Regio- and Enantioselective Allylation of Phenols via Decarboxylative Allylic Etherification of Allyl Aryl Carbonates Catalyzed by (Cyclopentadienyl)ruthenium(II) Complexes and Pyridine-Hydrazone Ligands

Article abstract of DOI:10.1002/adsc.201500534

(Cyclopentadienyl)tris(acetonitrile)ruthenium hexafluorophosphate [CpRu(CH₃CN)₃][PF₆] in combination with pyridine-hydrazone ligands efficiently catalyzes the asymmetric decarboxylative allylic rearrangement of allyl aryl carbonates. Formation of C-O bonds with high regio- and enantioselectivity ratios (up to 95:5 and 98% ee) is obtained. Good stereocontrol of the pseudotetrahedral geometry of the CpRu moiety is achieved by the hydrazone ligand and its "electron-poor" nature is evidenced through the epimerization of the hexacoordinated TRISPHAT-N anion.

Full text of DOI:10.1002/adsc.201500534

UPDATES
DOI: 10.1002/adsc.201500534
Regio- and Enantioselective Allylation of Phenols via
Decarboxylative Allylic Etherification of Allyl Aryl Carbonates
Catalyzed by (Cyclopentadienyl)ruthenium(II) Complexes and
Pyridine-Hydrazone Ligands
LØo Egger,a Cecilia Tortoreto,a Thierry Achard,a David Monge,b Abel Ros,c
Rosario Fernµndez,b JosØ M. Lassaletta,c,* and JØrôme Lacoura,*
a
DØpartement de Chimie Organique, UniversitØ de Gen›ve, Quai Ernest Ansermet 30, 1211 Gen›ve 4, Switzerland
E-mail: jerome.lacour@unige.ch
Departamento de Química Orgµnica and Centro de Innovación en Química Avanzada (ORFEO-CINQA), University of
b
Seville, C/Prof. García Gonzµlez, 1, 41012 Sevilla, Spain
Instituto Investigaciones Químicas (CSIC-US) and Centro de Innovación en Química Avanzada (ORFEO-CINQA),
c
AmØrico Vespucio 49, 41092 Sevilla, Spain
E-mail: jmlassa@iiq.csic.es
Received: June 2, 2015; Revised: September 4, 2015; Published online: October 14, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500534.
Abstract: (Cyclopentadienyl)tris(acetonitrile)ruthe-
nium hexafluorophosphate [CpRu(CH3CN)3][PF6]
in combination with pyridine-hydrazone ligands ef-
ficiently catalyzes the asymmetric decarboxylative
allylic rearrangement of allyl aryl carbonates. For-
As just mentioned, ruthenium complexes are able
to promote the regioselective attack of nucleophiles
onto unsymmetrical p-allyl fragments and lead to the
preferred formation of branched (b) over linear (l)
products.[10] In addition, stereogenic centers are usual-
ly created during the formation of the adducts. Elec-
tron-rich complexes such as Cp*Ru (Cp*=
C5Me5)[10–11] or Cp’Ru (planar chiral complexes)[12] are
generally preferred to the corresponding CpRu (Cp=
C5H5) complexes[13] due to their higher reactivity. Var-
ious substrates can be used but the most common are
primary or secondary allyl chlorides and carbonates.
From the point of view of selectivity, enantiopure
complexes of Cp*Ru developed by Bruneau gave
high enantioselectivity but modest regioselectivity for
the allylic etherification of allyl chlorides.[14] These
Cp*Ru complexes were generally used in combina-
tion with bis(oxazoline) ligands. Cp’Ru complexes can
also induce stereogenic information. They were found
À
mation of C O bonds with high regio- and enantio-
selectivity ratios (up to 95:5 and 98% ee) is ob-
tained. Good stereocontrol of the pseudotetrahe-
dral geometry of the CpRu moiety is achieved by
the hydrazone ligand and its “electron-poor” nature
is evidenced through the epimerization of the hexa-
coordinated TRISPHAT-N anion.
Keywords: allylic compounds; enantioselective cat-
alysis; etherification; ruthenium catalysts
The nucleophilic attack onto electrophilic allyl metal to be very efficient using allylic chloride or bromide
intermediates is a general tool in organometallic producing excellent regio- and enantioselectivities
chemistry.[1] This methodology allows the formation either with a tethered phosphane[12b] or sulfoxide li-
of C H, C O, C N, C S and C C bonds using the gand.[12a]
À
À
À
À
À
appropriate nucleophiles.[2] Several allylic building
Previously, our group has shown that combinations
blocks for natural product synthesis have been made of [CpRu(CH3CN)3][PF6] 1 (10 mol%) and enantio-
through this methodology.[3] Moreover, the nature of pure pyridine monooxazoline ligands can be efficient
the metal center plays a significant role in the control for the stereocontrol of such transformations.[13d–g]
of the regio- and enantioselectivity of the transforma- Using pymox L1 in particular, decarboxylative allylic
tions. Metals including Cu,[4] Fe,[5] Ir,[6] Mo,[7] Pd,[8] substitution of allyl aryl carbonates gave, for instance,
Rh[9] and Ru[10] have been utilized and display high relatively high enantiomeric excesses and regioselec-
levels of enantioselectivity or enantiospecificity, in ad- tivity (up to 87% ee and b:l>95:5, Scheme 1).[13g] In
dition to regioselectivity.
this process, it was however necessary to monitor the
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Scheme 1. Decarboxylative allylic etherification with CpRu
catalysts.
reactions carefully as branched products (b) were
shown to racemize and isomerize to the more thermo-
dynamically stable linear products (l), conducting
over time to a net loss of both enantio- and regiose-
lectivities. Since electronic and steric properties of the
N,N-bidentate ligands L are crucial for both reactivity
and asymmetric induction,[13b] the literature was sur-
veyed for “diimine” structures that could improve the
Scheme 2. Ligand screening (THF, 258C, 12 h unless other-
wise noted).
situation. The recent family of pyridine-hydrazone li- afforded in particular in comparison with L1 (conver-
gands developed by Lassaletta and co-workers was sion 36% in 36 h vs. >98% in 2 h, respectively). With
deemed particularly interesting. Achiral picolinalde- pyrrolidine L3, reactivity was improved (40% conver-
hyde N,N-dialkylhydrazones are effective in Ir-cata- sion after 12 h) while obtaining comparable regio-
lyzed nitrogen-directed borylation of arenes,[15] while and enantioselectivities. It was thus decided to retain
the chiral pyrrolidine derivative L3 is also a suitable the 2,5-diphenylpyrrolidine motif and investigate the
catalyst in enantioselective 1,2-addition of boronic influence of functional groups on the pyridine ring.
acids to cyclic sulfonimines and Suzuki–Miyaura As it could be expected, the presence of electron-do-
cross-coupling to biaryls.[16] Their use in allylic substi- nating groups increased reactivity to afford conver-
tutions was, however, unknown and care was thus sions up to 69% in 12 h (L4–L6, Scheme 2). Satisfac-
taken to attempt the decarboxylative etherification torily, improvements in regio- and enantioselectivity
reaction in their presence. The key structural ele- were also achieved with these three ligands. L6 (or its
ments that make these ligands appealing for this pur- antipode ent-L6), giving rise to the highest ee value,
pose are the strong conjugation along the hydrazone was selected for the remainder of this study.
system and the C2-symmetry at the pyrrolidine termi-
The scope of the reaction was then explored. First,
À
nus, making the rotation around N N bonds inconse- the influence of substituents of the aryloxy moiety
quential and always maintaining the chiral environ- was evaluated. Surprisingly, an increased reactivity
ment in close proximity to the metal center.[17]
was noticed with electron-donating groups on the
Using conditions developed previously for ligand phenol unit (p-Me or p-MeO) leading however to
L1 (dry THF, freshly distilled,[18] 258C), combinations a slightly lower enantiomeric excesses (Table 1, en-
of
pyridine-hydrazones
L2
to
L6
and tries 2 and 3). As expected, electron-withdrawing
[CpRu(CH3CN)3][PF6] 1 (10 mol% each) were thus groups such as CF3 or NO2 highly increased the rate
evaluated. To our satisfaction, all ligands allowed the of the reaction (entries 4 and 5). For instance, after
reaction to occur and afforded satisfactory results in only 3 h, complete conversion and a good level of
the first experiments. In detail, using piperidine-de- enantiomeric excess were obtained with the nitro sub-
rived L2, interesting levels of regio- and enantioselec- stituent, at the expense of b:l ratio nevertheless. After
tivities were achieved (b:l 93:7 and ee 96%, 12 h of reaction with this substrate, sharp decreases of
Scheme 2). However, only a moderate reactivity was both b:l ratio and enantiomeric excess were noted
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Regio- and Enantioselective Allylation of Phenols
Table 1. Substrate scope.[a]
Scheme 3. Mechanistic proposal.
provide a more effective discrimination than pymox
L1. In addition, the donor amino groups on L5 and
L6 are favorable for the reactivity promoting proba-
bly a faster oxidative addition (A!B) which is be-
lieved to be the rate-determining step.
Moreover, it was shown in previous studies that
allyl aryl ethers 3 continue to react under the [CpRu]/
L1 catalysis.[13d,g] In fact, both in-situ racemization of
[a]
Reaction conditions: 1 (10 mol%), L6 (10 mol%), THF,
258C, 12 h unless otherwise noted, c=0.5M; b:l ratios
and conversions (conv.) determined by 1H NMR
(400 MHz); ee determined by CSP-HPLC or CSP-SFC.
(b:l 57:43. 80% ee). This is explained by the better the products and isomerization to linear (thermody-
leaving group ability of the corresponding nitro-phen- namically preferred) regioisomers 4 happen via oxida-
oxide leading to a faster isomerization and racemiza- tive addition and reformation of p-allyl intermediates.
tion.[13d] On the other hand, substitutions on the cin- These unwanted pathways occur concomitantly to the
namyl part of electron-withdrawing groups led to main reaction. However, with ligand L6, these side re-
good selectivity levels but lower conversions (entries 6 actions are clearly slower allowing the isolation of
and 7). As an example, the nitro group was found to products 3 with high enantiomeric purity levels (ee up
have a negative effect on both enantiomeric excess to 98%) and good regioselectivity.[21]
and reactivity. Clearly, its presence induces a destabili-
At that stage, it was deemed interesting to prepare
zation of the cationic p-allyl complex (see below, in- allyl alkyl carbonates 5a and 5b and a couple of ex-
termediates B and C) and hence a slower formation. periments were performed under standard reaction
Cross-over experiments were further performed (see conditions to estimate, in the presence of L6, the dif-
the Supporting Information) and confirmed, with L6, ference of reactivity of these substrates with that of
the dissociative nature of the transformation.[13d]
A
the aryl derivatives (Scheme 4). While a much longer
rather classical mechanistic rationale can be thus pro- reaction time (96 h) was necessary for the partial con-
posed (Scheme 3). After the ruthenium-olefin com- version of 5a (55%), a total lack of reactivity was ob-
plex formation, the oxidative addition occurs to form served with 5b after four days.[22] In the case of 5a,
the p-allyl intermediate (A!B). At this step, decar- the expected product 6a was formed with good enan-
boxylation of the leaving group occurs to generate tioselectivity (ee 90% vs. 70% with L1) and a moder-
a nucleophilic phenoxide. Its subsequent attack on ate branched to linear ratio of 85:15.
the allylic complex occurs regio- and stereoselectively
Finally, our group has previously reported the syn-
at the most substituted (branched) position.[11j,19,20] thesis and resolution of TRISPHAT-N, a hexacoordi-
After decomplexation, the allylic ether product is ob- nated phosphorus anion (TTN, Figure 1). This anion
tained. The asymmetric induction is likely determined can be isolated in racemic and enantiopure forms; the
during the C!D elemental step. All ligands L2 to L6 D enantiomer being easier to separate by precipita-
Adv. Synth. Catal. 2015, 357, 3325 – 3331
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LØo Egger et al.
Scheme 4. Decarboxylative allylic etherification of allyl alkyl
carbonates.
Scheme 5. Synthesis of [CpRu(L)(TTN)] complexes; cat=
tetrachlorocatecholate.
to the a-pyridine protons H-6 (ligands L1, L6 or ent-
L6) of the four possible diastereomers were distin-
guishable (complex 7, spectrum a, Figure 2).[13g,26]
Then, derivative 8 made of ent-L6 and D-TTN was
monitored and two major peaks were observed for
Figure 1. TRISPHAT-N (D enantiomer shown) as counter-
ion/ligand for CpRu complexes; cat=tetrachlorocatecho-
late.
tion than the L antipode.[23,24] Importantly, TTN effi- this proton immediately after the dissolution of the
ciently coordinates to metal centers thanks to the pyr- complex in CDCl3 (spectrum b1, Figure 2).[27] Howev-
idine moiety. The resulting zwitterionic species are er, to our surprise, it was found that the composition
air- and moisture-stable. Associated with CpRu and of the mixture was changing over time (spectrum b2).
phen/L1 ligands,[13b] the TTN zwitterions become lipo- Knowing from previous studies[23] that the equilibrium
philic and are readily isolated by column chromatog- among the diastereomeric species is reached rapidly
raphy over silica gel.
with CpRu moieties, an epimerization of the TRI-
To gain some insight on the higher asymmetric in- SPHAT anion was considered. This was readily tested
duction with L6 over L1, the preparation of TTN and confirmed with complexes 9, 10 and 11 that af-
complexes was considered even if it was clear that forded the same spectra despite their different origin
this study could be challenging in regard of the pres-
ence of three stereogenic elements within the com-
plexes. In fact, the pseudo-tetrahedral Ru center, the
three-bladed propeller geometry of the anion, and the
enantiopure ligand L6 (or ent-L6) concur to generate
up to four equilibrating diastereomers in solu-
tion.[13c,23] In view of this situation, and hoping to sim-
plify it, care was taken to utilize not only the racemic
but also the available D source of the anion (rac- and
D-TTN, respectively) and this in combination with
both L6 and ent-L6 forms of the ligand.[25] In terms of
synthesis, the complexes were prepared by the reac-
tion of equimolar amounts of 1, ligands L1 or L6/ent-
L6 and [Bu3NH][rac-TTN] or [N-benzylcinchonidini-
um][D-TTN] salts in CH2Cl2 at 258C followed by
a chromatographic separation. The procedure and re-
sulting complexes 7, 8, 9, 10 and 11 are detailed on
Scheme 5.
Figure 2. 1H NMR spectra (500 MHz, CDCl3, 10.0 to
8.0 ppm): a) complex 7 (L1, rac-TTN); b1) complex 8 (ent-
L6, D-TTN) immediately after dissolution; b2) complex 8
after 12 h; c) complex 9 (ent-L6, rac-TTN); d) complex 10
First, for comparison purposes, the complex made
1
of pymox L1 and rac-TTN was analyzed by H NMR
spectroscopy and, as expected, signals corresponding (L6, D-TTN); e) complex 11 (L6, rac-TTN).
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Figure 3. Proposed geometries of the diastereomeric pseudo-tetrahedral [(RRu)-CpRuL6(CH3CN)] and [(SRu)-
CpRuL6(CH3CN)] complexes.
(spectra c, d and e, Figure 2). In these three instances, Experimental Section
only two signals were obtained with the same intensi-
ties (~86:14 ratio) irrelevant of the racemic or D General Procedure
nature of the anion or of the ent-L6 or L6 character
of the ligand. Normally, with the racemic anion, one
would have expected to see four peaks and not two
(see spectrum a for instance). Exchanging ent-L6 to
L6 in the complexes made with the enantiopure
anion, one would have expected to observe two new
signals at different chemical shifts which was not the
case (spectra b2 and d). As mentioned, the most logi-
cal explanation of this unexpected phenomenon is
a racemization of the anion induced by its binding to
the Lewis acidic Ru moiety. In fact, in the field of
chiral hexacoordinated phosphate chemistry, it is
known that Brønsted or Lewis acids provoke an equi-
librium between the D and L forms through partially
dissociative mechanisms.[28] The fact that complexes
made of L6 appear to be more Lewis acidic than that
derived from L1 can be explained by the more elec-
tron-poor nature of L6,[21] and the higher steric strain
induced onto the anion by the bulkier ligand. These
results also indicate that L6 (or ent-L6) leads to
a better stereocontrol of the pseudo-tetrahedral
CpRu geometry than ligand L1,[29] and this might be
In
a
2-mL vial under an N2 atmosphere, [Ru(h5-
C5H5)(CH3CN)3][PF6] 1 (6.3 mg, 14.4 mmol, 0.1 equiv.) and
ligand (14.4 mmol, 0.1 equiv.) were dissolved in 0.3 mL of an-
hydrous freshly distilled THF. The resulting deep red solu-
tion was stirred for 5 min at 258C before the addition of the
allyl aryl carbonates (0.144 mmol, 1 equiv.). The reaction
mixture was stirred under N2 at 258C until no trace of the
starting material could be detected on TLC (SiO2,
Et2O:pentane 8:2). The reaction mixture was diluted with
1 mL of an 8:2 mixture of ether and pentane. The precipitat-
ed metal salts were filtered on a short SiO2 column
(0.5 cm”4 cm, elution Et2O:pentane 8:2). The solvents were
evaporated under reduced pressure to afford the crude reac-
tion mixture as a pale yellow oil which was analyzed by
1H NMR and CSP-HPLC or CSP-SFC.
Acknowledgements
We thank the University of Geneva and the Swiss National
Science Foundation, the Spanish MINECO (grants
CTQ2013-48164-C2-1-P and -2-P, contract RYC-2013-12585
the reason for the higher asymmetric induction in the for A.R.), the European FEDER funds and the Junta de An-
dalucia (Grant 2012/FQM 1078) for financial support. We
are also grateful to Dr. Martina Austeri and Dr. David
Linder for fruitful discussions. We also acknowledge the con-
tributions of the Sciences Mass Spectrometry (SMS) platform
at the Faculty of Sciences, University of Geneva.
etherification. Accordingly, the predicted geometries
for the diastereomeric pseudo-tetrahedral [(RRu)-
CpRuL6(CH3CN)] and [(SRu)-CpRuL6(CH3CN)]
complexes[30] suggest that a considerable steric repul-
sion between the Cp ring and one of the phenyl
groups of the diphenylpyrrolidine terminus in the
latter might be responsible for a much favored forma-
tion of the (RRu) isomer (Figure 3).
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it was necessary to distil the THF obtained from sol-
vent purification systems (drying columns).
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Adv. Synth. Catal. 2015, 357, 3325 – 3331

UPDATES
Regio- and Enantioselective Allylation of Phenols
[19] The cross-over experiment supports a dissociative path-
way.
L6 form of the ligand to compensate for the lack of L
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obtained with a 96–99% enantiomeric purity for the
hexacoordinated phosphate.
[26] The diastereomeric ratio was constant over time in this
1
case (1.0:0.75:0.55:0.35, H NMR, 400 MHz).
[27] The minor peaks probably come from the presence of
a minimum amount of L enantiomer contained in the
N-benzylcinchonidinium salt.
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[25] In fact, in order to study all possible diastereomeric
combinations, it is necessary to use the antipodal ent-
Adv. Synth. Catal. 2015, 357, 3325 – 3331
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