Synthesis of Allenes Bearing Phosphine Oxide Groups and Investigation of Their Reactivity toward Gold Complexes

Article abstract of DOI:10.1002/adsc.201500345

A collection of phosphine oxide allenes has been prepared. Their coordination to gold(I) has been studied giving new coordination complexes when a pendant pyridine moiety was present. Alternatively, their gold(I)-catalyzed cycloisomerization has proven to

Full text of DOI:10.1002/adsc.201500345

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DOI: 10.1002/adsc.201500345
Synthesis of Allenes Bearing Phosphine Oxide Groups and
Investigation of Their Reactivity toward Gold Complexes
Avassaya Vanitcha,a Geoffrey Gontard,a Nicolas Vanthuyne,b Etienne Derat,a
Virginie Mouri›s-Mansuy,a,* and Louis Fensterbanka,*
a
Sorbonne UniversitØs UPMC Univ Paris 06, UMR CNRS 8232, Institut Parisien de Chimie MolØculaire, 4 place Jussieu,
C.229, F-75005 Paris, France
Fax: (+33) 1-4427-7360; e-mail: virginie.mansuy@upmc.fr or louis.fensterbank@upmc.fr
Aix-Marseille UniversitØ, Centrale Marseille, CNRS, iSm2 UMR 7313, 13397, Marseille, France
b
Received: April 6, 2015; Revised: May 21, 2015; Published online: July 14, 2015
Dedicated to Prof. Steve Buchwald on the occasion of his 60th birthday.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500345.
Abstract: A collection of phosphine oxide allenes
has been prepared. Their coordination to gold(I)
has been studied giving new coordination com-
plexes when a pendant pyridine moiety was present.
Alternatively, their gold(I)-catalyzed cycloisomeri-
zation has proven to be quite efficient. An almost
complete axial-to-central chirality transfer in the
cyclization process was observed, opening the po-
tential access to valuable enantiopure phosphorus
derivatives.
cient catalyst for the enantioselective arylation of a-
keto esters (Scheme 1).[5]
Because of our long-standing interest in gold catal-
ysis,[6] we wished to investigate the reactivity of new
allene scaffolds bearing phosphine oxide groups as co-
ordinating units. Coordination of gold(I) to allenes
has indeed been theoretically studied in order to ra-
tionalize some reaction pathways.[7] In addition, some
coordination complexes have also been isolated and
characterized by X-ray crystallography. Fürstner iso-
lated a complex between tetradimethylaminoallene
Keywords: allenes; gold carbenes; gold catalysis;
phosphines
Allenes are fascinating molecular substrates. Their
specific electronic configuration coupled to their ex-
tended tetrahedral spatial arrangement guarantee spe-
cial and valuable properties.[1] When substituted by
Lewis base substituents, the allene assembly can serve
as a ligand,[2] possibly conveying axial chirality. Illus-
tration of this principle was initially provided in 2008
by Krause, who reported the preparation of allenic bi-
pyridines I and the corresponding silver and copper
complexes.[3] In 2009, Ready developed chiral phos-
phine oxide allenes II as versatile organocatalysts for
the asymmetric ring opening of meso-epoxides.[4] The
same group then further adapted the parent structure
II to append phosphine groups bearing CF3 electron-
withdrawing groups in place of the phosphine oxide
ones. The resulting ligand could coordinate to Rh(I),
featuring coordination to the allene moiety, and pro-
vide complex III, which proved to be a highly effi-
Scheme 1. Allene ligands and complexes.
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Scheme 3. Formation of gold chloride complexes 11a and
11b.
lyzed SN2’ process which allows the introduction of
various R2 groups and furnishes allenes 5–7.[1a,b,11] Al-
ternatively, we also used the Pd(0) coupling reaction
between a zinc reagent and propargyl acetate 2 to
obtain 8 and 9, as well as pyridyl allenes 10a and 10b
from acetates 4a and 4b.[3] It should be noted that 10a
could also be prepared in slightly better yield (41%),
based on the addition of PyZnBr to 2.
Our initial attempts to obtain new allene-gold com-
plexes consisted in mixing 1 equiv. of allenes 5–7 with
1 equiv. of Me2SAuCl. However, we could not isolate
any complex from these substrates. So we turned our
attention to pyridyl-allenes 10a and 10b which incor-
porate an additional stronger Lewis base site of coor-
dination. In the presence of Me2SAuCl, a quantitative
reaction took place delivering gold complexes 11a
and 11b for which the structures were secured by
single-crystal X-ray diffraction (Scheme 3 and
Figure 1).[12] Careful examination of the literature
provided pieces of rationalization for the unforeseen
formation of complexes 11a and 11b. Gevorgyan
showed that similar allenylpyridine units could be ac-
tivated by gold providing indolizines through nucleo-
Scheme 2. Preparation of allene ligands.
and Ph3PAuSbF6 featuring h1 central coordination[8] philic addition of the pyridine nitrogen on the electro-
while Widenhoefer has also reported an X-ray crystal philic allene gold complex.[13] Quite recently, Kumar
structure of the h2 dimethylallene-Au+PPh3 com- and Waldmann described a synthesis of cyclic alkyl
plex.[9]
aminocarbene gold(I) complexes from the addition of
Our objective was to use allenylphosphine oxides Me2SAuCl to 1,7-enynes.[14]
IV as probes for two types of applications. The first
Post-functionalization of vinylgold complexes has
aim was the generation of new gold coordination been documented in the literature.[13b,15] However,
complexes, possibly implying the phosphine oxide
moiety.[10] These could be new precatalytic species or
models of proposed intermediates. The second objec-
tive was to involve these substrates in cycloisomeriza-
tion reactions when using appropriate R substituent
on IV in order to deliver new phosphorus-containing
chiral substrates.
Allenylphosphine oxide precursors were easily ob-
tained in a few steps as shown in Scheme 2. Ketone
1 is smoothly alkylated by the lithium anion of meth-
yldiphenylphosphine oxide. The resulting alcohol is
then acylated (2), setting the stage for a copper-cata- Figure 1. Crystal structures of complexes 11a and 11b.
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Synthesis of Allenes Bearing Phosphine Oxide Groups
Figure 2. Summary of calculations for complex 11b: (A) Mayer bond order, (B) molecular orbital showing the delocalization
between AuCl and carbene, (C) electrostatic potential (blue: positive; red: negative).
Table 1. HOMO and LUMO energy (in eV) for various gold
complexes 11a and 11b proved quite unreactive to
treatment with triflic acid to promote protodeaura-
tion,[15d] addition of electrophilic halide sources (I2,
NIS), and attempts of palladium couplings with aryl
halides.[16] This excited our curiosity and led us to LUMO
ligands.
PPh3
DIPP-NHC
SPhos
11b (no AuCl)
À0.77
À5.55
0.82
À5.75
À0.52
À4.85
À1.11
À4.55
HOMO
taking a closer look at the crystal structures of 11a
and 11b. Interestingly, gold carbenes without a stabiliz-
ing heteroatom in the a position have been isolated
À
only recently.[17] The Au C bond distances of ceptor. To further substantiate these findings, compar-
1.993(4) Š for 11a and 1.984(2) Š for 11b are in the isons were made with other classical gold ligands.
low range of previously isolated gold carbenes,[18] sim- Namely we compared the HOMO/LUMO energies of
À
ilar to the shortest 1.984(4) Š Au C bond so far ob- a typical phosphine (PPh3), a standard carbene
served by Miqueu, Amgoune and Bourissou for an o- (DIPP-NHC), a p-accepting phosphine (SPhos, Buch-
carborane diphenylphosphine gold(I) diphenylcarbene wald ligand[19]) and our new carbene. The data are
complex.[17d]
summarized in Table 1.
To get more insight into the mesomeric structures
It appears from Table 1 that the carbene derived
11a and 11b (vinylgold vs. gold carbene), we per- from 11b is more electrophilic than the usual carbene
formed DFT calculations using the Turbomole pro- (DIPP-NHC) and in the same range as a phosphine,
gram package (V6.4). B3LYP, supplemented by the especially the SPhos or Buchwald phosphine.
D3 correction which was used as functional and the
We then wanted to test the generality of the forma-
def2-SV(P) as basis set. Namely, structure 11b was op- tion of gold complexes such as 11a and 11b. We used
timized starting from X-ray crystallographic data. In a cationic gold precursor and exposed substrate 10b
order to better understand the electronic structure of to 0.7 equiv. of a 1:1 mixture of Ph3PAuCl and AgOTf
this unusual gold-carbene complex, various theoreti- (Scheme 4) in refluxing dichoroethane for 18 h. The
cal analyses were conducted. First, using the Mayer corresponding complex 12b was obtained in fair yield
bond order analysis (Figure 2A), it was possible to (63%) exhibiting characteristic spectral data by
conclude that the main mesomeric form for complex 31P NMR as shown in Scheme 4 and also a 13C NMR
2
11b is the vinylgold one, as written in Scheme 3. The resonance at 203 ppm (doublet, JC,P =114 Hz) diag-
specific reactivity of this complex can be better under- nostic of a vinylgold species.[20] We also tried to fur-
stood by looking at one of the orbitals involving inter- ther functionalize this complex. Protodeauration with
actions between the AuCl fragment and the carbene triflic acid smoothly provided indolizinium salt 13,
part (Figure 2B). p-bonding between gold and chlo- whose structure was confirmed by X-ray crystallogra-
ride is also delocalized on the pyridinium moiety of phy,[12] while iodolysis gave vinyliodine derivative 14.
the carbene, thus enhancing what is traditionally de-
Complexes 11a, 11b and 12b are chemical cul-de-
scribed as the back-donation. The fact that the car- sacs with no possible evolution in these reaction con-
bene is a good electron acceptor is further confirmed ditions. We hypothesized that the intervention of a nu-
by plotting the electrostatic potential (Figure 2C). cleophile on the activated allene that liberates
À
While the Au Cl fragment is found to be electron- a proton would trigger a protodeauration and regen-
rich, the in-situ created carbene moiety has a positive erate the gold catalyst. We therefore examined the re-
electrostatic potential and is thus a good electron-ac- activity of allenes 5 and 6 in the presence of a catalytic
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Scheme 5. Catalytic reactions.
Scheme 4. Ph3PAu–vinyl gold complex and reactivity.
with gold(I) salts. With a pendant pyridine moiety,
quantity of gold catalyst. However, these reactions some new examples of gold carbene complexes with
proved to be rather sluggish giving among a mixture no adjacent heteroatoms have been isolated and fully
of dienic products 15 and 16 as major components characterized, both analytically and theoretically. Al-
and single diastereomers. We deduced that the in- ternatively, with a nucleophilic component bearing
volved nucleophilic component was not electron-rich a labile proton, cycloisomerization takes place and de-
enough. Gratifyingly, allene 8 underwent the expected livers highly valuable scaffolds bearing a phosphine
hydroarylation reaction in satisfactory yield providing oxide moiety. The complete axial-to-central chirality
indenylphosphine oxide 17. Our next attempt focused transfer observed in one case augurs well for the
on the Au(I)-catalyzed rearrangement of vinylallene preparation of enantiopure phosphines.
9.[21] In that case, the expected cyclopentadiene prod-
uct 19 was formed in 71% yield (Scheme 5). All these
reactions must transit via gold intermediate 18 bear-
ing a stereogenic center. We surmised that if chirality
transfer[22] takes place in the cycloisomerization step,
then access to novel enantiopure phosphines whose
Experimental Section
chirality relies on a quaternary center b to the phos-
phorus atom should be possible. To probe this point,
we ran a preparative chiral HPLC on 8 and obtained
Synthesis of Ph3PAu–Vinyl Gold Complex 12b
A mixture of Ph3PAuCl (140 mg, 0.28 mmol, 1 equiv.) and
AgOTf (72 mg, 0.28 mmol, 1 equiv.) in DCE (4 mL) was
stirred for 3.5 h at room temperature. The mixture was fil-
tered to remove the AgCl and added to a solution of allene
both enantiomers with ees >98%. We engaged (–,
CD 254 nm)-8[23] in the same conditions as above and
gratifyingly isolated (–, CD 254 nm)-17 with a 97% ee
evidencing an almost total transfer of chirality. To the
best of our knowledge, this is the first example of
chirality transfer for a tetrasubstituted allene.
10b (200 mg, 0.39 mmol, 1.4 equiv.) in DCE (10 mL) and
heated at 508C for overnight. After 16 h, the reaction was
monitored by 31P NMR, the peak at ~45 ppm (Ph3PAuOTf)
had disappeared. The mixture was concentrated under re-
duced pressure to afford 12b as a brown solid; yield: 250 mg
In conclusion, we herein report the reactivity of
readily accessible phosphine oxides containing allenes (63%).
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Synthesis of Allenes Bearing Phosphine Oxide Groups
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Synthesis of Indolizinium Salt 13
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To a solution of gold complex 12b (140 mg, 0.13 mmol,
1 equiv.) in CH2Cl2 (2 mL) was added dropwise TfOH
(12 mL, 0.14 mmol, 1.08 equiv.). After 6 h at room tempera-
ture, a black solid precipitated. The reaction was monitored
by 31P NMR until disappearance of the peaks of starting
gold complex 12b. The black solid was removed by filtration
on Celiteꢁ. Water was added to the residue (10 mL) and
the resultant mixture was diluted with CH2Cl2 (5 mL). The
aqueous phase was extracted with CH2Cl2 (3”10 mL) and
the combined organic layers were washed with brine
(10 mL), dried over anhydrous MgSO4, filtered and concen-
trated under reduced pressure. The residue was purified by
precipitation in CH2Cl2 and Et2O to afford 13 as a pale
brown solid; yield: 84 mg (quant.).
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Synthesis of (À)-17 from Allene (À)-8
To a solution of Ph3PAuCl (4 mg, 0.009 mmol, 0.05 equiv.) in
dry and degassed CH2Cl2 (1.8 mL) was added AgSbF6
(3 mg, 0.009 mmol, 0.05equiv.). After 5 min stirring at room
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mixture was filtered through a short pad of Celiteꢁ and
washed with Et2O and CH2Cl2. The solution was concentrat-
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silica gel chromatography with EtOAc/Pentane (gradient:
from 3:7 until 1:1) as eluent to afford cyclic product (À)-17
as a pale yellow solid; yield: 83 mg (97% ee, 85%).
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[12] Crystallographic data for structures of compounds 11a,
11b and 13 were deposited at the Cambridge Crystallo-
graphic Data Centre with numbers CCDC 1055995
(11a), CCDC 1055996 (11b) and CCDC 1055997 (13).
These data can be obtained free of charge from The
We thank UPMC, CNRS, IUF, the Franco Thai Scholarship
Program administered by CAMPUS FRANCE. We are grate-
ful to Marc Petit (IPCM) for fruitful discussions, Omar
Khaled (IPCM) for mass spectrometry.
Cambridge
Crystallographic Data
Centre via
www.ccdc.cam.ac.uk/data_request/cif.
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[23] The sign has been determined by the circular dichroism
detector at 254 nm in the mobile phase used for the
chiral HPLC separation (see the Supporting Informa-
tion).
2218
asc.wiley-vch.de
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2015, 357, 2213 – 2218