages and limitations of these ligands are dis-
cussed.
(PMe3)2}].[7] Until 2006, a few more
of [(AuMe)2ACTHUNRGTENNUG{m-CCAHTUNGTRENNUGN
carbodiphosphorane-complexes of rhenium,[8] rhodi-
um,[9] nickel,[10] palladium,[9] platinum,[11] copper,[12,13]
and silver[12,13] were described, as well as the mono-
and digold species [AuCl{C
N
2ACHTUNGTRENNUNG{m-
C
N
ACHTUNGTRENNUNG
reported in 2006,[15] and the theoretical prediction of
carbodicarbenes in 2007,[16] created a renewed interest
for the coordination chemistry at carbon(0), and vari-
ous complexes of lithium,[17] boron,[18] ruthenium,[19]
rhodium,[20,21] iridium,[21] palladium,[9] copper,[22]
silver,[23] and gold[22,24] were synthesized. New theoreti-
cal studies aimed at understanding the bonding situa-
Keywords: carbenoids; carbodiphosphoranes; gold;
ligand effects; phosphane ligands
Tuning the stereo-electronic properties of catalyst li- tion and predicting the reactivity of carbon(0) deriva-
gands is a critical feature to ensure a higher activity tives were reported simultaneously.[25] All of this ex-
and to control the selectivity of a given reaction. One perimental and theoretical work converged toward
emblematic example of the impact of the ligand’s the idea that carbones are stronger donors than
nature on selectivity is the olefin metathesis catalyzed NHCs. Yet, to the best of our knowledge, these li-
by Grubbs first generation Ru-phosphine complexes gands have been tested in catalysis only once,[22] and
versus second generation NHC-based catalysts.[1]
A
wide array of ligands is available to chemists nowa-
days, including phosphites, phosphines, NHCs,
CAACs, NACs, and many others.[2] Due to their p-ac-
ceptor capabilities, phosphites are weaker electron
donors to metals than phosphines. On the other hand,
NHCs, which display a pseudo-filled p orbital, are
stronger donors than phosphines.[3] Beyond NHCs,
more powerful s-donors with even reduced p-accept-
or character have been designed.[2] Among these, car-
bodiphosphoranes and carbodicarbenes, also referred
Scheme 1. Resonance forms of carbodiphosphoranes and
to as carbones or bent allenes, feature a carbon(0) carbodicarbenes.
Adv. Synth. Catal. 2011, 353, 1865 – 1870
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Ahmad El-Hellani et al.
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Table 1. Au(I)-catalyzed benzylation of anisole.
Entry
[Au]
Yield[a] [%] (1/2)[b]
1
2
3
4
N
G
96 (42/58)
97 (43/57)
97 (43/57)
95 (38/62)
ACHTUNGTRENNUNG
N
ACHTUNGTRENNUNG
[a]
Isolated yields.
From H NMR.
5 mol%.
[b]
[c]
[d]
1
2.5 mol%.
Figure 1. Precatalysts used in this study and %Vbur of the
ligand when located 2 ꢁ away from the metal.
As expected, virtually no difference in the products
ratio was observed along this series.
To test the carbophilicity of the carbodiphosphor-
not with respect to their electronic properties. We ane-gold complexes, the hydroarylation of ethyl pro-
have carried out typical gold-catalyzed transforma- piolate by mesitylene was carried out next
tions[26] to evaluate the donor ability of a carbone (Table 2).[32] While all gold complexes gave the mono-
compared to other ligands.
The mono- and
[AuCl{C(PPh3)2}] and [(AuCl)
chosen as precatalysts and their selectivity compared used, one could expect that the most basic ligands
to (AuCl{P[O-(2,4-
(t-Bu)2C6H3]3})[27] and the commer- favor the reaction. This was actually shown in the
cially available [AuCl(PPh3)], and [AuCl (PEt3)] is a more active
(IPr)][28] original paper where [AuCl
[IPr=1,3-bis(2,6-diisopropylphenyl)-imidazol-2-yli- precatalyst than [AuCl
(PPh3)].[32] This trend seems to
dene] as prototypes of phosphite, phosphine, and car- be followed in our case as well, with PPh3 <IPr<
hydroarylation product as the predominant one, the
dinuclear
complexes dinuclear catalyst clearly outperformed the others in
A
E
U
A
ACHTUNGTRENNUNG
A
R
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
bene gold complexes (Figure 1).
Hexaphenylcarbodiphosphorane CAHCUTNGTRENNUNG
C
G
2ACHUTNGTREN{NGU m-CAHCUTGNTREN(NUGN PPh3)2}], the s-dona-
[29]
as the gold complexes [AuCl{C
[(AuCl)2A{m-C
ACHTUNGTRENNUNG
G
E
N
ACHTUNGTRENNUNG
literature procedures. To test the efficiency of these
species, selected gold(I)-catalyzed transformations
were carried out. In each case, the precatalysts were
Table 2. Au(I)-catalyzed hydroarylation of ethyl propiolate.
mixed with AgSbF6 in a 1:1 ratio for [AuCl{C
and in a 1:2 ratio with [(AuCl)2A{m-C(PPh3)2}] to gener-
ACHTUNGTRNE(NUNG PPh3)2}]
C
ACHTUNGTRENNUNG
ate the corresponding cationic and dicationic species.
For the sake of comparison, the same amount of gold
was maintained when using either [AuCl{C
ACHTUNGTRNE(NUNG PPh3)2}]
or [(AuCl)2A{m-C(PPh3)2}]. We began our investigations
C
ACHTUNGTRENNUNG
by probing the oxophilicity of the carbodiphosphor-
ane-complexes. A Friedel–Crafts reaction between
anisole (used as solvent) and benzyl alcohol was car-
ried out,[30,31] leading to ortho- and para-substitution
(Table 1). In such a reaction, the nature of the ligand
should have little influence on the product distribu-
tion, yet the result should be informative nonetheless
on the potential of Au-carbone complexes to serve as
catalysts. Gratifyingly, these species proved as effi-
cient as the phosphine- and carbene-complexes, pro-
viding the benzylation products in very high yields.
Entry
[Au]
Yield[a] [%] (3/4)[b]
1
2
3
4
A
N
R
G
R
R
78 (96/4)
81 (98/2)
30 (90/10)
quant (95/5)
E
G
ACHTUNGTRENNUNG
[a]
Isolated yields after 4 h, also correspond to conversions
(no degradation of the substrates).
From H NMR.
2 mol%.
1 mol%.
[b]
[c]
[d]
1
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Adv. Synth. Catal. 2011, 353, 1865 – 1870

Evaluation of the Electronic Properties of a Carbodiphosphorane through Gold Catalysis
gesting a limitation due to excessive electron-enrich- the starting material was reached in each case, except
ment. One could also argue that the metal center in with [AuCl{C
[Au{C
(PPh3)2}]+ is more sterically congested than in the crude mixtures, the expected compound 6 was ob-
[Au
(PPh3)]+ or [Au(IPr)]+. To shed light on this served, as well as exo and endo cycloisomers of 5.
matter, the percentage of buried volume (%Vbur) was We then focused on the intramolecular [4+2]cyclo-
estimated for each ligand (Figure 1).[33] In fact, with a addition of an arylenyne displaying an electron-rich
%Vbur of 44.7, C(PPh3)2 surrounds the metal almost alkene moiety (Table 4). It was shown that the cycli-
ACHTUNGTRNE(NUNG PPh3)2}] which proved unreactive. In
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
as much as IPr.[33b,34] For the dinuclear complex, be- zation of this compound is not stereoselective, and
cause the PCP angle is more acute, an estimate of that the use of a phosphine-complex leads to the trans
41.3 was obtained.[35] However, the free space of the stereoisomer as major component of the mixture,
sphere will have to be occupied by two gold atoms; whereas a phosphite-based catalyst rather furnishes
therefore the %Vbur of each metal center, which the cis stereoisomer.[39] These results were rational-
cannot be computed, will be obviously higher. Thus, ized by computing the bond distances of the putative
in spite of the quite high shielding of the gold atoms, intermediate. With a stronger donor ligand, the car-
the dinuclear complex is the most active of the series, benic structure dominates, leading to the trans prod-
and the lower activity of [AuCl{C
plained by the reduced electrophilicity of the alkyne- favor the open cationic structure. Because bond rota-
gold intermediate.[36]
tion is somewhat faster than nucleophilic attack of
ACHTUNGTREN(NUNG PPh3)2}] can be ex- uct. On the other hand, a weaker donor ligand would
To gain more insights, we selected LAu(I)-catalyzed the aromatic ring, the amount of cis product may in-
transformations for which the electronic parameters crease in this case. Using the described procedure for
of the ligand are known to play a major role. For in- such a reaction, i.e., by carrying it out under micro-
stance, in the intermolecular cyclopropanation of a wave irradiation, the cyclization products were
terminal 1,6-enyne with styrene, it was noticed that a formed in high yields. As expected, the weakest trans
phosphite-containing precatalyst gave the best results, selectivity was observed with the phosphite ligand
the transient gold-carbene being in this case more
electrophilic.[27,37] Accordingly, we found that the se-
Table 4. Au(I)-catalyzed intramolecular [4+2]cycloaddition
of an aryl-1,6-enyne.
lectivity for this process decreases as the metal be-
comes electron-richer (Table 3).[38] Full conversion of
Table 3. Au(I)-catalyzed intermolecular cyclopropanation of
a 1,6-enyne.
Entry [Au]
Yield of 6[a] [%] (cis/trans)[b]
Entry
[Au]
Yield[a] [%] (cis/trans)[b]
1
2
3
4
5
N
N
63 (2.7:1)
57 (1.5:1)
29 (1.7:1)
E
R
1
2
3
4
5
A
N
N
T
R
92 (1:1.7)
84 (1:3.2)
85 (1:4.6)
94 (1:5.7)
90 (1:4.3)
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
[e]
–
(PPh3)2}][d] 19 (3.7:1)
G
[a]
[b]
[c]
[d]
[e]
Isolated yields (full conversion).
From H NMR.
4 mol%.
2 mol%.
1
[a]
Isolated yields.
1
[b]
[c]
[d]
From H NMR (same ratio after purification).
2 mol%.
1 mol%.
No conversion.
Adv. Synth. Catal. 2011, 353, 1865 – 1870
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1867

Ahmad El-Hellani et al.
COMMUNICATIONS
(entry 1), and the highest with the monogold complex
In conclusion, we have shown that the gold com-
(entry 4).[40] This time, the digold complex proved less plexes of C
ACHTNUTRGNEUGN(PPh3)2 are valuable precatalyst for various
selective than the carbene-containing one. Neverthe- Au(I)-catalyzed transformations. The selectivities ob-
less, the result obtained with the monogold species re- served strongly suggest that this ligand can be used,
inforces the idea that CACHTUGNTRENNNUG
compared to IPr.
As an additional test reaction, we studied the cyclo-
CACHTUNGTRENNUNG
isomerization of eneallene 10. In this type of transfor- electron donation is partitioned between two metallic
mation, it was shown that the ancillary ligand exerts a centers, but also because a dual activation mechanism
dramatic influence on the outcome (Table 5).[41] As can be the operative one. We are now extending our
described, we obtained mixtures containing the [2+ study to carbodicarbene species to get a better over-
2]cycloadduct 11 and the regioisomeric [3+2]cyclo- view of the potential of these new gold complexes in
adducts 12 and 13. Under the experimental reactions catalysis.
conditions used, we noticed an increase in the ratio of
11 as the metal becomes more electron-rich. This
could be explained by a better stabilization of the car- Experimental Section
bocationic center of the vinylgold intermediate, lead-
Complete experimental procedures and full characterization
data are given in the Supporting Information.
ing to direct vinylation as previously reported in the
bis-allene series.[42] Here, the selectivity for the [2+
2]cycloadduct is much higher with the monogold
complex. Again, the digold species was found to be
less selective than the IPr complex.
Representative Procedure for the Au(I)-Catalyzed
Intramolecular [4+2]Cycloaddition of 7
A 5-mL microwave sealed tube was charged with the
gold(I) chloride (2 mol%, 0.0054 mmol), AgSbF6 (2 mol%,
1.86 mg, 0.0054 mmol) and CH2Cl2 (1 mL). The mixture was
stirred for 5 min to generate the cationic species. Then 7
(100 mg, 0.27 mmol) in CH2Cl2 (1 mL) was added and the
solution was heated at 808C under microwave irradiation
for 10 min. The mixture was passed through a short pad of
silica. The solvent was evaporated and the mixture of prod-
ucts 8 and 9 was separated from the starting material by
column chromatography (heptane/EtOAc, 94/6).
Table 5. Au(I)-catalyzed cycloisomerization of an eneallene.
Acknowledgements
A. E.-H. is thankful to UPS for a PhD grant (chaire d’excel-
lence). We also deeply thank Prof. Josꢀ Vicente and Prof.
Wolfgang Petz for their helpful advice for the synthesis of C-
AHCTUNTGRENN(GNU PPh3)2 and the corresponding gold complexes.
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ACHTUNGTRENNUNG
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ACHTUNGTRENNUNG
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G
ACHTUNGTRENNUNG
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[38] A similar product was tested by Echavarren (see
ref.[27]) with the same [AuCl(P
ACHTUNGTRENNUNG
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Ahmad El-Hellani et al.
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giving the products in 67% isolated yield (3.3:1 cis:-
trans ratio).
[39] E. Jimꢆnez-NfflÇez, C. K. Claverie, C. Bour, D. J. Cꢄrde-
nas, A. M. Echavarren, Angew. Chem. 2008, 120, 8010;
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[40] The cis:trans selectivity is different from that reported
by Echavarren with the phosphite ligand (1.7:1). How-
ever, in our case, the cationic species was generated in
situ with AgSbF6, which was not the case in ref.[39]
Changes in selectivity between Au/Ag and silver-free
protocols have been noted therein.
[41] a) M. Alcarazo, T. Stork, A. Anoop, W. Thiel, A. Fꢂrst-
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1870
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2011, 353, 1865 – 1870