Bis-phosphine allene ligand: Coordination chemistry and preliminary applications in catalysis

Article abstract of DOI:10.1039/c6cc02316a

A 1,3-bis-diphenylphosphine allene can give rise to new coordination complexes with palladium, platinum and gold metals. These complexes were fully characterized by NMR, HRMS and X-ray diffraction analysis. For gold(i), the corresponding dinuclear complex

Full text of DOI:10.1039/c6cc02316a

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Bis-phosphine allene ligand: coordination
chemistry and preliminary applications in
Cite this:DOI: 10.1039/c6cc02316a
catalysis†‡
Received 16th March 2016,
Accepted 12th April 2016
Avassaya Vanitcha,a Cecilia Damelincourt,a Geoffrey Gontard,a Nicolas Vanthuyne,b
Virginie Mouries-Mansuy*a and Louis Fensterbank*a
`
DOI: 10.1039/c6cc02316a
www.rsc.org/chemcomm
A 1,3-bis-diphenylphosphine allene can give rise to new coordination
complexes with palladium, platinum and gold metals. These com-
plexes were fully characterized by NMR, HRMS and X-ray diffraction
analysis. For gold(I), the corresponding dinuclear complex has been
used in a series of diagnostic catalytic reactions and gave promising
preliminary results in asymmetric catalysis.
Scheme 1 New prospects in coordination chemistry with allene ligands.
Coordination chemistry has been at the center of chemists’
preoccupations since the pioneering works of Christian Wilhem
Blomstrand, Sophus Mads Jorgensen and Alfred Werner in the
19th century. This very fundamental and intense domain of
investigation has continuously irrigated several fields of applica-
tions such as catalysis, material sciences, and supramolecular
and medicinal chemistry.1 Based on the interaction between a
ligand and a metal center, the game of combining both compo-
nents appears unlimited and the number of existing coordina-
tion adducts is still restricted, leaving a vast unexplored chemical
space. Nevertheless, a rational design is desirable to guarantee the
preparation of materials with optimized properties. This approach
generally requires one to start from elementary building blocks
(ligand or metal) with particular attributes.
the highly enantioselective formation of chlorohydrins from
meso-epoxides with SiCl4.4 Later, the same group devised chiral
allene-containing bisphosphines that, when coordinated to Rh(I),
were able to promote the asymmetric addition of arylboronic
acids to a-keto esters with high enantioselectivity.5
In line with this strategy, our attention was drawn to the
bis-phosphine 1,3-bis(diphenylphosphino)-1,3-diphenylallene
(1) featuring direct attachment of the phosphorus moieties on
the allene scaffold, which has been so far very rarely encountered.6
The Schmidbaur group previously described the synthesis of 1.7
Following their procedure, we obtained 1 in a consistent yield
of 48% (Scheme 2).
Thus, thanks to its unique stereoelectronic features, an allene
scaffold bearing a Lewis base and possibly conveying some chiral
information sites appeared to us as a valuable keystone (Scheme 1).2
As a further incentive, there has been, to the best of our knowledge,
a very limited number of allene-derived ligands involved in metallic
coordination complexes. Krause reported the formation of silver
and copper complexes from allene-containing bipyridine ligands,
but no catalytic activity was reported.3 Chiral diphosphine oxide
allenes were first used as organocatalysts by Ready who described
We then studied the unexplored coordination properties
of 1 and focused on the possibility of obtaining mononuclear
palladium(II) and platinum(II) complexes. Thus, heating at 80 1C
for 4 h, a 1 : 1 mixture of bis(acetonitrile)dichloropalladium and 1
in toluene afforded the coordination complex 3 (84% yield) that
was isolated as an orange solid after precipitation in cold ether.
31P NMR showed a singlet peak resonance at dP = 128.3 ppm, very
downfield compared to the starting allene 1 (dP = 33.2 ppm) and the
13C peak of the central allene carbon was observed as a triplet peak
(t, 2JCP = 3.8 Hz) at 193.3 ppm (d = 209.9 ppm, d, 2JCP = 3.5 Hz for 1).
a
´
Sorbonne Universites UPMC Univ Paris 06, CNRS, UMR 8232, Institut Parisien de
´
Chimie Moleculaire, 4 place Jussieu, C.229, F-75005 Paris, France
b
´
Aix-Marseille Universite, Centrale Marseille, CNRS, iSm2 UMR 7313, 13397,
Marseille, France
† Dedicated to Antonio Echavarren.
‡ Electronic supplementary information (ESI) available. CCDC 1448365 and
1448367–1448371. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c6cc02316a
Scheme 2 Allene bisphosphine 1 and its bis-seleno derivative 2.
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Scheme 3 Pd and Pt mononuclear complexes with ligand 1.
An initial crystallization attempt of the bis-P-coordinated
Pd allene complex 3 in MeOH provided suitable material for
a single-crystal X-ray diffraction (XRD) analysis. Interestingly,
the latter showed a co-crystallized mixture of MeOH and HCl
adducts on 3 (see ESI‡). While these findings strongly suggested
the formation of the Pd–allene complex 3, its structure was fully
confirmed after XRD analysis of crystals grown in a CH2Cl2/
cyclohexane mixture (Scheme 3).8 A square planar coordination
was observed. More striking was the severely bent character of
the allene moiety, exhibiting a CQCQC bond angle of 151.2(2)1.
The C1–C2 and C2–C3 bond lengths of respectively 1.303(3) and
1.315(3) Å lie in the typical range observed for cumulenes.9
Similarly, platinum complex 4 was formed quantitatively.
31P NMR shows a central resonance at dP = 98.7 ppm with two
Scheme 4 Mononuclear and dinuclear gold(I) complexes.
exclusively on the phosphine moieties, leaving the allene unaltered.
The two Au atoms lie in opposite direction, with an Au–Au
distance of 6.498(1) Å precluding any aurophilic interaction.19
The distance between the Au atom and the phenyl group of the
allene moiety is, on average, 3.71(2) Å suggesting no Z1 or Z2
interaction as described for the JohnPhos gold(I) chloride
complex and congeners.20
The evaluation of the electronic properties of allene 1 was
1
achieved by measuring the magnitude of JP–Se for the 77Se
isotopomer21 of the corresponding bis-selenide derivative 2
1
small satellite peaks due to 195Pt (1J
= 4371 Hz). Here
(Scheme 2). With a JP–Se value of 754 Hz, allene 1 exhibits an
31P–195Pt
intermediate s-donor ability, lower than that of PPh3 (1JP–Se
=
also, the 13C peak of the allene central carbon is more shielded
(d = 198.9 ppm, 2JCP = 5.1 Hz). The XRD analysis8 confirmed the
structure and also showed a square planar coordination with a
similar CQCQC bent angle of 151.8(4)1 (see ESI‡).
728 Hz) but much higher than that of P(PhO)3 (1JP–Se = 1027 Hz).22
We then evaluated the catalytic activity of gold complex rac-7
using the prototypical N-tethered 1,6-enyne precursor 8a. The
reaction was carried out in CH2Cl2 with a catalytic 1 : 1 mixture
of rac-7 (3 mol%) and AgSbF6 to afford diene 9a in 74% yield.
When 1,6-enyne 8b was treated under the same conditions, only
starting material was recovered even under refluxing conditions. An
additional 3 mol% of silver salt gave the expected product 9b in
52% yield. It is noteworthy that when 2 equiv. of AgSbF6 (6 mol%)
were introduced right from the start of the reaction, compound 9a
was obtained in similar yield as with 3 mol% but we observed
a better yield (85%) for compound 9b (Scheme 5).
A few bent allenes have previously been reported10 though
there has been some controversy in assigning an allenic character
to some of them.11 The most structurally related example to 3 or 4
corresponds to cyclic allene 5, previously isolated by Regitz and
exhibiting a CQCQC angle of 155.81.6c While palladium complex
3 appears quite stable, platinum complex 4 decomposed in a few
days upon standing on the bench at room temperature.
These preliminary findings validated the coordination ability
of allene 1 so we looked at the possible formation of gold
complexes in connection with our interest in gold catalysis.12,13
Our initial attempt focused on the formation of a tricoordinate
mononuclear complex as more and more examples14 of this
coordination mode have been reported and shown intriguing
properties.15 Interestingly, when mixing 1.1 equiv. of 1 with
1 equiv. of Me2SAuCl in CH2Cl2 at ꢀ20 1C, a new species
(dP = 12.5 ppm, broad peak) was selectively formed. The XRD
analysis revealed the polymeric gold complex 6.16,17 While the
properties of these macromolecular complexes will be studied,
we turned our attention to dinuclear complexes.
As previously reported in the literature23 and by us,24 we
suspected the detrimental formation of a chloride-bridged digold
complex. The latter was detected by ESI MS at 20 V (see ESI‡) from
an equimolar mixture of 1 and AgSbF6 in CH2Cl2, as attested by
+
the presence of a peak at m/z 989 (C39H30Au2ClP2 ). This complex
would be cleaved by more coordinating substrates like 8a or by
Thus, treatment of 1 with 2 equiv. of Me2SAuCl in CH2Cl2
provided quantitatively the chiral gold(I) complex rac-7. The
latter proved to be quite stable to air and moisture. Its structure
was also confirmed by XRD studies (Scheme 4).17 Despite the
known carbophilicity of gold(I) salts,18 coordination took place
Scheme 5 Preliminary catalytic testing.
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Scheme 7 Resolution and asymmetric catalysis.
by preparative HPLC remains limited,30 and to the best of our
knowledge none with gold,31 we tried this method on the gold
chloride complex rac-7. Gratifyingly, the enantiomers of complex
rac-7 were nicely separated by analytical chiral chromatography
on a Chiralpak IE column. Each enantiomer was obtained with
high ee of 98% and their absolute configuration determined
by anomalous XRD.32 We then evaluated the catalytic activity of
(aR)-(+)-7 for the asymmetric cycloisomerisation of 1,6-enynes 8b
and 20. The expected azabicyclo[4.1.0]heptenes 9b and 21 were
obtained in good yields and with promising ee values of 29 and
32%, respectively (Scheme 7).
This journey in the coordination of bis-phosphine allene 1
with palladium, platinum and gold salts has allowed the
formation of highly novel and original coordination complexes.
Physico-chemical properties of these complexes will be pursued.
Focusing on gold(I) salts, we could generate a new type of chiral
dinuclear gold precatalyst. The precatalyst rac-7 in the presence
of a silver salt proved to be very active and robust in a series
of prototypical gold-catalyzed reactions. Investigation of the
electronic properties suggests a moderately electrophilic gold
complex which was corroborated by catalytic results. Preliminary
testing of the corresponding optically pure complex obtained by
preparative chiral HPLC gave promising hits for asymmetric
catalysis. It is anticipated that fine electronic and steric tuning
of the allene bis-phosphine ligand by appropriate substitution
should boost the catalytic properties.
Scheme 6 Scope of catalysis with rac-7.
the addition of an excess of silver salt yielding a dicationic gold
catalytic species.
Following the previously defined reaction conditions, we
extended the investigation of the electrophilic catalytic proper-
ties of rac-7 by examining the reactivity of several representative
polyunsaturated substrates. Thus, enyne 10 provided regio-
25
selectively cyclopentadienic dienes 11a,a0 in 70% yield with
no cyclohexadienic product.13c Running the same reaction in
methanol as solvent did not affect the catalytic activity and gave
smoothly methoxycyclization adduct 12 as a single regioisomer.25
The quantitative conversion of enyne 13 into tricyclic derivative 14
confirmed the robustness of this catalytic system.26
Intermolecular gold-catalyzed additions offer interesting
synthetic opportunities. We evaluated the intermolecular cyclo-
propanation reaction between propargyl pivalate 15 and styrene
and were pleased to obtain cyclopropane adduct 16 in 60%
yield and with improved diastereoselectivity (420 : 1) in favor of
the cis-isomer than previously reported (46 : 1).27 The cyclo-
isomerization of allenediene 17 served as a very informative
probe. It showed that the chemoselective activation of an allene
was possible with our allene-based catalytic system since a 4 : 1
mixture of [4+2] and [4+3] cycloadducts 18 and 19 was obtained
in 67% yield. The major formation of 18 suggests a rather
electron-depleted catalytic species, more electrophilic than with
triphenylphosphine as ligand which gives a 2 : 1 ratio of 18
and 19.28 This product distribution is thus consistent with
our preliminary evaluation of the electronics with bis-seleno
derivative 2 (Scheme 6).
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